Investigations on energy efficiency of biomethane/biocrude production from pilot scale wastewater grown algal biomass

Real-time studies highlighting the actual bottlenecks of microalgae-based biofuels are needed to overcome the limitations of assumption-based approach. In the present study, an algal consortium consisting of Chlorella pyrenoidosa and Phormidium, was cultivated in municipal wastewater under outdoor conditions in a pilot scale (100 L) attached biofilm reactor during January-February 2018. No energy was used to maintain the cultivation conditions (temperature, light intensity/duration). After 6 days of hydraulic retention time, the consortium showed 53–87% reduction in the nutrients’ concentrations of the selected wastewater. Consistent biomass productivity of 3.48 ± 0.44 g m−2 d−1 was observed, and its biochemical composition showed that it was rich in lipids (35.20 ± 0.63% of total solids). The wastewater grown algal biomass was subjected to anaerobic digestion at 37 °C for 30 days and hydrothermal liquefaction at 27 bars and 230 °C for 20 min to produce biomethane or biocrude, respectively. On performing the biomethane potential assay, a cumulative biomethane production of 346.59 ± 5 mL g−1 VS was observed. The theoretical methane production and stoichiometric methane potential calculated for the biomass was 504 mL g−1 VS and 591 mL g−1 VS, respectively, showing a digestibility of 58.5%. Hydrothermal liquefaction of the wastewater grown biomass gave biocrude yield of 43 ± 2% (dried biomass basis) rich in aldehydes/ketones/fatty acids. Net energy ratios for the two processes were compared. Net energy ratio for algal cultivation integrated with anaerobic digestion was found to be 0.007 and for that with hydrothermal liquefaction was found to be 0.08, proving it to be a more energy efficient process.