Comparing PAC/UF and conventional clarification with PAC for removing microcystins from natural waters

PAC/UF was investigated for removing M. aeruginosa cells and microcystins from natural waters and its performance was compared with the conventional clarification with PAC addition (PAC+C/F/S). Ozonated and clarified waters from Tavira's Water Treatment Plant were used. Both processes achieved an absolute removal of chlorophyll-a, but greater turbidity and microcystins (intra and extracellular) removals were reached by PAC/UF. With PAC/UF, 10 mg/L PAC resulted in a cycle-averaged concentration of microcystins in the permeate (0.72 μg/L MC-LReq) below the WHO guideline value, while the water quality obtained with 15 mg /L PAC+C/F/S was far beyond that guideline value. However, the occurrence of cell lysis during UF (with subsequent release of microcystins and UV254nm absorbing substances) and the preferential removal of high molar mass compounds by coagulation yielded better UV254nm removals by PAC+C/F/S. Natural organic matter showed a small impact onto microcystins removal, with greater effect of some algogenic compounds, but especially of high concentrations of humic and tannic-like compounds.