Untangling the Effects of Hydraulic Design on Opportunistic Pathogen Growth Potential with an at-Scale Plumbing Rig

Abstract

A building plumbing rig experiment simultaneously examined how water temperature (cold/hot lines), influent disinfectant residual (0–1 mg/L chloramine), flow rate (0.5–2.2 gpm), and water retention time (WRT) of 0–17-days impacted water quality at the point of use. In cold water lines with no disinfectant, WRT was a key driver of bacterial growth, with total cell counts (TCC) in the water increasing by up to 20× relative to influent water at 6.7-days WRT. A chloramine residual in cold influent water suppressed the maximum TCC by about 50%, even after the residual was no longer measurable. When the water heater set point was warm (40 °C) with minimal or 1 mg/L Cl₂ chloramine, the majority of microbial growth occurred in the tank (WRT = 3 days). However, at a heater set point of 60 °C with 1 mg/L as Cl_2, growth was completely repressed in the tank, shifting growth to the distal pipes. Legionella spp. gene copies measured in cold bulk water increased with WRT, but not flow velocity. In hot water biofilms, Legionella spp. gene copies were highest at low WRT and high flow velocities. Mycobacterium spp. gene copies in hot water biofilms escalated after chloramines were introduced and were positively correlated to water velocity.

An at-scale pipe rig revealed that the location of optimal growth niches for Legionella spp., Mycobacterium spp., and total bacteria is dependent on water age, water heater set point, and influent disinfectant level.