Role of Hot Water System Design on Factors Influential to Pathogen Regrowth: Temperature, Chlorine Residual, Hydrogen Evolution, and Sediment

Abstract

Residential water heating is linked to growth of pathogens in premise plumbing, which is the primary source of waterborne disease in the United States. Temperature and disinfectant residual are critical factors controlling increased concentration of pathogens, but understanding of how each factor varies in different water heater configurations is lacking. A direct comparative study of electric water heater systems was conducted to evaluate temporal variations in temperature and water quality parameters including dissolved oxygen levels, hydrogen evolution, total and soluble metal concentrations, and disinfectant decay. Recirculation tanks had much greater volumes of water at temperature ranges with potential for increased pathogen growth when set at 49°C compared with standard tank systems without recirculation. In contrast, when set at the higher end of acceptable ranges (i.e., 60°C), this relationship was reversed and recirculation systems had less volume of water at risk for pathogen growth compared with conventional systems. Recirculation tanks also tended to have much lower levels of disinfectant residual (standard systems had 40-600% higher residual), 4-6 times as much hydrogen, and 3-20 times more sediment compared with standard tanks without recirculation. On demand tankless systems had very small volumes of water at risk and relatively high levels of disinfectant residual. Recirculation systems may have distinct advantages in controlling pathogens via thermal disinfection if set at 60°C, but these systems have lower levels of disinfectant residual and greater volumes at risk if set at lower temperatures.

Keywords: Legionella pneumophila; Mycobacteria avium complex; pathogen control; premise plumbing; water heaters.

FIG. 1.

Experimental design of head-to-head water heaters: A–J, sample taps; K, flush/waste line; L, 120 V energy meter; M, 220 V energy meter; N, gate check valve; O, pressure gauge. For experiments described in this article, copper tube was insulated. Picture modified from photo published in Brazeau and Edwards, .

FIG. 2.

To measure the internal temperature of the tanks, data loggers (1–5) were installed that automatically measured temperature every 30 min.

FIG. 3.

Temperature profiles during flushing for field observations of 50 gallon plus tanks and large-scale plumbing systems versus laboratory controlled measurements of 20 gallon tanks and small-scale plumbing systems.

FIG. 4.

Internal tank temperature with depth for RECIRC and STAND systems at 60°C and low volume use.

FIG. 5.

Internal tank temperature with depth for RECIRC and STAND systems at 49°C and low user pattern.

FIG. 6.

Internal tank temperature for RECIRC and STAND systems at 60°C and high user pattern.

FIG. 7.

Internal tank temperature for RECIRC and STAND systems at 49°C and high user pattern.

FIG. 8.

Dissolved oxygen (DO) concentrations at various sample ports (Fig. 1) in the bulk water of the systems taken at 60°C and high use.

FIG. 9.

Total metal concentrations in bulk water samples for the RECIRC and STAND systems.

FIG. 10.

Total chlorine decay during stagnation for the low use conditions.

FIG. 11.

Total metal concentrations in bulk water samples for the RECIRC and STAND systems.

FIG. 12.

Bench-top experiment results determining chlorine decay when mixed with a copper coupon.