Fortnightly kitchen waste collections blamed for poor AD results

To generate sufficient biogas, anaerobic digestion (AD) plants that depend on kerbside collections may need to work with authorities offering weekly kitchen waste collections to get enough putrescible material, trial results suggest. But the technology is set to boom for commercial food waste.

The Environment Department (DEFRA) launched the new technologies programme in 2004, with the intention of funding trials of ten new waste treatment technologies. It aimed to have the plants operating by the end of 2006, but it only managed two (ENDS Report 384, pp 14).

Greenfinch’s 5,000-tonne capacity AD plant in Ludlow, Shropshire, completed its first year of operation in March. It is one of two AD facilities in the UK using kerbside collected food and green waste as their sole feedstock. The other, Western Isles council’s 12,000-tonne capacity plant on the Isle of Lewis has only been running since October.

Biffa has a 50,000-tonne plant in Leicestershire, but this takes organic residues from the company’s ‘ball mill’ mechanical separation plant, as well as kerbside collected waste (ENDS Report 361, pp 25-28). The Greater Manchester Waste Disposal Authority intends to build five similar AD plants by 2011 (ENDS Report 385, pp 22).

Michael Chesshire, Greenfinch’s director, presented the plant’s first-year results at a Materials Recycling Week conference in April.

Because just 5-7% of its feedstock was food waste, the plant did not generate as much biogas as it hoped to convert into electricity. The plant has the potential to produce 200 kilowatts of electricity, but only generated 70kW.

Mr Chesshire blamed the low level of food waste arisings on householders being unwilling to keep it in a bin for a fortnight.

A recent report by consultancy Eunomia for the government’s Waste and Resources Action Programme, said fortnightly collections of organic waste do not bring in high volumes of food waste (ENDS Report 386, p 4). However, it said the level is usually closer to 15%.

Greenfinch also had problems with contamination - in one case a toaster was put in a bin.

"The plant has worked well," Mr Chesshire said, "but not to its throughput. We’ve also had lower biogas output than we expected, so we can’t say this is the way to go."

The company has made significant changes for the trial’s second year. Its feedstock will now be 90% food waste. About 1,000 tonnes will come from a weekly kerbside collection in Ludlow and another 1,000 tonnes from the Cwn Harry Land Trust, a community group running weekly kerbside collections in Newtown, Powys.

The remaining 3,000 tonnes will come from Viridor in Somerset, which runs weekly food collections but has surplus material. Mr Chesshire recognises this is contrary to the proximity principle, but said it had to get the material where it is available. Just over 50 councils currently collect food waste (ENDS Report 383, p 19).

In spite of the first year’s problems, Mr Chesshire is confident the technology is of use to local authorities. "Our work is coinciding with a time when there’s a big debate on food waste collections and local authorities have got to look at the feasibility of them. If they do, our technology becomes very attractive."

The Eunomia report concluded that separate food waste collections, coupled with anaerobic digestion, offer the best environmental performance among food waste treatment options. But the cost of adding food waste to an existing garden waste collection is "significant".

Some AD developers agree with Mr Chesshire that taking kitchen waste is viable. Biogen runs the Bedfordia plant, just north of Bedford, taking 30,000 tonnes of commercial food waste and 12,000 tonnes of pig slurry. It is currently trialling the processing of food waste from weekly collections in Bedford and Milton Keynes. AD should be of increasing interest to local authorities, it said, as targets under the landfill allowance trading scheme tighten.

But most AD companies ENDS spoke to said the difficulty of obtaining consistent material makes local authority waste unattractive. "I’ve seen some very clean material from local authorities," said Jake Prior, operations director for AD at Summerleaze, which runs an 80,000-tonne capacity plant in Holsworthy, north Devon. "But there are more risks with it than material from food processors. We don’t want to get involved in pre-processing."

Outside the municipal waste sector, AD appears to be taking off thanks to increased electricity prices and rising landfill tax. Marks and Spencer said in January it would trial AD for treating its food waste (ENDS Report 384, pp 5-6).

Summerleaze has two more plants in the pipeline - one at the site of its Cambridge research and development facility and one "not far" from Holsworthy. Both will be of 80,000-90,000-tonne capacity, and the company hopes they will be operating within two years.

Sita intends to have planning applications in by the end of the year for a 100,000-tonne facility in the Midlands and a 20,000-30,000-tonne facility in "the south".

The company is working in partnership with food processing companies for both, although it hopes to "include spare capacity to merchant in material from food waste collections", said Stuart Hayward-Higham, Sita’s head of business development. "There are also a couple of opportunities for single-client, bespoke systems for biological wastes," he added, such as brewery waste.

Biogen bullishly says it intends to build 15 plants by 2011, but it could provide no further details.

The only sticking point to AD’s growth, say these companies, involves the digestate, because spreading it to land requires a waste management licence.

The Renewable Energy Association has developed a digestate standard for the Scottish Environment Protection Agency. It will be tested using output from two plants over the next year to ensure it is economical and safe. There are discussions about extending it across the UK, although the Environment Agency would need a "quality protocol" to define when the digestate stops being "waste".

  • The Welsh Assembly Government has launched a programme to pay up to 30% of the cost of AD plants in Wales. The plants must take source-segregated biodegradable wastes, with at least 60% from commercial and industrial sources, and be capable of operation by 31 March 2009. Applications are required by 30 July.
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