Biogas as recycling product from anaerobic digestion of organic materials

Biotechnological processes can be established economically and ecologically advantageous for the substance recycling along with waste treatment. The center of interest hereby is the use of anaerobic or substance producing aerobic microorganisms. The best known substance recycling process of organic natural products, no matter in which composition they exist, is the recovery of biogas. By using that anaerobic part of the food-chain as an end product of the digestion of natural materials a mixture of carbon dioxide (CO₂) and methane (CH₄) known as biogas is released. Hereby, the methane part of the gas varies between 50-70 percent depending on the composition of the waste material.

Biogas anaerobic technique has already been used for a long time in technical dimensions, for example for the digestion of sewage sludge or the digestion of manure. In order to achieve a break-through for the digestion of organic wastes of municipal or industrial production, a digestion technique with a nearly complete conversion of natural compounds has to be created and realized instead of stabilization in order to compete with waste incineration.

A nearly complete digestion has to be specifically realized for each waste type, because of the different composition of the main waste streams as manure, bio-waste, sewage sludge and by-products of agroindustrial or food industries. Also, appropriate processes have to be developed waste-specifically. But as shown by many experiments, the digestion kinetics of the different waste streams follow a similar plan – a combination of zero order and first order kinetics. Therefore, concerning the process design, high digestion rates can only be realized by reactor cascades coupled with membrane-based separation technologies. Additionally, the nearly complete mineralization of organic waste as modelled in nature can only be achieved by a combined one-after-the-other application of anaerobic and aerobic treatments.

The decomposition of the organic fraction of some waste streams could already be increased significantly by the application of a two-step high-performance digestion process developed by the Fraunhofer IGB (Table 1). At the same time, the residence time could be decreased (2-5 days) and biogas yields could be enhanced.

Table 1: Digestion rates of the organic fraction of different wastes in a pure anaerobic cascade, achieved by the application of a two-step high-rate digestion process developed at Fraunhofer IGB.