Laboratory investigations on co-digestion of energy crops and crop residues with cow manure for methane production:: Effect of crop to manure ratio

Anaerobic co-digestion of grass silage, sugar beet tops and oat straw with cow manure was evaluated in semi-continuously fed laboratory continuously stirred tank reactors (CSTRs). Co-digestion of manure and crops was shown to be feasible with feedstock volatile solids (VS) containing up to 40% of crops. The highest specific methane yields of 268, 229 and 2131 CH4 kg(-1) VSadded in co-digestion of cow manure with grass, sugar beet tops and straw, respectively, were obtained with 30% of crop in the feedstock, corresponding to 85-105% of the methane potential in the substrates as determined by batch assays. Including 30% of crop in the feedstock increased methane production per digester volume by 16-65% above that obtained from digestion of manure alone. Increasing the proportion of crops further to 40% decreased the specific methane yields by 4-12%, while doubling the loading rate from 2 to 4 kg VS m(-3) day(-1) decreased the specific methane yields by 16-26%. The post-methanation potential of the digestates corresponded to 0.9-2.5 m(3) CH4 t(-1) wet weight of digestate and up to 12-31% of total methane production in northern climatic conditions, being highest after co-digestion of manure with straw. (c) 2006 Elsevier B.V. All rights reserved.