Optimal and strategic delivery of CO2 for Chlorella minutissima-mediated valorization of domestic wastewater with concomitant production of biomass and biofuel
Algal biorefinery has emerged as a sustainable solution towards addressing the current challenges in energy and environment. Mixotrophic mode of microalgal cultivation can help in nutrient removal from wastewater and capture CO2, along with the accumulation of metabolic products like lipids, starch, and pigments for biofuel and nutraceutical applications. In this study, bioremediation of domestic wastewater was attempted along with the strategic supply of CO2 for improved microalgal biomass and biodiesel production by Chlorella minutissima. On cultivation of C. minutissima in domestic wastewater, biomass productivity of 255 ± 10.0 mg L-1 d-1 and lipid productivity of 60.4 ± 2.7 mg L-1 d-1 was obtained with 98 % COD removal. Subsequently, the wastewater augmented with an optimal dose of mustard de-oiled cake hydrolysate (a low-cost nutrient-rich waste) was found to improve the biomass and lipid productivities by 42.9% and 33.1%, respectively along with 79% COD reduction. Further, strategic feeding of CO2 at different time intervals was evaluated, in order to reduce lag-phase and prolong log-phase in the growth cycle of microalgae for further enhancement of biomass and lipid productivities with more efficient nutrient removal. Among the intermittent CO2 feeding experiments, Strategy-III, where CO2 (3.5%, v/v) was sparged during the lag-phase and also from mid-log-phase to harvest, resulted in higher productivities of biomass (510 ± 26.0 mg L-1 d-1) and lipid (120.1 ± 2.8 mg L-1 d-1) with 85% COD removal. Thus, this study demonstrates the efficacy of employing suitable CO2 feeding strategy along with augmenting the low-cost nutrient-rich waste for improved algal biomass and lipid production with concurrent remediation of domestic waste and CO2.