Agricultural waste

Agricultural waste was brought within waste management regulations in 2006. The vast majority of agricultural waste was the slurries and manures. Of the remainder, the focus was on handling the plastics.[1]

There was growing consideration of anaerobic digestion, and more limited consideration of on-farm non-slurry agricultural waste, but little consideration of the two together, perhaps reflecting the reality that the quantities were modest and already used for practical purposes (e.g. animal feed or composting).

In the Anaerobic Digestate protocol produced by WRAP in Feb 2009, for instance, agricultural waste was treated as though it was entirely about slurries and manure.[2]

The 2007 report by the Biomass Task Force (chaired by a former NFU chairman) focused strongly on the role of agriculture, but did not identify any “wet” agricultural wastes other than slurries suitable for digestion.[3] It did highlight significant volumes of “dry” agricultural waste such as straw, suitable for thermal processes (e.g. gasification).

The Environment Agency’s recommendations for Agricultural Waste, published in 2001 offered the following figures:[4]

It is likely that a lot of the animal material would have needed to go to rendering. 100m m³ of biomethane looks like a stretch as the potential of this resource if it were all digested. And there was no reason to think that it would all be digested, rather than continuing to be used for the valuable and environmental uses (e.g. animal feed) to which it was being put.

It has not been possible to identify any quantification, nor much reference, to non-slurry agricultural wastes being used as AD feedstocks at the time of E&Y’s report. A small number of on-farm digesters fed primarily with slurry/manure were noted, and it is reasonable to infer that they would also have use any other putrescible material available. But it is hard to see on what basis a researcher could have concluded that there was any significant current contribution from this material in 2009, from which to project a large contribution in 2020.

Yet E&Y estimated the baseline contribution at 234m m³, and the “stretch” contribution at 967m m³ – nearly double the “stretch” contribution of the slurries and manures that dominated the statistics and analysis of the resource.

Fast forward to their target date, and WRAP recently estimated the amount of on-farm food waste and surplus produce as 3.6m tonnes.[5] Their objective, as always, was to minimise this waste, not to maximise its use for digestion. Most of it is already re-purposed, for example as animal feed. As the 2017 report for ClimateXChange, cited above, notes, there are few known instances of the digestion of this material to produce gas, whether for on-site use or grid injection. There is little reason to think now that a significant amount of gas will be produced from this material as efforts to minimise it proceed, and there was little reason in 2009 to expect much either.