Page last updated 30/08/2018
Models for Anaerobic Digestion
The process of anaerobic digestion involves the break-down of large food molecules into small molecules of gases. The process
is done by a range of micro-organisms that work symbiotically: the waste products of one set are the feed material for the
next. These microbes are mainly bacteria and archaea, but may also include fungi and yeasts. An effective digester needs to
contain a population of these microbes.
A good source of a suitable population is cattle dung, as the microbes help cattle break down food. Most of the biogas extension
programmes encourage users to feed their plants with animal dung, since the microbes are already present. The disadvantage
is that the animal has already used much of the energy in the food material for their own needs. Food materials offer more
energy, when placed in an anaerobic digester, although they need to be pre-processed, so that they are in a form that can be
exploited by the microbes. An animal does the pre-processing by chewing food materials with their teeth and then passing
them though chemical processes in their guts.
Various models can be used to estimate the amount of gas that can be generated from different feed materials. There are two
different modes for running a biogas plant: batch and continuous.
A batch plant is a container which is fed with a mixture of feed material and starter. The gas produced is collected. It takes
time for the process to start (the lag time); the gas production rises quickly to a peak; it then drops steadily over several
weeks. If a batch system is used for gas production, several plants are needed. These are started at different times, so that
sufficient gas is available all the time.
A continuous plant (usually semi-continuous) is a container partly filled with a slurry of feed material and microbes. The plant is
fed regularly (usually daily) with new slurry replacing the exhausted effluent, which is removed. Gas production is fairly steady,
as long as the same amount of similar feed is used each day.
Biogas models
There are a range of models to describe the amount of biogas generated from various feed materials. These range from very
simple models to much more complex ones.
Biogas Potential (or Biomethane potential)
The simplest model defines the total amount of biogas that can be generated from a feed material. This is usually measured in
small-scale laboratory batch plants. The carbon dioxide is usually removed from the biogas produced, so the amount of
methane is measured.
The theoretical amount of biogas that can be produced from a feed material can be calculated from the relative amounts of
carbon, hydrogen and oxygen in the material (Buswell’s formula). Starch, with a formula of (C
6
H
12
O
5
)100 when hydrolised with
water (H
2
O), gives equal amounts of carbon dioxide and methane. Not all of the biomass material can be processed, as some of
the constituent elements are bound up in the form of lignin, which cannot be digested by the microbes.
First order rate model
The weakness of the very simple model is that it does not explain how fast biogas is produced. A second constant can be
defined, the “rate” constant. The simple first order rate model allows a rate to be defined.
Complex models.
There is a growing number of other models developed by different researchers. These are much more complex, so require more
parameters to be defined and measured. A classic model was developed by Chen and Hashimoto (1978), which includes the
growth rate of the micro-organisms involved in the biogas process.