Abstract
Little is known about the forces that determine the assembly of diverse bacterial communities inhabiting drinking water treatment filters and how this affects drinking water quality. Two contrasting ecological theories can help to understand how natural microbial communities assemble; niche theory and neutral theory, where environmental deterministic factors or stochastic factors predominate respectively. This study investigates the development of the microbial community on two common contrasting filter materials (quartz sand and granular activated carbon-GAC), to elucidate the main factors governing their assembly, through the evaluation of environmental (i.e. filter medium type) and stochastic forces (random deaths, births and immigration). Laboratory-scale filter columns were used to mimic a rapid gravity filter; the microbiome of the filter materials, and of the filter influent and effluent, was characterised using next generation 16S rRNA gene amplicon sequencing and flow-cytometry. Chemical parameters (i.e. dissolved organic carbon, trihalomethanes formation) were also monitored to assess the final effluent quality. The filter communities seemed to be strongly assembled by selection rather than neutral processes, with only 28% of those OTUs shared with the source water detected on the filter medium following predictions using a neutral community model. GAC hosted a phylogenetically more diverse community than sand. The two filter media communities seeded the effluent water, triggering differences in both water quality and community composition of the effluents. Overall, GAC proved to be better than sand in controlling microbial growth, by promoting higher bacterial decay rates and hosting less bacterial cells, and showed better performance for putative pathogen control by leaking less Legionella cells into the effluent water.
Highlights
Water filters are populated by a high diversity of bacteria (Pinto et al, 2012), which may contribute in the removal of contaminants, such as dissolved organic carbon (DOC), nitrogen, and micropollutants (Richter et al, 2008), or might play a detrimental role, by harbouring potential pathogens or releasing DOC, with a subsequent increase in trihalomethane (THM) formation
We investigated the development of microbial communities on two drinking water filter materials receiving the same source water in laboratory-scale columns
The majority of the taxa employed in the NCM analyses, 54%, were either more frequent (24% of taxa) or less frequent (30% of taxa) than would be expected by their relative abundances if they were assembled by neutral stochastic processes alone (Table 1., Fig. 2a)
Summary
Bacterial communities are present with high abundance in drinking water treatment systems, in source waters (105106 cells/ml) (Hammes et al, 2008; Lautenschlager et al, 2014) and at different points of a Drinking Water Treatment Plant (DWTP) (105 cells/ml) (Hammes et al, 2008; Vital et al, 2012) and in the distribution networks (104-105 cells/ml) (Hammes et al, 2008; Lautenschlager et al, 2013). Water filters have a significant influence on the bacterial community composition in the post-filtration water and distribution network (Pinto et al, 2012).
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