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In composting plants the content of organic waste containers becomes nutrient-rich soil over the course of 6 to 8 weeks. In the process the waste is continuously mechanically ventilated.
© Sutco Recyclingtechnik GmbH
Utilising organic waste for generating energy
Projektinfo 17/2014

The organic waste is initially pulped
© Sutco Recyclingtechnik GmbH
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Combining compost and biogas

A new method improves the energy balance and processing capacity of composting plants. With the usual composting process, considerable energy is consumed to provide the required mechanical ventilation. In the new method, the liquid organic components of the organic waste are separated in advance and are used for producing biogas. This enables energy to be generated in the plants. For the project, a new biofilm digester has been designed which provides cost advantages relative to other digestion processes.

Composting is easy for hobby gardeners. Potato peelings and leaves from the garden are simply thrown onto a pile and, over time, nature transforms this into a nutrient-rich soil. If, however, the organic waste is instead collected and disposed of in organic waste containers, it is then taken to large, centralised composting plants, which requires significantly more effort. The waste is pre-treated, layered, shifted several times during the process, continuously mechanically ventilated and the leachate is collected and fed back into the process. This consumes energy and requires a certain technical expenditure. But this is the only way to create compost of a reliable quality, as is required by agriculture.

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In collaboration with the Entsorgungs-Gesellschaft Westmünsterland (EGW) waste disposal company and the Department of Urban Water and Waste Management at Duisburg-Essen University, the Sutco RecyclingTechnik company has developed a new process that combines composting with biogas production: a screw press separates organic matter from the pre-treated, fresh organic waste. The solid content of which the easily degradable organic matter has been removed is then added to the conventional composting process. This pre-pressing creates additional treatment capacity and the plants can process more material with their existing systems. This reduces the specific energy consumption per tonne of organic waste by 10 to 15 %. The squeezed-out liquid is treated in a new digestion plant, which produces biogas in the digesters. The gas can be used in engines to generate electricity and heat, or fed as biomethane into the natural gas network. Generating biogas from organic waste instead of from renewable resources such as maize, for example, has the advantage that no agricultural land is required.

Making better use of existing capacities

In 2012, approximately nine million tonnes of organic and green waste were produced separately from residual waste (“grey bins”) in Germany. The captured volume per resident varies within the federal states from 37 kg in Brandenburg to 151 kg in Lower Saxony. As of 1 January 2015, the amended German Closed Substance Cycle and Waste Management Act requires the separate collection of organic waste nationwide. According to forecasts, the volume will therefore grow by up to 30 %, as a large proportion of the organic waste was previously disposed of together with residual waste. This therefore requires concepts to increase the capacity of the approximately 1,000 German composting plants (2012).
The new composting method with the additional biogas stage is intended to be used in as many existing composting plants as possible. This will exploit the energy in the carbon content previously left unutilised during composting before it is converted into CO2. After pressing out the liquid, organic carbon compounds (organic matter), the organic waste is composted under aerobic conditions as before, whereby the quality of the compost remains almost unchanged. For the treatment of the press water, a biogas process has been selected that is as robust, reliable and economical as possible. This is necessary to limit the operational expenditure and to ensure safe disposal. In addition, the developers want to prevent digestate occurring that has to be disposed of externally. The gas yield is of secondary importance to meet these objectives.

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Addresses

Project management and process development
Sutco RecyclingTechnik GmbH

Scientific project support
Universität Duisburg-Essen, SiWaWi

Large-scale plant
EGW mbH

Links

Sutco Recyclingtechnik GmbH
Website of the project coordinator

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