Pathogen removal in high-rate algae pond: state of the art and opportunities

Abstract

High-rate algae ponds (HRAPs) are wastewater treatment systems that enable combining cost-efficient secondary treatment at small scale with the production of a harvestable biomass for subsequent valorisation (e.g. biofuel). However, there is still limited data on pathogen removal during long-term HRAP operation with real effluents. This critical review evaluates the potential significance of mechanisms driving pathogen removal in classical wastewater ponds in light of the specific environmental conditions occurring in HRAPs. We thus establish that the presence of algae (at high cell density) increases sunlight attenuation in HRAP and this attenuation negatively impacts sunlight-mediated pathogen removal. However, mixing may counteract the negative effect of light attenuation by increasing the frequency of pathogen exposure to high light intensity near the culture surface in HRAPs. The magnitude of sunlight-mediated pathogen removal is also likely increased at high pH and dissolved oxygen concentrations, two conditions frequently co-occurring in HRAPs harbouring intense algal activity. Exposure to high pH and toxic algae metabolites may further enhance pathogen decay in HRAPs but these mechanisms are still poorly understood. Finally, predation may be significant, but little is known about the quantitative impact of this mechanism in classical ponds and HRAPs. Overall, and with the exception of settling, all pathogen removal mechanisms known to occur in maturation ponds should remain significant in HRAPs. While this pathogen removal ability has been verified in a few studies, further research must now seek to develop a better mechanistic understanding based on the study of real systems.