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Microbial communities in premise plumbing systems were investigated after more than 2 months of long-term stagnation, during a subsequent flushing event, and during post-flush stagnation. Water samples were collected from showers in buildings supplied with chlorinated groundwater, untreated groundwater, and chloraminated surface water. The building supplied with chlorinated groundwater generally had the lowest bacterial concentrations across all sites (ranging from below quantification limit to 5.2 log copies/L). For buildings supplied with untreated groundwater, bacterial concentrations (5.0 to 7.6 log copies/L) and microbial community diversity index (ACE) values were consistent throughout sampling. Nontuberculous mycobacteria (NTM) and Legionella pneumophilawere not detected in any groundwater-supplied buildings. Total bacteria, Legionellaspp., and NTM were abundant in the surface water-supplied buildings following long-term stagnation (up to 7.6, 6.2, and 7.6 log copies/L, respectively). Flushing decreased these concentrations by ∼1 to >4 log units and reduced microbial community diversity, but the communities largely recovered within a week of post-flush stagnation. The results suggest that buildings supplied with disinfected surface water are more likely than buildings supplied with treated or untreated groundwater to experience deleterious changes in microbiological water quality during stagnation and that the water quality improvements from flushing with chloraminated water, while substantial, are short-lived.

There is concern about potential exposure to opportunistic pathogens when reopening buildings closed due to the COVID-19 pandemic. In this study, water samples were collected before, during, and after flushing showers in five unoccupied (i.e., for ∼2 months) university buildings with quantification of opportunists via a cultivation-based assay (Legionella pneumophilaonly) and quantitative PCR. L. pneumophilawere not detected by either method; Legionellaspp., nontuberculous mycobacteria (NTM), and Mycobacterium aviumcomplex (MAC), however, were widespread. Using quantitative microbial risk assessment (QMRA), the estimated risks of illness from exposure to L. pneumophilaand MAC via showering were generally low (i.e., less than a 10–7daily risk threshold), with the exception of systemic infection risk from MAC exposure in some buildings. Flushing rapidly restored the total chlorine (as chloramine) residual and decreased bacterial gene targets to building inlet concentrations within 30 min. During the postflush stagnation period, the residual chlorine dissipated within a few days and bacteria rebounded, approaching preflush concentrations after 6–7 days. These results suggest that flushing can quickly improve water quality in unoccupied buildings, but the improvement may only last a few days.

Bacterial communities in biofilters can improve drinking water quality through the biodegradation of dissolved contaminants but also pose potential risks by harboring and shedding microbes into the drinking water distribution system. In this study, pilot-scale granular activated carbon (GAC)-sand and anthracite-sand pilot-scale biofilters were investigated to determine the effects of filter design and operation on the microbiome of the filter media and its relationship to the microbiome in the filter effluent water. Bacterial abundance in the biofilters was relatively stable over time. Bacterial community composition exhibited spatial variation (i.e., with bed depth) and temporal variation linked to water quality changes. Bacterial community composition was significantly affected by the media type (GAC vs anthracite) and backwashing strategy (chloraminated water vs nonchloraminated water). The biofilters reduced bacterial abundance in the water (∼70%) but had only a minor effect on the bacterial community composition in the filtrate. Overall, our results suggest that the bacterial communities growing on biofilters affect filtered water quality primarily through the biotransformation of pollutants and nutrients rather than by altering the microbial community composition of the water as it passes through the filter.

Reed canary grass (Phalaris arundinaceaL.) is an invasive, cool‐season grass commonly dominating wetlands with high nutrient loads. Its impact on nitrogen removal via denitrification in wetlands is unknown. Most studies of denitrification in treatment wetlands have focused on the effects of physical or chemical variables and not on the effects of plant roots on the soil environment. The purpose of this study was to measure effects of plant type on denitrification rates in typical wetland soils of the midwestern United States by comparing wet prairie mix, switchgrass‐dominated, and reed canary grass plant communities. Nitrate (NO3−) removal and other parameters were measured in miniature wetlands, or mesocosms, containing each plant community transplanted from a small agricultural treatment wetland in southern Minnesota. Quantitative polymerase chain reaction analysis was used to quantify the total bacteria population (measured with 16S rRNA genes) and denitrifying gene abundance (measured with nosZgenes) from the rhizosphere of each plant community. The wet prairie mix mesocosms on average removed the most NO3−in each test (p= .01 and .08). Whereas the wet prairie mix removed the most NO3−from the surface water (p< .01), reed canary grass removed more from the subsurface (p< .01). Ratios of denitrifying to total bacteria were higher in the wet prairie mix than in the other communities’ root zones (p< .05). Results suggest that reed canary grass invasion could reduce denitrification in wetlands, especially during the spring and fall when it is growing but other plants are dormant.

Nontuberculous mycobacteria (NTM) are frequently found in chloraminated drinking water distribution systems (DWDSs) due to their chloramine tolerance. NTM were investigated in the water-main biofilms and drinking water of a chloraminated DWDS in the United States (initial chloramine residual = 3.8 ± 0.1 mg L–1) and a DWDS in Norway with minimal residual disinfectant (0.08 ± 0.01 mg L–1). Total mycobacteria and Mycobacterium aviumcomplex (MAC) were quantified by qPCR targeting, respectively, atpEgenes and the internal transcribed spacer region. Mycobacteria concentrations in drinking water did not differ between the two systems (P= 0.09; up to 6 × 104copies L–1) but were higher in the biofilms from the chloraminated DWDS (P= 5 × 10–9; up to 5 × 106copies cm–2). MAC were not detected in either system. Sequencing of mycobacterial hsp65genes indicated that the chloraminated DWDS lacked diversity and consisted almost exclusively of M. gordonae. In contrast, there were various novel mycobacteria in the no-residual DWDS. Finally, Mycobacterium- and Methylobacterium-like 16S rRNA genes were often detected simultaneously, though without correlation as previously observed. We conclude that, though residual chloramine may increase mycobacterial biomass in a DWDS, it may also decrease mycobacterial diversity.

The vast majority of bacteria in drinking water distribution systems (DWDSs) reside in biofilms on the interior walls of water mains. Little is known about how water quality conditions affect water-main biofilms because of the inherent limitations in experimenting with drinking water supplies and accessing the water mains for sampling. Bench-scale reactors permit experimentation and ease of biofilm sampling, yet questions remain as to how well biofilms in laboratory reactors represent those on water mains. In this study, the effects of DWDS pipe materials and chloramine residual on biofilms were investigated by cultivating biofilms on cement, polyvinyl chloride, and high density polyethylene coupons in CDC reactors for up to 28 months in the presence of chloraminated or dechlorinated tap water. The bench-scale biofilm microbiomes were then compared with the microbiome on a water main from the full-scale system that supplied the water to the reactors. The presence of a chloramine residual (1.74 ± 0.21 mg/L) suppressed biofilm accumulation and selected for Mycobacterium-like and Sphingopyxis-like operational taxonomic units (OTUs) while the destruction of the chloramine residual resulted in a significant increase in biomass quantity and a shift toward a more diverse community dominated by Nitrospira-like OTUs, which, our results suggest, may be complete ammonia oxidizers (comammox). Coupon material, however, had a relatively minor effect on the abundance and community composition of the biofilm bacteria. Although biofilm communities from the chloraminated water reactor and the water mains shared some dominant populations (namely, Mycobacterium- and Nitrosomonas-like OTUs), the communities were significantly different. This manuscript provides novel insights into the effects of dechlorination and pipe material on biofilm community composition. Furthermore, to our knowledge, it is the first study to compare biofilm in a tap water-fed, bench-scale simulated distribution system to biofilm on water mains from the full-scale system supplying the tap water.

The maintenance of a chlorine or chloramine residual to suppress waterborne pathogens in drinking water distribution systems is common practice in the United States but less common in Europe. In this study, we investigated the occurrence of Bacteriaand Legionellaspp. in water-main biofilms and tap water from a chloraminated distribution system in the United States and a system in Norway with no residual using real-time quantitative polymerase chain reaction (qPCR). Despite generally higher temperatures and assimilable organic carbon levels in the chloraminated system, total Bacteriaand Legionellaspp. were significantly lower in water-main biofilms and tap water of that system (p< 0.05). Legionellaspp. were not detected in the biofilms of the chloraminated system (0 of 35 samples) but were frequently detected in biofilms from the no-residual system (10 of 23 samples; maximum concentration = 7.8 × 104gene copies cm–2). This investigation suggests water-main biofilms may serve as a source of Legionellafor tap water and premise plumbing systems, and residual chloramine may aid in reducing their abundance.

Ammonia-oxidizing microorganisms (AOMs) include ammonia-oxidizing bacteria (AOB), archaea (AOA), and Nitrospiraspp. sublineage II capable of complete ammonia oxidation (comammox). These organisms can affect water quality not only by oxidizing ammonia to nitrite (or nitrate) but also by cometabolically degrading trace organic contaminants. In this study, the abundance and composition of AOM communities were investigated in full-scale biofilters at 14 facilities across North America and in pilot-scale biofilters operated for 18 months at a full-scale water treatment plant. In general, the relative abundance of AOM in most full-scale biofilters and in the pilot-scale biofilters was as follows: AOB > comammox Nitrospira> AOA. The abundance of AOB in the pilot-scale biofilters increased with increasing influent ammonia concentration and decreasing temperature, whereas AOA and comammox Nitrospiraexhibited no correlations with these parameters. The biofilters affected AOM abundance in the water passing through the filters via collecting and shedding but exhibited a minor influence on the composition of AOB and Nitrospirasublineage II communities in the filtrate. Overall, this study highlights the relative importance of AOB and comammox Nitrospiracompared to AOA in biofilters and the influence of filter influent water quality on AOM in biofilters and their release into the filtrate.