Rejection Of Cations Research Articles

Seawater Desalination Using Modified Ceramic Membranes

Nanofiltration has many applications such as pretreatment and partial demineralization in seawater desalination processes. This paper studies the performance of commercial ceramic membranes, modified by depositing either platinum or silver, to reject salts of seawater. The membrane used is multilayer structured with a titania layer (5 kD MWCO) manufactured by TAMI Industries (Nyons, France) and anionic impregnation was used to depose platinum or silver onto the membrane. The effect of salts rejection of membranes was investigated using seawater from the Mexican Pacific coast. The tests were carried out at pressure of 6 bar and the experimental results showed that modified membranes could achieve a salt rejection close to 30% in total dissolved solids (TDS) whereas the rejection in nonmodified membranes was less than 5%. Anion and cation rejection was also determined, with higher values obtained for cation removal.

Selective separation of contaminants from paper mill effluent using nanofiltration

Biologically treated newsprint mill effluent containing 57mgL−1 DOC and 1430 TDS was used in a screening study of nine commercial NF membranes for use as pretreatment for reverse osmosis in an end of pipe water recycling application. A salt-organic-separation (SOS) efficiency factor was developed to help rank the performance of the membranes. The SOS measures the ratio of the sum of the percentage rejection of organics and divalent cations over the percentage rejection of monvalents. It can be used to discriminate between NF membranes that are not too permeable to divalent cations or organics in which case the NF permeate will have a high chlorine demand due to the carryover of organics, or too retentive in which case all the material in the effluent will be retained and fouling problems are likely to occur. The optimum SOS efficiently for this study appeared to range from 3.5 to 5.6 for six membranes, DK, HPA-150, ESNA1-LF2, DL, TFC-SR2 and NF-270, which were categorised as membranes with an intermediate rejection. Out of these membranes ESNA1-LF2, TFC-SR2 and NF-270 were capable of operating up to 90% recovery with high permeabilities ranging from 17.7 to 22.3Lm−2h−1bar−1.Additionally, impact of membrane surface properties, such as molecular weight cut-off, surface charge and hydrophobicity, was assessed on the fouling and SOS efficiency. The molecular weight cut-off was found to have a correlation with the fouling potential of the membranes, while none of the surface properties had any correlation with the SOS efficiency.

Post-treatment of biologically treated wastewater of agrochemical industry by sulfated chitosan composite nanofiltration membrane

The objective of this study is to treat the biologically treated wastewater using sulfated chitosan/ polyacrylonitrile (PAN) composite nanofiltration (NF) membrane to improve agrochemical industry wastewater quality for reuse. Although biological treatment is quite efficient, the wastewater does not meet the reuse criteria. Hence, further treatment to improve the water quality is investigated. Sulfated chitosan composite NF membranes, having a PAN ultrafiltration membrane as the substrate, are prepared by coating and cross-linking methods. The effects of membrane preparation conditions on the rejection and permeation performance of the membranes are studied. The new membranes are characterized by NMR and scanning electron micrograph. Wastewater from agrochemical industry contains high concentrations of organic matter, color, heavy metals and other toxic substances. The operating variables studied are applied pressure (3-15 atm) and feed flowrate (4-16 L/min). It is found that the observed rejection (R(o)) increases with increase in feed pressure at constant feed flowrate. The rejection of cations follows the sequence: R(o)(Zn2+) > R(o)(Ni2+) > R(o)(CU2+) > R(o)(Cd2+) for wastewater. It is observed that the order of solute rejection sequence is inversely proportional to the diffusion coefficients.

Polypyrrole-based bilayer nitrate amperometric biosensor with an integrated permselective poly- ortho-phenylenediamine layer for exclusion of inorganic interferences

A bilayer amperometric nitrate biosensor with an integrated permselective layer has been developed for exclusion of inorganic anion and cation interferences. The inner PPy(polypyrrole)–NaR–NADH layer of the biosensor is formed by galvanostatic polymerization of pyrrole (Py) in presence of nitrate reductase (NaR) and nicotinamide adenine dinucleotide (NADH), followed by formation of the outer permselective poly- ortho-phenylenediamine (P- o-PDA) layer by potentiodynamic polymerization of ortho-phenylenediamine ( o-PDA). The exclusion efficiency ( E eff) of the outer layer in rejecting inorganic cation and anion interferences is evaluated by a new proposed relationship. 73–87% and 47–84% of anion and cation interferences, respectively, were efficiently rejected with the permselective layer. Further improvement in the exclusion efficiency for cations was accomplished by combining the use of the outer layer with the addition of 1 mM EDTA into the measurement solution. The addition of EDTA improved the E eff achieved for cation rejection by 10–40% to give net E eff of 89–94%. The inclusion of the outer layer also aided the retention of NaR and NADH in the inner PPy–NaR–NADH layer and, hence, enabled improved amperometric detection of nitrate, achieving a detection limit of 0.20 μM and a linear concentration range of 10–500 μM with a 3.4% rsd ( n = 10).

Novel polyamide-imide/cellulose acetate dual-layer hollow fiber membranes for nanofiltration

A novel dual-layer nanofiltration (NF) hollow fiber membrane was fabricated by the simultaneous co-extrusion of Torlon ® polyamide-imide and cellulose acetate dopes through a triple-orifice spinneret in a dry-jet wet phase inversion process. For the first time, the nanopores of dual-layer hollow fiber membranes were molecularly designed by controlling the phase inversion process with the aid of various non-solvent additives into the polymer solutions. Compared to ethanol and 2-propanol, the addition of methanol into the dope led to a significantly decreased pore size but dramatically increased pure water permeability. The improved NF performance may be attributed to (1) a controllable thin selective outer layer; (2) a less resistant interface between the outer and inner layers; and (3) a fully porous substructure with reduced transport resistance. In addition to non-solvent additives, spinneret temperature also plays an important role in designing dual-layer hollow fiber membranes with desirable NF performance. When the spinneret temperature was increased from 25 °C to 50 °C, the mean effective pore radius and the pure water permeability were simultaneously decreased, which was due to the formation of a denser surface skin and a more compact interface between the two layers. In addition to exhibiting a higher rejection of divalent anions than monovalent anions, and a lower rejection of divalent cations at pH 7.0, the newly developed NF dual-layer hollow fiber membranes with methanol as additive has a relatively high pure water permeability of 11.93 l m −2 bar −1 h −1 with a mean effective pore radius of 0.63 nm. These concepts hold great potential for the design of tailor-made NF membranes for various industrial applications.

Effect of quetiapine on the subjective estimates of sleep in the 8 weeks treatment of bipolar I and II Depression

Nanofiltration (NF) is a promising advanced treatment technique for oilfield wastewaters. In this study, we performed a crossflow NF on model boiler feed water (BFW) obtained from a thermal in-situ bitumen recovery process called steam assisted gravity drainage (SAGD) with the intent to remove dissolved organic matter and silica. Nanofiltration with a tight NF membrane was selected as the treatment technique since it provided reasonable product quality in a single-stage process and was found to be more energy efficient than reverse osmosis (RO). Two different feed pH values, 8.5 and 10.5, were investigated. Total organic carbon (TOC) and Si rejection of approximately 98% and divalent cation rejection greater than 99% was achieved at both pH values. Higher total dissolved solids (TDS) rejection, but lower flux rates were seen at pH 8.5 compared to pH 10.5. Surface characterization of the fouled membranes indicated the presence of organics (primarily carbon and oxygen) and inorganics (mainly silicon and iron) in the fouling deposits. Larger amounts of deposit were seen at pH 8.5 compared to pH 10.5. Increasing feed pH from 8.5 to 10.5 recovered the water flux more than 20% which demonstrated the critical role of pH as a pulsation technique to reduce membrane fouling. Fluorescence excitation emission matrix spectroscopy (FEEMs) on the feed and permeate showed that the organic matter that passed through the membrane was mainly hydrophilic compounds. Overall, this research indicated that NF is a viable treatment process for silica and TOC removal for SAGD BFW.

Treatment of landfill leachates by nanofiltration

Landfill leachate contains high concentrations of organic matter, color, heavy metals and toxic substances. This study presents the feasibility of a commercial nanofiltration membrane (NF-300) in the removal of pollutants from a landfill leachate generated from the Treatment Stabilization and Disposal Facility in Gujarat state of India. Two different leachate samples (Leachates A and B) were collected from the downstream side of closed landfill cells A and B. The average quality of the leachate was 67 719 mg/L COD, 217 mg/L ammonical nitrogen, 22 418 mg/L BOD, 3847 mg/L chlorides and 909 mg/L sulphate. The operating variables studied were applied pressure (4–20 atm), feed flowrate (5–15 L/min) and pH (2, 4, 5.5 and 6.7). It was observed that the solute rejection ( R O) increased with increase in feed pressure and decreased with increase in feed concentration at constant feed flowrate. In the present study, the rejection of cations followed the sequence: R O (Cr 3+) > R O (Ni 2+) > R O (Zn 2+) > R O (Cu 2+) > R O (Cd 2+) for leachates A and B. The order of solute rejection sequence is inversely proportional to the diffusion coefficients. The rejection of sulphate ions by the NF-300 membrane was 83 and 85%, while the rejection of chlorides was 62 and 65% for leachates A and B, respectively. The NF-300 membrane was characterized by using the combined-film theory-Spiegler–Kedem (CFSK) model based on irreversible thermodynamics and the ion transport model based on the extended Nernst–Planck equation. The membrane transport parameters were estimated using the Levenberg–Marquadt method. The estimated parameters were used to predict the membrane performance and the predicted values are in good agreement with the experimental results.

Nanofiltration and low energy reverse osmosis in rejection of radioactive isotopes and heavy metal cations from drinking water sources

While nanofiltration (NF) and low energy reverse osmosis (LERO) have only moderate rejection for monovalent salts, they have been shown to be highly effective in water dehardening and in removal of polyvalent contaminant ion species. The present work is a part of a long-term investigation of the treatment of contaminated ground waters and certain grades of industrial waste waters for rejection of radioactive isotopes and heavy metal cations (HMC) and for determination of efficient and environmentally safe waste disposal methods. In fact several technical challenges remain with regards to the efficiency and cost of conventional methods for removal of certain contaminants, NF is thought to offer higher efficiency and lead to lower cost. Separation of radium (Ra 2+ 226 and Ra 2+ 228), uranium 238 (UO 2+ 2 or its carbonate complex anions at pH 7.5 to 8,UO 2 (CO 3 ) 2- 2 /UO 2 (CO 3 ) 4- 3 , Cd 2+ , Cu 2+ , Hg 2+ and other cations from mixture salt solutions was investigated as a function of water composition and concentration by NF and LERO. Results were compared to those of separation by conventional methods of chemical precipitation, softening with the hot lime method (HLM), coagulation, and separation by chelating ion-exchange resins (IERs) determined under the same conditions. Membrane methods gave higher rejection of radionuclides and HMCs ranging from 92% to 99%. Contaminant concentration in permeate water was lowered than the maximum contaminant level (MCL) of the US Environmental Protection Agency at system recoveries which attained 90%. In case of separation by IERs a loss of process efficiency which attained 24% was observed in view of interference with separation of similar valency ions such as Ca 2+ and SO 2- 4 . The higher the efficiency of the resin to retain the radionuclide, the more its regeneration was difficult, resulting in a higher volume of contaminated spent solution. With NF and LERO, on the other hand, parallel rejection of polyvalent ions did not practically impact that of radionuclides or HMCs, and membranes do not require regeneration. Results of rejection of radionuclides and HMCs showed several significant advantages of membrane methods over that by IER, coagulation, chemical precipitation, and softening. These are the absence of chemical dosing specific to rejection, absence of sludge formation which is contaminated and requires disposal, mechanism does not include slow steps like settling, no need of subsequent filtration or of expensive installations, and the ability to realize parallel water desalination. The study will extend to the evaluation of the waste disposal alternatives. In view of the concentration of the contaminants in the waste stream, selection of environmentally safe disposal methods will be determined.

Experimental study of arsenic removal by direct contact membrane distillation

Arsenite (As(III)) and arsenate (As(V)) removal by direct contact membrane distillation (DCMD) were investigated with self-made polyvinylidene fluoride (PVDF) membranes in the present work. Permeability and ion rejection efficiency of the membrane were tested before the arsenic removal experiments. A maximum permeate flux 20.90 kg/m(2)h was obtained, and due to the hydrophobic property, the PVDF membrane had high rejection of inorganic anions and cations which was independent of the solution pH and the temperature. The experimental results indicated that DCMD process had higher removal efficiency of arsenic than pressure-driven membrane processes, especially for high-concentration arsenic and arsenite removal. The experimental results indicated that the permeate As(III) and As(V) were under the maximum contaminant limit (10 microg/L) until the feed As(III) and As(V) achieved 40 and 2000 mg/L, respectively. The 250 h simultaneous DCMD performance of 0.5mg/L As(III) and As(V) solution was carried out, respectively. The permeate arsenic was not detected during the process which showed the PVDF membrane had stable arsenic removal efficiency. Membrane morphology changed slightly after the experiments, however, the permeability and the ion rejection of the membrane did not change.

Nanofiltration of bivalent nickel cations — model parameter determination and process simulation

Nanofiltration is a pressure-driven membrane separation method that is also used for selective removal of salts from liquid streams. The separation by nanofiltration is based on both sieving and charge effects and operated typically at relatively low pressure. Although several mathematical models for nanofiltration are already known, the modeling of mass transfer in complex aqueous solutions remains still under discussion. In this work, the filtration of different single model solutions containing mono- and bivalent cations like Na + and Ni 2+ is studied with several membranes. First, the effect of solution pH on the rejection of the cations and on the permeate flux is investigated. Furthermore, the influence of pH on membrane characteristics due to change of the membrane charge is shown. The rejection of cations increases at pH values below the isoelectric point, in consequence of repelling interactions between cations and membrane which is positively charged at low pH. According to this finding, the optimal separation condition in terms of pH value for the purpose of recovery of Ni 2+ is ascertained for two membranes. From these experiments as well as from those with glycerol solution, three model parameters (pore radius, membrane charge and pore dielectric constant) are determined. These parameters are sufficient for simulating the process on the basis of a linearized transport model suggested by Bowen. The validity of this simplified approach is examined. The finally received model is applied for simulating a case study of a process integrated nickel recovery.

Mechanism of Ion Exclusion by Sub-2nm Carbon Nanotube Membranes

AbstractCarbon nanotubes offer an outstanding platform for studying molecular transport at nanoscale, and have become promising materials for nanofluidics and membrane technology due to their unique combination of physical, chemical, mechanical, and electronic properties. In particular, both simulations and experiments have proved that fluid flow through carbon nanotubes of nanometer size diameter is exceptionally fast compared to what continuum hydrodynamic theories would predict when applied on this length scale, and also, compared to conventional membranes with pores of similar size, such as zeolites. For a variety of applications such as separation technology, molecular sensing, drug delivery, and biomimetics, selectivity is required together with fast flow. In particular, for water desalination, coupling the enhancement of the water flux with selective ion transport could drastically reduce the cost of brackish and seawater desalting. In this work, we study the ion selectivity of membranes made of aligned double-walled carbon nanotubes with sub-2 nm diameter. Negatively charged groups are introduced at the opening of the carbon nanotubes by oxygen plasma treatment. Reverse osmosis experiments coupled with capillary electrophoresis analysis of permeate and feed show significant anion and cation rejection. Ion exclusion declines by increasing ionic strength (concentration) of the feed and by lowering solution pH; also, the highest rejection is observed for the salts (A=anion, C=cation, z= valence) with the greatest zA/zC ratio. Our results strongly support a Donnan-type rejection mechanism, dominated by electrostatic interactions between fixed membrane charges and mobile ions, while steric and hydrodynamic effects appear to be less important. Comparison with commercial nanofiltration membranes for water softening reveals that our carbon nanotube membranes provides far superior water fluxes for similar ion rejection capabilities.

Ion Rejection in Single and Binary Mixed Electrolyte Systems by Nanofiltration: Effect of Feed Concentration

The objectives of this work were to investigate and compare the separation behavior of ions in single and binary mixed electrolyte systems by NF70, with emphasis on the effect of feed concentration. Experimental results showed that the anion rejection is lower in the binary mixed system compared to the single salt system. Anion rejection decreases with the enhancement of feed concentration of co‐existing electrolyte. As for cation rejection, Na+ rejection in the mixed electrolyte system is generally greater than that in the single NaCl system; whereas the separation performance of Ca2+ at high feed concentrations shows the opposite trend, and cannot be explained by the Donnan theory. Several possible mechanisms have been evaluated. Finally, the Spiegler‐Kedem equation was used and parameters were calculated.

Ion Beam Irradiation Modifications of a Commercial Polyether Sulfone Water-Treatment Membrane

Environmental Context.The demand for more efficient and effective water-treatment processes is increasing. A popular method is the use of membrane filtration, to separate water from contaminants such as ions and microorganisms, however more selective membranes have a lower flux (permeate produced per unit area of membrane per unit time) and are more susceptible to fouling (an accumulation of surface-blocking materials). To aid flux and limit fouling, and thereby make membrane processes competitive with conventional technologies, a post-synthesis treatment is applied to alter the microstructure and surface of the water-treatment membrane. Abstract.A commercial polyether sulfone (PES) water-treatment membrane was modified by ion beam irradiation. Bench-scale cross-flow filtration experiments were conducted to investigate the transport properties and fouling potential of the modified membrane with respect to the four major constituents of raw water: monovalent cations, divalent cations, natural organic matter (NOM), and bacterial presence. Results indicated modification led to a reduction in the charge of the membrane, as observed by lower rejection of monovalent cations and increased cross-linking of divalent cations on the membrane’s surface, along with a hardening of membrane pores, as observed by increased organic matter removal. The most significant result was with respect to NOM fouling, which was shown to become more reversible.

Rejection efficiency of water quality parameters by reverse osmosis and nanofiltration membranes.

The objective of this study was to evaluate the effectiveness of reserve osmosis (RO) and nanofiltration (NF) membranes, under various solution chemistries, on water quality. The effects of organic carbon, divalent and monovalent cations, bacteria, and permeate drag on the rejection efficiencies of three different membranes were investigated through a series of laboratory bench-scale experiments. Quantitative models were successfully developed to predict the rejection of turbidity, divalent and monovalent cations, ultraviolet absorbance at 253.7 nm (UV254), and dissolved organic carbon (DOC) by membrane filtration. It was found that mechanical sieving (measured as molecular weight cutoff, MWCO) and electrostatic interactions were the most significant parameters since they were found to be important in nearly all models developed. For negatively charged membranes, under high ionic strength solution environments that repress electrostatic interaction between charged compounds and membranes, passage of compounds was mainly a function of size exclusion (i.e. MWCO). Further, of the feedwater parameters tested, bacteria concentration was observed to be the most significant influence on UV254, divalent cation and monovalent cation rejections. The developed models revealed that interactions between feedwater composition and membrane properties impacted the rejection efficiency of membranes as significantly as water composition and membrane properties individually.

Separation of inorganic solutes from oil-field produced water using a compacted bentonite membrane

We investigated the possibility of purifying oil-field produced water using a bentonite clay membrane. Experiments were performed in a bench-scale cell which incorporates a piston to compact the clay membrane. We diluted a produced water sample with an original total dissolved solids (TDS) of 196,250 mg l −1 obtained from a facility near Loco Hill, New Mexico, operated by an independent and tested the separation efficiency of the bentonite membrane for each of four dilutions. The results indicate that membrane efficiency for inorganic solutes decreases with increasing solute concentration and with increasing TDS. The rejection of SO 4 2− was greater than Cl −. This may be because the SO 4 2− concentration was much lower than the Cl − concentration in the waters tested. The cation rejection sequence varied with solute concentration and TDS. The solute rejection sequence for multi-component solutions is difficult to predict for synthetic membranes; it may not be simple for clay membranes either. The permeate flows in our experiments were 4.1–5.4% of the total flow. Both the flux through the membrane (approximately 2.5×10 −3 m 3 m −2 per day) and the inorganic solute rejections were low. This suggests that clay membranes, as tested, are not suitable for purification of high TDS waters.

Salt separation and hydrodynamic permeability of a porous membrane filled with pH-sensitive gel

The effect of pH on permeability and the pressure-driven ionic separation of a microporous membrane incorporating a pH-sensitive cross-linked poly(4-vinylpyridine) gel in the pores was examined. Membranes were prepared with cross-linked poly(4-vinylpyridine) constrained in the pores of a poly(ethylene) microporous host membrane. The degree of ionization (protonation) of poly(4-vinylpyridine) in the membranes as a function of pH was determined by potentiometric titration. The pressure-driven flux and salt separation of these membranes were monitored as a function of pH and, consequently, the degree of ionization of the pore-filling polyelectrolyte. The pure water flux was found to decrease reversibly by an order of magnitude when the pH of water was changed from 5.5 to 2.6 by the addition of HCl. Similar reversible flux changes were found with pH-adjusted municipal tap water used as a feed. On the other hand, the rejection of cations in tap water increased sharply with addition of HCl and increase of ionization of the incorporated poly(4-vinylpyridine). The results obtained in this study are explained by microphase transitions taking place in the gel network enmeshed with the porous structure of the support membrane.

Effect of the Univalent Cation Adsorption on the Nanofiltration Membrane Selectivity

The selective, filtration, and electrokinetic properties of an OPMN-KMZ nanofiltration membrane are studied with respect to the aqueous solutions of the chlorides of alkali metals and ammonium within a concentration range from 10–4 to 10–1 mol/l. The dependences of the charge of the membrane pore surface and the rejection of cations of centinormal solutions of the aforementioned salts and protons on pH are obtained. The membrane isoelectric points corresponding to the centinormal solution of each salt are determined. The minima of the membrane selectivity with respect to the corresponding cations of salts are revealed in the region of isoelectric points. It is shown that, for univalent cations in the regime of convective diffusion, the coefficient of electrolyte rejection by the membrane correlates with the adsorption potential of cations (counterions) at the pore surface of a selective layer.

Nanofiltration of seawater: fractionation of mono- and multi-valent cations

First the capabilities of six nanofiltration membranes (cut-offs of between 100 to 1000 daltons) to selectively demineralize salt water containing the same cations as seawater (monovalent: Na +, K +; divalent: Ca 2+, Mg 2+) were assessed and compared. The AFC-30 membrane displaying monovalent cation rejection rates of about 50% and divalent cations rejection rates higher than 90% was chosen for the second part of the study dealing with seawater. The second part shows that water with the desired overall cation content and desired monovalent over divalent cation ratio can be obtained from raw seawater, without using foreign water, through a series of processes: (1) diafiltration, using osmosed seawater as washing solution, with a diavolumes number of 3; (2) concentration, with a volume reduction factor of 1.5; and (3) light final dilution of the aforementioned osmosed water. Potential applications of the selectively demineralized water are in the field of health (preparation of nasal sprays, medical dietetics, etc.).

Cost factors and chemical pretreatment effects in the membrane filtration of waters containing natural organic matter

This paper compares the membrane processes available for water treatment. Membranes have the advantage of currently decreasing capital cost, a relatively small footprint compared to conventional treatment, generally a reduction in chemicals usage and comparably low maintenance requirements. Three membrane processes applicable to water treatment, micro- (MF), ultra- (UF), and nanofiltration (NF), are compared in terms of intrinsic rejection, variation of rejection due to membrane fouling and increase in rejection by ferric chloride pretreatment. Twelve different membranes are compared on the basis of their membrane pore size which was calculated from their molecular weight cut-off. A pore size of <6 nm is required to achieve substantial (>50%) organics removal. For a fouled membrane this pore size is about 11 nm. UV rejection is higher than DOC rejection. Coagulation pretreatment allows a higher rejection of organics by MF and UF and the cut-off criterion due to initial membrane pore size is no longer valid. A water quality parameter (WQP) is introduced which describes the product water quality achieved as a function of colloid, DOC and cation rejection. The relationship between log (pore size) and WQP is linear. Estimation of membrane costs as a function of WQP suggests that open UF is superior to MF (similar cost at higher WQP) and NF is superior to tight UF. Chemical pretreatment could compensate for the difference between MF and UF. However, when considering chemicals and energy costs, it appears that a process operated at a higher energy is cheaper at a guaranteed product quality (less dependent on organic type). This argument is further supported by environmental issues of chemicals usage, as energy may be provided from renewable sources.

Fouling effects on rejection in the membrane filtration of natural waters

Membrane processes in drinking water applications are micro- (MF), ultra- (UF) and nanofiltration (NF). These processes remove turbidity and bacteria (MF), viruses and macromolecules (UF) and small molecules and hardness (NF). Of particular concern in water treatment is the removal of natural organic matter (NOM) which contains potential disinfection by-product precursors. The presence of colloids, multivalent ions and organics in surface waters may cause substantial fouling of membranes. A study was carried out which looked at the rejection abilities of a range of membranes targeting hematite colloids (40–500 nm), NOM and cations, fouling conditions and cost of treatment of these processes with consideration of chemical pretreatment with ferric chloride [1]. In this paper the effect of membrane fouling on rejection is presented. The study was based on experiments with two MF membranes (GVWP, GVHP, 0.22 μm, Millipore), six UF membranes (1, 3, 5, 10, 30, 100 kDa, regenerated cellulose, Millipore), and four organic NF membranes (TFC-SR, TFC-S, TFC-ULP, CA-UF, Fluid Systems, US). Three different types of organics (IHSS humic acid, IHSS fulvic acid and an Australian concentrated NOM) in a carbonate buffer containing calcium chloride and a background electrolyte were used. Experiments were carried out in perspex (MF, UF) and stainless steel (NF) stirred cells of a volume of 110–185 mL and a membrane area of 15.2–21.2×10 −4 m 2 at transmembrane pressures of 1, 1–3, and 5 bar for MF, UF, and NF, respectively. UF removes 10–95% of NOM depending on the molecular weight cut-off (MWCO) of the membrane. Pore sizes of <6 nm are required to remove about 80% of NOM, where a 6 nm pore size corresponds to a MWCO of about 10 kDa. Colloids are fully rejected. NF removes NOM effectively (70–95% as dissolved organic carbon (DOC) and 85–98% as UV absorbance). Cation rejection is very membrane dependent and varies for the investigated membrane types between 13 and 96% for calcium and 10–87% for sodium. Fouling was also dependent on pore size and was caused by large colloids (250 nm) or coagulant flocs in MF, small colloids, organic-calcium flocs and aggregates with a dense structure (formed slowly) in UF, and by a calcium-organic precipitate in NF. The fouling influenced the rejection of colloids in MF and that of NOM in UF and NF. If a highly charged layer was deposited on the NF membranes, cation rejection was also influenced. The characterisation of permeate organics revealed that low molecular weight acids passed through the NF membranes and that the rejection of these acids was also dependent on the deposit on the membrane. The mechanisms which can explain such an increase in rejection are different for the three membrane processes. In MF, pore plugging and cake formation was found responsible for fouling. This reduces the pore size and increases rejection. In UF, internal pore adsorption of calcium-organic flocs reduces the internal pore diameter and subsequently increases rejection. In NF, the key factor appears to the charge of the deposit. This was investigated with the deposition of a ferric chloride precipitate. If the precipitate was of high positive charge, the rejection of cations increased and that of negatively charged low molecular weight acids decreased compared to more neutral or negative precipitates. In essence, the rejection characteristics of membranes depend more on the fouling state of the membranes and the nature of the foulants than on the initial membrane characteristics.