Factors affecting mass inflow of quaternary ammonium compounds into Japanese sewage treatment plants.

Quaternary ammonium compounds (QACs), ecotoxic organic chemicals linked to multidrug resistance, are being used increasingly, for example to prevent the transmission of infections such as covid-19, in households, hospitals, and industry. To understand the locations, fluctuations, and fractions of QACs entering sewers, we monitored 14 QACs (benzalkonium chloride [BAC]-C8, C10, C12, C14, C16, and C18; dialkyldimethylammonium chloride [DDAC]-C8, C10, and C12; alkyltrimethylammonium chloride [ATAC]-C12, C16, and C18; benzethonium chloride; and cetylpyridinium chloride), and a disinfectant (chlorhexidine) in influent at four Japanese sewage treatment plants (STPs) five times throughout a year. Mass inflows were relatively stable throughout the year, indicating that the recent seasonal covid-19 epidemic did not greatly influence them. The differences in mass inflows among the STPs were normalized successfully by sewered residential population (most relative SDs were <30%), implying households to be the main source. Per-capita mass inflows accounted for 58%-73% of the per-capita consumption of BAC-C12 + C14 + C16, 28%-59% of that of DDAC-C10, 52%-70% of that of ATAC-C12, 86%-99% of that of ATAC-C16, and 64%-82% of that of ATAC-C18, indicating that a large proportion of their consumption entered sewers. The high contribution of ATAC-C16 agreed with its limited use in primary and secondary industries, little of whose wastewaters enter sewers, whereas the low contribution of DDAC-C10 is attributable to its substantial use in animal husbandry. Our first observation of fractions of QACs entering sewers will advance the management of environmental risks.