Dry fermentation

Until now, biogas technology has mainly concentrated on ”wet fermentation” of agricultural waste. Processability of materials with a high dry matter content solid waste were limited. The recently developed dry fermentation process allows methane yields from stackable organic matter with a high dry matter content, requiring no conversion into a pumpable liquid substrate. This method renders fermentation of biowaste possible with up to 50 % dry matter content. Biowaste is inoculated with substratum that has already been fermented. It is then filled into the tank and fermented under airtight conditions. Continuous inoculation with bacterial matter occurs per recirculation of percolation liquid, which is sprayed over the organic matter in the digester. During the fermentation process, no further mixing, pumping or stirring is necessary inside the digester, nor is further material added. Excessive percolation liquid is collected in a drainage system, temporarily stored in the percolation tank, and then re-sprayed over the biomass in the digester.

Photo. Dry fermentation plant

Fermentation occurs at mesophilic temperature of 38° C, which is regulated through heated floors and walls. The dry method is a single-step batch process. The different stages of degradation (i.e., hydrolysis, acid, and methane formation) take place in the same box digester. Batch process means that, during fermentation, no further material is added or subtracted, and that the biomass is left to ferment in the digester until the end of retention time. The digesters (i.e., fermenters) are gas-tight, concrete, oblong, garage-like chambers and can be filled and emptied with wheel or front-end loaders. Several digesters may be built next to one another and run simultaneously in progressive biogas production stages, to guarantee continuous biogas production. Temperatures within the isolated digesters are regulated via heated floors and walls, thus exploiting the entire contact area between fermenting substratum and digester. Heating tubes are integrated into the cement floor and walls during construction. Accordingly, no intrusive parts bar the digester’s interior. In addition, a heat exchanger warms the self-fed percolation liquid.

Loading of dry fermentation biogas reactor

Digesters have hydraulically operated, gas-tight, steel gate-like doors. They have a seal which, when inflated towards the cement entry wall, make the entrance gas-tight. Before the gates are opened, the air in the seal is released. These are then opened top-down, thus preventing the wheel- loader from colliding into and damaging them. The inflatable seal lies within the gate’s edge and is therefore also protected from damage. The system is run under light overpressure, thus categorically preventing any potential form of gas-air explosion, even in the case of leakage. The advantage of the dry technology is that constant mixing of the biomass is unnecessary. Pumping and stirring units are not needed. The fermentation substratum rarely requires pre-treatment. The technology is therefore much simpler and robust to that of wet fermentation plants. The dry digesters have no moving parts. Therefore, wear and tear costs are low, as well as maintenance costs and staff costs. Process energy consumption is also minimal, making it easy to treat biomass with high dry matter content and interfering substances. Wood, plastic, sand, shrubs, etc. are not a problem.

Photo. Dry fermentation plant