BACKGROUNDThis study aims to simultaneously reduce biogas slurry nutrient and upgrade biogas via microalgal production. Marine microalgae were grown in a culture medium containing a mixture of seawater and biogas slurry with CO2 from synthetic biogas. The effects of different ratios of biogas slurry and various influent concentrations of CH4 in synthetic biogas on the growth of Nannochloropsis sp. were examined. Nutrient removal and biogas upgrading were also studied under different influent CH4 concentrations. RESULTSNannochloropsis sp. showed optimal growth in 40% biogas slurry and significantly enhanced growth in synthetic biogas with 50% CO2. The optimal range of influent CH4 concentration for the removal of biogas slurry nutrients was 45% to 50%. Notably, the influent CH4 concentration of 60% (v/v) provided an optimal CO2 concentration for Nannochloropsis sp. cultivation. Advantageous CO2 removal and biogas upgrading were observed under this condition. This treatment increased CH4 concentration in synthetic biogas from 60% to 93.58% after 6 days cultivation. CONCLUSIONAppropriate control of the biogas slurry or seawater and CH4 concentrations in the influent is crucial to simultaneously optimize nutrient removal and biogas upgrading. (c) 2015 Society of Chemical Industry