The formation and control of disinfection by-products by two-step chlorination for sewage effluent: Role of organic chloramine decomposition among molecular weight fractions

•Two-step chlorination formed less DBPs than one-step for wastewater effluent.•Decomposition of organic chloramines was responsible for the lower HAN formation.•High and medium MW DON tend to form organic chloramines during first chlorination.•The low MW DON were the predominate precursors of decomposable organic chloramines.•Two-step chlorination lowers the microbiological impacts of receiving water. With the increasing discharge of wastewater effluent to natural waters, there is an urgent need to achieve both pathogenic microorganism inactivation and the mitigation of disinfection by-products (DBPs) during disinfection. Studies have shown that two-step chlorination, which injected chlorine disinfectant by splitting into two portions, was more effective in inactivating Escherichia coli than one-step chlorination under same total chlorine consumption and contact time. In this study, we observed a substantial reduction in the formation of five classes of CX3R-type DBPs, especially highly toxic haloacetonitriles (HANs), during two-step chlorination of secondary effluent when the mass ratio of chlorine-to-nitrogen exceeded 2. The shift of different chlorine species (free chlorine, monochloramine and organic chloramine) verified the decomposition of organic chloramines into monochloramine during second chlorination stage. Notably, the organic chloramines generated from the low molecular weight (< 1 kDa) fraction of dissolved organic nitrogen in effluent organic matter tended to decompose during the second step chlorination leading to the mitigation of HAN formation. Furthermore, the microbiological analysis showed that two-step chlorinated effluent had a slightly lower ecological impact on surface water compared to one-step chlorination. This work provided more information about the two-step chlorination for secondary effluent, especially in terms of organic chloramine transformation and HAN control. [Display omitted]