Manure - dairy & cattle

One of the first commercial AD plants in the UK, Holsworthy Biogas, was originally obliged to take 80% of its feedstock from farm slurry. It was not economic under this constraint, and went into administration. It only became viable when its new owners (Summerleaze) broke the obligation and switched the plant to mostly food waste.

Cattle manure and slurry produce a fraction of the gas per tonne of (wet) feedstock, compared to food waste, and less still than energy crops. Waste attracts a gate fee (albeit significantly diminished nowadays due to over-capacity), whereas farmers will not generally pay to get rid of their manure. The manure intake therefore represents a cost not an income, because it has to be transported to the digester.

On-farm digesters may mitigate some of these disadvantages by minimising the transport distance and utilising the energy on-site, which offers a higher value than export as gas or electricity. Specialist farm digesters can also be cheaper, as they can be simpler than digesters that handle waste. And digestate disposal directly on the farm may be cheaper.[1]However, they will tend to be smaller, and will often not be located on the gas grid. In the rare cases where the benefits of localism are sufficient to outweigh the poor economic fundamentals of slurry digestion, the opportunities for grid injection may be limited.

There is thought to be a substantial resource of manure and farm slurry. In England and Wales, it is estimated that approximately 72m tonnes is collected for spreading, and another 73m tonnes is deposited directly in the fields by the animals.[2] Of this, dairy and beef accounts for nearly 60m tonnes of the spread material and 42m tonnes of the directly-deposited material. At around 15 m³/tonne, this implies a theoretical potential of 900m m³ of methane from AD, if all the collected dairy and beef manure/slurry were digested before spreading.

Yet the production of biogas from this material was negligible at the time of the NG/E&Y report, and remains insignificant today, despite the stimulus efforts. A 2017 report by Ricardo for ClimateXChange identified:[3]

Bioenergy Europe note that:[4]

Yet biogas as a whole constitutes 2.1%, 0.4% and 1.3% of those countries’ primary energy consumption. Agricultural residues constitute 51%, 68% and 49% (by mass) of their biogas feedstock. Given the lower gas-producing potential of the feedstock, that means that in the European countries that have placed greatest emphasis on manure as a biogas feedstock, it is responsible for around 0.75%, 0.2% and 0.4% of their primary energy consumption. That is particularly striking in Denmark, which is noted for its successful promotion of renewables, has the infrastructure (e.g. municipal district heating schemes) to maximise the value of AD plants, and has large quantities of pig (and other) slurries to digest.

If we combine the fact that farm slurries are generally uneconomic but may have limited potential as a complement to higher-gas feedstocks, with the experience in the countries that have tried hardest to use this material, we may conclude that the maximum practical proportion of the feedstock is around 50% by mass, which equates to around 25% of the gas potential (i.e. if we work out the potential of other feedstocks, we could add up to 33% for co-digestion of this material).

The reality of digesting manure is that, while the theoretical potential is quite large, the economic potential is small and always has been. NG/E&Y were not making abstract claims about the theoretical potential. They were claiming not only that biomethane could supply nearly half our domestic gas, but that the level of support needed to achieve that was modest. The combination of these two claims with regard to manure is disingenuous.