A Case Study on Converting Organic Farm Waste Vegetables to Biogas Using a Cartridge Design Anaerobic Digester

Anaerobic digestion of multiple waste vegetables collected from an organic farm in Central Illinois was carried out using a new cartridge design anaerobic digestion system. Waste vegetables, including carrot, cucumber, bell pepper, onion, lettuce, and potato, were chopped and then mixed together to be used as the digestion feedstock. Three cartridges in the digestion chamber were rotated every week. Results showed that the system was stable, in terms of biogas and methane yields, ammonium-nitrogen concentration, and pH value, throughout the 90-day operation. On average, the daily and accumulative methane yield were 23.38 L/day/kg-VS and 490.98 L/kg-VS (21-day retention time), respectively. Rotation of cartridge significantly affected methane yield, methane concentration in biogas, and hydrogen sulfide concentration in biogas. Especially, the average hydrogen sulfide concentration decreased from 1145 ppm, to 695 ppm, and then to 539 ppm, in biogas samples taken on 2 days, 4 days, and 7 days after rotation. No liquid waste was generated throughout the test. A rough estimate of the potential biogas yield shows that if all the waste vegetables and crop residues collected from this farm were used in this new anaerobic digestion system, US$4711 in energy can be saved in a year.