Impact of Drinking Water Treatment and Pipe Biofilms on Bacterial Dynamics in the Distribution System

Abstract: This thesis addresses drinking water quality and microbiology in full-scale drinking water distribution systems (DWDSs). It examines how UV irradiation and slow sand filters (SSFs) alter the water bacteriome, and how the biofilm in the DWDS affects the drinking water quality.In addition, the effects of installing a combined ultra­filtration and coagulation treatment stage on the pipe biofilm com­mun­ity in a DWDS were assessed.PCR-based methods were assessed and developed to be able to monitor the effects of UV irradiation. The impact of UV doses of 250, 400, and 600 J/m2, delivered to water at a full-scale drinking water treatment plant (DWTP), was investigated using 16S rRNA gene amplicon sequenc­ing. Phylogenetic analysis, including differential abundance analysis using DESeq2, showed that Actinobacteria were more resis­tant to UV irradiation, whereas Bacteroidetes were sensitive to UV irradiation. Amplicon sequence variants (ASVs) resistant to UV had a greater average guanine-cytosine(GC) content than ASVs sensitive to UV irradiation: 55% ± 1.7 (n = 19) vs. 49% ± 2.5 (n = 16), respectively. UV irradiation may affect the microbial dynamics and the biostability throughout the DWDS, as the composition of a bacterial community in irradiated water stored for 6 days at 7 °C to approximate conditions in the DWDS, changed compared to the non-irradiated controls.Full-scale SSFs were studied using flow cytometry (FCM) and cytometric histogram image comparison (CHIC) analysis. An established, well-functioning SSF removed coliforms and Escherichia coli, and reduced the pH and the amount of total organic carbon, even when the schmutzdecke of the SSFs was removed. This was in contrast to two new filters, which showed compromised performance, including breakthrough of coliforms and E. coli. FCM analysis showed that well-functioning SSFs changed the microbial community of the influent water to include more low nucleic acid (LNA) bacteria in the filter effluent. The SSF with a mixture of new sand plus sand from established SSFs on top exhibited better performance than a SSF with new sand, indicating that priming with sand from established SSFs may be favorable when constructing new SSFs. Monitoring the SSFs with FCM and CHIC analysis was demonstrated to be a fast, reliable and informative method of monitoring the bacterial community in water.An ultrafiltration and coagulation step was installed at a DWTP (hereafter defined as UF start). This removed almost all the bacteria from the finished water, and reduced the total cell concentration (TCC) in the distributed water from 6.0 × 105 (± 2.3 × 105) cells/mL to 6.0 × 103 (± 8.3 × 103) cells/mL, taking seasonal variations into account. After the UF start, almost all the bacteria in the drinking water leaving the DWDS originated from the pipe biofilm, although no significant biofilm detachment was observed. The removal of cells by UF allowed the identification of the bacteria released from the mature pipe biofilm, which included Sphingomonas, Nitrospira, Mycobacterium, and Hyphomicrobium. The biofilms of excavated pipe sections were analyzed over a period of 27 months in order to study how the biofilm adapted to the new UF water quality. It was observed that the bacterial community was dominated by Nitrosomonadaceae, Nitrospira, Hyphomicrobium and Sphingo­monas, confirming the previous results. DNA sequences classified as belonging to the opportunistic pathogens Mycobacterium and Legionella were also detected in the pipe biofilms. The high relative abundance of the nitrifying bacteria Nitro­somonadaceae and Nitrospira, together with the fact that the turnover of nitrogen compounds was unchanged by UF start indicated that nitrification in the DWDS was localized to the pipe biofilm. The bacterial community on the pipes changed following UF start and a stable community was reached after 18 months, while still maintaining the turnover of nitrogen compounds. The bacteria leaving the biofilm after a shorter residence time (<25 h) were high nucleic acid (HNA) bacteria, and a shift to an increased relative abundance of LNA bacteria was observed with longer residence times of up to about 170 h.