Effects of temperature on post-methanation of digested dairy cow manure in a farm-scale biogas production system

A post-methanation process that could be adopted at farm-scale, operating at temperatures prevailing in farm manure digester post-storage tanks, was evaluated. Digested manure samples from a farm digester (35 degrees C) and post-storage tank (5-10 degrees C) were incubated in parallel batches at 5-20 degrees C and as reference at 35 and 55 degrees C. Specific methane yields (kg(-1) volatile solids (VS)(added waste)) were 0.20-0.26 m3 at 35-55 degrees C and 0.085-0.09 m3 at 10-20 degrees C for digester material (345 days of incubation) and 0.16-0.21 m3 at 35-55 degrees C, 0.053-0.087 kg(-1) VS(added waste) m3 at 15-20 degrees C and 0.026 m3 at 10 degrees C for post-storage tank material (250 days). Both materials produced less than 0.005 m3 at 5 degrees C. However, an increase in temperature to 35 degrees C (40 days) improved methane production in assays pre-incubated at 5-20 degrees C (9 months). These results suggest that the untapped methane potential of the digested manure cannot effectively be recovered at temperatures prevailing in farm digested manure storage tanks during the winter in Northern latitudes. Nevertheless, as ambient temperatures increase during the late spring, an increase in methanogenesis can be expected.