Assessment of the mobile biofilm microbiome in distributed drinking water following installation of hybrid ultrafiltration process, in Varberg, Sweden.

• Drinking water is delivered from the treatment plant to the consumer through kilometres of pipes. Lining these pipes are communities of bacteria living as biofilms. Little is known about the microbial ecology of these biofilms, as access to drinking water pipes for sampling is often limited to pipes sporadically removed for repair or replacement. Bacteria are constantly exchanged between the biofilm and flowing water, however with bacteria in the water leaving the treatment plant reaching numbers as high as 700 000 cells per millilitre, identifying those cells originating from the biofilm is difficult.Membrane hybrid processes—coagulation coupled with ultrafiltration (UF)—have become a common method to comply with the legal, chemical, and microbiological requirements for drinking water. The main advantages of integrating coagulation with membrane filtration are the enhanced removal of natural organic matter (NOM) and reduced membrane fouling. Therefore, in November 2016, the Kvarnagården WTP in Varberg, Sweden was upgraded with a UF facility (capacity of 1080 m3 h−1 net permeate flow rate). The commissioning of the UF treatment process at Kvarnagården WTP, provided the opportunity to observe the microbial consequences of exposing a distribution system previously exposed to high cellular counts, to a virtually cell-free water phase. By observing which bacteria entered this new water phase, particularly those appearing after an extended time within the distribution system, a snapshot would be obtained of which bacteria can enter the water phase from the pipe biofilm.Water samples were taken before, three days after, and for one month after, installation of UF for flow cytometry (FCM) and Illumina 16S rRNA gene sequencing. FCM gives total cell counts and some additional parameters (%intact cells, %HNA) to describe the bacterial community, while DNA sequencing provides detailed genetic descriptions. Samples from the UF feed showed an average of 8 x105 cells/mL while filtered water contained 2.4 x 104 cells/mL. FCM also showed that UF removed intact cells, but that filtration by this method did not show a preference for either high (HNA) or low (LNA) nucleic acid-containing bacteria. Water samples from three locations at increasing distance from the treatment plant were taken within the distribution system. These samples showed an increase in total cells, with, on average, an addition of 0.6 x 104 cells/ mL, contributed from bacteria leaving the biofilm. These bacteria from the biofilm were further characterized by FCM as intact cells containing an increased percentage of HNA cells, relative to the water leaving the treatment plant. DNA sequencing of all samples, followed by the bioinformatics pipeline QIIME, further revealed the community present in the filtered water originating from the biofilm. Community content in the distributed water was distinct from the finished water, and also differed with respect to location within the distribution system. Both species richness and diversity in the distributed water decreased following installation of UF, as measured by Chao´s richness and Shannon diversity index. Differential analysis of the sequencing reads counts by DESeq2 were used to detect statistical significant operational taxonomic units (OTUs) that have changed over the travel across the pipes. Several groups of bacteria were identified associated with filtered water that had, and had not been, in contact with biofilm. These included Hyphomicrobium, Pedomicrobium, Nitrospira, Sphingomonas, Mycobacterium and Rhodobacter as well as several unidentified genera. This suggests that these are the most mobile members of the pipe biofilms in this distribution system. How the mobility of these groups change over seasons or how the overall microbiome of both the biofilm and the water phase in this distribution system will adapt to the filtered water over a longer time period will provide additional information about how pipe biofilms respond to large changes in the microbiology of distributed water.

Subject headings

TEKNIK OCH TEKNOLOGIER  -- Samhällsbyggnadsteknik -- Vattenteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Civil Engineering -- Water Engineering (hsv//eng)

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