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The new process is integrated within an existing composting facility. The digestion takes place in four digesters connected in series.
© Sutco Recyclingtechnik GmbH

The daily biogas production depends on how much press water is fed in. The plant was at a standstill between the 79th and 91st days.
© Sutco Recyclingtechnik GmbH

The two biofilm digesters in Gescher are 12.5 metres high with a 3-metre diameter. Left and top right: Digester in Gescher. Below right: The plant in the laboratory.
© Sutco Recyclingtechnik GmbH
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Testing in practice and in the laboratory

The practical test for the combined process is running in the Gescher composting plant belonging to the EGW company. Each year the plant processes about 55,000 tonnes of organic waste that has a low organic matter content when compared nationally. This limits the possible gas yield. Under laboratory conditions, the gas production amounted to 74 Nm³/t of input in the digestion; in the plant it amounted to an average of 50 m³. It took about four weeks to convert the plant, which was carried out during normal operation. Two screw presses and two cylindrical and slender digesters connected in series were installed and a continuous stirredtank reactor (CSTR) was converted into a secondary digester. A smaller mock-up of the entire plant was built at a laboratory scale at Duisburg-Essen University. Here data is determined that would otherwise be disproportionately difficult to measure in the large system, including the activities, nature and scope of the microbiology on the fixed-bed in the individual stages. The aim of these investigations is to further optimise the process and to speed up the detection of changing operating phases in both qualitative and quantitative terms. One goal is to run the process in the thermophilic range (55 – 60 °C) instead of the previously used mesophilic range (30 – 35 °C). This would be beneficial for sanitising the digestate.

Pressing the organic waste

A screw press separates liquid organic matter from the pulped organic waste (maximum grain size: 80 mm). It works with a pressure of 2 to 5 bar and an electrical output of 37 kW. The squeezed organic waste is then still sufficiently permeable to air to enable mechanical ventilation during the aerobic composting. If there is excessive compression, the air resistance increases and thus the energy required for the ventilation. After pressing, the material is homogeneously moist and stripped of organic matter, and therefore rots several days quicker. This effect increases the capacity of composting plants by 10 to 15 % for the same energy requirement and without having to expand them.
During pressing, filter baskets with hole sizes from 10 to 20 mm are used. On average Gescher can produce around 45 kg of organic matter from one tonne of organic waste annually. The proportion of dissolved organic carbon in the organic waste varies during the course of the year. It is higher in winter than in summer because then the waste bins are predominantly filled with kitchen waste.

Biofilm promotes gas formation

Wet processes are mostly suitable for digesting press water. The project team decided to use a newly developed biofilm digester with a steep funnel in the base. In this digester, the microorganisms adhere to a fixed-bed consisting of special textile materials and thus form a large surface area. This creates a biofilm that resembles a layer of slime, whereby only small amounts of bacteria are flushed out of the process. The large surface area makes it possible to shorten the substrate’s retention time in the digester, which in turn enables a high throughput in the plant. The method converts up to 86 % of the contained organic matter.
A considerable amount of earth and sand adheres to garden waste. This has to be continuously separated and removed so as not to clog or reduce the filling volume ofrf the digester. For this purpose, collection containers were attached to the bases of each individual digester (Fig. 2) from which the sand can be removed regularly without interrupting the process. Large quantities of sand soon collected in a CSTR digester that was tested at the start of the project. To remove the sand, the process had to be stopped, the substrate drained and the sand then removed mechanically while ensuring occupational safety and providing protection against explosions.
The digestate can be mainly used for washing the fresh organic waste and thus recycled. This avoids external disposal. Otherwise it would have to be sanitised for use as a fertiliser with additional energy expenditure. The selected biofilm digester meets the aforementioned requirements best of all. Although other dry and wet digestion methods would enable a greater gas yield, they would incur serious disadvantages in terms of the investment, wear and operating costs or the process stability. A film digester available on the market with a tubular fixed-bed was used in the first phase of the project but did not prove to be successful and, following an accident, was replaced with the current system.

Projektinfo 17/2014:
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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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