Anaerobic Digestion (AD)

Anaerobic Digestion is the breakdown of organic matter by living processes (bugs) in the absence of Oxygen.  It has been going on since the first blob of primaeval stuff looked at his neighbour and thought "I wonder what he tastes like?".

The first reliable report of deliberate Anaerobic Digestion by humans comes from 10th century BC Assyria where rotting manures were used to heat bathwater.  In the 17th century Jan Babtita Van Helmont discovered that rotting animal manures produced flammable gases, and in 1808 Sir Humphrey Davy determined that the gas methane (CH4) was the flammable constituent.

In 1859 a leper colony in Bombay, India built the first digestion plant, and in 1895, Exeter in Britain, became the first town whose gas lights were powered by sewage gas (now referred to as Biogas).

It was not until the 1930s that the link to anaerobic bacteria was established.

There are three identifiable stages to the Anaerobic Digestion process:

1. Hydrolysis

Carbohydrates, cellulose, proteins and fats are broken down by enzymes produced by hydrolytic bacteria.  During this phase Carbon Dioxide (CO2) is given off and about 20% Hydrogen (H2).  There is considerable work being carried out attempting to isolate this H2, but commercialisation is some way in the future.

2. Acidogenesis

Organic acids formed in the previous hydrolysis phase are converted by acetogenic bugs to acetic acid (vinegar) and other organic acids.  Production of CO2 and H2 drops to nearly zero during this phase.

3. Methanogenesis

Methanogenic bacteria proliferate and using the organic acids produced in the Acidogenesis phase they produce methane at around 60% and CO2 at around 40%.

Systems and Processes

Batch

A batch system is one in which a quantity of material is introduced to the reactor, processed and then removed.

Advantages of batch processing

  • Simplicity
  • Lower capital costs
  • Flexibility
  • No flow pattern problems
  • Accurate control over digestion times
  • Ease of processing high dry matter materials
  • Ease of isolating problems
  • Ease of maintenance
  • Less dependence on regularity of feedstock
  • Possible to start and stop the digester
  • Possible to gain access to the digester between batches

Constant flow

A constant flow system is one in which material is constantly added to the reactor at one end, is processed as it passes through, and then ejected at the other end.

Advantages of constant flow

  • Easily automated
  • Low operating costs
  • Low maintenance
  • High gas yields
  • Odour free sealed process
  • Small footprint
  • Regular spike free energy requirements

Wet and dry Anaerobic Digestion

Wet Anaerobic Digestion

When dry matter (DM) concentrations are below 15% the process is described as wet. A wet digester will typically process a slurry of around 10% DM which has the consistency of thin soup and must be constantly stirred to prevent suspended solids from precipitating. Stirring is usually either mechanical in which case a rotating paddle is the preferred method, or gas, in which case the biogas given off is redirected through the substrate to ensure stirring. Wet digesters are nearly always constant flow digesters.

Wet digesters are ideally suited to the processing of low DM feedstock such as farm slurries and source separated food wastes.

Dry Anaerobic Digestion

The material processed during dry AD is normally around 25 – 30% DM and has the consistency of thick porridge. In constant flow digesters this must always be mechanically stirred, not only to prevent settling and unequal pocketing, but also to assist the flow of the material through the reactor.

Dry digesters are ideally suited to high DM feedstock such as energy crops, garden wastes and mechanically recovered municipal wastes in MBT plants.

Mesophilic and Thermophilic Digestion

Mesophilic Digestion

Mesophilic digestion is the most commonly used process for anaerobic digestion, in particular waste sludge treatment. Digestion takes place over a period of 15 to 40 days at a temperature of 30 to 40oC

It is usually more robust than the thermophilic process, but the biogas production tends to be less, and additional sanitisation such as pasteurisation is usually required.

Thermophilic Digestion

Thermophilic digestion is less common and not as mature a technology as mesophilic digestion. The reactor is heated to 55oC and held for a period of 12 to 14 days.

Thermophilic digestion systems provide higher biogas production, faster throughput and an improved pathogen and virus ‘kill’, but the technology is more expensive, more energy is needed and it is necessary to have more sophisticated control & instrumentation.

Products of Anaerobic Digestion

Biogas

which is normally 40% CO2 and 60% methane. This is usually burnt to produce heat and electricity, but can also be stored in liquid form for use as a vehicle fuel.

Digestate

the solid material left over. A very high quality stable soil conditioner, this can be spread straight to land or stacked for later use. It is resistant to leaching and can be stacked on fields. The nitrogen within has a low availability, and repeated applications can build up a high level of slow release soil fertility.  Digestate can also be simply processed into biomass briquettes or pellets enabling it's use in biomass heating systems. 

Liquor

the liquid left over. This is a super concentrated liquid fertiliser that (subject to inputs) is appropriate for use on organic as well as conventionally cultivated land. The analysis will depend upon inputs, but can be tailored by selecting food waste types.