Flow-through electrode system (FES): An effective approach for biofouling control of reverse osmosis membranes for municipal wastewater reclamation.

Emerging electrochemical disinfection techniques provide a promising pathway to the biofouling control of reverse osmosis (RO) process. However, the comparative effectiveness and mechanism of it under flow-through conditions with low voltage remains unclear. This study investigated the effect of a flow-through electrode system (FES) with both direct current (DC) and alternating pulse current (AC) on RO biofouling control compared with chlorine disinfection. At the initial stage of biofouling development, the normalized flux of AC-FES (67% on Day 5) was saliently higher than the control group (56% on Day 5). Subsequently, the normalized fluxes of each group tended similarity in their differences until the 20th day. After mild chemical cleaning, the RO membrane in the AC-FES group reached the highest chemical cleaning efficiency of 58%, implying its foulant was more readily removable and the biofouling was more reversible. The biofouling layer in the DC-FES group was also found to be easily cleanable. Morphological analysis suggested that the thickness and compactness of the fouling layers were the major reasons for the fouling behavior difference. The abundance of 4 fouling-related abundant genera (>1%), which were Pseudomonas, Thiobacillus, Sphingopyxis, and Mycobacterium exhibited a salient correlation with the biofouling degree. The operating cost of FES was also lower than that of chlorine disinfection. In summary, AC-FES is a promising alternative to chlorine disinfection in RO biofouling control, as it caused less and easy-cleaning biofouling layer mainly due to two advantages: a) reducing the regrowth potential after disinfection of the bacteria, leading to alleviated initial fouling, (b) reshaping the microbial community to those with weaker biofilm formation capacity.

Comparison of the reverse osmosis membrane fouling behaviors of different types of water samples by modeling the flux change over time.

Fouling of RO membranes has long been a complex but inevitable problem in wastewater reclamation. In this study, a modified intermediate blocking model with two parameters was applied to describe the flux change of RO membranes treating various water samples, including municipal secondary effluent, treated industrial wastewater, surface water, and groundwater. The model was validated by 55 sets of data reported by 13 articles, and the results were promising, with 90% of the determination coefficient (R2) exceeding 0.90. Relatively large flux and high operational pressure were found likely to aggravate membrane fouling. Treated industrial wastewater had the highest fouling potential (fouling constant k: 0.061-2.433) compared to municipal wastewater secondary effluent, surface water, and groundwater, even with similar dissolved organic carbon concentration. With industrial wastewater excluded, water samples exhibited lower fouling potential than organic matter solutions, with the majority (25%∼75%) of k distributing in 0.03-0.12, much lower compared to the major k range of the latter (0.05-0.28). This suggested a deviation in fouling behaviors between model organic matters and real water samples. Xanthan gum and guar gum were proposed to be model polysaccharides based on their model parameters, which were relatively close to real water samples.

Comparison of disinfection-residual-bacteria (DRB) after seven different kinds of disinfection: Biofilm formation, membrane fouling and mechanisms.

Membrane fouling is the Achilles' heel of the reverse osmosis (RO) system for high-quality reclaimed water production. Previous studies have found that after the significant selection effect of traditional disinfection, the remaining disinfection-residual bacteria (DRB) may possess more severe biofouling potentials. To provide more constructive advice for the prevention of biofouling, we compared the RO membrane fouling characteristics of DRB after using five commonly used disinfection methods (NaClO, NH2Cl, ClO2, UV, and O3) and two novel disinfection methods (K2FeO4 and the flow-through electrode system (FES)). Compared with the control group (undisinfected, 21.1 % flux drop), the UV-DRB biofilm aggravated biofouling of the RO membrane (23.4 % flux drop), while the FES, K2FeO4, and NH2Cl treatments showed less severe biofouling, with final flux drops of 6.9 %, 8.1 %, and 8.1 %, respectively. Adenosine triphosphate (ATP) was found to be a capable indicator for predicting the biofouling potential of DRB. Systematic analysis showed that the thickness and density of the DRB biofilms were most closely related to the different fouling degree of RO membranes. Moreover, the relative abundance of bacteria with higher extracellular polymeric substance (EPS) secretion levels, such as Pseudomonas and Sphingomonas, was found closely related with the biofouling degree of RO membranes.

Pretreatment for alleviation of RO membrane fouling in dyeing wastewater reclamation.

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.

Metagenomics analysis of the key functional genes related to biofouling aggravation of reverse osmosis membranes after chlorine disinfection.

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-Cl2/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-Cl2/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.

Effect of ultraviolet disinfection on the fouling of reverse osmosis membranes for municipal wastewater reclamation.

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.