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Consider the 'other AD'

The Government is very interested in renewable energy, both from the point of view of cost and security.  AD – along with wind turbines and solar panels – has been pushed by various Government departments. But a technology that is as old as AD, offering an alternative, has been neglected.

Thermophilic aerobic digestion (TAD) processes organic waste in a similar way to AD but produces a much less odorous liquor. This means it can be applied to compost heaps without risking complaints and planning objections from local people about bad smells.

Composting also helps to improve the quality of the material as it digests the soluble nutrients so that when the farmer spreads it onto the land, nutrients don’t run away when it rains. Leaching costs the farmer in terms of lost nutrients that need to be replaced as well as polluting the ground water.  Some people argue that AD makes nutrients more ‘available’ but is this really the case if it means it can just run off the land? Available Nitrogen, for example, is usually measured by testing for the ammonium ion (“ammonia Nitrogen”), which is rain-soluble and can pollute groundwater. 

AD can also be composted to achieve the same results but because AD liquor is much more smelly, local residents are more likely to object. Currently most compost heaps are not covered, so the smell would not be contained, although this is beginning to change. Composting the liquor also provides a storage solution as farmer don’t need to place it in tanks over the winter months - when they are not allowed to apply the liquor to the land.

The liquor produced from AD is full of anaerobic micro-organisms yet the soil is populated by aerobic organisms such as Penicillin and other fungi that protects crops from disease. I am concerned that there is a risk of changing the naturally occurring aerobic population protecting the crops, even if only temporarily.

The TAD process works by blowing air through the mixture of food waste. This mixes the material and builds up a population of aerobic microogranisms that digest the food and kill human pathogens by raising the temperature to that required by regulations. The end product is a liquid compost with a much lower odour. The problem is then how to produce electricity.

Lee Sutton farms with his parents at Grendon, near Atherstone off the A5 in Warwickshire and together they aim to eliminate mineral fertiliser purchasers within five years. From the start, they have been discussing technical developments with colleague members of Land Network, the farmer-owned group set up to recycle urban wastes to land. 

“We much prefer TAD because it is aerobic and that is what our soils are,” says Sutton.  “This process is much more attractive to agriculture because it has lower capital costs and is easier to manage. The problem has been that TAD does not itself produce methane and, therefore, cannot be used to produce electricity or gain support to produce “renewable” electricity.”

However, Land Network as an organisationhas developed a solution that combines TAD and AD. The hybrid is currently under test by Land Network with its associated technology and engineering companies including Tanks and Vessels Industries.  The TAD unit produces a hot and uniform liquor that is fed into a methane extracting unit. It is the methane that can then be used to drive a generator to produce electricity.

“The situation is not entirely logical nor is it predictably certain because some local authorities are moving to co-mingling food waste with green garden waste,” says Lee Sutton. “This reduces their capital costs because they can collect garden waste and food waste with their current systems.  However, it dramatically raises the total gate fees they have to pay.”

Gate fees for food waste is around double that for green waste so by mixing them together, the higher fee for food waste has to be charged.

“To tender for this co-mingled material, we would have to build an in-vessel system which is basically a big shed over what we do outside,” says Sutton. Under the Animal By-Products Regulations, all food waste must be covered during treatment. “We will try to offer whatever local authorities want. However, in the group, we think that separate food collection fed into a TAD facility gives both the council and the farm a better deal.  Adding the hybrid to the TAD produces renewable electricity and then the country gets a better deal.”

Bill Butterworth is senior advisor to Land Network, the national farmer-owned group which recycles urban wastes to land. He is also the author of “How to Make On-Farm Composting Work”, and “Reversing Global Warming for Profit”, both published by MX Publishing.

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