Improving drinking water quality by integrating ClO₂ pre-oxidation and UV/chlorine post-oxidation with conventional treatment units

Abstract

Increasing worldwide contamination of freshwater systems with micropollutants is a critical environmental problem. To address this problem, drinking water utilities are interested in upgrading their treatment facilities to enhance micropollutant removal and byproduct control. The UV/chlorine advanced oxidation process (AOP) effectively degrades a wide range of recalcitrant micropollutants in drinking water. However, the generation of chloro-organic disinfection byproducts (DBPs) in this process remains a major concern. This thesis work proposed an advanced treatment train by applying pre-oxidation by chlorine dioxide (ClO₂) followed by coagulation-flocculation-sedimentation (i.e., conventional treatment units, CTUs) and the UV/chlorine AOP to simultaneously enhance micropollutant abatement and reduce byproduct formation in drinking water treatment. The impacts of ClO₂ pre-oxidation on the formation of byproducts (e.g., chloro-organic DBPs, chlorite (ClO₂-), and chlorate (ClO₃-)) and the generation of reactive species in the post-CTUs and UV/chlorine AOP were investigated. Experimental results showed that the ClO₂ pre-oxidation reduced the six investigated chloro-organic DBP formation by 16.3%–37.6%, while DCAA and TCAA decreased by 26.1% and 13.2%, respectively. As a result, the toxicity associated with these DBPs decreased by 25.1% in the post-UV/chlorine AOP. The CTUs also have a major role in the DBP control, reducing the DBP formation by 21.6%–52.7% compared to the integrated process without the employment of CTUs. However, ClO₂ pre-oxidation decreased the degradation rate constant of a model micropollutant (carbamazepine) by 53.5%. The inhibition of carbamazepine degradation was verified to be attributed to the reduction of the steady-state concentrations of radicals (HO^•, Cl^•, and ClO^•) in the UV/chlorine AOP. The reduction of radical concentration was attributed to the ClO₂- formed from ClO₂ pre-oxidation, which is a strong radical scavenger and could not be removed by the conventional coagulation process using aluminum sulfate. Moreover, ClO₂- was oxidized by the radicals and enhanced chlorate (ClO₃-) formation in the UV/chlorine AOP and increased the associated health risks of the treated water. To reserve the merits of using ClO₂ pre-oxidation for reduction of chloro-organic DBPs in the UV/chlorine AOP and meanwhile eliminate the negative impacts of the ClO₂- formed from ClO₂ pre-oxidation, this thesis work proposed to dose micromolar-level ferrous iron (Fe(II)) into aluminum-based coagulants. Such modification of the coagulation process effectively eliminated the ClO₂- generated from ClO₂ pre-oxidation, which significantly reduced the radical scavenging and ClO₃- formation from ClO₂-, without much affecting the formation chloro-organic DBPs. Experimental results showed that the addition of 52.1-μmol/L FeSO₄ effectively eliminated the 10.8 ± 0.4-μmol/L of ClO₂- generated from the pre-oxidation using 1.0 mg/L (14.8-μmol/L) of ClO₂. Furthermore, ClO₂- reductions increased the degradation rate constant of carbamazepine by 55.0% in the post-UV/chlorine process. The enhanced degradation was attributed to the increased steady-state concentrations of HO^• and ClO^• by Fe(II) addition. Moreover, Fe(II) addition also decreased the ClO₃- formation by 53.8% in the UV/chlorine process, and its impact on the formation of chloro-organic byproducts was minor. The findings in this thesis work demonstrated a promising strategy to improve drinking water safety and quality by adding low-level Fe(II) in coagulation in an advanced drinking water treatment train (ClO₂ pre-oxidation + CTUs + UV/chlorine AOP).

Date of Award	2022
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Chii SHANG (Supervisor)