Assessing the Efficacy of Coagulation (Al3+) and Chlorination in Water Treatment Plant Processes: Inactivating Chironomid Larvae for Improved Tap Water Quality

Abstract

In response to the finding of chironomid larvae in tap water, South Korea’s water treatment system has stepped up water quality monitoring. However, due to the challenging nature of larval behavior, effective elimination remains difficult despite the use of various purification techniques such as coagulation, sedimentation, filtration, and disinfection. Based on it, the aim of this study is to evaluate the effectiveness of coagulation and chlorination in inactivating chironomid larvae and investigate their behavior. The coagulation experiment tested how the behavior of the larvae changed in water with a high turbidity level of ±100 mg/L and 2 mg/L Al3+ as a coagulant, compared to water with a lower turbidity level of ±30 mg/L and 1 mg/L of Al3+ as a coagulant. The larvae were exposed to various doses of chlorine (0.5–20 mg/L as Cl) in 500 mL beaker glasses. The behavioral activity of the larvae was observed at different time points for 5 days. It was found that chironomid larvae exhibit different responses to exposure to coagulant and chlorine, with coagulation causing the formation of flocs that cover the larval body as a protective measure. Conversely, exposure to chlorine causes a decrease in activity and growth, leading to the death of the larvae and subsequent melting. The results showed that the first instar larvae dead after 48 h of exposure to coagulation treatment, while the first instar larvae exposed to chlorination experienced mortality after a mere 5 min of exposure to 10–20 mg/L as Cl. The larvae can still grow and transform into pupae and adults, especially during the third and fourth instars, even after exposure to coagulant and chlorine with low dosage. These findings suggest that the floc generated during coagulation must be thoroughly cleaned, as it may contain larvae that can persist and develop further. Furthermore, the presence of larvae during the chlorination process highlights the need for alternative, more effective oxidants to be utilized in place of the conventional chlorine treatment.

Comprehensive Solutions to Prevent Larvae Breakout in Water Filtration Systems

“…On the other hand, chlorination serves as a potent oxidizing agent capable of penetrating the cell walls of Chironomidae larvae. It then oxidizes crucial cell components, including enzymes and genetic material, disrupting the larvae's vital functions and rendering them inactive [8,20].…”

“…Chironomidae larvae are known for their highly active behavior, characterized by their lopping swimming movements [8]. These larvae exhibit undulating movements, which further enhance their oxygen-capturing capabilities [24].…”

“…Efforts to improve water quality included filtration methods, yet challenges persisted [6,7]. In 2000, the emergence of small larvae midges in water treatment plants and tap water worldwide became a growing concern, continuing into 2020 in Korean facilities [8]. The presence of Chironomidae larvae in tap water results from multiple factors, including accidental entry through intake pipes [9].…”

“…Chironomidae larvae have unique adaptations that enable them to survive water treatment processes even resisting disinfection methods like chlorination [11]. Despite undergoing standard disinfection procedures, some Chironomidae larvae may remain viable and persist in the water supply [8]. Understanding the factors contributing to the presence of Chironomidae larvae in tap water is crucial for ensuring the safety and quality of drinking water.…”

RT-CLAD: Artificial Intelligence-Based Real-Time Chironomid Larva Detection in Drinking Water Treatment Plants

“…Recognized as indicator species, these larvae are sensitive to various environmental changes and exposure to harmful substances [ 4 ]. Therefore, it is imperative to control their emergence and reproduction during water treatment processes [ 5 ].…”