Impact of Premise Plumbing Design, Velocity, and Operational Factors on Microbial Activity During Stagnation in Pipes

Abstract

Water systems using chloramine as a disinfectant are vulnerable to nitrification, which can degrade water quality by enhancing the loss of residual disinfectant, lowering pH, and providing substrates for the growth of heterotrophs, leading to a more active biofilm that may support opportunistic pathogen growth in the premise plumbing (i.e., piping within a building). This study examined the influence of a variety of domestic plumbing factors, specifically: use frequency (pipes used once per week vs. five times per week), pipe material (copper vs. CPVC vs. PEX-b), pipe diameter (3/8″ vs. 1/2″ vs. 3/4″), and flow rates (0.2 gal/min vs. 3.2–3.7 gal/min), on indicators of water quality and nitrification, specifically: residual disinfectant, ammonia, nitrite, nitrate, turbidity, pH, and adenosine triphosphate (ATP, a non-specific proxy for biological activity). Experiments were conducted on a set of 28 pipes connected to a chloraminated public drinking water supply. Fourteen of the pipes were operated at fast flow rates of 3.2–3.7 gal per minute (12.4–13.9 L/min), which corresponded to 2.7, 5.7, and 9.5 ft/s (0.81, 1.7, 2.9 m/s) for the 3/4, 1/2, and 3/8 in. pipes, respectively. The remaining 14 pipes were operated at slow flow rates of 0.2 gal per minute (0.8 L/min), which corresponds to 0.15, 0.36, and 0.56 ft/s (0.045, 0.11, and 0.17 m/s) for the 3/4 in. (19 mm), 1/2 in. (13 mm), and 3/8 in. pipes (9.5 mm), respectively. Faster velocity flows were associated with higher residual disinfectant, less degradation of ammonia, lower turbidity and lower ATP. These results suggest that the faster flow velocity altered the biofilm and reduced the amount of microbial activity and nitrification occurring. Differences were particularly strong in the 3/8 in. (9.5 mm) pipes, suggesting that a velocity of 9.5 ft/s (2.9 m/s) offered additional benefits in biofilm control, beyond those achieved at the velocities of 2.7 ft/s (0.81 m/s) and 5.7 ft/s (1.7 m/s) in 3/4 in. (19 mm) and 1/2 in. (13 mm) pipes, respectively. Pipes flushed once per week tended to reach a similar end state of loss of residual and progression of nitrification, with limited differences due to velocity, pipe material, or diameter. Better understanding of the conditions that are associated with nitrification offers the potential to better control nitrification. This in turn may help maintain residual disinfectant as a protection against harmful microorganisms in drinking water.