Solid-State Anaerobic Co-digestion of Organic Substrates for Biogas Production

Abstract

Solid waste management is an important problem in the developing countries due to the rapid quantity of waste generation as they urbanize. As the demand increases for bioenergy, biofuels produced from waste biomass replicates as a supplementary energy resource to satisfy the requirements. Most of these generated wastes consist of biodegradable organic matter, which could be utilized as a source for biofuel generation. These biodegradable wastes are highly opted by suitable treatment method, which is known by anaerobic biodegradation. This study investigated the performance of organic waste digestion in laboratory-scale for biogas production. Also, it focuses on the effects of process parameters such as pH, alkalinity and volatile acids on biogas yield performance by batch and semi-continuous digestion. Food waste and switchgrass is used as the feedstock in the present study, which is collected from NITK campus. The objective (1) of this study aimed to investigate the effects of pretreatment of switchgrass on biogas production. Switchgrass is used as a feedstock, which is subjected to physical and chemical pretreatment for batch digestion at mesophilic condition. Batch experimental results from raw switchgrass yields 248 mL CH4/g VS at mesophilic condition. The biomethane potential of pretreated SG is 53%, 52% and 12% higher for alkali, organosolv and thermal pretreatments respectively, and 44% and 20% lower at acid and liquid hot water pretreatments in comparison to raw SG yield. Highest biomethane yield confirms the enhanced biodegradability of switchgrass by alkaline and organosolv pretreatments. The objective (2) aimed at co-digesting the food waste (FW) and switchgrass (SG) by batch and semi-continuous mode for biogas production. The performance of batch codigestion is determined with FW and SG as a feedstock with different mix ratio (0:1; 1:1; 0:1 FW: SG) at mesophilic and thermophilic temperatures. Semi-continuous digestion is conducted by varying the loading from 4-8 g/L with mix ratios (100:0, 12:88, 25:75, 50:50 and 0:100 FW: SG) at mesophilic conditions. The process parameters (pH, alkalinity and volatile fatty acids) are monitored frequently for their interactive effects on biogas production by batch and semi-continuous digestion.iv The highest methane yield is observed with 1:1 FW: SG as 267 mL/g VS at mesophilic (32-day retention time) and 234 mL/g VS at thermophilic (18-day retention time) condition during batch digestion. Methane yield has a positive response on co-digestion and confirmed by digestion performance index (DPI). Results reveal that co-digestion at 1:1 ratio yields an enhanced performance with both FW and SG in mesophilic as well as thermophilic condition. This study confirms that the presence of slow and fast biodegradable organic matters has an equal contribution to methane yield. A t-VFA/Alk ratio maintains the consistency between acidification and methanation phase. The t-VFA/alk ratio is 0.2 to 0.9 for mesophilic and 0.3-1.5 for thermophilic condition. The release of volatile acids at shorter retention time is observed with thermophilic owing to, faster hydrolysis than at mesophilic conditions. The maximum biogas yield is 628 mL/g VS for 4 g /L loading for semi-continuous mode. The methane content obtained is around 65% that shows the stable performance at varying ratios of FW and SG. Average value of methane yield is 320 mL CH4/g VS which is estimated about 32,000 m3 that produces the energy of 320, 000 kW-h. Results well agreed to implement the combined heat and power system, as electrical and thermal efficiencies by 35% and 50% are widespread across many countries for the energy conversion.