Anaerobic Digestion Science

The Process

At its simplest bacteria working in four distinct stages breaks down the feedstock's cells to produce methane dominated gas.

four stages

The Feedstock

The graph below shows the approximate differing methane yield from differing feedstock. Although possible its unusual to use one single ingredient; at its most basic slurry/manure provides the bacteria which breaks down the higher methane yielding feedstock.

Methane Yield

When choosing feedstock it is common sense to prioritise those that can be sourced for the minimum cost. This could/ should include manure, bedding, feed waste and slurry.

Virtually no farmer or business owner actually feels they have unutilised waste, in fact it is likely that they pride themselves on wasting very little and using their waste profitably.

So describing Anaerobic Digestion as using waste can be misleading; using waste more profitably is more accurate. It is an extra step in using the waste possibly requiring small changes to existing practices. However the revenue is substantial and Anaerobic Digestion income has different characteristics to the main source of income; its predictable and not reliant on global commodity prices/weather. Furthermore Anaerobic Digestion once installed opens the door to other potential income streams and capital receipts.

There are four main characteristics of any given feedstock that affect the energy yield:

  • TOTAL SOLIDS (DRY MATTER)
    The amount of available energy from a material rises with increased dry matter content.
  • VOLATILE SOLIDS (ORGANIC DRY MATTER)
    In addition to the basic dry matter content the ability of that material to be broken down effectively in the digester must be considered. A higher VS value typically results in a higher energy yield.
  • BIOGAS OUTPUT PER TONNE OF VOLATILE SOLIDS
    A higher value of biogas output per tonne of volatile solids gives a higher energy yield.
  • METHANE CONTENT IN BIOGAS
    Although all Biogas is always dominated by Methane this may vary between 55-65%.