Fate and Biotransformation of Quaternary Ammonium Compounds in Biological Treatment Processes

Quaternary ammonium compounds (QACs) are organic, cationic surfactants extensively used in domestic, agricultural, healthcare, and industrial applications as surfactants, emulsifiers, fabric softeners, disinfectants, pesticides, corrosion inhibitors, and personal care products. As a result, QACs are ubiquitous micropollutants found worldwide in both engineered and natural systems. Most uses of QACs lead to their release into wastewater treatment systems or the environment. Total QACs concentrations from low to high μg/L have been reported. Although aerobic biodegradation of QACs has been observed, the extent of biodegradation varies depending on the QAC concentration, structure, microbial acclimation and presence of QAC resistant/degrading microorganisms. However, because QACs adsorb onto (bio)solids, adsorption generally outcompetes biodegradation and results in the transfer of QACs to anaerobic digesters in engineered systems as part of the primary and waste activated sludge or to sediments in natural systems. In spite of the fact that QACs accumulate in anaerobic engineered and natural systems, their fate and effect under anaerobic conditions have not been studied comprehensively. In addition, QACs contribute to the induction and co-selection of antibiotic resistance genes, a serious and emerging environmental problem.The presentation will focus on results of research being conducted relative to the fate, toxicity and biotransformation of representative QACs. Nine QACs, belonging to three homologous groups — monoalkonium, dialkonium and benzalkonium chlorides – have been the target QACs. The QACs critical micelle concentration (CMC) was determined and then used as a descriptor to derive relationships between QAC structure and partitioning to biosolids as well as acute Microtox® toxicity. The biotransformation potential of benzalkonium chlorides (BAC) — the most commonly used QACs found in engineered and natural systems — under aerobic, methanogenic, nitrate reducing, and fermentative conditions was evaluated using bioenergetics and batch bioassays. The aerobic BAC biotransformation involved sequential dealkylation and debenzylation steps resulting in the formation of benzyl dimethyl amine, and dimethyl amine, respectively. The bacterial community involved in the aerobic BAC degradation was mainly composed of species belonging to the Pseudomonas genus. All QACs tested were recalcitrant under methanogenic conditions and inhibited methanogenesis at and above 25 mg QAC/L. Under nitrate reducing and fermentative conditions, BAC was transformed to alkyldimethyl amines via an abiotic reaction known as modified Hofmann degradation and a biotic reaction known as fumarate addition, respectively. Both reactions are based on a mechanism known as nucleophilic substitution. The discovery of these two novel (bio)transformation pathways is the first ever report documenting QAC transformation under anoxic/anaerobic conditions and delineate the transformation pathway. The implications of such transformations will be discussed. This research has contributed to a better understanding of the environmental and human health risks associated with QACs by providing systematic information on physical, chemical and biological processes that determine the fate and effect of QACs in engineered and natural systems.