Why has Anaerobic Digestion (AD) become the dominant technology for recycling food waste in the UK

In the first ten years of this century, In-Vessel Composting (IVC) was the leading technology for recycling food waste in the UK. Indeed, Agrivert designed, built, owned and operated three of these plants, providing food waste recycling to over 15 local authorities. However, almost no new IVC plants have been built in the UK for over a decade, and some have ceased to operate, being replaced by Anaerobic Digestion. As a result, AD now dominates the food waste recycling landscape, with composting used only for garden waste. Why is this?

Why has Anaerobic Digestion (AD) become the dominant technology for recycling food waste in the UK

As recycling targets for Local Authorities were introduced, IVC provided a quick and easy route to achieving high waste capture and recycling. IVC offered Local Authorities the ability to collect garden waste, food waste and sometimes cardboard on a single Refuse Collection Vehicle (RCV). Local Authorities were attracted to the relatively low cost of recycling against escalating landfill costs. Ease of conversion was another driver; often, no new bins were required, expensive collection vehicles did not need to be changed, and the messaging to households was comparatively simple.

The challenges of operating IVC and its benefits became clear over the next decade, hence a move away from IVC. The main challenges were:

  • IVC is difficult to operate in harmony with the community, especially with high food waste inputs. Odour emissions led to some IVCs being closed or restricted by the Environment Agency
  • IVCs do not enjoy high concentrations of food waste as they need woody carbon-rich garden waste inputs for structure and to balance the high nitrogen food wastes. Consequently, few IVCs operate well with more than twenty-five per cent of food waste inputs. For IVC, this is particularly challenging in the winter months when carbon-rich garden waste collections are reduced (or stop entirely) from December to March
  • IVC is a very aggressive process, with the plant infrastructure life often being less than 15 years. Therefore, IVC capacity closed as they required uneconomic refits to the plants to extend their lives
  • Food capture rates were significantly lower than AD capture rates. Most believe this is because the public didn't think it was worth putting only 2kg of food waste in a 240-litre bin, especially in winter when little green waste was collected
  • Compost quality was also a significant problem, especially in cardboard-inclusive collections. Many fragments of contamination were finding their way into the product making compost difficult to market. This attracted much attention from regulatory bodies and waste collection authorities
  • IVC are very energy intensive. Large fans are required to aerate compost and power odour control systems, and heavy machinery is needed to turn and move compost. These have heavy monetary and environmental costs
  • Emerging studies highlighted that bioaerosols pose a significant risk to staff. As a result, employers were increasingly open to litigation

AD has several advantages over IVC, making it the technology of choice.

  • Local Authorities have a legal obligation to consider the waste hierarchy when selecting waste recycling solutions. Consequently, AD is higher up the order and should be considered first
  • AD is much more sympathetic to contaminated wastes. The separation of contaminants is more straightforward and more comprehensive in AD. Consequently, enabling plants to accept a broader range of commercial and industrial organic wastes and allows AD to produce a clean nitrogen-rich fertiliser
  • The AD financial model is underpinned by energy revenues not available to IVCs. As a result, IVC swiftly became expensive in comparison to AD as IVC depended almost wholly on a gate fee to support the financial model
  • AD can operate in a more or less completely enclosed environment. Therefore, the odour signature of an AD plant is generally significantly less than a compost site enabling them to operate within the community
  • Whilst not free of bioaerosols, the wetter environment of AD and the enclosed nature of treatment reduced exposure to staff

The only significant barrier to changing from IVC to AD, in more rural areas especially, is that food waste collection can be cheaper if collected commingled with green waste. However, with split bodied RCVs, this gap is rapidly narrowing.

Whilst there is still a place for composting cleaner green wastes, it is perhaps understandable that AD is replacing IVCs. However, many countries are still procuring IVC as a waste solution. It will be interesting to see if these are still the primary treatment technology in decades to come or if they encounter the same issues experienced in the UK.

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