Effect of heat flushing on the concentrations of Legionella pneumophila and other heterotrophic microbes in hot water systems of apartment buildings

The decontamination of Legionella pneumophila and other heterotrophic microbes by heat flushing in four legionellae-positive hot water systems was studied. Before the decontamination procedure, the concentration of legionellae varied from 3.0 x 10(3) to 3.5 x 10(5) cfu/L and the hot water temperature from 43.6 to 51.5 degrees C. During the contamination the temperature was raised to 60-70 degrees C. All taps and showers were cleaned from sediments and flushed with hot water twice a day for several minutes. The decontamination lasted for 2-4 weeks. In a few weeks the heat-flushing method reduced the concentration of legionellae below the detection limit (50 cfu/L) in the hot circulating water system just before and after the heat exchanger. The high hot water temperature also decreased the viable counts of heterotrophic bacteria, fungi, and total microbial cells determined by the epifluorescent microscopy. However, the eradication of legionellae failed in a water system where the water temperature remained below 60 degrees C in some parts of the system. After the decontamination, the temperature of hot water was lowered to 55 degrees C. Thereafter, all the studied hot water systems were recolonized by legionellae within a few months, showing that the decontamination by heat flushing was temporary. Also, the contamination of other bacteria increased in a few months to the level before decontamination.