Chlorination of toxic cyanobacterial cells and their associated toxins

Cyanobacteria (Blue-green algae) are photosynthetic micro-organisms commonly existing in most aquatic systems. Because of climate change and increasing eutrophication of waters, the occurrence of algae blooms is becoming more frequent worldwide. Cyanobacteria are widely considered as sources of taste and odour in drinking water supplies. Cyanotoxins produced by a number of cyanobacteria species are a hazard for the health of human and animals. Therefore, cyanobacteria are a global concern.;Pre-chlorination is still practiced in many North American water treatment plants. Direct chlorination of cyanobacteria can increase the removal of their cells and toxins. However, it may also cause some water quality problems, such as cell lysis, which leads to toxin and DOC (Dissolved organic carbon) release, an increase in chlorine demand and disinfection by-products formation.;This study is to examine the effects of pre-chlorination on cyanobacteria (cultured Microcystis aeruginosa and natural bloom) and their associated toxins. The major objectives of this work are: (1) to determine and to model M. aeruginosa cell lysis, toxin and DOC release, and MCs oxidation during chlorination of ultrapure and natural waters spiked with laboratory cultures; (2) to estimate the M. aeruginosa cellular chlorine demand when cell lysis occurs due to chlorination; (3) to study the DBP formation associated with the chlorination of M. aeruginosa cell-bound materials; (4) to evaluate the efficiency of chlorination of toxic cyanobacterial cells in natural bloom samples.;Chlorine exposure (CT) values of 110.7 mg.min/L (for cultured M. aeruginosa spiked in Mille-Ile River water) and 130.3 mg.min/L (for cultured M. aeruginosa spiked in ultrapure water) were required to achieve 76% cell lysis and oxidation of released cell-bound toxins at levels below the WHO (World Health Organazation) guideline value of 1.0 mug/L for MC-LR in drinking water. In ultrapure water, the rates of toxin oxidation were similar or faster than the rates of cell lysis. A constant value of 5.6+/-0.2 pgCl2/cell is estimated as the M. aeruginosa cellular chlorine demand when cell lysis occurs due to chlorination. There is no significant DBP formation in chlorination of lysed M. aeruginosa suspension (500,000 cells/L) in ultrapure water. Natural blooms are more resistant than cell suspension of laboratory cultures..