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Feature: Clever composting

During 2001-2002, Donarbon was involved in windrow composting of green waste derived from Cambridgeshire civic amenity sites.

In order to meet the growing requirements of the county council for composting source-separated household garden and kitchen waste, the company started investigating the use of in-vessel composting systems and trialled a variety of systems based around concrete A-blocks.

Four main findings were ascertained from the research. First, the roof had to be removable to allow large machinery easy and safe access to load or unload the contents. Fixed roofs required high walls so that machines could move in and out.

They also created a large void above the waste that dissipated the heat from the process that should have been recirculated back through the waste and assisted in the decomposition process.

Second, the air system had to be more reliable; earlier systems were of crude design and frequently blocked with material and leachate at the ventilating apertures. The loading and unloading door mechanism had to be simple. finally, there was an issue with the Health and Safety Executive (HSE) regarding the Confined Spaces Act.

In early 2003, these requirements were given to Copperfield Engineering in Suffolk, along with additional needs of a low cost, low maintenance, easy to use, compact and reliable system that could be extended easily and quickly if the amounts of waste increased. The new system had to be in place before the summer because concrete for the new composting site was being laid.

By June 2003, the first four vessels were built. In July 2003, the Animal By-Products Regulations 2003 (ABPR) came into force, requiring any system that treated household garden and kitchen waste to achieve a particular time and temperature and to achieve it twice in two separate units: a two-barrier system.

After some minor upgrades, the second set of vessels were built in September 2003, opposite the first set.

In March 2004, as required under the ABPR, the site was given a temporary approval and started its validation period. During this time, the site and vessels had to show that they could work under the strict requirements of the ABPR and achieve the required times and temperatures. To deal with the increasing amounts of waste, a further four vessels, two each side, were built in April 2004. Full approval was received in July 2004, the first in England under the new regulations.

Between December 2003 and December 2004, 30,000 tonnes of source-separated household garden and kitchen waste was treated through the system to produce a soil conditioner. This is used mainly in the agricultural industry. Donarbon has since increased its vessel numbers to 16, eight each side, which should treat up to 40,000-50,000 tonnes per annum of source-separated household garden and kitchen waste.

Bearing in mind the convenient and readily available concrete Ablocks manufactured by Poundfield in Suffolk, it seemed appropriate to develop a building utilising these units. The system needed to meet stringent guidelines if it was to be ABPR-validated.

To reach the mandatory minimum temperature of 60ºC within the material, and hold it there for a minimum of two days, required the void between the top of the material and the roof be kept to a minimum. This was accomplished by designing the RetractaRoof, a retractable roof that would allow complete filling of the vessel.

Once filled, the roof manually or electrically would close over the material to create a small void between itself and the material.

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