Microalgae impact on inactivation of indicator virus in a large-scale wastewater treatment system using microalgae

Wastewater treatment systems using microalgae called High Rate Algal Ponds (HRAP) have been increasingly considered as alternative solutions to classical activated sludge systems. In these paddlewheel-mixed shallow raceways (30 cm depth), oxygenation of the HRAP by microalgae replaces artificial aeration for aerobic degradation of organic matter. In HRAP, pathogen removal mainly relies on ultraviolet (UV) radiation from the sun. UV radiation induces photochemical modifications of DNA and RNA, leading to pathogen inactivation. However, high turbidity due to microalgae and detritus from the wastewater reduces UV penetration in HRAP. Paddlewheel mixing has then a profound impact on the treated water quality by exposing microbial pathogens to higher UV irradiation at the pond surface. Microalgae are expected to contribute significantly to turbidity in HRAP, however, they are also responsible for high oxygen concentration, high pH and, in the presence of UVA, production of Reactive Oxygen Species (ROS) favoring disinfection, questioning the relative impact of microalgae on pathogen inactivation. The purpose of this study was to investigate, in a laboratory UVA cabinet, the impact of microalgae on indicator viruses’ inactivation, in terms of UVA attenuation (inhibition of inactivation) and production of ROS (enhancement of inactivation). This study highlighted a significant negative impact of microalgae due to UVA attenuation over 30 cm depth together with a strong inherent capacity to produce ROS for virus inactivation, confirming the relevance of vertical mixing for disinfection in Peterborough HRAP.