Fate of toxic cyanobacterial genera from natural bloom events during ozonation

Intense accumulation of toxic cyanobacteria cells inside plants, unsuccessful removal of cells and consequent breakthrough of cells and toxins into treated water have been increasingly documented. Removal or destabilisation of cells in the pre-treatment stage using pre-ozonation could be an efficient practice as ozonation has been proven to be effective for the removal of cells and toxins. However, several unknowns including the ozone demand, the potential release of cell-bound toxins and organic matter and their impact on treatment train needs to be addressed. The general objective of this work was to study the impact of direct ozonation on different potentially toxic cyanobacteria genera from natural blooms. Water samples from five cyanobacterial bloom events in Lake Champlain (Canada) were ozonated using 2-5 mg/L O3 for a contact time of maximum 10 min. Cyanobacterial taxonomic enumeration, cyanotoxins, organic matter and post-chlorination disinfection by-product formation potential analyses were conducted on all samples. Anabaena, Aphanizomenon, Microcystis and Pseudanabaena were detected in bloom water samples. Total cell numbers varied between 197,000 and 1,282,000 cells/mL prior to ozonation. Direct ozonation lysed (reduction in total cell numbers) 41%-80% of cells and reduced released toxins to below detection limits. Microcystis was the genus the least affected by ozonation. However, DOC releases of 0.6-3.5 mg/L were observed leading to maximum 86.92 μg/L and 61.56 μg/L additional total THMs (four trihalomethanes) and HAA6 (six haloacetic acids) formation, respectively. The results of this study demonstrate that vigilant application of pre-ozonation under certain treatment conditions would help to avoid extreme toxic cells accumulation within water treatment plants.