Ability of a mutant strain of the microalga Chlorella sp. to capture carbon dioxide for biogas upgrading

In this study, a culture system with outdoor microalgae-incorporating photobioreactors was utilized to upgrade biogas produced from the anaerobic digestion of swine wastewater. Using ethyl methane sulfonate (EMS) random mutagenesis, we isolated a mutant strain of microalga, Chlorella sp. MB-9, which was tolerant to high CH₄ and CO₂. In the field study of outdoor operation, the maximum growth rates of Chlorella sp. MB-9 aerated with desulfurized biogas at 0.05, 0.1, 0.2 and 0.3vvm were 0.32, 0.311, 0.275 and 0.251gL^-1d^-1. In addition, ∼70% of the CO₂ in desulfurized biogas (∼20% CO₂, ∼70% CH₄, and H₂S<50ppm) could be captured by the Chlorella sp. MB-9 cultures. The CH₄ concentration in the effluent biogas from the Chlorella cultures increased from its original 70% up to 85-90%. The established outdoor microalgae-incorporating culture system with a gas cycle-switching operation could be efficiently used as a CO₂ capture model for biogas upgrading.