Effects of the anaerobic biowaste treatment processes Management on the digestate quality (residues from Digestion/compost, process water/wastewater)

Regarding the municipally-collected biowaste amount in Germany, a quantity of 90 - 100 kg/ per annum per citizen is registered (Gallenkemper and Doedens, 1994). That results in an annual biowaste amount of 6.3 to 7 million tons in the Federal Republic of Germany (FRG), whereby also sewage sludges and biogenic, industrial wastes are added. According to Hochrein (1994) about 3 million tons of organic industrial wastes arise annually, besides the bio and green wastes, which on account of their properties are suitable for digestion.

Prior to other methods aerobic processes (i.e., composting) have been used for the biological treatment of biowastes. As biowastes coming from large cities with a high amount of water content are suitable for digestion only to some extent, the technique of digestion became in recent years more and more important. Even at a medium digestion capacity of 700.000 tons per annum, only little information has been published about the quality of anaerobic compost and the arising wastewater.

The quality of the anaerobic composts is, however, an essential factor for its utilisation and marketing. Accepted treatment methods would enhance marketing of the end products. In particular, the avoidance of possible odour emissions during subsequent decomposition of the digest residues has to be considered.

Prior to the anaerobic treatment the digestion materials are usually subjected to a solid-liquid separation, whereby the arising wastewater amount depends on the water amount of the input material, the quantity of additional water and the water content of the digestion residues. Hitherto little notice has been taken of the composition of wastewater from digestion plants. This can have a severe effect on the treatment outcomes.

First intermediate results of an actual Deutsch Bundesstiftung Umwelt (DBU) research project about the quality of digestion residues and composts, produced in cooperation with the LASU, Münster, and the PlanCoTec, Neu- Eichenberg, are pointed out in this report. Moreover, the question of special features, as compared to those of aerobic composts, is discussed. On the other hand results of a project that has been sponsored by the Oswald Schulze Foundation considering the composition of wastewater from biowaste digestion plants and the influences of processing owing to water ingredients are presented.

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