Low-risk cyanobacterial bloom sources: Cell accumulation within full-scale treatment plants

Toxic cyanobacteria in drinking water sources and within drinking water treatment plants (DWTPs) have been increasingly documented, even in regions with historically infrequent blooms. The main objective of the authors' research was to study the accumulation of potentially toxic cyanobacteria within two full-scale DWTPs (Eastern Canada) fed by water sources considered to have a low risk of cyanobacterial presence. Intensive in vivo measurements were conducted on raw, clarified, filtered, and chlorinated waters. Samples were also taken for microscopic counts, toxin analyses, and water characterization. Cyanobacterial profiles were mapped in the sedimentation and filtration basins using a fluorescence probe. Even though cell numbers at the water intake were below 400 cells/mL, an accumulation of cyanobacterial cells was observed in the sludge bed of clarifiers (more than 1 × 106cells/mL) and on the surface of the sedimentation and filtration basins. Microcystis and Gloetrichia were the dominant genera. Preozonation of raw water helped with the removal of cells in the clarification process.