tel: +44 (0)1275 858386
fax: +44 (0)1275 858387
Unit 36, Southfield Rd Trading Estate, Nailsea, Bristol, BS48 1JE, England
The Latest Plant (Cumbernauld — April 2006)
Operation & Control
Before the biodegradable organic waste is loaded into the composting tunnel it must be shredded and provided with the correct level of moisture.
The control of the Composting Process is based on two parameters – the oxygen level and the lowest temperature in the composting waste.
Both these parameters are monitored by the PLC continuously. Each is kept within the operator preset ranges of oxygen level and compost temperature by increasing/decreasing the compost blower speed and admitting fresh air into the compost blower suction duct.
To assist in providing a flexible control system each compost blower speed may be varied.
The speed of each Biofilter blower may also be varied to maintain the required headspace pressure within the tunnel. This is under PLC control and completely automatic once set up.
The control of the plant must rely upon instrumentation to detect conditions throughout the plant.
In-compost temperature monitoring devices are installed at 8 positions along the length of each composting tunnel, plus in the inlet and exhaust ducts for each composting blower, the outlet of each biofilter blower, and the scrubber outlets.
Oxygen level monitoring is provided in the roof of each tunnel.
In each tunnel the pressure in the air space above the compost is monitored and compared with atmospheric air pressure. If the difference exceeds maximum process limits an actuated butterfly valve is automatically opened to atmosphere so that the pressure in the tunnel may remain within the pre-set range. An excessive pressure differential could cause damage to the tunnel doors.
previous pageDesign & Build
This plant was constructed for Scottish Water to a design by Enviros Consulting of Shrewsbury. It is designed to compost 20,000 tonnes of class 3 waste per year.
The composting plant structure is made up of 4 adjacent tunnels each of which is 25 metres long, 5.3 metres wide, and 5.3 metres high. These tunnels are constructed in reinforced concrete. The roofs are also in reinforced concrete. Each composting tunnel is filled by front loader through the full width and length, and to the required batch height. Each composting tunnel may be operated independently.
The air to each tunnel is supplied through a series of ducts cast into the floor, the air emerging into the compost mass through an array of cast-in plastic nozzles.
A centrifugal blower is provided for each tunnel. This blower may provide up to 8000 m3/hour of air into the underfloor aeration system.
The suction side of the blower is connected to the airspace above the composting material so that the composting blower acts to continuously re-circulate the same mass of air through the compost. Fresh air is provided by a branch pipe on the suction side of the composting blower. Control of the fresh air inlet is effected using a modulating square damper in the branch pipe.
A second centrifugal blower (3000 m3/hr capacity) is provided for each tunnel to draw off exhaust air displaced by any fresh air introduced into the system.
These four exhaust blowers discharge into two air ducts which lead into two proprietary Scrubbers — designed, manufactured and installed by Forbes of Norfolk. The Scrubbers act to remove ammonia and particulates from the airflow and to ensure that its humidity remains high. The Scrubbers also cool the airflow. This ensures that the air outflow from the Scrubbers may be successfully deodorised by the bacteria living on the media within the Biofilter tunnels.
The whole of the Biofilter Blower airflow therefore passes through the Scrubbers and the Biofilters, and is then exhausted to atmosphere.
The Biofilter consist of 2 adjacent rooms — constructed in pre-cast concrete sections - which contain permeable media upon which de-odourising bacteria are grown.
All above-ground air ducting was fabricated in polypropylene pipe (SDR 33) using the butt fusion process, and incorporating various flanged connections to facilitate disassembly. The discharge duct from each Biofilter blower (into the Scrubber) is in round stainless steel ducting, flanged to suit. The manifold connecting each Composting Blower discharge to the four cast-in re-circulating air pipes per tunnel is also fabricated in stainless steel.