Nutrient recovery and biogas generation from the anaerobic digestion of waste biomass from algal biofuel production

Microalgae are gaining popularity as a source of biodiesel. Recycling fertilizer nutrients is critical to sustain large-scale biodiesel production because the global supply of surplus fertilizer is limited. This study demonstrates that anaerobic digestion of residual algal biomass from biodiesel production can provide additional nutrients and energy. Anaerobic digestion of Chlorella sorokiniana 1412 whole cell algae (WCA), sonicated algae (SA), and SA subjected to lipid extraction (LEA) in bench-scale batch reactors operated at 30 ± 2 °C for 42 days released a considerable amount of the nitrogen and phosphorus in the algal cells. Digestion of WCA, SA, LEA released 48.1, 77.4, and 61.5% of the total algal nitrogen as NH₄ ⁺-N, and 87.7, 99.4, and 93.6% of the total algal P as soluble P, respectively. The energy recovery from algae biomass was quantified through the methane yield. The biochemical methane potential of WCA, SA and LEA was 0.298, 0.388 and 0.253 L methane per gram algal volatile solids, respectively. The conversion of LEA and WCA biomass to methane was very similar (38 and 41% on a COD basis, respectively), indicating that the energy yield was not significantly lowered by extraction of the lipid fraction (which accounted for 9% of algal dry weight). Sonication improved the access of hydrolytic enzymes to algal biopolymers (compensating in part for the energy lost due to lipid extraction). The results taken as a whole indicate that anaerobic digestion of LEA can provide considerable yields of methane and soluble nutrients.