Nutrient removal from biogas slurry and biogas upgrading of crude biogas at high CO₂ concentrations using marine microalgae

BACKGROUND This 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 CO 2 from synthetic biogas. The effects of different ratios of biogas slurry and various influent concentrations of CH 4 in synthetic biogas on the growth of Nannochloropsis sp. were examined. Nutrient removal and biogas upgrading were also studied under different influent CH 4 concentrations. RESULTS Nannochloropsis sp. showed optimal growth in 40% biogas slurry and significantly enhanced growth in synthetic biogas with 50% CO 2 . The optimal range of influent CH 4 concentration for the removal of biogas slurry nutrients was 45% to 50%. Notably, the influent CH 4 concentration of 60% (v/v) provided an optimal CO 2 concentration for Nannochloropsis sp. cultivation. Advantageous CO 2 removal and biogas upgrading were observed under this condition. This treatment increased CH 4 concentration in synthetic biogas from 60% to 93.58% after 6 days cultivation. CONCLUSION Appropriate control of the biogas slurry or seawater and CH 4 concentrations in the influent is crucial to simultaneously optimize nutrient removal and biogas upgrading.