29 March 2002

Something for nothing – with added benefits

Faced with processing

35,000t of green waste

each year, one Devon firm

now recycles over 20,000t

on to agricultural land,

supplied as a nil-cost

organic fertiliser for

farmers. Geoff Ashcroft

looks at the operation

EVERYONE likes something for nothing, but when that something comes with a multitude of hidden values, the gesture becomes even greater, as many Devon farmers who have received compost from Ecological Sciences, might confirm.

"The potential for organic compost extends far beyond its soil conditioning properties," says Matthew Lawson of Ecological Sciences (EcoSci). "Applying 50t/ha of green waste compost can reduce bought-in fertiliser requirements by up to 20%."

Based in Exeter, Devon, EcoSci processes over 35,000t of green waste a year for Devon county council. About 12,000t finds its way into 50-litre bags as fine grade compost for garden centres with the remaining 23,000t being processed and spread directly on to agricultural land. And the quantity being processed is growing each year.

"People produce more and more waste each year and the government is trying to restrict how much goes into landfill sites," says Mr Lawson. "By segregating waste at local civic amenity sites, operations like ours can recycle green waste into a useful organic compost."

For EcoSci, the task of transforming green waste into compost is a slow, but continual process. From initial shredding of waste to final screening of mature compost, the process can take between six and nine months.

"We do not add anything to the mix – we rely on natural bacteria and good management skills to produce a valuable organic compost," he says.

EcoSci collects green waste – mostly hedge/grass clippings and tree branches – from civic amenity sites throughout Devon, then transports it to four of its own shredding depots strategically located in the county.

"By taking waste to the shredder, we can monitor the quality of material that is used to produce organic compost," he adds.

At this point, material is put through a Tim Envipro flail shredder, where a 435hp V8 Mercedes engine pulverises between 30 and 45t/hour into a variety of sizes – essential to promote the aerobic digestion process needed to produce compost. Shredded material is then distributed to farms, where on-farm composting begins.

Because EcoSci is paid by the council to recycle the material, the firm only needs to ensure it can find a home for the final product and is why the compost is supplied to farmers within a five mile radius of a shredding site, at no cost.

"We are continually adding farms to our composting database, though they need to be reasonably close to a shredding site otherwise they will be required to contribute to transport costs," he says. "And each farms cropping regime along with soil samples are taken into account before we can determine how much shredded material can be delivered."

Once shredded waste is delivered, the composting process begins. EcoSci monitors the heaps on a regular basis – which heat up naturally through bacterial activity – to ensure the correct aerobic activity takes place.

"Temperature is critical to promote bacterial activity. If a heap becomes too hot it will promote fungal activity – if it becomes too cold, then anaerobic digestion will start and that produces smells and acidity," he says. "The best biological activity occurs between 45C and 65C, which also helps to kill off weed seeds in the heap."

Compost heaps are regularly probed to record their temperature and to determine the level of bacterial activity taking place. Such information is essential to heap management, says the firm, and dictates when compost heaps are "turned". "We use telehandlers or excavators to turn the heaps," he says. "The key is to leave a heap loose, so air can pass through and encourage further aerobic activity."

Mr Lawson says that when a heaps temperature stabilises at about 45C – usually between six and nine months after the heap was first formed – the compost is mature and ready for screening.

"We pass the material over a Trommel screen to remove anything over 25mm in size," he says.

"Oversized screenings are removed from the farm and recycled again via the shredder, while the final screened product is left in a heap at the farm." &#42

A 435hp shredder is used by EcoSci to pulverise green waste. Shredded material is then stockpiled, ready for transportation to nearby farms, where it is frequently turned to aerate and promote the composting action.

Matthew Lawson: "Green waste compost is a nil-cost product for farmers. It can enable inorganic fertiliser needs to be reduced by up to 20%."