Comparative evaluation of disinfection processes for inactivation of Chironomid larvae in drinking water treatment SOEUNG Sreynich

Abstract

Chironomid larvae have recently been increasingly detected in tap water supplies in Korea and other countries, raising public concerns over drinking water safety. As emerging biological contaminants, effective control of these larvae during drinking water treatment is essential, particularly through physical and chemical disinfection processes. Although several studies have evaluated larval inactivation by water disinfectants, their applicability has been limited by a lack of consideration for realistic water conditions and residual disinfectant concentrations. This study quantitatively compared the inactivation efficiency of Chironomid larvae exposed to four disinfectants: free available chlorine (FAC; HOCl/OCl-), chlorine dioxide (ClO₂), ozone (O₃), and ultraviolet (UV254) irradiation at pHs 7.0 and 8.5 (10 mM phosphate buffer) and at 15°C and room temperature (23±2°C). Inactivation was assessed based on C x T values for chemical disinfectants (mg/L × min) and UV fluence (mJ/cm²) for UV254 irradiation, based on microscopic observation of larval movement. All treatments exhibited sigmodal inactivation curves fitted by non-linear regression, with a noticeable lag phase in fourth-instar larvae, suggesting physical and biochemical resistance. Complete inactivation required C x T values of 129.34 ± 43.20 and 662.5 ± 132.6 mg/L × min for FAC, 1262.81 ± 128.23 and 2565.68 ± 164.20 mg/L × min for ClO2, and 8.19 ± 0.60 and 10.76 ± 1.72 mg/L × min for O3 in first- and fourth-instar larvae, respectively. UV254 fluences of 2750 ± 354 and 5786 ± 303 mJ/cm² were required for complete inactivation of first- and fourth-instar larvae. Overall, O₃ and ClO₂ demonstrated the highest efficacy, while first-instar larvae were more susceptible to oxidative disinfectants due to their thinner cuticles and less developed defense mechanisms. These findings provide quantitative insights useful for optimizing and upgrading oxidation and disinfection processes in drinking water treatment plants to effectively control chironomid larvae and enhance biological safety.