Co-digestion of organic wastes together with livestock manure has received growing interest in Germany during the last 5 years because some waste acts have been changed in order to achieve a better protection of the environment and to promote the utilisation of wastes for the formation of renewable energy and secondary products which can substitute mineral fertilisers and conventional soil conditioners. The technical instructions on urban wastes (TA Siedlungsabfall, 1993) prohibits the dumping of wastes with more than 5 % organic dry matter (oTS) which makes a treatment necessary. The recirculation and waste act (Kreislaufwirtschafts- und Abfallgesetz, 1994) stipulates the closing of production cycles and recommends the energetic use of wastes and the recycling of nutrients. Both can be achieved only by anaerobic digestion which enables a high energetic yield at a relatively small energy input. According the recirculation idea, the nutrient rich organic wastes from food industry, agro-industry, markets and households should be recycled to agriculture. In order to guarantee a sufficient quality of the digester residues for land application a special biowaste ordinance (Bioabfallverordnung, 1998) was issued in October last year which defines the demands on the treatment processes, products and land application of the treated residues on arable and grass land.
Many organic wastes are structureless and contain large amounts of water, so that composting processes cannot be applied. Wet anaerobic digestion processes enable the treatment of such wastes but often, the anaerobic process is inhibited due to the unbalanced composition of the waste matter or due to the lack of essential trace elements. Most of these problems can be solved if industrial and municipal wastes are treated together with pig or cow manure in co-digestion processes. Co-digestion results in stable process conditions and higher gas yields due to the mixture of nutrients and trace elements, co-metabolism and other synergetic effects of the mixed digestion (Weiland and Karle, 1999).
Co-digestion shows also several advantages regarding waste management and economy, because the utilisation of the digested residues can be done on regional basis as a service of the farmers. Therefore, co-digestion is becoming of increasing importance in Germany by farmers, communities and waste disposal companies.