Formation and transformation of pre-chlorination-formed disinfection byproducts in drinking water treatment process

As pre-chlorination is increasingly adopted in drinking water treatment plant (DWTP), an attractive question emerged: how the disinfection by-products that formed during pre-chlorination (preformed DBPs) would be transformed in the drinking water treatment process? This study investigated the DBP formation kinetics and molecular characteristics in chlorinated source water, DBP transformation and removal in practical DWTP. It was found that the formation of trihalomethanes (THMs) followed pseudo first-order kinetic model and the intensified Br^- exposure facilitated the transformation of TCM into TBM. As Br^- concentration shifted from 0.5 mg L^-1 to 2.0 mg L^-1, the predicted maximum yield of TBM was doubled to 53.7 μg L^-1 with the increase of formation rate constant (k-value) from 0.249 h^-1 to 0.336 h^-1. Besides known DBPs, the molecular-scale investigation unveiled that the preformed unknown Cl-DBPs were a cluster of unsaturated aromatic DBPs ((DBE-O)/C_wa = 0.16, AI_{mod, wa} = 0.36) with high H/C (H/C_wa = 1.25). Pre-ozonation exhibited a preferential removal pattern towards condensed aromatic preformed Cl-DBPs with high H/C (AI_mod ≥ 0.67, H/C > 1.2 and O/C < 0.3). However, the removal of Cl-DBPs in coagulation-clarification process was limited with 56 more unknown Cl-DBP formulas identified. O₃-biological activated carbon process exhibited effective removal of preformed DBPs featured with low MW (carbon number ≤ 13), high unsaturation (DBE ≥ 7), condensed aromaticity (AI_mod ≥ 0.67), and higher H/C (H/C > 1.6). When the pre-chlorination process is adopted, the removal of preformed DBPs during the conventional treatment process is limited, while advanced treatment process can effectively remove these preformed DBPs.