In this study, recent trends in off-flavor problems were investigated by a nationwide questionnaire targeting 142 drinking water treatment plants (DWTPs) in Japan. The results indicated that 46 of 142 DWTPs in Japan had experienced off-flavor problems in a recent 5-year period. Cyanobacteria such as Anabaena spp., which produce musty/earthy odors, are the most common microorganisms that were presumed by DWTPs to be responsible for producing off-flavor compounds. Other microorganisms, such as diatoms, chrysophytes, and actinomycetes, are also involved in producing off-flavors. To eliminate off-flavor compounds, most facilities use powdered activated carbon treatment, which greatly increases operational costs. In addition, an interview survey targeting one typical DWTP affected by off-flavor problems was performed to investigate operational costs increased by off-flavor problems and the effects of water temperature on the occurrence of off-flavors. The operational costs when an off-flavor problem occurred were 2.9 times those when the problem did not occur, mainly due to the cost of powdered activated carbon. Therefore, off-flavors are a very serious problem in water treatment systems in Japan. The interview survey suggested that water temperature would be one of the important factors influencing the occurrence of off-flavor compounds in water sources.

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