“Methano-compost”, a booster and restoring agent
for thermophilic anaerobic digestion of energy
crops
The influence of plant litter-compost of the hot rotten-phase as additional inoculum for
anaerobic batch digestion of sugar beet silage (SBS) was studied. Four simultaneously
driven batch-fermenters were inoculated with sewage sludge. Two of the fermenters were
inoculated additionally with the same amount of organics by compost of the hot rottenphase.
Two of the fermenters were mesophilic (40 _C) and the other two were thermophilic
(60 _C). The impact on the gas production rate and gas yield was observed to be
boosted for thermophilic (60 _C) and only a minor effect of 6e13% for mesophilic (40 _C)
digestion. The gas yield increased considerably up to 26.5% at 60 _C (batch). Also the
methane content increased from 57.4% to 62.3% by adding compost (continuously run
mesophilic digestion). Fluorescence In Situ Hybridization (FISH) indicated that a microbial
effect was responsible for the observed stimulation of gas production rates, but not simply
by increasing the bacterial counts. By analysing each fermenter for its mineral and trace
element content a mineralic effect could be excluded. However, the bacterial counts by
FISH of 10 different groups were somewhat ambiguous. But an effect on the presence of
Chloroflexi could be demonstrated. They nearly doubled to 15e16% by supplementation
with compost. Furthermore, under thermophilic conditions, the added compost induced a
significant shift in the microbial composition towards hydrogenotrophic Methanobacteriales.
The suggestive conclusion drawn is that this explicitly increase in hydrogenotrophic
activity could alone or in combination with accompanying fermentative bacteria forces the
microbial food chain towards stimulation of methane generation.
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