The influence of biomass agitation on biogas and methane production using the high-solids thermophilic anaerobic digestion

Abstract

In this study, we tested the high-solids thermophilic anaerobic digestion of separated cattle slurry solids inoculated by liquid digestate collected from the 1st stage of the wet process operated in an agricultural mesophilic biogas plant. The process without batch agitation (stirring) was carried out in a barrel bioreactor located in the incubator, whereas the process with batch agitation was carried out in a rotating drum bioreactor (rotation speed: 0.1 min-1). In both cases, the biomass batch was non-liquid and the content of total solids was 18.8 wt%. The processes were conducted discontinuously without any addition for 21 days. The total solids content of both batches decreased to about 15 wt%, with only slight liquefaction. The bulk density increased from 500 to 750 kg m-3. The highest biogas production was achieved consistently on the 7th day. During the 21-day period, the unagitated batch produced 0.120 mN3 of biogas, and the agitated batch produced 32.5% extra. The average CH4 content in biogas from the unstirred batch amounted to 47.7 vol.%, and biogas from the stirred batch reached 46.2 vol.%. The cumulative methane production rates from the unagitated and agitated batches were 0.095 and 0.121 mN3 kgVS-1, respectively. Thus, the agitation resulted in 28.5% higher methane yield.