Dosing low-level ferrous iron in coagulation enhances the removal of micropollutants, chlorite and chlorate during advanced water treatment.

Drinking water utilities are interested in upgrading their treatment facilities to enhance micropollutant removal and byproduct control. Pre-oxidation by chlorine dioxide (ClO) followed by coagulation-flocculation-sedimentation and advanced oxidation processes (AOPs) is one of the promising solutions. However, the chlorite (ClO) formed from the ClO pre-oxidation stage cannot be removed by the conventional coagulation process using aluminum sulfate. ClO negatively affects the post-UV/chlorine process due to its strong radical scavenging effect, and it also enhances the formation of chlorate (ClO). In this study, dosing micromolar-level ferrous iron (Fe(II)) into aluminum-based coagulants was proposed to eliminate the ClO generated from ClO pre-oxidation and benefit the post-UV/chlorine process in radical production and ClO reduction. Results showed that the addition of 52.1-µmol/L FeSO effectively eliminated the ClO generated from the pre-oxidation using 1.0 mg/L (14.8 µmol/L) of ClO. Reduction of ClO increased the degradation rate constant of a model micropollutant (carbamazepine) by 55.0% in the post-UV/chlorine process. The enhanced degradation was verified to be attributed to the increased steady-state concentrations of HO and ClO by Fe(II) addition. Moreover, Fe(II) addition also decreased the ClO formation by 53.8% in the UV/chlorine process and its impact on the formation of chloro-organic byproducts was rather minor. The findings demonstrated a promising strategy to improve the drinking water quality and safety by adding low-level Fe(II) in coagulation in an advanced drinking water treatment train.