Characterisation of biogas plants on organic farms and potentials for improvement

Abstract

Biogas plants on organic farms contribute little to the total installed electrical capacity of biogas in Germany (<1.0 %). To increase the energy efficiency, parameters of biogas plant specifications, fermentation characteristics and feedstock composition were examined for inherent problems in 13 biogas plants. Biogas plants with two reactor stages were found to have significantly higher substrate degradation efficiency and better overall process performance than single-stage biogas plants. The main factor for low volatile solid (VS) degradation was process temperature (range 17 to 49 °C) before hydraulic retention time (HRT) and organic loading rate (OLR). Low VS degradation was found in biogas plants with only one reactor stage, low fermentation temperature and a high share of livestock manure. The total feedstock of the biogas plants comprised mainly of livestock residues (61 %) and grass silage (14 %). The individual biogas plant feedstocks showed strong intake variability and, in general, low specific methane yields (median 256.34 m3 kg−1 VS) when compared to German average. The presented study specified several optimisation pathways for biogas production within the organic farming. Residue storage covers would lessen ecological and economic disadvantages of high VS contents in the effluents considerably. Technical changes in combined heat and power (CHP) units and reactor content recirculation show a way to increase process temperatures and HRT which, in turn, would improve VS degradation efficiency. Moreover, the German Renewable Energies Sources Act 2014 offers possibilities for new small-scaled (≤75 kWel) manure processing (≥80 % fresh weight (FW)) biogas plant installations, an opportunity to increase the total installed electrical capacity of biogas plants on organic farms.