Breaking it down: Unveiling the roles of chemical structure, concentrations and technology on micropollutant biotransformation in wastewater treatment plants

Complex mixtures of anthropogenic micropollutants (MPs) pose risks to ecosystems and health by entering the environment through multiple pathways. Microbial biotransformation is a major removal process, yet the key drivers remain poorly understood. Activated sludge (AS) treatment in wastewater treatment plants (WWTPs) acts as a partial barrier and reduces MP emissions into aquatic ecosystems, yet removal efficiencies vary widely across MPs. Here, we studied the biotransformation of 189 MPs in AS from six Swiss WWTPs operated at three treatment conditions: carbon elimination (Celim), nitrification-denitrification (DeNit), and moving bed biofilm reactors (MBBR). MPs were spiked into lab-scale assays with AS sampled from those WWTPs and analyzed by high-resolution mass spectrometry to derive biotransformation rate constants. Overall, biotransformation efficiency followed the order of solids retention time (MBBR>DeNit>Celim), and, across MPs, their average influent concentration (C_inf) was the most decisive factor for increasing observed transformation. Using a multivariate analysis approach, we further identified structural features that were key determinants of MP biotransformation. Presence of functional groups amenable to broadly co-metabolic processes (e.g., amides, esters, sulfonamides) resulted in fast biotransformation and consistently low variability across treatment technologies. In contrast, we found ample evidence for catabolic degradation of many of the MPs, which mostly exhibited high variance across treatment technologies, and identified specific structural features that increased the likelihood of catabolic degradation. Contrary to prevailing assumptions, catabolic degradation was found to be widespread, especially in MBBR, and not limited to high-C_inf MPs. These findings provide new insights into the fate of MPs during microbial biotransformation, and highlight the need to distinguish co-metabolic from catabolic pathways when further investigating the structural determinants that lead to increased biotransformation. As such, our study is in line with the EU's Safe and Sustainable by Design (SSbD) framework, emphasizing proactive chemical design for full biodegradability.