Anaerobic co-digestion of waste microalgal biomass with cattle dung in a pilot-scale reactor: effect of seasonal variations and long-term stability assessment

Abstract

The present work is first of its kind to attempt on year-round biogas production utilizing microalgal biomass from lake water. The analyses showed that the lake water contain ample nutrients (TAN: 3.6–16; TDP: 4.88–20.38 and NO3-N: 0.3–19.63 mg L−1) to support microalgal growth, with an average biomass concentration of ≈ 2.0 g L−1, throughout the year. Significant correlation (p < 0.05) was observed between microalgal biomass and nutrient concentration in the lake water. Further, co-digestion studies in the pilot-scale plant (0.5 m3) showed good performance with a volumetric and specific biogas yield in the range of 0.45 to 0.65 m3 day−1 and 0.72 to 1.04 m3 kg−1 VSfed day−1 respectively, during the summer. However, during winters, the volumetric and specific biogas yield were found to be in the range of 0.06 and 0.21 m3 day−1 and 0.096–0.336 m3 kg−1 VSfed day−1, respectively, with an average 55–65% CH4. Detailed analyses of digestate indicated long-term stability of the pilot-scale process. This study established the feasibility of utilizing waste algal biomass from eutrophic lake water for year-round biogas production.