Model-based analysis of greenhouse gas emission reduction potential through farm-scale digestion

An anaerobic digestion model was set up and applied to estimate desired methane production in the form of biogas as well as unwanted methane emissions associated with farm-scale digestion of manure. The generally accepted Anaerobic Digestion Model No. 1 was simplified assuming that hydrolysis is rate-limiting during anaerobic digestion of manure, mainly consisting of non-readily biodegradable compounds. Simulations were performed to demonstrate the effect of temperature and retention time on methane emissions resulting from long-term manure and digestate storage. Moreover, the overall carbon footprint of several manure management scenarios for Flemish dairy farms was assessed based on model simulations and literature data. The scenarios assessed, differed in the possible presence of a digester as well as in the manure collection and storage method. A reduction in methane emissions was achieved for lower manure storage temperatures (through external storage) and by decreasing the stored manure volume and thus the storage time before (controlled) anaerobic digestion. At the same time, feeding fresh manure induced an increased methane production in the digester. The lowest carbon footprint could be achieved on dairy farms with fresh manure collection by a manure scraper, followed by controlled digestion and storage of the digestate in a gas-tight tank, located outside. The controlled digestion must take place in a properly managed and correctly dimensioned reactor as high digester methane losses and low digester retention times increase the carbon footprint significantly.