Inactivation of natural assemblages of freshwater phytoplankton with UV-LED: reduction of viable organisms, regrowth, species composition and potential release of cyanotoxins

Abstract

Lentic freshwater systems such as ponds, lakes and reservoirs play an important role as drinking water sources or recreational venues. Algal and cyanobacterial blooms may pose a risk for water consumers and users, particularly in episodes of proliferation of toxin-produces species. The inactivation with ultraviolet (UV) light is currently used in drinking water purification as well as for ecological restoration of small water bodies. Traditional UV mercury lamps are expected to be replaced by recently developed UV-LEDs. The objective of this study is to evaluate the efficacy of the UV irradiation with LEDs emitting at 275 nm for the inactivation of phytoplanktonic organisms in two reservoirs, using different determination approaches and focusing on the changes on the species composition as well as on the potential production of cyanotoxins after UV exposure. The results revealed that the concentration of viable phytoplankton was reduced by more than 99.9% when applying UV doses of 55.7 mJ cm−2. However, the UV treatment up to 100 mJ cm−2 promoted the predominance of cyanobacteria with respect to other species, and triggered the production of microcystins, if the treated water is release into a confined environment with availability of light and nutrients.