Control of Inhibition Through Anaerobic Co-digestion of Algae with Sugarcane Bagasse

Anaerobic digestion (AD) of algal-bacterial biomass grown on wastewater has been used successfully for bio-methane production. However, challenges with AD of microalgae include inhibition due to accumulation of free ammonia (FA) and volatile fatty acids (VFAs), alkalinity depletion, and/or pH outside optimal values. In this study, algal-bacterial biomass was cultured on high-strength wastewater and then co-digested with sugarcane bagasse, an agricultural waste product that is widely available in tropical regions of the world. Biochemical methane potential (BMP) assays were set up at approximately 2% solids content with varying ratios of algal-bacterial biomass and sugarcane bagasse and varying substrate to inoculum ratios (S/I). Although methane content of biogas was similar in all digestion sets (61–67%), co-digestion sets with algae and bagasse produced higher methane yields (145 and 101 mL CH₄/g VS) than algae alone (61 and 82 mL CH₄/g VS) or bagasse alone (74 mL CH₄/g VS). A significant correlation (r² = 0.88, p = 0.012) was observed between algal-bacterial biomass content of the substrate and total gas production and therefore methane yield. Environmental conditions, including pH, alkalinity, total ammonia nitrogen (TAN), and VFA concentrations, were in the favorable range in all BMPs. However, co-digestion BMPs had greater initial release of COD, VFA, and TAN and greater VS reduction, indicating enhanced biodegradability of the substrates. Additional research is needed on the potential for using waste resources, such as wastewater and agricultural residuals, for biogas production in tropical regions.