Microbial and physicochemical water quality changes within distribution and premise plumbing systems during a chlorine conversion

Cu- and Ag-mediated inactivation of L. pneumophila in bench- and pilot-scale drinking water systems

Legionella pneumophila (Lp) is an opportunistic drinking water pathogen that can cause infections through the inhalation of Lp-containing aerosols and can occur in premise plumbing systems. In this work, the use of copper (Cu) and silver (Ag) ions was evaluated at the bench and pilot scale to determine (i) the effective independent concentrations of copper and silver that are efficacious in inactivating Lp, (ii) the impact of various water quality parameters on the efficaciousness of copper and silver ions, and (iii) the effectiveness and practicality of using dissociation to produce ions at the pilot scale. At the bench scale, it was determined that 0.3 ppm and 0.03 ppm of Cu and Ag, respectively, achieved 6-log inactivation of Lp in 5 h in experimental buffer. But, in dechlorinated filter-sterilized tap water, the same concentrations of Cu were not effective, and the effectiveness of Ag was slower. pH and dissolved inorganic carbon content were found to be important parameters in determining if the use of Cu and Ag ions is appropriate. At the pilot scale, dissociation was successfully used to produce Cu and Ag ions. Target levels of ions were met at the pilot scale but were difficult to achieve, and no impact was observed on Lp concentrations. Results from this study suggest that there are important caveats in the application of this technology when applied in a drinking water matrix and prior understanding of a system's water chemistry may be important to determine the effectiveness of Lp disinfection using Cu and Ag.IMPORTANCEThis work sheds light on the effectiveness of using Cu and Ag ions to inactivate (or kill) Legionella pneumophila. Legionella is an opportunistic drinking water pathogen of public health concern. This work demonstrates that there are important caveats in the application of using Cu and Ag ions to inactivate Legionella pneumophila.

Impact of residential hot water heater type and water stagnation on drinking water quality within a full-scale premise plumbing system

Premise plumbing systems (PPSs), which are connected to the main drinking water distribution system via service lines, are a primary source for human exposure to chemical and microbial contaminants through inhalation, ingestion, and skin contact. To understand the occurrence and distribution of drinking water contaminants in these systems, this study utilized a full-scale PPS and monitored operational parameters and water quality changes for 26 weeks. The PPS contained natural gas- and electric-powered instantaneous and tank hot water heaters (HWHs), each supplying separate sinks and a single shower stall. Total water, gas, and electric usage was monitored daily. Over 400 water samples were collected at 11 timepoints across a 26-week period. Samples represented 4 types: Supply, Hot, Cold, and Shower water, with either stagnant (after an 8-hour period of no water usage), 10-15 second, and/or 5-10-minute flushed samples collected. Lower heterotrophic plate count (HPC) levels were observed in Shower and Hot water samples compared to those in the Supply Line, with most Hot water samples displaying decreases in adenosine triphosphate (ATP) levels post flushing. Water quality differences were also observed between Hot water samples supplied by instantaneous and tank HWHs, such as temperature, oxidation-reduction potential (ORP), conductivity, chlorine levels, and US regulated disinfection byproducts (DBPs). Spearman tests indicated very strong to perfect negative correlations between DBPs, ORP, and free chlorine in Hot and Shower water samples. Principal component analysis revealed distinct clustering of Cold and Hot water samples, supplied either by instantaneous or tank HWH samples, which were driven by differences in temperature, HPC/ATP, DBPs, conductivity, and disinfectant residual. These differences were also impacted by water stagnation as observed by the separate clustering of respective stagnant and flushed HWH samples. Collectively, HWH system and water age (i.e., stagnation) and type strongly influenced chemical and microbial water quality and furthers the understanding of the impacts PPS design and engineering parameters have on water quality.

Development of a machine learning model to support low cost real-time Legionella monitoring in premise plumbing systems

Legionella pneumophila (L. pneumophila) is a pathogenic bacterium primarily known for causing Legionnaires' Disease which is known for high mortality rates, particularly in the elderly. With caseloads continuing to increase, further research is needed to improve our understanding of optimized sampling schema and safe limits of L. pneumophila, in part to target improved treatment options and realistic population-level risk modeling. Particularly in healthcare and other high-risk locations these become crucial and time sensitive needs. Therefore, we conceptualized this research as a means of incorporating easily measured physiochemical water quality parameters and generalization of the unique ecology of building water systems to build a computational model that can allow for more rapid and accurate decision making. This research uses the specific machine learning (ML) method called statistical learning theory to incorporate concentration of host cells, such as native amoeba, and physiochemical water quality parameters to estimate the probability of observing ranges of Legionella gene copy concentrations. Using data from previously published research on Legionella prevalence in a large building, our ML method trains the model on the relative impacts of physiochemical parameters on likely amoeba host cell occurrences. The model is expanded to estimate host cell concentrations using correlations and regressions operated through LASSO algorithms. After categorization variables from these results are then used to inform a logistic regression to provide an estimate of the probability of Legionella gene copy concentration ranges. In summary, conventional results generated by logistic regression and multiple linear regression quantified the associations among ecological conditions in the water and ability to predict a likely range of Legionella concentration in a management focused way. Further, two ML methods, PCA and LASSO, demonstrated feasibility in accurate real-time monitoring of Legionella through physiochemical indicators as evidenced with good accuracy of predictions based for validation results. Furthermore results demonstrate the vital need to account for the impact of water quality on building on host cells, and via their quantified water microbial ecology, not just Legionella concentrations.

Reconsider the burn: The transient effect of a chlorine burn on controlling opportunistic pathogens in a full-scale chloraminated engineered water system

Nitrification is a serious water-quality issue in chloraminated engineered water systems (EWSs). Nitrification is often remediated by a chlorine burn (i.e., a free‑chlorine conversion), a short-term switch from chloramination to chlorination in EWSs. Opportunistic pathogens (OPs) are the dominant infectious agents in EWSs. However, the responses of OPs to a chlorine burn are unknown. This study for the first time assessed how a chlorine burn affected OPs in a full-scale EWS. We determined the impact of a 1.5-month chlorine burn on four dominant OPs (Legionella, Mycobacterium, Pseudomonas, and Vermamoeba vermiformis) in a representative full-scale chloraminated EWS in the United States. Legionella and Mycobacterium were the most abundant OPs. In the water main, the summed concentration of the four OPs during the chlorine burn [3.27 ± 1.58 log10(GCN·L-1); GCN: genome or gene copy number] was lower (p ≤ 0.001) than before the burn [4.83 ± 0.50 log10(GCN·L-1)]. After the burn, the summed concentration increased to 4.27 ± 0.68 log10(GCN·L-1), comparable to before the burn (p > 0.05), indicating a transient effect of the chlorine burn in the water main. At the residential sites, the summed concentrations of the four OPs were comparable (p > 0.05) at 5.50 ± 0.84, 5.27 ± 1.44, and 5.08 ± 0.71 log10(GCN·L-1) before, during, and after the chlorine burn, respectively. Therefore, the chlorine burn was less effective in suppressing OP (re)growth in the premise plumbing. The low effectiveness might be due to more significant water stagnation and disinfectant residual decay in the premise plumbing. Indeed, for the entire sampling period, the total chlorine residual concentration in the premise plumbing (1.8 mg Cl2·L-1) was lower than in the water main (2.4 mg Cl2·L-1). Consequently, for the entire sampling period, the summed concentration of the four OPs in the premise plumbing [5.26 ± 1.08 log10(GCN·L-1)] was significantly higher (p < 0.001) than in the water main [4.04 ± 1.25 log10(GCN·L-1)]. In addition, the chlorine burn substantially increased the levels of disinfection by-products (DBPs) in the water main. Altogether, a chlorine burn is transient or even ineffective in suppressing OP (re)growth but raises DBP concentrations in chloraminated EWSs. Therefore, the practice of chlorine burns to control nitrification should be optimized, reconsidered, or even replaced.

Evaluation of distribution system water quality during a free chlorine conversion

Chloraminated drinking water systems commonly use free chlorine conversions (FCCs) to prevent or control nitrification, but unintended water quality changes may occur, including increased disinfection by-product and metal concentrations. This study evaluated water quality in a chloraminated drinking water system and residential locations before, during, and after their annual, planned FCC. Water quality alternated between relatively consistent and variable periods when switching disinfectants. During the FCC, regulated four trihalomethane and five haloacetic acid concentrations increased by four and seven times, respectively, and exceeded corresponding maximum contaminant levels. Implications of disinfection by-product sampling during an FCC were assessed, and an approach to account for increased FCC disinfection by-product concentrations was proposed. For metals, the FCC had minor impacts on distribution system concentrations and did not appear to impact residential concentrations. Overall, observed variable water quality appeared primarily associated with switching disinfectants and depended on distribution system hydraulics.