Different response pattern of cyanobacteria at development and maintenance stage to potassium permanganate oxidation

Highlights: • Elevated protein contents resulted in lower oxidant exposure at maintenance stage. • Cyanobacteria at maintenance stage held higher resistance to KMnO₄ attack than development stage. • Elevated cyanobacterial biomass decreased toxin degradation efficiency for maintenance stage. • Practical strategy of KMnO₄ treatments was depending on the stage of cyanobacteria. Occurrence of successive cyanobacterial blooms in source waters can continuously impair drinking water quality. Previous studies have separately investigated potassium permanganate (KMnO₄) to treat high-viability cyanobacteria at just one stage of either development or maintenance. However, maintenance stage exhibited significantly higher cell-density and extracellular organic matters (EOMs) than development stage, which may result in a different KMnO₄ oxidation pattern. In this study, kinetics of oxidant decay, membrane integrity loss, and toxin degradation of high-viability cyanobacteria at both stages were compared. Results showed that cyanobacteria at maintenance stage became more resistant to KMnO₄ oxidation than that at development stage, since elevated cell-density and more proteins involved in EOMs resulted in lower oxidant exposure at this stage. Meanwhile, elevated cyanobacterial biomass became the main competitors to decrease toxin degradation efficiency at maintenance stage, leading to incapacity to degrade extracellular toxin to below safety guideline of 1 μg L⁻¹. Consequently, comparing with the best strategy for development stage (6 mg min L⁻¹, no membrane damage), a higher oxidant exposure (12 mg min L⁻¹) was recommended to treat cyanobacteria at maintenance stage even with slight membrane damage (19%), since it degraded extracellular toxin to below safety guideline and achieved the highest removal ratio of EOMs. Overall, this study demonstrated that stage of cyanobacteria can strongly affect KMnO₄ oxidation pattern, and it is necessary for water supplies to optimize KMnO₄ treatments depending on bloom stage.