Fouling Of Reverse Osmosis Membranes Research Articles

Fouling of Reverse Osmosis (RO) and Nanofiltration (NF) Membranes by Low Molecular Weight Organic Compounds (LMWOCs), Part 1: Fundamentals and Mechanism.

Reverse osmosis (RO) and nanofiltration (NF) are ubiquitous technologies in modern water treatment, finding applications across various sectors. However, the availability of high-quality water suitable for RO/NF feed is diminishing due to droughts caused by global warming, increasing demand, and water pollution. As concerns grow over the depletion of precious freshwater resources, a global movement is gaining momentum to utilize previously overlooked or challenging water sources, collectively known as "marginal water". Fouling is a serious concern when treating marginal water. In RO/NF, biofouling, organic and colloidal fouling, and scaling are particularly problematic. Of these, organic fouling, along with biofouling, has been considered difficult to manage. The major organic foulants studied are natural organic matter (NOM) for surface water and groundwater and effluent organic matter (EfOM) for municipal wastewater reuse. Polymeric substances such as sodium alginate, humic acid, and proteins have been used as model substances of EfOM. Fouling by low molecular weight organic compounds (LMWOCs) such as surfactants, phenolics, and plasticizers is known, but there have been few comprehensive reports. This review aims to shed light on fouling behavior by LMWOCs and its mechanism. LMWOC foulants reported so far are summarized, and the role of LMWOCs is also outlined for other polymeric membranes, e.g., UF, gas separation membranes, etc. Regarding the mechanism of fouling, it is explained that the fouling is caused by the strong interaction between LMWOC and the membrane, which causes the water permeation to be hindered by LMWOCs adsorbed on the membrane surface (surface fouling) and sorbed inside the membrane pores (internal fouling). Adsorption amounts and flow loss caused by the LMWOC fouling were well correlated with the octanol-water partition coefficient (log P). In part 2, countermeasures to solve this problem and applications using the LMWOCs will be outlined.

Biofouling control of reverse osmosis membrane in municipal wastewater recycling plants using urine as a cleaning agent

Biofouling is a major issue for reverse osmosis (RO) membranes during wastewater recycling, which requires significant chemical inputs for its cleaning. This study investigated the feasibility of using urine, a human waste that can be directly obtained from the community, as a novel and low-cost cleaning agent for RO membranes subjected to biofouling. Lab-scale immersion cleaning tests and cross-flow cleaning tests were performed on four fouled RO membranes collected from municipal wastewater recycling plants (MWRPs). In the immersion cleaning test, urine solutions (15–25 %) showed higher removal efficiency for ATP, proteins, and polysaccharide than the traditional alkaline solution (NaOH, pH=11), with a cleaning efficiency of 82 %, 53 %, and 79 %, respectively. In the cross-flow cleaning test, urine solutions (15 % − 25 %) also exhibited comparable or better performance than the conventional alkaline cleaning (NaOH, pH=11) in ATP, proteins and polysaccharide removals, reaching a removal rate of 79 % − 100 %, 61 % − 90 %, and 70 % − 100 %, respectively, and permeability increase by 7.8 % − 16.0 %. Urine solutions (15–25 %) demonstrated to be a promising agent for biofouling cleaning on RO membranes in MWRPs, providing a cost-effective and environmentally friendly alternative.

Intensive coagulation of secondary effluent to mitigate reverse osmosis membrane fouling at low temperatures: Strategies of pre-oxidation and coagulant aids addition

Conventional coagulation-sedimentation, typically used as a pretreatment for ultrafiltration-reverse osmosis, is ineffective in solving the problem of reverse osmosis membrane fouling at low temperatures. In this study, the coagulation enhancement effect of pre-oxidation and the addition of coagulant aids on conventional coagulation with polyaluminum chloride as a coagulant was investigated. The results showed that the addition of fly ash, powdered activated carbon, or diatomite as coagulant aids improved the reduction of turbidity, dissolved organic carbon (DOC), and ultraviolet absorbance at 254 nm (UV254), and also reduced the residual content of dissolved aluminum ions. Among them, diatomite was superior. Pre-oxidation before conventional coagulation increased the reduction of turbidity and UV254 by about 50 % and doubled the removal of proteins, polysaccharides, and DOC compared to conventional coagulation. Ozone was superior to sodium hypochlorite. The combination of ozone pre-oxidation and diatomite addition with conventional coagulation further increased the pollutant removal rate, with the removal rates of turbidity, UV254, polysaccharides, proteins, and DOC reaching 82.20 %, 52.35 %, 26.34 %, 24.89 %, and 26.38 %, respectively. In addition, the residual content of dissolved aluminum ions decreased to 0.84 mg/L. The reverse osmosis filtration simulation test showed that pretreating the feed water with the enhanced coagulation by the combination of ozone pre-oxidation and diatomite addition resulted in a slower decrease in membrane flux, reduced foulant content on the membrane surface, and the smallest increase in contact angle. The results of this research may offer a viable pretreatment method to reduce fouling in reverse osmosis membranes operating at low temperatures.

Optimized removal of silica during manure treatment by electrocoagulation-flotation (EC-F) in view of fouling prevention of reverse osmosis membranes

Reverse osmosis (RO) membrane fouling threatens the performance of manure treatment. In this regard, silica is considered one of the most challenging inorganic foulants. Removing silica fouling from RO membranes is difficult, ultimately leading to performance deterioration, including reduced permeability and premature membrane failure. This study examines the removal of silica during manure treatment using a novel tubular electrocoagulation-flotation (EC-F) reactor with an Al electrode. The EC-F was positioned between an ultrafiltration and a RO membrane filtration system. The operational parameters—current density, electrolysis residence time, and initial pH—were optimized for maximum silica removal efficiency and minimum operational cost. At the optimum conditions (pH of 8.2, current density of 160 A/m2 and electrolysis residence time of 23 s), 88 % of silica (initial concentration of 104 mg/L) was removed. Subsequent membrane fouling assessment revealed that EC-F pretreatment decreased the fouling potential by 28 % (from 7.11 × 1016 to 5.10 × 1016 Pa·s/m2·t), indicating efficient foulant removal. SEM-EDX analysis of the membrane confirmed the lack of scale formation when using EC-F pretreated manure as a feed stream. This study demonstrates that the use of innovative EC-F pretreatment significantly reduces the operational expenditure (1.75 €/m3) compared to prior EC-F research for silica removal from manure, thereby reducing the fouling of RO membranes.

Molecular weight insight into critical component contributing to reverse osmosis membrane fouling in wastewater reclamation

Molecular weight (MW) of organics was one of the important factors influencing membrane fouling propensity. This study identified critical foulants of reverse osmosis (RO) membranes in reclaimed water by MW fractionation. MW > 10 kDa component was identified as the critical fouling contributor (CFC) in secondary effluent (SE), which accounted for only 13 ± 5% of dissolved organic carbon (DOC) but contributed to 86 ± 11% of flux decline. Throughout 12-month monitoring, SE and MW > 10 kDa component showed a similar fouling variation tendency: apparently higher fouling potential in winter and lower in summer, while MW < 10 kDa component presented minor fouling changes. Morphology of membrane fouled by CFC characterized a smooth and thick foulant layer on membrane surface. CFC was mainly composed of proteins and polysaccharides, and a protein-polysaccharide-protein “sandwich” fouling layer structure was preferentially formed on membrane surface. extended Derjaguin–Landau-Verwey–Overbeek (xDLVO) analysis demonstrated that strong attractive interactions between CFC and membrane surface dominated the fouling process. Furthermore, computational fluid dynamics (CFD) simulation revealed strong filtration resistance of CFC, confirming its significant fouling potential. Dual effects including attractive interactions and advantageous ridge-and-valley surface appearance accounted for the significant fouling propensity of MW > 10 kDa component and glean valuable insights into RO fouling mechanisms of reclaimed water in practical application.

Application of the multi-wavelength UV-LED/chlorine process to improve reverse osmosis membrane performance for reused water treatment in the steel industry

ABSTRACT Membrane fouling is a prominent issue that affects the stable and efficient operation of reverse osmosis (RO) in reused water treatment. In this study, a zero-discharge RO system was adopted to treat the ultrafiltration permeate from a steel plant with the combined multi-wavelength UV-LED/chlorine process, focusing on organic structure modification and membrane fouling control. The results showed that the UV-LED/chlorine process could not only efficiently remove the dissolved organic carbon and the total nitrogen of the RO influent but also alter the organic substances from large molecules to small ones. In addition, the longer wavelength of a 295 nm UV-LED/chlorine process exhibited a greater RO permeate flux of 158 LMH, as compared to the shorter wavelength of 255 nm with the flux of 152 LMH. Moreover, compared to the single-wavelength, the dual-wavelength UV-LED/chlorine process played a more significant role in RO filtration performance, which induced a looser and thinner foulant structure, resulting in an 8% larger permeate flux and recovery at 275 + 295 nm than at 295 nm. This study demonstrated that the combined UV-LED/chlorine process could effectively alleviate RO membrane fouling. Our findings can provide theoretical and technical support for the sustainable development of membrane-based reused water treatment in the steel industry.

Alleviation of RO membrane fouling in wastewater reclamation plants using an enhanced acid-base chemical cleaning method

Membrane fouling has always been a critical constraint in the operation of the reverse osmosis (RO) process, and chemical cleaning is essential for mitigating membrane fouling and ensuring smooth operation of the membrane system. This paper presents an optimized chemical cleaning method for the efficient cleaning of RO membranes in full-scale applications. Compared to the regular cleaning method (cleaning with 0.1 % NaOH + 1 % ethylenediaminetetraacetic acid + 0.025 % sodium dodecyl benzene sulfonate followed by 0.2 % HCl), the optimized cleaning method improves the cleaning efficiency by adding sodium chloride to the alkaline cleaning solution and citric acid to the acid cleaning solution. Notably, the membrane flux recovery rate with the optimized cleaning method is 45.74 %, and it improves the cleaning efficiency by 1.65 times compared to the regular cleaning method. Additionally, the optimized cleaning method removes 30.46 % of total foulants (organic and inorganic), which is 2.11 times higher than the regular cleaning method. The removal of inorganic ions such as Fe, Ca, and Mg is significantly improved with the optimized cleaning method. For organic matter removal, the optimized cleaning method effectively removes more polysaccharides, proteins, and microbial metabolites by disrupting the complex structures of organic matter. Furthermore, it also changes the microbial community structure on the RO membrane surface by eliminating microorganisms that cannot withstand strong acids, bases, and high salt environments. However, Mycobacterium can adapt to these harsh conditions, showing a relative abundance of up to 84.13 % after cleaning. Overall, our results provide a new chemical cleaning method for RO membranes in full-scale applications. This method effectively removes membrane foulants and enhances the understanding of the removal characteristics of foulants on RO membrane surfaces by chemical cleaning.

Full-length 16S rRNA gene sequencing reveals the operating mode and chlorination-aggravated SWRO biofouling at a nuclear power plant.

Reverse osmosis (RO) membrane fouling and biological contamination problems faced by seawater desalination systems are microbiologically related. We used full-length 16S rRNA gene sequencing to assess the bacterial community structure and chlorine-resistant bacteria (CRB) associated with biofilm growth in different treatment processes under the winter mode of a chlorinated seawater desalination system in China. At the outset of the winter mode, certain CRB, such as Acinetobacter, Pseudomonas, and Bacillus held sway over the bacterial community structure, playing a pivotal role in biofouling. At the mode's end, Deinococcus and Paracoccus predominated, with Pseudomonas and Roseovarius following suit, while certain CRB genera still maintained their dominance. RO and chlorination are pivotal factors in shaping the bacterial community structure and diversity, and increases in total heterotrophic bacterial counts and community diversity in safety filters may adversely affect the effectiveness of subsequent RO systems. Besides, the bacterial diversity and culturable biomass in the water produced by the RO system remain high, and some conditionally pathogenic CRBs pose a certain microbial risk as a source of drinking water. Targeted removal of these CRBs will be an important area of research for advancing control over membrane clogging and ensuring water quality safety in the future.

Comparison of Coagulation-Integrated Sand Filtration and Ultrafiltration for Seawater Reverse Osmosis Pretreatment.

The removal of dissolved organic matter (DOM) from seawater before the reverse osmosis (RO) processes is crucial for alleviating organic fouling of RO membranes. However, research is still insufficiently developed in the comparison of the effectiveness of integrating coagulation with ultrafiltration (UF) or sand filtration (SF) in the pretreatment stage of seawater reverse osmosis (SWRO) for the removal of DOM. In this study, we investigated the effect of pretreatment technologies on RO fouling caused by DOM in seawater, including the integration of coagulation and sand filtration (C-S pretreatment) and the integration of coagulation and ultrafiltration (C-U pretreatment). Both integrated pretreatments achieved comparable DOM removal rates (70.2% for C-U and 69.6% for C-S), and C-S exhibited enhanced removal of UV-absorbing compounds. Although C-U was more proficient in reducing the silt density index (below 2) compared to C-S (above 3) and improved the elimination of humic acid-like organics, it left a higher proportion of tyrosine-protein-like organics, soluble microbial by-product-like organics, and finer organics in the effluent, leading to the formation of a dense cake layer on RO membrane and a higher flux decline. Therefore, suitable technologies should be selected according to specific water conditions to efficiently mitigate RO membrane fouling.

Combined impacts of aluminum and silica ions on RO membrane fouling in full-scale ultrapure water production facilities

In this study, a fouled reverse osmosis (RO) membrane from a full-scale ultrapure water (UPW) facility was autopsied and investigated to propose the fouling mechanism and mitigation strategies. Although the influent was introduced to the coagulation and ultrafiltration as the pre-treatment processes, thick fouling layers were found over the entire RO surface. The foulant was mainly composed of aluminum silicates due to the elevated Al concentration (up to 100 μg/L) originated from the coagulation process utilizing polyaluminum chloride, and high recovery (up to than 90%) of RO process. Lab-scale fouling tests also confirmed that the elevated Al concentration in the RO influent contributed to significant flux decline due to a bridging effect with negatively charged organic matter and the formation of aluminum silicates. The application of polydiallyldimethylammonium chloride as an alternative coagulant successfully mitigated the organic and silicate fouling. Therefore, control of the residual aluminum ions originated from the coagulation process is critical to mitigate both organic and inorganic fouling on RO membrane surfaces during the production of UPW.

Prediction of reverse osmosis membrane fouling in water reuse by integrated adsorption and data-driven models

In contemporary society, the reverse osmosis (RO) process is important for water treatment and reuse industry. However, membrane fouling remains a challenging issue during operation. To date, RO system operation mainly relies on the operator's knowledge, and the maintenance is carried out based on schedule or pre-defined criteria due to our limited understanding of RO fouling in real applications. To better understand the process and enable system optimization, an integrated data-driven coupled with adsorption model has been proposed in this study. The data-driven model calibrates the mechanistic model and predicts future values, while the adsorption model provides predictions on transmembrane pressure (TMP). By integrating these two models, an accurate prediction with high robustness and an insight into the detailed fouling mechanisms in real plants were achieved. In addition to the model itself, multiple regression analysis (MRA) had also been applied to identify the dominant fouling mechanisms. This analysis confirms that it is highly possible that membrane fouling is developed from an intermediate pore blockage to cake filtration.

Fouling of reverse osmosis membrane in sugar mill condensate purification under sub- and super-boundary flux conditions

The sugar production process, inherently water-intensive, is characterized by pronounced condensate formation during the evaporation and boiling phases. Given the environmental and resource implications, there is a scholarly emphasis on the potable reuse of such condensate by reverse osmosis (RO) in sugar mills. However, membrane fouling presents a significant challenge to the consistent operation of the RO process. This research examines the influence of operating parameters, namely transmembrane pressure and crossflow velocity, on the performance and fouling tendencies of an RO membrane during the purification of sugar mill condensate. The boundary flux theory was employed to model the permeate flux throughout this process. A boundary flux (5.92 L h−1m−2) and its corresponding operating pressure (10 bar) were determined. Moreover, a comparative analysis of the fouling mechanisms and the composition of the foulant layer under both sub-boundary and super-boundary conditions was undertaken. Under sub-boundary conditions, the fouling mechanism was identified as cake-intermediate blocking, while the super-boundary condition exhibited intermediate blocking. Under both conditions, organic fouling emerged as the primary fouling type. The TOC of surface contaminants ranged from 1.07 to 1.86 g/m2. Within this, proteins (484.0 to 850.3 mg m−2) and polysaccharides (201.5 to 379.2 mg m−2) were the dominant components. However, when operating conditions surpassed the boundary, there was a noticeable rise in inorganic fouling such as Ca (from 44.95 to 149.43 mg m−2) and Fe (from 4.40 to 112.09 mg m−2), which is likely due to the co-deposition of organic materials with inorganic ions. This research contributes to a more nuanced understanding of how operating conditions influence fouling mechanisms and provides foundational insights for addressing fouling challenges in sugar mill condensate purification.

Biofouling control of reverse osmosis membrane using free ammonia as a cleaning agent

Reverse osmosis (RO) is an important and widely-used membrane separation process for water recycling. However, biofouling is extensively considered a major problem for RO membranes due to the biofilm formation on the membrane surfaces. This study proposed and demonstrated a novel and sustainable chemical cleaning approach using a free ammonia (FA) solution for the removal of biofouling on RO membranes. The feasibility of FA solution for biofouling removal was investigated through a series of lab-scale soak cleaning tests and cross-flow cleaning tests on four fouled RO membranes (M1-M4) collected from municipal wastewater recycling plants. In soak cleaning tests on M1, FA concentrations of 65–560 mg NH3–N/L (pH = 8.9) can remove adenosine triphosphate (ATP) by 32–75 %, remove proteins by 18–47 % and remove polysaccharides by 31–74 %, which was up to 3.4 times of the removals by using NaOH solution under the same pH. FA solution of 310–560 mg NH3–N/L (pH = 8.9) even showed higher removals than NaOH solution with a higher pH of 11. In the cross-flow cleaning tests on M2-M4, FA solution of 310–560 mg NH3–N/L (pH = 8.9) removed the ATP by 82–100 %, removed proteins by 58–87 % and removed polysaccharides by 68–100 % in the fouling layers and increased the permeability by 8–16 %. Such cleaning effects in cross-flow tests were also positively correlated (R > 0.9, p < 0.05). Compared to the conventional anti-biofouling agent of NaOH solution (pH = 11), FA solution (310–560 mg NH3–N/L) showed significantly better cleaning performance. A high FA concentration of 560 NH3–N/L (pH = 8.9) could achieve comparable cleaning effect to 1 % EDTA solution (pH = 10). Additionally, increasing the frequency of cleaning or using a higher concentration of FA solution for biofouling removal will be more advantageous to prolong the membranes’ lifespan. The findings provide a promising alternative to using FA as a cost-effective and environmentally friendly solution for cleaning biofouling on RO membranes.

Identifying artifacts in EDS of fouled RO membranes through Raman and ICP-MS analysis

Fouled reverse osmosis (RO) membranes, primarily characterized by Scanning Electron Microscope and Energy Dispersive X-ray Spectrometer (SEM-EDS), face several limitations including spectral inaccuracies and misidentification of elements caused by automatic peak assessment software. This work addresses the latter by analyzing fouled polyamide membranes from community RO plants of Rajasthan treating brackish groundwater. Initially, EDS analysis revealed unexpected elements including Sc, Ti, V, and Cr, alongside anticipated foulants like Ca, Mg, Al, Si, Fe, and Mn. However, upon manual reassessment guided by spectral libraries and elemental likelihood, the peaks originally identified by the software as Sc, Ti, and V were reattributed to Ca and Ba, emphasizing the importance of careful peak assignment. Further, Raman spectroscopy of the same membranes detected calcite, dolomite, and molybdenum-bearing minerals. A peak initially assigned to Mo by the software was reinterpreted as interlayer carbonate, supported by literature on hydrotalcite structures. Subsequent ICP-MS analysis of the deposited salts showed that both Sc and Mo were below the detectable threshold, validating the reassessed findings. This study underscores the challenges of accurately identifying the inorganic scales to develop the right descaling approach by emphasizing the importance of integrating multiple analytical methods for comprehensive analysis and effective membrane cleaning solutions.

Fouling characteristics and flux prediction model of reverse osmosis membrane based on hydrophobic fractions in reclaimed water

During the operation of the reverse osmosis (RO) system, the identification of key fouling components and the prediction of flux changes based on these components are crucial for minimizing the costs associated with flux recovery. In this study, the DAX-8 resin fractionation method was employed to separate the organic matter in secondary effluent (SE) into hydrophobic acid substances, hydrophobic base substances, hydrophobic neutral substances (HON), hydrophilic substances, and hydrophobic substances (HOS). The primary objective was to discern critical membrane fouling components. The results revealed that HOS constituted 50.0%–60.0% of the dissolved organic carbon concentration in the SE, emerging as the pivotal fraction responsible for RO membrane fouling, contributing to 90.0% of the flux drop induced by SE. Additionally, a fouling prediction model employing two parameters quantified the fouling characteristics of distinct hydrophilic/hydrophobic fractions in RO treatment. Model fitting results indicated a flux decrease of 61.3%, 54.3%, and 46.7% caused by SE, HOS, and HON, respectively. Notably, 90.0% of the decrease in RO membrane flux induced by SE was attributed to the presence of HOS, with HON accounting for about 90.0% of this decrease. The results of the extended Derjaguin-Landau-Verwey-Overbeek theory demonstrated higher adhesion energy and cohesion energy between HOS and RO membrane surfaces compared to hydrophilic substances. This finding suggests that hydrophobic pollutants are more closely bound to RO membrane surfaces. Subsequently, an RO membrane flux prediction method based on HOS in reclaimed water was established. Predicting the RO membrane fouling characteristics of SE became feasible by measuring the concentration of HOS in the influent. Furthermore, resin pre-treatment mitigated the membrane fouling by removing HOS, and the HOS concentration was proposed as a new influent quality index for RO system design.

Adsorptive chito-beads for control of membrane fouling

Chitosan has excellent antimicrobial, adsorption, heavy metal removal, and adhesion properties, making it a good substitute for microplastic-based cleaners. Here, chitosan microbeads (chito-beads) of various sizes ranging from 32 μm to 283 μm were prepared via emulsion using a liquid on oil method and the feasibility of using them as an essential constituent in a chemical cleaning solution for a reverse-osmosis (RO) membrane-fouling-control process was assessed. Prior to the assessment the cleaning efficiency of a solution containing chito-beads, the interaction energy between chitosan and a representative organic foulant (humic acid (HA)) in a RO membrane fouling was analyzed using colloidal atomic force microscopy, and the strongest attraction between chitosan and HA was observed in an aqueous solution. When comparing the membrane cleaning efficiency of cleaning solutions with and without chito-beads, smaller chito-beads (32 μm and 70 μm) were found to have higher cleaning efficiency. Applications of chito-beads to the membrane cleaning process can enhance the cleaning efficiency through the physicochemical interaction with organic foulants. This study can widen the use of chito-beads as an additive to membrane chemical cleaning solutions to control membrane fouling in other membrane processes as well.

A comprehensive evaluation of the temporal and spatial fouling characteristics of RO membranes in a full-scale seawater desalination plant

The fouling of seawater reverse osmosis (SWRO) membranes remains a persistent challenge in desalination. Previous research has focused mainly on fouling separately; however, organic, inorganic, and biofouling can coexist and influence each other. Hence, in-depth study of the spatiotemporal changes in actual combined fouling in full-scale seawater desalination will provide more effective information for fouling investigation and control. In this study, we monitored (i) the operational performance of a full-scale desalination plant for 7 years and (ii) the development and characterization of membrane and spacer fouling at different locations of spiral-wound membrane modules sampled after 2.5-, 3.5-, and 7-year operation. The findings showed that (i) operational performance indicators declined with time (normalized flux 40 % reduction, salt rejection 2 % in 7 years), with a limited effect of the 20-day cleaning frequency, (ii) fouling accumulation in the membrane module mainly occurred at the feed side of the lead module and the microbial community in these area exhibited the highest diversity, (iii) the dominant microbial OTUs belonged mainly to Proteobacteria (43–70 %), followed by Bacteroidetes (10–11 %), (iv) Phylogenetic molecular ecological networks and Spearman correlation analysis revealed that Chloroflexi (Anaerolineae) and Planctomycetes were keystone species in maintaining the community structure and biofilm maturation and significantly impacted the foulant content on the SWRO membrane, even with low abundance, and that (v) fouling accumulation was composed of polysaccharides, soluble microbial products, marine humic acid-like substances, and inorganic Ca/Fe/Mg/Si dominate the fouling layer of both the membrane and spacer. Overall, variation partitioning analysis quantitatively describes the increasing contribution of biofouling over time. Ultimately, the organic‒inorganic-biofouling interaction (70 %) significantly contributed to the overall fouling of the membrane after 7 years of operation. These results can be used to develop more targeted fouling control strategies to optimize SWRO desalination plant design and operation.

Synergistic effect of ozone and non-oxidizing bacteriostatic agent on biofouling control of reverse osmosis membranes

Biofouling was the critical flaw of the reverse osmosis (RO) process. Both strong oxidizing and non-oxidizing bacteriostatic agents had obvious defects facing this problem. This study firstly compared the single and synergistic effect of a strong oxidizing agent (ozone) and a commercial non-oxidizing bacteriostatic agent (FR110) on RO membrane fouling control of reclaimed water, to find a more high-efficiency and practical approach. Consequently, the synergistic application of ozone and FR110 at low dosages effectively reduced the fouling of the RO membranes (18% normalized flux alleviation than the control group). Further analysis suggested that FR110 could induce a thinner fouling layer, while ozone pretreatment could lead to a loose fouling layer. The bacterial community analyses suggested that the relative abundance of biofouling-related genera was the key reason causing fouling differences. Phenotypic prediction confirmed the advantages of the synergetic application of FR110 and O3 in reducing the biofilm-forming capacity. Pure-strain experiments showed the synergetic application of FR110 and O3 could inhibit the overgrowth and secretion of certain biofilm-forming strains, such as Pseudomonas aeruginosa PAO1. In summary, synergistic disinfection was a promising way to regulate the bacterial community structure and biofilm-forming potentials, hoping for high-efficiency biofouling control in the RO system.

Detailed organic characterization of process water to evaluate reverse osmosis membrane fouling in industrial wastewater treatment

One of the major waste streams within the oil and gas (O&G) industry is produced and process water generated during hydrocarbon production and treatment, respectively. In gas production facilities, process water typically has lower salinity (i.e., <10,000 mg/L) which present opportunities for these wastewaters to be treated for beneficial reuse applications via advanced water treatment technologies, such as reverse osmosis (RO) membranes. One of the key challenges for RO membranes application to industrial wastewater treatment is fouling which is frequently attributed to the soluble organics present in the water and/or associated with field chemicals. In this study, a detailed organic characterization methodology, using liquid chromatography with organic carbon detector (LC-OCD) was applied to characterize the organics on a real process water collected from industrial wastewater treatment plant at a gas production facility. Additionally, a rigorous bench-scale testing procedure was implemented to assess the performance of a full-scale RO system with and without activated carbon filter (ACF) pretreatment, making this study the first one to apply LC-OCD methodology on real wastewater to evaluate the fouling of RO membranes deployed at an industrial treatment plant. Bench-scale RO results showed that in the absence of ACF pretreatment, a 12 % decline in membrane permeability (from 1.78 to 1.57 L/(m2-bar-h)) was observed, while no permeability decline was measured after ACF treatment. The organic fouling was confirmed by mass balance calculations on the bench scale experiments as well as Fourier Transform Infrared (FTIR) analysis on the membrane coupons. Moreover, LC-OCD analysis showed that the ACF inlet has a TOC concentration of approximately 3.06 mg/L; of those 1.29 mg/L (42 %) are hydrophobic, and 1.77 mg/L (58 %) are hydrophilic. After ACF treatment, the hydrophobic organics decreased to 0.65 mg/L, revealing that the ACF is removing >50 % of hydrophobic organics which are likely responsible for membrane fouling. Bench-scale operational and water quality results were comparable to the full-scale RO performance data validating the lab testing procedure implemented in this study.

Mitigation of reverse osmosis membrane fouling by coagulation pretreatment to remove silica and transparent exopolymer particles

The dissolution of silica and transparent exopolymer particles (TEP) can deposit on the membrane surface and cause serious membrane fouling in reverse osmosis (RO) technology. Coagulation, as a common pretreatment process for RO, can effectively intercept pollutants and alleviate membrane fouling. In this study, FeCl3 and AlCl3 coagulants and polyacrylamide (PAM) flocculants were used to explore the optimal coagulation conditions to reduce the concentration of silica and TEP in the RO process. The results showed that the two coagulants had the best removal effect on pollutants when the pH was 7 and the dosage was 50 mg/L. Considering the proportion of reversible fouling after coagulation, the removal rate of pollutants, and the residual amount of coagulation metal ions, the best PAM dosage was 5 mg/L for FeCl3 and 1 mg/L for AlCl3. After coagulation pretreatment, the Zeta potential decreased, and the particle size distribution increased, making pollutants tend to aggregate, thus effectively removing foulants. The removal mechanisms of pollutants by coagulation pretreatment were determined to be adsorption, electric neutralization and co-precipitation. This study determined the best removal conditions of silica and TEP by coagulation and explored the removal mechanism.