30 November 2001

Mobile digester for composting biodegradables

A Welsh company has

developed a trailer-mounted

system for composting

biodegradable materials,

ranging from manures to

restaurant leftovers

VITAL Earth of Brecon has worked with Shropshire-based trailer maker Harry West on the construction of a 40cu m tippable stainless steel vessel, through which air is blown to maintain aerobic fermentation of pre-chopped materials by bacteria.

Blown air is also used to manipulate temperature inside the vessel. Processing is controlled and monitored by a computer using specially written software. At times the temperature is allowed to rise high enough to kill pathogens and weed seeds.

When the breakdown process has reached the required stage, which varies according to whether or not worms are to be used for further digestion, the vessel is moved inside a building and tipped under controlled conditions. Processing takes 2-3 weeks if worms are not to be used.

The material is screened and any pieces that are too large are returned to the vessel for further processing.

Where worms are used, the digester is emptied after seven days and the material placed in timber trays. The worms are added and allowed to work for a week to 10 days. The nutrient rich worm casts are blended with the other product to make a compost that is packed for sale through garden centres, and directly to local authorities. Pure worm casts are also marketed as a plant "vitaliser" containing nitrogen, phosphate, potash and a host of macro and trace elements.

Currently, Vital Earth is looking for farmers to breed and grow replacement worms on contract.

Founder director Malcolm Rich says the mobile digester can cope with any biodegradable material as long as the mixture put in has the right proportions of carbon and nitrogen, and contains 65% moisture.

When operational fine tuning of the prototype digester is completed the first commercial composing site with eleven trailers will be set up at Lampeter, Cardiganshire.

"Others will follow with up to 50 trailers, but most will have 20 units processing a total of 50t/day," he forecasts. "Sites have to be located where the waste stream will provide enough material without excessive transport costs."

While the company anticipates retaining ownership and operational charge of the units, Mr Rich sees potential for joint venture agreements with farmers who have sites with suitable access. To reduce the number of lorry journeys Vital Earth also plans to part process some bulky wastes where they are produced.

"Processing is controlled and clean and odour free. Leachate from the vessel is collected, and air leaving it goes through a biological filter. Processing areas will not be unsightly like landfill sites. There will be no infestation by vermin and birds, and no methane production."

Because operations will not be subject to landfill tax, and there is a marketable end product, the company can also bid to take waste material at competitive prices.

But the project involves massive investment. A 20 trailer site will cost about £1m. Mr Rich is convinced that interest in recycling will grow, and pressure on existing disposal systems will increase.

"Using landfill we are just passing the buck to another generation," he said. &#42

Malcolm Rich: "Processing is clean and odour free, and areas used will not be unsightly like landfill sites."

Testing compost pH level.