How composting is closing the loop
23 March 2012, source LAWR
The West London composting site
The heavy snowfall may have caused disruption to the council waste collections, but as far as the operations on-site are concerned, it is business as usual for West London Composting.
Sue Grundon, site manager at the family-run business at Harefield, near West Ruislip, Middlesex, has been out early, helping shovel snow from the entrance, ready for the morning deliveries.
Since opening its doors for business in 2004, West London Composting has been receiving a mixture of garden waste, kitchen and canteen leftovers, and cardboard and paper, processing the material through in-vessel composting to produce a PAS 100 compliant compost for agriculture, landscape gardeners and the general public.
Licensed to process up to 50,000 tonnes of waste a year, five local authorities - the London boroughs of Hillingdon, Brent and Harrow plus Three Rivers District Council and Watford Borough Council - transport material daily to the site, which produces around 30,000 tonnes of compost a year.
Because each council has its own collection regime, deliveries can vary from a simple garden waste service to mixed waste streams. One of the main challenges is reducing contamination caused by non-conforming and non-compostable paper and cardboard.
"It's fine in the winter when we've got plenty of time to pull contamination out, but in the summer when we've got everything coming in, we don't have the luxury of time and space to be able to sort the loads properly," says Grundon.
"The whole issue of paper and cardboard is a massive problem. Any cardboard that comes into us has to be compostable."
As site manager, Grundon sees "problem" items on a daily basis - anything from Tetra Paks to Sunday supplements still in their plastic bags to microwave meal sleeves complete with plastics pots.
"We have got a picking line on-site and we do put the worst loads over it but even then we are picking out the rubbish, more than the paper and cardboard," she says.
"Most of the card that doesn't compost will come out at the screening stage at the end of the composting process. We have to ensure we get enough of the contamination out so that the compost is PAS 100 compliant."
Grundon says that it's a juggling act, especially at this time of year when the ratio between cardboard and garden waste is particularly low.
Nevertheless, despite the contamination issues, the in-coming material passes through a vigorous process before it emerges as fine-quality compost. After the council vehicles have tipped the material and the contaminated items largely removed, it is shredded into smaller pieces.
"With the composting process, you've got to get the mix right, you need the correct carbon to nitrogen ratio, and you also need larger pieces of material to help create air spaces because it's an aerobic process and oxygen needs to be ever present," she says.
To speed up the composting and to control odour, an additive is added at this stage. The shredded material is then transferred to barrier one, comprising 16 large sealed vessels, each capable of holding up 200 tonnes of material.
As a legal requirement, governed by the 2003 EU Animal By-Products Regulations (ABPR), the material has to be in-vessel for a minimum of 7-10 days, during which time it has to reach 60°C for two consecutive days to kill off any pathogens. The exact process is then repeated in a second barrier.
The material is then moved to a maturation site for the final composting stage. Six vessel loads are heaped in to a single windrow and, using specialist machinery, it is turned regularly for up to eight weeks, before being analysed to ensure it meets the PAS 100 standard.
"It's a family-run business and we all live in the community so we want people to know what we are doing," she says, reflecting on Grundon's efforts to promote recycling locally.
"A lot of people don't know where their green waste goes. It's not until they come and look around that they can see what should be put in their waste. It helps if they have seen it first-hand."
Nick Warburton is editor of LAWR
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