Polymeric Substances and Microbial Communities in Source Water and in Biofiltration Processes in the Treatment for Drinking Water
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The application of membrane filtration for drinking water treatment is limited by membrane fouling due to the accumulation of biopolymers. Biofiltration may be an effective pretreatment approach to reduce the biopolymers, thus improving the performance of membrane filtration. Biological treatment with the microbial community associated with drinking water treatment systems can potentially play a positive role on chemical contaminant removal and biopolymer reduction through biodegradation. However, extracellular polymeric substances (EPS) developed inside biofilters through microbial secretions may have adverse effects on downstream membrane filtration if an EPS sloughing event occurs. In this study, the efficacy of biofiltration to remove biopolymers was investigated. Methods developed for the analyses of EPS associated with microbial flocs and biofilms in wastewater were adapted to analyze surface waters where biopolymers are typically found at lower concentrations. Freeze-drying was found to be an effective method for concentrating water samples and to recover and analyze neutral and acidic polysaccharides (PS), but was ineffective for protein (PN) quantification. EPS extracted from the biofilter media was analyzed as part of the characterization of the microbial community associated with biofiltration. Results of water samples suggest that a passive biofilter was not effective for PS removal at the Peterborough drinking water pilot plant (DWPP). However, coupled with a roughing filter, a consistent removal of PS was observed in the passive biofilter at the Mannheim DWPP. Nutrient addition, hydrogen peroxide supplementation, inline coagulation and GAC were found to have impacts on PS removal and total EPS. Transparent exopolymer particles (TEP) are a group of acidic PS, potentially playing an important role in membrane fouling. A significant reduction (over 20%) of TEP was observed in the passive biofilter at the Mannheim DWPP, suggesting the positive role of biofilter in removing TEP. Subsequently, microbial community analysis using denaturing gradient gel electrophoresis (DGGE) and 16S rRNA sequencing have been undertaken to understand the microbial stability of biofiltration system by studying the microbial community association between source water and biofilter systems. High similarities of microbial communities (30-65%) based on DGGE analysis were found between source waters and biofilters, suggesting microbial community shift inside biofilters are due to the changes in the microbial community of source water. Biofilters do not appear to develop into stable compartmentalized communities. It appears these are subject to fluctuation or shifts linked to the conditions of the source water. Therefore, fluctuation of biofilter performance may be expected.