Abstract
Advanced treatment facilities for potable water reuse of wastewater are designed to achieve high removal levels of specific pathogens, as well as many other constituents. However, changes to the microbial community throughout treatment, storage, and distribution of this water have not been well characterized. We applied high-throughput amplicon sequencing, read-based, assembly-based, and genome-resolved metagenomics, and flow cytometry to investigate the microbial communities present in a pilot-scale advanced water treatment facility. Advanced treatment of secondary-treated wastewater consisted of ozonation, chloramination, microfiltration, reverse osmosis (RO), advanced oxidation (UV/H2O2), granular activated carbon (GAC) filtration, and chlorination. Treated water was fed into bench-scale simulated distribution systems (SDS). Cell counts and microbial diversity in bulk water decreased until GAC filtration, and the bacterial communities were significantly different following each treatment step. Bacteria grew within GAC media and contributed to a consistent microbial community in the filtrate, which included members of the Rhizobiales and Mycobacteriaceae. After chlorination, some of the GAC filtrate community was maintained within the SDS, and community shifts were associated with stagnation. Putative antibiotic resistance genes and potential opportunistic pathogens were identified before RO and after advanced oxidation, although few if any members of the wastewater microbial community passed through these treatment steps. These findings can contribute to improved design of advanced treatment trains and management of microbial communities in post-treatment steps.
Highlights
Regions facing water scarcity are turning to new sources of drinking water, including wastewater that has been purified through advanced treatment to meet potable water quality standards (Raucher and Tchobanoglous, 2014)
To examine potential persistence of bacteria from secondary wastewater and early treatment steps into post-NF/reverse osmosis (RO) samples, we investigated the reoccurrence of specific amplicon sequence variant (ASV), and used metagenomic read-mapping to assemblies, to unique RpsCcontaining scaffolds, and to non-redundant metagenome assembled genome (MAG)
Based on amplicon sequencing and metagenomic analyses of microbial communities in a pilot advanced treatment facility for potable reuse, we found that: (1) bulk water bacterial diversity paralleled cell counts through treatment; (2) bacterial communities were significantly different by location in treatment; (3) few or no community members were shared between secondary wastewater and simulated distribution systems (SDS); (4) absolute abundances of potential opportunistic pathogens dropped substantially during treatment and neither opportunistic pathogens nor antibiotic resistance gene (ARG) present in treated water were likely due to passage of bacteria through advanced treatment; and (5) microbial growth occurred after NF/RO and could have been due to metabolism of small carbon compounds
Summary
Regions facing water scarcity are turning to new sources of drinking water, including wastewater that has been purified through advanced treatment to meet potable water quality standards (Raucher and Tchobanoglous, 2014). In direct potable reuse systems, purified water is blended into the drinking water distribution system after minimal storage time, and microbial risks must be considered carefully. Current regulations focus on virus and protozoan cyst removal, and ensuring that the risk level for these pathogens in advanced treated water is as low as for conventional drinking water. To understand how advanced treatment affects microbial water quality, it is necessary to examine removal and growth of bacteria across treatment trains and in distribution
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