Control of inhibition through anaerobic co-digestion of algae with sugarcane bagasse

Abstract 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 co-digested with sugarcane bagasse, an agricultural waste product, to increase biogas production by controlling inhibition. Algal-bacterial biomass was cultured in bench-scale photo-sequencing batch reactors (PSBRs) used to treat high ammonia strength wastewater. Biochemical methane potential (BMP) assays were set up at approximately 2% solids content with varying ratios of algal-bacterial biomass and sugarcane bagasse to achieve carbon to nitrogen (C/N) ratios between 4.5 and 60. Addition of sugarcane bagasse helped balance the high nitrogen content of algae and control pH/Alkalinity, VFA and FA in BMPs. Methane content of the biogas was similar for all BMPs (~66 %). The highest total biogas production was observed for BMPs with substrates composed of algae (AL) and sugarcane bagasse (BG) at C/N ratios of 17 and 18. When the C/N ratio was maintained at about the optimal ratio, a significant correlation (r2 = 0.88, p = 0.012) was observed between algae biomass content of the substrate and total gas production and thereby methane yield. The results show that sugarcane bagasse addition to algal-bacterial biomass reduced AD inhibition and led to greater methane yields.