Trihalomethane, haloacetic acid, and haloacetonitrile behaviors in water heater storage tanks

Abstract

Increasing the operating temperature has been recommended for controlling opportunistic premise plumbing pathogen (OPPP) growth in buildings. However, tradeoffs with operation and both disinfectant residuals and disinfection byproducts (DBPs) are not well understood. Water heater storage units were operated to evaluate the impact of use pattern (high and low), operation temperature (∼45 °C and ∼60 °C) and disinfectant type (chlorine and chloramines) on disinfectant residual and DBP concentrations at the showerhead point of use (POU). Control studies with chlorinated water yielded a loss of residual by 72 hours at 22 °C and 3 hours at 48 °C and 60 °C. In the tank heater studies, free chlorine residuals decayed to below the limit of quantitation (LOQ) for all POU samples. The concentrations of total trihalomethanes (TTHMs) and five haloacetic acids (HAA5) at the POU and control sample locations increased by factors of 2.01 ± 0.21 and 1.46 ± 0.11, respectively, relative to the feed and peaked at the first point that the chlorine residual was below the LOQ, indicating a chlorine-limited reaction. The TTHM concentrations did not change thereafter, just as the HAA concentrations remained unchanged at 45 °C, but the HAA concentrations decreased at 60 °C, indicating thermal degradation. Strong decay patterns were found for four haloacetonitriles (HAN4) at elevated temperatures, and the decay was faster at 60 °C. In the chloramination system, total chlorine decreased passing through the tank, but a detectable residual was measured in all samples. No additional DBP formation relative to the feed was found. Under all scenarios, there was no indication of a tradeoff between controlling OPPPs and DBP formation when increasing the operating temperature to 60 °C.