Keeping it in the neighbourhood

Agrivert's latest in-vessel composting facility has opened for business in Potters Bar, a sustainable set-up that will keep carbon impacts to a minimum. Katie Coyne paid the site a visit.

Where there's muck there's brass certainly appears to be the case when it comes to Agrivert. The organics specialist has just opened its third UK in-vessel composting site in Potters Bar, Hertfordshire. The site spans across two hectares, has an annual capacity of 50,000 tonnes, and has already secured a 15-year contract worth £27M taking green and food waste from Hertfordshire County Council.

While the facility also has the capacity to take waste from other operators - it will begin to take some commercial waste from Essex this summer - most of the waste to the site is travelling only about five miles. Agrivert plans to sell on the compost to the neighbouring farm, which literally adjoins the composting facility.

Above the roar of the M25 next door to the site, commercial director Harry Waters explains: "When we look for a new site, the first thing we look for is where the compost is going to go. The moment you put anything in a truck these days, even if you're only moving it a mile, it costs £5. But you're also increasing the carbon footprint."

For a composting site, the four striking things about it are its cleanliness, relative lack of odour and noise, and energy efficiency. Although the site is only just up and running, having opened in April, Waters insists: "It will still look like this if you visited it in a year's time."

He points out that every running area is swept each day to keep it clean, which also helps to keep odours at bay. The door to the reception centre, where the waste is received and shredded, swiftly opens and closes, thus reducing odour. A ramp has also been built in so that the trucks can dump the waste over the side so it won't contaminate the wheels. Despite costing double that of a typical in-vessel composting site, Agrivert was able to put in a competitive bid because of its low energy usage.

Carbon impacts
"We are trying to do this in a way that is environmentally friendly and low in carbon," explains Waters. "Costs come down if you are carbon friendly because it tends to mean lower energy use and energy costs."

He adds that the site employs Biodegma technology; gore membranes as the sides and roofs of the in-vessel structure, wrapped around an aluminium frame. The gore membrane allows around 10-15,000 tonnes of water - annually, over the whole site - to escape. Waters says: "If we didn't lose the water we would have to treat the leachate produced."

He adds that an in-vessel composting site of a similar size might need to spend around £300,000 a year just to process the leachate, and points out that treating leachate is also very energy intensive. The small amount that is produced on the Potters Bar site is recycled and put back onto the raw material as it is shredded in the reception centre. The leachate is collected during the composting process through drains located on the sulphur resistant concrete floors of the vessels.

The material coming into the Potters Bar site comprises around 30% food waste and 70% green waste with some cardboard. The shredder currently used at the facility is diesel-powered, but Agrivert is looking to replace this with an electric machine, which is more reliable and energy efficient. Waters says there are plans to bring electricity to the
site shortly.

Turning on the electrics
"Electrics can be costly - it depends where you're getting it in from. We don't have to get it in from very far and as this is a long term project we thought we should do it early on."

Once the waste has gone through the vessel process it is put onto an open maturation pad. This has a drainage tank underneath for storing rainwater. The material will stay there for about 12 weeks before it is screened where the plastics contaminants are removed and either recycled or landfilled.

Larger bits are taken out and put back on to the maturation pad. The resulting compost screened to 30mm is then sold to the farmer for around £2.50 a tonne. Although this might not sound like a lot of money, Waters believes that the market for compost will soon pick up.

"There is a global shortage of phosphate predicted in 10 years' time," he explains. "It will be depleted so the value of compost will have to improve significantly because it's an ingredient that everyone relies on."

Katie Coyne is a freelance journalist

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