Controls of disinfection byproducts and biotoxicity in chlorinated post-denitrifying effluents: Optimization of the carbon-to-nitrogen ratio and hydraulic retention time.

As an effective control strategy, optimizing operational parameters of wastewater treatment processes prior to disinfection can reduce disinfection byproduct (DBP) precursors, thereby controlling DBP formation and associated biotoxicity. For the widely applied tertiary post-denitrification process, however, the effectiveness and underlying mechanisms of two key parameters-the carbon-to-nitrogen (C/N) ratio and hydraulic retention time (HRT)-in controlling DBPs and biotoxicity remain unclear. In this study, the formation of 32 halogenated DBPs and adsorbable organic halogen (AOX), as well as the acute toxicity of luminescent bacteria, were investigated under different C/N ratios (3-6) and HRTs (7-24 h). Results showed that HRT had a more pronounced effect than the C/N ratio on DBPs, AOX, and toxicity. At 30 °C with a C/N ratio of 6, extending HRT from 7 h to 24 h reduced carbonaceous DBPs, nitrogenous DBPs, AOX, and toxicity by 73%, 45%, 19%, and 17%, respectively. Correlation analyses suggested that AOX exhibited a significant positive linear correlation with the total fluorescence intensity of post-denitrifying effluents, and that tryptophan-like and humic-like components were identified as primary precursors of AOX. Additionally, AOX was found to be the main toxicity driver. Based on established linkages between operational parameters, microbial communities, and fluorescence components, a sequential pathway was proposed: operational parameters (primarily HRT) regulate specific microbial taxa, which mediate fluorescent precursor production, further influence DBPs and AOX formation, and ultimately affect biotoxicity. This pathway preliminarily provides actionable guidance from engineering and microbiological perspectives for the source control of DBPs and biotoxicity in chlorinated wastewater.