Linking the occurrence, biodegradation, and N-nitrosodimethylamine formation of N,N-dimethylbenzylamine in natural waters and wastewater treatment systems.

N,N-Dimethylbenzylamine (DMBA), a persistent intermediate from the biodegradation of benzalkonium chlorides, was identified as a potent precursor of the carcinogenic disinfection byproduct N-nitrosodimethylamine (NDMA). However, its environmental persistence and reactivity under biological treatment and chlorine-based disinfection remain unclear. This study examined DMBA and its degradation product N-methylbenzylamine (MBA) across natural waters, a full-scale wastewater treatment plant (WWTP), and lab-scale bioreactors. Field monitoring showed that DMBA was consistently more abundant than MBA in a connected river-lake system in southern Taiwan (2.4-6.2 µg/L), indicating persistence and transport potential. In the WWTP, anaerobic units retained DMBA (up to 8.3 µg/L) and generated MBA, whereas aerobic units reduced but did not completely remove DMBA. Lab-scale experiments confirmed these patterns: anaerobic reactors enriched Clostridium spp. and produced MBA as a stable intermediate, while aerobic reactors dominated by Pseudomonas spp. achieved more complete DMBA degradation. NDMA formation experiments revealed that DMBA is a reactive but partial precursor, with yields governed by chlorine-to-nitrogen ratios, seasonal hydrology, and treatment type. Wet-season waters exhibited higher NDMA yields (7.0-10.5 %), while dry-season matrices suppressed formation. Chloramination consistently produced greater NDMA yields (2.6-6.2 %) than chlorination, and anaerobic effluents yielded more than aerobic effluents. These results establish DMBA as an environmentally relevant NDMA precursor, emphasizing the need to optimize disinfection conditions and limit precursor persistence to mitigate NDMA risks in water treatment and reuse.