[Mechanism of Pre-ozonation Enhanced Coagulation on DON in the Secondary Effluent]

Municipal wastewater effluent is increasingly used as reclaimed water and source water for downstream areas. Scholars are beginning to study the potential of dissolved organic nitrogen (DON) as a precursor to the formation of nitrogen disinfection by-products (DBP). In order to explore the mechanism of pre-ozonation coupled with enhanced coagulation on DON in the secondary effluent, the distributions of molecular weight and hydrophobicity/hydrophilicity of DON were analyzed. Disinfection by-product formation potential from chlorination was also investigated. Lastly, the composition and chemical structure of DON before and after coagulation were studied in combination with 3DEEM. Our results showed that pre-ozonation followed by coagulation with PACl significantly enhanced the removal of DON and mitigated DBP formation potentials. The following conclusions can be drawn from this study:① The tested effluent was very hard as indicated by total hardness and high calcium concentrations, and the pH had an important effect on pre-ozonation enhanced coagulation. As pH was decreased, the rates of DON, DOC and UV₂₅₄ removal increased. At pH 6.2, the rate of DON removal was maximal, and a 3.7 fold increase in removal-rate was achieved. ② Calcium played a critical role in pre-ozonation enhanced coagulation. Zeta potentials and calcium concentrations were substantially affected at higher ozone doses. At ozone dosage of 8 mg·L^-1, Zeta potentials increased from -33 to -8 mV, and calcium concentrations decreased from 116 to 89 mg·L^-1. The ratio of calcium hardness to DOC was relatively high in this study, approaching 24 mg·mg^-1 (CaCO₃/DOC). Thus, pre-ozonation promoted the removal of DON by coagulation. ③ Large molecular weight and hydrophobic DON were easier to remove by coagulation. Ozonation modified the DON characteristics to become more hydrophilic and smaller in molecular weight. Coagulation reduced overall DBP formation potentials to less than 20%. Pre-ozonation enhanced coagulation decreased overall DBP formation potentials by more than 45%. ④ Finally, the 3DEEM spectra indicated that the formation potentials and DON characteristics, due to disinfection, were related to three main peaks, which corresponded to tryptophan-based proteins, aromatic proteins, and fulvic acids.