A longitudinal study of mass loads of quaternary ammonium compounds in sludge associated with COVID-19 Stringency Index indicators in Beijing, China

This study estimated population normalized mass loads (PNML) of quaternary ammonium compounds (QACs) including alkyltrimethylammonium chloride (ATMAC), benzylalkyldimethylammonium chloride (BAC), and dialkyldimethylammonium chloride (DADMAC) and assessed their associations with COVID-19 containment policies. We collected daily sludge specimens from five wastewater treatment plants in Beijing from July 2020 to May 2022. Using liquid chromatography-tandem mass spectrometry, we determined QAC concentrations and backward-estimated their PNML. We used COVID-19 Oxford Stringency Index and its metrics to measure containment policies. We analysed the relationship between QACs and containment measures using linear regression models. Among QACs categories, ΣBAC had median PNML of 94.54 μg/person/day, ΣDADMAC had 282.36 μg/person/day, and ΣATMAC had 802.55 μg/person/day. The ΣQACs median PNML was 1186.12 μg/person/day. The PNML of QACs appeared to have general increasing trend over time and were significantly different across seasons. PNML increased on average by 11 μg/person/day (95 % CI: 3.2, 19) for ΣBAC per 10-unit increase in Stringency Index score. Notably, higher stringency levels in three index indicators — workplace closures, internal movement restrictions, and public gathering restriction were positively associated with higher ΣQACs PNML. Sludge derived QACs may reflect the magnitude of disinfectant use at different stages of pandemic control response.