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It is a challenge to adapt anaerobic fermentation technologies for the direct treatment of organic waste with solid contents of 20 % and more. While wet anaerobic digestion is well established, research on dry anaerobic fermentation technology for agricultural substrates has been rather scarce. Suitable materials are, e.g., grass and maize silage, dung, or green forage. A batch fermentation system has been proposed where the input is stacked up in a gas-tight container and left there for fermentation for about 50 days, while being sprayed intermittently with a heated, circulating inoculate.

The aim of our work was to investigate the performance of a dry fermentation plant (system "Bioferm") operating in batch-mode, and compare it to common "wet fermentation" processes.

The substrates used in this investigation were grass silage, green forage from landscape care and cattle dung. The production of gas was of 191.38 l/kg of organic dry matter (ODM), 188.64 l/kg ODM and 218.48 l/kg ODM respectively. The content of methane in the gas was similar for the different substrates, being an average of 51.89%. Comparing the production of the gas measured with the one simulated in wet fermentation, it was found that this investigated system ( Bioferm) has an efficiency of 32% to 37%. This is consequent with the low degradability that the starch presented. These results were related to the use of the anaerobic pre-treatment and of the non-uniform percolate spray system.

It can be concluded that the anaerobic pre-treatment should be eliminated from the system. The percolate spray system should be improved to ensure that all the substrate is homogeneously wet during the process. This will insure enough moisture to support an anaerobic microbial flora.

With this investigation it was possible to evaluate the efficiency of the Bioferm system of dry fermentation, and to determinate the improvements necessary to optimize the system.