In this study, we analyzed the actual situation of water suppliers in Japan in measuring heterotrophic plate count (HPC) bacteria that has been included in the complementary items of drinking water quality standards since the fiscal year of 2008 using the Water Supply Statistics. As a result, we found that the number of suppliers measuring HPC increased year by year, and in the fiscal year of 2019, 34% of the suppliers measured HPC in tap water and 57% measured HPC once a year. In addition, a strong correlation was found between standard plate count and HPC in raw water. Furthermore, HPC in raw water tended to be higher in surface water than in groundwater, while in the outlet water of water purification plants and tap water, HPC tended to be higher in systems using groundwater as raw water. Annually approximately 50 water supply systems reported that the arithmetic mean of HPC in a water supply tap increased by more than one order of magnitude compared to that in the outlet of water purification plant.

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