In this work, a cooperative algal-bacterial system that efficiently upgrade biogas, simultaneously reduce the biogas slurry nutrient, and exhibits high biomass productivity, was developed. The method about removing H2S and CO2 from biogas by three microalgal strains (Chlorella vulgaris, Scenedesmus obliquus, and Neochloris oleoabundans) mixed with activated sludge using biogas slurry as nutrient medium. A CO2 concentration of 45% (v/v) was considered optimum to support CO2 and H2S removals of 74.11%-80.57% and 99.04%-99.42%, respectively. At the CO2 concentration of 45%-55%, the mixed culture containing S. obliquus and C vulgaris efficiently removed COD (chemical oxygen demand) and TP (total phosphorus), respectively. The mixed culture containing S. obliquus demonstrated high N removal efficiency at CO2 concentration of 45%. Biomass productivity increased at increasing CO2 concentration in a certain range, with a maximum of 0.177 g L-1 d-(1) at CO2 concentration of 45% (v/v) for the mixed culture containing S. obliquus, whereas the C, N, and P biomass contents remained constant at 46.73%-52.31%, 7.59%-9.08%, and 0.91%-1.08%, respectively. This study showed the potential of the combination of alga and bacteria to serve as a treatment for nutrient removal and biogas upgrading in algal-bacterial processes. (C) 2016 Elsevier Ltd. All rights reserved.