Biogas production from micro-algal biomass obtained from nutrient removal of domestic wastewater

This study comparatively evaluated the effect of co-substrates on anaerobic digestion (AD) and biochemical methane potential of wastewater-derived microalgal biomass, with an emphasis on carbon-to-nitrogen (C:N) and substrate-to-inoculum (S:I) ratios. A semi-continuous photobioreactor was inoculated with Chlorella vulgaris and the nutrient recovery potential was investigated. Then, the derived microalgal slurry was subjected to AD in the absence and presence of co-substrates, namely, model kitchen waste (MKW) and waste activated sludge (WAS). The results revealed that up to 99.6% of nitrogen and 91.2% of phosphorus removal could be achieved from municipal wastewater using Chlorella vulgaris. Biomethane yields obtained were successfully improved by co-digestion with both MKW and WAS. The maximum biomethane yield was observed as (523 ± 25.6) ml CH4 g VSadded-1, by microalgal biomass and MKW co-digestion in 50:50 ratio, at an initial chemical oxygen demand (COD) concentration of (14.0 ± 0.1) g l-1, C:N ratio of 22.0, and S:I ratio of 2.2. The observed biomethane yield was 80.7% higher than that of the mono-digestion. The highest improvement achieved by 50:50 co-digestion of microalgal biomass and WAS was 15.5%, with biomethane yield of (272 ± 11.3) ml CH¬4 g VSadded-1 at an initial COD concentration of (14.0 ± 0.1) g l-1, C:N ratio of 13.0, and S:I of 2.3. The findings reveal that the microalgal biomass AD must be optimized individually for different co-substrates by adjusting C:N and S:I ratios. This study demonstrated feasibility of an integrated waste management and bioenergy production process with significant improvement on the system performance by co-digestion.