Are quaternary ammonium compounds emitted into surface waters through wastewater or urban runoff?

Quaternary ammonium compounds (QACs) such as benzalkonium chlorides (BACs) are used in human hygiene, food production and building protection. However, the use of these compounds on roofs is popular, but illegal. There are, thus, two main pathways for QACs to enter recipients: 1) legal use in human hygiene and emission via wastewater and 2) illegal use on rooftops and emissions via runoff water. However, the relative relevance of these two pathways for the occurrence and levels of QACS observed in surface waters is unknown. To resolve this issue, a process study was conducted on a small riverine system. QAC concentrations were below 0.1 ng/L during dry weather while they reached 90 ng/L during rainy weather. The concentration patterns of the parent QACs indicate that they originate mainly from surface runoff. Additionally, tributaries without wastewater treatment discharges delivered significant or even major QAC loads during rain events, also suggesting surface runoff from treated buildings as the predominant source. This interpretation is further solidified by using metoprolol as a marker for the river's wastewater fraction, thus calibrating a mass balance model. Both support the hypothesis that urban runoff dominates wastewater treatment plants as a source of QACs. On top of the parent compounds, QAC metabolites, originating from microbial degradation processes, were also detected.