Effects of Sodium Hypochlorite on Daphnia spp. Populations and Resting Eggs Hatching in Urban Wastewater Treatment

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.