Evaluation of Ceramics Adsorption Filter as a Pretreatment for Seawater Reverse-Osmosis Desalination.

Seawater reverse osmosis (SWRO) is the most energy-efficient process for desalination to produce drinking water from seawater. However, its sustainability is still challenged by membrane fouling. Appropriate feed water quality is one of the crucial prerequisites for SWRO operation. In the current study, a ceramic adsorption filter (CAF), which was predominantly coated with an aluminum-based adsorbent (i.e., Alumina, Al2O3), was employed to enhance the pretreatment performance of SWRO. The fouling performance of SWRO pre-treated with a CAF was evaluated by feeding with real ultrafiltration (UF)-filtrated seawater collected from a seawater desalination R&D facility in Singapore. The flux decline profile showed that the presence of CAF after UF could mitigate around 10-30% of SWRO fouling. Based on the autopsy of the fouled SWRO membranes, it was observed that SWRO with CAF pre-treatment and daily regeneration could alleviate around 77.5% of Ca-induced inorganic fouling as well as 76% of lower biofouling. The present work highlights the potential of applying adsorption technology to enhance pre-treatment performance to extend the lifespan of SWRO membranes. Coupling the adsorbents on a ceramic filter should be a useful way to ease their implementation, i.e., inline adsorption and re-generation.

Biocarriers facilitated gravity-driven membrane filtration of domestic wastewater in cold climate: Combined effect of temperature and periodic cleaning.

In this study, two lava stone biocarrier facilitated gravity-driven membrane (GDM) reactors were operated at ~8 °C and ~22 °C in parallel for treating primary wastewater effluent. Although the biocarrier reactor at 8 °C displayed less efficient removals of biodegradable organics than that at 22 °C, both GDM systems (without cleaning) showed comparable fouling resistance distribution patterns, accompanying with similar cake filtration constants and pore constriction constants by modelling simulation. Compared to the GDM at 8 °C, more foulants were accumulated on the GDM at 22 °C, but they presented similar soluble organics/inorganics contents and specific cake resistances. This indicated the cake layers at 22 °C may contain greater-sized foulants due to proliferation of both prokaryotes and eukaryotes, leading to a relatively less-porous nature. In the presence of periodic cleaning (at 50 °C), the cleaning effectiveness followed a sequence as ultrasonication-enhanced physical cleaning > two-phase flow cleaning > chemical-enhanced physical cleaning > physical cleaning, regardless of GDM operation temperature. However, significantly higher cake resistances were observed in the GDM system at 22 °C than those at 8 °C, because shear force tended to remove loosely-attached foulant layers and may compress the residual dense cake layer. The presence of periodic cleaning led to dissimilar dominant prokaryotic and eukaryotic communities in the cake layers as those without cleaning and in the lava stone biocarriers. Nevertheless, operation temperature did not influence GDM permeate quality, which met EU discharge standards.

Fouling mitigation in reverse osmosis processes with 3D printed sinusoidal spacers.

Feed spacers are an essential part of spiral wound modules for reverse osmosis (RO). They create flow channels between membrane sheets and manipulate hydrodynamic conditions to control membrane fouling. In this work, additive manufacturing (Polyjet) was used to print novel sinusoidal spacers with wavy axial filaments connected by perpendicular (ST) or slanted (SL) transverse filaments. When tested with 2 g/L NaCl solution, conventional and SL spacers had similar flux while the ST spacer had about 5-7% lower flux. The pressure losses for ST and SL spacers increased by up to 3 folds depending on the flow condition. In the colloidal silica fouling and biofouling tests, the sinusoidal spacers showed lower membrane permeability decrease of 46% for ST, 41% for SL vs 56% for conventional and 26% for ST, 22% for SL vs 33% for conventional, respectively. Optical coherence tomography images from colloidal silica fouling and confocal images from biofouling tests revealed that fouling patterns were closely associated with the local hydrodynamic conditions. Overall, sinusoidal spacers showed promising results in controlling membrane fouling, but there is potential for further optimizations to reduce channel pressure loss.

Enhancing performance of biocarriers facilitated gravity-driven membrane (GDM) reactor for decentralized wastewater treatment: Effect of internal recirculation and membrane packing density.

This study aims to investigate the effect of internal recirculation and membrane packing density on the performance (water quality, membrane performance, and microbial community) of a biocarriers facilitated gravity-driven membrane (GDM) reactor under intermittent aeration condition. The results revealed that the presence of internal recirculation in the GDM reactors could effectively improve water quality (especially increasing nitrogen removal) and membrane performance (especially reducing cake layer resistance) compared to those without internal recirculation. In addition, compared to a high packing density membrane module (1150 m2/m3), a lower packing density membrane module (290 m2/m3) benefited to improve 15% of nitrogen removal and 44% of permeate flux due to the effective aeration scouring effect and less-limited eukaryotic activity, as well as reduce 20% of total treatment cost. In addition, the presence and absence of internal recirculation could lead to dissimilar microbial community compositions of the biofilms in the GAC layers and on the membrane surfaces. However, the membrane packing density could play an insignificant effect on the microbial community compositions of the biofilms in the GDM reactors with internal recirculation.

A review on spacers and membranes: Conventional or hybrid additive manufacturing?

Over the past decade, 3D printing or additive manufacturing (AM) technology has seen great advancement in many aspects such as printing resolution, speed and cost. Membranes for water treatment experienced significant breakthroughs owing to the unique benefits of additive manufacturing. In particular, 3D printing's high degree of freedom in various aspects such as material and prototype design has helped to fabricate innovative spacers and membranes. However, there were conflicting reports on the feasibility of 3D printing, especially for membranes. Some research groups stated that technology limitations today made it impossible to 3D print membranes, but others showed that it was possible by successfully fabricating prototypes. This paper will provide a critical and comprehensive discussion on 3D printing specifically for spacers and membranes. Various 3D printing techniques will be introduced, and their suitability for membrane and spacer fabrication will be discussed. It will be followed by a review of past studies associated with 3D-printed spacers and membranes. A new category of additive manufacturing in the membrane water industry will be introduced here, known as hybrid additive manufacturing, to address the controversies of 3D printing for membrane. As AM technology continues to advance, its possibilities in the water treatment is limitless. Some insightful future trends will be provided at the end of the paper.

Impact of salt accumulation in the bioreactor on the performance of nanofiltration membrane bioreactor (NF-MBR)+Reverse osmosis (RO) process for water reclamation.

The impacts of salt accumulation, through adjusting the solid retention time (SRT), in the bioreactor on the bioprocess as well as membrane performance of a high retention nanofiltration membrane bioreactor (NF-MBR) and subsequent reverse osmosis (RO) process for water reclamation are addressed in this study. The build-up of salts (i.e., Ca, Mg, PO4) is a function of SRT, hydraulic retention time (HRT) and membrane rejection. Despite the accumulation of salts, both NF-MBRs at SRT of 30 and 60 days, achieved (i) similar biodegradation efficiency; (ii) excellent organic removal (> 97%); and (iii) excellent ammonia removal (> 98%). Extending the SRT could improve the microbial bio-flocculation capability, but did not influence the microbial activity, viability, and community structure. However, more severe membrane fouling was observed in the NF-MBR with elevated salt levels, which was attributed to the greater formation of calcium phosphate scale and Ca-polysaccharides complex (i.e., irreversible fouling layer) as well as the cake-enhanced-osmotic-pressure (CEOP) effect. Although both NF-MBRs produced comparable quality of permeate, a higher RO membrane fouling rate was observed when the permeate of NF-MBR with SRT at 60 days was fed to the RO system, implying organic compositions in NF-MBR permeate may influence RO performance.

Integration of an anaerobic fluidized-bed membrane bioreactor (MBR) with zeolite adsorption and reverse osmosis (RO) for municipal wastewater reclamation: Comparison with an anoxic-aerobic MBR coupled with RO.

This study compared the performance of an anaerobic fluidized bed membrane bioreactor (AFMBR)-zeolite adsorption-reverse osmosis (RO) system and an anoxic-aerobic MBR-RO system for municipal wastewater reclamation. Both MBR-RO systems were operated in parallel with the same operating conditions. The results showed that the MBR systems achieved excellent organic removals (>95%) and the anoxic-aerobic MBR could also remove ∼57% of soluble total nitrogen. Compared to the aerobic MBR, the AFMBR displayed better membrane performance with less energy consumption, attributed to effective membrane scouring by liquid-fluidized GAC particles. Furthermore, a zeolite column was employed to remove ammonia in the AFMBR permeate, which ensured comparable organic and nitrogen levels in the feeds to RO units in the two processes. Although less organic substances and microbial cells were accumulated on the RO membrane fed with AFMBR-zeolite column effluent, its fouling rate (∼6.5 ± 2.2 bar/day) was significantly greater than that fed with anoxic-aerobic MBR permeate (∼1.1 ± 1.5 bar/day). This may be associated with more severe inorganic colloidal fouling on the RO membrane, illustrated by an electrical impedance spectroscopy fouling monitoring system.

Impact of isolated dissolved organic fractions from seawater on biofouling in reverse osmosis (RO) desalination process.

The biofouling potential of three isolated dissolved organic fractions from seawater according to their molecular weights (MWs), namely, fractions of biopolymers (F.BP, MW > 1000 Da), humic substances and building blocks (F.HS&BB, MW 350-1000 Da), and low molecular weight compounds (F.LMW, MW < 350 Da) were characterized by assimilable organic carbon (AOC) content. The AOC/DOC ratio was in the order of F.LMW (∼35%) > F.BP (∼19%) > F.HS&BB (∼8%); AOC/DOC of seawater was ∼20%; organic compositions of seawater were BP ∼6%, HS&BB ∼52% and LMW ∼42%; LMW accounted for >70% of AOC in seawater. Their impact on SWRO biofouling in term of flux decline rate was in the order of F. LMW (∼30%) > F.BP (∼20%) > F.HS&BB (<10%). Despite being the major organic compound in seawater, HS&BB showed marginal effect on biofouling. The role of indigenous BP was less critical owing to its relatively low concentration. LMW, which was the major AOC contributor, played a significant role in biofouling by promoting microbial growth that contributed to the build-up of soluble microbial products and exopolymeric substances (i.e., in particular BP). Therefore, seawater pretreatment shall focus on the removal of AOC (i.e., LMW) rather than the removal of biopolymer.

Biocarriers facilitated gravity-driven membrane (GDM) reactor for wastewater reclamation: Effect of intermittent aeration cycle.

This study investigated the performances of gravity-driven membrane (GDM) reactors integrated with granule activated carbon (GAC) biofilm process for wastewater treatment under different intermittent aeration cycles (intensity and frequency). The results showed the removal efficiencies of dissolved organic carbon, total nitrogen, ammonia were significantly improved under intermittent aeration conditions (~86-87%, ~29-37%, and ~83-99%, respectively) compared to non-aeration condition (~72% and ~18%, and ~17%, respectively). In addition, it was found that the intermittent aeration significantly reduced the cake layer resistance and therefore improved ~130-300% the permeate flux compared to control without aeration. Microbial community analysis indicated that prokaryotic and eukaryotic compositions in the cake layer biofilm were significantly influenced by aeration condition. Lastly, energy consumption analysis revealed that GAC + GDM with shorter aeration period and low aeration intensity could be promising as a decentralized wastewater treatment process in terms of water quality and operating energy.

Fouling behavior of isolated dissolved organic fractions from seawater in reverse osmosis (RO) desalination process.

Organic fouling is still elusive in seawater reverse osmosis (SWRO) desalination process. Classifying organics in seawater will provide an in-depth understanding of the important fraction on RO fouling. In this study, dissolved organic matter (DOM) in seawater was fractionated and concentrated by membrane technique into three major fractions (i.e., biopolymer fraction, humic substance with building block fraction, and low molecular weight fraction) by their molecular weight (MW) according to the definitions in liquid chromatography with organic carbon detection (LC-OCD) method. Overall recovery of >80% was attained. The isolated organic fractions were compared with common model foulants such as sodium alginate (SA), bovine serum albumin (BSA), and humic acid (HA), in terms of chemical analyses using fluorescence-excitation emission matrix (FEEM) and LC-OCD, as well as their fouling potentials. SWRO fouling experiments were carried out and fouling mechanism was investigated by atomic force microscopy (AFM) method and extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. Results showed that initial fouling (i.e., foulant-membrane interaction) was the main driver in SWRO organic fouling with biopolymer fraction as the major contributor followed by low molecular weight fraction. In addition, divalent ions was found to enhance the RO fouling by increasing the adhesion and cohesion forces between foulant-membrane and foulant-foulant.

Quorum quenching in anaerobic membrane bioreactor for fouling control.

Quorum quenching (QQ) is an effective method to control membrane biofouling in aerobic membrane bioreactors (AeMBRs). However, it is not clear if QQ is feasible in an anaerobic membrane bioreactor (AnMBR). In this study, Microbacterium. sp that has QQ capability was embedded in alginate beads, known as QQ beads (QQB), and applied in a lab-scale AnMBR to investigate their potential in fouling control. With the addition of QQB, the operating period of AnMBR-QQB reactor was prolonged by about 8-10 times at constant flux operation before reaching the pre-set maximum transmembrane pressure (TMP). The concentration of Acyl-homoserine lactones (AHLs) in the bulk liquid was significantly higher during the 'TMP jump' period compared to QQB and control phases, while AHLs in the membrane foulants were remarkably lower in QQB phase compared to control phase. Furthermore, a much lower level of soluble microbial production (SMP) was observed in QQB phases. Extracellular polymeric substance (EPS), protein in particular, was reduced by 39.73-80.58% in the cake layer of the membrane from QQB phases. Significant changes of organic functional groups were observed in cake layer from QQB membrane as compared with that from control membrane. At the end of operation, bio-polymer (BP), building blocks (BB) and low molecular weight (LMW) organic matters increased in the foulant from control phases but such increase was not observed in QQB phase. After long-term operation, revival of QQB is required due to the declined activity for AHLs degradation.

A comparison of gravity-driven membrane (GDM) reactor and biofiltration + GDM reactor for seawater reverse osmosis desalination pretreatment.

In this study, permeate quality, membrane performance, and microbial community in a gravity-driven microfiltration (GDM) reactor and a biofiltration (BF) + GDM reactor for seawater reverse osmosis (RO) desalination pretreatment were compared at both lab-scale and pilot-scale. The presence of BF column was more efficient in removing soluble organic substances by biosorption/biodegradation, leading to superior permeate quality from BF + GDM and subsequently lower RO fouling than GDM. Compared to the biofilm-saturated anthracite media, the granular activated carbon media in BF improved the assimilable organic substances removal in BF + GDM. Although less organic substances and microbial cells were accumulated on the membrane in BF + GDM, its permeate flux was 10-20% lower than GDM. Furthermore, BF lowered the amounts and diversity of prokaryotes (due to less organic substances) and eukaryotes (due to BF media rejection and lacking of prokaryotic preys) in the membrane biofilm of BF + GDM, but did not cause significant shifts of predominant species. Thus, the lower flux in BF + GDM was attributed to the limited predation and movement of eukaryotes in membrane biofilm, which may result in the formation of less porous and compact biofilm layer. The cost analysis indicated that BF + GDM-RO requires 5.2% less operating cost and 1.5% less water production cost than GDM-RO.

Recycling rainwater by submerged gravity-driven membrane (GDM) reactors: Effect of hydraulic retention time and periodic backwash.

Rainwater recycling has been considered as an alternative cost-effective decentralized water supply. The low cost and effective gravity-driven membrane (GDM) filtration technology has been introduced to treat the rainwater prior use. In this study, we investigated the effects of hydraulic retention time (HRT; 27 h, 51 h, and 156 h) and periodic backwash durations (2 min, 5 min, 10 min, and 30 min per 2-3 days' filtration) on the permeate quality, flux and fouling mechanism in lab-scale submerged GDM reactors. Compared to the performance at HRT of 51 h (40% of DOC removal and ~2.9 L/m2 h), better permeate quality and higher membrane flux were achieved at HRT of 27 h (51% of DOC removal and ~4.2 L/m2 h) and 156 h (48% of DOC removal and ~5.0 L/m2 h). Although the hydraulically reversible resistance was predominant (up to 90% of the total fouling resistance), the permeate flux could not be fully recovered by periodic backwash, regardless of the backwash durations. After several filtration-backwash cycles, the stabilized flux of GDM reactor with backwash was even worse than those without backwash. However, no correlation can be established between the stabilized flux (i.e., cake layer resistance) and the soluble organics and microbial cells in the cake layer of the GDM system during rainwater treatment.

Online monitoring of transparent exopolymer particles (TEP) by a novel membrane-based spectrophotometric method.

The presence of transparent exopolymer particles (TEP) in water bodies has been related to several adverse impacts in various water treatment processes. In recent years, there have been an increasing number of publications relating to TEP. Unfortunately, this increased interest in TEP measurement has not been accompanied by significant improvement in the analysis method or TEP monitoring. Currently, the most common method to analyze and quantify TEP only allows offline, and often offsite measurement, causing delays and slow response times. This paper introduces an improved method for TEP monitoring using a membrane-based spectrophotometric technique to quantify TEP in various water bodies. The proposed TEP monitor involves a crossflow filtration unit, reagent injection and a spectrophotometer system. The TEP retained on the membrane surface is stained by Alcian blue and the amount deposited is quantified directly using an optic fibre reflectance probe coupled with a spectrophotometer. The novel method shows a linear relationship with various concentrations of Xanthan gum (a model representing TEP). When tested with various water samples, the proposed method was found to correlate well with the conventional method. Several advantages of this novel method are shorter analysis time, increased accuracy, and the potential to be further developed into an online system.

The feasibility of nanofiltration membrane bioreactor (NF-MBR)+reverse osmosis (RO) process for water reclamation: Comparison with ultrafiltration membrane bioreactor (UF-MBR)+RO process.

This study examines the feasibility of a novel nanofiltration membrane bioreactor (NF-MBR) followed by reverse osmosis (RO) process for water reclamation at 90% recovery and using an ultrafiltration MBR (UF-MBR)+RO as baseline for comparison. Both MBRs adopted the same external hollow fiber membrane configurations and operating conditions. The collected permeates of the MBRs were subsequently fed to the respective RO systems. The results showed that the NF-MBR (operated at a constant flux of 10 L/m2h) achieved superior MBR permeate quality due to enhanced biodegradation and high rejection capacity of the NF membrane, leading to lower RO fouling rates (∼3.3 times) as compared to the UF-MBR. Further analysis indicated that the cake layer fouling that caused the cake-enhanced osmotic pressure (CEOP) effect contributed predominantly to the transmembrane pressure (TMP) increase in the NF-MBR, while irreversible pore fouling was the major reason for UF membrane fouling. Furthermore, it was found that the biopolymers (i.e., organics with MW > 10 kDa) were the main components present in the foulants of the NF/UF membranes and RO membranes. The analysis indicated that the NF-MBR + RO system at recovery of 90% has comparable energy consumption as the UF-MBR + RO system at recovery of 75%. Our findings proved the feasibility of the NF-MBR + RO for water reclamation at a high recovery rate.

Improved performance of gravity-driven membrane filtration for seawater pretreatment: Implications of membrane module configuration.

As a low energy and chemical free process, gravity-driven membrane (GDM) filtration has shown a potential for seawater pretreatment in our previous studies. In this study, a pilot submerged GDM reactor (effective volume of 720 L) was operated over 250 days and the permeate flux stabilized at 18.6 ± 1.4 L/m2h at a hydrostatic pressure of 40 mbar. This flux was higher than those in the lab-scale GDM reactor (16.3 ± 0.2 L/m2h; effective volume of 8.4 L) and in the filtration cell system (2.7 ± 0.6 L/m2h; feed side volume of 0.0046 L) when the same flat sheet membrane was used. Interestingly, when the filtration cell was submerged into the GDM reactor, the flux (17.2 L/m2h) was comparable to the submerged membrane module. Analysis of cake layer morphology and foulant properties indicated that a thicker but more porous cake layer with less accumulation of organic substances (biopolymers and humics) contributed to the improved permeate flux. This phenomenon was possibly associated with longer residence time of organic substances and sufficient space for the growth, predation, and movement of the eukaryotes in the GDM reactor. In addition, the permeate flux of the submerged hollow fibre membrane increased with decreasing packing density. It is thought that the movement of large-sized eukaryotes could be limited when the space between hollow fibres was reduced. In terms of pretreatment, the GDM systems effectively removed turbidity, viable cells, and transparent exopolymer particles from the feed seawater. Importantly, extending the reactor operation time produced a permeate with less assimilable organic carbon and biopolymers. Thus, the superior quality of the GDM permeate has the potential to alleviate subsequent reverse osmosis membrane fouling for seawater treatment.

Effects of spacer orientations on the cake formation during membrane fouling: Quantitative analysis based on 3D OCT imaging.

Spacer design plays an important role in improving the performance of membrane processes for water/wastewater treatment. This work focused on a fundamental issue of spacer design, i.e., investigating the effects of spacer orientations on the fouling behavior during a membrane process. A series of fouling experiments with different spacer orientation were carried out to in situ characterize the formation of a cake layer in a spacer unit cell via 3D optical coherence tomography (OCT) imaging. The cake layers formed at different times were digitalized for quantitatively analyzing the variation in the cake morphology as a function of time. In particular, the local deposition rates were evaluated to determine the active regions where the instantaneous changes in deposit thickness were significant. The characterization results indicate that varying the spacer orientation could substantially change the evolution of membrane fouling by particulate foulants and thereby result in a cake layer with various morphologies; the competition between growth and erosion at different locations would instantaneously respond to the micro-hydrodynamic environment that might change with time. This work confirms that the OCT-based characterization method is a powerful tool for exploring novel spacer design.

Physiological Responses of Salinity-Stressed Vibrio sp. and the Effect on the Biofilm Formation on a Nanofiltration Membrane.

This study evaluated the effects of salinity on the physiological characteristics of Vibrio sp. B2 and biofilm formation on nanofiltration (NF) membrane coupons used in the high recovery seawater desalination process. The test conditions were at 0.6, 1.2, and 2.4 M sodium chloride (NaCl), equivalent to salinity of seawater, brine at 50% and 75% water recovery, respectively. High salinity inhibited the cell growth rate but increased the viability and bacterial membrane integrity. In addition, protein and eDNA concentrations of salinity-stressed bacteria were increased at 1.2 and 2.4 M NaCl. In particular, protein concentration was linearly correlated with the NaCl concentration. Similarly, less biofilm formation on the NF membrane coupon (without permeation flux) was observed by the salinity-stressed bacteria; however, the production of extracellular polymeric substances (EPS) was significantly increased as compared to control, and protein was an influential factor for biofilm formation. This study shows that salinity-stressed bacteria have a high potential to cause biofouling on membrane surface as the bacteria still maintain the cell activity and overproduce EPS. The potential of biofilm formation by the salinity-stressed bacteria has not been reported. Therefore, the findings are important to understand the mechanisms of membrane biofouling in a high salinity environment.

Analyzing the Evolution of Membrane Fouling via a Novel Method Based on 3D Optical Coherence Tomography Imaging.

The development of novel tools for studying the fouling behavior during membrane processes is critical. This work explored optical coherence tomography (OCT) to quantitatively interpret the formation of a cake layer during a membrane process; the quantitative analysis was based on a novel image processing method that was able to precisely resolve the 3D structure of the cake layer on a micrometer scale. Fouling experiments were carried out with foulants having different physicochemical characteristics (silica nanoparticles and bentonite particles). The cake layers formed at a series of times were digitalized using the OCT-based characterization. The specific deposit (cake volume/membrane surface area) and surface coverage were evaluated as a function of time, which for the first time provided direct experimental evidence for the transition of various fouling mechanisms. Axial stripes were observed in the grayscale plots showing the deposit distribution in the scanned area; this interesting observation was in agreement with the instability analysis that correlated the polarized particle groups with the small disturbances in the boundary layer. This work confirms that the OCT-based characterization is able to provide deep insights into membrane fouling processes and offers a powerful tool for exploring membrane processes with enhanced performance.

Dynamics of biofilm formation under different nutrient levels and the effect on biofouling of a reverse osmosis membrane system.

Pseudomonas aeruginosa PAO1 wild type and a mucoid derivative (FRD1) which over produces alginate were used to foul reverse osmosis (RO) membranes. When operated at a constant flux, biofilm formation on the RO membrane resulted in a slow rise in transmembrane pressure (TMP) of 22% for the initial four days of operation, followed by a sharp increase of 159% over the following two days. The initial slow increase in TMP was probably due to the formation of a biofilm on the membrane surface, which then accelerated the rate of biofouling through the effect of concentration polarization. At later stages of operation, most of the bacterial biomass consisted of dead cells. The amount of extracellular polymeric substances appeared to correlate positively with the number of dead cells. The results indicate that prolonging the initial stage of slow TMP increase and avoiding the latter stage of accelerated TMP increase would provide a sustainable operation of the RO system. These results suggest that nutrient limitation could reduce biofilm accumulation and delay the increase in TMP.