Effects of chlorine dose on the composition and characteristics of chlorinated disinfection byproducts in reclaimed water.

During chlorination of reclaimed water, the dose of chlorine used can influence the formation of chlorinated disinfection by-products (Cl-DBPs). We used non-targeted screening by Orbitrap mass spectrometry to identify Cl-DBPs in samples of chlorinated reclaimed water, and found that chlorination was likely to form slightly oxidized unsaturated aliphatic compounds and polycyclic aromatic compounds. Increasing the chlorine dose increased the proportion of polycyclic aromatic Cl-DBPs containing one chlorine atom (Cl-DBPs) and highly oxidized unsaturated aliphatic Cl-DBPs containing two chlorine atoms (Cl-DBPs). In addition, increasing the chlorine dose first decreased and then increased the proportion of Cl-DBPs with aromatic index values >0.5 and increased the proportion of Cl-DBPs with aromatic index values <0.5. Increasing the chlorine dose increased double bond equivalent minus oxygen values of Cl-DBPs and decreased the double bond equivalent minus oxygen values of Cl-DBPs, while the nominal oxidant state of carbon decreased for Cl-DBPs and increased for Cl-DBPs. In considering the possible precursors of Cl-DBPs and their reaction pathways, substitution reactions occurred more readily with aliphatic compounds and addition reactions occurred more readily with aromatic precursors. When the chlorine dose is increased, more Cl-DBPs may be formed by substitution. Overall, the chlorine dose influences Cl-DBP formation and composition and should be taken into account during water treatment.