Mama Mia

When people eat, they generate food waste. It seems fitting to take that waste and recycle it to farm land – to produce some more food. (Photo courtesy of Mama Mia restaurant, one of the best in Malta.)


  • Fertilisers have an enormous energy cost.
  • Wastes can be used to eliminate import/purchase of mineral fertilisers.
  • AD liquor is one option.
  • Logistics of AD liquor have problems.
  • AD liquor needs an economic dryer.

By Bill Butterworth

15 March 15


United Nations research looking at feeding the expanding global population concluded that one tonne of Nitrogen nutrient, made in a modern, relatively efficient, USA factory takes over 21,000 kWh (or “units”) of electricity to manufacture. Every farmer knows that fertilisers are not going to get cheaper but this figure does concentrate the mind.

Similarly, diesel will not get cheaper, nor electricity, nor heating and, frankly, any and all other costs are going to rise. Hydrocarbon fuels have gone down in price very significantly over the last year or so. They are, however, creeping back up again. Thirty years ago, a tonne of fertiliser was around £100 per tonne and so was wheat – plus or minus twenty quid. Now, fertiliser is roughly twice the value of wheat.

So, what are the options?

This blog, 7 December 2014, “Reversing global warming”, looked at two farms that were eliminating purchase and use of mineral fertilisers. It took several years of composting and building up the soil biological activity to get there but, by 2010, the first of those farms had eliminated the need for mineral fertilisers and, in 2011, saved around £130,000 on not buying any mineral fertilisers for 335 ha (880 acres) of heavy clay, average 10 tonnes per ha across-the-farm land.

So, it can be done and these farms each run a profitable business, standing on its own, where the gate fees for “wastes” entering the site justify the investment and less fertiliser purchase is a bonus.


There is a problem; outdoor composting is increasingly restricted by regulation and, in some areas, there is competitive composting capacity holding the gate fees down. However, despite the growth in AD (Anaerobic Digestion), there is still a shortage in most areas of capacity to process food waste.

The big plus for AD, and the reason for its backing by DEFRA, is that it will produce electricity which is “renewable”. However, the liquor output, is smelly and a large amount (more than half the annual capacity of the facility) needs to be spread on winter-sown crops in the spring. The logistics of handling these fluids are not attractive to farming. Further, the nutrients are there but soluble and potentially lost in rainfall and pollute groundwater. A further environmental negative is that most of the organic Carbon has been used to produce methane to drive the generator, ultimately producing more Carbon dioxide. So, AD liquor will have very limited effect on soil organic matter.

The answer? A new breed of energy efficient driers is needed. These must dry the liquor from an AD plant (or, better still, a TAD or Thermophilic Aerobic Digester hybrid with an AD plant) and produce a granule which can be stored easily until next crop window for cultivation and planting, in August/September. However, there is yet again a catch. Many of the new offerings will not be very energy efficient. Anyone can pour energy in and create a dryer. The secret is to use energy from the processes themselves and then top up with efficient energy input to minimise over-all energy consumption. The technology for this more efficient energy drying is now under development. It can be done and some of the new breed will be a significant step forward. Asking for figures and make comparisons remains the maxim for the prudent farm entrepreneur.

This, then, is farm-sensitive engineering and it opens another door for farming to cut costs and increase production and income by farming better; farming with an organic base.


  1. Energy in Farm Production, Volume 6 (ed. R.C. Fluck), pp. 177–201. New York: Elsevier.