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

Summary

The availability of organic animal feed for monogastric animals and organic fertilizer is in many regions a limiting factor for the further spread of animal breeding and crop cultivation in organic farming. The production of these two commodities is the main target of a green biorefinery concept based on regionally organically grown grass biomass, called Organofinery. Anaerobic digestion of the residual streams after protein separation for the organic feed product 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 from the processing of red clover and clover grass (press cake obtained from screw pressing to produce green juice and brown juice after precipitation of protein concentrate from the green juice) where determined in batch and reactor experiments and a mass balance of the nutrients was established based on analysis of N, P, K, and S from the different process stages of the concept. 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. The methane yields, the nutritional value of the digestates from these experiments as well as the mass balances for the whole process will be presented.
In mesophilic co-digestion (50:50 ratio based on VS) of press cake and brown juice from red clover at 20d hydraulic retention time (HRT) methane yields of 237 to 283 L-CH4/kg-VS were achieved while mesophilic treatment of brown juice from clover grass in the high-rate reactor yielded on average 202 L-CH4/kg-VS at 3 days HRT.
For red clover, concentrations of N, P, K and S were 5.07, 0.45, 4.58, and 0.25 g/-kg-biomass, respectively while the values for these nutrients were 2.97, 0.40, 5.51, and 0.23 g/-kg-biomass for clover grass. After screw press separation and protein precipitation of red clover 52, 53, 43 and 52% of the respective nutrient input were left in the press cake while 15, 30, 38, and 13% were found in the brown juice. After processing of clover grass it was 60, 56, 39 and 55% of the respective nutrient input left in the press cake while it was 11, 27, 31, and 26% in the brown juice. Nitrogen in the digestate from the co-digestion process with 20 days HRT was made more bioavailable with an increase of the share of NH4+-N of total-N from 13% to 62%. During 3 days HRT in the UASB process this effect was much lower with only an increase from 11% to 16%.