Acute inhibition of methane production by quaternary ammonium compounds in anaerobic digestion varies widely based on chemical structure

Quaternary ammonium compounds (QACs) are widely used in consumer products and industrial processes as disinfectants. In water resource recovery facilities, a substantial portion of QACs reside with solids, and thus pass through anaerobic digesters. As antimicrobial agents, QACs present a potential risk to digester operation, yet we have limited understanding of their toxicity to this unit operation. This research combined experimental analysis with advanced machine learning to evaluate acute QAC exposure impacts on anaerobic digestion and to identify the main molecular drivers of toxicity. Among the QACs tested, C₈-DADMAC exhibited the strongest inhibitory effect on the maximum methane production rate in anaerobic digesters, causing a 10 % reduction at an average concentration of 2040 mg/kg TSS. QAC mixtures were found to exhibit synergistic inhibition, while long-chain benzalkyldimethyl ammonium compounds (BACs) showed less inhibitory effects compared to other commonly used QACs such as dialkyldimethylammonium compounds (DADMACs). A robust mechanistic model, validated by both Random Forest and CatBoost algorithms, revealed that the intrinsic toxicity of QACs is fundamentally driven by a non-linear interplay of three foundational features: molecular size, flexibility, and conformational strain. Together, these findings advance understanding of the acute toxicity mechanisms of QACs under methanogenic conditions and provide predictive approaches to guide the development of QACs with reduced impacts on anaerobic digestion processes. Moreover, the inhibitory threshold values (IC₁₀ & IC₅₀) for the 13 QACs tested can inform guidelines on QAC concentrations in anaerobic digesters that have potential to impact methane production.