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Composting Class

Europe's largest in-vessel composting facility is being built at a farm in west London. John McKenna investigates.

Jump on any underground train heading west out of the capital and once it has emerged from the tunnels the grey concrete jungle gradually
gives way to a greener landscape. Now largely a commuter zone for central London, Uxbridge still has a sufficiently large patchwork of fields to accommodate working and non-working farms.

To term Highview Farm in Harefield "non-working" would be an injustice. True, there are no longer crop harvests or multitudes of livestock to tend to. But this farm is in the process of becoming Europe's largest in-vessel composting facility.

Established in 2004 as West London Composting (WLC), the 2.4ha site now takes green waste from five west London boroughs and four district councils in Hertfordshire.

"As soon as we were built we were inundated by all the councils," says WLC managing director Martin Grundon.

Hardly surprising, given that local authority targets to divert waste from landfill are weight-based and compost is one of the heaviest categories of recyclable material.

WLC is also able to take organic materials like garden waste as well as food waste, which is estimated to make up 30% of the domestic
waste left once other green waste and materials such as paper and glass have been recycled. It is one of only two facilities within the M25 that takes food waste and it is able to do so because it uses in-vessel composting technology (see below).

This year's waste strategy for England, published by the Department for Environment, Food and Rural Affairs in May, identified food and organics as priority material for composting thanks to their propensity to propel councils up the recycling rankings.

The ability to handle a mixture of domestic and garden waste means that after busy summer bank holidays WLC can be the scene of
tailbacks of trucks waiting to offload tonnes of hedgerow cuttings and half-eaten sausages.

"The rationale behind increasing the size of the facility here at Highview Farm is simply to give us more room to operate, particularly during seasonal peaks where the amount of waste brought in is often in abundance," says Grundon.

Prior to construction, WLC was capable of handling 50,000t of waste per year and turning it into 30,000t of compost.

When material arrives on site it is shredded, then taken to one of the eight 200t-capacity vessels known collectively as Barrier One.

When a vessel is full, seven temperature probes are inserted into the compost, along with two oxygen probes. Then, a retractable roof is wound over the vessel and secured.



At the end of the seven to 10-day composting period, the compost is moved across to Barrier Two, which comprises eight 200t-capacity vessels. Here, the entire process carried out in Barrier One is repeated. The compost is then transported to WLC's maturation site, heaped up in windrows and left to mature for six to eight weeks. During this period, the compost is turned one or twice a week, depending on the weather, to keep it aerobic.

The new £2.7M facility, partly funded by the Waste and Resources Action Plan, adds another 16 vessels, taking WLC's capacity up to 100,000t a year.

These new vessels will be used for the Barrier One function process and 16 old vessels will act as Barrier Two.

Designed by WLC, in collaboration with manufacturer, The Composting Company, the new vessels are basic concrete structures with steel-framed fabric-covered roofs.

To construct them, half-tonne reinforced concrete blocks are cast on site by contractor ARD Construction and put together to form vessels measuring 7.5m wide by 14m in length and 3.1m in height.Once the concrete shell of the vessel is complete, its roof is assembled. The roof consists of 14 stainless steel cages measuring 2m by 4m that are bolted together during construction with a 10mm polycarbonate sheet pulled over the top.

Clearance with the roof closed is only 4m but this rises to 5.2m with the roof allowing free access for the trucks. "The roofs need to be jacked up to allow the dumper trucks in," says Composting Company managing director Chris Field.

The vessels are 4m high with the roof closed but are 5.2m high when the roof is open.

All along the underside of the roof is a rubber seal that locks out air once the roof is fully closed and the doors are shut.

"By sealing the roof you are not only controlling odour but also controlling the aeration process," says Field.

A control panel at the front door of each bay operates the entire opening and closing process for the roofs. These doors are steel frames clad on the inside with stainless steel. This use of stainless steel, also employed on the roof cages, is necessary due to the high levels of acidity present in the compost, which makes it a very corrosive environment.

Air is pumped into the vessels through aeration pipes, which are laid before the concrete floor is cast. Sealing the vessels and pumping air in allows WLC a greater degree of certainty and control over the composting process.

WLC is now applying for the British Standards Publicly Available Specification 100 (PAS 100) for its compost, which it currently sells to farmers, landscape gardeners and garden centres.

If it achieves this quality benchmark, Grundon is hopeful that sales of his compost will rocket, as the PAS 100 mark means that the compost is certified by the Composting Association as quality assured, traceable, safe and reliable.

WHAT IS IN VESSEL COMPOSTING?

Composting is a biological process in which micro-organisms convert biodegradable organic matter into a stabilised residue known as compost.

The process uses oxygen from the air and produces carbon dioxide and water vapour as by-products.

The term "in-vessel composting" is used to cover a wide range of composting systems. These systems feature the enclosed composting of waste, therefore allowing a higher degree of process control than is possible with windrow composting.

Composting in this way complies with the 2003 EU Animal By-Products Regulations (ABPR), the only method accepted by the Department for Environment, Food and Regulatory Reform for processing kerbside-collected kitchen waste.

Under ABPR regulations, material must be left in the vessel for a minimum of seven days. And during this time, it has to reach a temperature of 60ˇC for two consecutive days. This ensures the destructionof bugs such as salmonella.

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