Anaerobic digestion of lipid-extracted Auxenochlorella protothecoides biomass for methane generation and nutrient recovery

• Lipid extracted algal residues were found to be suitable for anaerobic digestion.
• Average specific methane yield reached 0.25 L/gVS or 50% of the predicted maximum.
• Solvent residues present in the LEA could cause inhibition especially at higher OLR.
• 30–60% of N, P, Mg, Ca, and S, and 15–25% of Mn and Fe were available for recycling.
• Nutrient recycling with AD effluent may reduce the supplied fertilizer cost up to 45%.

This study evaluated methane production and nutrient recovery from industrially produced, lipid extracted algal biomass (LEA) of Auxenochlorella protothecoides using semi-continuous anaerobic digestion (AD) at different organic loading rates (OLRs) and hydraulic retention times (HRTs). It was shown, that AD can improve biofuel production efficiency and sustainability, especially for scaled processes, through up to 30% increase in energy generation (up to 0.25 L of methane per g of LEA volatile solids) and partial nutrient recovery and recycling. The nutrient recycling with the AD effluent may reduce the cost of the supplied fertilizers by up to 45%. However, methane production was limited to nearly 50% of theoretical maxima potentially due to biomass recalcitrance and inhibition effects from the residual solvent in the LEA. Therefore, further AD optimization is required to maximize methane yield and nutrient recovery as well as investigation and elimination of inhibition from solvent residues.