Tunnel composting
The tunnel composting process is a modular, completely enclosed and individually controllable or controlled technology, whereas the process runs fully automated by means of a positive aeration.Tunnel composting is already viable with waste inputs from approx. 3,000 t/a, but can be used equally for very high throughputs by simply increasing the number of modular units. It is suitable for the aerobic biological treatment of bio waste or mixed household and kitchen waste, as well as for the further treatment of materials difficult to handle such as sewage sludge or digestate. The technology achieves proven best results at short retention times not only for composting, but also for stabilisation and biological drying.
For composting plants which comprise both, intensive and post-composting sections, the tunnel sections are designed preferably as intensive composting modules. Generally speaking, tunnel composting can be implemented in either of two design variants, which differ in terms of equipment and level of technology:
- Tunnel with air circulation and automatic process control
(HP tunnel/ high performance)
With this variant, the optimum conditions for the desired biological process are set for each tunnel by varying the fresh air, circulation air and exhaust air flows on the basis of temperature and oxygen content measurements (in the material or the exhaust air from each individual tunnels); this is achieved by way of a closed loop process control for the gate valve positions and frequency converters to control the fan unit of each individual tunnel.
In combination with intensive humidification using microbially active process water, and thus reliable control of the water content during the composting process (no risk of drying-out), the energy-efficient operating mode with air recirculation ensures maximum decomposition rates within very short retention times. The aeration floor is usually designed as a slotted floor with an air distribution chamber underneath. - Standard tunnel
(NS tunnel = normal standard)
With this variant, which is characterised by a lower level of technology and equipment, the biological process is controlled automatically as open loop system according to the temperature and oxygen content, or else by the temperature alone.
There is generally no air recirculation. The tunnel aeration, which is normally a positive aeration, can also be switched to an intermittent mode and controlled accordingly. With this tunnel variant, the necessary retention time to attain the desired decomposition or the required degree of stabilisation is longer compared to an HP tunnel, but still much shorter compared to other composting methods such as aerated static piles, composting boxes, windrows or others.
As far as the structural engineering is concerned, and especially with regard to the possible tunnel dimensions and construction, there are no differences compared to the HP tunnel, apart from an optionally different design for the aeration floor.
With this tunnel variant, the aeration floor can be also realised in a simplified form and thus more cost effective, with a series of aeration channels or pipes.
Process characteristics and benefits
- Optimum conditions for the biological process ensure precisely adjustable parameters in combination with intensive humidification
- Automatic process control, wherein each tunnel can be controlled separately
- Maximum decomposition rates with very short retention times -> reduced space requirement
- No electrical or mechanical equipment in the tunnel -> maximum operating reliability, high availability and a long service life
- Modular design allow low-cost plant extension
- High flexibility in terms of equipment and level of automaton; simple integration of automatic loading and unloading equipment
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