Thursday, 9 August 2007


Photos : Plenty of digestion going on at the Clonmany agricultural show.

What is Anaerobic Digestion and can it be used on the farm?
Anaerobic Digestion plants are springing up around the country. They are a way of converting animal waste into biogas energy (typically composed of 65% methane and 35% carbon dioxide). Some plants supply the farm with enough to power the house and sheds as well as selling the excess to the national grid.

The process of Anaerobic Digestion (AD) involves the breakdown of organic waste by bacteria in an oxygen-free environment. Anaerobic Digestion can take place in a specially designed AD plant (or naturally at landfill sites).

AD Plants
Farm, municipal or industrial-based AD plants convert waste material into biogas. Waste/feedstock is pumped into a closed vessel (digester), which has been inoculated with suitable bacteria. Anaerobic (0% oxygen) conditions are then maintained in the vessel and the temperature is held at a constant value (typically 370C).

The biogas produced can be upgraded to fossil ('natural') gas quality, but is normally used on site to generate heat and power.

The AD process residue can be separated into a liquid and fibrous part. The liquid can be returned to the land as a fertiliser and the solid fibre used as a soil conditioner.

What Types of manure are suitable?

In general, all manures can be digested.

Digestion of dairy and cattle manure has been successfully implemented
Digestion of only poultry or swine manure may present more challenges — other materials may be added to optimise the blend.

Sand or other inorganic materials will settle out in the digester and must be considered in the design. Many digesters will require shutdown and removal of built up materials after 10 years of usage.

AD systems work best with fresh manure — manure stored under barn may not be as suitable.

AD systems are not effective with highly diluted manure. Processes such as bypassing milk house wash water should be considered.

Some experts suggest that it is not reasonable to convert solid manure into a liquid in order anaerobically digest it.


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