Anaerobic digestion protocol progressing

Anaerobic digestion (AD) has received much attention after being identified in the Waste Strategy 2007 as an effective way of recycling food waste (see letsrecycle.com story). However, one of the main obstacles to AD is that the digestate, which can be put on soil as a conditioner, is subject to waste regulations which limit its use.

WRAP is devising a Quality Protocol, similar to the protocol published for compost in March, this year (see letsrecycle.com story), and is carrying out tests as part of a process to define when the digestate becomes a product rather than waste, which means it would not have to comply with waste regulations.

Results of risk assessments to see if the digestate is suitable to spread on soil are due to be published in the next couple of weeks. After this, WRAP will begin defining the standards the digestate must reach under the Publicly Available Specification (PAS), to be classified as a product. If all continues according to plan, WRAP will be putting the protocol out to consultation in February 2008.

Nina Sweet, who is heading up WRAP's AD protocol team, said: “The protocol aims to define at which point the material stops being a waste, to give more confidence to users of the digestate and to increase the market for the product”.

Speaking at last week's anaerobic digestion conference in London, organised by the Environmental Services Association, she also said that the focus of the Quality Protocol programme could shift to providing clarification over existing waste management legislation on anaerobic digestion instead. This would happen if the digestate carries too high a risk to be spread onto soil, or if the knowledge gap about the product is too wide.

The anaerobic digestion quality protocol is being funded as part of Defra's Business Resource Efficiency and Waste Programme (BREW) programme for 2007/08.

Its advisory group includes representatives from the Department for the Environment, Food and Rural Affairs, the Environment Agency, the Environment and Heritage Service Northern Ireland, the Scottish Environmental Protection Agency and The Composting Association.

AD interest

However, even without a Quality Protocol, AD operators have said interest in the technology is booming.

Michael Chesshire, director of Greenfinch, which operates the UK's only AD plant processing primarily food waste, said that the potential of increased subsidies for biogas production from the Renewables Obligation Certificates is really where the revenue for AD will come from (see letsrecycle.com story).

He explained: “We have assigned a negative value to digestate – we get our income from the gate fee and from the biogas we produce.”

“Anaerobic digestion is not as expensive a technology as it is made out to be – this is one of the big myths in the industry. People forget that AD plants treating food waste don't need to be as big as in-vessel composters (IVC), which need room to co-compost food and bulky green waste.”