Biogas is an environment-friendly fuel but that must be upgraded before being utilized: The method about removing CO2 from biogas by microalgal culturing using biogas effluent as nutrient medium in this study could effectively upgrade biogas and simultaneously reduce the biogas effluent nutrient. Results showed that the optimum parameters for microalgal growth and biogas effluent nutrient reduction was moderate light intensity with middle photoperiod. While low light intensity with long photoperiod and moderate light intensity with middle photoperiod obtained the best biogas CO2 removal and biogas upgrade effects. Therefore, the optimal parameters were moderate light intensity 350 mu mol m(-2) s(-1) with middle photoperiod 14 h light:10 h dark. Under this condition, the microalgal dry weight, CH4 concentration, reduction efficiency of chemical oxygen demand, total nitrogen, and total phosphorus was 615.84 +/- 33.07 mg L-1, 92.16 +/- 2.83% (v/v), 88.74 +/- 3.45%, 83.94 +/- 3.51%, and 80.43 +/- 4.17%, respectively. (C) 2013 Elsevier Ltd. All rights reserved.