Co-fermentation of sewage sludge and biowastes

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Courtesy of ORBIT e.V.

The anaerobic treatment of sewage sludge has a long history in Germany. It began early in the Twentieth Century with the development of the Imhoff tank and has, since then, been con-tinuously further developed. Today, anaerobic sludge digestion is the most widely used process for stabilizing sewage sludges. Digesters, in which a portion of the sewage sludge is converted into utilizable biogas under the exclusion of air, are to be found at almost all medium-sized and larger wastewater treatment plants (from approx. 20,000 to 30,000 PE onwards).

Compared to sewage sludge digestion, the systematic production of biogas by fermenting biowastes is a relatively new sphere. Only in the last few years have a variety of fermentation processes been developed as an alternative to conventional composting. The positive energy and CO2 balances, as well as other advantages of the anaerobic process, have led to a sharp increase in treatment capacities in Germany in recent years (Kern, 1999). The relationship to traditional sewage sludge treatment becomes particularly distinct in the wet process (solids content less than 15 %).

Nowadays, except for a receiving and processing station, the facilities necessary for treating the biowastes are already available at all larger wastewater treatment plants. For this reason alone, it seems practical and feasible to incorporate biowastes in existing sewage sludge treatment plants. In this way, the necessity to build new, high-cost fermentation plants could be precluded, the energy content of the biowastes could, in addition, be utilized and the product used to advantage in agriculture.

Due to the fact that, apart from extremely cost-effective solutions, the concept of co-fermenting sewage sludge and biowastes offers a number of further, interesting advantages, the process was intensively tested. The Emscher and Lippe Associations collaborated with the Department of Waste at the Bauhaus University, Weimar, under the direction of Prof. Bidlingmaier, on the project. The objective was to utilize existing digesters for the co-treatment of municipal and industrial/commercial biowastes.

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