Fouling Of Reverse Osmosis Membranes Research Articles

Enhanced resistance to organic fouling in a surface-modified reverse osmosis desalination membrane

Membrane surface modification with the aim of lowering foulant to surface affinity, has recently gained considerable attention. In this article, we report improved performance (permeate flux, salt rejection, and resistance to alginate fouling) of surface-modified reverse osmosis (RO) membranes, under cross-flow filtration conditions. The surface of RO membranes was modified by amphiphilic hydroxethyl methacrylate-co-perfluorodecyl acrylate (HEMA-co-PFDA) copolymer films. The amphiphilic coatings were deposited via an all-dry and solventless vapor deposition technique, termed as initiated chemical vapor deposition. Scanning electron microscopy revealed that a dense and continuous layer of alginate formed on the surface of the unmodified membranes, whereas foulant deposition on the surface-modified membranes was found to be more sporadic and discontinuous. The coatings were found stable even after 6 h of exposure to sodium alginate at higher pressure (800 psi), as evidenced by ATR-FTIR analysis of the post-fouled membranes.

Assessment of hardness, microorganism and organic matter removal from seawater by electrocoagulation as a pretreatment of desalination by reverse osmosis

Electrocoagulation (EC) process was used as an alternative pretreatment in order to assess its applicability to replace the conventional pretreatments used to mitigate membrane fouling prior to seawater desalination by reverse osmosis process, such as chemical coagulation, chlorination and scale inhibitors. Electrocoagulation was conducted in a batch cell using aluminum electrodes driven in the galvanostatic mode. Absorbance (UV254nm) and DOC were measured to follow the effectiveness of EC process for removing organic matter from seawater. Likewise, cultivable heterotrophic bacteria were enumerated to assess the disinfection ability of the process and total hardness was monitored. The influence of various operating parameters, namely mixing speed, current density, initial pH and inter-electrode distance, was investigated. The removal of organic matter from seawater by electrocoagulation was improved with higher current density and lower pH. Electrocoagulation removed 70.8% DOC, while absorbance abatement was 89.7% with a complete removal of microorganisms at high current density. Conversely, the abatement of total hardness was weak, about 10%, so that electrocoagulation could not be used as a softening process. Finally, experimental results showed the high potential of electrocoagulation as a pretreatment method to mitigate potential organic fouling and biofouling of reverse osmosis membrane owing to its ability to remove effectively dissolved organic matter and microorganisms from seawater.

Influence of bulk concentration on the organisation of molecules at a membrane surface and flux decline during reverse osmosis of an anionic surfactant

Surfactants are extensively used in household and industrial products. Several processes exist to treat industrial wastewaters, including membrane filtration such as ultrafiltration, nanofiltration and reverse osmosis (RO). We studied fouling of RO membranes during filtration of aqueous anionic surfactant solutions under different conditions. The aim was to describe the local organisation of the surfactant at the membrane interface. To this end, the typical surfactant sodium dodecyl sulfate (SDS) and a polyamide membrane (SG, GE Water & Process Technologies) were selected. A marked surfactant mass loss was experimentally quantified and attributed to the accumulation of surfactants on the membrane surface and adsorption on the non-membrane materials in the filtration system. The concentration of surfactant in the polarisation layer compared with the SDS phase diagram, combined with contact angle measurements and flux decline analysis, enabled us to deduce a structure for the fouling. The fouling layer presented different structures according to the surfactant concentration: from a dense hydrophobic layer at very low concentration to a lamellar hexagonal phase in the gel layer at concentrations above 35wt% in water.

Biofouling and scaling control of reverse osmosis membrane using one-step cleaning-potential of acidified nitrite solution as an agent

Biofouling is generally regarded as a major issue in reverse osmosis (RO) membrane filtration. Two-step chemical cleanings with alkaline and acidic agents are typically applied to restore the treatment capacity. In this study, the feasibility of one-step cleaning using free nitrous acid (FNA) was investigated as a novel low cost cleaning agent. The FNA cleaning solution was prepared by acidification of a sodium nitrite solution with hydrochloric acid. Seven fouled RO membranes collected from full-scale wastewater recycling and desalination plants were used to perform lab-scale cleaning trials. Membrane fouling characterisation revealed six of out of seven membranes were mainly bio-fouled, while one membrane was severely fouled by calcium carbonate. This study showed the feasibility of using FNA at pH 3.0 for biomass removal as well as for calcium carbonate scaling removal. The results from the lab-scale cleaning tests suggested that FNA can be used as a single cleaning agent for both biofouling and scaling removal. Cost analysis showed that FNA is a cost-effective solution for biofouling and scaling removal in RO filtration applications.

A new flat sheet membrane bioreactor hybrid system for advanced treatment of effluent, reverse osmosis pretreatment and fouling mitigation

This paper introduces a new hybrid electro membrane bioreactor (HEMBR) for reverse osmosis (RO) pretreatment and advanced treatment of effluent by simultaneously integrating electrical coagulation (EC) with a membrane bioreactor (MBR) and its performance was compared with conventional MBR. Experimental results and their statistical analysis showed removal efficiency for suspended solids (SS) of almost 100% for both reactors. HEMBR removal of chemical oxygen demand (COD) improved by 4% and membrane fouling was alleviated according to transmembrane pressure (TMP). The average silt density index (SDI) of HEMBR permeate samples was slightly better indicating less RO membrane fouling. Moreover, based on the SVI comparison of two reactor biomass samples, HEMBR showed better settling characteristics which improved the dewaterability and filterability of the sludge. Analysis the change of membrane surfaces and the cake layer formed over them through field emission scanning electron microscopy (FESEM) and X-ray fluorescence spectrometer (XRF) were also discussed.

Diminished swelling of cross-linked aromatic oligoamide surfaces revealing a new fouling mechanism of reverse-osmosis membranes.

Swelling of the active layer of reverse osmosis (RO) membranes has an important effect on permeate water flux. The effects of organic- and biofouling on the swelling of the RO membrane active layer and the consequent changes of permeate flux are examined here. A cross-linked aromatic oligoamide film that mimics the surface chemistry of an RO polyamide membrane was synthesized stepwise on gold-coated surfaces. Foulant adsorption to the oligoamide film and its swelling were measured with a quartz crystal microbalance, and the effects of fouling on the membrane's performance were evaluated. The foulants were extracellular polymeric substances (EPS) extracted from fouled RO membranes and organic compounds of ultrafiltration permeate (UFP) from a membrane bioreactor used to treat municipal wastewater. The adsorbed foulants affected the swelling of the cross-linked oligoamide film differently. EPS had little effect on the swelling of the oligoamide film, whereas UFP significantly impaired swelling. Permeate flux declined more rapidly under UFP fouling than it did under EPS. Foulant adsorption was shown to diminish swelling of the aromatic oligoamide surfaces. Among the already known RO membrane fouling mechanisms, a novel RO fouling mechanism is proposed, in which foulant-membrane interactions hinder membrane swelling and thus increase hydraulic resistance.

Surface chemical heterogeneity of polyamide RO membranes: Measurements and implications

The chemical heterogeneity (charge distribution) of polyamide reverse osmosis (RO) membrane surfaces and its influence on membrane fouling were investigated using atomic force microscopy (AFM). Negatively charged carboxyl modified latex (CML) and positively charged aliphatic amine latex (AAL) particles were employed to make AFM colloidal probes. The adhesive forces were significantly higher with the AAL probe due to negatively charged membrane surfaces. The chemical heterogeneity measured by AFM colloidal probes indicated that RO membranes could have chemically heterogeneous surfaces even if they exhibited similar average surface properties including zeta potential, contact angle and surface roughness. By performing fouling experiments with two commercial RO membranes of different chemical heterogeneities, it was demonstrated that RO membrane with high chemical heterogeneity was more prone to fouling, suggesting that surface charge distribution is one of the key factors governing RO membrane fouling. Consequently, surface chemical heterogeneity provides a new tool for accessing membrane fouling mechanisms and a novel concept for developing fouling resistant RO membranes.

Real time fouling monitoring with Electrical Impedance Spectroscopy

Electrical Impedance Spectroscopy (EIS) was explored as an in situ tool for the detection of fouling of reverse osmosis membranes, during filtration of industrial (molasses) wastewater and model feeds containing silica and BSA foulants. The electrical capacitance measured at low frequencies (<100Hz) was found to be the most sensitive electrical parameter for detecting fouling, during the early stage of flux decline. The effective gain (the percentage of change in capacitance divided by the percentage change in the flux) was ~3 for molasses waste water, 1.4 for silica and 2.5 for BSA. The formation of the reversible fouling layer occurred in conjunction with concentration polarization during this period and EIS was more sensitive than flux decline in monitoring this process. Hence EIS has potential in industrial application for fouling detection. Further the impedance spectra after cleaning were consistent with the flux recovery.The impedance spectra obtained following macromolecular (BSA and molasses wastewater) fouling was found to be significantly different from those obtained with colloidal (silica) fouling. The decrease in capacitance observed for BSA and molasses wastewater, suggests the formation of a gel like layer on the membrane surface, while the increase in capacitance of the silica suggests a loose cake which enhances concentration polarization. Hence EIS may also be capable of indicating the type of fouling which is occurring at the membrane surface.

Improving the fouling resistance of brackish water membranes via surface modification with graphene oxide functionalized chitosan

Fouling of reverse osmosis (RO) membrane is a challenging and restricting aspect in membrane filtration performance, therefore new methods are required to enhance membrane antifouling properties. In this study, a novel surface modification technique for commercial brackish water (BW30LE) thin-film composite (TFC) membrane was applied by using graphene oxide (low GO1 and high GO2) functionalized chitosan (GO/f-Cs). All the membranes including the unmodified one were treated with 200ppm of sodium hypochlorite and were further characterized by contact angle, scanning electron microscope, atomic force microscopy, UV spectrophotometer and ATR-FTIR. The obtained GO/f-Cs/polyamide (PA) composite membranes showed a superior performance compared to the unmodified PA (TFC) membrane in terms of hydrophilicity, surface smoothness, NaCl rejection, water flux and antifouling property. The unmodified membrane, GO1/f-Cs/PA (M1) and GO2/f-Cs/PA (M2), exhibited permeation fluxes of 56.1, 48.5 and 61.5L/m2h and salt rejections of 88.7%, 95.4% and 95.6%, respectively, using 1500mg/L NaCl concentration as a feed solution under 14bar. To examine the membrane's dynamic fouling resistance, cross-flow filtration tests were conducted using bovine serum albumin (BSA). M2 membrane had less flux decline than the unmodified one, and achieved 97% water flux recovery ratio by only water cleaning.

Pilot-scale study on the reverse osmosis treatment of oil sands tailings pond water: Impact of pretreatment on process performance

The oil sands industry has recently focused on both reducing their freshwater usage and maximizing the reuse of process water. This study is one of the few pilot-scale investigations reporting the ability of reverse osmosis (RO) to treat recycle water (RCW) from an oil sands facility. Two distinct treatment trains were assessed to evaluate the impacts of pretreatments on the RO performance. Treatment train 1 consisted of coagulant addition, ceramic ultrafiltration (CUF), antiscalant, and a single-pass RO system operated at natural pH, while the treatment train 2 included softening, coagulant addition, CUF system, weak acid cation ion exchange, antiscalant addition, and an RO system operated at alkaline pH. RO permeate fluxes normalized to 25°C of approximately 31–39L/m2·h at 72% recovery and 38–52L/m2·h at 85% recovery were recorded for treatment trains 1 and 2, respectively. At these conditions, the two treatment trains resulted in total dissolved solids lower than 18mg/L, while the dissolved sodium concentrations were below 7mg/L. During the pilot tests, clean-in-place procedures were not required for both treatment configurations, highlighting the effectiveness of the pretreatment steps to reduce the RO membrane foulants.

Fouling and Cleaning Characteristics of Reverse Osmosis (RO) Membranes

This work deals with fouling and successive cleaning of RO membrane fouled by an organic foulant, sodium alginate using a laboratory-scale cross flow test unit. First, spiral-wound RO membrane was fouled with sodium alginate solution up to 10% and 15%, respectively at an applied pressure of 1380 kPa with flow rate of 10 lit/min. An anionic surfactant, sodium dodecyl sulfate (SDS) was used as a chemical cleaning agent for cleaning of RO membrane. The effect of cleaning chemical dose and cross-flow velocity on the membrane chemical cleaning duration to achieve 100% cleaning efficiency (i.e., to get original water flux) was also investigated. As the SDS concentration increases, the membrane chemical cleaning time decreases due to increase in the solubility of the foulant (when the surface tension decreases by an increase in the SDS concentration). Furthermore, the membrane chemical cleaning time decreases with increasing cross-flow velocity of the cleaning chemical solution (SDS). Higher cross-flow velocity enhances the turbulence at the fouling layer and hence the mass transfer of the foulant from the fouling layer to the bulk solution is improved, then the SDS has weakened the structural integrity of the fouling layer. It is observed that better cleaning is occurred with higher concentration of SDS and flow rate. The obtained results clearly reveal that SDS cleaning is proved to be an efficient cleaning method for RO membranes fouled with organic foulant.

Comparison of reverse osmosis membrane fouling characteristics in full-scale leachate treatment systems with chemical coagulation and microfiltration pre-treatments.

Fouling characteristics of reverse osmosis (RO) membrane with chemical coagulation and microfiltration (MF) pre-treatment were investigated at full-scale leachate treatment systems. In chemical coagulation pre-treatment, solid separation from stabilized leachate was performed by ferric chloride coagulation followed by sand filtration. Meanwhile, MF pre-treatment and the RO system utilized direct filtration using a 0.03 µm membrane without chemical addition. MF pre-treatment yielded better pollutant removals in terms of organics and nitrogen. The study on effect of pre-treatment on RO membrane fouling revealed that accumulated foulant on the RO membrane in MF pre-treatment was significantly lower than that of chemical coagulation. Nevertheless, NaOH cleaning of the fouled RO membrane after chemical coagulation pre-treatment could better recover its permeate flux, thus suggesting that the formation of a loose-structure cake layer by chemical coagulation pre-treatment could allow effective penetration of chemical cleaning and detachment of foulant layer from the membrane surface.

Advanced treatment of textile dyeing secondary effluent using magnetic anion exchange resin and its effect on organic fouling in subsequent RO membrane

Strict regulations are forcing dyeing factory to upgrade existing waste treatment system. In this study, advanced treatment of dyeing secondary effluent by magnetic anion exchange resin (NDMP) was investigated and compared with ultrafiltration (UF); NDMP as a pre-treatment of reverse osmosis (RO) was also studied. NDMP resin (20mL/L) gave higher removal of dissolved organic carbon (DOC) (83.9%) and colority (94.9%) than UF with a cut-off of 10kDa (only 48.6% and 44.1%, respectively), showing that NDMP treatment was effective to meet the stringent discharge limit of DOC and colority. Besides, NDMP resin (20mL/L) as a pretreatment of RO increased the permeate flux by 12.5% and reduced irreversible membrane fouling by 6.6%, but UF pretreatment did not mitigate RO membrane fouling. The results of excitation-emission matrix fluorescence spectra and resin fractions showed that NDMP had more efficient removal than UF for transphilic acid and hydrophilic fraction, such as protein-like organic matters and soluble microbial products, which contributed to a significant proportion of RO membrane fouling. In sum, NDMP resin treatment not only gave effective removal of DOC and colority of dyeing secondary effluent, but exhibited some improvement for RO membrane flux and irreversible fouling.

Suitability of cationic flocculants for RO membranes performance improvement

As it has already been published and demonstrated, one of the main causes of membranes failures is related to a poor plant pretreatment. Data from more than 600 autopsies demonstrated that both biofilm and colloidal matter are the most common components on reverse osmosis (RO) membrane fouling; therefore, main lack on membrane pretreatment is related to disinfection and small size of the particles. The main deficiency in membrane pretreatment is related to the dosing of chemicals such as biocides, and flocculants, which are the only tool for membrane pretreatment improvement. Although on previous papers, cationic flocculants effectiveness was already proved, there is still a reluctance to use these kinds of products on RO pretreatments. Genesys Membrane Products laboratories has broad experience on the optimization of cationic flocculants use which proves the effectiveness of these products for RO pretreatment with no risk for the membranes. This paper will demonstrate the compatibility of some cationic flocculant with membranes, as well as the compatibility of novel formulations including biocides in the flocculant formulation. On this paper, the authors will review data from laboratory tests, case studies, and compatibility tests carried out through different analytical techniques (SEM-EDX, ATR/FTIR, etc.) for the different flocculants developed by Genesys International Limited and will try to demonstrate the safe use of these products for the best performance of RO systems.

On the cleaning procedure of a hydrophilic reverse osmosis membrane fouled by secondary-treated olive mill wastewater

Abstract The membrane cleaning protocol has a major importance to permit the economic feasibility and reduce the environmental impacts of industrial scale membrane processes, especially in the treatment of highly polluted industrial effluents like olive mill wastewater (OMW), which typically implies deleterious fouling problems on the used membranes. In the present study, the most adequate cleaning protocol was examined for a hydrophilic reverse osmosis (RO) membrane fouled by organic and inorganic matter after the treatment of OMW previously subjected to a secondary treatment (OMWST). The HR-SEM microphotographs of the fouled RO membrane layer reveal the presence of organic fouling as well as inorganic deposits in the form of residual colloidal iron and scaling mainly due to calcium carbonate, chloride and sulfate. This is supported by the positive saturation index value (0.44–0.90) and the elemental microanalyses performed on the fouled membrane surface. The physical cleaning did not achieve a significant recovery of the initial permeability of the RO membrane. An integrated alkaline-detersive plus acid cleaning procedure consisting in two cleaning stages in series, that is, (i) acid cleaning with citric acid followed by (ii) alkaline-detersive cleaning with NaOH + SDS solution, provided the maximum cleaning efficiency upon 0.1% (w/v) dosage. Finally, by performing the cleaning procedure upon turbulent tangential velocity over the membrane (4.01 m s−1, equivalent to NReynolds = 2.1 · 104) at a cleaning operating temperature ranging from 30 to 35 °C during 20–25 min, complete restoration of the membrane permeability was successfully achieved.

Autonomous operation and maintenance of small-scale PVRO systems for remote communities

Solar-powered reverse osmosis desalination (Photovoltaic-powered reverse osmosis [PVRO]) is a technically feasible method of providing fresh water to many remote communities with saline water sources. To be practical, these systems must be well operated and maintained by non-experts. Their productivity is a complex function of their locations, water chemistry and demand, and the solar radiation history at their locations, which is quite variable with time. A key aspect of the maintenance program is the cleaning of the reverse osmosis (RO) membranes, including system flushing and chemical cleaning. Guidelines for cleaning from membrane manufacturers do not consider the complex, variable operating conditions for these small solar-powered systems. Local operators do not have the expertise to determine how and when cleaning should be done. While cleaning will generally improve clean water production, it is costly, requires the system to be shut down, and uses some of the clean water produced. Here, simple, physics-based models of RO membrane fouling and remediation are used to find maintenance schedules that maximize water produced by a small-scale PVRO system under deterministic conditions, such as found in large RO plants using conventional grid power. However, it is shown that for small PVRO systems working in remote locations, the large uncertainties have a significant impact on optimal cleaning schedules.

Using modelling approach to validate a bench scale forward osmosis pre-treatment process for desalination

Forward osmosis (FO) has recently attracted growing attention in wastewater, brackish groundwater and seawater desalination, and power generation. This study evaluates the potential of using a batch laboratory-scale FO system as a pre-treatment for the reverse osmosis (RO) process. FO is a low pressure-driven process that offers many advantages compared to the conventional pre-treatment for RO especially for brackish water with high potential of scaling and fouling. FO can help to reduce the RO process cost by avoiding RO membrane fouling and achieving higher water recovery. An experimental modelling has been employed to describe the FO process taking into account water flux, water recovery and final draw solution. Based on this experimental modelling, the energy consumption for RO has been estimated. It has been found that the treatment time for the FO process and the initial draw solute concentration are important parameters that have an interrelated effect on FO and RO efficiency. The optimal conditions for this FO pre-treatment process are determined by modelling and are experimentally validated by using real brackish water as feed in a bench-scale FO system.

Fouling of reverse osmosis membrane for municipal wastewater reclamation: Autopsy results from a full-scale plant

A fouled RO membrane from a full-scale municipal wastewater reclamation plant was autopsied to elucidate fouling characteristics and behavior associated with feed water quality. Organic pollution (occupied 75% of the deposit) was the major problem for the RO membrane. The deposit dissolved in NaOH solution was 2.37g-DOC/m2 and largely comprised of microbial-derived organic matter (OM) and humic-like OM. Hydrophobic acids (HOA) and hydrophilic neutrals (HIN) were the two largest fractions in the deposit among the six fractions of HOA, hydrophobic bases (HOB), hydrophobic neutrals (HON), hydrophilic acids (HIA), hydrophilic bases (HIB) and HIN. HOA fraction could deposit on the membrane easily and should be monitored as key fractions to predict the organic fouling of RO membrane. HIN fraction occupied 34.2% of the total DOC in the deposit, but it was suggested that HIN fraction in the deposit was produced by the microorganisms on the membrane instead of depositing. The inorganic scaling of the RO membrane was mainly caused by element Fe, Ca and Si. The content of Fe was the highest (349.18mg/m2), since Fe deposited on the RO membrane much more easily than other elements. Ca presented the lowest deposition ratio (0.0017%) due to the effective function of the antiscalant.

Fouling of nanofiltration membranes in full- and bench-scale systems treating groundwater containing silica

To better understand full-scale fouling of nanofiltration and reverse osmosis membranes, it is common to autopsy a membrane element from a specific location in the full-scale system of interest, or conduct fouling tests at bench and pilot scale. However, several factors that may impact fouling at full scale vary along full-scale membrane systems and are difficult to recreate at bench and pilot scale. Thus, we performed an in-depth comparison of fouling in membranes at the lead and tail ends of a full-scale nanofiltration system treating a groundwater containing silica, as well as in a membrane fouled at bench scale with concentrate water from the full-scale system. We characterized the elemental composition, chemistry and structure of the foulant layers, and water permeability recovery upon chemical cleaning. Results showed that while there were apparent physico-chemical differences between the foulant layers of membranes fouled at the lead and tail ends of the full-scale system, water permeability recovery upon cleaning was similar for the two membranes. Bench-scale fouling tests produced foulant layers that were different in composition from those generated at full scale and led to the incorrect conclusion in terms of which was the optimum cleaning solution at full-scale. The dissimilarities between bench and full scale results were likely caused by the various challenges in reproducing full-scale conditions at bench-scale, particularly the inability to maintain anoxic conditions and use a one-pass filtration scheme. Thus, to determine the optimum cleaning solutions for the full-scale system, it was preferable to conduct bench-scale cleaning tests on a membrane fouled at full scale than to attempt recreating fouling at bench scale; this finding may hold true for other anoxic waters. In this study, we also introduce Rutherford backscattering spectrometry (RBS) for elemental and structural characterization of foulant layers. Our results showed that RBS is able to depth profile the elemental composition of the first few microns from the surface of foulant layers, which is information that the more commonly used X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy analyses cannot provide.

The roles of Pseudomonas aeruginosa extracellular polysaccharides in biofouling of reverse osmosis membranes and nitric oxide induced dispersal

The roles of Pseudomonas aeruginosa polysaccharides (Pel, Psl, alginate) in reverse osmosis (RO) membrane biofouling and nitric oxide (NO) induced dispersal were investigated. While mutants deficient in Psl formed significantly lower biofilm (total cell and polysaccharide) biovolumes than the PAO1 wild-type on glass surfaces, total cell biovolumes were similar during fouling of RO membranes. However, biofilms of the Psl deficient mutants exhibited a striated pattern, leaving large areas of membrane unfouled and contained up to 70% less polysaccharide and 24% less protein than the wild-type. Membranes fouled by the psl mutants exhibited a 69% reduction in the rate of biofouling (pressure rise over a given period), while the pel and alginate mutants were similar to the wild-type, suggesting functional differences in the polysaccharides. Overproduction of alginate by a PDO300 mutant increased the biofouling rate (59%) relative to wild-type, highlighting the ability of this polysaccharide to promote biofilm adherence and increase hydraulic resistance to permeate flow in an RO system. These results emphasize the importance of attachment specific polysaccharides for bacteria when fouling industrial RO membranes. When exposed to NO, dispersal of the PDO300 mutant biofilm was 25% lower than the wild-type, whilst dispersal of the alginate deficient mutant was 11% greater. Alginate thus appears to play an important role in NO induced dispersal of PAO1 biofilms.