Transporting live fish under high-density and prolonged conditions poses significant challenges due to rapid deterioration of water quality, primarily from ammonia accumulation. This study introduces a novel water-treatment approach for extended holding and transport of live fish and seafood, demonstrated here for 96 h with juvenile gilthead seabream (Sparus aurata) at bio-density of ∼65 kg/m³ , which is roughly 15 times more intensive than the standard practice. The system integrates oxygen enrichment, CO₂ stripping, pH regulation, turbidity control, and an innovative cation-exchange material made of PES-coated zinc hexacyanoferrate (ZnHCF) for efficient ammonia removal in saline water. ZnHCF demonstrated very high selectivity for NH₄⁺, full regenerability with NaCl, and stable performance under seawater conditions. Four semi-commercial scale trials confirmed that key water-quality parameters remained within safe predetermined limits (TAN ≤ 7 mgNH₄⁺/L; NH₃ ≤ 0.05 mg/L; turbidity ≤ 8 NTU), while plasma cortisol analysis indicated minimal stress except when intentionally induced. Survival rates exceeded 98.8%, with no post-transport mortality. This novel and scalable zero-liquid-discharge technology has a potential to become a breakthrough in improving the sustainability of modern aquaculture logistics and the live seafood supply chain, enabling longer transport durations and higher stocking densities without compromising fish welfare.

Desalinated water, while increasingly relied upon as a sustainable source of drinking water, is deficient in essential minerals, out of which the most important macronutrient is magnesium. Magnesium deficiency has been linked to cardiovascular diseases and other health risks in humans, as well as negative effects on agricultural productivity when desalinated water is used for irrigation. This review covers current technological approaches for enriching desalinated water with Mg²⁺, ranging from simple techniques such as dilution with seawater or other brines, direct chemical supplementation and dissolution of quarry dolomite, to advanced seawater-separation-based methods including ion-exchange, nanofiltration followed by diananofiltration and ultrafiltration, hybrid electrodialysis/nanofiltration, and flow-electrode capacitive deionization. Case studies from Israel and Saudi Arabia highlight practical challenges, normalized costs, and policy drivers. While no single method is universally applicable, separation-based technologies offer promising scalability and cost-effectiveness for achieving regulatory targets between 5 and 20 mg/L as Mg in the product water. A comparative analysis of operational, economic, and water quality implications is presented, together with answers to common questions and perspectives for future research and implementation.

Rb⁺ is a critical metal ion with increasing demand and limited terrestrial resources. In this study, a multi-step ion-exchange and thermal separation process was developed to extract nearly pure RbCl_(s) from the raffinate stream produced during industrial phosphoric acid manufacturing. The semi-pilot-scale process consisted of an initial adsorption step using a Zn-hexacyanoferrate (Zn-HCF) ion exchanger, followed by chromatographic desorption (with NH₄Cl as the eluent), drying of the regenerant, and sublimation-based separation of RbCl_(s) and NH₄Cl_(s) at > 350 °C. The process offers a practical and scalable route for converting industrial brines into a high-value RbCl_(s) product salt with a purity of ≥ 99.5 %. A comprehensive cost assessment, based on experimental data, mass balances, and operational parameters, indicated a normalized production cost of approximately 25 % of the 2024 market price of RbCl_(s), corresponding to a return on investment (ROI) of less than 2.5 years. These results demonstrate both the technical feasibility and economic viability of selectively recovering rubidium salts from complex industrial waste streams.

Extending the recovery ratio (RR) of brackish water reverse osmosis (RO) plants to zero liquid discharge (ZLD, i.e., ≥95%) is vital, particularly inland, where the cost of safe retentate disposal is substantial. Various suggestions appear in the literature; however, many of these are impractical in the real world. Often, the limiting parameter that determines the maximal recovery is the SiO₂ concentration that develops in the RO retentate and the need to further desalinate the high osmotic pressure retentates produced in the process. This work combines well-proven treatment schemes to attain RR ≥ 95% at a realistic cost. The raw brackish water undergoes first a 94% recovery nanofiltration (NF) step, whose permeate undergoes a further 88-RR RO step. To increase the overall RR, the retentate of the 1st RO step undergoes SiO₂ removal performed via iron electro-dissolution and then a 2nd, 43% recovery, RO pass. The retentate of this step is combined with the NF retentate, and the mix is treated with mechanical vapor recompression (MVR) (RR = 62.7%). The results show that >95% recovery can be attained by the suggested process at an overall cost of ~USD 0.70/m³. This is ~60% higher than the USD 0.44/m³ calculated for the baseline operation (RR = 82.7%), making the concept feasible when either the increase in the plant’s capacity is regulatorily requested, or when the available retentate discharge method is very costly. The cost assessment accuracy was approximated at >80%.

This study presents a new, sustainable, high-pH recirculating batch process for the regeneration of NH₄⁺-laden cation exchange resins, while recovering the ammonia as a pure, reusable, solid salt (NH₄Cl). The process efficiently recycles a minimal volume of regeneration solution while reducing NaCl consumption between cycles. The regeneration solution, containing a high concentration of NaCl maintained at high pH (pH > 11) allows Na⁺ to replace the desorbed NH₄⁺, which, due to the high pH, completely transforms into NH₃. Real time pH monitoring of the regeneration solution and the column effluent provided accurate tracking of the regeneration rate and allowed concise endpoint determination. Ammonia was recovered by evaporation of the exhausted regeneration solution followed by sublimation (and thereafter deposition) of NH₃ and HCl, which produced pure NH₄Cl salt. The optimal working conditions for the cation exchange regeneration were identified as 1 M NaCl, pH 12.0, and regeneration solution volume of 1 bed volume. Single-column application of six adsorption-regeneration cycles (no zeolite replacement) showed that the process conditions do not degrade the resin's adsorption performance. Cost assessment of two possible ammonia recovery options indicated that while stripping NH₃ from the regeneration solution and reabsorbing it in an acidic solution is operationally easier, the sublimation-based approach yields a more valuable product. For an optimized sublimation approach, a low regeneration solution volume, combined with a high resin capacity for NH₄⁺, are essential. This process demonstrates a sustainable solution for ammonia salt recovery from wastewater, offering a circular approach that does not only add an important control to the regeneration step, but also harvests ammonia as a reusable product.

The transport of live aquaculture species, both throughout the culture period and during marketing as live products, is far from optimized. In wet transports, the bottleneck is the total ammonia nitrogen (TAN) concentration that develops in the transport water. The effect of actively removing TAN from the water on the survival of fish and crabs during wet transport is herein reported. The TAN concentration was controlled by exchanging NH₄⁺ with Na⁺ using self-synthesized polymer-coated zinc-hexa-cyanoferrate (Zn-HCF) cation-exchange material. Theoretical simulation followed by empirical experiments were carried out to mimic the transports of Gilthead seabream and European Brown crab, in the presence and absence of Zn-HCF. The Zn-HCF proved very efficient: while the TAN concentration in the seabream test tanks did not exceed the predetermined 5 mgN/L value (throughout the work), the control tanks showed much higher values. In the Brown Crab experiment the control had to be stopped after 2.3 days to avoid animal welfare issues, while the test completed the full 4.8 d. Yet, the crabs from the Test tanks exhibited much higher survival rates following 24 and 48 h dry packing. The results show unequivocally that the addition of Zn-HCF to wet transfer tanks can improve the vitality of the shipped animals, allow to extend the transfer periods and/or increase the shipment density.

A new hydrothermal hot isostatic pressing (HHIP) approach, involving hydrothermal water conditions and no usage of inert gas, was hypothesized and tested on 3D-printed Al-10%Si-0.3%Mg (%Wt) parts. The aluminum-based metal was practically inert at the applied HHIPing conditions of 300–350 MPa and 250–350 °C, which enabled the employment of a long (6–24 h) HHIP treatment with hardly any loss of material (the overall loss due to corrosion was mostly <0.5% w/w). Applying the new approach on the above-mentioned samples resulted in an 85.7% reduction in the AM micro-pores, along with a 90.8% reduction in the pores’ surface area at a temperature of 350 °C, which is much lower than the 500–520 °C applied in common argon-based aluminum HIPing treatments, while practically maintaining the as-recieved microstructure. These results show that better mechanical properties can be expected when using the suggested treatment without affecting the material fatigue resistance due to grain growth. The proof of concept presented in this work can pave the way to applying the new HHIPing approach to other AM metal parts.

The operation of municipal wastewater treatment plants (WWTPs) invariably results in significant emission of greenhouse gases (i.e., CH₄, N₂O, and CO₂) into the atmosphere. We propose to consider a radical change in the way municipal WWTPs are operated, with the aim of minimizing GHG emissions while recycling most of the nutrient mass. The means to this end are to reduce the WWTP energy demand while maximizing the recovery of resources (phosphorus, ammonia, methane). The suggested concept involves operating the activated sludge process at a low sludge retention time (SRT < 2 d), i.e., under conditions that maximize the heterotrophic mass yield and eliminate nitrification. The ammonia concentration that remains in the water (considering N in the excess sludge and struvite production in the sludge-dewatering supernatant line) would be separated from the WWTP effluents using a unique ion-exchange material (ZnHCF), which would be regenerated using a low-volume 4 M NaCl solution. The ammonia would be then stripped at high pH and re-adsorbed by an acidic solution for reuse as fertilizer. The high bacterial yield and lack of nitrification in the aerobic step are expected to boost methane yield 3–4-fold, induce lower oxygen consumption, and most importantly, yield much lower N₂O release. An approximate energy mass balance shows the concept to merit further consideration, owing to the potential significant reduction in N₂O_(g) emissions and recovery of resources. Empirical work followed by LCA is required to corroborate the hypothesis presented herein.

A new concept is presented for eliminating off-flavor from cold-water RAS-grown fish, while feeding, and as a part of the normal grow-out period. The technology is based on disconnecting the nitrification biofilter, and instead passing the water through an electrolysis system, which both oxidizes the ammonia and disinfects the water, while also removing the off-flavor compounds from the water, which thereby results in the purging of the fish. The purging period was expected to last up to 2 weeks and the fish are fed throughout it. Laboratory and pilot plant experiments were performed to prove the new concept. Lab experiments included quantification of the removal of MIB and geosmin by electrooxidation and stripping, together and separately, in the presence and absence of organic matter. A pilot plant experiment was performed using Rainbow trout to determine the rate at which the off-flavor compounds were removed from the water and the fish flesh (both skin and muscle were tested). The results show that the treatment process eliminated off-flavors in the water after ∼7 days and that the fish were below taste and odor threshold for geosmin and MIB after a maximum of 11 days. Detachment from the biofilter and the fact that the water was vigorously disinfected during the electrooxidation step guaranteed that no further off-flavor compounds would be generated during the operation. Aquacultural-management assessment indicates that RAS farms can increase both their annual production and their income by more than 10%, by implementing the suggested concept as part of the grow-out period.

Electrochemical water treatment for recirculating aquaculture systems (RAS) is a promising approach for replacing the biological water treatment methods and establishing a new RAS generation with improved cost-effectiveness, lower environmental footprint, and no start-up periods. On top of ammonia oxidation directly into N_2(g), electrochemical oxidation results in effective disinfection, and in the removal of organic matter, including specific organic constituents such as off-flavour agents. The paper provides an overview of incentives for the implementation of electrochemical methods in RAS. It covers the electrochemical principles relevant to aquaculture applications, the effects of physical and chemical parameters, as well as design considerations. In addition, the research performed to date for integrating electrochemical methods in RAS operation is reviewed and the variety of designs and operational configurations described. The electrochemical water treatment is perceived beneficial over biological water treatment especially in cold saline-seawater aquaculture (e.g., Atlantic salmon), where large nitrification reactors are required and the large water consumption for purging processes can be curtailed. It is also beneficial for the culturing of nitrate-sensitive species (e.g., L. vannamei). The paper points out the gaps to be overcome for allowing commercial breakthroughs based on electrochemical water treatment, including the need for expanding the practice and improving engineering practices by operating pilot systems for growing fish at both small and large scales; adjusting of electrochemical cell designs for reducing both capital and operational costs; developing full-proof malfunction-free dechlorination strategies, and evaluating and optimizing the disinfection abilities for inactivating typical pathogens in aquaculture.

A new general method is presented for separating pure RbCl_(s) from solutions rich in Na⁺ and K⁺. The method relies on Rb⁺ adsorption via ion exchange performed by self-synthesized PES coated Zn-Hexa-Cyanoferrate material. The procedure starts by passing the wastewater through an ion exchange column, which is thereafter regenerated with 1 M NH₄Cl. If the Rb⁺ absorbed on the column does not reach a minimal predetermined value (e.g., 8%, eq-based), the ammonia is removed by sublimation and the remaining salts are passed again through a Na⁺-preadsorbed column. Once the adsorbed Rb⁺ is substantial (>8%), a chromatography-based separation between Rb⁺ and Na⁺/K⁺ is performed, using a 2^nd column, fully pre-adsorbed with NH₄⁺. First, 0.05M NH₄⁺-solution is used to extract Na⁺ and K⁺ out of the first column, along with a small Rb⁺ mass, which is thereafter partly re-adsorbed on the second column, while Na⁺/K⁺ ions are not. Once the exiting eluent solution is devoid of the competing ions, 1M NH₄⁺-solution is used to extract all the remaining Rb⁺ into the regeneration solution, which is thereafter subjected to water evaporation followed by NH₃/HCl sublimation to result in pure RbCl_(s) product. We used theoretical simulations corroborated by empirical results to present proof of concept for the suggested approach. A detailed cost analysis (Capex and Opex) reveals that the RbCl_(s) production cost does not exceed ∼25% of the current salt price.

A new approach is presented for pretreating very hard, divalent-ion-laden brackish-water, to allow applying reverse-osmosis desalination at reasonable recovery-ratio and competitive cost. The approach consists of a cation-exchange step applied on the raw-water to reduce the divalent-cation concentrations thereby enabling operation at high recovery (>80 %) without chemical scaling. For cost-effectiveness, the exhausted ion-exchange regeneration solution undergoes nanofiltration separations to recover the NaCl, enabling multi-cycle usage. The process was exemplified on brackish groundwater from New-Mexico's National-Desalination-Research-Facility. Two treatment alternatives were considered for the exhausted regeneration brine, consisting of several NF in series (alternative 1) and additional NF-retentate treatment (alternative 2). Following simulation and selection of alternative 1, empirical-evaluation and cost-assessment ensued. The results showed that when the regeneration-solution contained total-hardness < 2 % of the total-cation concentration (eq/eq), the cation-exchange breakthrough curves did not deteriorate. To maintain this criterion the exhausted regeneration-solution should undergo 3–4 sequential NF passes after each regeneration cycle. This approach enables operating the BWRO plant with RR = 81 % at additional cost of ∼US$0.4/m³ (CAPEX+OPEX), which ∼doubles the overall brackish desalination cost, but also produces more desalted water and much less retentate. The approach is competitive when the brackish water is laden with divalent ions and the retentate discharge-cost is high.

This paper reports on the physico-chemical removal of NH₄⁺ from the supernatant line in municipal wastewater treatment plants (WWTPs), using zinc-hexa-cyano-ferrate (ZnHCF) beads. The work is divided into three parts: First, the characteristics of three (Zn-, Co-, Ni-) types of HCF beads were determined, with a finding that ZnHCF was the most suitable for the purpose of this work. Second, synthetic and actual supernatant wastewater was passed through a ZnHCF column for many cycles until apparent steady-state results were attained. Due to the very high affinity of the beads toward NH₄⁺ and the much lower affinity toward competing cations, the same regeneration solution could be used for many cycles (20 cycles in this work) without affecting the following adsorption breakthrough curve efficiency and the operational capacity, which was >88% at the end of all adsorption steps. Finally, a cost analysis was performed, revealing that the cost of removing ~500 mg/L of ammonia from the supernatant line is ~$0.02 per m³ of raw wastewater flowing into the plant if the ammonia is recaptured and sold as NH₄Cl. This may be cost-effective when the WWTP receives a higher-than-planned load, and an incentive exists for alleviating the ammonia load on the oxidation reactor.

The continuous removal of TOC and the degradation efficiency of carbamazepine and 17β-estradiol were investigated using actual secondary municipal-effluent RO-retentate solutions. A specific set of operating parameters were applied within the supercritical water oxidizing conditions: temperature range 420–480 °C, 25.1 MPa, hydraulic retention time (HRT) of 1–2 min, excess oxidant molar-ratio of 3–10 and presence of a homogenous catalyst (IPA) at 50–100 mg/L. > 99% organic carbon mineralization, along with complete degradation of model pollutants, was observed at 450 °C/1 min/OC= 5–10 and 100 mgIPA/L. The outlet estrone concentration, 1.03 ± 1.14 ng/L, representing estrogenic pollutants, dropped to the “no effect” range. A model for a SCWO plant treating secondary-municipal-effluent-RO-retentate for a city of 100,000 capita-equivalent was developed, based on a shell & tube SCWO flow reactor, showing > 75% energy-efficiency. The model yielded that for the extreme case of a zero caloric-value feed-solution, the total OPEX and CAPEX would be < $6.0 ± 2.5 per m³ of secondary effluents, i.e., two orders of magnitude lower than the reported environmental shadow-price associated with CECs (contaminants of emerging concern). Further work is required on the continuous and efficient separation of the salt-matrix, which can lead to higher overall heat transfer coefficients and enable further reduction in capital costs.

Continuous subcritical and supercritical water oxidation experiments were conducted on dilute carbamazepine- and estradiol-containing synthetic solutions used to simulate the removal of model emerging pollutants from secondary municipal effluents. The operating conditions comprised 340–500 °C, retention time of 24–453 s and a stoichiometric oxidant ratio (O.C.) between 4 and 64. The transformation of the various species was determined at the outlet and by modeling a segmented non-isothermal reaction system. Four empirical power law kinetic models were established to represent both the pollutants' degradation and TOC removal efficiencies, using nonlinear multiple regression coupled with bootstrapping and K-fold cross-validation. The mineralization and degradation models for both pollutants yielded a R² of 67–80.5% vs. the experimental results. Discussion on the various model assumptions revealed that attributing full model deviations to the constant oxygen concentration or to the laminar reactor flow, yielded a deviation of 6% and 15% in the removal efficiencies, respectively. However, the expected deviation of the models was lower than 0.32% at conditions leading to (almost) full mineralization (45–60 s, 480–500 °C and O.C.s of 5–10). The methodologies developed in the study are useful for interpreting future results obtained from SCWO of actual secondary effluent solutions.

An onboard zero-discharge water treatment process, aimed at the well-boat industry, is described and demonstrated as an alternative to current techniques. Two live-fish holding trials were carried out with Arctic char for 8 days (84 kg/m³) and 6 days (154 kg/m³). The trials were conducted in a zero-discharge pilot system with a unique water treatment unit relying on electrochemical water-treatment for oxidizing ammonia into N_2(g), disinfecting the water, removing fine particles/organic matter and chlorine-species via activated-carbon filtration. No professional skills are required to operate the system. The TAN excretion rate of adult Arctic char under starvation at 5°C amounted to ~22 mgN/(kg·d). Sensory evaluation showed that Arctic char could be successfully held in a system based on the suggested technology without adverse effects relating to mortality and product quality. The costs of operating the system were assessed at 0.30 USD per one ton of standing stock per 24-h journey.

Typical product water from desalination plants is depleted of salts and important minerals that are essential for its final consumption as well as its conveyance in the distribution system. In this study, we propose a hybrid nanofiltration (NF)-reverse osmosis (RO) filtration scheme that can produce desalinated brackish water with improved mineral composition. Here, the monovalent-divalent ion selectivity of NF membranes is used in a preliminary step to remove and “store” divalent minerals in the NF retentate stream, which are later dosed back to the mineral-free RO permeate to replenish the product water. We combine experimental data from pilot-scale NF with RO simulation to demonstrate the feasibility and economic viability of the proposed treatment scheme. In particular, we show the potential to operate the hybrid filtration scheme at high recovery ratios (>85%) and minimum saturation indices of potential scaling solids. A cost assessment further supports the applicability of the proposed treatment scheme, estimating the total capital and operating cost at $0.24/m³ product, which is on par with existing brackish-water desalination costs.

Reverse osmosis (RO) is currently the most cost-efficient method for seawater (SW) desalination; however, producing high-quality water with a low boron concentration typically requires a two-pass process, which increases the required area and chemical consumption. We propose a sustainable and economic pathway for boron removal in a single RO step, thus reducing the area footprint. At the same time, chemicals are produced onsite from the RO brine using bipolar membrane electrodialysis (BMED), thus reducing the chemical footprint. We conducted BMED using natural and synthetic feed solutions and studied the acid and base production kinetics and electricity consumption to assess the feasibility. In terms of energy efficiency, the divalent cationic impurities in the feed are more detrimental than the anionic ones. We found that monoselective cation-exchange membranes are not efficacious in eliminating these, and hence, precipitation/nanofiltration before BMED is essential. As a BMED feed, the nanofiltered SWRO brine was the best option over SW or nanofiltered SW. Economical analysis shows that as compared to purchasing chemicals, BMED integration can reduce the process cost by 45%. In addition, the results point to the flexibility of the proposed design that increases its robustness toward fluctuation in chemicals and electricity prices.

The maximal recovery ratio of brackish water RO desalination plants located on the coastline can be significantly increased by exchanging Ca²⁺ with Mg²⁺ and Na⁺ ions in the raw water by passing the water through a cationic ion-exchange (CIX) resin which is thereafter regenerated with seawater. The concept allows also enriching the product water with Mg²⁺, at no additional cost. The Ca²⁺ fraction (equivalent-based) attached to the CIX resin at equilibrium with seawater was merely ~10% of its total capacity, much lower than Na⁺ (59%) and Mg²⁺ (28%). The process was simulated on the Sabha-Eilat BWRO plant (Israel) water with the goal of reducing [Ca²⁺] from ~850 to ~450 mg/L using a breakthrough/seawater-regeneration sequence. The goal was met by applying a 60 bed-volume (BV) breakthrough-step followed by 20-BV seawater regeneration (HRT = 6 min). ~2% of the CIX-mended water bypassed the RO step to provide 10 mg Mg/L in the product water. Simulating the maximal recovery ratio showed that it could be raised to 89.2%. A general design and cost-analysis revealed that the cost of each additional m³ of desalinated water produced by the new method was ~$0.23, ~14% more water is produced and retentate production is ~50% lower.

A new water treatment technology is presented for extending the longevity and increasing the maximal bio-load of container-bound, lucrative live seafood transportations. The technology is designed for removing ammonia and minimizing the bacterial concentration that develop in the water during the transport. This paper focuses on the characteristics of self-synthesized polyether-sulfone (PES) coated Zn-HCF composite beads, which have a high adsorbing capacity for NH₄⁺ in seawater and constitute the heart of the developed technology. Adsorption isotherms show that the operational capacity of the composite material (PES = 20% w/w) at NH₄⁺ concentration of 10 mgN/L at 3.5 °C is ∼3 mgN/g Zn-HCF. The kinetics of the PES-coated beads were shown to be considerably slower than the bare Zn-HCF, but since the retention time in the transport is long (many days), this does not detract from the effectiveness of the adsorption. Simulation experiments with and without live fish showed that the adsorbing material behaved as expected during a 21-d trip and that it did not have any effect on the fish. Repeated adsorption/regeneration (3 and 6 M NaCl) tests proved the composite material's stability and ion-exchange robustness. Electrooxidation of the ammonia in the exhausted regeneration solution was carried out with high efficiency and the treated solution could be used effectively in the following chemical regeneration step. The cost of a treatment unit installed in a 40-foot container was estimated at $40,000 and the ROI at 6 to 12 months.

Intrusion of toxic heavy-metal cations into water-distribution systems (WDS) may cause severe adverse health-effects on large populations, along with an undesirable psychological impact. The corrosion (scale) layer, that invariably develops on the pipes’ inner walls, is capable of adsorbing a significant mass of metal-cations and releasing them thereafter via diffusion to the water once operation is resumed, thereby causing a secondary contamination event. To overcome this, the contaminant should be completely removed, in a controlled fashion, from both the aqueous and scale phases, with minimum damage to the pipe’s physical stature. This study determined the range of the Cd(II) adsorption capacity of corrosion-scales and quantified alternative treatments for desorbing it, using an assortment of metal water-pipes, extracted from the WDS. Batch, water-recirculation and flow-through experiments were conducted to determine the extent of Cd(II) adsorption and the best way to desorb it. Corrosion-scales showed substantial Cd(II)-absorption capacity (up to 0.75 mg Cd(II)/g scale) with an approximately linear relation between the aqueous Cd(II) concentration and the adsorbed mass. Desorption experiments included dosages of various acids. Sequential rinsing (eight pipe-volumes) by pH3 solution was found to be the best approach, releasing close to ∼100% of the adsorbed Cd(II), with only a minor effect on the pipes’ integrity.

A new analytic approach is presented for determining the total volatile fatty acids (VFA_T) concentration in anaerobic digesters. The approach relies on external determination of the inorganic carbon concentration (C_T) in the analyzed solution, along with two strong-acid titration points. The C_T concentration can be determined by either a direct analysis (e.g., by using a TOC device) or by estimating it from the recorded partial pressure of CO₂ (g) in the biogas (often a routine analysis in anaerobic digesters). The titration is carried out to pH 5.25 and then to pH 4.25. The two titration results are plugged into an alkalinity-mass-based equation and then the two terms are subtracted from each other to yield an equation in which VFA_T is the sole unknown (since C_T is known and the effect of the total orthophosphate and ammonia concentrations is shown to be small at this pH range). The development of the algorithm and its verification on four anaerobic reactor liquors is presented, on both the raw water and on acetic acid-spiked samples. The results show the method to be both accurate (up to 2.5% of the expected value for VFA_T/Alkalinity >0.2) and repetitive when the total orthophosphate and ammonia concentrations are known, and fairly accurate (±5% for VFA_T >5 mM) when these are completely neglected. PHREEQC-assisted computation of C_T from the knowledge of the partial pressure of CO₂ (g) in the biogas (and pH, EC and temperature in the liquor) resulted in a very good estimation of the C_T value (±3%), indicating that this technique is adequate for the purpose of determining VFA_T for alarming operators in case of process deterioration and imminent failure.

A new technology for live seafood transportation has been developed and tested at real conditions. The technology, whose purpose is to treat the water to allow for increased transportation periods and biomass loads, comprises the following steps: (i) pH control to maintain low NH₃ concentration; (ii) electrochemical removal of ammonia; (iii) water disinfection; and (iv) separation of dissolved and particulate organic matter. A fraction of the rearing water is separated several times a day from the live seafood vessels and electrolyzed in a separate reactor to generate active chlorine, which oxidizes ammonia into nitrogen gas, in parallel to disinfecting the water. The electrolyzed water is then dechlorinated by a metabisulfite-based solution and pumped back to the holding tanks through an activated carbon column, applied to remove traces of both chlorine residuals and soluble organic matter. The water treatment process is thereafter repeated with a new portion of water, in a batch manner, as required to maintain the ammonia concentration at a predetermined value. The proposed technology was tested with European brown crab (Cancer pagurus). Four experiments were conducted to simulate transportation in cold aerated water using contemporary technology (two “control” tanks) and a similar setup was operated in parallel, but in which the rearing water was treated, using the tested technology. The tanks that were operated with the new system showed significant reduction in un-ionized ammonia and dissolved and particulate organic matter concentrations. Accordingly, the treatment resulted in much higher survival rates of the crabs, in experiments lasting for 16 days, conducted at a high crab density (~170 kg/m³). Economic analysis conducted using the data obtained in this Proof-of-concept study showed that the proposed technology has the potential to significantly increase the profitability of seafood producers and retailers and to decrease the price of live seafood to the consumer.

A hybrid nanofiltration-electrodialysis process was developed and tested for highly-selective separation of MgSO₄ from seawater. The magnesium-rich solution was produced primarily for replenishing Mg²⁺ into desalinated water within the post-treatment step in seawater desalination plants. The new three-step process consists of: (1) seawater nanofiltration (NF) for generation of a magnesium-rich retentate; (2) a Dia-NF step in which the NF retentate is used as feed, which is aimed at minimizing unwanted solutes (primarily B, Na⁺ and Cl⁻); and (3) a selective electrodialysis (ED) step on the retentate from step 2, aimed at further lowering the Cl⁻:Mg²⁺ mass ratio to below 0.2, to produce a high-purity MgSO₄-rich product solution. Results are presented with an emphasis on the ED step, for which the effects of flow velocity, current density and two operation modes (constant-current and constant-potential) were assessed. The results show a highly-dependent selectivity of the monovalent-selective ion-exchange membrane to the cell potential, suggesting that it can serve as a fine-tuning tool for selective ED processes. The cost of adding 20 mgMg/L to one m³ of desalinated water was estimated at ~$0.014/m³, which is competitive compared to other separation techniques, and certainly versus the alternative of direct dosage of food-grade magnesium salts.

This work focuses on applying supercritical water oxidation (SCWO) for eliminating residues of pharmaceutical, endocrine-disruptor and pesticide species from the reject of reverse-osmosis-treated secondary municipal wastewater. SCWO, a well-known treatment method for organic sludge and highly concentrated wastewaters, has not yet been investigated with reverse osmosis (RO) retentates of municipal secondary effluents. The results shown here, focusing on batch SCWO of municipal effluent RO retentate spiked with emerging pollutants, demonstrate the potential of utilizing this single advanced-oxidation generic treatment to fully-mineralize the vast variety of organic species encountered in municipal wastewater effluents, while at the same time lowering its total organic carbon concentration by up to 99%. The chosen model pollutants (carbamazepine and 17β-Estradiol both in synthetic solution and within a real effluent RO retentate matrix) and the effluent’s background organic matter, started to degrade already at subcritical conditions, however near-complete mineralization of the full matrix (including transformation products) was invariably attained only at fully-established supercritical conditions (450°C–540°C). The extreme hydrothermal oxidation conditions applied in this work allowed for simultaneous oxidation of target molecules and background organic species throughout the applied temperature range, while the phenomenon known as “radicals scavenging” by the background organic matter, was not observed. A new technique is introduced in this work for controlling the pressure within semi-batch SCWO experiments.

Biofouling of seawater reverse osmosis (SWRO) membranes is a major problem that SWRO freshwater production facilities face worldwide. A three-step pretreatment scheme, initially designed to assist the SWRO process in boron rejection, was examined as a potential membrane biofouling inhibitor. The conventional pretreatment procedure in this study includes acidification, CO₂ stripping and pH elevation, and its effect on planktonic cell mortality and biofilm formation on the membrane was analyzed. An in-situ electrolytic production of free chlorine in seawater, during the low pH phase, was suggested as an additional pretreatment step, with the goal of serving as an aggressive disinfection course. In addition, 48-hour enhanced biofouling experiments using an RO membrane cell assembly were conducted in order to observe the flux decline pattern as affected by the various treatments. The conventional pretreatment scheme was found to have a moderate effect on membrane biofouling, increasing the membrane permeability by 25% relative to an untreated experimental run. Moreover, the conventional pretreatment did not significantly promote cell mortality; however, it caused an increase in extracellular polysaccharide production during biofilm formation. The additional chlorination step increased the biofouling phenomenon, causing the production of particulate matter, likely due to cell wall oxidation.

A new process is presented for replacing the typical NaCl-based with a KCl-based solution, for regenerating water-softening ion-exchange resins. The incentive is three-fold: to minimize the release of detrimental brines to the environment, to augment K⁺ in humans’ diet at the expense of harmful Na⁺ and to improve the worth of the treated water with respect to irrigation and soil properties. Since KCl is 3–5 times more expensive than NaCl, the proposed process targets the recycling of the spent regeneration solution through a sequence of membrane processes (nanofiltration, diananofiltration, reverse osmosis) aimed at separating the divalent cations (hardness species) from K⁺, followed by concentration of a fraction of the K⁺ solution using RO to prepare it to serve as the next-cycle regeneration solution. Detailed results of all separation steps (apart from the relatively-simple RO step) are presented, along with a comprehensive cost analysis, that shows that the process becomes cost competitive (vs. the traditional NaCl-based regeneration) after merely five ion exchange/regeneration cycles. The three-step recycling technique resulted in the recovery of 92% of the K⁺ mass, along with 60% of the fresh water used in the process. After five adsorption/regeneration cycles the ion exchange breakthrough curve showed only slight deterioration due to a low divalent cations concentration that accumulated in the recycled regeneration solution ([Ca²⁺] + [Mg²⁺] ≈ 23 meq/L vs. ∼1000 meq/L of K⁺).

An innovative physicochemical approach for treating the recycled water within a seawater recirculating aquaculture systems (RAS) has been recently developed, which includes inherent disinfection within the ammonia electro-oxidation cycle. The efficiency of the inherent disinfection was tested in comparison to a RAS consisting of a bio-treatment unit (which did not include a disinfection component), both systems operating under similar fish densities and makeup water flow rates. Neural necrosis virus (NNV) was chosen as a model pathogen, due to its pathogenicity to sea bass (Dicentrarchus labrax) and many other aquaculture species. The course of infection (exposure → immune response → infection) was monitored in a cohabitation experiment, testing the horizontal transmission of NNV using immunological and molecular methods, in addition to recording clinical signs. Results indicate that under the conditions prevailing in the RAS in which the physicochemical method was applied, the NNV horizontal transmission pathway was blocked. No immune response to viral neural necrosis (VNN) was detected in the fish sampled during the experiment, in contrast to a significant and classic immune response observed in the fish held in the RAS based on bio-treatment. Challenged fish in both systems showed typical outbreak profile, however in the bio-treated RAS, a slow disease course was observed, hypothesized by us to be the result of a high concentration of humic substances present in the water. The results indicated that the off-host phase of the NNV can be controlled by effective chlorine-based disinfection. The disinfection effect in the physico-chemically-treated RAS and C∙t (chlorine concentration times the retention time) value quantified in the present work are most likely sufficient to reduce infection of a wide range of other aquaculture pathogens.

This book provides chemical concepts as well as crucial steps for inorganic water and wastewater treatment. Examples and tools help to understand and to guide through industrial and natural water process engineering. Chemical and environmental engineers, researchers and professionals, as well as students benefit from this concise and explanatory book. • Explains and deepens fundamentals of the chemistry of aqueous solutions; • Describes water and wastewater treatment processes as well as desalination processes.

Replacing bio-filtration with an electro-chemical method for treating water within recirculating aquaculture systems (RAS) has the potential to solve various obstacles stemming from the very existence of the bio-filter. A recently developed RAS operational approach relies on electrooxidation of seawater for in-situ production of chlorine, which is used for simultaneous ammonia oxidation and water disinfection. While being the most commonly used disinfectant, the reactions between chlorine and organic matter have the potential to result in high amount of carcinogenic by-products, particularly in seawater, which contains a high Br⁻ concentration. Trihalomethanes (THM) are the major by-products of seawater chlorination. More specifically, highly-brominated THM have been reported to bio-accumulate in fish tissues. The current work focused on the implications of operating the new approach on the formation and accumulation of THM both in the rearing water and the fish tissues. The role of bromine/chlorine/organic matter interactions leading to THM formation the under unique RAS conditions are described and quantified. THM concentrations developing under the system's conditions were found low, likely due to the particular characteristics of the organic matter present in the system. Low concentrations of THM precursors were recorded in the physicochemical RAS operating in the absence of a biofilter, stemming from the low microbial activity and the continuous breakdown of the organic matter by chlorine. A method based on SPME separation followed by GC/MS analysis was developed for quantifying THM in the fish tissues. The bioconcentration factor (BCF) of bromoform and dibromochloromethane in the tissues amounted to 1.35 and 0.0 (mg/kg)/(mg/l) in the fish muscle and 0.23 and 0.91 (mg/kg)/(mg/l) in the liver, respectively. Assessment of the THM content in the fish tissues revealed that fish grown in the system meet both EPA regulation for edible fish and recommendations for THM oral exposure.

Contaminant(s) intrusion into water distribution systems (WDS) may have an adverse effect on large populations. The pipeline corrosion scale has the capability to adsorb the contaminant and thereafter release it to water once the system is returned to operation, causing secondary contamination. Therefore, overcoming contamination events should remove the contaminant from both aqueous and scale phases in a controlled fashion, while not jeopardizing the WDS integrity, nor causing red-water events. This study examined the adsorption and subsequent release of cadmium, as a representative heavy-metal ion, from representative pipeline corrosion scales. Adsorption/desorption batch experiments were conducted on corrosion scales peeled off from old domestic WDS pipes. The effect of water quality (pH, alkalinity, ionic composition, [Cd²⁺]) on Cd(II) adsorption and release was examined. The corrosion scale showed high Cd²⁺absorption capacity and linear relation between [Cd²⁺] and the adsorbed Cd. Desorption experiments included dosages of various acid types, Na2S, and KCl. HCl dosage to pH3 was found a suitable technique, releasing ~90% of the adsorbed Cd(II). The work conclusions will serve to design continuous adsorption/desorption experiments, with the aim of determining the optimal rehabilitation treatment.

The paper describes results from operating a new 3-step membrane-based process targeted at separating Mg²⁺ from seawater in an inexpensive way, with the purpose of using it to enrich desalinated water with magnesium, with as little as possible Cl⁻ and Na⁺ addition. To this end, seawater undergoes a series of processes aimed at increasing the Mg²⁺ concentration from ~1350 to ~4000 mg/L through nanofiltration while the monovalent ion concentrations are reduced by a nanofiltration-diananofiltration sequence, in which the diluent is RO produced water from a desalination plant. A dense ultrafiltration (UF) step precedes the nanofiltration-diananofiltration (NF-DiaNF) cycles. In this step sulfate in seawater is rejected better than divalent cations hence the retentate of this step has a ratio of total hardness to sulfate (([Ca²⁺] + [Mg²⁺])/[SO₄²⁻] → 1) which enables attaining an almost complete washout of monovalent ions in the DiaNF step. The paper is concluded with presentation of general design of the process steps and a cost assessment, which shows the process to be both flexible in the quality of the rich Mg solution generated, and cost competitive, relative to other alternatives.

Operation of seawater reverse-osmosis (RO) desalination using high flux membranes at pH > 9 was shown to be an energy-efficient approach for single-pass boron removal. This operational approach was previously studied only at 25 °C, however since RO desalination is highly temperature-dependent, the current work tested the high-pH approach throughout the typical temperature range of the Mediterranean Sea. Since total dissolved solids (TDS) removal in a high-flux-membranes single-pass is limited, the effect of applying a 2nd RO pass was also examined. Finally, the process cost was assessed at varying operational conditions. Results showed 1st pass permeate TDS to be ∼440, 290 and 200 mg/l at 54% recovery-ratio and 31, 25 and 15 °C, respectively. Boron removal was adequate (i.e. <0.4 mgB/l in permeate) at pH 9.5 throughout the temperature range. However, at 31 °C antiscalant had to be added to prevent Mg(OH)₂ scaling. TDS and boron concentrations in the 2nd-pass permeate met the threshold limit (30 mg/l and 0.5 mgB/l) at 90% recovery ratio. The cost of applying the single-pass alternative process was lower by about $0.03/m³ permeate than the conventional alternative, for temperatures lower than 28 °C. The addition of a second pass increased the operational cost merely by a ∼$0.03/m³, making the described two-pass process competitive.

Dolomite dissolution is often mentioned as an option for post-treatment of desalinated water for supplying calcium, magnesium and alkalinity to the product water and comply with non-aggres-siveness criteria. This paper uses reliable literature-based dolomite dissolution data to compute and discuss various options for utilizing this process to meet two common sets of desalinated water quality criteria. Alkalinity mass-balance was performed to corroborate literature-data correctness. Discussion shows that CO₂-enhanced dolomite dissolution is impractical due to slow dissolution rates encountered at the relatively high pH values induced by the CO₂ dosage, resulting in low Mg²⁺ concentrations at reasonable retention time. In the H₂ SO₄-enhanced dolomite dissolution the main difficulty arises from the low alkalinity value attained in the water following its blending with raw desalinated water. This, in turn, necessitates very high NaOH dosages to meet required alkalinity and LSI. At relatively low H₂ SO₄ dosages, the post-dilution alkalinity value can be increased, but at the expense of treating the majority of the desalination plant flow, resulting in excessively-high dissolution-reactor volumes. The overall conclusion is that dolomite dissolution is markedly inferior (cost-and quality-wise) to competing Mg²⁺ addition alternatives, including simple dissolution of off-the-shelf chemicals, such as MgSO₄.

A new concept is introduced for separating and concentrating (predominantly) Mg²⁺ and SO₄²⁻ from seawater, along with low monovalent ion concentrations. Seawater is first subjected to cationic ion exchange aimed at balancing the equivalent concentration ratio between divalent cations and anions. This step's effluent undergoes nanofiltration, and the MgSO₄-rich retentate of the nanofiltration step undergoes further semi-batch diananofiltration for reducing the monovalent ion concentrations. Regeneration of the ion exchange resin is carried-out by permeate attained from nanofiltrating SWRO desalination retentate. The paper presents detailed results from all process steps followed by cost-analysis amounting to ∼$1.6 kg⁻¹ of separated Mg²⁺.

The quality with which water is released from desalination plants increases continuously. However, prior to the post-treatment step, desalination permeates are slightly acidic, contain very low buffering capacity, and are very soft. As such, the water may be aggressive and corrosive to infrastructure, resulting in adverse health and economic effects. Consequently, a post-treatment step in which the chemical stability, buffering capacity and mineral content of the water is adjusted, is invariably practiced. This chapter reviews the knowledge accumulated in the last decades on desalination post-treatment processes. It covers fundamental chemistry aspects related to the aqueous, gaseous and solid phases relevant to post-treatment processes. The chapter also provides a thorough explanation on required water quality criteria; the advantages and disadvantages of currently applied post-treatment processes are detailed, engineering and cost considerations are discussed, state of the art post-treatment alternatives are covered, and contemporary research trends are listed.

We report on selective separation of monovalent and divalent cations (Na⁺ and Mg^{2 +}) and anions (Cl⁻ and SO₄^{2 −}) from aqueous solutions using the flow electrode capacitive deionization (FCDI) process, operated with ion-exchange and nanofiltration membranes (NF). For the selective separation of cations and anions the FCDI module was operated with an NF membrane (NF270) and an anion-exchange or cation-exchange membrane, respectively, at varying applied cell potentials (0.6, 0.8 and 1.23 V) and initial mono- to di-valent ions molar concentration ratios of 1, 10 and 20. The permselectivity of the NF270 membrane, calculated as a ratio between measured ionic fluxes, was found highly dependent on the initial molar concentration ratios of the mono- to the di-valent ions. Concentration-normalized Na⁺ to Mg^{2 +} permselectivity was 0.69–1.04, indicating that the NF270 membrane does not pose selectivity for the separation of sodium and magnesium in the studied process. Conversely, the concentration-normalized permselectivity between Cl⁻ and SO₄^{2 −} was found between 1.28 and 7.03 depending on the applied cell potential, indicating high potential for implementing the proposed NF-FCDI method for selective separation of anions.

Ion-exchange (IX) is common for separating NO₃⁻ from drinking water. From both cost and environmental perspectives, the IX regeneration brine must be recycled, via nitrate reduction to N_2(g) . Nano zero-valent iron (nZVI) reduces nitrate efficiently to ammonia, under brine conditions. However, to be sustainable, the formed ammonia should be oxidized. Accordingly, a new process was developed, comprising IX separation, nZVI-based nitrate removal from the IX regeneration brine, followed by indirect ammonia electro-oxidation. The aim was to convert nitrate to N_2(g) while allowing repeated usage of the NaCl brine for multiple IX cycles. All process steps were experimentally examined and shown to be feasible: nitrate was efficiently separated using IX, which was subsequently regenerated with the treated/recovered NaCl brine. The nitrate released to the brine reacted with nZVI, generating ammonia and Fe(II). Fresh nZVI particles were reproduced from the resulting brine, which contained Fe(II), Na⁺, Cl⁻ and ammonia. The ammonia in the nZVI production procedure filtrate was indirectly electro-oxidized to N_2(g) at the inherent high Cl⁻ concentration, which prepared the brine for the next IX regeneration cycle. The dominant reaction between nZVI and NO₃⁻ was described best (Wilcoxon test) by 4Fe_(s) + 10H⁺ + NO₃⁻ → 4Fe²⁺ + NH₄⁺ + 3H₂O, and proceeded at >5 mmol·L⁻¹·min⁻¹ at room temperature and 3 < pH < 5.

A new process is presented for producing metallic magnesium from seawater. Ion exchange is used to selectively extract Mg^{2 +} from seawater, without introducing Na⁺ and K⁺ impurities into the product Mg. The magnesium extracted from the resin is reacted with CaO and precipitates on the surface of fine ferrosilicon suspension to produce a Mg(OH)₂-Ca(OH)₂-FeSi solids cake, which, after further wash and decantation, yields raw material at a quality similar to that used in the classical dolomite-based Pidgeon silicothermic reduction technique. The filtrate of the raw cake decantation step, comprising predominantly of CaCl₂, is used as the ion-exchange regeneration solution. Ca^{2 +} ions are adsorbed on the resin and thereafter replenished through CaO dosage, applied for the Mg(OH)₂ precipitation. The close contact between the Mg(OH)₂ and the FeSi in the dry cake enables direct usage of the cake in ~ 1150 °C retort which is applied for single-step dehydration and thermal reduction of the Mg^{2 +}, thereby minimizing the problem of heat and mass transfer through the briquettes and eliminating the need for separate calcination, briquetting and reduction. Feasibility dehydration and reduction tests yielded 99.0–99.5% pure Mg. Materials and energy cost assessment revealed that the proposed process is ~$0.75 more expensive per kg of magnesium than the dolomite-based process, assuming equal thermal reduction yields. The cost estimation stimulates further optimization and parametric study of the proposed process and suggests that production of Mg in places where dolomite is not available, requiring seawater as the sole raw resource, is feasible.

A membrane-based process is presented for separating divalent ions (namely Mg²⁺, Ca²⁺ and SO₄²⁻) from seawater in a highly selective fashion. The main goal is to selectively and cost-effectively separate Mg²⁺ from seawater with the intention of either dosing it into desalinated water in the desalination post-treatment stage or generating a Mg²⁺/Ca²⁺/SO₄²⁻ solution that can be used for, e.g., precipitating struvite from high-strength wastewaters prior to anaerobic-digestion, with minimal addition of detrimental Cl⁻/Na⁺ ions. The process comprises three steps: First, seawater undergoes a high-recovery nanofiltration (NF) step with an “open” NF membrane aimed at reducing the molar ratio between divalent-cations and SO₄²⁻ in the NF retentate. The retentate of this step is then subjected to further ∼65% recovery (conventional) NF step aimed at increasing the Mg²⁺ concentration in the retentate, which is thereafter subjected to a diananofiltration step aimed at reducing the monovalent ion concentrations while maintaining high Mg²⁺/SO₄²⁻ concentrations in the product solution. The hypothesis (which was fully substantiated in the work) was that the reduction of the molar ratio between total hardness and SO₄²⁻ in the retentate of the 1st NF step would result in a lower Cl⁻ to Mg²⁺ concentration ratio in the product solution (retentate) of the NF-NF-DiaNF process sequence. Results are presented for a variety of operational conditions using both seawater and seawater reverse osmosis brine as raw solutions. The cost of separating one kg of Mg²⁺ from seawater using the method is significantly lower than the equivalent cost of the chemical MgSO₄ in all the presented scenarios. However, reducing the Cl⁻ concentration in the product solution by ∼62% more than doubled the cost. For example, dosage of 20 mg Mg²⁺/l to desalinated water was estimated at $0.006/m³ and $0.017/m³ for addition of 32 and 12 mg Cl⁻/l along with the Mg²⁺, respectively.

Boron removal from desalinated seawater is essential for obtaining high quality water, suitable for irrigation. Removal of boron by RO (reverse osmosis), the leading desalination technology, is significantly affected by variations in feed water temperature. Nevertheless, a widely agreed quantitative method for describing the temperature effect on the boric acid permeability constant, an important parameter in RO process modeling, was thus far unavailable. In this paper, different methods for describing permeability constants as a function of temperature were systematically evaluated against empirical results. It was demonstrated that non-specific temperature correlations, which are based on a single permeability measurement at a reference temperature, result in increased deviations from the measured permeability as the temperatures shift away from the reference value. A more accurate approach is to determine membrane-specific temperature correlations based on measured permeabilities at the relevant temperature range. Subsequently, the influence of accurate temperature correction on process modeling was assessed by comparing experimental boron rejections at practical conditions to simulation results. It was found that a reliable boric acid permeability coefficient is particularly significant at warm temperature, where boron rejection is less effective. Finally, implications to process design are discussed in light of accurate temperature dependent boron removal simulations.

Wastes from concentrated animal feeding operations (CAFOs) are challenging to treat because they are high in organic matter and nutrients. Conventional swine waste treatment options in the U.S., such as uncovered anaerobic lagoons, result in poor effluent quality and greenhouse gas emissions, and implementation of advanced treatment introduces high costs. Therefore, the purpose of this paper is to evaluate the performance and life cycle costs of an alternative system for treating swine CAFO waste, which recovers valuable energy (as biogas) and nutrients (N, P, K⁺) as saleable fertilizers. The system uses in-vessel anaerobic digestion (AD) for methane production and solids stabilization, followed by struvite precipitation and ion exchange (IX) onto natural zeolites (chabazite or clinoptilolite) for nutrient recovery. An alternative approach that integrated struvite recovery and IX into a single reactor, termed STRIEX, was also investigated. Pilot- and bench-scale reactor experiments were used to evaluate the performance of each stage in the treatment train. Data from these studies were integrated into a life cycle cost analysis (LCCA) to assess the cost-effectiveness of various process alternatives. Significant improvement in water quality, high methane production, and high nutrient recovery (generally over 90%) were observed with both the AD-struvite-IX process and the AD-STRIEX process. The LCCA showed that the STRIEX system can provide considerable financial savings compared to conventional systems. AD, however, incurs high capital costs compared to conventional anaerobic lagoons and may require larger scales to become financially attractive.

Magnesium (Mg²⁺) appears at high concentration in seawater and seawater-reverse-osmosis brines. In contrast, desalinated water is almost completely depleted of Mg²⁺, a mineral perceived essential for human health and agricultural irrigation. The paper introduces a cost-effective method to enrich desalinated water with an almost pure Mg(II) solution, originating from seawater. The method uses seawater nanofiltration (or nanofiltration of seawater-reverse-osmosis brine) to produce brine characterized by high Mg²⁺ concentration, accompanied by relatively low B, Cl⁻, and Na⁺ concentrations. Subsequently, Mg(II) is separated from the produced nanofiltration brine by precipitating Mg(OH)_2(s) and adsorbing it onto the surface of micro-magnetite particles. Finally, the solid slurry (Fe₃O₄ + Mg(OH)₂) is magnetically separated from the brine and the Mg(OH)_2(s) is re-dissolved into the desalinated water in a separate reactor. Application of the method results in a relatively pure Mg(II) addition to the desalinated water product. For example, for Mg(II) addition of 10 mg/L (as recommended by the World Health Organization), the following negligible concentrations of unwanted species are added to the water (in mg/L units): Na⁺: 0.04, Cl⁻: 0.18, Ca²⁺: 0.05, and B: 0.0094. The cost of adding 10 mg Mg/L was estimated at 0.76 cent$/m³ of desalinated water, i.e. competitive with previously suggested processes.

A new technique is introduced for extracting Mg²⁺ (and Ca²⁺) ions from seawater to produce a solution characterized by high concentrations of required ions (i.e. Mg²⁺, Ca²⁺ and SO₄^2-) along with minimal concentrations of unwanted species (i.e. Cl^-, B, Br^- and Na⁺), to be dosed to soft waters. The technique comprises of nanofiltration of seawater (or 1st-pass seawater reverse osmosis brine) followed by a DiaNanofiltration step, applied on the nanofiltration brine. DiaNanofiltration is a procedure involving dosage of low-TDS water to the nanofiltration feed solution, to improve the "wash out" of unwanted components. The purpose of the preceding nanofiltration step is to produce low-volume brine retaining most of the Mg²⁺, Ca²⁺ and SO₄^2- from the seawater, thereby minimizing the cost of the DiaNanofiltration diluting solution. The paper describes results of experimental and theoretical examination of the process under various operational conditions. The results show that the Cl^- to Mg²⁺ concentration ratio in the produced solution varies between 1.52 and 3.27 (w:w), depending on the operational alternative. A cost assessment (operational and capital expenses) is provided, showing the cost of all examined scenarios to be lower than $0.01 for supplying soft (desalinated) water with 10 mg/l of Mg²⁺.

A new operational approach for seawater recirculating aquaculture systems (RAS), which is based solely on physicochemical water treatment techniques, is described. Within this concept the fish are grown at very high TAN concentration and slightly acidic pH, the latter calculated to maintain the NH₃ concentration lower than a predetermined threshold (typically <0.1 mgN/l). The inherently high Cl^- concentration in seawater enables efficient electro-generation of Cl_2(aq) species and consequent electrooxidation of ammonia directly to N_2(g). Fish pond water is constantly recycled between electrolysis tanks and the pond, supplying both disinfected water and most of the acidity required to maintain the necessary low pH in the fish pond. For establishing proof-of-concept gilthead seabream (Sparus aurata) was grown at the pilot scale for 133 consecutive days. Throughout this period the fish showed excellent growth rate and survival, along with excellent general health condition. 88% of the NH₃ mass excreted by the fish was electro-oxidized in the pilot system, while at the same time the pond water was efficiently disinfected. The high TAN concentrations in the rearing water resulted in high current efficiency in the electrooxidation step, reduced water treatment units volume and generation of very low trihalomethane (THM) concentrations, attributed to the low pH allowed to develop during the electrolysis stage. The pilot system was operated in two stages differing in TAN concentration (30 and 65 mgN/l) and pH (6.7 and 6.4, leading to NH_3(aq) concentrations of 0.047 and 0.052, respectively). Average current efficiency in the TAN electrolysis step was 68%. Cost analysis showed that both capital and operational costs were highly competitive, as compared with conventional RAS operation. Specifically, the cost of the water treatment component within the pilot operation (TAN removal to N_2(g) and water disinfection) was estimated at ~$0.205 per kg feed, out of which $0.079 was attributed to electricity costs.

Acid-base properties such as pH, alkalinity and boron speciation are essential factors for the design and operation of seawater reverse osmosis desalination, as they influence critical processes such as membrane scaling, biofouling and boron permeation. However, due to the high complexity of acid-base dynamics in both the retentate and permeate streams of reverse osmosis applications, the evolution of acid-base related properties in these streams is not yet completely understood. This often leads to overly simplified models resulting in inaccurate predictions, which may impede optimal design and thus the cost-effectiveness of the process. This paper introduces a unique computerized model dedicated to modeling acid-base dynamics within seawater reverse osmosis processes based on a reactive-transport algorithm, which couples transport and equilibria phenomena. The WATRO (Weak Acid Transport in Reverse Osmosis) simulation program based on this algorithm (freely available in the ESI) is experimentally tested here and shown to extend the predictive capacity and accuracy for these important quantities beyond previous work, and also beyond the capabilities of available commercial software. Implications related to 1st and 2nd pass CaCO₃ scaling and boron rejection are exemplified and discussed. From the practical standpoint, the advantages of the suggested approach are particularly prominent for 2nd RO pass modeling and design, mainly due to the passage of hydronium and hydroxide ions through the membrane.

The transport of hydronium and hydroxide ions through reverse osmosis membranes constitutes a unique case of ionic species characterized by uncommonly high permeabilities. Combined with electromigration, this leads to complex behavior of permeate pH, e.g., negative rejection, as often observed for monovalent ions in nanofiltration of salt mixtures. In this work we employed a rigorous phenomenological approach combined with chemical equilibrium to describe the trans-membrane transport of hydronium and hydroxide ions along with salt transport and calculate the resulting permeate pH. Starting from the Nernst-Planck equation, a full non-linear transport equation was derived, for which an approximate solution was proposed based on the analytical solution previously developed for trace ions in a dominant salt. Using the developed approximate equation, transport coefficients were deduced from experimental results obtained using a spiral wound reverse osmosis module operated under varying permeate flux (2-11 μm/s), NaCl feed concentrations (0.04-0.18 M) and feed pH values (5.5-9.0). The approximate equation agreed well with the experimental results, corroborating the finding that diffusion and electromigration, rather than a priori neglected convection, were the major contributors to the transport of hydronium and hydroxide. The approach presented here has the potential to improve the predictive capacity of reverse osmosis transport models for acid-base species, thereby improving process design/control.

A new operational approach is presented, which has the potential to substantially cut down on the energy and cost demand associated with seawater reverse osmosis (SWRO) desalination, without changing the currently-installed infrastructure. The approach comprises acidification/decarbonation of the feed seawater followed by high-pH single RO pass using high-flux membranes. Since the limitation imposed by CaCO_3(s) precipitation is overcome, the recovery ratio can be significantly increased. This work presents a new operational concept aimed at maximizing the benefits that can be obtained from new low-energy RO membranes available on the market. Results obtained from operating a pilot RO system revealed that following an acidification and decarbonation step, recovery ratio of 56% could be practically attained, along with effluent TDS and boron concentrations of 375 and 0.3 mg/l, respectively (feed water pH was adjusted to pH9.53 following the decarbonation step). The specific energy consumption (SEC) of this operation was calculated to be 5%-10% lower than the SEC typically associated with "conventional" SWRO operation. Two further scenarios were theoretically considered, under which the limiting operational parameter became Mg(OH)_2(s) and BaSO_4(s) precipitation. It was concluded that despite the fact that higher recovery ratios could be obtained, the high pressure required in these scenarios made them less appealing from both the SEC and cost standpoints. The normalized cost of the suggested approach was found to be ~$0.07 ± 0.02/m³ cheaper than the currently-practiced SWRO approach for obtaining product water characterized by TDS < 500 and B < 0.5 mg/l.

A novel and potentially cost effective filtration scheme for removal of nitrate from groundwater, characterized by production of low salinity waste brine that can be easily discharged to sewerage systems and high product-water recovery, is proposed. The inherent preference of particular NF membranes for rejecting chloride and sodium over nitrate ions is utilized in a preliminary NF stage to remove Na⁺, Cl⁺, Ca²⁺ and Mg²⁺ to a side stream. In a second stage, RO is applied to remove NO₃^- and the RO permeate is mixed with the side stream of the NF stage to create product water low in nitrate, yet with a balanced composition consisting all the required species and minerals. The number of NF stages depends mainly on the rejection efficiency of the NF membrane. Based on Israeli regulations for both drinking water and required composition of brines discharged to the sewage, a treatment scheme composed of a single and double NF stages followed by RO is shown to reach water recoveries of 91.6% and 94.3%, respectively. Each NF stage raises the energy cost by approximately 0.5cent/m³ product water. However, this cost is easily paid back by the inherent additional advantages of the combined scheme, i.e., less water treated by the RO, significant increase in total recovery ratio, no need in re-mineralization of the product water and minimization of calcium carbonate precipitation potential on the RO membrane. The principles for process design are described, making the specific treatment scheme proposed here easily adjustable to other regulatory requirements and other water characteristics. A provisional patent has been filed.

Deliberate struvite (MgNH₄PO₄) precipitation from wastewater streams has been the topic of extensive research in the last two decades and is expected to gather worldwide momentum in the near future as a P-reuse technique. A wide range of operational alternatives has been reported for struvite precipitation, including the application of various Mg(II) sources, two pH elevation techniques and several Mg:P ratios and pH values. The choice of each operational parameter within the struvite precipitation process affects process efficiency, the overall cost and also the choice of other operational parameters. Thus, a comprehensive simulation program that takes all these parameters into account is essential for process design. This paper introduces a systematic decision-supporting tool which accepts a wide range of possible operational parameters, including unconventional Mg(II) sources (i.e. seawater and seawater nanofiltration brines). The study is supplied with a free-of-charge computerized tool (http://tx.technion.ac.il/∼agrengn/agr/Struvite-Program.zip) which links two computer platforms (Python and PHREEQC) for executing thermodynamic calculations according to predefined kinetic considerations. The model can be (inter alia) used for optimizing the struvite-fluidized bed reactor process operation with respect to P removal efficiency, struvite purity and economic feasibility of the chosen alternative. The paper describes the algorithm and its underlying assumptions, and shows results (i.e. effluent water quality, cost breakdown and P removal efficiency) of several case studies consisting of typical wastewaters treated at various operational conditions.

Seawater nanofiltration (SWNF) generates a softened permeate stream and a retentate stream in which the multivalent ions accumulate, offering opportunities for practical utilization of both streams. This study presents an approach to simulation of SWNF including all major seawater ions (Na⁺, Cl^-, Ca²⁺, Mg²⁺, and SO₄^2-) based on the Nernst-Planck equation, and uses it for permeate and retentate streams composition prediction. The number of degrees of freedom in the system was reduced by assuming a very high ionic permeability for Na⁺, which only weakly affected the other parameters in the system. Two alternatives were examined to analyze the importance of concentration dependence of ion permeabilities: The assumption of constant ion permeabilities resulted in a reasonable fit with experimental data. However, for the permeate composition the overall fit was significantly improved (P < 0.0001) when the permeabilities of Ca²⁺ and Mg²⁺ were allowed to depend on the ratio of their total concentration to Na⁺. This type of dependence emphasizes the strong interaction of divalent ions with the membrane and its effect on the membrane fixed charge through screening or charge reversal. When this effect was included, model predictions closely matched the experimental results obtained, corroborating the phenomenological approach proposed in this study.

Abstract Disposing of nitrate-containing effluents from seawater-fed intensive aquacultural applications is a major environmental problem. A possible solution is to mix nitrate-rich effluents from marine recirculating aquaculture systems (RASs) with citrate-rich liquid wastes (CLW), a common by-product of the food industry. Where possible, such strategy can alleviate two environmental problems simultaneously, in a cost-effective fashion. However, concerns are often raised regarding secondary pollution stemming from the use of CLW, particularly related to phosphorus and heavy metals. This work showed that both phosphorus and heavy metal were completely absorbed by the bacterial sludge generated in the process, indicating low environmental risk associated with the disposal of the treated effluent to the environment. Operation of continuous stirred-tank reactor (CSTR) single-sludge denitrification reactor with CLW as electron and carbon donor resulted in high nitrate removal efficiency (>95 %) and denitrification rate of up to 1.6 g NO₃-N L^-1 reactor day^-1 along with low bacterial biomass yield [0.23 g chemical oxygen demand (COD) new cells g^-1 COD citrate]. Moreover, the use of CLW was found to be environmentally safe and equally efficient to the use of traditional, costly carbon sources such as methanol and acetic acid, rendering this alternative attractive for treatment of nitrate-rich saline effluents.

Placement of water quality sensors in a water distribution system is a common approach for minimizing contamination intrusion risks. This study incorporates detailed chemistry of organophosphate contaminations into the problem of sensor placement and links quantitative measures of the affected population as a result of such intrusions. The suggested methodology utilizes the stoichiometry and kinetics of the reactions between organophosphate contaminants and free chlorine for predicting the number of affected consumers. This is accomplished through linking a multi-species water quality model and a statistical dose-response model. Three organophosphates (chlorpyrifos, malathion, and parathion) are tested as possible contaminants. Their corresponding by-products were modeled and accounted for in the affected consumers impact calculations. The methodology incorporates a series of randomly generated intrusion events linked to a genetic algorithm for minimizing the contaminants impact through a sensors system. Three example applications are explored for demonstrating the model capabilities through base runs and sensitivity analyses.

The boron isotopic composition of calcium carbonate skeletons is a promising proxy method for reconstructing paleo-ocean pH and atmospheric CO₂ from the geological record. Although the boron isotope methodology has been used extensively over the past two decades to determine ancient ocean-pH, the actual value of the boron isotope fractionation factor (ε_B) between the two main dissolved boron species, ¹¹B(OH)₃ and ¹⁰B(OH)^-₄, has remained uncertain. Initially, ε_B values were theoretically computed from vibrational frequencies of boron species, resulting in a value of ~19‰. Later, spectrophotometric pH measurements on artificial seawater suggested a higher value of ~27‰. A few independent theoretical models also pointed to a higher ε_B value. Here we provide, for the first time, an independent empirical fractionation factor (ε_B=26.0±1.0‰25 °C), determined by direct measurements of B(OH)₃ in seawater and other solutions. Boric acid was isolated by preferential passage through a reverse osmosis membrane under controlled pH conditions. We further demonstrate that applying the Pitzer ion-interaction approach, combined with ion-pairing calculations, results in a more accurate determination of species distribution in aquatic solutions of different chemical composition, relative to the traditional two-species boron-system approach. We show that using the revised approach reduces both the error in simulating ancient atmospheric CO₂ (by up to 21%) and the overall uncertainty of applying boron isotopes for paleo-pH reconstruction. Combined, this revised methodology lays the foundation for a more accurate determination of ocean paleo-pH through time.

The paper addresses two potential applications for electrochemical ammonia oxidation within the operation of recirculating aquaculture systems, in which nearly complete removal of N species is required. In one described application, a physical-chemical ammonia oxidation method is suggested to entirely replace conventional biological treatment methods (i.e. nitrification/denitrification). The second described method is suggested as a final polishing step for removing ammonia from effluents of denitrification reactors supplied with intrinsic organic matter, prior to the discharge of the water. Empirical results and cost assessment are reported for the second alternative, while the first, which was recently published, is discussed with respect to improvements, operational conditions and field tests required to induce its commercial application. The polishing alternative was shown capable of efficiently removing TAN in the effluents of RAS denitrification reactors fed with intrinsic organic solids. The cost for treating denitrification reactor effluents with TAN concentration of 10mgN/L was estimated at 6.67cent/m³ of discharged water. Since the chloride ion concentration in seawater and in most brackish waters is high, combining the intrinsic organic carbon denitrification process with subsequent ammonia polishing by electrochemically produced active chlorine may be a competitive approach for the removal of nitrogen species from seawater and brackish water RAS.

Efficient removal of seawater boron species in reverse osmosis (RO) plants is gaining momentum as the guidelines for maximal B concentrations in desalinated water are becoming stricter, driven by the adverse effects that boron has on agricultural crops at concentrations higher than ∼0.7. mg/L. This chapter explores alternatives for reducing B concentration in the permeate water to below 0.7. mg/L in a single RO pass operated at high pH, without promoting chemical fouling of the membrane elements. The second part of the chapter is dedicated to the challenge of modeling the performance of RO units with respect to boron removal at high pH. Special attention is given to the role of acid-base equilibria and B speciation, which are also discussed with respect to the difficulties related to pH measurements in highly concentrated brines. It is shown that available B simulation models do not yield accurate results and an alternative simulation approach is suggested.

Certain soluble heavy metals are known to accumulate in the human body, resulting in (inter alia) toxicity to the kidney, liver, lungs, brain, heart and central nervous system. Water quality sensors can monitor small changes in water quality properties such as pH, TOC, turbidity, temperature, free chlorine concentration, and alkalinity. Heavy metals neither react with free chlorine nor consist of organic carbon; therefore, unless the solubility threshold is surpassed, the contaminant presence is distinguishable only by a change in the pH value. This characteristic makes the detection of heavy metal contamination events relatively tricky. In this work, a detailed aquatic chemistry multi-species model was developed within EPANET-MSX for the purpose of simulating the changes in water quality induced by cadmium contamination events. The model was applied on an example application network and the possible effects of various contamination events were explored.

Intrusion of contaminants into a water distribution system (WDS), deliberately or accidentally, is a potential risk facing water authorities and utilities. Deployment of methodologies for water quality sensor placements in water distribution systems is an approach for mitigation of such risks. This study incorporates a methodology for optimal placement of water quality sensors, through including contaminants detailed chemistry reactions. This is performed through using multispecies water quality model (EPANET-MSX) coupled with a statistical dose-response model. At the first stage, a series of contamination events are simulated, and the number of incident consumers' are evaluated. For each contamination event three decision variables are selected: injection location, contaminant type, and injection time. At the second stage, a genetic algorithm (GA) is invoked for selecting the optimal sensor placements through minimizing the average expected incidents. The method is demonstrated on an example application and two methods for events generation are compared and discussed.

Measuring and modeling pH in concentrated aqueous solutions in an accurate and consistent manner is of paramount importance to many R&D and industrial applications, including RO desalination. Nevertheless, unified definitions and standard procedures have yet to be developed for solutions with ionic strength higher than ~0.7M, while implementation of conventional pH determination approaches may lead to significant errors. In this work a systematic yet simple methodology for measuring pH in concentrated solutions (dominated by Na⁺/Cl^-) was developed and evaluated, with the aim of achieving consistency with the Pitzer ion-interaction approach. Results indicate that the addition of 0.75M of NaCl to NIST buffers, followed by assigning a new standard pH (calculated based on the Pitzer approach), enabled reducing measured errors to below 0.03 pH units in seawater RO brines (ionic strength up to 2M). To facilitate its use, the method was developed to be both conceptually and practically analogous to the conventional pH measurement procedure. The method was used to measure the pH of seawater RO retentates obtained at varying recovery ratios. The results matched better the pH values predicted by an accurate RO transport model. Calibrating the model by the measured pH values enabled better boron transport prediction. A Donnan-induced phenomenon, affecting pH in both retentate and permeate streams, was identified and quantified.

As a complementary step towards solving the general event detection problem of water distribution systems, injection of the organophosphate pesticides, chlorpyrifos (CP) and parathion (PA), were simulated at various locations within example networks and hydraulic parameters were calculated over 24-h duration. The uniqueness of this study is that the chemical reactions and byproducts of the contaminants' oxidation were also simulated, as well as other indicative water quality parameters such as alkalinity, acidity, pH and the total concentration of free chlorine species. The information on the change in water quality parameters induced by the contaminant injection may facilitate on-line detection of an actual event involving this specific substance and pave the way to development of a generic methodology for detecting events involving introduction of pesticides into water distribution systems. Simulation of the contaminant injection was performed at several nodes within two different networks. For each injection, concentrations of the relevant contaminants' mother and daughter species, free chlorine species and water quality parameters, were simulated at nodes downstream of the injection location. The results indicate that injection of these substances can be detected at certain conditions by a very rapid drop in Cl₂, functioning as the indicative parameter, as well as a drop in alkalinity concentration and a small decrease in pH, both functioning as supporting parameters, whose usage may reduce false positive alarms.

Despite appearing at relatively low concentrations, weak acids of various types have a significant impact on the design, operation and the quality of the product water in many RO (reverse osmosis) applications. During the RO process, weak acid species are simultaneously involved in interconnected transport and chemical reactions. This complex behavior requires the use of a "reactive transport" modeling approach. In this paper we present a systematic method for combining commonly used cross-membrane transport and concentration polarization models with thermodynamic aqueous phase equilibrium models. The new approach can be used for the prediction of the acid-base thermo-chemical condition that develop within the membrane module affecting the final permeate and brine characteristics in a variety of RO applications. A computerized algorithm was developed, described and applied for simulating two case studies. The case study results demonstrate the practical significance of the approach in the context of boron removal and chemical scaling control within seawater RO applications.

Grey mullet fingerlings were grown in different acclimation solutions for three consecutive years, with the aim of developing a procedure for their adjustment to commercial growth in very low salinity (EC=0.37dS/m) and Cl^-/Na⁺-deficient inland water (Dan River water, Northern Israel). Tested acclimation conditions composed of simulative solutions of varying ionic compositions (EC range 1.0-1.1dS/m) and a solution made up by dissolving quarry dolomite beads into the target (Dan River) water by H₂SO₄ without and with a small NaCl addition (1.5mM). The latter alternative was favored because of its low production costs and minimal environmental impact (minute addition of extrinsic Na⁺ and Cl^- ions to the environment). The results obtained in the three experimental sets showed that stocking the fingerlings in a ~1dS/m acclimation solution for 1 week, followed by 20% daily dilution by the target water, was appropriate for attaining survival rates higher than 70% in all acclimation solutions tested (growth periods varied in the range 43-77d). Survival rate in brackish water control experiments was 85-95%. In the absence of an acclimation period the fish survival rate was ~35%. No significant difference was observed in fish growth rates between treatments conducted in a given year, suggesting that fish that survived the acclimation period would grow in the low salinity water at a rate similar to fish grown in brackish water. The minor differences in survival and growth results between the different acclimation solutions seemed to indicate that the role of specific ionic constituents in the water was inconsequential and that the most significant parameter is the overall water salinity. However, histological analyses revealed that applying the acclimation period with very low Cl^- and Na⁺ concentrations (<~75mg/L and ~45mg/L, respectively) caused non-reversible adverse effects on the fish, whereas in all of the other treatments recorded adverse effects were of a reversible nature. Considering all the results, the best acclimation solution was defined as dissolution of dolomite by H₂SO₄ into the target low-salinity water plus addition of 1.5mM of NaCl. The cost of producing the acclimation solution was shown to amount to only ~0.03% of the overall culture cost.

Wastewaters generated during hydraulic fracturing of the Marcellus Shale typically contain high concentrations of salts, naturally occurring radioactive material (NORM), and metals, such as barium, that pose environmental and public health risks upon inadequate treatment and disposal. In addition, fresh water scarcity in dry regions or during periods of drought could limit shale gas development. This paper explores the possibility of using alternative water sources and their impact on NORM levels through blending acid mine drainage (AMD) effluent with recycled hydraulic fracturing flowback fluids (HFFFs). We conducted a series of laboratory experiments in which the chemistry and NORM of different mix proportions of AMD and HFFF were examined after reacting for 48 h. The experimental data combined with geochemical modeling and X-ray diffraction analysis suggest that several ions, including sulfate, iron, barium, strontium, and a large portion of radium (60-100%), precipitated into newly formed solids composed mainly of Sr barite within the first ∼10 h of mixing. The results imply that blending AMD and HFFF could be an effective management practice for both remediation of the high NORM in the Marcellus HFFF wastewater and beneficial utilization of AMD that is currently contaminating waterways in northeastern U.S.A.

A new approach is presented for cost effective recovery of Mg(II) from 1st stage seawater reverse-osmosis brines (salinity: twice seawater concentration). The process is based on precipitation of Mg(OH)_2(s) on the surface area of self-synthesized magnetite (Fe₃O₄) micro-particles and magnet-assisted separation of the solids-slurry from the Mg(II)-depleted brine. Once separated from solution, the solids slurry is subjected to acidic conditions (pH~4-6) under which Mg(OH)_2(s) is recovered as Mg(II) with the counter anion being either SO42-, Cl^- or HCO3-, depending on the choice of strong acid used in the dissolution step. The magnetite solids are then used in the following adsorption cycle. This paper focuses on proof-of-concept of the suggested process and on defining ranges for the major process operational conditions (Fe₃O₄ particle concentration; pH range maintained during Mg(OH)_2(s) dissolution step; determination of the favorable solid-aqueous separation technique, etc.). Once defined, the chosen operational conditions were applied and shown to result in three high purity (>97%) Mg(II) solution products at costs which are comparable with equivalent commercial products.

The importance of supplying drinking water with a balanced mineral composition, including a minimal concentration of Mg(II) ions, has been recently acknowledged by many publications, as well as in official WHO guidelines. The issue is relevant to naturally occurring soft waters and lately to the rapidly increasing volume of supplied desalinated water. This paper presents an enhancement of a recently developed nanofiltration-based method for the selective separation of soluble Mg(II) species from seawater. The generated rich-Mg(II) brine is demonstrated to be suitable for supplementing soft waters with magnesium ions. The brine, generated using a commercial membrane (DS-5 DL, Osmonics) at various operational conditions is characterized by high Mg(II) concentrations (~8.5 g/L) and low Cl:Mg and Na:Mg molar concentration ratios (1.6 and 0.6, respectively, at 28-bar operation). A food-grade antiscalant is dosed to the feed seawater to prevent scaling; however, since the Mg(II) concentration in the brine is high, for attaining 10 mg Mg/L of desalinated water, the dilution ratio with the desalinated water is ~1:850, resulting in maximal additional concentrations of 0.024 antiscalant, 34.9 Cl(-I), 12.9 Na(I), 0.05 Sr(II) and 0.003 B (all concentrations in mg/L). The overall cost of 1 kg of Mg(II) separated by the presented process amounts to between $0.05 and $0.07, i.e., much cheaper than the estimated costs of alternative processes for Mg(II) addition to desalinated water.

An algorithm for finding the optimal sensor placement within water distribution systems is presented herein. The calculations are based on simulation injections of the organophosphate pesticide chlorpyrifos (CP) at arbitrary locations within Net1 of the EPANET software package. Hydraulic parameters and chemical concentrations of CP and its daughter compounds were calculated over 72-h duration. The methodology herein integrates an existing hydraulic analysis with a chemical analysis that accounts for the actual contaminant threshold value of significant impact on human health and its toxicity level. A pollution matrix, which provides an estimation of the affected population, is generated from the simulations and then utilized through genetic algorithms to determine the optimal sensor placement and the minimal exposed consumers.

The effects of sub-lethal CO_2(aq) concentrations were tested for the first time on gilthead seabream (Sparus aurata) juveniles (4-25g; 64 growth days) and adult (~300-400g; 71d) fish, both in fully controlled pilot tests and the latter also as part of full-scale RAS (recirculating aquaculture system) operation. In the pilot experiments (concentration range 5.2-56.3mg CO₂/L) the specific growth rate, mortality rate, and physical fish disorders were monitored. In the full scale experiment, two groups of fish, originally from the same batch, were exposed for 197d to controlled (by NaOH dosage) and uncontrolled pH conditions, resulting in exposure of the fish to significantly different CO_2(aq) concentrations. The pilot results showed, as expected, that the seabream fish grew faster at the lower CO₂ concentrations and that the growth rate of both juveniles and adult fish was only minimally inhibited up to roughly 20mg CO₂/L (compared to a previously published curve). Mortality rate was considerable only at the highest CO₂ concentration (~56mg CO₂/L). Physical irregularities were not observed, apart from abnormally high absence of swim bladder at the highest CO_2(aq) treatment. The (statistically significant) results from the full-scale RAS operation showed that growing gilthead seabream for 197d at roughly constant and relatively low (~16mg/L) CO_2(aq) concentration resulted in fish with ~10% larger mean weight relative to the fish grown in ponds in which CO_2(aq) was not controlled and its concentration fluctuated daily between 19 and 37mg/L.

Dissolution of quarry CaCO₃ has become very common as means of alkalinity and calcium supply in the post treatment step of desalination plants. This paper addresses theoretical and practical aspects pertinent to the design of H₂SO₄-based calcite dissolution processes, such as the maximum acid dosage possible without gypsum precipitation (1400mg/L); factors dominating the product water quality; and effect of given operational scenarios on the expected operational (namely the chemicals' demand) and capital (namely reactors size) expenses. The conclusions presented in the paper were based on theoretical calculations combined with extensive empirical results obtained from operating a pilot scale calcite dissolution reactor under six different operational conditions, differing from each other in flow velocity (10, 20 and 30m/h) and in H₂SO₄ dosages (to attain calcium carbonate dissolution potential values of 778 and 1075mg/L as CaCO₃). Operating the reactor under these conditions, the product water quality was only slightly affected by changes in the operation parameters. The overall post treatment cost (CAPEX and OPEX) was estimated at between 0.0491 and 0.054$/m³ product water with low sensitivity towards the operational parameters tested. The CAPEX accounted for between 3% and 31% of the overall cost in the various scenarios tested.

A new simulation approach is presented for predicting boron concentrations in the product water of seawater reverse osmosis operations. The new (numerical) approach links traditional mass-transfer models (the solution-diffusion transport approach and the concentration polarization film-layer model) with full aqueous-phase thermodynamic species characterization, performed by chemical equilibrium software (PHREEQC), based on the Pitzer approach. The new approach results in a more accurate calculation of the boric acid (B(OH)₃) molar fraction which develops close to the membrane wall, on the feed side, thereby improving the prediction accuracy of B(OH)₃ permeation. Specifically, acknowledging that the pH value of the feed invariably changes as seawater brine progresses through the membranes' train, calculation of this pH change, as performed in the new approach, enables a more physically-accurate and better simulation of the boric acid fraction. The new approach is shown in the paper to result in a prediction that matches better empirical results obtained from the operation of a pilot-scale SWRO plant, as compared to the traditional approach.

A new process, based on ion-exchange and electrochemical regeneration, was tested for removing ammonia from effluents of swine-waste anaerobic lagoons. The process, consisting of a daily sequential operation of adsorption (180min), chemical regeneration (125min) and electrooxidation of ammonia in the regeneration solution (8h, applied during low-cost electricity hours), was shown feasible for reducing the ammonia concentration in the wastewater from ∼1000 to ∼60mg/L with a total cost estimated at ∼$3/kgN. The work focused on identifying the best operational conditions enabling continuous operation of the ion-exchange column and recycling of the regenerant solution without the need for replenishment of either. Chemical additions were restricted to in-line addition of NaOH to maintain constant pH during the electrolysis step and periodical addition of NaCl to compensate for Cl^- and Na⁺ losses. It was found that removing NH4+ (by chabazite-zeolite) from swine wastewater characterized by NH4+:K⁺:Ca²⁺:Mg²⁺:Na⁺ ratio of 1:0.66:0.1:0.06:0.30 (g/g) could be carried out by the proposed treatment sequence at conditions of pseudo steady state, with an adsorption stage of 12.5 bed volumes and 14.5min hydraulic retention time. Applying such conditions, the concentration of K⁺ (the main competing cation) stabilized both in the regenerant solution and on the chabazite, enabling >90% NH4+ removal without need for replacing the regenerant solution. Electrooxidation efficiency constantly exceeded 90% due to the high Cl^- concentration (>17g/L) maintained in the regeneration solution and because only a small mass of organic matter was transferred to the regenerant solution following the adsorption step.

A new physico-chemical process for ammonia removal from fresh-water recirculated aquaculture systems (RASs) is introduced. The method is based on separating NH₄⁺ from RAS water through an ion-exchange resin, which is subsequently regenerated by simultaneous chemical desorption and indirect electrochemical ammonia oxidation. Approach advantages include (1) only slight temperature dependence and no dependence on bacterial predators and chemical toxins; (2) no startup period is required and the system can be switched on and off at will; and (3) the fish are grown in much lower bacterial concentration, making the potential for both disease and off-flavor, lower. A small pilot scale RAS was operated for 51d for proving the concept. The system was stocked by 105 tilapia fish (initial weight 35.8g). The fish, which were maintained at high TAN (total ammonia nitrogen) concentrations (10-23mgNL^-1) and fish density of up to 20kgm^-3, grew at a rate identical to their established growth potential. NH_3(aq) concentrations in the fish tank were maintained lower than the assumed toxicity threshold (0.1mgNL^-1) by operating the pond water at low pH (6.5-6.7). The low pH resulted in efficient CO₂ air stripping, and low resultant CO_2(aq) concentrations (<7mgL^-1). Due to efficient solids removal, no nitrification was observed in the fish tank and measured nitrite and nitrate concentrations were very low. The system was operated successfully, first at 10% and then at 5% daily makeup water exchange rate. The normalized operational costs, calculated based on data derived from the pilot operation, amounted to 28.7 $ cent per kg fish feed. The volume of the proposed process was calculated to be ∼13 times smaller than that of a typical RAS biofilter. The results show the process to be highly feasible from both the operational and economical standpoints.

A different approach for the operation of seawater reverse osmosis desalination plants, in which the boron concentration in the product water should not exceed 0.3 mgB/L, was recently introduced. The new approach is based on strong acid (either H₂SO₄ or HCl) dosage to the feed seawater to attain pH~4.3, followed by almost complete CO₂ stripping and sub-sequently strong base addition to pH 9.0-9.25. At this high pH range, a high B removal efficiency can be attained even by the new generation of ultra-low energy (high-flux) membranes. This paper addresses the energy saving potential stemming from the elimination or size reduction of the 2nd reverse osmosis (RO) pass and from the use of high-flux elements, both made possible by the new approach. Additionally, total dissolved solids removal from the 1st RO pass permeate can be obtained by operating a smaller, more energy efficient, 2nd RO pass.

The paper describes the application of an alternative method for generating struvite (MgNH₄PO₄) solids from wastewater, using a low-cost Mg(II) solution, previously separated from seawater by nanofiltration (NF). Mg²⁺ ions are present at high concentrations in the oceans (∼1400 mg/L) and are well rejected by nanofiltration membranes, therefore, NF enables obtaining a concentrate that is rich in Mg(II) at a low cost. Since the largest cost component in conventional methods for struvite precipitation from wastewaters stems from the cost of the Mg chemicals, the reduction in the overall struvite generation cost in the presented method is considerable. However, the NF separation method includes inherent potential disadvantages: along with Mg(II), other ions are also separated from the seawater (although with lower rejection values) e.g. Cl(-I) and Na(I), which contribute to the salinity of the wastewater effluents, and Ca(II) that may promote precipitation of unwanted calcium-phosphate solids, rendering the obtained struvite product less homogeneous thus less valuable. Consequently, the work focused on determining the best operational alternatives to attain the purest struvite product possible, using a fluidized bed (FB) reactor. The results show the approach to be highly feasible. A theoretical simulation was first carried out to determine the best operational conditions (pH, Mg(II) dosage through NF brine) to attain 90% P removal from the actual supernatant of a domestic-sludge dewatering facility. Total ammonia and orthophosphate supernatant concentrations were adjusted to simulate high (∼300 mg P/L and 600 mg N/L) and low (100 mg P/L and 450 mg N/L) nutrient supernatant concentrations. The FB reactor was fed with a mixture of supernatant and NF brine solutions, while pH was maintained constant via NaOH addition, and the hydraulic retention time was tested at 20, 30 and 60 min. In the six scenarios tested, P removal was higher than 90% and the struvite purity obtained was very high (∼95%), as demonstrated by both dissolution experiments and XRD analyses. The struvite precipitation experiments were carried out in the presence of an antiscalant agent, which was added to the NF brine to prevent the clogging of the NF membrane which was operated at 90% recovery to attain a concentrated Mg(II) brine.

A new, simple and accurate method is introduced for determining H ₂CO₃^* alkalinity in fresh waters dominated by the carbonate weak-acid system. The method relies on a single H₃PO₄ dosage and two pH readings (acidic pH value target: pH∼4.0). The computation algorithm is based on the concept that the overall alkalinity mass of a solution does not change upon the addition of a non-proton-accepting species. The accuracy of the new method was assessed batch-wise with both synthetic and actual tap waters and the results were compared to those obtained from two widely used alkalinity analysis methods (titration to pH∼4.5 and the Gran titration method). The experimental results, which were deliberately obtained with simple laboratory equipment (glass buret, general-purpose pH electrode, magnetic stirrer) proved the method to be as accurate as the conventional methods at a wide range of alkalinity values (20-400 mg L^-1 as CaCO₃). Analysis of the relative error attained in the proposed method as a function of the target (acidic) pH showed that at the range 4.0<pH<4.5 the error was minimal. A suggested experimental setup for continuous alkalinity measurement is also described.

A different approach is presented for the operation of seawater RO desalination plants in which the boron concentration in the product water should not exceed 0.3mgB/l. The approach is based on strong acid (either H ₂SO ₄ or HCl) dosage to the feed water to attain pH~4.3, followed by CO ₂ stripping and subsequently strong base addition to pH 9.00-9.25. At this high pH range, a high B removal efficiency is attained, and since the water is practically devoid of carbonate species, no CaCO _3(s) scaling takes place, and pH elevation is limited by Mg(OH) _2(s) precipitation, expected only at pH>9.45. The approach enables operation in the absence of antiscalants. Furthermore, CO ₂ stripping is effected in stripping towers in two steps: the high CO _2(aq) concentration is first stripped by vacuum-operated stripping towers and the CO ₂-rich air is used for dissolution of calcite in the post treatment stage. The remaining CO ₂ mass is stripped to the atmosphere using blower-assisted stripping towers. This paper aims at introducing the new concept and providing " proof of concept" The paper addresses experimental and theoretical aspects of the proposed process, as well as engineering and economic evaluation. The proposed approach is shown to be both technically feasible and cost effective, as compared with conventional boron removal alternatives.

An alternative approach is presented for the operation of the pre- and post-treatment stages of desalination plants fed by brackish waters characterized by a high carbonate concentration. A strong acid (either HCl or H ₂SO ₄) is dosed to the feed water with the aim of converting HCO ₃ ^- to CO _2(aq) thereby eliminating potential precipitation of solids and scale inhibitor requirement. CO _2(aq) is a small, uncharged molecule, which readily passes RO membranes. Consequently, since the CO _2(aq) concentration in the feed/brine water closely equals the CO _2(aq) concentration that develops in the permeate water, the corresponding negative calcium carbonate precipitation potential values in the permeate is further utilized in the post treatment stage for enhancing CaCO _3(s) dissolution, resulting in supply of carbonate alkalinity, Ca ²⁺ ions and also (indirectly) Mg ²⁺ ions, using the "calcite dissolution-ion exchange" process introduced in previous works. Thereby, the dosage of the strong acid to the feed water accomplishes two goals, making the approach cost effective. The paper addresses experimental (at both laboratory and full scale) and theoretical aspects of the proposed process, as well as assessment of engineering and economic feasibility.

The governing mechanism of indirect ammonia electrooxidation has been often described similarly to breakpoint chlorination. However, comparison of the chloramine concentrations which develop in batch indirect ammonia electrolysis and classical breakpoint chlorination experiments (performed under similar conditions) suggests that the governing reactions are different. Three experimental sets were carried out with excess-ammonia solutions, with the aim of elucidating the mechanism of indirect electrochemical ammonia oxidation: (1) chloramination with Cl _2(g) and NaOCl; (2) batch- and (3) single-pass electrolysis experiments. Based on the results we propose a new mechanism for indirect ammonia electrooxidation, according to which trichloramine, rather than monochloramine, is the initial and primary product. NCl ₃ apparently forms from a reaction between NH ₄ ⁺ and Cl _2(aq), which occurs in the near anode area where pH is <2 and the bulk Cl ^- concentration is high. At such conditions Cl _2(aq) is the dominant active chlorine species in the anode vicinity. Upon formation in the near anode area NCl ₃ decomposes to N ₂, NH ₂Cl and NHCl ₂ in the bulk solution or/and close to the cathode surface area, where pH > 12. Under batch operation and/or single-pass electrolysis characterized by long contact times both NH ₂Cl and NHCl ₂ that form in the bulk electrolyte are oxidized to NCl ₃ by Cl _2(aq) upon return to the near-anode zone.

This study is on the inclusion of chemical water stability considerations in optimizing the operation of water distribution systems. The problem of chemical water instability arises in systems supplied by a mixture of desalinated, surface, and ground water. Such circumstances are commonly found in countries which utilize large scale seawater desalination plants within their water supply systems to mitigate water scarcity problems (e.g., Israel). The most known and problematic occurrence related to unstabilized water is the phenomenon of "red water" which describes a situation in which a layer of (mostly) iron oxides is detached from the internal surface of metal pipes into the water, which then reaches the consumer's taps with a characteristic yellow-brown-red color. Another well known problem is the deterioration of metal pipes due to slow corrosion. Beyond destroying the pipes, the products of corrosion consume chlorine products, rendering disinfection less efficient, it creates scales on the pipe 's surface that increase the energy required for pumping, it supports Biofilm growth and may produce suspensions of (mainly) iron particles that result in water that is not appealing to the consumer. The developed methodology in this work links a genetic algorithm, a hydraulic and water quality simulator, and a numerical scheme for computing the calcium carbonate precipitation potential (CCPP) [which is the quantitative measure of the precise potential of a solution to precipitate (or dissolve) CaCO3(s)], and the pH of the water. The model minimizes the cost of pumping and treatment of the water for an operational time horizon subject to required quantities, pressures, and CCPP andpH constraints. The methodology is demonstrated on an example application through base runs and sensitivity analysis.

This work presents a model for the inclusion of chemical water stability in optimizing the operation of water distribution systems. When desalinated water is mixed with surface water and/or groundwater, the blend can become chemically unstable. Such a state can cause the phenomena of "red water," an increase in corrosion rates, and a reduction in disinfection efficiency. In this study, a methodology is developed that links a genetic algorithm, a hydraulic and water quality extended period simulator, a numerical scheme for computing the calcium carbonate precipitation potential (CCPP) [the quantitative measure of the precise thermodynamic potential of a solution to precipitate (or dissolve) CaCO_3(s)], and the pH of the water. The model minimizes the cost of pumping and treatment subject to quantities, pressures, CCPP, and pH constraints. Two example applications are utilized for demonstrating the methodology capabilities. Although the model provides a new tool for the explicit inclusion of chemical water stability in optimal operation of water distribution systems, it overlooks variations in pump efficiency at operational points, does not constrain the number of pump switches, the minimum pump operation and off times, the durations between pump start and shutoff, and the plant or source capacity. Those limitations should be considered in possible extensions of this study.

A nanofiltration-based method is presented for selectively separating soluble Mg(II) species from seawater, with the aim of using the Mg-loaded brine for either enriching desalinated water with magnesium ions or for enhancing precipitation of struvite from wastewater steams. Two 2.4' commercial NF membranes were tested under varying operational conditions. The membrane that was chosen for further investigation (DS-5 DL, Osmonics) showed lower Cl:Mg and Na:Mg concentration ratios in the brine, and improved performance (with respect to the investigated process) at high recovery values. Since the addition of antiscalants was perceived detrimental to the downstream uses of the brine, the aquatic chemistry program PHREEQC was used to simulate the critical (highest) recovery values at which no CaSO₄ would precipitate, assuming two concentration polarization factors. To prevent CaCO₃ precipitation at the critical recovery values a theoretical calculation was performed (PHREEQC) to determine the required strong acid dosages to the raw seawater. Using the DS-5 DL membrane at 64% recovery, the attained Mg(II) concentration in the brine was 3500mg/l. Therefore, for attaining 12.15mg Mg/l of desalinated water the brine should be dosed to the water at a 1:288 ratio, resulting in additional concentrations of 32.5mg SO₄^-2/l, 89.3mg Cl^-/l, 39.4mg Na⁺/l, 3.3mg Ca⁺²/l, and 0.01mg B/l. The overall cost of the proposed process was estimated at 0.00098$/m³ product water, i.e. approximately five times lower than two assessed alternative processes and more than one order of magnitude cheaper than implementing direct dissolution of chemicals, using either MgCl₂ or MgSO₄.

The formation and minimization of bromate ions as a by-product of the operation of a non-thermal plasma reactor for the removal of refractory organic species from drinking water was investigated. BrO₃^- formation kinetics was found to be 1st order with respect to the Br^- concentration. BrO₃^- formation increased when the organic pollutant concentration fell below a threshold value, intrinsic to each organic species. Formation also increased when dissolved ozone concentration was elevated by injection of O₃-rich air, generated inside the reactor, back to the water, although measured O₃ concentrations were low (~0.1mg/l) BrO₃^- formation significantly decreased at high temperatures (>35°C) and low pH (~pH6.0) values and was found insensitive to the carbonate alkalinity concentration. Dosage of hypochlorite followed by ammonia to the inlet water was found to be more effective in minimizing bromate formation than ammonia dosage alone. A general method was developed for assessing the relative importance of the various possible bromate formation pathways within plasma reactors, and a probable dominant BrO₃^- formation reaction sequence was suggested for the particular conditions prevailing in non-thermal plasma systems.

The quality with which water is released from desalination plants is continuously increasing. Since desalination permeates are slightly acidic, contain very low buffering capacity and are very soft, post-treatment is always required. This paper reviews the knowledge accumulated in the last decades on desalination post-treatment processes. It covers fundamental chemistry aspects, required water quality criteria, advantages and disadvantages of currently applied processes, engineering and cost considerations, recent full-scale project experience and up-to-date research trends.

The ability of many noniron metals to be incorporated into the structure of ferrites is being utilized in numerous industrial and environmental applications. The incorporation of some of these metals during Fe(II) oxidation-induced precipitation at moderate temperatures (80-100°C) appears to be limited, for reasons not fully understood, and to extents not always agreed (e.g., Ni²⁺, Cr³⁺). In this paper, the incorporation maxima of six metals into the structure of precipitated ferrites (in terms of x in Me_xFe_3-xO₄, Me represents a noniron metal) were concluded to be 1.0, 1.0, 0.78, 0.49, 0.35, and 0.0 for Zn²⁺, Co²⁺, Ni²⁺, Al³⁺, Cd²⁺ and Cr³⁺, respectively. With the exception of the much larger Cd²⁺, these values were associated with kinetic considerations controlled by the H₂O exchange rate between the hydration shells surrounding the dissolved metal ion.

Desalinated brackish water is becoming an important water source for agricultural irrigation, a fact which raises the incentive to include a minimal Mg²⁺ concentration in the product water, which would minimize fertilization requirements. Both the mineral content and chemical stability of desalinated water are attained through the post treatment stage at the desalination plant. A modified post treatment process is introduced, in which Mg²⁺ ions are separated from the brackish water fed to the desalination plant via a selective cation exchange resin, and subsequently released to the product water through further exchange with Ca²⁺ ions previously generated in a common H₂SO₄-based calcite dissolution process. Calcite dissolution provides the water with carbonate alkalinity, Ca²⁺ and SO₄^2-, while the ion exchange step adds Mg²⁺. Laboratory and theoretical investigations of the process show that it results in a high water quality, attained in a cost effective and environmentally friendly manner. Three case studies are presented in which the additional cost of supplying Mg²⁺ concentration of either 12 or 18 mg L^-1 using the process is estimated at less than $0.006 m^-3 product water. The results indicate that the process is feasible for brackish waters characterized by Mg²⁺ to Ca²⁺ concentration ratio higher than 1:1.

Incomplete oxidation of Fe(II) species released from the anode to Fe(III) may impede iron electrocoagulation processes conducted under low dissolved oxygen and/or pH<7 conditions, accompanied by the typically high buffering capacity of wastewater. This paper introduces a new approach to overcome this drawback by applying a second electrochemical cell (Ti/RuO₂ anode and Ti cathode) to be operated in parallel to the electrocoagulation cell. The second unit oxidizes Cl^- ions invariably present in the water to HOCl, which is capable of oxidizing Fe(II) species at a high rate, irrespective of pH or O_2(aq) concentration. An electrolytic cell with a Ti/RuO₂ anode and Ti cathode was shown to successively operate in parallel to a sacrificial electrocoagulation cell (Fe anode and Ti cathode) to attain complete Fe(II) conversion to Fe(III) under low-pH conditions, in which, in the absence of the 2nd cell, unwanted Fe(II) species would have dominated the dissolved iron species. Current efficiency for Cl₂ production was 12.4% and 45.7% at 200 and 1000mg Cl/l, respectively. Under three practical conditions (pH 6, [Cl^-]=200mg/l; pH 6, [Cl^-]=400mg/l; pH 5, [Cl^-]=600mg/l) the power demand of the combined system was 25.29, 12.7 and 8.1kWh/kg Fe(III)_produced, respectively, suggesting that the presented approach is competitive at [Cl^-]>~600mg/l.

Struvite (MgNH₄PO₄) is emerging as a potential fertilization product for agriculture. As other phosphate minerals that can be precipitated from wastewater are known to have lower value as fertilizers, the overall fertilization quality of the solids precipitated in struvite recovery systems depends largely on the percentage of struvite in the mixture of precipitated solids. This work focused on determining the most appropriate operating conditions (with a focus on pH and hydraulic retention time [HRT]) for removing >90% of the dissolved phosphate and, at the same time, attaining a precipitate with the highest possible struvite content from typical filtrates of sludge dewatering belt press systems. To this end, a continuous laboratory-scale completely mixed reactor was operated at different pH values and HRTs and the precipitant composition was determined. Theoretical calculations showed that the most cost-effective MgCl₂ dosage to typical Israeli belt press filtrate is ∼10 mM. Using this dosage, ∼8.0 mM of phosphate precipitated under all the operating conditions studied. The shortest retention time (15 min) and lowest pH value (pH 7.4) applied were found to be the most favorable for the attainment of the most homogeneous struvite precipitate (∼85% struvite). At higher pH values and longer HRTs, the overall precipitated mass (in P units) was slightly higher, but unwanted calcium-phosphate and magnesium-phosphate precipitates were observed at higher percentages alongside the struvite crystal. This finding implies that the typical high pH values, previously perceived to be optimal for struvite precipitation, actually result in a less-valuable product.

New water quality standards, specific for desalinated water, have been recently approved in Israel. To satisfy the new quality criteria a novel post-treatment process has been developed, aimed at cost-effectively meeting the new standards as well as supplying Mg²⁺ ions, required for both health and agricultural reasons. The paper introduces a modification to the original process. In the modified process calcite is dissolved using CO₂ (instead of H₂SO₄, as in the original process), and/or a combination of CO₂ and H₂SO₄. An additional presented feature is the option to elevate product water pH by CO₂ stripping rather than NaOH dosage. The modified process can be implemented as an add-on to existing CO₂-based calcite dissolution post-treatment systems or in cases where a restriction is posed on the total hardness value in the product water, since the modification extends the flexibility in the product water quality, with respect to the ratio attained between total hardness and alkalinity concentrations. Cost estimation reveals that upgrading a conventional post-treatment process based on CO₂ dissolution of CaCO₃ to the modified process for attaining [Mg²⁺] of 10 mg/l, can result in an increase in the operational costs of between 0.15 and 0.69 $cents per m³ of product water, depending on the cost of chemicals.

New water quality standards, specific for desalinated water, have been recently approved in Israel. Consequently, a novel post-treatment process was developed, aimed at meeting the new standards in a cost effective way, as well as supplying Mg²⁺ concentration of at least 10 mg Mg/l, required for both health and agricultural reasons. A pilot plant, capable of post-treating 1600 m³/d of desalinated water was operated for *6 months to optimize economic and engineering aspects of the process. The article presents results from three operational scenarios differing from each other by the percentage of water that undergoes treatment, out of the total flow rate. The results indicate that the required set of water quality parameters can be produced in a stable manner in all the three scenarios tested. All scenarios were found similar with respect to both operational costs and resultant water quality. However, in terms of capital costs, treating a smaller fraction of the total flow rate was found advantageous. The article also introduces a modification to the original process, which was tested at the laboratory scale. In the modified process calcite is dissolved using CO_2(g) (instead of H₂SO₄, as in the original process). The modified process can be implemented as an add-on to existing CO₂-based calcite dissolution post-treatment systems or in cases where a restriction is posed on the total hardness value in the product water.

Over 90% of the water supplied in the coastal region in Israel in 2013 (600Mm³ y^-1) will be from desalination plants. The wastewater generated from this water (>400Mm³ y^-1) is planned, after proper treatment, to be reused for agricultural irrigation, making this low-salinity water the main agricultural-sector future water source. In this respect both the Mg²⁺ concentration and the Sodium Adsorption Ratio value of the water are of concern. We show that the typical Na⁺ concentration addition to wastewater (between ∼100 and ∼165mgL^-1) is much higher than the combined addition of Ca ²⁺ and Mg²⁺ (between 0 and several mgL^-1). Since desalinated water is typically supplied with low Ca²⁺ and Mg²⁺ concentrations (∼35 and 0mg L^-1 respectively), the treated wastewater is characterized by very low Mg²⁺ concentrations, low salinity and very high SAR values, typically >6 and up to 10 (meq L^-1)^0.5. SAR values can be lowered by adding either Ca²⁺ or Mg²⁺ to desalinated water. Adding Mg ²⁺ is preferable from both health (minimizing cardiovascular disease hazards) and agriculture (inexpensive Mg fertilization) aspects. The low cost of Mg²⁺ addition at the post-treatment stage of desalination plants corroborates the request for Mg²⁺ addition in regions where treated wastewater from desalinated water origin is planned to be reused for irrigation.

A modified pH 1.0 liquid redox sulfur recovery (LRSR) process, based on reactive absorption of H₂S_(g) in an acidic (pH 1.0) iron solution ([Fe(III)] = 9-8 g L^-1, [Fe(II)] = 1-2 g L^-1) and electrochemical regeneration of the Fe(III)/Fe(II) catalyst couple, is introduced. Fe(II) was oxidized in a flow-through electrolytic cell by Cl _2(aq) formed on a Ti/RuO₂ anode. pH 1.0 was applied to retard the potential precipitation of predominantly jarosite group Fe(III) species. At pH 1.0, the presence of chloride ions at [Cl^-] = 30 g L^-1 allows for both efficient (indirect) electrochemical oxidation of Fe(II) and efficient H₂S_(g) reactive absorption. The latter observation was hypothesized to be associated with higher concentrations of Fe(III)-Cl complexes that are more highly reactive toward H₂S _(aq) than are free Fe(III) ions and Fe-SO₄ complexes that otherwise dominate pH 1.0 Fe(III) solutions in the absence of a significant Cl^-concentration. At the described operational conditions the rate of Fe(II) oxidation in the experimental system was 0.793 kg Fe h^-1 per m² anode surface area, at a current efficiency of 58%. Electricity cost within the electrochemical step was approximated at $0.9 per kg H₂S_(g) removed.

Blending desalinated water with surface and/or ground water may result in water that has a negative precipitation potential with respect to CaCO_3(s), rendering it chemically unstable. In this paper a simulation tool for calculating the pH and calcium carbonate precipitation potential (CCPP) values at the nodes of a water distribution system is introduced. This computerized tool is then used to simulate the CCPP values that would develop in a schematic distribution system fed by three water sources (desalinated, surface and ground waters) under a simulative water consumption pattern. The simulation demonstrates, for a case study that is based on typical Israeli conditions, that an increase in the alkalinity value of the desalinated water from 50 to 100 mg/L as CaCO₃ results in a positive CCPP value at all times whereas at the low alkalinity value (which is the concentration which is currently supplied by the 100 million-m³/y and 30 million-m³/y Ashkelon and Palmachim plants in Israel) the CCPP values at the nodes are often negative as a result of blending the desalinated water with groundwater. The conclusion is that there is a need to increase the alkalinity value in desalinated waters. This request is augmented by additional arguments in support of this approach. The negative effect of high alkalinity values on copper-tubing corrosion rates is also noted.

Desalinated water is expected to become the major source of drinking water in many places in the near future, and thus the major source of wastewater to arrive at wastewater treatment plants. The paper examines the effect of the alkalinity value with which the water is released from the desalination plant on the alkalinity value that would develop within the wastewater treatment process under various nitrification-denitrification operational scenarios. The main hypothesis was that the difference in the alkalinity value between tap water and domestic wastewater is almost exclusively a result of the hydrolysis of urea (NH₂CONH₂, excreted in the human urine) to ammonia (NH₃), regardless of the question what fraction of NH_3(aq) is transformed to NH₄⁺. Results from a field study show that the ratio between the alkalinity added to tap water when raw wastewater is formed (in meq/l units) and the TAN (total ammonia nitrogen, mole/l) concentration in the raw wastewater is almost 1:1 in purely domestic sewage and close to 1:1 in domestic wastewater streams mixed with light industry wastewaters. Having established the relationship between TAN and total alkalinity in raw wastewater the paper examines three theoretical nitrification-denitrification treatment scenarios in the wastewater treatment plant (WWTP). The conclusion is that if low-alkalinity desalinated water constitutes the major water source arriving at the WWTP, external alkalinity will have to be added in order to avoid pH drop and maintain process stability. The results lead to the conclusion that supplying desalinated water with a high alkalinity value (e.g. ≥ 100 mg/l as CaCO₃) would likely prevent the need to add costly basic chemicals in the WWTP, while, in addition, it would improve the chemical and biological stability of the drinking water in the distribution system.

The activity of anammox bacteria in a denitrification reactor in a recirculating aquaculture system (RAS) for gilthead seabream production was investigated. Organic matter, extracted from the pond's solid filter, was used as the electron donor and carbon source for the denitrification reaction. The reactor was operated at four solid retention times (SRT). At steady state, anammox activity showed similar activity at SRTs of 12.5, 8 and 6 days (approximately 35 mg N l reactor^{- 1} day^{- 1}), and a much lower activity of 12 mg N l reactor^{- 1} day^{- 1} at a SRT of 4 days, indicating that anammox bacteria were washed out of the reactor at SRT< 6 days. These results were corroborated by fluorescence in situ hybridization (FISH) that showed that at SRT of 12.5, 8 and 6 days the anammox bacteria population in the denitrification reactor was on the order of 10⁸ cells ml^{- 1} as compared with 10⁶ cells ml^{- 1} at SRT of 4 days. The reportedly long cell division time of anammox bacteria together with the relatively short SRT for anammox washout in the denitrification reactor suggested that a substantial quantity of anammox bacteria were being supplied on a daily basis to the denitrification reactor with the solids captured in the pond's filter system. Since close-to-saturation conditions for oxygen prevailing in both the pond and the solids filter do not favor anammox growth, it was further conjectured that anammox bacteria propagate in fish intestines. This paper shows qualitative evidence that anammox bacteria are indeed present in significant quantities in both the feces of seabream fish and in the solids backwashed from the pond's filter.

Recent WHO publications recommend a minimum concentration of 10 mg Mg/L in desalinated water. Dolomite dissolution was investigated as means of adding magnesium ions to desalinated water at the post treatment stage, in addition to Ca²⁺ ions and carbonate alkalinity. The results show that dolomite dissolution per se is not feasible for post treatment purposes, because dolomite stops dissolving at a relatively low pH, rendering the alkalinity and Calcium Carbonate Precipitation Potential values very low and negative, respectively. To overcome this problem three combined dolomite-calcite dissolution alternatives were investigated. The results show that the most promising method is to dissolve dolomite and then use the CO_2(aq)-rich effluent to further dissolve calcite. Applying such in-series dissolution, it is possible to produce water with the following quality criteria: alkalinity = 75 mg/L as CaCO₃, [Mg²⁺] = 12.4 mg/L, [Ca²⁺] = 120 mg/L as CaCO₃, pH 8.17. However, the resultant total hardness value is high (170 mg/L as CaCO₃). The operational costs of this alternative were approximated at 0.042 $ m^!3 product water.

Corona discharge is emerging as a promising advanced oxidation process (AOP) for the treatment of a variety of organic contaminants, including compounds that are not effectively destroyed by more common AOPs. This paper presents laboratory and field results describing the destruction of regulated and Contaminant Candidate List (CCL) compounds in tertiarytreated wastewater effluent and contaminated groundwater during the operation of a novel Hydro-Non-Thermal-Plasma (HNTP) AOP system. The system generates a plasma discharge above the target water matrix, which emits an “electron wind”, ultraviolet (UV) irradiation, O_3(g) and hydroxyl radicals (•OH) into a relatively thin water layer. The synergism between these oxidizing agents results in efficient degradation of refractory organics (typically >95%) rendering further chemical dosage unnecessary. Batch experiments revealed the dominating kinetics to be first order for MTBE (k = 7.5 × 10^-4 s^-1) and TCE (k = 4.8 × 10^-4 s^-1). This study is the first report of pilot-scale HNTP destruction of (mainly) TCE, 1,4-dioxane and NDMA from a contaminated water source (groundwater in California). The pilot-scale HNTP reactor showed high removal efficiencies of 95.3%, 91.7% and 95.3%, for these three contaminants, along with energy efficiency (E_EO) values comparable to other AOP systems.

The recent supply of large volumes of seawater desalinated water in Israel prompted both the development of new water quality standards and the development of a novel post treatment process, designed to comply with the new standards at a cost effective price. The new process is designed to supply water with alkalinity, Ca²⁺ and calcium carbonate precipitation potential values as required in the new criteria, along with the addition of a threshold Mg ²⁺ concentration recently recommended by the WHO. The current paper describes the process in general, and focuses in particular on attaining these criteria while maintaining a low total hardness concentration (120 mg/L as CaCO³). The process is based on dissolving calcite using H ₂SO₄ and replacing the excess calcium ions generated in this process by Mg²⁺ ions (using a specific cation exchange resin - Amberlite) and by Na⁺ (using a second cation exchange resin - chabazite, from the zeolite group). Once exhausted the resins are re-loaded with Mg²⁺ and Na⁺ by the brine generated in the RO process, thus no unwanted brines are generated. A case study is presented for which operational costs were approximated at 0.034 $US/m³ product water.

Analytical methods used for determining dissolved Fe(II) often yield inaccurate results in the presence of high Fe(III) concentrations. Accurate analysis of Fe(II) in solution when it is less than 1% of the total dissolved Fe concentration (Fe_T) is sometimes required in both geochemical and environmental studies. For example, such analysis is imperative for obtaining the ratio Fe(II)/Fe(III) in rocks, soils and sediments, for determining the kinetic constants of Fe(II) oxidation in chemical or biochemical systems operating at low pH, and is also important in environmental engineering projects, e.g. for proper control of the regeneration step (oxidation of Fe(II) into Fe(III)) applied in ferric-based gas desulphurization processes. In this work a method capable of yielding accurate Fe(II) concentrations at Fe(II) to Fe_T ratios as low as 0.05% is presented. The method is based on a pretreatment procedure designed to separate Fe(II) species from Fe(III) species in solution without changing the original Fe(II) concentration. Once separated, a modified phenanthroline method is used to determine the Fe(II) concentration, in the virtual absence of Fe(III) species. The pretreatment procedure consists of pH elevation to pH 4.2-4.65 using NaHCO₃ under N₂(g) environment, followed by filtration of the solid ferric oxides formed, and subsequent acidification of the Fe(II)-containing filtrate. Accuracy of Fe(II) analyses obtained for samples (Fe(II)/Fe_T ratios between 2% and 0.05%) to which the described pretreatment was applied was >95%. Elevating pH to above 4.65 during pretreatment was shown to result in a higher error in Fe(II) determination, likely resulting from adsorption of Fe(II) species and their removal from solution with the ferric oxide precipitate.

Refreshment (make-up) water is used in recirculating aquaculture systems (RAS) mainly to purge off-flavors, to add alkalinity and sometimes for temperature control. Alternatively, alkalinity may be added by means of a chemical base and heat may be supplied by a heating system. The objective of this study is to show how the optimal (minimizing cost) mix of the three controls: water, base and heat, can be found for given temperatures and water prices. The optimal solution varies over the temperature space and also depends on the price of water. For conditions at Eilat, Israel (on the Red Sea), using supplementary heating to maintain a constant temperature may become prohibitively expensive. If heating is given up, the remaining choice is between the supply of alkalinity via the refreshment water and adding a base. The supply of alkalinity with the water requires ∼2.0 m³[water]/kg[feed], much more than the minimum refreshment rate required to purge off-flavors, which is thought to be ∼0.3 m³[water]/kg[feed]. If the price of water is more than ∼0.03 USD/m³, the use of sodium bicarbonate for alkalinity control is justified.

The paper describes a novel approach to reduce ammonia emissions from Concentrated Animal and Feeding Operations (CAFO) in general, and from poultry houses in particular. The approach is based on installing a dedicated air capturing system on the feeding infrastructure that draws air from close to the litter. Air at these locations has NH_3(g) concentrations an order of magnitude higher than at the vents of the ventilation system. Moreover, while the dedicated waste air drawing system can work continuously, the operation of the ventilation system is intermittent and directed towards maintaining the birds climatically-comfort. The NH_3(g) rich waste air is conveyed to an acidic (0<pH<∼5) bubble column reactor in which ammonia is converted to NH₄⁺. The reactor operates in a batch mode, starting at pH0 (1N HCl solution) and is switched to a new acidic absorption solution just before NH_3(g) breakthrough occurs, at around pH 5. Experiments with a wide range of NH_3(g) concentrations showed that the absorption efficiency is practically 100% throughout the process as long as the face velocity is below 4 cm/s. The advantages of the method include high absorption efficiency, lower NH_3(g) concentrations in the vicinity of the birds, generation of a valuable product (a high concentration ammonia solution) and the separation between the ventilation and ammonia treatment systems. A small scale pilot operation conducted for 5 weeks in a broiler house showed the approach to be technically feasible. A larger scale pilot study is required for fine-tuned cost estimation.

Removal of heavy metals (HM) from industrial wastewater is of primary environmental importance. The seeded ambient temperature ferrite process, in which heavy metals are removed from solution by their incorporation into a magnetite structure, is an attractive alternative for both complete metals separation and generation of stable magnetic sludge. Despite its potential, the effects of various operational parameters on the composition and stability of the final product are not fully understood to-date. The current paper addresses the combined effect of selected operational parameters on Co²⁺ incorporation efficiency. Co²⁺, which is used in many industries (e.g., plating, alloys manufacturing, catalytic converters, and paint pigments) and hence may pose a significant environment risk, was shown previously to be incorporated successfully into ferrite structures. Co²⁺-bearing ferrites were synthesized from Fe(II) solutions at ambient temperature (20 and 30°C), applying slow oxidation, pH 10.5, 20 g (as Fe) l^-1 ferrite seed, and different Fe²⁺ to Co²⁺ influent ratios. Pure magnetite was used as an initial seed, which was gradually exchanged, as the reaction proceeded, by the Co²⁺-bearing ferrite generated within the reactor (until full exchange at steady state). Under all conditions investigated >99% of the Co²⁺ was removed from solution (yielding Co ²⁺ levels below 20 μg/l in the effluent) while the generated precipitates were composed predominately of magnetite/ferrite (based on XRD analysis). Favorable results in terms of Co²⁺ incorporation efficiency and sludge stability were obtained when the following operational conditions were applied: 1:10 Co²⁺:Fe²⁺ molar influent ratio, 30°C, 1,500 mgFe²⁺ l^-1 metal intermediate concentration, and 16 h aging period (pH 10.5).

In many places desalinated water is becoming a significant component of the overall water supply. Notwithstanding its superior quality, un-stabilized desalinated water may be corrosive to water distribution systems, and mixing such water with other water sources in the distribution system can result in corrosion of metallic constituents and in "red water" events. To date, no explicit quality criteria for desalinated water can be found in the literature. In this work, such criteria are discussed from various perspectives, including chemical stability, bio-stability, effect on wastewater treatment, water palatability, health and economic effects, and post-treatment engineering considerations. The first part of the work was carried out for the Committee for the Update of Water Quality Standards, appointed in 2005 by the Israeli Ministry of Health. As a result of the study, the following set of quality criteria was proposed and approved: Alkalinity > 80, 80 < [Ca²⁺] < 120, 3 < CCPP < 10 (all concentrations in mg/L as CaCO₃), and pH < 8.5. The second part of the paper focuses on the implications of the new criteria on the post treatment process. A comparison between existing alternative post treatment processes is presented, and a novel, cost effective and unique post treatment process is introduced. The new process was developed not only to comply with the new criteria in a cost effective way, but also to result in a supply of Mg²⁺ ions, which are required in desalinated water for both health and agricultural reasons.

Removal of sulfide species from municipal sewage conveyance systems by dosage of iron salts is a relatively common practice. However, the reactions that occur between dissolved iron and sulfide species in municipal sewage media have not yet been fully quantified, and practical application relies heavily on empirical experience, which is often site specific. The aim of this work was to combine theoretical considerations and empirical observations to enable a more reliable prediction of the sulfide removal efficiency for a given dosing strategy. Two main questions were addressed, regarding the dominant sulfur species that results from the oxidation of sulfide by Fe(III) and the dominant precipitation reaction between Fe(II) and sulfide species. Comparison of thermodynamic prediction obtained by an equilibrium chemistry-based computer program (MINEQL+) with experimental results obtained by dosing ferrous salts showed that the product of precipitation is FeS under all operational conditions tested. Regarding the reaction between ferric salts and sulfide species, analysis of thermodynamic data suggested that the dominant product of sulfide oxidation under typical pe/pH conditions prevailing in municipal raw wastewater is SO₄^2-. However, comparison between sulfide removal in laboratory experiments conducted with multiple samples of raw municipal sewage with a varying composition, and the prediction of MINEQL+ showed the main sulfide oxidation product to be S⁰. In order to reduce sulfide in sewage to < 0.1 mgS/l a minimal molar ratio of around 1.3 Fe to 1 S should be applied when ferrous salts are used, as compared with a minimal ratio of 0.9 Fe to 1 S required when ferric salts or a mixture of ferrous and ferric salts (at a 2 Fe(III) to 1 Fe(II) ratio) are used. It appears that the high Fe to S(-II) ratios often recommended in practice can be reduced considerably by applying tight in-line control.

Damage to crops after irrigation with extremely pure water from the world's largest reverse-osmosis desalination plant reveals a need for revised treatment standards.

A novel post-treatment approach for desalinated water, aimed at supplying a balanced concentration of alkalinity, Ca²⁺, Mg²⁺ and SO₄^2-, is introduced. The process is based on replacing excess Ca²⁺ ions generated in the common H₂SO₄-based calcite dissolution post-treatment process with Mg²⁺ ions originating from seawater. In the first step, Mg²⁺ ions are separated from seawater by means of a specific ion exchange resin that has high affinity toward divalent cations (Mg²⁺ and Ca²⁺) and an extremely low affinity toward monovalent cations (namely Na⁺ and K⁺). In the second step, the Mg²⁺-loaded resin is contacted with the effluent of the calcite dissolution reactor and Mg²⁺ and Ca²⁺ are exchanged. Consequently, the excess Ca²⁺ concentration in the water decreases while the Mg²⁺ concentration increases. The process is stopped at a predetermined Ca²⁺ to Mg²⁺ ratio. All water streams used in the process are internal and form a part of the desalination plant sequence, regardless of the additional ion exchange component. The proposed process allows for the supply of cheap Mg²⁺ ions, while at the same time enables the application of the cheap H₂SO₄-based calcite dissolution process, thus resulting in higher quality water at a cost-effective price. A case study is presented in which additional cost of supplying a Mg²⁺ concentration of 12 mg/L using the process is estimated at $0.004/m³ product water.

The spontaneous chemical oxidation of Fe(II) to Fe(III) by O₂ is a complex process involving meta-stable partially oxidized intermediate species such as green rusts, which ultimately transform into a variety of stable iron oxide end-products such as hematite, magnetite, goethite and lepidocrocite. Although in many practical situations the nature of the end-products is of less interest than the oxidation kinetics, it is difficult to find in the literature a description of all the basic steps and principles governing the kinetics of these reactions. This paper uses basic aquatic-chemistry equilibrium theory as the framework upon which to present a heuristic model of the oxidation kinetics of Fe(II) species to ferric iron by O₂. The oxidation rate can be described by the equation (in units of mol Fe(II)/(l min)): - d [Fe^{2 +}] / d t = 6 × 10^{- 5} [Fe^{2 +}] + 1.7 [Fe (OH)⁺] + 4.3 × 10⁵ [Fe (OH)₂⁰]. This rate equation yields a sigmoid-shaped curve as a function of pH; at pH values below ∼4, the Fe²⁺ concentration dominates and the rate is independent of pH. At pH > ∼5, [Fe (OH)₂⁰] determines the rate because it is far more readily oxidized than both Fe²⁺ and FeOH⁺. Between pH 5 and 8 the Fe (OH)₂⁰ concentration rises steeply with pH and the overall oxidation rate increases accordingly. At pH values > ∼8 [Fe (OH)₂⁰] no longer varies with pH and the oxidation rate is again independent of pH. The paper presents a heuristic overview of the pH dependent kinetics of aqueous ferrous oxidation by O_2(aq) which we believe will be useful to professionals at both research and technical levels.

In 2010 desalinated water is expected to provide approximately 25% of Israel's fresh water supply. Since desalination is cost-effective only if operated constantly, areas adjacent to the desalination plants may receive unblended desalinated water for prolonged times while other sources are added only at peak demand. Notwithstanding that desalinated water is of superior quality, it is widely accepted that soft waters may be corrosive to water distribution systems, and that soft waters mingling with other sources can cause a variety of adverse effects, namely metal corrosion and red water events. Despite this, no unambiguous quantitative criteria have been proposed to-date to address the required quality of desalinated water, following the post treatment stage. In this paper the water quality criteria are considered from various angles (chemical stability, bio-stability, effect on wastewater treatment, water palatability, health and economic effects, and post-treatment engineering considerations) and the following set of quality criteria for desalinated water is suggested: Alkalinity > 80, 80< [Ca²⁺] <120, 3< CCPP <10 (all concentrations in mg/L as CaCO₃), and pH <8.5. The work was carried out for, and approved by, the Committee for the Update of Water Quality Standards, appointed by the Israeli Ministry of Health.

The variety of kinetics expressions encountered in the literature and the unreasonably broad range of values reported for the kinetics constants of Acidithiobacillus ferrooxidans underscore the need for a unifying experimental procedure and for the development of a reliable kinetics equation. Following an extensive and critical review of reported experimental techniques, a method based on batch pH-controlled kinetics experiments lasting less than one doubling time was developed for the determination of extant kinetics constants. The Fe(II) concentration in the experiments was measured by a method insensitive to Fe(III) interference. Kinetics parameters were determined by nonlinear fitting of the integrated form of the Monod equation to yield a K_S of 31 ± 4 mg Fe²⁺ liter^-1 (mean ± standard deviation), a K_P of 139 ± 20 mg Fe³⁺ liter ^-1, and a μ_max of 0.082 ± 0.002 h^-1. The corresponding kinetics equation was as follows: dS/dT=(-0.082/2. 3·10⁷)S·X/31(1+P₀+S₀-S/139)+S where S represents the Fe(II) concentration in mg liter^-1, P ₀ represents the initial Fe(III) concentration in mg liter ^-1, X represents the suspended bacterial cell concentration in cells ml^-1, and t represents time in hours. The measured data fit this equation exceptionally well, with an R² of >0.99. Fe(III) inhibition was found to be of a competitive nature. Contrary to previous reports, the results show that the concentration of Acidithiobacillus ferrooxidans cells has no affect on the kinetics constants. The kinetics equation can be considered applicable only to A. ferrooxidans cells grown under environmental conditions similar to those of the inoculum tested in the study. In contrast, the experimental and computational procedure is completely general and can be applied to A. ferrooxidans irrespective of the culture history.

A model aimed at calculating the aeration efficiency of a passively aerated biological vertical bed unit is presented. The model allows the calculation of the mass of air that would flow via convection into the vertical bed, therefore enabling the prediction of the maximal capacity of the bed as an aerobic biological reactor. Aeration efficiency, defined as the volume of air that would enter the bottom of the bed as a function of the volume of water that is drained from it, is predicted in the model as a function of the mean particle size of the gravel media, and the diameter and number of the aeration tubes installed. The model was calibrated in the laboratory and verified using results from a pilot scale vertical bed treating secondary municipal wastewater effluents. The principal model equation is: EPAVB = NP Dp4 (NP Dp4 +0.285p Dr2 δ- 2), where EPAFB =efficiency of the passive air pump (-); NP =number of aeration pipes (-); Dp =aeration pipes' diameter (m); p =medium porosity (-); and Dr =vertical bed diameter (m).

This manuscript describes the methodology and application of a genetic algorithm scheme tailor-made to EPANET for optimizing the operation of water distribution systems with desalinated water sources, under unsteady water quality conditions. Many studies exist that describe potential problems that might occur when waters that have different chemical characteristics are blended, and especially when desalinated water sources are present. It can be shown that the chemical stability of the blend, as manifested by the Calcium Carbonate Precipitation Potential (CCPP) of the water, can become negative (i.e. un-stabilized water) when desalinated water are blended with ground water, even if both sources have a positive CCPP. The objective in this study is to minimize both the cost of pumping and of water treatment related expenses for a selected operational time horizon, while delivering the consumers the required quantities at acceptable qualities and pressures. The methodology is demonstrated on a small illustrative example.

This paper summarizes the findings of a survey (256 participants) conducted to determine the attitude of the Israeli urban public towards various urban water reuse options. Israel is known for its long and successful agricultural water reuse scheme, but to date no large-scale urban reuse projects have been implemented. The survey included 21 reuse options, which were clustered into three reuse categories, namely: low, medium, and high contact levels. Results show that a high proportion of the participants supported medium contact reuse options such as sidewalk landscaping (95%), domestic WC flushing (85%) and firefighting (96%). Higher contact reuse options such as domestic laundry (38%), preserved food (13%), and potable aquifer recharge (11%) found much lesser support. Less than expected support was found for low contact reuse options with 86% for field crop irrigation, 62% for aquifer recharge for agricultural irrigation, and as low as 49% for orchard irrigation. This low support is surprising, since all three options have been practiced on a large scale for over three decades in Israel without any adverse effects to the public. No correlation was found between any biographical characteristic examined (education, gender, income, marital status, having young children, and age) and support for medium contact options. For the medium contact options, the results suggest that perceived financial gain (individual and/or communal) and positive public opinion enhances support, while perceived health effects negatively affects the degree of support. Technology, trust in authorities and awareness of water and environmental issues were found to not have a significant effect on support for medium contact reuse options. Analyzing the four possible reasons for support given by participants who identified themselves as supporters of wastewater reuse revealed that the most important reason for support was "water saving", followed by "minimization of importing water from abroad". These were followed by "infrastructure cost saving" together with "environmental improvement".

In the near future, the discharge of nitrate from recirculating aquaculture systems (RAS) to receiving water bodies is expected to be constrained by environmental regulations. Following wastewater treatment terminology, nitrate removal that makes use of the organic solid wastes generated within the RAS as the energy source for denitrification, may be termed 'single-sludge denitrification'. In this approach, the costs associated with the addition of an external carbon source, the treatment of solid-wastes generated in the RAS, and the supplementation of alkalinity are reduced. The simple and economical operation of such a process can be realized by adopting the conventional activated sludge (AS) methodology. Organic solid-wastes taken from the solids-filter of a RAS growing gilthead seabream were characterized for their chemical and biodegradation properties. The results were used to generate a conceptual model to predict the performance of single-sludge denitrification in RAS. The model was run under typical operational conditions, employing the mean solids retention time (SRT) as the key operational parameter. Results indicated that in order to attain high denitrification rates, the ratio of the flux of organic solids (as COD) supplied to the denitrification reactor to the flux of NO₃^- reduced should be between 4.0 and 6.0 g COD (g NO₃-N)^{- 1} and the SRT values should be lower than 10 d. At these conditions, sludge production was estimated to be between 40% and 60% of the solids feed mass (in terms of COD), and NH₃-N production as a result of ammonification was estimated to be less than 10% of the NO₃^--N removed. Empirical verification of the model is presented elsewhere. The model can be used as a design tool and for predicting the performance of the process at any operational conditions.

A lab-scale activated-sludge type reactor was used to induce denitrification by using the organic solid waste of a typical Recirculating Aquaculture System (RAS) as the electron donor ('Single-sludge denitrification'). The results were compared with the predictions of a stoichiometry-based model. As predicted by the model, reactor's performance was found to be strongly related to the mean solids retention time (SRT) employed. Measured denitrification rates conformed very well to model predictions. High nitrate removal rates of up to 590 mg N (L_reactord)^{- 1} were recorded at a relatively low SRT of 4 d. Oxygen, that entered the reactor via both atmospheric diffusion and with the stream used to simulate the influent from a fish tank, reduced the amount of organic matter available for denitrification, resulting in lower denitrification rates. This interference was more significant when the system was operated at the longer SRTs. Most of the excess ammonia released to the aqueous phase through ammonification was oxidized (presumably by anammox bacteria) under the prevailing anoxic conditions, resulting in very low effluent TAN concentrations. Phosphate release to the aqueous phase was significantly lower than predicted, suggesting above-typical microbial P assimilation. Reaction kinetics was found to be zero order with respect to nitrate at concentrations of above 1.5 to 2.0 mg N L^{- 1}. Taken together the findings indicate that intensive single-sludge denitrification for treating RAS effluents is technically feasible, and that the process appears to be a cost-effective solution to reducing both the nutrient and the organic loads generated by intensive fish farms. The main advantages of the method include minimal formation of undesired by-products, small reactor volume and simple control and operation. Furthermore, the process is well described by a conceptual mathematical model, allowing its application as a part of any RAS design.

A new semiempirical approach is presented for predicting air-to-water oxygen transfer rates in mixed tanks and gravity sewers, using methods adopted from mixing theory. First, a flocculation unit was used to impart selected mean velocity gradients (G) into a completely mixed tank, from which oxygen was first removed, and dissolved oxygen concentrations were measured with time. Regression analysis was used to fit the rate of oxygen transfer equation against G. The reaeration rate in completely mixed reactors was found to be proportional to G² (R² = 0.987). Subsequently, G was linked to headless in sewers, and the equation was calibrated using a slope-adjustable, 27-m-long, gravity-flow, experimental sewer (internal diameter, D = 0.16 m). Here, the reaeration rate was proportional to G ¹ (R² = 0.981). The equation was compared with existing oxygen transfer models and validated against experimental data from the literature, to which the overall mass transfer coefficient for oxygen, K _La, derived by the new approach, conformed well.

Fish excrete two principal toxic metabolites to the water: NH₃ and CO₂, the former being typically toxic to fish at low (< 0.1 mg N l^{- 1}) concentrations. However, allowing the accumulation of metabolic CO_2(aq) results in pH reduction, thereby reducing the fraction of NH₃ from TAN. Such operation strategy may allow increasing the design criteria for TAN, which can result in reduced water flow requirements in flow-through systems. In this study, growth parameters of sea bream Sparus aurata grown in high TAN and low pH values, were monitored. TAN toxicity was first tested in 27-l aquariums, where sea bream fingerlings were grown in TAN values of up to 20 mg N l^{- 1} and pH 6.8, without showing any significant negative effects. Following that, two 100 m³ marine fish-culture tanks were stocked with 84 g fish and supplied with the same daily feed for 250 days. Liquid oxygen enrichment was affected and paddlewheel aerators were used for CO₂ stripping. Seawater was supplied to the high TAN experimental system at an average rate of 5.25 m³ (kg feed)^{- 1} and to a control tank at an average rate of 22.9 m³ (kg feed)^-1 (normal flow-through practice). The experimental system included a solids filter, but not a nitrification unit. TAN concentrations measured in the experimental system were much higher than those in the control system (5.44 ± 1.2 mg N l^{- 1} and 1.34 ± 0.6 mg N l^{- 1} on average, respectively), however fish growth and fish mortality rates in both systems were statistically identical. The inorganic carbon mass balance differed significantly between the two systems emphasizing the important role of the CO₂ stripping device. The choice of stripping device allows controlling the CO_2(aq) concentration, which in turn controls the pH value for a given alkalinity value. Such control over the CO_2(aq) concentration enables operating the system at relatively high TAN concentrations while maintaining NH_3(aq) below the threshold concentration. An aquatic-chemistry model was developed to predict pH value, and consequently CO_2(aq) and NH_3(aq) concentrations, assuming steady state conditions. Model results were used to determine the minimal makeup water flow-rate that would allow safe operation with regard to the threshold metabolite concentrations. Model results indicated that under the conditions tested, a flow-through system could be operated safely with a ratio as low as 4.4 m³ seawater (kg feed)^{- 1} with no need for a nitrification biofilter. The implications of growing fish at high TAN concentrations are extensive, the most important being a significant reduction in water treatment costs.

All detrimental phenomena (malodors, metal corrosion, concrete disintegration, health hazard) associated with hydrogen sulfide in gravity sewers depend on the rate of H₂S emission from the aqueous phase to the gas phase of the pipe. In this paper a different approach for predicting H₂S_(g) emission rates from gravity sewers is presented, using concepts adapted from mixing theory. The mean velocity gradient (G=γSV/μ; S is the slope, V the mean velocity), representing mixing conditions in gravity flow, was used to quantify the rate of H₂S _(g) emission in part-full gravity sewers. Based on this approach an emission equation was developed. The equation was verified and calibrated by performing 20 experiments in a 27-m gravity-flow experimental-sewer (D=0.16m) at various hydraulic conditions. Results indicate a clear dependency of the sulfide stripping-rate on G1 (R2=0.94) with the following overall emission equation:-d[ST]dt=8×10-7γSV/μwAcs1.024(T-20)(ST1+Ks110- pH+Ks1Ks210-2pH-PpH2SKH), where S_T is the total sulfide concentration in the aqueous phase, mg/L; w the flow surface width, m; A_cs the cross-sectional area, m²; T the temperature,°C; K_H the Henry's constant, mol L^-1 atm^-1; and P_pH2S the partial pressure of H₂S_(g) in the sewer atmosphere, atm.

Public support is crucial for successful implementation of wastewater reuse projects. This paper analyses the findings of a questionnaire-type survey (256 participants) conducted to determine the attitude of Israeli urban public towards possible urban reuse options. The paper summarises the support / objection to 13 reuse options and the correlation between support and environmental awareness and perceptions. Results show that a high proportion of the participants supported options perceived as low-contact, such as irrigation of public parks (96%), sidewalk landscaping (95%) and use in the construction industry (94%), while higher-contact reuse options found less support (e.g. commercial launderettes, 60%). No correlation was found between biographical characteristics and support (education, gender, income, age). Based on the results, public campaigns in Israel should focus on disseminating information regarding wastewater treatment technologies, discuss health related issues, highlight the positive economic impact of water reuse and generate a positive public opinion, as these factors tend to influence individuals to support reuse projects.

Advances in the seeded ambient temperature ferrite process for treatment of acid mine drainage (AMD) are described. Magnetite formation requires that the oxidation rate of ferrous to ferric does not exceed the rate at which ferrous iron is incorporated into the crystal structure (dehydroxylation- crystallization). If the oxidation rate is too high, then ferric-only oxides form, an effect exacerbated by the presence of calcium. NaOH was used to raise the pH of simulated AMD, which also contained calcium so as to simulate the use of lime (i.e., the dissolved Ca/Fe²⁺ concentration was maintained at 1:1 by the coaddition of CaClJapanese source2 because this is the Ca/Fe ratio that occurs when pH is elevated by the dissolution of lime), Raising the pH to 10.5 causes Fe²⁺ to precipitate as "ferrous intermediate" (FI), which is then partially oxidized to magnetite (Fe²⁺-Fe ³⁺₂O₄). The inhibitory effect of calcium is overcome by a combination of magnetite seed particles, high FI concentrations, and aging, High FI concentrations are easily obtained, even from AMD low in Fe²⁺, by a contact stabilization reactor-settler sequence. Results for simultaneous removal of cobalt, a metal found in significant concentrations in South African AMD, are also presented.

A titration approach was developed to measure low concentrations of citric acid (C₆H₈O₇) in a mixture of other weak acid/bases. Two methods were tested. The first and more practical method (a 4-point titration procedure) is applicable in conditions where volatile fatty acids (VFAs) are not normally present. The second method (a 5-point titration procedure) was developed for anaerobic environments where VFAs may be encountered. Generally, fairly accurate and repetitive results (precision >95%) were obtained for both situations although stability and accuracy were better in the absence of VFA. Both methods can be used for routine monitoring of biological reactors where citric acid is added as a carbon source and electron donor. Mg²⁺ and Ca²⁺ form complexes with citric acid and thus inhibit the use of the method. To overcome this, a sodium-saturated cationic ion exchanger was used to exchange these cations with Na⁺. Following cation exchange, citric acid concentration was determined accurately using the method.

A novel pilot-scale passively aerated vertical bed (PAVB) for the treatment of municipal wastewater was investigated. Two modes of PAVB operation were compared. In the first mode, "continuous feeding," an uninterrupted flow of wastewater was applied to the bed until clogging, followed by a "rest period" to regenerate pore space. During the rest period only passive aeration was provided by recirculation of clean effluent through the bed. In the second mode, "intermittent feeding," a daily schedule of 8 h of waste-water flow to the bed followed by a rest period of 16 h was employed. The results indicate that continuous operation is advantageous over intermittent operation. A high COD removal rate of about 800 g/m²/day was obtained when the system operated in the "continuous feeding" mode and the COD removal efficiency was about 50%. Clogging of the bed with apparent ponding occurred at a free pore space of 30 to 35% from its initial value. During both continuous and intermittent modes of operation, an irreversible clogging of 40% was observed. Higher organic removal efficiency was achieved by operating three vertical bed units in a series. The overall average COD and BOD reductions in the vertical bed system were 83 and 88%, respectively, with effluent COD₁ concentrations between 50 and 128 mg/L and BOD concentrations were between almost zero and 45 mg/l. In contrast to PAVB for tertiary treatment, no significant advantage was shown in using passive aeration when the PAVB was used for secondary treatment with a relatively large particle size (15 mm).

An increase in volatile fatty acids (VFA) concentration (or the proportional decrease in carbonate alkalinity concentration) is the first practical measurable indication that an anaerobic treatment system is in a state of stress. If the system is not rectified at this early stage, failure is likely. Current methods for VFA measurement include distillation, colorimetry, gas chromatography and various titration techniques. In terms of simplicity, speed and cost-effectiveness it is generally accepted that titration methods are superior for the purpose of on-site routine monitoring and control, particularly in developing countries. This paper reviews the methods published in the last four decades concerning on-site titration measurement of VFA and carbonate alkalinity concentrations. The review encompasses the following: aquatic chemistry related to the theory on which most of the methods are based, and a detailed description of each of the principal methods published followed by critical and comparative evaluation.

The factors affecting sulfide buildup in gravity sewers are complex, consisting of biological and physical processes, both in the aqueous and the gas phases of the sewer. The rate of each of these processes varies (among other parameters) according to flow characteristics, temperature, and pH. Under fast and turbulent flow conditions, the stripping of hydrogen sulfide into the gas phase may become the dominant process. The paper presents a semiempirical approach to the problem of quantifying hydrogen sulfide emission rates in sewers. Kinetics of hydrogen sulfide emission as a function of hydraulic parameters was measured in the laboratory using methods adopted from flocculation theory. A flocculation unit was used to impart a selected velocity gradient (G) into the water, and sulfide concentration was measured with time. The process was repeated for a number of G values. Regression analysis was then used to fit the rate of hydrogen sulfide emission equation against G. An equation was developed linking G to H_L (head loss) in sewers assuming plug flow conditions. The hydraulic model and the kinetic model were linked (via G) to give the desired rate equation for hydrogen sulfide emission along a sewer line. The model was used to predict H2S emission from a uniform flow sewer and the effect of parameters such as pH, sewer slope and degree of fullness was studied. As expected, results show that low pH, high slope, and low degree of fullness enhance emission rates. Reasonable agreement was attained when the model output was compared with measured results from a field test sewer in Virginia, South Africa, under conditions where sulfide stripping was the rate-dominant process.

The potential of using soil rich in iron for controlling hydrogen sulfide concentrations in aquaculture systems was investigated. The sulfide removal capacity of two local soils (Terre Rosse and Bazalt) was compared with four commercially available hematite ores. Terre Rosse soil, with iron content of 8% by mass, showed high reactivity towards sulfide. ⁵⁷Fe Mössbauer analysis of the soil revealed that the iron content is distributed as hematite (43%), goethite (33%) and iron silicates (24%). Dissolution potential experiments conducted at a high sulfide concentration (1200 mg S l^-1 at the end of the reaction) showed that 1 kg of Terre Rosse soil can remove up to 25 g of H₂S-S at pH values typical in aquaculture practice (near neutral), and that the reactive iron phase (i.e. the part that reacts with sulfide) is ∼40% of the total iron mass in the soil. The dissolution potential experiments were considered in conjunction with the analysis of the temperature evolution (300-5 K) of the Mössbauer data that also revealed ∼40% of the iron content to exist as nano-structured iron-oxides phases. The anticipated high surface-to-volume ratio and enhanced reactivity of such nanophase iron oxides, compared to bulk "micronic" materials is supposed to be the likely cause of the high reactivity of this soil. In an experiment performed with gradual addition of low sulfide concentrations (up to about 10 mg S l^-1) to simulate biological sulfide formation in anaerobic sediments, sulfide breakthrough occurred after the removal of 9.5 g S (kg soil)^-1. Under anaerobic conditions, sulfide removal was found to proceed via a redox reaction (end product: elemental sulfur) followed by FeS precipitation. Reaction kinetics was found to be pseudo-first order with respect to the total sulfide concentration. The iron oxides in the soil react rapidly in the presence of sulfide - the reactive iron fraction in the soil dissolves almost completely after about 1 day in the presence of a high sulfide concentration (>0.9 g S l^-1). At low concentrations, 2 g of Terre Rosse reduced the sulfide concentration from 200 to about 5 mg S l^-1 in around 120 h (solution volume=63 ml). In practical terms, it is hypothesized that an addition of 3 to 4 kg-soil m^-2-sediment prior to seasonal de-stratification of fish farming reservoirs will significantly reduce the toxic effects and rapid oxygen consumption rates associated with sulfide dispersal following abrupt mixing of the water column.

The aim of this research was to investigate the feasibility of treating liquid dairy wastes by a vertical bed equipped with an innovative passive aeration system. The vertical bed (32 liter) was operated by recirculating consecutive batches of liquid waste in the column. Batches of liquid waste were applied at two different rates: 1) each batch was recirculated for 72 hours, and 2) each batch was recirculated for 24 hours. Settled liquid dairy wastes (5000 mg 1^-1 COD, 2000 mg 1^-1 BOD and 2500 mg 1^-1 TSS) were used in the experiments. When the reactor operated with each batch recirculating for 72 hours, the BOD and COD reduction were 66% and 40%, respectively. The vertical bed operated successfully without the need for an additional rest period. The main removal was observed to take place during the first 20 hours. No biomass or solids accumulation was observed indicating that the remaining 52 hours of recirculation were actually used for bed regeneration, i.e. integrated rest period. When the reactor operated with each batch recirculating for 24 hours, the system clogged after 21 days. An additional 24 day rest period was needed in order to free 94% of the initial void space. In this mode, the BOD and COD reduction were 67% and 47%, respectively. The overall COD removal in a complete operational cycle (feeding period followed by a rest period) was 467 g COD m^-3 d^-1 (996 g COD m^-2 d^-1). This value is 1.4 higher than the COD removal obtained in the 72 hour per batch mode and shows the advantage of conventional vertical bed operation of intensive feeding followed by rest period rather than a rest period integrated into the feeding cycle.

An ambient temperature ferrite process has been developed for the removal of iron and non-ferrous metals from AMD waters. The process involves the controlled formation of magnetite (Fe₃O₄) that has the capacity to substitute divalent and trivalent cations as part of the lattice, thus forming a stable easy-to-separate ferrite. This paper reports on continuous operations of the process in the absence and presence of Ca²⁺, which is well known to impede ferrite formation. In the first instance, the process involves the precipitation of hydroxy-metals at pH 10.5 and their subsequent adsorption onto magnetite seed in a contact stabilisation reactor. Second, liquid-solid separation is effected and the solid fraction is subsequently treated in an oxidising reactor in which a fraction of the ferrous species is oxidised to an intermediate ferric precipitate. Finally, both ferrous and ferric species undergo crystal-chemical processing and are incorporated into stable magnetite. Results indicate that Ca²⁺ interference can be overcome by maintaining a high ratio of precipitated ferrous species to dissolved Ca²⁺. It was found that in order to attain the required high Fe²⁺:Ca²⁺ ratio, the solid ferrous-hydroxy species concentration in the oxidation reactor should be maintained at above 1 200 mg Fe/l. Ferrous to calcium ratios greater than 3 were found to favour magnetite formation. In the absence of Ca²⁺, a solid ferrous-hydroxy species concentration of approximately 500 mg/l was sufficient for magnetite formation. Operating the process at ferrous-hydroxy concentrations of lower than 1 200 and 500 mg/l in the presence and absence of calcium respectively enhanced the formation of other iron oxides, primarily goethite. In all experiments the iron concentration in the effluent was less than 1 mg/l, the sludge volume index (SVI) extremely low (< 4 ml/g) and the percentage of ferrous-hydroxy species in the sludge can be reduced to about 1%. These features, together with the potential to incorporate heavy metals into a stable compound, make the process very promising for AMD treatment.

A novel approach toward the removal of iron and nonferrous metals from typical South African acid mine drainage (AMD) waters was investigated. The approach involves the controlled oxidation of ferrous-containing AMD water at ambient temperatures in the presence of magnetite seed. The resulting oxidation product is the ferrite (M1³₂⁺M2^2+O₄) magnetite (Fe₃O₄) which has the capacity for nonferrous metal removal by cation substitution. Mössbauer spectroscopy, x-ray diffraction, and scanning electron microscopy analyses confirmed the precipitant to be magnetite. The effects of four parameters are reported: airflow rate, seed concentration, pH, and temperature. All of these independently affect the % ferrous in the final precipitant. In all experiments, the airflow rate was found to be rate limiting with respect to the kinetics of ferrous removal. The retention time for the complete removal of 1,200 mg Fe/L was 0.3-1.6 h (corresponding to airflow rates of 0.05-0.6 L/min, respectively). The precipitant settled well and showed complete stability at pH 5. The total iron concentration in the raw effluent was always less than 1 mg/L, representing an iron removal efficiency of greater than 99.9%.

This paper presents a new simple, rapid, and accurate method suitable for on-site measurement of volatile fatty acids (VFA) and carbonate alkalinity in anaerobic reactors. This titrimetric method involves eight pH observations, and typically, the full procedure takes approximately 15 min. An important feature of the method is a built-in quality control mechanism allowing the user a rapid means of assessing the reliability of the experimental procedure. To evaluate the accuracy of the method, both laboratory-made waters and industrial UASB effluent were tested. High accuracy for both VFA and carbonate alkalinity measurements (error within 2% and 1%, respectively) plus good repetition (average standard deviation of 6.7% and 1.45%, respectively) was obtained. The method takes into account the effects of the phosphate, ammonium, and sulfide weak acid subsystems. Appraisal of the effect of an input error in these subsystems revealed that VFA measurement is fairly insensitive to phosphate and ammonium concentrations. It is, however, sensitive to H₂S loss during titration where the sulfide concentration is higher than approximately 100 mg/L as S. With regard to the carbonate alkalinity measurement, error in concentration of either phosphate or sulfide or H₂S loss might result in a significant error. Short guidelines for correct execution of the method are given in an appendix.

The production of gaseous nitrogen compounds, particularly the greenhouse gas nitrous oxide, was investigated in a novel process for ammonium removal from wastewater. The process is based on the adsorption of ammonium on zeolite followed by bioregeneration. The zeolite serves the dual purpose of an ion exchanger and a physical carrier for nitrifying bacteria which bio-regenerate the ammonium saturated mineral. An analysis of the nitrifying population composition in the reactor fed with simulated secondary effluent (NH₄⁺ = 50 mg/l) revealed that about half of the bacteria in the biofilm were common ammonium oxidizers Nitrosococcus mobilis and Nitrosomonas, while the other half were nitrite oxidizers. The amount of nitrogen losses, under different conditions, and the identification of the emitted gases (N₂ or N₂O) were investigated in two sets of experiments: (I) batch experiments using biomass originating from the ion exchange reactor with and without the addition of nitrite, and (II) continuous experiments using the ion exchange reactor with zeolite as the biomass carrier. In the batch experiments, nitrite and oxygen concentrations were determined as the major parameters responsible for the formation of gaseous nitrogen gas during ammonia oxidation by autotrophic bacteria. Continuous experiments showed that the major parameter significantly affecting nitrogen losses was the amount of ammonium adsorbed by the zeolite during the ion exchange phase. The amount of ammonium adsorbed determines the ammonium concentration during the initial period of bioregeneration, which in turn directly influences oxygen demand and the resulting concentrations of oxygen and nitrite. It was concluded that the formation of nitrogen gas compounds in the ion exchange/bioregeneration process can be eliminated by adjusting the operational regime to have a shorter adsorption phase resulting in smaller amounts of ammonium adsorbed per cycle.

A novel approach towards the removal of iron and heavy metals from South African acid mine drainage (AMD) waters is presented. The approach involves the controlled oxidation of ferrous-containing AMD water at ambient temperatures in the presence of magnetite seed. The resulting oxidation product is the ferrite (M1³⁺₂M2²⁺O₄) magnetite (Fe₃O₄), which has the capacity for nonferrous metal removal and which forms a stable sludge that is easily separated from the effluent. Sludge characterisation studies (XRD, SEM and dissolution tests) show that oxidation of ferrous solutions under controlled pH and oxidation conditions (pH 10.5, air flow rate = 0.05 ℓ/min) in the presence of magnetite seed (initial seed:ferrous ratio = 7:1) yields almost pure magnetite at ambient temperature. It was found that magnetite seed channels the end products of the AMD oxidation reaction towards magnetite. Under identical conditions, but in the absence of magnetite seed, a poorly characterised mixture of largely amorphous iron oxides are formed with magnetite comprising not more than 17% of the total iron. The kinetics of the reaction under the investigated conditions were found to be very favourable, with magnetite forming at a rate of 12.8 mg Fe/ℓ/min. The total iron concentration in the effluent was always less than 1 mg/ℓ representing an iron removal efficiency of 99.9%. The precipitant settled well (SVI 8 mℓ/g) and showed substantial stability at pH 3 (dissolution of 1.1% after 120 h). An outline for a one-step ambient temperature ferrite process is presented.

A novel vertical bed process for the removal of ammonium from secondary effluents, using a 'passive air pump', has been developed. The process is based on convective aeration caused by a fill and draw operational sequence, and combines the advantages of the vertical wetlands concept with the high loading rates typically associated with trickling filters. Experiments were carried out in a 500-l reactor using simulative effluents and actual municipal secondary effluents. A maximal ammonium removal rate of 1100 g N/m² reactor/d was achieved using simulative effluents and an effective gravel size of 0.96 mm. At all hydraulic loads applied, the nitrification rate was found to be limited by the oxygen transfer rate. The small-size medium used with simulative effluents clogged when using actual municipal secondary effluents. Two other media (2.46 mm and 4.31 mm) did not clog during the entire experimental period and a maximum removal load of 300 g N/m² reactor/d was achieved. This value is still much higher than typical rates reported for conventional vertical beds. (C) 2000 Elsevier Science Ltd.

A fluidized bed reactor for nitrification with chalk as the biomass carrier and the sole buffer agent was studied. Chalk dissolution in the reactor was found to follow the stoichiometric ratio of 1 mole of CaCO₃ dissolved for each mole of NH⁺₄ oxidized. Three batches of chalk, each one having a different dissolution rate, were used to replace the dissolved chalk. The three dissolution rates resulted in three different steady state pH levels in the reactor (4.7-6.6) and nitrification rates. Nitrification was found to be limited by either the chalk dissolution rate or dissolved oxygen concentration depending on the type of chalk used. A maximal nitrification rate of 1.44 g NH⁺₄-N/l reactor . d was observed. The average cell yield was 0.1 g cells/g N oxidized, similar to the cell yield during reactor start-up when the pH was 7. The specific ammonium oxidation rates varied between 0.08 and 0.15 mg NH⁺₄-N oxidized/mg protein . h, values which are in the reported range for nitrification at pH 7 to 8. Oxygen update rate (OUR) results indicated that the major mechanism responsible for the high nitrification rate observed in the reactor operating at low pH seems to be the favorable microenvironment provided by the chalk. Copyright (C) 2000 Elsevier Science Ltd.

Conventional characterisation of low alkalinity waters via pH measurement and titration of total alkalinity to a prescribed end-point invariably leads to large errors. These errors result from instability of the pH probe and an unknown titration end-point. In this paper two direct methods (termed the "double Gran function" and the "blend" method) for the characterisation of such waters are evaluated critically. A blend composed of the raw water, sodium chloride (to increase conductivity), and standard bicarbonate (to increase buffering capacity) was titrated with standard strong acid in two pH regions: 6.3 < pH < 7.0, and 3.5 > pH <4.0. In both methods, total alkalinity was determined using the latter set of points, and the first Gran Function. In the double Gran function method the upper set of titration points was used to determine CO₂ acidity using the second Gran Function. In the "blend" method, equilibrium chemistry data were used to calculate total acidity for each point based on the known total alkalinity, pH reading, temperature ionic strength. The two methods gave excellent results (in terms of both repetition and accuracy) as compared to characterisation based on total alkalinity and inorganic carbon analysis. A detailed procedure for the execution of the two approaches is given in an appendix.

The 5-point titration method proposed by Moosbrugger et al. (1993) provides a cheap and rapid means for measuring inter alia short-chain volatile fatty acids. However, output from the analysis requires invoking a 'systematic pH error'. The authors ascribed this to either residual liquid junction potential effects or pH calibration errors. However, from a scientific standpoint this detracts from confidence in the method. In this paper, it is shown that Moosbrugger et al's 'systematic pH error' is an artefact of the numerical techniques employed in their analysis. An alternative numerical approach is presented which also gives excellent results, without invoking the pH error affect.

A new process for ammonium removal from wastewater using zeolite has been developed. The zeolite (chabazite) serves the dual purpose of an ion exchanger and a physical carrier for nitrifying bacteria which bio-regenerate the ammonium-saturated mineral. The entire process is carried out in a single, compact reactor and takes place in two phases: ion-exchange phase and bioregeneration phase. This paper describes the effects of the biofilm on ion-exchange capacity and kinetics. Batch and continuous experiments showed a reduction of about 25 to 30% in the ion-exchange rate in biofilm covered chabazite as compared to virgin chabazite, while the ion-exchange capacity did not change. Experiments conducted indicated that the rate-controlling step for ion exchange shifted from pore diffusion in the virgin chabazite to film diffusion in the biofilm-covered chabazite. The diffusion rate of NH₄⁺ inside biofilms is of the same order of magnitude as diffusion rate of NH₄⁺ in water and 3 to 4 orders of magnitude greater than typical pore diffusion rates reported in zeolites. Therefore, the biofilm coverage of the chabazite was originally not expected to affect the ion-exchange rate. In addition, chemical precipitation was experimentally found not to be the cause for the ion-exchange rate reduction. It was hypothesised that the rate-limiting factor for ion exchange was caused by the part of the biofilm adjacent to the chabazite which differs from the rest of the biofilm and is characterised by a much higher density which impedes diffusion.

A new process for ammonia removal from sewage effluents is presented. The process uses an ion exchange material, zeolite, both as the separator of NH₄/⁺ from the wastewater and also as the carrier for a nitrifying biomass. The process is carried out in a single reactor operating in two modes: An adsorption mode in which the zeolite column acts as a typical ion exchanger and a bioregeneration mode in which the bacteria attached to the zeolite oxidizes the NH₄/⁺ to NO₃/^-. The separation between carbonaceous removal and NH₄/⁺ removal enables the exclusive selection of nitrifiers at high concentrations attached to the zeolite, thus achieving high bioregeneration rates. This paper summarizes three years of research on the process and focuses on the operation with actual secondary and primary effluents. Process operation showed: (1) No bed clogging occurred due to suspended solids accumulation; (2) residual BOD from the adsorption phase resulted in only minimal heterotroph competition and thus, no fall in the rate of bioregeneration; (3) only a small deterioration in exchange efficiency due to zeolite biofilm coverage as compared to the ion exchange efficiency of 'virgin' zeolite. Results show that both secondary and primary effluents can be successfully treated by the process.

A new concept for ammonium removal from secondary effluent by zeolite followed by bioregeneration has been studied. In contrast to other studies of hybrid biological-ion exchange multireactor systems, the proposed process uses the ion exchange material, zeolite, as a carrier for the nitrifying biomass. Therefore, the entire process is carried out in a single reactor. Since all the ammonium from the original effluent is concentrated in the zeolite and released gradually during regeneration, nitrification is carried out in a small volume reactor in an almost batch mode where optimal conditions for nitrification can easily be maintained. Moreover, the conversion of ammonium cations to nitrate anions allows for regenerate recycle, where the amount of chemicals added for desorption is reduced to the amount of sodium bicarbonate added as a buffer for nitrification. As a result, operational costs and production of large volumes of brine are minimized. To achieve sufficient NH₄⁺ concentration in the solution to allow for high rate nitrification, the cation-rich regenerant solution (or part of it) is reused from one cycle to the next. A theoretical model including ion exchange and bioregeneration modes, indicates that the total cation concentration and each cation in the recycled regenerant should reach constant values after several cycles of adsorption-regeneration and remain constant as long as the influent characteristics and operation conditions stay similar. Experiments results verified the predicted values.

A new concept for ammonium removal from secondary effluent by zeolite followed by bio-regeneration has been studied. In contrast to other studies of hybrid biological-ion exchange multireactor systems, the proposed process uses the ion exchange material, zeolite, as the carrier for the nitrifying biomass. This enables the two mode process to be carried out in a single reactor. ammonium is released gradually and converted to nitrate by the active biomass residing on the zeolite. Nitrification is carried out batchwise and in a small volume reactor where optimal conditions can easily be maintained. Moreover, the addition of chemicals for the desorption of ammonium is minimal due to regenerant reuse during several cycles of nitrification. As a result, operational costs and production of large volumes of brine are minimized. Batch and breakthrough experiments showed that the amount of ammonium adsorbed on the chabazite is strongly affected by the presence of competing cations present in secondary effluent. A reduction of about 75% was observed when using a typical Israeli sewage ion composition. The attached biomass did not significantly effect the efficiency of the ion exchange column. Ammonium desorption experiments showed that regeneration with 10,000 mg/L Na⁺ is much faster than with 2,440 mg/L (more than 90% ammonium recovery after 40 and 70 bed volumes, respectively). A nitrification rate of 6 g NH₄-N/(L reactor (*)day) was obtained in a fluidized bed reactor with chabazite as the carrier. Although this rate is in the high range of reported values for biofilm reactors, desorption experiments proved that nitrification will be the process's rate limiting step, rather than the desorption rate when regenerant solutions as low as 2,440 mg/L Na⁺ are used.