This study examines the influence of chlorine disinfectant type, water use frequency (stagnation time), and water heater temperature on the growth and persistence of microbes, including nontuberculous mycobacteria (NTM), in water heater storage tanks and downstream plumbing. The experimental setup consists of four 50-gallon electric water heater tanks at two different locations that provide different types of secondary disinfectant (chloramine and free chlorine). The water heaters were operated at two different temperature settings (45°C ± 3°C and 60°C ± 3°C), and with two different use patterns (once per 24 h and three times per 24 h), which allowed for sampling at three different stagnation times: overnight, 16-h stagnation, and 3-h stagnation. Microbial analyses were conducted on water samples collected at the tank effluent and the shower head to compare the water quality in the tank before the stagnation period and in the piping downstream of the tank after the stagnation period. The results indicated that culturable NTM was not detected under any of the conditions studied in the free chlorine disinfectant system. In the chloramine system, higher water use frequency (equivalent to 180 gallons/day) and higher water heater temperature (60°C ± 3°C) decreased the number of heterotrophic bacteria in the tank and the piping downstream to the tank but failed to control NTM. For both heterotrophic plate count and NTM, the piping downstream was not more microbially active than the tank.
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