Anaerobic digestion for closing the loop of a biorefinery for organic farming: Production of biogas and organic fertilizer from process residues.

The availability of organic animal feed for monogastric animals and organic fertilizer is in many regions a limiting factor for the further spread of organic farming. The production of these two commodities based on regionally organically grown grass biomass is the main target of a green biorefinery concept called Organofinery. After extraction of leaf protein concentrate from the grass, anaerobic digestion (AD) of the residual streams is a key process to extract both the energetic and the nutritional value of the residual biomass in the biorefinery. Biogas potentials of the residual streams were determined in batch and reactor experiments and mass balances of the nutrients were established based on analysis of N, P, K, and S from the different process stages (screw pressing, protein precipitation and AD process). The AD process was tested in lab-scale both as co-digestion of press cake and brown juice in a continuous stirred tank reactor (CSTR) and of the brown juice alone in an up-flow anaerobic sludge blanket (UASB) reactor. Both reactor processes showed stable performance without signs of inhibition or nutrient deficiency. In mesophilic co-digestion (50:50 ratio based on VS) of press cake and brown juice at 20d hydraulic retention time (HRT) methane yields of 237 to 283 L-CH4/kg-VS were achieved while mesophilic treatment of brown juice in the high-rate reactor yielded on average 202 L-CH4/kg-VS at 3 days HRT. Nutrient analysis showed that most of the nutrients of the fresh crop were left in the press cake with a similar nutrient composition as in the input biomass, whereas the brown juice contained only 13-38% of the input nutrients with lower proportions of N and S. A full evaluation of the AD process including biogas yields and of the nutritional value of the digestate from the processes will be presented at the conference.