Release and Removal of Microcystins from Microcystis during Oxidative-, Physical-, and UV-Based Disinfection

Cyanotoxins released from cyanobacteria (or blue-green algae) pose an increasing public health risk worldwide. In this study, the release of the cyanotoxin microcystin, from Microcystis aeruginosa due to oxidative, ultraviolet (UV), and physical impacts during water treatment was studied. Additionally, the relative and absolute rates of chemical oxidation of the six microcystins were determined for selected oxidants. Cell viability was measured based on treatment dosage using a fluorescence method. The specific chemical oxidants studied were free chlorine (HOCl/OCl−), chlorine dioxide, ozone, permanganate, and monochloramine. UV energy was at 254 nm. For chemical oxidants and UV, the exposures or doses examined were selected based on typical disinfection dosages. Other treatments examined included low and high salinity, ultrasonics, and physical blending. Free chlorine, permanganate, chlorine dioxide, monochloramine, and ozone were observed to at least partially disinfect the cyanobacteria, while the other disinfectants used as treatments did not. Significant concentrations of microcystin-LR (MC-LR) were observed in treated water after treatment of Microcystis aeruginosa with chlorine dioxide, low salinity, sonication, and blending. In these cases, the rate of cell lysis due to treatment was greater than the subsequent removal of the chemical from solution. For the other oxidants and UV, no significant buildup of the cyanotoxin was observed. These results suggest that permanganate was shown to be the most effect disinfectant for achieving both disinfection and removal of the released cyanotoxins with typical disinfectant dosages. However, it should be noted that the use of permanganate leads to the formation of particulate manganese dioxide in solution which can cause potential problems in treated drinking water.