Disadvantages of currently used biogas technologies
Common biogas plants typically use two or more large digesters, with volumes between 2,000 and 6,000 cubic meters. They require significant space and the investment costs are high.
The large volume of the bioreactors is required because the retention time of the applied substrate is long. Fermentation starts using liquid manure or other animal slurries. But the fermentative performance of the biocenosis in these bioreactors is poor.
Animal slurries contain little energy. To increase performance it is necessary to add in substrates from different sources (i.e. from the food industry, municipal waste, etc.).
Biogas plants usually apply specially cultivated energy crops. The microbial system in the bioreactors only decomposes the readily soluble sugars, starch, proteins and fat. Therefore it is necessary to use the fruit-parts of the cultivated plants to obtain energy.
Maize, rye and sugar beets are amongst the most suitable energy crops. Producing energy crops leads to competition around the use of land. Farmers have to decide whether they want to cultivate food, feed or energy crops.
Unfortunately common biogas plants have enormous problems decomposing one of the most important parts of the plants – the fibrous components.
Despite the very long retention times, common biogas plants are unable to decompose the fibrous and cellulose based non-food-parts of the plants to create energy. Thus, between 20 to 50% of the applied substrate remains as a fibrous residue from the fermentation process. Even when farmers use maize which is considered as an excellent substrate, between 20 an 25% of the primarily applied substrate will remain as a residue. This is clear once the solid fibrous residue will be separated from the liquid output.
All practical data indicates that the amount of biogas depends in a direct and negative correlation with the content of fibrous materials in the substrate. The only key to an economic and sustainable biogas production is the utilization of organic waste materials. Cellulose and hemicellulose are representing the world's most frequent renewable raw material. Accordingly, we have focused our technological developments on biogas plants which are able to use this potential on an economically reasonable way.