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 VS_fed 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 VS_fed 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.

Graphical abstract