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Reverse osmosis (RO) is widely used in wastewater reclamation to alleviate the increasingly global water shortage. However, it has an inconvenient defect of biofouling. Some disinfection processes have been reported to select certain undesirable disinfection-residual bacteria (DRB), leading to severe long-term biofouling potential. To provide constructive guidance on biofouling prevention in RO systems, this study performed a 32-day experiment to parallelly compared the biofouling characteristics of RO membranes of DRB after five mature water disinfection methods (NaClO, NH2Cl, ClO2, UV, and O3) and two recently developed water disinfection methods (K2FeO4 and flow-through electrode system). As a result, the DRB biofilm of K2FeO4 and O3 caused a slight normalised flux drop (22.4 ± 2.4% and 23.9 ± 1.7%) of RO membrane compared to the control group (non-disinfected, ~27% normalised flux drop). FES, UV, NaClO and ClO2 caused aggravated membrane flux drop (29.1 ± 0.3%, 33.3 ± 7.8%, 34.6 ± 6.4%, and 35.5 ± 4.0%, respectively). The biofouling behaviour showed no relationship with bacterial concentration or metabolic activity (p > 0.05). The thickness and compactness of the biofilms and the organics/bacterial number ratio in the biofilm, helped explain the difference in the fouling degree between each group. Moreover, microbial community analysis showed that the relative abundance of typical highly EPS-secretory and biofouling-related genera, such as Pseudomonas, Sphingomonas, Acinetobacter, Methylobacterium, Sphingobium, and Ralstonia, were the main reasons for the high EPS secreting ability of the total bacteria, resulting in aggravation of biofouling degree (p < 0.05). All types of disinfection except for NaClO and ClO2 effectively prevented pathogen reproduction in the DRB biofilm. [ABSTRACT FROM AUTHOR]

• How EPS function in RO membrane biofouling is shown for the first time. • EPS in bio-foulants facilitate the adhesion of bacteria to membranes and each other. • High compactness and firm adhesion of bio-foulant indicate severe biofouling. • High EPS content and MW indicate high interaction energy and severe biofouling. The interaction of bio-foulants at the interface of reverse osmosis (RO) membranes is highly correlated with the degree of fouling during wastewater treatment. However, there are few observations showing the micro/nanoscale bio-foulant morphology at the interface and its relationship with the degree of fouling. In this study, two typical bacterial strains (Escherichia coli CGMCC1.3373 and Bacillus cereus CR19) present in reclaimed water were used to test their fouling potential and reveal their interactions with RO membranes. B. cereus , a typical chlorine-resistant bacterium, exhibits significantly higher extracellular polymeric substance (EPS) content and higher interaction energy with membranes. The rate of decrease in membrane flux in the B. cereus group was 1.5 times higher than that in the E. coli group. By rapidly freezing the bio-foulants and examining the micro/nanoscale morphology around their interface with RO membranes, it was revealed that the membrane was fouled by stacked bacterial cells with EPS adhering to each other and to the RO membrane surface. B. cereus secreted more EPS, stacked more compactly, and adhered to the RO membranes more firmly than E. coli , resulting in more severe fouling. To our knowledge, this is the first study to demonstrate that the micro/nanoscale morphology of the interface between bio-foulants and RO membranes indicates the degree of fouling. [ABSTRACT FROM AUTHOR]

Adsorption and coagulation were commonly used to alleviate reverse osmosis (RO) membrane fouling caused by dissolved organic matters (DOM), but the effects of changed composition and structure of DOM in dyeing wastewater after adsorption and coagulation on RO membrane fouling have seldom been studied. This study aimed at resolving the mechanism how the RO membrane fouling during dyeing wastewater treatment was alleviated by using adsorption and coagulation. The dyeing wastewater caused serious RO membrane fouling. Pretreatment with granular activated carbon (GAC), polyferric sulfate (PFS) and polyaluminum chloride (PACl) were conducted. It was shown that GAC could remove most of the DOM (95%) and preferred to adsorb protein, hydrophobic neutrals and fluorescent compounds. Both coagulants of PFS and PACl preferred to remove polysaccharides (the removal rate was 9–19% higher than that of DOM), high-MW compounds and these compounds with high fouling potential. Afterwards, the RO membrane fouling potential of the dyeing wastewater was tested. The GAC and PFS performed well to alleviate fouling. After GAC treatment, the decline rate of RO flux was similar to that of raw wastewater after 6-fold dilution. With pretreatment by PFS or PACl, the fouling potential of dyeing wastewater was much lower than that of raw wastewater after diluted to the same DOM content. Changes in polysaccharides content in the DOM had more effects on RO membrane fouling than that of proteins after these pretreatment. Although the DOM changed significantly after pretreatment, the fouling type was still intermediate blocking. [Display omitted] • Activated carbon preferred to remove fluorescent compounds and low-MW DOM. • Coagulants of PFS and PACl preferred to remove polysaccharides and high-MW DOM. • PFS and PAC preferred to remove compounds with high fouling potential. • PFS and GAC effectively alleviated RO membrane fouling caused by dyeing wastewater. • Polysaccharides had more negative effects on RO membrane fouling than proteins. [ABSTRACT FROM AUTHOR]

Extracellular polymeric substances (EPS) were the main cause of membrane biofouling in advanced treatment of wastewater. Disinfection was usually used to reduce microbial concentration in the feed water and prevent biofouling. However, the disinfection residual bacteria (DRB) could regrow to produce EPS, and there was still a paucity of information on EPS productivity and properties from DRB and their further interaction with membranes. In this study, three commonly-used disinfection methods (free chlorine, chloramine and ozone disinfection) were used to investigate their effects on microbial EPS production and the further effects on membrane fouling. It was found all the disinfection treatment increased the relative abundance of typical bacterial genus with high EPS production ability, and thus significantly enhanced EPS volumetric productivity (by 26%–82%). The molecular weight and fluorescence intensity of EPS produced by DRB also increased. The fouling potential of these EPS on ultrafiltration (UF) membranes and reverse osmosis (RO) membranes was then tested. EPS of DRB showed significantly higher fouling potential. Compared with the control group, the time for transmembrane pressure (TMP) increased to 3 bar decreased by 43%–49% in the constant-flow UF system in the disinfection groups. As for the constant-pressure RO systems, the final flux decreased by 33%–41% in the disinfection groups compared with the control group. The thickness of foulants on UF and RO membranes in the disinfection groups was all higher than that in the control group. The extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory further showed that the EPS of DRB had significantly higher interaction energy with membranes than that in the control group. Therefore, caution should be exercised when oxidizing disinfection was used as pre-treatment for membrane filtration in the wastewater reclamation process. [Display omitted] • Disinfection by chlorine, chloramine and ozone enhanced EPS productivity by 26%–82%. • Disinfection enhanced the molecular weight and fluorescence intensity of EPS. • EPS of disinfection residual bacteria showed higher UF/RO membrane fouling potential. • In disinfection group, the thickness of foulants on membranes significantly increased. • The interaction energy between EPS and membrane was much higher in disinfection group. [ABSTRACT FROM AUTHOR]

Soluble microbial products (SMP) are one of the main causes of reverse osmosis (RO) membrane fouling in wastewater reclamation, and coagulation and adsorption are commonly used to prevent such fouling. However, the mechanism by which typical coagulants and adsorbents affect the RO membrane fouling caused by SMP remains unclear. In this study, two bacterial strains isolated from fouled RO membranes in a full-scale wastewater reclamation plant were used to produce SMP. Both bacterial strains were found to generate high SMP yield (0.23–0.25 g/g), which caused severe RO membrane fouling. Coagulation by polyaluminum chloride (PACl) and ferric chloride (FeCl 3) and adsorption by granular activated carbon (GAC) were applied as pretreatment methods to alleviate the membrane fouling caused by SMP. Compared with PACl, FeCl 3 performed better in removing SMP, decreasing fluorescence intensity, and reducing molecular weight (MW). GAC preferentially adsorbed protein and humic substances in SMP and unselectively and efficiently removed fluorescent compounds; however, it could only remove a limited amount of the high-MW compound. The RO membrane fouling potential of treated SMP was investigated, and FeCl 3 and GAC were found to effectively alleviate the fouling. In addition to a decrease in the dissolved organic carbon content of SMP, a decrease in specific fluorescence intensity, MW, and polysaccharide content caused by coagulation or adsorption could result in lower RO membrane fouling. [Display omitted] • Bacteria on RO membranes had high SMP yield and the SMP severely fouled RO membranes. • FeCl 3 performed better in SMP removal than PAC. • Activated carbon prefers to remove fluorescent compounds and low-MW SMP. • FeCl 3 and GAC could effectively alleviate RO membrane fouling caused by SMP. • Reducing polysaccharide content of SMP significantly alleviate RO membrane fouling. [ABSTRACT FROM AUTHOR]

• Five typical CRB were isolated from RO membranes in pilot and full-scale systems. • Fouling potential of all the strains increased monotonically with chlorine resistance. • EPS was decisive factor to chlorine resistance and fouling potential of CRB. • CFD simulation demonstrated the contribution of EPS to filtration resistance. • Membrane adhesion potential and fouling potential of CRB were relatively independent. Chlorine disinfection is often used as a pretreatment technology to control biofouling of reverse osmosis (RO) membranes. However, previous studies showed that biofouling of the RO system was aggravated after chlorine disinfection. Chlorine-resistant bacteria (CRB) were presumed to be closely related to the aggravation of fouling caused by chlorine disinfection. In order to analyze the membrane fouling mechanisms of CRB, 5 CRB strains were isolated from the surface of fouled RO membranes for wastewater reclamation, and 3 reference bacterial strains, Sphingopyxis soli BM1–1, Pseudomonas aeruginosa PAO1 and Escherichia coli CGMCC1.3373, were selected for comparative study. The chlorine resistance, membrane fouling potential, secretion and adhesion characteristics of these strains were evaluated. Among these isolated strains, 3 strains showed much higher chlorine resistance than PAO1 under the condition of 0.5, 2, 5 mg/L-Cl 2 , especially Bacillus CR19 and Bacillus CR2. Furthermore, a significant positive correlation was found between membrane fouling potential and chlorine resistance of all the strains in this study. The membrane fouling potential of the above 8 strains increased monotonically with the increase of chlorine resistance (under the condition of 0.5 mg/L-Cl 2). Serious fouling caused by extracellular substances was observed in biofouling layers of the strains with high chlorine resistance, which lead to more severe flux decline. Extracellular polymeric substances (EPS) amount per cell was found to be the main factor related to the chlorine resistance as well as the fouling potential. Computational fluid dynamics (CFD) simulation was used to demonstrate the filtration resistance induced by the secretion of EPS. However, CRB with higher EPS amount may not show higher membrane adhesion potential, and thus may not be the dominant strain on the RO membranes before chlorine disinfection. These CRB with high fouling potential but low membrane adhesion potential, such as Bacillus CR19 and Bacillus CR2, may become the dominant bacteria on the membrane surface after chlorine disinfection and thus aggravate membrane fouling significantly. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Pressure-time curves were classified according to curve shape and mechanisms. • New constant-flux RO pressure–time model was developed: the Integrated SDC Model. • Integrated SDC Model showed better fitting performance than previous models. • The proposed model could indicate the fouling tendency of a full-scale RO system. • Integrated SDC Model could predict pressure growth of RO systems in NEWater plant. Reverse osmosis membranes have been used to produce high-quality reclaimed water for portable and industrial use, but face inevitable fouling problems. In the constant flux RO membrane operation, fouling is manifested as increase in operational pressure. However, the pressure–time curves have no consistent form, and there's a lack of pressure–time models that is capable of fitting various shapes of curves. In this study, the pressure–time curves of RO membranes were summarized into 3 types according to their change in pressure increasing rate. Each type of curve was speculated to be caused by a specific density change pattern of the fouling layer. An Integrated Spherical Diffusion Cell (SDC) Model was developed for the pressure–time relationships of Type I curves (pressure increasing rate increases with time) and Type III curves (pressure increasing rate decreases with time). Time-course pressure data of Type I and III curves were collected from literature, and the Integrated SDC model fitted these data better than other models derived from literature. Over 70% of R2 exceeded 0.90 for Type I curves and over 85% of R2 exceeded 0.88 for Type III curves. Furthermore, the Integrated SDC Model also showed superior fitting performance to the operational data of the full-scale RO system in the Changi NEWater plant. The model could characterize the fouling tendency by the model parameters, and predict subsequent pressure growth with the pressure data within 20% increase compared to the original pressure. For 10 out of 14 sets of data, the relative error of the predicted final pressure to the actual final pressure was below 10%. The findings are well-applied to industrial practice and may enable further research in the pressure growth pattern of constant flux RO filtration, assuring stable operation of full-scale RO systems. [ABSTRACT FROM AUTHOR]

Chlorine disinfection is a common technology to control biofouling in the pretreatment of the reverse osmosis (RO) system for wastewater reclamation. However, chlorine disinfection could even aggravate the RO membrane biofouling because of the changes of microbial community structure. In this study, the mechanism of biofilm formation and EPS secretion after chlorine disinfection was investigated by analyzing the genes coding quorum sensing, exopolysaccharide biosynthesis, and amino acid biosynthesis. After 1, 5, and 15 mg-Cl 2 /L chlorine disinfection, the relative abundances of the functional genes all increased significantly. Compared with the control group, chlorine-resistant bacteria (Acidovorax , Arenimonas , and Pseudomonas) also harbored higher relative abundances of these functional genes. The high relative abundances of these genes might provide the bacterial community after chlorine disinfection with high potential of biofilm formation and EPS secretion and then cause severe RO membrane biofouling. In the sample with 5 mg-Cl 2 /L chlorine disinfection, the correlation coefficients (r) between each two of the three kinds of functional genes were more than 0.9 and much stronger than that in the control group. These results indicated that the bacterial community selected by chlorine disinfection could build more stable biofilm to resist chlorine but also could cause more severe RO membrane biofouling. [Display omitted] • Chlorine disinfection increased genes coding biofilm formation and EPS biosynthesis. • Percentages of functional genes harbored by different genera were changed by chlorine. • Chlorine-resistant bacteria harbored high abundance of genes related to biofouling. • Chlorine selected bacteria with more functional genes related to biofouling. • Strong correlation between quorum sensing and EPS biosynthesis was observed. [ABSTRACT FROM AUTHOR]

• Pretreatment by high dosage of UV aggravated RO membrane fouling. • The organic matter amount in the foulants increased after UV disinfection. • The content of proteins and polysaccharides in foulants increased with high UV dosage. • Typical UV-resistant bacteria were selected in the foulants with high UV dosage. • Residual bacteria after UV disinfection secreted more amounts of EPS. Membrane fouling is a prominent problem that hinders the stable and efficient operation of the reverse osmosis (RO) system for wastewater reclamation. Previous studies showed that chlorine disinfection, which was commonly used in industrial RO systems as pretreatment, could lead to significant change in microbial community structure and resulted in serious biofouling. In order to prevent biofouling during wastewater reclamation, the effect of ultraviolet (UV) disinfection on RO membrane fouling was investigated and the mechanism was also revealed in this study. With the disinfection pretreatment by UV of 20, 40 and 80 mJ/cm2, the bacteria in the feed water were inactivated significantly with a log reduction of 1.11, 2.55 and 3.61-log, respectively. However, RO membrane fouling aggravated with higher UV dosage. Especially, in the group with the UV dosage of 80 mJ/cm2, the normalized RO membrane flux decreased by 15% compared with the control group after 19-day operation. The morphology of the fouled RO membranes indicated serious biofouling in all groups. The analysis on the microbial amount of the foulants showed that the heterotrophic plate counts (HPC) and ATP content on the fouled RO membranes with and without UV disinfection were at the same level. However, the total organic carbon content of the foulants with the UV dosage of 40 and 80 mJ/cm2 was significantly higher than the control group, with higher content of proteins and polysaccharides as indicated by EEM and FTIR spectrum. Microbial community structure analysis showed that some typical UV-resistant bacteria were selected and remained on the RO membrane after disinfection with high UV dosage, including. These residual bacteria after disinfection with high UV dosage showed higher extracellular polymeric substances (EPS) secretion compared with those without UV disinfection, and thus aggravated RO membrane fouling. Thicker EPS could decrease the transmission of UV rays, and thus bacteria with higher EPS secretion might be selected after UV disinfection. [Display omitted] [ABSTRACT FROM AUTHOR]