Context. Understanding how chlorination impacts bacteria in the drinking water distribution system (DWDS) and biofilm can support decision making and microbial risk assess.
Gap. A number of studies compare between different DWDS with different chlorination regimes. At full scale, however, the number of variables can complicate interpretation of the impact of chlorination specifically on microbial communities. This study looks at the bacterial community in one DWDS, before and after removal of monochloramine. As the water in this system is prepared using ultrafiltration, all cells originate in the biofilm within the DWDS.
Aim. To describe the microbial implications of removing monochloramine disinfection from a full scale DWDS, including species level information.
Methods. Bacteria were harvested by filtration and DNA extracted from water in a DWDS in Varberg, Sweden. As drinking water production includes ultrafiltration, all cells sampled originated from DWDS biofilm. Sampling was conducted for 15 months at 6 locations. 16S SSU ribosomal DNA amplicons of the v3-v4 region were sequenced and pre-processed using standard pipelines. Differential abundance analysis was performed using ALDEx2 to minimize the compositional effect of varying total abundance.
Findings. Changes were most dramatic in regions of the DWDS originally having a measurable chlorine residual. Abundance of taxa Nitrospira, Nitrosomonas, Legionella, Pseudomonas, Mycobacteria and Ferribacterium decreased following removal of monochloramine (Fig 1 B-E). Similar to previous findings, Nitrosomonas abudance was greater upstream with Nitrospira more abundant downstream, before removal of monochloramine. Nanopore long-read sequencing of selected samples identified potential pathogens such as Mycobacterium gordonae and Mycobacterium angelicum present only before monochloramine removal. A transition community developed, then ultimately shifted to a community with greater abundance of taxa Bdellovibrio, Curvibacterium and Methylonetera in the absence of monochloramine.
Utilization. These findings: can support removal of extraneous disinfection; and, suggest specific bacterial species are selected in biofilm through use of monochloramine.
