Effects of Sodium Hypochlorite on Daphnia spp. Populations and Resting Eggs Hatching in Urban Wastewater Treatment
Year: 2026
Type: article
Abstract: The proliferation of daphnids in secondary clarifiers of urban wastewater treatment plants (WWTPs) can compromise effluent quality and disrupt treatment stability. This study evaluated the effectiveness of sodium hypochlorite (NaOCl) for controlling daphnid populations and assessed its influence on dormant eggs hatching. A pilot-scale oxidation ditch activated sludge system was operated under conditions simulating a full-scale WWTP. Acute toxicity tests were performed in clarified water (CW) and mixed liquor (ML) using NaOCl concentrations between 0.76 and 5 mg L−1, with mortality monitored over 96 h and LC50 values determined. In CW, concentrations ≥ 3.125 mg L−1 caused 100% mortality within 24 h (24 h LC50 = 1.75 mg L−1). In ML, toxicity was significantly reduced (24 h LC50 = 7.43 mg L−1). Statistical analysis confirmed NaOCl concentration as the main driver of mortality, with additional contributions from operational parameters such as electrical conductivity, total dissolved solids, and dissolved oxygen. Hatching assays revealed that higher NaOCl concentrations and prior cold exposure (4 °C) increased ephippia hatching, reaching 40% under combined conditions. Although NaOCl effectively inhibits active organisms, it may stimulate dormant egg hatching, potentially sustaining populations. Optimized control strategies are therefore required to ensure effective and sustainable daphnid management in WWTPs.
Source: Urban Science
Authors: Pedro Esperanço, Carolina Coelho, Olímpia Sobral, Verónica Oliveira, António Luís Amaral, Carla Rodrigues
Language: English
Subfield: Health, Toxicology and Mutagenesis
Field: Environmental Science
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,930
OpenAlex ID: W7167042330