Distribution of bacteria in a domestic hot water system in a Danish apartment building

Bacterial growth in hot water systems seems to cause problems such as bad odor of the water, skin allergies and increased heat transfer resistance in heating coils. In order to establish a basis for long-term suppression of bacterial growth, we studied the distribution of bacteria in a Danish domestic hot water system. Heterotrophic plate counts (HPC) were measured in both water and biofilm samples from various sampling sites in the system. In hot water samples, where the temperature was 55–60°C, the HPC were 103–104 CFU/mL at incubation temperatures of 25°C or 37°C and 105 CFU/mL at 55°C or 65°C. In the cold water (10°C) supplying the hot water system, the HPC at 25°C or 37°C was lower than in the hot water, and no bacteria were found after incubation at 55°C or 65°C. HPC constituted from 38% to 84% of the AODC results in hot water but only 2% in cold water, which showed a high ratio of culturable bacteria in hot water. Biofilm samples from the hot water tank and the inner surface of the pipes in the cold and hot water distribution system were collected by specially designed sampling devices, which were exposed in the system for 42 days. The quasi-steady-state number of bacteria in the biofilm, measured as the geometric mean of the HPC obtained between 21 and 42 days, was five-fold higher in the hot water pipe (13×105 CFU/cm2 at 55°C) than in the cold water pipe (2.8×105 CFU/cm2 at 25°C). There was no significant difference between the number of bacteria in the biofilm samples from the top, middle and bottom of the hot water tank, and the number of bacteria in the biofilm counted at 55°C ranged from 0.6×104 to 1.7×104 CFU/cm2. The surfaces of the sacrificial aluminum anodes and the heating coils in the hot water tank also contained high bacterial numbers. The measured number of bacteria in water and biofilm samples was related to the dimensions of the hot water system, and calculations showed that the majority of bacteria (72%) were located in the biofilm especially in the distribution system, which accounts for the greatest surface area. Free-living bacteria accounted for 26% and only a minor part of the bacteria were in the sludge in the hot water tank (2%).