Occurrence and source identification of quaternary ammonium compounds in Japanese catchments with substantial unsewered areas

Quaternary ammonium compounds (QACs)—ecotoxic biocidal contaminants linked to multidrug resistance—enter sewers owing to their domestic and commercial applications. Though high rates of removal of QACs in sewage treatment plants (STPs) were reported, little is known about their fates in on-site systems used in unsewered areas. Four times during a year, we monitored 14 QACs (benzalkonium chloride [BAC]-C8, C10, C12, C14, C16, C18; benzethonium chloride; cetylpyridinium chloride; dialkyldimethylammonium chloride [DDAC]-C8, C10, C12; and alkyltrimethylammonium chloride [ATAC]-C12, C16, C18), a disinfectant (chlorhexidine), and 15 drugs at 15 sites along river stretches and at an inlet and outlet of an STP in densely populated but still largely unsewered Japanese catchments. The maximum river-water concentrations of 2 ATACs were in the range of 10–100 μg/L; those of 3 BACs, 3 DDACs, 1 ATAC, and 3 drugs were 1–10 μg/L; and those of the others were < 1 μg/L. The higher mean river-water concentrations of 13 QACs removed in the STP at >1.5 log (>96.8 %) than those in STP effluent were attributable to untreated or poorly treated wastewater, consistent with the significant correlation of analyte composition between river water and raw sewage (r_s = 0.83, P < 0.01). A principal component analysis with tracer drugs linked 8 QACs of STP removal >2.5 log (>99.7 %) with untreated or poorly treated household wastewater, and the other 6 QACs with the total quantity of household wastewater or with both household and commercial wastewaters. Our findings will advance the understanding of managing QACs in unsewered areas.