Legionella is the causative agent of Legionnaires\' disease, and its prevalence in potable water is a significant public health issue. Water stagnation within buildings increases the risk of Legionella. However, there are limited studies investigating how stagnation arising through intermittent usage affects Legionella proliferation and the studies that are available do not consider viable but non culturable (VBNC) Legionella. This study used a model plumbing system to examine how intermittent water stagnation affects both VBNC and culturable Legionella. The model plumbing system contained a water tank supplying two biofilm reactors. The model was initially left stagnant for ≈5 months (147 days), after which one reactor was flushed daily, and the other weekly. Biofilm coupons, and water samples were collected for analysis at days 0, 14 and 28. These samples were analysed for culturable and VBNC Legionella, free-living amoebae, and heterotrophic bacteria. After 28 days, once-a-day flushing significantly (p < 0.001) reduced the amount of biofilm-associated culturable Legionella (1.5 log10 reduction) compared with weekly flushing. However, higher counts of biofilm-associated VBNC Legionella (1 log10 higher) were recovered from the reactor with once-a-day flushing compared with weekly flushing. Likewise, once-a-day flushing increased the population of biofilm-associated Vermamoeba vermiformis (approximately 3 log10 higher) compared with weekly flushing, which indicated a positive relationship between VBNC Legionella and V. vermiformis. This is the first study to investigate the influence of stagnation on VBNC Legionella under environmental conditions. Overall, this study showed that a reduction in water stagnation decreased culturable Legionella but not VBNC Legionella.

Small-scale distributed water purifiers (SSDWPs), providing better quality drinking water, are popularly used both in homes and in the public domain. Non-continuous operation leads to water stagnation and ultimately induces microbial contamination. However, information related to such contamination in these purifiers is reported scarcely. In the present study, an SSDWP, consisting of sand filtration (SF), granular activated carbon (GAC), and ultrafiltration (UF) processes, was established to explore microbial changes induced by water stagnation, based on the aspects of bacterial count, microbial size, microbiome and pathogenic communities. Our results primary showed that: first, compared with drinking water distribution system (DWDS), bacterial counts increased more rapidly in SSDWPs, growing to > 500 cfu/mL after 2.5 h stagnation. The proportion of intact cells also increased with stagnation time. Conversely, microbial size decreased with stagnation time according to changes in forward scatter detected using flow cytometry. Second, microbiome evolution followed the isolated island model, while in stagnated DWDS, microbiome evolved according to the continent island model, and the former had higher abundance of biodiversity. Furthermore, stagnation evidently caused microbiome changes in each unit, and spatial differences contributed to microbiome dissimilarity more significantly than temporal differences. Third, Mycobacterium was the dominant pathogenic genus in the SF and GAC units while Acinetobacter was the most abundant in the UF unit. Pathogenic risks increased with water stagnation time and lower nutrients level contributed to pathogenic community richness. Therefore, terminal disinfection of SSDWPs is strongly advised.

This paper studied stagnation-induced changes of disinfectant and bacteria using an orthogonal test and kinetic analysis, and then proposed a disinfection strategy. Tap water from a drinking water distribution system and ultrafiltrated water were collected and disinfected with four disinfectants (concentrations were set 0.2-1 mg/L as Cl2. The study had several findings. First, disinfectants expanded lag phases and shortened generation times of the microbiome. Reduction in culturability, substrate responsiveness, respiratory activity, membrane potential and integrity subsequently occurred with increasing disinfection concentration. Second, the disinfectant decay rate decreased with initial disinfection concentration, and the effective disinfection phase (heterotrophic plate count (HPC) was less than 100 cfu/mL) was longer in water samples with lower organic matter. Moreover, the disinfection process was divided into an effective phase and an invalid phase (HPC>100 cfu/mL). Then a disinfection efficiency model was built and the regulation of disinfection by-products (DBPs) production was studied in chlorinated water samples, which provides a general method for other disinfectant studies. The average trihalomethanes (THMs) production during the effective phase (marked as THM/th) and THMs production during the invalid phase (marked as ΔTHM) were proposed to evaluate the DBPs production. The level of THM/th and ΔTHM were lower in ultrafiltrated water than those in tap water. THM/th were negatively correlated with initial chlorine concentration while ΔTHM were positively correlated with initial chlorine concentration. Finally, for the purpose of raising disinfection efficiency and decreasing DBPs, we propose periodic pulse disinfection.

The drinking water quality changes during the transport through distribution systems. Domestic drinking water systems (DDWSs), which include the plumbing between the water meter and consumer\'s taps, are the most critical points in which water quality may be affected. In distribution networks, the drinking water temperature and water residence time are regarded as indicators of the drinking water quality. This paper describes an experimental research on the influence of stagnation time and temperature change on drinking water quality in a full-scale DDWS. Two sets of stagnation experiments, during winter and summer months, with various stagnation intervals (up to 168 h of stagnation) were carried out. Water and biofilms were sampled at two different taps, a kitchen and a shower tap. Results from this study indicate that temperature and water stagnation affect both chemical and microbial quality in DDWSs, whereas microbial parameters in stagnant water appear to be driven by the temperature of fresh water. Biofilm formed in the shower pipe contained more total and intact cells than the kitchen pipe biofilm. Alphaproteobacteria were found to dominate in the shower biofilm (78% of all Proteobacteria), while in the kitchen tap biofilm Alphaproteobacteria, Betaproteobacteria and Gammaproteobacteria were evenly distributed.