Dissolved organic matter treatability and disinfection byproducts formation potential: Role of floc aging

Enhanced coagulation is a cost-effective method for removing particulates and organic matter in drinking water treatment. In full-scale operation, flocs may be subject to extended retention in sedimentation basins or sludge storage tanks, or be recirculated, processes that can trigger the release or re-adsorption of organic matter and subsequently influence disinfection byproducts (DBPs) formation. This study examines the interfacial migration of dissolved organic matter (DOM) during the aging of polyaluminum chloride (PACl) flocs and its impact on DBP generation. It revealed the effects of aging time and pH on organic matter adsorption dynamics. Over 5 days of aging, humic substances removed by coagulation were slowly re-released into the supernatant with aluminum nanoparticles, with an average release intensity of 9% at all pH levels; while proteins occupied the vacant active sites left by the released humic substances. Due to enhanced floc crystallization and electrostatic repulsion, the release of humic acid at alkaline pH (7 - 8) was 1.83 times higher than at acidic pH (6 - 7). The released humic substances, which mainly belonged to the lignins and tannins, exhibited high unsaturation, high aromaticity, and low molecular weight (<380 Da). This reactive DOM pool directly influenced DBPs formation. After 5 days of aging, the concentrations of known DBPs (THMs, HAAs, HALs, HANs) increased by 27%, and the cytotoxicity increased by up to 31%, while the number of unknown DBPs (including C-DBPs and N-DBPs) rose by 180%. Mass difference reaction network analysis indicated that 89% of the DBPs precursors in the released DOM originated from lignins and tannins. These findings suggest that increasing the frequency of floc discharge from sedimentation tanks, maintaining slightly acidic pH conditions, and treating sludge storage tank supernatant separately are effective strategies for mitigating DBPs and enhancing finished water safety.