To upgrade biogas produced from the anaerobic digestion of swine wastewater, an outdoor photobioreactor was established in this study. A mutant strain of microalga Chlorella sp. MM-2 was firstly isolated by ethyl methane sulfonate-induced random mutagenesis. The Chlorella sp. MM-2 grew in the presence of gas containing H2S < 100 ppm, and the growth capacity of the microalgal culture aerated with 80% CH, was similar to 70% that of the control culture (0% CH4). In the field study, CO2 capture efficiency of the Chlorella cultures, at a biomass concentration of 1.2 g L-1, from the desulfurized biogas (-20% CO2, 70% CH4 and H2S < 100 ppm) was approximate 70% on cloudy days and 80% on sunny days. CH4 concentration in the effluent biogas from the Chlorella cultures was increased to approximate 84% on cloudy days and 87% on sunny days from its original 70%. The established outdoor photobioreactor system using a gas cycle-switching operation could be used as a CO2 capture model for biogas upgrading. Crown Copyright (C) 2011 Published by Elsevier Ltd. All rights reserved.