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Casting a wide net

“In the composting process there are two major enemies: odour and bioaerosols,” Agrivert commercial director Harry Waters explains. Indeed, these are two big issues for the industry, which currently faces possible policy changes imposed by the Environment Agency to curb the possible health risks from bioaerosols produced at composting sites.

For Agrivert, the risk of odour and bioaerosols being exposed to nearby homes and workplaces is considerably diminished as a result of the special Gore membrane technology it uses at its in-vessel composting (IVC) sites. It is the UK’s sole agent for the technology, which uses the Gore membrane - made from a similar material to that found in some outdoor clothing - in the structure of its vessels that allows water vapour out while keeping odour molecules in.

Agrivert has recently opened its third IVC facility, although one now belongs to LondonWaste. All three use the Gore material and are the only ones in the UK to do so, although it is widely used in Europe and the US. The newest facility, which has a capacity to take 55,000 tonnes of food and garden waste, is situated in South Mimms, Hertfordshire. It serves households in St Albans, Hertsmere, Welling Hatfield, Stevenage and Broxbourne, but 10% of the capacity will also come from local businesses and a small amount from Essex.

“The advantages [in using Gore] are significant and materialise in a number of ways,” says Waters. “In terms of odour, Gore will reduce it by roughly 98%, which is a better performance than most biofilters.

“Most plants using Gore operate with no or very low leachate disposal cost”

“The pore size of the membrane, combined with a condensate layer formed on the underside of the Gore, forms an effective barrier against odour and bioaerosols. Odour is then reabsorbed into the compost, which continues to break down complex fatty acids that are often the cause of smells. So the Gore allows the odour and bioaerosols to be properly contained.”

But while Gore stops any odour or bioaerosols from escaping, it allows air molecules to pass through, therefore naturally carrying out the work of conventional IVC facilities, which have to re-circulate the air using pumps.

This also means that water vapour can pass out of the membrane as the compost heats up and warms the liquid contained it. According to Waters, 65-70% of a batch of organic waste could be liquid so, while the compost heats up to 60°C, the water slowly evaporates.

Waters explains: “In conventional composting systems, all the moisture is contained in composting vessels, resulting in a build up of leachate. Leachate is extremely expensive to treat and represents a significant operations cost for many composters.

“But for those using Gore, the membrane allows water vapour through it, so the condensation passes freely out of the vessel. So most plants using Gore operate with no or very low leachate disposal cost.”

For Agrivert, however, the carbon savings made through using the membrane technology are just as important as the cost savings. In many conventional systems, energy consumption is anything from two to three times that of the Gore systems. The energy cost savings are significant but, perhaps more importantly, so are environmental benefits.

“It is an excellent technology, not only because of its strong environmental controls, but it also results in lower gate fees which make the technology competitive,” says Waters. “It’s a quality technology with an associated price tag, but operators reap the costs back through low operational cost.”

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