Biofilm impacts on water quality in drinking water distribution systems

• Main Author: Shi, Yi
• Published: Cardiff University 2018
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.742894

Drinking water distribution systems (DWDSs) account for the majority of the infrastructure for transporting water from treatment plants to customers’ tap. During the transportation, water quality deteriorates due to the unavoidable accumulation of biofilm within the pipelines. The microbial activity and ecology within the biofilm have great impact on the water quality degradation process. Within DWDSs using chloramine as disinfectant, nitrification caused by nitrifying bacteria is increasingly becoming a concern as it poses a great challenge for maintaining water quality. In order to control nitrification in DWDSs, it is essential to consider both the nitrifying bacteria and their shelter. Hence, the overall aim of this study is to investigate nitrification properties under different operational conditions, in addition to biofilm characteristics in chloraminated water distribution systems. To achieve the aim, nitrifying biofilm was firstly incubated within a flow cell experimental facility. A total of four test phases were conducted to investigate the effects on the extent of nitrification of five flow rates (2, 4, 6, 8 and10 L/min) and four disinfection strategies (total chlorine=1mg/L, Cl2/NH3=3:1; total chlorine=1mg/L, Cl2/NH3=5:1; total chlorine=5mg/L, Cl2/NH3=3:1; and total chlorine=5mg/L, Cl2/NH3=5:1). Physico-chemical parameters and nitrification indicators were monitored during the tests. The main results from the study indicate that nitrification is affected by hydraulic conditions and the process tends to be severe when the fluid flow transforms from laminar to turbulent (2300 < Reynold number < 4000). Increasing disinfectant concentration and optimizing Cl2/NH3 mass ratio were found to have limited efficacy for controlling nitrification. Furthermore, several nitrification indicators were evaluated for their prediction efficiency and the results suggest that the change of nitrite, together with total organic carbon (TOC) and turbidity can indicate nitrification potential more efficiently. At the end of the tests, genomic DNA from biofilm and bulk water from each flow cell unit running at different operational conditions were subjected to a next generation sequencing (NGS) analysis by Illumina MiSeq. The results obtained showed that the microbial community and structure was different between biofilm and water samples. There was no statistical difference in microbial community in biofilm identified between different hydraulic regimes, suggesting that biofilm is a stable matrix to environment. Results further showed that Cl2/NH3 mass ratio had obvious effect on microbial structure in biofilm. This suggests that excessive ammonia is an influencing factor for microbial activity within biofilm. Within bulk water, species richness and diversity tended to be higher at lower hydraulic regimes. This confirms the influence of hydraulic condition on biofilm mechanical structure and further material mobilization to water. Opportunic pathogens such as Legionella and Mycobacterium were detected in abundance in the experimental system. This confirms that nitrification can lead to a decrease of water quality and microbial outbreaks. The characteristics of extracellular polymeric substance (EPS) from biofilm conditioned under different operational conditions were also analysed. Carbohydrate was found to be the main components within biofilm’s EPS. EPS composition and structure were found to be governed by operational conditions, but no simple linear relationship was found. This suggests the interactive effects of EPS properties, hydraulics and disinfectant strategies. EPS effects on disinfection were evaluated via disinfectant decay tests. EPS was confirmed to have an influencing in biofilm overcoming disinfection.