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

Abstract

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.