Early-stage targeted low-dose chlorine-bentonite treatment for cyanobacterial bloom control in deep reservoirs

Cyanobacterial blooms can threaten water supply safety worldwide. Previous studies have employed various chemical treatments (e.g., natural/modified clays, oxidants) for cyanobacterial bloom control when blooms were already clearly visible with high biomass. However, the early/initiation stages of blooms have been ignored, with a lower-biomass and a higher sensitivity to chemicals agents. In this study, we used a novel approach: low-dosage chlorine, combined with bentonite, applied at the early bloom stage for bloom control in a deep drinking water Reservoir Kengkou. Then, cyanobacterial biomass, water quality parameters, and micro-eukaryotic community were analyzed before and after the treatment. Results showed that the optimized combined treatment using extremely low-dosage of chlorine (0.002 mg/L) and bentonite (0.2 mg/L), effectively eliminated the cyanobacterial bloom through flocculation-settling removal and photosynthetic inactivation. Total and dissolved cyanotoxins risk did not occur after the treatment. Meanwhile, the water quality significantly improved following the treatment, including a decrease in pH, turbidity, and chemical oxygen demand (COD). The α-diversity of the micro-eukaryotic community increased by 11.5% after seven weeks, and the relative abundance of Chlorophyta, Cryptophyta, and Bacillariophyta, increased by 2.7, 2.4, and 6.3-folds respectively, implying the successful ecological restoration following the treatment. Overall, the application of low-dosage combined chlorine and bentonite could be a promising approach for cyanobacterial bloom control in the early stage, and with significant environmental and ecological benefits.