Book Description

Since biofilm has been implicated in the deterioration water quality and the increase of public health risks, various efforts have been made to minimize biofilm regrowth in drinking water distribution systems. Although traditional water treatment processes can greatly remove a large fraction of disinfection by-products (DBPs) precursors, a small portion of natural organic matter (NOM) may still enter water distribution systems. Untreated NOM can serve as nutrients for biofilm growth while also consuming maintained disinfection residuals, which can result in microbial contamination in drinking water. To suppress biofilm formation, water utilities maintain disinfectant residuals for the distribution system. However, upon disinfectant addition, toxic DBPs are inevitably produced. Biofilm and its secreted extracellular polymeric substances (EPS) produce toxic DBPs, due to the very similar chemical composition compared to traditional investigated DBP precursors. This research investigated the role of biofilm on DBP formation and decay in simulated drinking water distribution systems with four objectives. The first objective was to investigate the influence of chemical composition and quantity of bacterial EPS on the biosorption of NOM in drinking water. Results indicated that both protein and polysaccharide based EPS adsorbed existing NOM. Biosorption capacity was mainly determined by divalent ion (Ca2+ and Mg2+) concentrations. Mechanistically, the presence of a diffuse electrical double layer inhibited NOM biosorption by potential energy barriers, however, presence of divalent ions in the aquatic environment enhanced biosorption processes, permitting functional group interactions between EPS and NOM. In addition, hydrophobic interactions, EPS characteristics and quantity can also be used to explain biosorption results. Bridging between hydrophilic carboxyl groups on alginate EPS and NOM appeared to be the dominant form of biosorption, while hydrophobic interactions enhanced biosorption for protein-based EPS. The second and third objectives of this study were to investigate the role of biofilm EPS on the formation of both carbonaceous DBPs (C-DBPs) and nitrogenous DBPs (N-DBPs). DBP yield (formation potential) tests of both bacterial culture and extracted EPS indicated that the chemical composition and quality of EPS played a critical role for DBP formation. In general, protein based EPS possessed higher DBP yields compared to polysaccharide based EPS, especially for N-DBPs. To further determine the relative contribution of each biomolecule in EPS to DBP formation and speciation, detailed chemical compositions of biomolecules in EPS (amino acids, polysaccharide monomers, and fatty acids) from both pure culture and mixed species biofilm isolated from a water utility were analyzed. DBP yield results from both extracted EPS and EPS surrogates (amino acids and polysaccharide monomers) indicated that proteins in EPS have a greater impact on DBP formation, where amino acids containing unsaturated organic carbon or conjugated bonds in R-group produced higher amount of DBPs. However, DBP yields of polysaccharide monomers were lower than those of tested amino acids groups and the DBP yields were not significantly influenced by their chemical structures. The last objective of this study was to understand the influence of biofilm on DBP formation and decay in a simulated water distribution system using lab scale annular reactors. For Cl2 disinfection at 0.5 mg L-1 Cl2 residual concentration, no obvious DBP formation was observed. This was mainly due to the combination of low DBP formation, DBP volatilization, and biodegradation. However, when high Cl2 residuals were maintained, the formations of both C-DBPs and N-DBPs increased dramatically beyond the DBP formation potential of the feed solution. This suggests higher Cl2 residual not only reacted with humic acid (HA) in feed solution but also reacted with biofilm and produced extra DBPs, especially the high formation of N-DBPs (haloacetonitriles). For NH2Cl disinfection, the DBP levels were much lower than those of Cl2 disinfection and differences in DBP formation were not significant under different NH2Cl residual concentrations. Combined results suggested that biofilm can impact both C-DBP and N-DBP formation and decay in water distribution systems, where biomolecules in EPS affect DBP speciation.