Characterisation and removal of recalcitrants in reverse osmosis concentrates from water reclamation plants

Comparative assessment of treatment technologies for minimizing reverse osmosis concentrate volume for industrial applications: A review

Desalination of seawater, brackish water, and reclaimed water is becoming increasingly prevalent worldwide to supplement and diversify fresh water supplies. However, particularly for industrial wastewater, the need for environment-friendly and economically viable alternatives for concentrate management is the major impediment to deploying large-scale desalination. This review covers various strategies and technologies for managing reverse osmosis concentrate (ROC) and also includes their disposal, treatment, and potential applications. Developing energy-efficient, economical, and ecologically sound ROC management systems is essential if desalination and wastewater treatment are being implemented for a sustainable water future, particularly for industrial wastewater. The limitations and benefits of various concentrate management strategies are examined in this review. Moreover, it explores the potential of innovative technologies in reducing concentrate volume, enhancing water recovery, eliminating organic pollutants, and extracting valuable resources. This review critically discusses concentrate management approaches and technologies, including disposal, treatment, and reuse, including new technologies for reducing concentrate volume, boosting water recovery, eliminating organic contaminants, recovering valuable commodities, and minimizing energy consumption.

Removal of Organics with Ion-Exchange Resins (IEX) from Reverse Osmosis Concentrate

Reverse osmosis concentrate (ROC) produced as the by-product of the reverse osmosis process consists of a high load of organics (macro and micro) that potentially cause eco-toxicological effects in the environment. Previous studies focused on the removal of such compounds using oxidation, adsorption, and membrane-based treatments. However, these methods were not always efficient and formed toxic by-products. The impact of ion-exchange resin (IEX) (Purolite^®A502PS) was studied in a micro-filtration-IEX hybrid system to remove organics from ROC for varying doses of Purolite^® A502PS (5-20 g/L) at a flux of 36 L/m²h. The purolite particles in the membrane reactor reduced membrane fouling, evidenced by the reduction of transmembrane pressure (TMP), by pre-adsorbing the organics, and by mechanically scouring the membrane. The dissolved organic carbon was reduced by 45-60%, out of which 48-81% of the hydrophilics were removed followed by the hydrophobics and low molecular weight compounds (LMWs). This was based on fluorescence excitation-emission matrix and liquid chromatography-organic carbon detection. Negatively charged and hydrophobic organic compounds were preferentially removed by resin. Long-term experiments with different daily replacements of resin are suggested to minimize the resin requirements and energy consumption.

PLA-ZnO/TiO2 Nanocomposite Obtained by Ultrasound-Assisted Melt-Extrusion for Adsorption of Methylene Blue

Access to fresh water is an increasing concern worldwide. The contamination of this vital liquid is largely caused by discharges of pollutants into rivers and seas from different types of industries. Waste dyes from different industries have been classified as harmful to health. In this study, polymeric nanomaterials based on polylactic acid (PLA) and nanoparticles of titanium dioxide (TiO₂) and zinc oxide (ZnO) modified by ultrasound-assisted extrusion were obtained. These materials were evaluated by FTIR, DRX, TGA, DSC, SEM and methylene blue adsorption. From the results of the physicochemical characterizations, it was possible to observe the presence of TiO₂ and ZnO nanoparticles dispersed in the polymeric matrix, increasing the crystallinity and thermal stability of the polymer. In addition, a good dispersion of the nanoparticles could be seen by means of SEM, due to the extrusion assisted by ultrasound. The methylene blue dye adsorption tests revealed that the best result was 98% dye adsorption in a time of 13 min for the 1.5% PZT sample. Additionally, this material could be used for 3 adsorption cycles without affecting its adsorbent properties.

Treating reverse osmosis concentrate to address scaling and fouling problems in zero-liquid discharge systems: A scientometric review of global trends

Currently, reverse osmosis concentrate (ROC) treatment is one of the most promising techniques for its disposal because it produces freshwater with high recovery and valuable materials such as salts and reduces waste volume and environmental pollution. Public attention to the severe consequences of water pollution and strict environmental regulations on wastewater discharge has pushed water-polluting industries toward zero-liquid discharge (ZLD). However, scaling and fouling problems increase energy consumption and limit permeate flux at high salt concentrations, mainly due to calcium, magnesium, and silica precipitation, ultimately decreasing ZLD performance. Therefore, this study discusses drivers and ROC pretreatment technologies to improve ZLD efficiency and presents a scientometric review of global trends. The advantages, disadvantages, and economic and environmental aspects of conventional and emerging pre-treatment technologies were studied. Traditional treatment of chemical processes combined with precipitation removes a large amount of scaling ions; however, high operation and maintenance costs and limited full-scale plant experience are the main drawbacks. Softening and coagulation are most commonly applied to treat large volumes at a moderate cost; however, substantial sludge production and increased conductivity are major operational issues. Moreover, emerging technologies efficiently remove scale-forming ions with high capital and operating costs. New variations in standard reverse osmosis technologies have improved ZLD efficiency; nonetheless, scaling and fouling are of concern. Therefore, this review presents the studies on ROC pre-treatment technologies for removing scaling ions to enhance ZLD efficiency, which can help in future research.

Current status and future prospects of membrane separation processes for value recovery from wastewater

Resource constraints and deteriorating environment have made it necessary to look for intensification of the industrial processes, to recover value from spent streams for reuse. The development of reverse osmosis has already established that water can be recovered from aqueous streams in a cost-effective and beneficial manner to the industries. With the development of several membrane processes and membrane materials, the possibility of recovering value from the effluents looks like a workable proposition. In this context, the potentialities of the different membrane processes in value recovery are presented. Among the pressure-driven processes, reverse osmosis can be used for the recovery of water as value. Nanofiltration has been used for the recovery of several dyes including crystal violet, congo red, methyl blue, etc., while ultrafiltration has been used in the fractionation of different solute species using membranes of different pore-size characteristics. Diffusion dialysis is found useful in the separation of acids from its salt solutions. Bipolar membrane electrodialysis has the potential to regenerate acid and base from salt solutions. Thermally driven membrane distillation can provide desalinated water, besides reducing the temperature of hot discharge streams. Passive membrane processes such as supported liquid membranes and membrane-assisted solvent extraction have been found useful in separating minor components from the wastewater streams. The details are discussed to drive home that membrane processes can be useful to achieve the objectives of value recovery, in a cost-effective manner through process intensification, as they are more compact and individual streams can be treated and value used seamlessly.

Reverse osmosis concentrate treatment by microbubble ozonation-biological activated carbon process: Organics removal performance and environmental impact assessment

Reverse osmosis (RO) is being used in many water reclamation facilities to produce high quality water that can be reused for different purposes. As a part of the RO process, a reject stream is produced as the reverse osmosis concentrate (ROC), which contains elevated levels of contaminants compared to the source water. Effective treatment and safe disposal of ROC via cost-effective means is very challenging. This study aims to develop a robust microbubble ozonation-biological process for industrial ROC treatment with a target effluent chemical oxygen demand (COD) lower than 60 mg/L. As compared to macrobubble ozonation, microbubble ozonation exhibited better ozone dissolution and 29% higher COD removal efficiency with the same ozone dosage. Under the optimum operating conditions with ozone dosage of 30 mg/L, ROC natural pH of 8.67 and ozonation duration of 1 h, microbubble ozonation achieved 42% COD removal efficiency while increasing the BOD₅/COD ratio (ratio of biological oxygen demand over 5 days to the corresponding chemical oxygen demand) in ROC from 0.042 to 0.216. A biological activated carbon (BAC) column with an empty bed contact time (EBCT) of 120 min was combined with microbubble ozonation for continuous ROC treatment. Over the 100-day operation, the combined system performed consistent organics removal with an average effluent COD of 45 mg/L. Both LC-OCD data and fluorescence EEM spectra confirmed humic substances were the dominant organic species in ROC. Ozone pre-treatment could achieve significant removal of humic substances in raw ROC. ATP analysis found that ozone pre-treatment enhanced BAC biofilm activity by around 5 folds. 5 min acute toxicity assessment with Aliivibrio fischeri showed 4 times reduction of bioluminescence inhibition in ozone treated ROC. From the environmental point of view, Life cycle assessment (LCA) results demonstrated that Ozone-BAC system had significant environmental burdens on climate change and human toxicity due to the electricity production process. These environmental impacts can be mitigated by optimizing the ozonation process with reduced ozone dosage or utilizing renewable energy sources for electricity generation.

UV-based advanced oxidation of dissolved organic matter in reverse osmosis concentrate from a potable water reuse facility: A Parallel-Factor (PARAFAC) analysis approach

Disposal of reverse osmosis concentrate (ROC) from advanced water purification facilities is a challenge associated with the implementation of reverse osmosis-based treatment of municipal wastewater effluent for potable reuse. In particular, the dissolved organic matter (DOM) present in ROC diminishes the quality of the receiving water upon environmental disposal and affects the toxicity, fate, and transport of organic contaminants. This study investigates UV-based advanced oxidation processes (UV-AOPs) for treating DOM in ROC using a Parallel Factor Analysis (PARAFAC) approach. DOM composition and degradation were tested in UV-only and three UV-AOPs using hydrogen peroxide (H₂O₂), free chlorine (Cl₂), and persulfate (S₂O₈^2-). The four-component PARAFAC model consisted of two terrestrial humic-like components (C_UVH and C_VisH), a wastewater/nutrient tracer component (C_NuTr), and a protein-like (tyrosine-like) component (C_PrTy). Based on the observed loss in the maximum fluorescence intensity of the components, DOM degradation was determined to be dependent on UV fluence, oxidant dose, and dilution factor of the ROC (i.e., bulk DOM concentration). C_VisH was most the photolabile component in the UV-only system, followed by C_NuTr, C_PrTy, and C_UVH, respectively. Furthermore, UV-H₂O₂ and UV-S₂O₈^2- displayed faster overall reaction kinetics compared to UV-Cl₂. The degradation trends suggested that C_NuTr and C_PrTy consisted of chemical moieties that were susceptible to reactive oxygen species (HO^•) but not reactive chlorine species; whereas, C_VisH was sensitive to all reactive species generated in the three UV-AOPs. Compared to other components, C_PrTy was recalcitrant in all treatment scenarios tested. Calculations using chemical probe-based analysis also confirmed these trends in the reactivity of DOM components. The outcomes of this study form a foundation for characterizing ROC reactivity in UV-AOP treatment technologies, to ultimately improve the sustainability of water reuse systems.

Simulation of FBR-Fenton/GAC process for recalcitrant industrial wastewater treatment with a computational fluid dynamics-kinetic model framework

An integrated computational fluid dynamics (CFD)-kinetic model framework was developed to numerically describe the hydrodynamic and kinetic phenomena in a liquid-solid two phases Fluidized-bed reactor Fenton/granular activated carbon (FBR-Fenton/GAC) system. The model obtained excellent accuracy for predicting chemical oxygen demand (COD) removal in reverse osmosis concentrate (ROC) treatment under different operation conditions. Hydrodynamic evaluation demonstrated that under the quasi-steady state, the GAC particles were uniformly circulated in the bed region with two pairs of counter-rotating recirculation cells, and a clear interface layer formed between the solid and the liquid phases. Superficial liquid velocity highly affected the fluidized bed expansion and solid volume fraction, while its impact on the overall COD removal efficiency was negligible. Chemical evaluation revealed that GAC/H₂O₂ catalytic reaction enhanced the ^•OH production in FBR-Fenton/GAC process by 2.7 folds as compared to homogenous Fenton process. Fenton reaction mainly occurred in the upper liquid region and its kinetics for ^•OH generation significantly diminished by 75% within the first 10 min. GAC/H₂O₂ reaction took place in the fluidized bed region for continuous ^•OH generation with a relatively stable rate from 1.21 × 10^-6 to 0.60 × 10^-6 M/s. Along the ROC treatment with FBR-Fenton/GAC process, the simulated COD degradation rate decreased along the reaction time with 2.05 × 10^-6 M/s and 2.93 × 10^-7 M/s at 2 min and 60 min, respectively. Faster COD removal was attained in the fluidized bed region due to combining effects of ^•OH oxidation and GAC adsorption. The overall predicted COD concentration reduced from 122 to 35 mg/L, ^•OH oxidation and GAC adsorption contributed 59% and 41%, respectively, to the total COD removal.

Electrochemical oxidation of reverse osmosis concentrates using macroporous Ti-ENTA/SnO2-Sb flow-through anode: Degradation performance, energy efficiency and toxicity assessment

Due to poor mass transfer performance and high energy consumption of the traditional electrochemical flow-by mode, this study developed a high-efficiency electrochemical oxidation system in flow-through mode based on three-dimensional macroporous enhanced TiO₂ nanotube array/SnO₂-Sb (MP-Ti-ENTA/SnO₂-Sb) anode. The effects of initial pH, current density and flow rate on the COD degradation of reverse osmosis concentrates (ROCs) from reclaimed wastewater plant were investigated. Besides, the energy efficiency, biodegradability and acute biotoxicity were studied during electrochemical flow-through process. Compared with the flow-by mode, the flow-through mode based on the MP-Ti-ENTA/SnO₂-Sb anode had a COD removal rate of 0.38 mg min^-1 (current density: 5 mA cm^-2) and an electrical efficiency per order (EE/O) of 5.3 kW h m^-3. The three-dimensional fluorescence spectrum showed that the fulvic acids, humic acids and soluble microbial metabolites of ROCs could be effectively removed by the flow-through anode. In addition, the luminescence inhibition rate of the effluent was 22.4 %, indicating that the acute biotoxicity was reduced by more than 40 %. The electrochemical flow-through process of ROCs treatment required relatively low energy consumption without extra chemical agent addition, showing a broader application prospect.

Increasing net water recovery of reverse osmosis with membrane distillation using natural thermal differentials between brine and co-located water sources: Impacts at large reclamation facilities

Maximizing water recovery and minimizing the volume of RO concentrate (i.e., brine) produced is a growing challenge, especially for inland communities that lack ocean disposal options. In such regions, transitioning towards zero liquid discharge (ZLD) can avoid detrimental impacts associated with salt disposal via regional sewer discharge or deep-well injection. On-site ZLD energy requirements are proportional to the RO brine flowrate. Thus, system-level strategies that reduce RO brine flows will lower ZLD costs while simultaneously increasing the overall water recovery for beneficial reuse in reclamation facilities. We investigated a membrane distillation (MD) system operating using co-located, cooler source water to treat warmer wastewater RO brine. Using experimentally-quantified MD fluxes based on observed monthly water temperatures of co-located water and RO brine at a facility in central Arizona, and based on the previously reported performance of large-scale MD systems, energy consumption and operating cost were estimated to evaluate the potential capabilities of MD to treat RO brine at full scale facilities. When the RO unit was combined with MD brine treatment, net water recovery at the full-scale facility can increase from 85% to up to 91% while brine flow can be reduced by 26%. A 25% lower thermal energy was required to achieve RO net water recovery of 95% when using co-located water, compared against conventional MD without using co-located water. Overall, this work demonstrates the potential to use local thermal gradients to reduce RO brine volumes, thereby reducing ZLD costs.

Effect of extracellular polymer substances on the tetracycline removal during coagulation process

In this study, the effect of extracellular polymer substances on the tetracycline removal under hydroxyl aluminium treatment was investigated, and the molecular mechanisms of extracellular polymeric substances mediated coagulation of tetracycline were also explored. The results show that the presence of extracellular polymeric substances could significantly enhance the removal efficiency of tetracycline in hydroxyl aluminium coagulation. Findings suggest that tyrosine and tryptophan in extracellular proteins acted as binding sites to capture tetracycline. Evidences provided by the density functional theory calculations in combination with spectroscopy analysis indicated that two main mechanisms accounted for tetracycline removal in the presence of extracellular polymeric substances and polyaluminum chloride: (1) amino group in proteins and carbonyl in tetracycline were bridged by Al³⁺; (2) benzene rings in tryptophan and tyrosine were π-π stacked with tetracycline, and the amino group in complexes were further coordinated with Al³⁺.

Speciation and conversion of carbon and nitrogen in young landfill leachate during anaerobic biological pretreatment

There is an increasing need for landfill leachate pretreatment prior to discharge to wastewater treatment plants due to increasingly stringent sewer discharge limits. Lab-scale tests have shown that the anaerobic biological processes can effectively remove chemical oxygen demand and dissolved organic carbon from landfill leachate. Our work expands the knowledge in anaerobic leachate pretreatment by systematically studying the conversion of carbon and nitrogen species, particularly their recalcitrant fractions in a submerged anaerobic biofilm reactor using real-world leachate from a typical young municipal solid waste landfill. After reaching steady state, the reactor removed 41.7% of the fulvic acids (i.e., 1290 mg C/L). While compounds with a low degree of oxidation (O:C < 0.2) and compounds with a low degree of saturation (H:C < 1) were removed, compounds that were more oxidized (O:C > 0.2) and more saturated (H:C > 1) were produced. At steady state, 98% of recalcitrant dissolved organic nitrogen (i.e., rDON = 222 mg N/L) was removed. Compared to the DON in the raw leachate, the produced DON in the pre-treated leachate were more oxidized (O:C > 0.35) and more bioavailable (N:C > 0.07). The submerged anaerobic biofilm reactor may be an efficient leachate pretreatment method if rDON removal is needed.

A comparative study of biological activated carbon based treatments on two different types of municipal reverse osmosis concentrates

A study was conducted to understand the impact of reverse osmosis concentrate (ROC) characteristics on the efficacy of biological activated carbon (BAC) based treatments for removing organics and nutrients from two ROC streams (ROC_a derived from municipal waste input with high salinity, and ROC_b derived from domestic waste plus industrial trade waste with markedly lower salinity). Fluorescence excitation and emission matrix spectra and molecular weight analysis demonstrated that ROC_a and ROC_b had a significantly different composition of organic compounds due to the petrochemical processing and abattoir waste compounds in ROC_b. Although the sequence of coagulation, UV/H₂O₂ and BAC gave the highest organic removal from the two ROCs (67% DOC for ROCa and 62% for ROCb), UV/H₂O₂ followed by BAC achieved satisfactory removal (>55%) for both ROC types. Sequential treatment involving coagulation gave better phosphorus removal (>90%) than any single treatment (<65%). Total nitrogen (TN) removal was fairly low (<50%) for all the treatment options and the salinity level had insignificant impact on nitrogen removal. Analysis of bacterial communities suggested that higher phosphorus removal and lower total nitrogen and nitrate removal from ROC_b than ROC_a was related to the presence of various denitrifying or phosphorus accumulating bacteria in the BAC.

Crystallization of CaCO3 in Aqueous Solutions with Extremely High Concentrations of NaCl

The effect of NaCl at extremely high concentrations from 3.5 to 14 wt. % on the crystallization of CaCO3 was investigated in depth. The static test experiment verified that the Ca2+ retention efficiency (η) of NaCl on CaCO3 scale increased from 31.06% (3.5 wt. %) to 41.56% (14 wt. %). Based on the calculation of supersaturation rations, the high concentration of NaCl could reduce the activity coefficients of [Ca2+] and [CO32−], thus reducing the actual concentration of CaCO3. The CaCO3 deposition rate constants (k) showed that NaCl slowed down the rate of CaCO3 crystallization. The X–ray diffraction (XRD) testing disclosed that the growth of (1 0 4) and (1 1 0) faces from calcite was impeded, while the formation of (1 1 1) face from aragonite was induced by the increasing concentration of NaCl. The inductively coupled plasma optical emission spectrometry (ICP–OES) results indicated that Na+ could be doped into CaCO3, leading to the one–dimensional crystal growth. It was further proved that NaCl heightens the efficiency of the typical phosphate inhibitors (2–phosphonobutane–1,2,4–tricarboxylic acid (PBTCA) and 1–hydroxyethane–1,1–diphosphonic acid (HEDP)) on prohibiting the scale of CaCO3.

Potential of the integration of coagulation and ozonation as a pretreatment of reverse osmosis concentrate from coal gasification wastewater reclamation

Combination of coagulation and ozonation was used to treat brine derived from a three-stage reverse osmosis (RO) process during coal gasification wastewater reclamation. Effects of operating parameters on the removals of total organic carbon (TOC), color and UV absorbance at 254 nm (A₂₅₄) were investigated during coagulation and ozonation. All the removal efficiencies of TOC, A₂₅₄ and color of FeCl₃ coagulant are about twice those of AlCl₃ coagulant at the same molar dose since almost all the molecular weight fractions of RO concentrate (ROC) could be removed effectively by FeCl₃ coagulant while only the fractions of molecular weight > 3 k Da could be removed effectively by AlCl₃ coagulant. The TOC removal increases with the increasing of ozone dose and reaction temperature during ozonation of ROC after coagulation pretreatment. TOC and color of ROC after pretreated by coagulation could be further removed effectively during ozonation since ozonation can significant reduce the fluorescence response of all the fractions of effluent organic matter in ROC. It is unexpectedly found that the increase of A₂₅₄ is observed after ozonation, this is because the intensity of absorbance at 254 nm by the low molecular weight transformation products (<2 k Da) increases significantly with the reaction time after 30 min. The coagulation coupling with ozonation is efficient in the removals of both TOC and color of ROC.

Membrane-based separation of potential emerging pollutants

The potential emerging pollutants (PEPs) such as hazardous chemicals, toxic metals, bio-wastes, etc., pose a severe threat to human health, hygiene and ecology by way of polluting the environment and water sources. The PEPs are originated from various industrial effluent discharges including pharmaceutical, food and metal processing industries. These PEPs in contact with water may pollute the water and disturb the aquatic life. Innumerable methods have been used for the treatment of effluents and separating the toxic chemicals/metals. Of these methods, membrane-based separation processes (MBSPs) are effective over the conventional techniques for providing clean water from wastewater streams at an affordable cost with minimum energy requirement. Microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), and forward osmosis (FO) methods as well as hybrid technologies are discussed citing the published results of the past decade.

Promotion by humus-reducing bacteria for the degradation of UV254 absorbance in reverse-osmosis concentrates pretreated with O3-assisted UV-Fenton method

The primary pollutants in reverse-osmosis concentrates (ROC) are the substances with the UV absorbance at 254 nm (UV₂₅₄), which is closely related to humic substances that can be degraded by humus-reducing bacteria. This work studied the degradation characteristics of humus-reducing bacteria in ROC treatment. The physiological and biochemical characteristics of humus-reducing bacteria were investigated, and the effects of pH values and electron donors on the reduction of humic analog, antraquinone-2, 6-disulfonate were explored to optimize the degradation. Furthermore, the O₃-assisted UV-Fenton method was applied for the pretreatment of ROC, and the degradation of UV₂₅₄ absorbance was apparently promoted with their removal rate, reaching 84.2% after 10 days of degradation by humus-reducing bacteria.

Coagulation of surface water: Observations on the significance of biopolymers

Although the treatment of natural surface waters by coagulation has been investigated extensively, the detailed interaction between natural organic matter and alum is still not adequately understood or predictable, owing to the complexity of different components of the organic matrix and the conditions during coagulation. In this paper, we present the results of a novel approach to the study of the topic, which involved the progressive separation of organic components according to size, followed by coagulation of the filtrate solution, in order to expose the influence of particular organic fractions. Using two natural water sources, representative of lake and river waters, solutions of different organic content were obtained by progressively filtering the source waters using membranes of decreasing pore size; viz. microfiltration (MF), ultrafiltration (UF), and two grades of nanofiltration (NF). While MF had little impact on the range of organics present, UF was able to separate biopolymers (MW > 100 kDa), and NF had a substantially impact on the separation of medium-high MW (1-10 kDa) substances. The results of the coagulation tests showed that the size of flocs was substantially greater when biopolymers were present, suggesting their beneficial role in bridging precipitated Al(OH)₃ nanoparticles. For the smaller organic fractions (<10 kDa), the results showed a trend of increasing floc size with decreasing organic MW and concentration, but the trend was minor and may be explained by charge effects. Very similar results were found with both water sources, which support the main finding that biopolymers have an important influence on floc formation.

Simultaneous removal of organic matter and salt ions from coal gasification wastewater RO concentrate and microorganisms succession in a MBR

A lab-scale membrane bioreactor (MBR) with intermittent aeration was operated to treat the reverse osmosis concentrate derived from coal gasification wastewater. Results showed intermittent aeration represented slight effect on organic matter reduction but significant effect on nitrite and nitrate reduction, with 6h aeration and 6h non-aeration, removal efficiencies of organic matter, chloride, sulfate, nitrite and nitrate reached 48.35%, 40.91%, 34.28%, -36.05% and 64.34%, respectively. High-throughput sequencing showed a microorganisms succession from inoculated activated sludge (S1) to activated sludge in MBR (S2) with high salinity. Richness and diversity of microorganisms in S2 was lower than S1 and the community structure of S1 exhibited more even than S2. The most relative abundance of genus in S1 and S2 were unclassified_Desulfarculaceae (9.39%) and Roseibaca (62.1%), respectively. High salinity and intermittent aeration represented different influence on the denitrifying genus, and non-aeration phase provided feasible dissolved oxygen condition for denitrifying genera realizing denitrification.

A review on the application of constructed wetlands for the removal of priority substances and contaminants of emerging concern listed in recently launched EU legislation

The presence of organic pollutants in the aquatic environment, usually found at trace concentrations (i.e., between ng L^-1 and μg L^-1 or even lower, known as micropollutants), has been highlighted in recent decades as a worldwide environmental concern due to their difficult elimination by conventional water and wastewater treatment processes. The relevant information on constructed wetlands (CWs) and their application for the removal of a specific group of pollutants, 41 organic priority substances/classes of substances (PSs) and 8 certain other substances with environmental quality standards (EQS) listed in Directive 2013/39/EU as well as 17 contaminants of emerging concern (CECs) of the Watch List of Decision 2015/495/EU, is herein reviewed. Studies were found for 24 PSs and 2 other substances with EQS: octylphenol, nonylphenol, perfluorooctane sulfonic acid, di(2-ethylhexyl)phthalate, trichloromethane, dichloromethane, 1,2-dichloroethane, pentachlorobenzene, benzene, polychlorinated dibenzo-p-dioxins, naphthalene, fluoranthene, trifluralin, alachlor, isoproturon, diuron, tributyltin compounds, simazine, atrazine, chlorpyrifos (chlorpyrifos-ethyl), chlorfenvinphos, hexachlorobenzene, pentachlorophenol, endosulfan, dichlorodiphenyltrichloroethane (or DDT) and dieldrin. A few reports were also published for 8 CECs: imidacloprid, erythromycin, clarithromycin, azithromycin, diclofenac, estrone, 17-beta-estradiol and 17-alpha-ethinylestradiol. No references were found for the other 17 PSs, 6 certain other substances with EQS and 9 CECs listed in EU legislation.

Application of ozonation for pharmaceuticals and personal care products removal from water

Due to the shortening on natural water resources, reclaimed wastewater will be an important water supply source. However, suitable technologies must be available to guaranty its proper detoxification with special concern for the emerging pharmaceutical and personal care products that are continuously reaching municipal wastewater treatment plants. While conventional biological systems are not suitable to remove these compounds, ozone, due to its interesting features involving molecular ozone oxidation and the possibility of generating unselective hydroxyl radicals, has a wider range of action on micropollutants removal and water disinfection. This paper aims to review the studies dealing with ozone based processes for water reuse by considering municipal wastewater reclamation as well as natural and drinking water treatment. A comparison with alternative technologies is given. The main drawback of ozonation is related with the low mineralization achieved that may lead to the production of reaction intermediates with toxic features. The use of hydrogen peroxide and light aided systems enhance ozone action over pollutants. Moreover, scientific community is focused on the development of solid catalysts able to improve the mineralization level achieved by ozone. Special interest is now being given to solar light catalytic ozonation systems with interesting results both for chemical and biological contaminants abatement. Nowadays the integration between ozonation and sand biofiltration seems to be the most interesting cost effective methodology for water treatment. However, further studies must be performed to optimize this system by understanding the biofiltration mechanisms.

Tertiary treatment of a municipal wastewater toward pharmaceuticals removal by chemical and electrochemical advanced oxidation processes

This study focuses on the degradation of pharmaceuticals from a municipal wastewater after secondary treatment by applying various advanced oxidation processes (AOPs) and electrochemical AOPs (EAOPs) like UVC, H₂O₂/UVC, anodic oxidation (AO), AO with electrogenerated H₂O₂ (AO-H₂O₂), AO-H₂O₂/UVC and photoelectro-Fenton (PEF) using either UVC radiation (PEF-UVC) or UVA radiation (PEF-UVA). The municipal wastewater after secondary treatment was spiked with 5.0 mg L^-1 of trimethoprim (TMP) antibiotic. The efficiency of processes to remove TMP followed the order UVC < AO-H₂O₂ < PEF-UVA << AO ≈ PEF-UVC < AO-H₂O₂/UVC < PEF-UVA (pH = 2.8) < H₂O₂/UVC ≈ PEF-UVC (pH = 2.8), using neutral pH, except when identified. While the UVC radiation alone led to a very low TMP removal, the H₂O₂/UVC process promoted a very high TMP degradation due to the production of hydroxyl radicals (OH) by H₂O₂ cleavage. In the AO-H₂O₂/UVC process, the electrogeneration of H₂O₂ can avoid the risks associated with the transportation, storage and manipulation of this oxidant and, furthermore, OH at the anode surface are also formed. Nevertheless, low contents of H₂O₂ were detected mainly at the beginning of the reaction, leading to a lower initial reaction rate when compared with the H₂O₂/UVC system. In the PEF-UVC, the addition of iron at neutral pH led to the visible formation of insoluble iron oxides that can filter the light. At pH 2.8, the iron remained dissolved, thereby promoting the Fenton's reaction and increasing the organics removal. The UVA-driven processes showed limited efficiency when compared with those using UVC light. For all processes with H₂O₂ electrogeneration, the active chlorine species can be scavenged by the H₂O₂, diminishing the efficiency of the processes. This can explain the lower efficiency of AO-H₂O₂ when compared with AO. Moreover, the degradation of the MWWTP effluent spiked with 18 pharmaceuticals in μg L^-1 during AO process was assessed as well as the influence of the following operational variables on the process efficiency: (i) H₂O₂ concentration on H₂O₂/UVC, (ii) current density on AO, AO-H₂O₂, AO-H₂O₂/UVC, PEF-UVC and PEF-UVA, and (iii) pH on PEF-UVA.

Changes in the components and biotoxicity of dissolved organic matter in a municipal wastewater reclamation reverse osmosis system

The characteristics of dissolved organic matter (DOM) and the biotoxicity of these components were investigated in a municipal wastewater reclamation reverse osmosis (mWRRO) system with a microfiltration (MF) pretreatment unit. The MF pretreatment step had little effect on the levels of dissolved organic carbon (DOC) in the secondary effluent, but the addition of chlorine before MF promoted the formation of organics with anti-estrogenic activity. The distribution of excitation emission matrix (EEM) fluorescence constituents exhibited obvious discrepancies between the secondary effluent and the reverse osmosis (RO) concentrate. Using size exclusion chromatography, DOM with low molecular weights of approximately 1.2 and 0.98 kDa was newly formed during the mWRRO. The normalized genotoxicity and anti-estrogenic activity of the RO concentrate were 32.1 ± 10.2 μg4-NQO/mgDOC and 0.36 ± 0.08 mgTAM/mgDOC, respectively, and these values were clearly higher than those of the secondary effluent and MF permeate. The florescence volume of Regions I and II in the EEM spectrum could be suggested as a surrogate for assessing the genotoxicity and anti-estrogenic activity of the RO concentrate.

Evaluation of the treatment of reverse osmosis concentrates from municipal wastewater reclamation by coagulation and granular activated carbon adsorption

Reverse osmosis concentrate (ROC) from municipal wastewater reclamation reverse osmosis (mWRRO) contains elevated concentrations of contaminants which pose potential risks to aquatic environment. The treatment of ROC from an mWRRO using granular activated carbon (GAC) combined pretreatment of coagulation was optimized and evaluated. Among the three coagulants tested, ferric chloride (FeCl3) presented relatively higher DOC removal efficiency than polyaluminium chloride and lime at the same dosage and coagulation conditions. The removal efficiency of DOC, genotoxicity, and antiestrogenic activity concentration of the ROC could achieve 16.9, 18.9, and 39.7 %, respectively, by FeCl3 coagulation (with FeCl3 dosage of 180.22 mg/L), which can hardly reduce UV254 and genotoxicity normalized by DOC of the DOM with MW <5 kDa. However, the post-GAC adsorption column (with filtration velocity of 5.7 m/h, breakthrough point adsorption capacity of 0.22 mg DOC/g GAC) exhibited excellent removal efficiency on the dominant DOM fraction of MW <5 kDa in the ROC. The removal efficiency of DOC, UV254, and TDS in the ROC was up to 91.8, 96, and 76.5 %, respectively, by the FeCl3 coagulation and post-GAC adsorption. Also, the DOM with both genotoxicity and antiestrogenic activity were completely eliminated by the GAC adsorption. The results suggest that GAC adsorption combined pretreatment of FeCl3 coagulation as an efficient method to control organics, genotoxicity, and antiestrogenic activity in the ROC from mWRRO system.

Occurrence and removal of organic micropollutants: An overview of the watch list of EU Decision 2015/495

Although there are no legal discharge limits for micropollutants into the environment, some regulations have been published in the last few years. Recently, a watch list of substances for European Union-wide monitoring was reported in the Decision 2015/495/EU of 20 March 2015. Besides the substances previously recommended to be included by the Directive 39/2013/EU, namely two pharmaceuticals (diclofenac and the synthetic hormone 17-alpha-ethinylestradiol (EE2)) and a natural hormone (17-beta-estradiol (E2)), the first watch list of 10 substances/groups of substances also refers three macrolide antibiotics (azithromycin, clarithromycin and erythromycin), other natural hormone (estrone (E1)), some pesticides (methiocarb, oxadiazon, imidacloprid, thiacloprid, thiamethoxam, clothianidin, acetamiprid and triallate), a UV filter (2-ethylhexyl-4-methoxycinnamate) and an antioxidant (2,6-di-tert-butyl-4-methylphenol) commonly used as food additive. Since little is known about the removal of most of the substances included in the Decision 2015/495/EU, particularly regarding realistic concentrations in aqueous environmental samples, this review aims to: (i) overview the European policy in the water field; (ii) briefly describe the most commonly used conventional and advanced treatment processes to remove micropollutants; (iii) summarize the relevant data published in the last decade, regarding occurrence and removal in aqueous matrices of the 10 substances/groups of substances that were recently included in the first watch list for European Union monitoring (Decision 2015/495/EU); and (iv) highlight the lack of reports concerning some substances of the watch list, the study of un-spiked aquatic matrices and the assessment of transformation by-products.

Impact of salinity on organic matter and nitrogen removal from a municipal wastewater RO concentrate using biologically activated carbon coupled with UV/H2O2

The concentrate streams generated from reverse osmosis (RO)-based municipal wastewater reclamation processes contain organic substances and nutrients at elevated concentrations, posing environmental and health risks on their disposal to confined receiving environments such as bays. The impact of salinity (TDS at 7, 10 and 16 g/L) of a RO concentrate (ROC) on the treatment efficiency of a biological activated carbon (BAC) system after pre-oxidation with UV/H2O2 was characterised in terms of removal of organic matter and nitrogen species, and the bacterial communities. Organic matter removal was comparable for the ROC over the tested salinity range, with 45-49% of DOC and 70-74% of UVA254 removed by the combined treatment. However, removal in total nitrogen (TN) was considerably higher for the ROC at the high salinity (TDS ∼ 16 mg/L) compared with the low (∼7 g/L) and medium salinity (∼10 g/L). Effective nitrification with high ammonium removal (>90%) was achieved at all salinity levels, whereas greater denitrification (39%) was obtained at high salinity than low (23%) and medium salinity (27%) which might suggest that the bacterial communities contributing to the greater denitrification were more halotolerant. Microbiological characterisation using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and culture based techniques showed that diversified bacterial communities were present in the BAC system as evident from different 16S rDNA. The major bacterial groups residing on the BAC media belonged to Bacillus (Firmicutes), Pseudomonas (γ-Proteobacteria), and Rhodococcus (Actinobacteria) for all salinity levels, confirming that these microbial communities could be responsible for carbon and nitrogen removal at the different salinity levels. This has implications in understanding the effectiveness and robustness of the BAC system over the salinity range of the ROC and so would be useful for optimising the treatment efficiency of the BAC system.

Removing organic and nitrogen content from a highly saline municipal wastewater reverse osmosis concentrate by UV/H2O2-BAC treatment

Reverse osmosis (RO) concentrate (ROC) streams generated from RO-based municipal wastewater reclamation processes pose potential health and environmental risks on their disposal to confined water bodies such as bays. A UV/H2O2 advanced oxidation process followed by a biological activated carbon (BAC) treatment was evaluated at lab-scale for the removal of organic and nutrient content from a highly saline ROC (TDS 16 g L(-1), EC 23.5 mS cm(-1)) for its safe disposal to the receiving environment. Over the 230-day operation of the UV/H2O2-BAC process, the colour and UV absorbance (254 nm) of the ROC were reduced to well below those of the influent to the reclamation process. The concentrations of DOC and total nitrogen (TN) were reduced by approximately 60% at an empty bed contact time (EBCT) of 60 min. The reduction in ammonia nitrogen by the BAC remained high under all conditions tested (>90%). Further investigation confirmed that the presence of residual peroxide in the UV/H2O2 treated ROC was beneficial for DOC removal, but markedly inhibited the activities of the nitrifying bacteria (i.e., nitrite oxidising bacteria) in the BAC system and hence compromised total nitrogen removal. This work demonstrated that the BAC treatment could be acclimated to the very high salinity environment, and could be used as a robust method for the removal of organic matter and nitrogen from the pre-oxidised ROC under optimised conditions.

Modeling full-scale osmotic membrane bioreactor systems with high sludge retention and low salt concentration factor for wastewater reclamation

A full-scale model was developed to find optimal design parameters for osmotic membrane bioreactor (OMBR) and reverse osmosis (RO) hybrid system for wastewater reclamation. The model simulates salt accumulation, draw solution dilution and water flux in OMBR with sludge concentrator for high retention and low salt concentration factor. The full-scale OMBR simulation results reveal that flat-sheet module with spacers exhibits slightly higher flux than hollow-fiber; forward osmosis (FO) membrane with high water permeability, low salt permeability, and low resistance to salt diffusion shows high water flux; an optimal water recovery around 50% ensures high flux and no adverse effect on microbial activity; and FO membrane cost decreases and RO energy consumption and product water concentration increases at higher DS flow rates and concentrations. The simulated FO water flux and RO energy consumption ranges from 3.03 to 13.76LMH and 0.35 to 1.39kWh/m(3), respectively.

Novel technologies for reverse osmosis concentrate treatment: a review

Global water shortages due to droughts and population growth have created increasing interest in water reuse and recycling and, concomitantly, development of effective water treatment processes. Pressured membrane processes, in particular reverse osmosis, have been adopted in water treatment industries and utilities despite the relatively high operational cost and energy consumption. However, emerging contaminants are present in reverse osmosis concentrate in higher concentrations than in the feed water, and have created challenges for treatment of the concentrate. Further, standards and guidelines for assessment and treatment of newly identified contaminants are currently lacking. Research is needed regarding the treatment and disposal of emerging contaminants of concern in reverse osmosis concentrate, in order to develop cost-effective methods for minimizing potential impacts on public health and the environment. This paper reviews treatment options for concentrate from membrane processes. Barriers to emerging treatment options are discussed and novel treatment processes are evaluated based on a literature review.

Application of solar photo-Fenton at circumneutral pH to nanofiltration concentrates for removal of pharmaceuticals in MWTP effluents

In view of the inefficient elimination of micro-pollutants by today's conventional biological treatments and new legislation requiring elimination of at least 80 % of their concentration, the application of an advanced tertiary treatment must be studied. A good option would be advanced oxidation processes (AOPs), which have very often been combined with physicochemical pre-treatSments to increase efficiency or reduce operating costs. This study focused on the combination of membrane nanofiltration and solar photo-Fenton for the main purpose of removing five pharmaceuticals (sulfamethoxazole, ibuprofen, ofloxacin, carbamazepine and flumequine) from real MWTP effluents under realistic conditions (μg L(-1)). This research also included tests performed with modified photo-Fenton using a low iron concentration at circumneutral pH and a low hydrogen peroxide dose, in an attempt to reduce major treatment costs. Over 80 % of dissolved organic carbon, chemical oxygen demand and turbidity were also retained during nanofiltration, making pharmaceutical removal less efficient in terms of concentrate treatment time than direct treatment, i.e. the concentrate illumination time was around 150 min while direct treatment was around 40 min. Nevertheless, it should be highlighted that, although no savings in installation costs was observed for the combined system (nanofiltration/solar photo-Fenton), the reaction rate improved and so, there was a savings in reagent costs (mainly hydrogen peroxide and sulfuric acid).

Selective removal of photocatalytic non-degradable fluorosurfactants from reverse osmosis concentrate

Recently photocatalytic treatment of municipal reverse osmosis concentrate (ROC) has drawn increasing attention due to its relatively high efficiency and low cost. However, photocatalytic reactions by commercially available TiO₂ are not able to degrade fluorosurfactants in the ROC sample due to the absence of photoreactive groups in these compounds. Here we investigated adsorption and coagulation methods and their efficiencies in removing fluorosurfactants. The analysis and characterization methods included mass spectrometry (LC-QToF), total organic carbon (TOC), fluorescence & UV–Visible spectra, SEM, IR, N2 sorption, zeta potential, and elemental analysis. Ferric chloride (FER) coagulation was found to be quite efficient in removing fluorosurfactants, while powdered activated carbon (PAC) adsorption was inefficient. The FER pre-treatment process was found to perform better than the post-treatment process in removing the fluorosurfactants. FER selectively removed the bulky fluorosurfactants with long branches but not the slim ones with short or no branches. At a concentration of 10.60 mM, FER could efficiently remove 62.19% fluorosurfactants in total from the ROC sample. The applicability of Freundlich and Langmuir models for the adsorption processes was also investigated. FER was able to remove fluorosurfactant while PAC unable. While the PAC removal mechanism was adsorption, the FER coagulation mechanism was far more complicated.

Characterization and biotoxicity assessment of dissolved organic matter in RO concentrate from a municipal wastewater reclamation reverse osmosis system

Reverse osmosis (RO) concentrate from municipal wastewater reclamation reverse osmosis (mWRRO) system containing organic compounds may associate with toxic risk, and its discharge might pose an environmental risk. To identify a basis for the selection of feasible technology in treating RO concentrates, the characteristics and biotoxicity of different fractions of dissolved organic matter (DOM) in RO concentrates from an mWRRO system were investigated. The results indicated that the hydrophilic neutrals (HIN), hydrophobic acids (HOA) and hydrophobic bases (HOB) accounted for 96% of the dissolved organic carbon (DOC) of the total DOM in the RO concentrate. According to the SEC chromatograph detected at 254 nm wavelength of UV, the DOM with molecular weight (MW) 1-3 kDa accounted for the majority of the basic and neutral fractions. The fluorescence spectra of the excitation emission matrix (EEM) indicated that most aromatic proteins, humic/fulvic acid-like and soluble microbial by-product-like substances existed in the fractions HOA and hydrophobic neutrals (HON). The genotoxicity and anti-estrogenic activity of the RO concentrate were 1795.6 ± 57.2 μg 4-NQOL(-1) and 2.19 ± 0.05 mg TAM L(-1), respectively. The HIN, HOA, and HOB contributed to the genotoxicity of the RO concentrate, and the HIN was with the highest genotoxic level of 1007.9 ± 94.8 μg 4-NQOL(-1). The HOA, HON, and HIN lead to the total anti-estrogenic activity of the RO concentrate, and HOA occupied approximately 60% of the total, which was 1.3 ± 0.17 mg TAM L(-1).

Removal of pharmaceuticals from MWTP effluent by nanofiltration and solar photo-Fenton using two different iron complexes at neutral pH

In recent years, membrane technologies (nanofiltration (NF)/reverse osmosis (RO)) have received much attention for micropollutant separation from Municipal Wastewater Treatment Plant (MWTP) effluents. Practically all micropollutants are retained in the concentrate stream, which must be treated. Advanced Oxidation Processes (AOPs) have been demonstrated to be a good option for the removal of microcontaminants from water systems. However, these processes are expensive, and therefore, are usually combined with other techniques (such as membrane systems) in an attempt at cost reduction. One of the main costs in solar photo-Fenton comes from reagent consumption, mainly hydrogen peroxide and chemicals for pH adjustment. Thus, in this study, solar photo-Fenton was used to treat a real MWTP effluent with low initial iron (less than 0.2 mM) and hydrogen peroxide (less than 2 mM) concentrations. In order to work at neutral pH, iron complexing agents (EDDS and citrate) were used in the two cases studied: direct treatment of the MWTP effluent and treatment of the concentrate stream generated by NF. The degradation of five pharmaceuticals (carbamazepine, flumequine, ibuprofen, ofloxacin and sulfamethoxazole) spiked in the effluent at low initial concentrations (μg L(-1)) was monitored as the main variable in the pilot-plant-scale photo-Fenton experiments. In both effluents, pharmaceuticals were efficiently removed (>90%), requiring low accumulated solar energy (2 kJUV L(-1), key parameter in scaling up the CPC photoreactor) and low iron and hydrogen peroxide concentrations (reagent costs, 0.1 and 1.5 mM, respectively). NF provided a clean effluent, and the concentrate was positively treated by solar photo-Fenton with no significant differences between the direct MWTP effluent and NF concentrate treatments.

Effect of coagulation on treatment of municipal wastewater reverse osmosis concentrate by UVC/H2O2

Disposal of reverse osmosis concentrate (ROC) is a growing concern due to potential health and ecological risks. Alum coagulation was investigated as pre-treatment for the UVC/H2O2 treatment of two high salinity ROC samples (ROC A and B) of comparable organic and inorganic content. Coagulation removed a greater fraction of the organic content for ROC B (29%) than ROC A (16%) which correlated well with the reductions of colour and A254. Although the total reductions after 60 min UVC/H2O2 treatment with and without coagulation were comparable, large differences in the trends of reduction were observed which were attributed to the different nature of the organic content (humic-like) of the samples as indicated by the LC-OCD analyses and different initial (5% and 16%) biodegradability. Coagulation and UVC/H2O2 treatment preferentially removed humic-like compounds which resulted in low reaction rates after UVC/H2O2 treatment of the coagulated samples. The improvement in biodegradability was greater (2-3-fold) during UVC/H2O2 treatment of the pre-treated samples than without pre-treatment. The target DOC residual (≤ 15 mg/L) was obtained after 30 and 20 min irradiation of pre-treated ROC A and ROC B with downstream biological treatment, corresponding to reductions of 55% and 62%, respectively.

Pharmaceuticals and organic pollution mitigation in reclamation osmosis brines by UV/H2O2 and ozone

One significant disadvantage of using reverse osmosis (RO) for reclamation purposes is the need to dispose of the RO retentates. These retentates contain a high concentration of micropollutants, effluent organic matter (EfOM) and other inorganic constituents, which are recalcitrant to biological treatment and may impact the environment. The occurrence of 11 pharmaceuticals (concentrations ranging from 0.2 to 1.6 μg L(-1)) and their mitigation in RO retentates by a UV/H2O2 process and ozonation was studied using a wide range of oxidant dosages. Eleven pharmaceuticals were identified at. Initial observed kinetic constants (kobs) were calculated for the different pharmaceuticals. Other typical wastewater parameters were also monitored during the UV/H2O2 and ozonation reactions. The range for kobs was found to be 0.8-12.8L mmol O3(-1) and 9.7-29.9 L mmol H2O2(-1) for the ozonation and UV/H2O2 process, respectively. For ozonation, Atenolol, Carbamazepine, Codeine, Trimethoprim and Diclofenac showed the lowest initial kobs (in the order mentioned). Atenolol and Carbamazepine appeared as the most ozone resistant pharmaceuticals, exhibiting the lowest percentage of elimination at low ozone doses. On the other hand, despite the non-selectivity of HO, differences in the initial kobs were also observed during the UV/H2O2 process. Trimethoprim, Paroxetine and Sulfamethoxazole exhibited the lowest initial kobs values (in the order mentioned). Trimethoprim and Paroxetine also exhibited the lowest percentage removal when low H2O2 doses were assayed. The compounds that were identified as problematic during ozonation were more efficiently removed by the UV/H2O2 process. UV/H2O2 generally appeared to be a more efficient technology for removing pharmaceuticals from RO brines compared to ozonation.

Potential of BAC combined with UVC/H2O2 for reducing organic matter from highly saline reverse osmosis concentrate produced from municipal wastewater reclamation

The organic matter present in the concentrate streams generated from reverse osmosis (RO) based municipal wastewater reclamation processes poses environmental and health risks on its disposal to the receiving environment (e.g., estuaries, bays). The potential of a biological activated carbon (BAC) process combined with pre-oxidation using a UVC/H2O2 advanced oxidation process for treating a high salinity (TDS~10000 mg L(-1)) municipal wastewater RO concentrate (ROC) was evaluated at lab scale during 90 d of operation. The combined treatment reduced the UVA254 and colour of the ROC to below those for the influent of the RO process (i.e., biologically treated secondary effluent), and the reductions in DOC and COD were approximately 60% and 50%, respectively. UVC/H2O2 was demonstrated to be an effective means of converting the recalcitrant organic compounds in the ROC into biodegradable substances which were readily removed by the BAC process, leading to a synergistic effect of the combined treatment in degrading the organic matter. The tests using various BAC feed concentrations suggested that the biological treatment was robust and consistent for treating the high salinity ROC. Using Microtox analysis no toxicity was detected for the ROC after the combined treatment, and the trihalomethane formation potential was reduced from 3.5 to 2.8 mg L(-1).