Fouling Of Reverse Osmosis Membranes Research Articles

Removal of silica from brackish water by electrocoagulation pretreatment to prevent fouling of reverse osmosis membranes

Desalination of seawater and brackish water by reverse osmosis (RO) has become increasingly important for drinking water supply in a greater part of the world. The presence of high silica concentrations in some brackish water, however, limits the application of RO desalination due to the potential formation of silica scales that irreversibly deteriorate the membrane material and performance. This study investigates the feasibility of electrocoagulation as a pretreatment process to remove silica from the source brackish water. The effects of several electrical parameters, including electrode arrangement, current intensity and hydraulic retention time, were studied on the basis of silica removal efficiency. Bipolar configuration attained greater extent of silica removal as compared to monopolar configuration. Increases in charge loading generally improved the silica removal efficiency, but excessive hydraulic retention time (60 min) was detrimental to the system performance. In this study, with no modification of the source water, silica removal efficiency up to 80% was achieved under a current intensity of 0.5 A and a hydraulic retention time of 30 min. The subsequent nanofiltration studies demonstrated severe flux declines over the first 3 h, yielding only 70% of its initial flux for brackish water containing 100 mg/L silica, and progressively lower with higher silica concentrations. For the pretreated water by electrocogulation, the extent of flux decline was markedly improved, suggesting that the pretreatment was effective for the attenuation of the flux decline. Electron micrograph images of the membrane autopsy also confirmed the lack of scale formation for the pretreated water as compared to those without pretreatment.

An autopsy study of a fouled reverse osmosis membrane element used in a brackish water treatment plant

The fouling of a spiral wound reverse osmosis (RO) membrane after nearly 1 year of service in a brackish water treatment plant was investigated using optical and electron microscopic methods, Fourier transform infrared spectroscopy (FTIR) and inductively coupled plasma atomic emission spectrometry (ICP-AES). Both the top surface and the cross-section of the fouled membrane were analysed to monitor the development of the fouling layer. It has been found that the extent of fouling was uneven across the membrane surface with regions underneath or in the vicinity of the strands of the feed spacer being more severely affected. Fouling appeared to have developed through different stages. In particular, it consisted of an initial thin fouling layer of an amorphous matrix with embedded particulate matter. The amorphous matrix comprised organic–Al–P complexes and the particulate matter was mostly aluminium silicates. Subsequently, as the fouling layer reached a thickness of about 5–7 μm, further amorphous material, which is suggested to include extracellular polymeric substances such as polysaccharides, started to deposit on top of the existing fouling layer. This secondary amorphous material did not seem to contain any particulate matter nor any inorganic elements within it, but acted as a substrate upon which aluminium silicate crystals grew exclusively in the absence of other foulants, including natural organic matter (NOM).

Colloidal fouling of RO membranes following MF/UF in the reclamation of municipal wastewater

Major municipal wastewater reclamation plants in California, USA, Singapore, and many built or planned for other regions of the world use the high pathogen reduction properties of microfiltration/ultrafiltration (MF/UF) membranes followed by reverse osmosis (RO) membranes. Operational experiences in these plants suggest that while MF and UF membranes encounter pore and cake fouling by colloidal particles (0.1 micron down to molecular sizes, ie. nanoparticles), a significant fraction of the smaller colloidal particles pass through the MF and UF membranes, and end up on RO membranes as cake-layer foulants. Autopsies, foulant analyses and cleaning studies performed on fouled RO membranes from the plants and pilot plants showed that colloidal natural organic matter, colloidal calcium phosphate and some times colloidal silicates are the main components. These colloidal particles have great affinity towards aggregation with each other. Due to incomplete removal by MF and even UF, fouling of RO membranes downstream becomes measurable by trend-charts of normalized values of permeate flow, differential pressure and salt passage. Normalized permeate flow is the most sensitive, and an early indicator of such fouling. In this paper we will provide some details of our studies and provide literature evidences that support the conclusion that calcium phosphate in foulants originated as nanoparticles in the wastewater.

Investigation of microbial communities on reverse osmosis membranes used for process water production

In the present study, the diversity and the phylogenetic affiliation of bacteria in a biofouling layer on reverse osmosis (RO) membranes were determined. Fresh surface water was used as a feed in a membrane-based water purification process. Total DNA was extracted from attached cells from feed spacer, RO membrane and product spacer. Universal primers were used to amplify the bacterial 16S rRNA genes. The biofilm community was analysed by 16S rRNA-gene-targeted denaturing gradient gel electrophoresis (DGGE) and the phylogenetic affiliation was determined by sequence analyses of individual 16S rDNA clones. Using this approach, we found that five distinct bacterial genotypes (Sphingomonas, Beta proteobacterium, Flavobacterium, Nitrosomonas and Sphingobacterium) were dominant genera on surfaces of fouled RO membranes. Moreover, the finding that all five "key players" could be recovered from the cartridge filters of this RO system, which cartridge filters are positioned before the RO membrane, together with literature information where these bacteria are normally encountered, suggests that these microorganisms originate from the feed water rather than from the RO system itself, and represent the fresh water bacteria present in the feed water, despite the fact that the feed water passes an ultrafiltration (UF) membrane (pore size approximately 40 nm), which is able to remove microorganisms to a large extent.

Protein (BSA) fouling of reverse osmosis membranes: Implications for wastewater reclamation

Effluent organic matter (EfOM) has been known to contribute significantly to organic fouling of reverse osmosis (RO) membranes in advanced wastewater reclamation. In this study, the effects of feed solution chemistry (calcium concentration, ionic strength, and solution pH) and feed foulant composition on fouling of RO membranes by bovine serum albumin (BSA) – selected to represent proteins in EfOM – are investigated. Crossflow fouling experiments show that RO membrane fouling by BSA is enhanced at higher calcium concentration and at a solution pH at the BSA isoelectric point (pH 4.7). It is further demonstrated that BSA fouling of RO membranes is significantly enhanced in the presence of alginate (a model polysaccharide) as co-foulant. There appears to be an initial synergistic fouling effect when RO membranes are fouled by both BSA and alginate, as compared to fouling by BSA or alginate alone. Foulant–foulant adhesion forces, determined by AFM force measurements under solution chemistries identical to those of the crossflow fouling experiments, further confirm the trends of the fouling profiles. It is consistently shown that solution chemistries and foulant compositions that induce higher fouling rates are associated with greater foulant–foulant adhesion forces.

Effect of Flux (Transmembrane Pressure) and Membrane Properties on Fouling and Rejection of Reverse Osmosis and Nanofiltration Membranes Treating Perfluorooctane Sulfonate Containing Wastewater

Perfluorooctane sulfonate (PFOS) is an emergent contaminant of substantial environmental concerns. In this study, reverse osmosis (RO) and nanofiltration (NF) membranes were used to remove this toxic and persistent compound from PFOS-containing wastewater. Five RO membranes and three NF membranes were tested at a feed concentration of 10 ppm PFOS over 4 days, and the PFOS rejection and permeate flux performances were systematically investigated. PFOS rejection was well correlated to sodium chloride rejection. The rejection efficiencies for the RO membranes were > 99%, and those for the NF membranes ranged from 90-99%. Improvement in PFOS rejection, together with mild flux reduction (< 16%), was observed at longer filtration time. Such shifts in rejection and flux performance were probably due to the increased PFOS accumulation at longer duration, as shown by X-ray photoelectron spectroscopy and liquid chromatograph and tandem mass spectrometry results. A fraction of PFOS molecules might be entrapped in the polyamide layer of the composite membranes, which hindered the further passage of both water and other PFOS molecules. In a similar fashion, PFOS rejection and fouling were enhanced for greater initial flux and/or applied pressure, where PFOS accumulation was promoted probably due to increased hydrodynamic permeate drag. Flux reduction was also shown to correlate to membrane roughness, with the rougher membranes tend to experience more flux reduction than the smoother ones.

The pretreatment with enhanced coagulation and a UF membrane for seawater desalination with reverse osmosis

The application of reverse osmosis (RO) for desalination process has increased rapidly with the construction of large RO plants. Although there have been considerable improvements in membrane materials and operation experience, the fouling of membranes is a significant problem up to the present. There have been many instances of fouling of RO membranes caused by the presence of iron and silica. Biomineralization is usually believed to be caused by microorganisms metabolizing at iron and silica present. Its formation process was studied and described first in the present work, then the enhanced coagulation with Fe(VI) and UF membrane treatment process for pretreatment of reverse osmosis for desalination has been investigated in a laboratory for 3–4 months. The main aim is to reduce the feed water pollution, such as turbidity, iron, silica and aglae, microbial contamination in order to control biofouling and mineralization on the membrane surface. The results showed that the biomineralization formation process is the adsorption of organism and the biosorption of inorganics onto the organic matrix. The pretreatment results show that turbidity is less than 0.5 NTU, iron concentration never exceeds 0.2 mg/l, silicon concentration must not exceed 0.1 mg/l; and the removal rate of aglae and microbial is more than 98%.

Fouling of reverse osmosis membranes by biopolymers in wastewater secondary effluent: Role of membrane surface properties and initial permeate flux

Reverse osmosis (RO) is being increasingly used in treatment of domestic wastewater secondary effluent for potable and non-potable reuse. Among other solutes, dissolved biopolymers, i.e., proteins and polysaccharides, can lead to severe fouling of RO membranes. In this study, the roles of RO membrane surface properties in membrane fouling by two model biopolymers, bovine serum albumin (BSA) and sodium alginate, were investigated. Three commercial RO membranes with different surface properties were tested in a laboratory-scale cross-flow RO system. Membrane surface properties considered include surface roughness, zeta potential, and hydrophobicity. Experimental results revealed that membrane surface roughness had the greatest effect on fouling by the biopolymers tested. Accordingly, modified membranes with smoother surfaces showed significantly lower fouling rates. When Ca 2+ was present, alginate fouled RO membranes much faster than BSA. Considerable synergistic effect was observed when both BSA and alginate were present. The larger foulant particle sizes measured in the co-existence of BSA and alginate indicate formation of BSA-alginate aggregates, which resulted in greater fouling rates. Faster initial flux decline was observed at higher initial permeate flux even when the flux was measured against accumulative permeate volume, indicating a negative impact of higher operating pressure.

Fouling of reverse osmosis and nanofiltration membranes by humic acid—Effects of solution composition and hydrodynamic conditions

Fouling of reverse osmosis (RO) and nanofiltration (NF) membranes by humic acid, a recalcitrant natural organic matter (NOM), was systematically investigated. The membrane flux performance depended on both hydrodynamic conditions (flux and cross-flow velocity) and solution composition (humic acid concentration, pH, ionic strength, and calcium concentration), and was largely independent of virgin membrane properties. While increasing humic acid concentration and ionic strength, and lowering cross-flow velocity affected flux performance moderately, severe flux reduction occurred at high initial flux, low pH, and high calcium concentration. At a calcium concentration of 1 mM, all the membranes exhibited an identical stable flux, independent of their respective intrinsic membrane permeabilities. The effect of solution composition was more significant at higher fluxes. Improved salt rejection was observed as a result of humic acid fouling, which was likely due to Donnan exclusion by humic material close to membrane surfaces. Greater rejection improvement was observed for membranes with rougher surfaces.

Effect of surface roughness on fouling of RO and NF membranes during filtration of a high organic surficial groundwater

Fouling characteristics of various thin film composite polyamide reverse osmosis (RO) and nanofiltration (NF) membranes were systematically investigated using a high organic surficial groundwater obtained from the City of Plantation, Florida. Prior to bench-scale fouling experiments, surface properties of the selected RO and NF membranes were carefully analysed in order to correlate the rate and extent of fouling to membrane surface characteristics, such as roughness, charge and hydrophobicity. More specifically, the surface roughness was characterized by atomic force microscopy, while the surface charge and hydrophobicity of the membranes were evaluated through zeta potential and contact angle measurements, respectively. The results indicated that membrane fouling became more severe with increasing surface roughness, as measured by the surface area difference, which accounts for both magnitude and frequency of surface peaks. Surface roughness was correlated to flux decline; however, surface charge was not. The limited range of hydrophobicity of the flat sheet studies prohibited conclusions regarding the correlation of flux decline and hydrophobicity.

Effects of polyether–polyamide block copolymer coating on performance and fouling of reverse osmosis membranes

The effects of a water-permeable polymer coating on the performance and fouling of high-flux (ESPA1 and ESPA3) and low-flux (SWC4) polyamide reverse osmosis (RO) membranes were investigated. It was anticipated that the coating would create a smoother hydrophilic surface that would be less susceptible to fouling when challenged with a motor-oil/surfactant/water feed emulsion (used as a model foulant). AFM and FT-IR analyses confirm that a 1 wt.% polyether–polyamide (PEBAX ® 1657) solution applied to ESPA and SWC4 membranes produces a continuous polymer coating layer and, thereby, provides smoother membrane surfaces. However, pure-water permeation data combined with a series-resistance model analysis reveal that the coating does not only cover the surface of the polyamide membrane, but also penetrates into its porous ridge-and-valley structure. During a long-term (106-day) fouling test with an oil/surfactant/water emulsion, the rate of flux decline was slower for coated than for uncoated membranes. This improvement in fouling resistance compensated for the decrease in permeate flux for SWC4 over a period of approximately 40 days. However, the coating material is believed to penetrate more deeply into the polyamide surface layer of the high flux, high surface area ESPA membranes relative to the low-flux SWC4, resulting in significant water flux reduction.

Control of residual aluminum from conventional treatment to improve reverse osmosis performance

Soluble aluminum (Al 3+) may react with both ambient silica and antiscalant components to form colloidal foulants during reverse osmosis (RO) treatment. Whereas conventional treatment (coagulation/filtration/sedimentation/dual-media filtration) was being used prior to RO, aluminum sulfate (alum) and polyaluminum chloride (PACl) coagulants were evaluated at ambient pH (pH 7.8–7.9) and suppressed pH (pH 6.7) in an effort to lower the total aluminum to below 50 μg/L — a level previously observed to prevent RO membrane fouling. Additional tests were conducted with 5 mg/L citric acid added to the RO influent to chelate the soluble aluminum fraction. All tests were conducted with 1.5–2.5 mg/L chloramines present. Testing of a RO process fed with optimized alum- or PACl-coagulated water showed that PACl outperformed alum regardless of pH. Alum coagulation at ambient pH resulted in 184–273 μg/L total aluminum passing through the filtration process. Only by lowering the mean influent water pH to 6.7 was the mean soluble aluminum residual (45 μg/L) for alum coagulation reduced to below the 50 μg/L aluminum goal. Regardless of pH, for alum-coagulated waters, the higher aluminum carryover resulted in severe RO membrane fouling within 500 h of operation. Only when a chelating agent (citric acid) was added to the RO feed was the loss in productivity and selectivity arrested. However, PACl consistently met the 50-μg/L goal for both total and soluble aluminum for all pH levels tested, which resulted in more stable membrane performance over time. Further research on the compatibility of PACl and polyamide membranes in the presence of chloramines is needed as data from this project suggest PACl coagulation may facilitate membrane oxidation.

Fouling of reverse osmosis membranes by hydrophilic organic matter: implications for water reuse

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.

Relating Organic Fouling of Reverse Osmosis Membranes to Intermolecular Adhesion Forces

Organic fouling of reverse osmosis (RO) membranes and its relation to foulant--foulant intermolecular adhesion forces has been investigated. Alginate and Suwannee River natural organic matter were used as model organic foulants. Atomic force microscopy was utilized to determine the adhesion force between bulk organic foulants and foulants deposited on the membrane surface under various solution chemistries. The measured adhesion force was related to the RO fouling rate determined from fouling experiments under solution chemistries similar to those used in the AFM measurements. A remarkable correlation was obtained between the measured adhesion force and the fouling rate under the solution chemistries investigated. Fouling was more severe at solution chemistries that resulted in larger adhesion forces, namely, lower pH, higher ionic strength, presence of calcium ions (but not magnesium ions), and higher mass ratio of alginate to Suwannee River natural organic matter. The significant adhesion force measured with alginate in the presence of calcium ions indicated the formation of a crossed-linked alginate gel layer during fouling through intermolecular bridging among alginate molecules.

Fouling and cleaning of seawater reverse osmosis membranes in Kalpakkam nuclear desalination plant

A seawater reverse osmosis plant of 1800 m³/day capacity is part of the 6300 ³/day capacity nuclear desalination demonstration project at Kalpakkam. The plant was commissioned in October 2002 and is in continuous operation. This paper deals with types of foulants, membrane cleaning procedures and the improvement in the reverse osmosis system after cleaning. This paper also describes the analysis of foulants, which may consist of adsorbed organic compounds, particulate matter, micro-organisms and metallic oxides, and describes the chemical cleaning procedure to be adopted, which is specific the seawater used because the membrane foulants are very specific with respect to the seawater constituents. The cleaning of the membranes in the Kalpakkam nuclear desalination plant was taken up because the quality of the permeate had deteriorated and the differential pressure across the membrane had gone up. This paper essentially deals with the selection of cleaning chemicals, the experience gained with the cleaning procedure adopted and the effects of cleaning on the membrane system.

Chemical and physical aspects of cleaning of organic-fouled reverse osmosis membranes

The role of chemical and physical interactions in cleaning of organic-fouled reverse osmosis (RO) membranes is systematically investigated. Fouling and cleaning experiments were performed with organic foulants (alginate and natural organic matter) that simulate effluent organic matter (EfOM) and selected cleaning agents using a laboratory-scale crossflow test unit. Following accelerated organic fouling runs with divalent calcium ions, cleaning experiments were performed at various chemical and physical conditions. The results showed that a metal chelating agent (EDTA) and an anionic surfactant (SDS) were able to clean the fouled RO membrane effectively by optimizing chemical (dose and pH) and physical (time, crossflow velocity, and temperature) conditions during cleaning. The permeate flux was, however, poorly recovered when cleaning was performed with NaOH (pH 11). Interfacial force measurements (by AFM) further confirmed the cleaning results, demonstrating the influence of cleaning agents on reducing the foulant–foulant adhesion force. The results showed that the adhesion force caused by calcium-induced foulant–foulant interaction was reduced significantly with EDTA and SDS cleaning, while substantial adhesion force still remained following NaOH cleaning. It is proposed that an efficient cleaning agent and favorable cleaning conditions could be selected by considering two important mechanisms: chemical reaction between cleaning agents and foulants in the fouling layer, and mass transfer of chemical agents (from the bulk phase to the fouling layer) and foulants (from the fouling layer to the bulk phase).

A Novel Method for Investigating the Influence of Feed Water Recovery on Colloidal and NOM Fouling of RO and NF Membranes

A systematic investigation on the role of feed water recovery (or concentration factor) in colloidal and natural organic matter (NOM) fouling of reverse osmosis (RO) and nanofiltration (NF) membranes is reported. Fouling experiments were conducted using a laboratory-scale crossflow test unit with continuous permeate disposal to simulate concentration factor (CF) and feed water recovery as commonly observed in full-scale RO/NF systems. With this novel method, the deterioration in membrane performance due to fouling and recovery-related osmotic pressure buildup can be distinguished. For colloidal fouling of RO membranes, the so-called "cake enhanced osmotic pressure" is a key fouling mechanism resulting in severe decline in permeate flux and significant decrease in salt rejection with increasing CF. On the other hand, NOM–calcium complexation rather than enhanced osmotic pressure is the key mechanism for NOM fouling of NF membranes. The decline in permeate flux and decrease in solute rejection with increasing CF were significant under favorable conditions for fouling, namely at higher initial permeate flux, ionic strength, and calcium ion concentration.

Assessing Ultrafiltration performance problems

Using membrane autopsy, foulant analyses, and related cleaning studies in assessing reverse osmosis membrane fouling is well known. Fouling in Ultrafiltration (UF) and microfiltration (MF) membranes, however, with their multiple configurations, membrane material compositions, and filtration applications, is more difficult to determine. Nalco's Michael Kamrath and Deepak Musale show how autopsies of tubular and hollow fibre UF membranes - as well as a cleaning study on a tubular membrane - can help identify and solve such fouling problems.

Influence of filtration conditions on the performance of nanofiltration and reverse osmosis membranes in dairy wastewater treatment

Filtration performance and fouling of nanofiltration (NF) and reverse osmosis (RO) membranes in the treatment of dairy industry wastewater were investigated. Two series of experiments were performed. The first one involved a NF membrane (TFC-S) for treating the chemical-biological treatment plant effluents. The second one used a RO membrane (TFC-HR) for treating the original effluents from the dairy industry. The permeate flux was higher at higher transmembrane pressures and higher feed flowrates. The curves of permeate flux exhibited a slower increase while the feed flowrate decreased and the pressure increased. Membrane fouling resulted in permeate flux decline with increasing the feed COD concentration. Furthermore, the flux decline due to the COD increase was found higher at higher pressures for both NF and RO membranes.

Early discovery of RO membrane fouling and real-time monitoring of plant performance for optimizing cost of water

RO plant operators, end-users, membrane manufacturers and system suppliers have been facing two major problems since the advent of commercial membrane technology applications for water desalination utilizing reverse osmosis (RO) and other membrane processes: how to reliably monitor their membrane system performance and how to detect membrane fouling and scaling development in real time and before significant or irreversible loss of performance efficiency occurs, resulting in lower plant availability and significantly higher OM costs. The current industry-standard performance analysis and evaluation technique is based on trending RO flux decline characteristics of membranes via normalizing system operating data in accordance with ASTM D-4516 standard method. This paper discusses the shortfalls of this technique, and introduces a practical new technology for measuring and monitoring membrane fouling and flux performance in real-time. The new Silent Alarm™ technology, designed as an early-warning system, allows the discovery of any fouling or scaling development on the RO membranes in the very early stages, thus providing a valuable tool for the plant operator to take immediate corrective measures before it is too late. Two major brackish and seawater Arabian Gulf RO plant case studies with and without a fouling history are discussed. This proven capability to monitor RO plant performance in real-time and measure the actual development of any membrane fouling or scaling very early on has a direct and dramatic positive impact on optimizing the total cost of desalinated water.