Direct and indirect inactivation of Microcystis aeruginosa by UV-radiation

Excessive algal growth in drinking water sources like lakes and reservoirs is responsible for filter-clogging, undesirable taste and odor, disinfectio n-by-product formation and toxin generation. Although various methods are c urrently being used to control algal bloom, their successes are limited. So me water utilities routinely use copper sulfate to control excessive algal growth. But there is a growing concern against its use mainly because it is non-specific to target algae and kills many non-target species. In this st udy. the scope of using UV-radiation to control algal growth was assessed u sing Microcystis aeruginosa as test species. A UV-dose of 75 mW scm(-2) was found to be lethal to M. aeruginosa. A smaller dose of 37 mW scm(-2) preve nted growth for about 7 days. It was found that UV-radiation may increase t he specific gravity of the cells and thus may adversely affect the ability of the cells to remain in suspension. Three days after a UV-dose of 75 mW s cm(-2). almost an the cells settled to the bottom of the incubation tubes, whereas all the unirradiated cells remained in suspension. It was also obse rved that UV-radiation on algal extracellular products has a significant re sidual effect and can contribute to alga growth control. The extent of resi dual effect depends on the UV-dose and can continue even for 7 days. UV-rad iation was found to produce H2O2 in the muM level concentration. But at suc h level, H2O2 itself is not likely to cause the residual effect that was fo und in this study. (C) 2001 Elsevier Science Ltd. All rights reserved.