Codigestion

Co-digestion is the process of anaerobically digesting two or more organic feedstocks in a single digester to improve methane production and process stability. By combining substrates with complementary properties, such as varying carbon-to-nitrogen ratios, moisture content, and nutrient profiles, co-digestion can enhance biodegradability and increase total biogas yield compared with digestion of a single substrate.

Common inputs include livestock manure, sewage sludge, food and catering waste, agricultural residues, and certain industrial organic wastes. The practice helps balance imbalances in carbon and nitrogen content and buffering capacity, improves mixing characteristics, and can allow higher organic loading while maintaining reactor stability.

Co-digestion is carried out in mesophilic or thermophilic conditions, using continuous or batch reactors. Key operating considerations include pre-treatment or shredding, thorough mixing, pH control near neutral, and buffering. Performance is described by methane yield, hydraulic retention time, and organic loading rate. Co-digestion may enable shorter retention times for certain wastes and can support energy-positive operation.

Benefits include increased biogas production, improved energy balance, reduced waste volumes, and stabilization of digestion for challenging materials. Risks involve inhibition from ammonia, sulfides, or long-chain fatty acids when substrate balance is not maintained, as well as odor, regulatory issues, and operational complexity due to feedstock variability.

Applications range from municipal wastewater plants co-digesting sewage sludge with source-separated organics to agricultural facilities combining manure with food waste, and industrial plants processing mixed organic wastes. Environmental and economic outcomes include renewable energy generation and reduced greenhouse gas emissions, with digestate usable as fertilizer, subject to nutrient and management considerations. See also anaerobic digestion, biogas, digestate.