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Recovery of natural gas and oil from unconventional (shale) reservoirs relies on horizontal drilling and hydraulic fracturing to make it economical. Hydraulic fracturing generates vast quantities of flowback and produced water (FPW) and its composition exhibits huge spatial and temporal variations among shale plays. This review focuses on the characteristics and management of wastewaters originating for oil and gas extraction. Wastewater characteristics, including the quantity and chemical composition of the FPW, are discussed. The future of unconventional oil and gas industry hinges on effective management of FPW. Membrane technologies have the potential to offer solutions to sustainable reuse of this water resource. The performance of a range of membrane processes is evaluated and compared. Emerging membrane-based technologies employed in similar fields are also discussed. The results in peer-reviewed publications could offer a guide for the selection of appropriate technologies based on the desired application. Membrane fouling, lack of pilot- and full-scale experience and high energy consumption are primary challenges for membrane applications in FPW. Then challenges and future research needs are addressed, advances in membrane materials, systematic analyses of organics and electric generation from salinity gradient are promising approaches to address the issues.

Capacitive deionization (CDI), which is based on the electrosorption of ions by porous electrodes, is an emerging technology for brackish water desalination. Understanding the key drivers of energy consumption in CDI and benchmarking CDI with reverse osmosis (RO), the current state-of-the-art for brackish and seawater desalination, is crucial to guide the future development of desalination technologies. In this study, we develop system-scale models to analyze the energy consumption and energy efficiency of CDI and RO over a wide range of material properties and operating conditions. Using our models, we explore how the energetic performance of CDI and RO compare as a function of feed salinity, water recovery, salt rejection, and average water flux, which is normalized by electrode and membrane area in CDI and RO, respectively. Our analysis shows that RO is significantly more energy efficient than CDI, particularly when targeting higher salinity feed streams and higher salt rejection values. For brackish water with a salt concentration of 2000 mg L−1, achieving 50% water recovery and 75% salt rejection, with an average water flux of 10 L m−2 h−1 using CDI requires a specific energy consumption of 0.85 kWh m−3, more than eight times that of RO (0.09 kWh m−3). Importantly, our results also indicate that current efforts to improve electrode materials can only marginally reduce the energy consumption of CDI. We conclude with a discussion highlighting other important factors, such as capital cost, electrode stability, and membrane fouling, which affect the efficacy of CDI and RO for low-salinity desalination.

Asymmetric membranes with a thin, small pore size upper layer have the potential to facilitate a high vapor flux while maintaining high liquid entry pressure, which is critical for membrane distillation (MD) and thermo-osmotic energy conversion (TOEC) processes. Here, asymmetric mixed cellulose ultrafiltration membranes were modified with perfluorodecyltrichlorosilane to produce 50 nm and 25 nm pore size membranes with highly hydrophobic surfaces (contact angle >115°) and unprecedented liquid entry pressures >24 bar. The 50 nm membrane performance was evaluated in a series of MD and TOEC mode experiments, where the membrane dense layer faces the hot feed stream or the cold permeate stream, respectively. Our results demonstrate that the membrane water vapor permeability coefficient is significantly higher when operating in MD mode (1.7 × 10−7 kg m−2 s−1 Pa−1) compared to TOEC mode (0.9 × 10−7 kg m−2 s−1 Pa−1) at a similar temperature difference of 39 °C, suggesting an additional resistance to vapor flux in TOEC mode. We developed a model for mass and heat transfer through the membrane to explain the change in performance due to reversing the asymmetric membrane orientation. As expected, the model and the experimental results show a linear increase in the water vapor permeability coefficient with respect to temperature difference across the membrane for the MD orientation. However, for the TOEC orientation, the water vapor permeability coefficient was relatively constant across all the temperature differences investigated. Our results predict that the additional mass transport resistances in TOEC mode decrease as the transmembrane temperature gradient decreases. We conclude with a discussion on the implications of using asymmetric membranes for MD and TOEC processes.

Diffusion cannot be a major water transport mechanism in osmotic membranes because of the lack of true water concentration gradient within the membrane. Due to the semipermeable property of osmotic membranes, water concentration in the membrane is virtually constant because of the absence of salts. The recently confirmed porous structure of the skin layer of osmotic membranes cannot support the basis to exclude bulk water flow in the membrane as assumed in the classic solution-diffusion model. Herein we demonstrate that the concentration difference of water at the membrane-solution interface manifests itself as a negative hydraulic pressure in the membrane. Hence, the only possible driving force for water movement in osmotic membranes is hydraulic pressure gradient. Osmotically driven membrane processes are characterized with negative pressure within the membrane below the water vapor pressure, inevitably leading to the formation of vapor or small bubbles within the membrane matrix. This phenomenon is expected to markedly reduce the effectiveness of osmotic pressure as a driving force for water transport. Delineation of the breakdown and possible restoration of water continuity under negative pressure is essential for proper understanding of the principles governing water transport in osmotic membranes.

Electrodialysis (ED) is an electro-driven desalination technology that relies on the selective transport of ions through ion exchange membranes. Though several approaches have been developed to model and evaluate the performance of ED, mechanistic ion-transport models, which rigorously solve the fundamental Nernst-Planck equation, remain some of the most reliable and utilized. However, complexity of the involved transport phenomena prevents analytical solutions of such models, and numerical solutions can be prohibitively intensive. Here, we use an equivalent circuit analogue to derive a simple correlation equation that predicts the energetic performance of ED for brackish water desalination. Specifically, our correlation equation predicts the specific energy consumption of ED for a given productivity, set of desalination parameters (i.e., feed salinity, salt removal, water recovery), and system properties. The correlation equation demonstrates robustness in predicting the specific energy consumption across a wide range of operational parameters, showing excellent agreement with a Nernst-Planck ion-transport model and literature-reported experimental data. Furthermore, we use the developed correlation equation to show the dependence of the specific energy consumption on the productivity, highlighting the tradeoff between the thermodynamic energy efficiency and desalination rate of the ED process. Overall, our developed correlation equation provides a convenient alternative to computationally intensive mechanistic models for performance analysis of the ED process.

Optimizing system design and operation of reverse osmosis (RO) systems requires an in-depth comprehension of the intrinsic tradeoff between RO mass transfer kinetics and energetics. In this study, we demonstrate that this critical trade-off can be quantified using the relationship between the average water flux and the specific energy consumption (SEC). We derive analytical expressions to quantify the average water flux and SEC for single stage, two stage, and closed circuit RO processes. These analytical expressions are useful for system design and operation optimization as they facilitate direct comparison of the kinetic and energetic efficiencies between RO processes with different operation conditions and system configurations. Finally, we compare the kinetics and energetics of the three system configurations using these analytical expressions and discuss their relative advantages and disadvantages in RO desalination.

Pressure retarded osmosis (PRO) is an osmotically driven membrane process that utilizes the energy of mixing between streams of high and low chemical potential to generate electrical energy. High power density of a PRO membrane is essential to maximize process efficiency and minimize the capital and operating costs. Thus, robust PRO membranes that can support high pressure and have high water flux, low reverse salt flux, low structural parameter, and a good membrane support structure are needed. In this study, four commercial forward osmosis (FO) membranes for use in PRO were compared. The effect of operating pressures, membrane spacers (type, orientation, and arrangement), and flow velocities on process performance were investigated. A thin film composite polyamide membrane from Hydration Technology Innovations was found to be the most robust and selective membrane. Compared to other spacer configurations, the use of unique feed channel spacer orientations was found to increase PRO power density by up to 46%, yielding a power density of 22.6 W/m2 (3 M NaCl draw solution and 4.1 MPa hydraulic pressure). However, membrane deformation was observed when operating pressures exceeded 3.5 MPa. The use of unique spacers coupled with decreased draw solution cross-flow velocities was found to increase PRO process efficiency.

Biofouling may lead to severe operational challenges that can significantly impair membrane desalination processes. In recent years, copper-based nanoparticles (Cu-NPs) have gained increased attention as a potentially viable anti-biofouling agent in membrane processes, due to their strong antibacterial activity and relatively low cost. This study presents a novel and facile method to attach biocidal Cu-NPs on the surface of a thin-film composite reverse osmosis membrane. Herein, we suggest a method for membrane surface functionalization with Cu-NPs that is performed without disassembling the membrane module, which highlights its practicality and potential application for reverse osmosis desalination plants. The loading of Cu-NPs on the membrane was confirmed both by scanning electron microscope imaging and X-ray photoelectron spectroscopy analysis, indicating that the deposited nanoparticles were composed of either metallic copper or copper-oxide. The impact of the in situ Cu-NP modification on membrane transport properties was found to be minor, with only a slight increase of the water and salt permeability. Furthermore, except for a slight increase in hydrophobicity, the modified membrane exhibited surface properties comparable to those of the pristine membrane. Finally, the in situ formed Cu-NPs imparted a strong antibacterial activity to the membrane surface, leading to 90% reduction in the number of attached live Escherichia coli bacteria on the modified membrane compared to the pristine reverse osmosis membrane. This study demonstrates that in situ grafting of Cu-NPs on reverse osmosis membranes is a potential alternative to reduce biofouling.

A biocidal coating was developed in order to reduce biofouling on a reverse osmosis (RO) membrane using a quaternary ammonium (QA) functionalized polymer. The synthesis of a series of polysulfone (PS) ionomers with QA groups is described, and a method for spraying these QA ionomers as an alcoholic solution, which dried into water insoluble coatings. Contact angle and streaming potential were used to analyze the coating's hydrophilicity and surface charge. Both PS-QA1 and the commercial RO membrane had an apparent contact angle of 68° that increased to 126° for PS-QA12 corresponding to alkyl chain length. A negatively charged particle-probe was used to measure coated and uncoated RO membrane interaction forces. Measured interaction forces correlated strongly with the length of alkyl chains or hydrophobicity of the coated surfaces. Uncoated RO membranes and ones coated with PS-QA were exposed to suspensions of Escherichia coli cells. All four PS-QA coatings showed significant biotoxicity and killed 100% of the E. coli cells, but uncoated RO membranes had metabolically active biofilms. However, coatings tested in a RO crossflow system showed a flux reduction that is attributed to mass transfer resistance due to excessively thick films.

Thermal desalination technologies are required for minimal and zero liquid discharge (MLD/ZLD). However, conventional and emerging thermal desalination technologies, such as mechanical vapor compression (MVC) and membrane distillation (MD), are usually highly expensive to implement or/and energy intensive to operate. In this study, we develop a novel desalination technology by using a vapor-compression pump to assist membrane distillation (MD). Comparing the energy efficiencies of the novel heat-pump assisted MD (HPMD), MVC, and conventional MD under similar operating conditions, demonstrates that HPMD is an energy-efficient thermal desalination technology. Furthermore, through process modeling, we provide guidelines for HPMD system design and show that the HPMD can theoretically obtain low energy consumption (~10 kWh of electrical energy per cubic meter of produced fresh water or gain output ratio, GOR, of ~60) and high water vapor flux (i.e., >60 L m−2 h−1). We conclude by highlighting promising applications of HPMD for MLD/ZLD, enabled by its high energy efficiency, low capital cost, and modularity.

Reverse osmosis (RO), currently the most energy efficient desalination process, is inherently more energy intensive compared to conventional fresh water treatment technologies, as it is constrained by the thermodynamics of separation of saline solutions. Therefore, pushing the energy consumption towards the thermodynamic limit of separation would lead to significant long-term savings in energy cost. In this work, we quantitatively demonstrate the potential of energy reduction for RO desalination using staged operations with both multi-stage direct pass and closed-circuit configurations. We relate the minimum specific energy of desalination (i.e., the minimum energy required to generate a unit volume of permeate water) to the number of stages in each configuration, and elucidate the fundamental difference between the two configurations. Our analysis suggests that although it is theoretically impossible to reach the thermodynamic minimum energy of separation with closed-circuit RO, this configuration is robust and much more practical to implement than the multi-stage direct pass RO.

Forward osmosis (FO) has been extensively investigated in the past decade. Despite significant advancements in our understanding of the FO process, questions and challenges remain regarding the energy efficiency and current state of the technology. Here, we critically review several key aspects of the FO process, focusing on energy efficiency, membrane properties, draw solutes, fouling reversibility, and effective applications of this emerging technology. We analyze the energy efficiency of the process, disprove the common misguided notion that FO is a low energy process, and highlight the potential use of low-cost energy sources. We address the key necessary membrane properties for FO, stressing the importance of the structural parameter, reverse solute flux selectivity, and the constraints imposed by the permeability–selectivity tradeoff. We then dispel the notion that draw solution regeneration can use negligible energy, highlighting the beneficial qualities of small inorganic and thermolytic salts as draw solutes. We further discuss the fouling propensity of FO, emphasizing the fouling reversibility of FO compared to reverse osmosis (RO) and the prospects of FO in treating high fouling potential feed waters. Lastly, we discuss applications where FO outperforms other desalination technologies and emphasize that the FO process is not intended to replace RO, but rather is to be used to process feed waters that cannot be treated by RO.

Confocal laser scanning microscopy (CLSM) is often used to evaluate biofilm development or biofouling mitigation in membrane systems. However, several methods of CLSM sample preparation exist. In this paper, we evaluate the effects of three preparation techniques — dry, confined (wet), and unconfined (immersed) mounting — on CLSM-derived biofilm architecture and dimensions. Although placing a wet or dry biofilm between a slide and a coverslip before viewing is relatively common, our results show that this confinement significantly alters the biofilm observed. Therefore, biofilms should be viewed in an unconfined and hydrated state that allows for full extension of the biofilm structure in a media-filled viewing well of fixed depth (~ 250 μm). Pseudomonas aeruginosa biofilms were grown on thin-film composite reverse osmosis membranes and glass coupons. Dry and confined mounting of 24 and 48 h biofilms resulted in biofilms with low 3-D complexity and thickness (14 and 18 μm, respectively). Measured biofilm thickness was significantly higher on samples prepared using unconfined mounting (55 μm). Additionally, the reduction in biofilm thickness and biovolume observed after treatment with biocidal compounds was significantly less on the dry and confined biofilms than the unconfined samples. Our results strongly suggest that biofilms on membranes be prepared for microscopy using unconfined mounting to accurately assess biofilm structure and dimensions. Unconfined mounting will allow for accurate CLSM assessment of membrane biofilm structure, dimensions, and biofouling mitigation measures in membrane systems.

Fouling behaviour and its impact on the rejection of trace organic contaminants (TrOCs) by forward osmosis (FO) were investigated. Membrane fouling was simulated using humic acid and colloidal particles as model foulants at different initial permeate water fluxes. Water flux decline was insignificant at an initial permeate flux of 9 L/m 2 h and the fouling layer was loose and fluid-like. By contrast, the water flux decline was substantial at an initial permeate flux of 20 L/m 2 h, resulting in the formation of a compact fouling layer. Water flux recovery after physical cleaning for both humic acid and colloidal particle fouled membranes was consistently higher at an initial permeate flux of 9 L/m 2 h compared to 20 L/m 2 h. The results suggest that the fouling layer structure varied from a fluid-like loose layer at low initial permeate flux to a more cohesive and compact layer at high initial permeate flux. We surmise that the fluid-like loose layer formed at low initial permeate flux contributed to pore blockage and thus enhanced steric hindrance, thereby leading to an increase in TrOC rejection. By contrast, the cohesive and compact fouling layer formed at high initial permeate flux exacerbated cake-enhanced concentration polarisation, resulting in a decrease in TrOC rejection.

In this study we provide a comprehensive evaluation of the economic performance and viability of solar membrane distillation (MD). To achieve this goal, process models based on mass and energy balances were used to find the minimum cost of water in MD systems. Three MD configurations: direct contact, sweeping gas, and vacuum MD, were compared in terms of economic cost and energy requirements in optimized, solar-driven desalination systems constrained to produce 10 m3 d−1 of distillate from 3.5% or 15% salinity water. Simulation results were used to calculate the water production cost as a function of 13 decision variables, including equipment size and operational variables. Non-linear optimization was performed using the particle swarm algorithm to minimize water production costs and identify optimal values for all decision variables. Results indicate that vacuum MD outperforms alternative MD configurations both economically and energetically, desalting water at a cost of less than $15 per cubic meter of product water (both initial salt levels). The highest fraction of total cost for all configurations at each salinity level was attributed to the solar thermal collectors—approximately 25% of the total present value cost. Storing energy in any form was economically unfavorable; the optimization scheme selected the smallest battery and hot water tank size allowed. Direct contact MD consumed significantly more energy (primarily thermal) than other MD forms, leading to higher system economic costs as well.

This study investigated the silica scaling and cleaning behavior in forward osmosis (FO) and how it compared with that in reverse osmosis (RO). The comparison between FO and RO modes shows that, under the hydrodynamic conditions tested, the flux decline rates under silica scaling are very similar in the two modes, but the flux recovery is close to 100% in the FO mode while it is only around 80% in the RO mode. Cellulose acetate (CA) and polyamide (PA) membranes were used to study the effects of membrane materials on silica scaling and cleaning. It is found that the flux decline rates for both membranes are similar, but the flux recovery of the CA membrane is 30–40% higher than that of the PA membrane. AFM force measurements indicate that membrane surface roughness increases the adhesion force between the PA membrane and a silica gel layer, significantly decreasing the cleaning efficiency of the PA membrane. Results from dynamic light scattering and energy-dispersive X-ray spectroscopy indicate that silica scaling is initiated as monosilicic acid deposits on the membrane surface, followed by polymerization/condensation that forms an amorphous silica gel layer at the interface between the membrane and silica particles.

TFC membranes with a cellulose triacetate (CTA) support layer were prepared using a modified interfacial polymerization (IP) method. In this method, a linking molecule covalently binds the polyamide (PA) active layer to the CTA support. The effects of three linking molecules (trimesoyl chloride (TMC), succinyl chloride and malonyl chloride) on membrane performance were investigated. The membrane prepared using TMC as the linking molecule displayed a strong decrease in salt rejection as a function of time in reverse osmosis (RO) and a strong increase in reverse salt flux as a function of draw solute concentration in forward osmosis (FO). The membranes made with bifunctional succinyl and malonyl chloride displayed a more stable performance. Confocal laser scanning microscopy (CLSM) images of the membranes indicate changes in CTA support morphologies upon exposure to high salt concentrations, especially for the TMC membrane. The different behavior of the trifunctional TMC membrane could be attributed to the higher number of charged groups on the support membrane, which causes anisotropic swelling/deswelling in the polymer. Our results suggest that bifunctional linkers can be used to ^fabricate membranes with performance characteristics less dependent upon salt concentration. The implications of the results for FO and pressure retarded osmosis are discussed.

Re-engineering the support layers of membranes for forward and pressure retarded osmosis is critical for making these technologies commercially viable. Real-world applications of forward and pressure retarded osmosis, especially those involving natural and waste waters, will require membranes to withstand significant stresses. Therefore, structural changes to the support layer, which are necessary in minimizing internal concentration polarization, must not compromise its critical abilities to resist mechanical stress and provide a suitable surface for the interfacial polymerization of a robust and selective active layer. Electrospinning can provide nanofibers for support layers to potentially overcome the limitations of traditional membrane fabrication techniques in fulfilling these challenging design criteria. In this work, we present the fabrication and evaluation of thin-film composite membranes composed of electrospun polyethylene terephthalate nanofibers, a phase separation formed microporous polysulfone layer, and a polyamide selective layer formed by interfacial polymerization. These membranes have active and support layer transport properties that are suitable for engineered osmosis, with water permeability of 1.13 L m − 2 h − 1 bar − 1 (3.14 × 10 − 7 m s − 1 bar − 1 ), salt permeability of 0.23 L m − 2 h − 1 (6.4 × 10 − 8 m s − 1 ), and a structural parameter of 651 μm. Relevant and easily reproducible tests for membrane resistance to mechanical stress were performed. The use of electrospun fibers in the support layer enhanced membrane resistance to delamination at high cross-flow velocities because the 340 nm diameter electrospun fibers enmesh with the microporous polysulfone layer. A broader discussion of the most promising approaches for using electrospun materials to improve membranes for engineered osmosis is provided.

The energy requirements of ammonia–carbon dioxide forward osmosis (FO) desalination are predicted by the use of chemical process modeling software (HYSYS). The FO process is modeled using single or multiple distillation columns to separate draw solution solutes from the product water for solute recycling within the FO system. Thermal and electrical energy requirements of the process are calculated, as well as a combined term for equivalent electrical work. The results of the simulations are compared to the energy requirements of current desalination technologies. Energy savings of FO compared to current technologies, on an equivalent work basis, are projected to range from 72% to 85%. Forward osmosis desalination is in an early stage of its development, and several areas of future work promise opportunities to improve its energy utilization and cost.

The mechanisms governing internal concentration polarization (ICP) were studied using well-controlled forward osmosis experiments. The relationship between osmotic pressure and water flux was observed across a range of solute concentrations and molecular weights. The effect of membrane orientation on ICP was also studied. Two regimes of ICP — dilutive and concentrative — were described and characterized, and their governing equations were tested. Resistances to solute diffusion within the membrane porous support layer were calculated under each regime and found to be consistent across a wide variety of experimental parameters.

Effluent organic matter (EfOM) is suspected as a major cause of fouling of reverse osmosis (RO) membranes in advanced wastewater reclamation. Among the main constituents in EfOM, polysaccharides are the most ubiquitous. The influence of solution chemistry and hydrodynamics on RO membrane fouling with alginate — a model for polysaccharides in secondary wastewater effluent — was systematically investigated. Results of fouling runs with alginate demonstrate that RO membrane fouling increases with decreasing pH, increasing ionic strength, and addition of calcium ions. At fixed solution ionic strength and pH, the presence of divalent calcium ions, at concentrations typical of those found in secondary wastewater effluent, had a dramatic effect on membrane fouling. However, for similar concentrations of divalent magnesium ions, fouling was negligible. The severe fouling in the presence of calcium is attributed to the formation of a thick, dense alginate gel layer on the membrane surface via calcium-alginate complexation and crosslinking (bridging) of alginate macromolecules by calcium. In addition to solution chemistry, hydrodynamic operating conditions — initial permeate flux and crossflow velocity — were also shown to influence RO membrane fouling with alginate.

A novel forward (direct) osmosis (FO) desalination process is presented. The process uses an ammonium bicarbonate draw solution to extract water from a saline feed water across a semi-permeable polymeric membrane. Very large osmotic pressures generated by the highly soluble ammonium bicarbonate draw solution yield high water fluxes and can result in very high feed water recoveries. Upon moderate heating, ammonium bicarbonate decomposes into ammonia and carbon dioxide gases that can be separated and recycled as draw solutes, leaving the fresh product water. Experiments with a laboratory-scale FO unit utilizing a flat sheet cellulose tri-acetate membrane demonstrated high product water flux and relatively high salt rejection. The results further revealed that reverse osmosis (RO) membranes are not suitable for the FO process because of relatively low product water fluxes attributed to severe internal concentration polarization in the porous support and fabric layers of the RO membrane.

The influence of colloidal fouling and feed water recovery (or concentration factor, CF) on salt rejection of thin-film composite reverse osmosis (RO) and nanofiltration (NF) membranes was investigated. Fouling experiments were carried out using a laboratory-scale crossflow test unit with continuous permeate disposal to simulate the CF and recovery as commonly observed in full-scale RO/NF systems. For feed waters containing only salt (NaCl), permeate flux declined linearly as CF was increased and salt rejection was nearly constant for both RO and NF membranes. On the other hand, a sharp decrease in permeate flux and significant decline in salt rejection with increasing CF were observed under conditions where colloidal fouling takes place. For both RO and NF membranes, the marked permeate flux decline was attributed to the so-called “cake-enhanced osmotic pressure”. The decline in salt rejection when colloidal fouling predominated was much more substantial for NF than for RO membranes. In all cases, the decline in salt rejection was higher under conditions of more severe colloidal fouling, namely at higher ionic strength and initial permeate flux.

The production rate of permeate in a reverse osmosis (RO) process controlled by mass transfer is proportional to the net driving pressure and the total membrane surface area. This linear relationship may not be the only mechanism controlling the performance of a full-scale membrane process (typically a pressure vessel holding six 1-m-long modules in series) which utilizes highly permeable membranes. The mechanisms that control the performance of an RO process under various conditions were carefully examined in this study. It was demonstrated that thermodynamic equilibrium can impose a strong restriction on the performance of a full-scale RO process under certain circumstances. This thermodynamic restriction arises from the significant increase in osmotic pressure downstream of an RO membrane channel due to the accumulation of rejected salt within the RO channel as a result of permeate water production. Concentration polarization is shown to have a weaker influence on the full-scale RO process performance than the thermodynamic restriction. The behavior of the process under thermodynamic restriction is quite different from the corresponding behavior that is controlled by mass transfer. The transition pressure for an RO process to shift from a mass transfer controlled regime to a thermodynamically restricted regime was determined by the basic parameters of the full-scale RO process.

Reverse osmosis membranes processing natural and waste waters are often exposed to low concentrations of chlorine in feed water. This biocide is chemically aggressive toward most commercial high performance membrane polymers. Chemical attack by chlorine ultimately results in membrane failure as measured by enhanced passage of both salt and water. Membrane failure is due to certain structural changes within the polymer in response to chlorine exposure. These changes in polyamide type membranes result from chlorine attack on amide nitrogen and aromatic rings. The resulting substitution products may cause deformation in the polymer chain or cleavage at amide linkages. The exact chemical mechanism of chlorine-polymer interaction and subsequent membrane failure is not, as yet, clearly understood. A review of published work on membrane-chlorine interaction will be presented here. Experimental evidence supporting various models for membrane failure will also be documented. In addition, certain common structural features known to enhance chlorine resistance of polymeric membranes are identified. It is anticipated that this paper will stimulate research efforts toward development of polymeric reverse osmosis membranes with high levels of chlorine resistance.

The use of a novel streaming potential analyzer to measure the zeta potential of cellulose acetate and composite polyamide reverse osmosis membranes is reported. Zeta potentials of these membranes were measured at various solution chemistries. These include effects of salt (NaCl) concentration, solution pH, and the presence of dissolved humic substances. It is demonstrated that streaming potential is a useful tool to measure zeta potential of reverse osmosis membrane surfaces. Results indicate that solution chemistry has a marked effect on the electrokinetic properties of reverse osmosis membranes. Humic substances strongly adsorb onto the surface of reverse osmosis membranes and thus alter the surface charge of the membranes. Furthermore, the zeta potential of reverse osmosis membranes becomes more negative as the NaCl concentration in solution increases, in a marked contrast to conventional electric double layer theories. It appears that the zeta potential of reverse osmosis membranes is strongly influenced by the presence of unreacted chemical substances or impurities on the membrane surface. Various explanations for the behavior of the membranes at the above solution chemistries are evaluated and discussed.