A quite different approach is the dry#fermentation. This method is characterized
by the substrate that is, during the fermentation of dry material (manure, grass,
etc.), wetted again from above with leaking liquid (percolate). Thus, the digestate
is repeatedly inoculated with bacteria and in this way the fermentation is kept
going. For approximately two years, at the University of Applied Sciences
Weihenstephan experiments for solid state fermentation similar to the batch
process are carried out. These are mobile containers that are filled with fresh
material and sealed airtight. Because at batch operation, the gas production and
gas composition are not constant, at least three fermenters are charged in parallel
and out of phase. The digestion time is dependent on the substrate, two to four
weeks. The biogas earnings correspond to the gas yields of wet fermentation. After
fermentation, the fermenter is dumped and refilled with fresh biomass.