Novel application of aerobic granular biofilm systems for treating nitrate-polluted groundwater at low temperature: Microbial community and performance

An aerobic granular biofilm process was operated in a sequential batch reactor for treating nitrate-pollute groundwater at 6 oC, which was fed with a progressively reduced carbon concentration to understand the nitrate removal efficacy in cold regions by denitrifying microorganisms. The evaluation of the performance reflected a quick granulation promoted by hydraulic stress and a high organic loading rate (OLR). The granules were dense and compact and kept stable conformation until the end of operation, despite the low temperature. Nitrate removal in the stage with high OLR was close to 100%, while the stage with the lowest OLR reached efficiencies higher than 87% in steady-state, suppose a successful advantage against other biological processes for treating drinking water. The bacterial community in the mature granules were colonized by cold-adapted microorganisms such as Holophagaceae and Candidatus Microthrix. Methanospirillum and Bathyarchaeota were the most abundant archaeal phylotypes, which showed a more stable consortium and Nectriaceae and Trichosporonaceae were the most dominant fungal phylotypes for all stages. The qPCR revealed that the crucial role was played by Fungi and Bacteria, while Archaea lost abundance in granular biomass, despite extreme conditions. Aerobic granular sludge is a robust biological technology able to remove nitrate in oligotrophic waters bodies.