Identification of microorganisms for optimization of manure processing at biogas plant

The objective of the project was to improve the efficiency of anaerobic digestion of dairy cattle manure. The project was a collaboration between Xergi and Novozymes and studied the effect of biological treatment and a patented thermo-chemical treatment (NiX®) either separately or in combination. The tests were carried out at lab scale (biological treatment) and pilot scale (combination of the two treatments). The substrate used was manure fibres separated from the degassed effluent from a full-scale plant operated on dairy cattle manure. The biological treatment consisted of aeration of the manure fibres after application of microorganisms (bacteria or fungi) with known ability to degrade fibres. The NiX® treatment consists of pressure cooking at 140°C and 4 bars with base addition. The best results of the lab-scale trials were obtained with fungal strain Coprinus (NN055974) on autoclaved fibers which resulted in methane potential increases of 30-40 ml CH4/g-VS. The success criterion for moving from lab-scale to pilot-scale was that 25% of the theoretical methane potential was converted to methane in batch tests, compared to batch testing of water-treated fibers. In lab-scale batch testing, this corresponds to an increase of 100 - 113 ml-CH4/g-VS, assumning a theoretical methane potential of 400 - 450 ml-CH4/g-VS. Although the results did not meet the original criterion for moving to pilot-scale testing, the possibility of a synergistic effect between the thermo-chemical treatment NiX and a microbial treatment was apparent. Therefore, it was decided to move to pilot scale testing. The pilot scale test studied the effect of the treatments on the efficiency of a 2-step CSTR biogas digesting dairy cattle manure. The NiX® treatment improved by 48% the methane yield of the second step of the biogas process (from 65 +- 7 to 96 +- 7 L-CH4/kg-VS) and by 16% the total methane yield of the overall 2-step process (from 180 +- 13 to 209 +- 15 L-CH4/kg-VS, where VS is the organic matter content of the raw substrate cattle manure). The results from CSTR experiments were confirmed by batch tests. Combination of NiX® and biological treatment did not increase the methane yield. NiX® treatment proved to be suitable to increase the methane yield of cattle manure for long-term operation (steady state) or to temporarily boost the biogas production. The low catalyst dosage makes thermo-chemical NiX treatment attractive for commercial-scale applications