Challenges and ecological implications of residual PAC and SPFS coagulants in harmful algal bloom control

This study examines the impact of micro-coagulants, which refer to residual coagulants remaining in the water after treatment, on algal blooms, particularly in controlling harmful algal blooms (HABs) caused by cyanobacteria such as Microcystis spp. Using sodium polyferric sulfate (SPFS) and polyaluminum chloride (PAC) as model coagulants, we evaluated their effects at varying concentrations on algal growth, nutrient dynamics, and community structure. Results revealed that low concentrations of coagulants promoted Microcystis growth and microcystin production, likely due to trace elements serving as essential micronutrients and highlighting an overlooked ecological risk of coagulant discharge. On the contrary, high concentrations effectively inhibit the proliferation of cyanobacterial by reducing bioavailable nitrogen and phosphorus, promoting nutrient flocculation, and promoting the growth of green algae and diatoms rather than cyanobacteria. High concentration coagulants also increase dissolved oxygen and pH values, further supporting non-toxic algal species. This study emphasizes the dual role of coagulants in HABs management: while high concentrations mitigate HABs, residual coagulants discharged into natural water bodies need to be further considered in environmental management to prevent unexpected ecological impacts.