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Anaerobic digestion is the biological degradation of organic material without
oxygen present. This results in the production of biogas, a valuable (energy
containing) product.
Biogas is a mixture of several gases and vapours, mainly
methane and carbon dioxide. Methane also is the main component in natural
gas and contains the bulk energy value of the biogas. Biogas occurs naturally,
hence its name, amongst others in swamps and lakes when conditions are
right. Anaerobic digestion can be used to produce valuable energy from
waste streams of natural materials or to lower the pollution potential
of a waste stream.
The definition above contains four parts: biological,
degradation, organic material and without oxygen.
- Biological:
This implies that the process is carried out by bacteria, which have
to be kept in a healthy condition and in good living conditions.
The bacteria have to be grown and nurtured in the process to get
a good production of biogas.
- Degradation:
This means that the substrate/organic material is broken down into
its building blocks and subsequently for a large part into biogas.
- Organic material:
The (dry) material in greenhouse residues consists of organic, or
natural, material and anorganic materials like sand. Part of the
organic material is broken down into biogas.
- Without oxygen:
This means that air is not allowed to interact with the greenhouse
residues. In degrading greenhouse residues there are a few competing
biological processes: with and without the presence of oxygen.
To promote the production of biogas as a valuable product of the
degradation, oxygen must be kept away from the reactor contents.
Biological steps
The biological anaerobic degradation of greenhouse residues can be divided
into four steps:
- hydrolysis: high weight organic molecules (like proteins, carbohydrates,
fat, cellulosis) are broken down into smaller molecules like sugars,
aminoacids, fatty acids and water.
- acidogenesis: further breakdown of these smaller molecules into
organic acids, carbondioxide, hydrogen sulfide and ammonia occurs.
- acetagenesis: the products from the acidogenesis are used for
the production of acetates, carbondioxide and hydrogen.
- methanogenesis: methane (finally), carbondioxide and water are
produced from the acetates, carbondioxide and hydrogen (products
of acidogenesis and acetagenesis).
There are several groups of bacteria that perform
each step; in total dozens of different species are needed to degrade
a heterogeneous stream completely.
Process parameters
The anaerobic digestion process can be carried out quite different conditions.
All of these conditions have specific influences on the biogas production.
Additionally, from a technological viewpoint, the biological process can
also be carried out in more than one reactor, which has some, mainly economical,
implications.
"Dry" digestion vs "wet" digestion
In digestion processes water is an important parameter. Water is needed for
life in general and for digestion bacteria too. It is the transport medium
for nutrients, for (half-)products and it is a very good reaction medium
for digestion.
Digestion is practised in two different ranges of
water content: dry digestion, with a typical dry solids content of
25-30% and wet digestion, with a dry solids content of less than 15%.
These ranges have technological and economic reasons: higher solid
contents lead to smaller (and thus cheaper?) reactors, lower solids
contents (more water) lead to much better mixing possibilities but
to a higher energy input (more water to be heated) and a bigger reactor.
Natural wastes from plants (like greenhouse residues)
have an estimated dry solids content of 25%. This dry solids content
opens the possibility to perform the digestion without addition of
water.
Thermophilic vs mesophilic digestion
(Digestion) bacteria have a temperature range in
which they are most productive in terms of production rates, growth
rates and substrate degradation performance. The several groups of
bacteria involved in anaerobic digestion all have (slightly) different
temperature optimums. This results in two main temperature ranges in
which digestion usually can be performed optimally and most economically.
These ranges are: 25-38°C called the mesophilic range, and 50-70°C
called the thermophilic range.
These ranges have different characteristics, advantages
and disadvantages of which the most important ones are: compared to
the mesophilic process, the thermophilic process usually results in
a higher degradation of the substrate at a faster rate at the expense
of a less stable process. It is less attractive from an energetic point
of view since more heat is needed for the process.
Batch processes vs continuous processes
In process technology the two main types of process (models) are used, the
batch process and the continuous process. In the batch process the substrate
is put in the reactor at the beginning of the degradation period after which
the reactor is closed for the entire period without adding additional substrate.
In the continuous process, the reactor is filled continuously with fresh
material and also emptied continuously.
As explained before, digestion consists of several
consequetive steps. In a batch reactor all these reaction steps occur
more or less after each other. The production of biogas (endproduct)
is non-continuous: at the beginning only fresh material is available
and the biogas production will be low. Half-way through the degradation
period the production rate will be highest and at the end, when only
the less easily digestible material is left, production rate will drop
again.
In a continuous process, fresh substrate is added
continuously, and therefore all reactions will occur at a fairly constant
rate resulting in a fairly constant biogas production rate. Several
mix forms of these two models are developed in process technology including
the so-called plug-flow reactor and the sequencing batch-reactor all
of which try to combine the advantages of the two extremes.
Residence time
The longer a substrate is kept under proper reaction conditions the more complete
its degradation will become. But the reaction rate will decrease with increasing
residence time. The disadvantage of a longer retention time is the increasing
reactor size needed for a given amount of substrate to be treated. A shorter
retention time will lead to a higher production rate per reactor volume unit,
but a lower overall degradation. These two effects have to be balanced in
the design of the full scale reactor.
Acidity or pH-value
The groups of bacteria needed for digestion not only have an optimum temperature
but also an optimum acidity at which they are most productive. Unfortunately,
for the different groups of bacteria the optimum pH-value (measure for acidity)
is not the same. The complexity of the entire system is increased by the
fact that the intermediate products of the digestion have a tendency to lower
the pH, making the later steps in the process more difficult. This makes
balancing the pH in the reactor an important design and operation issue.
Organic loading
Bacteria have a maximum production rate depending on the type of reactor, substrate,
temperature etc. Organic loading is one of parameters used to describe this
production rate. It is the amount of organic material put into the reaction
medium per time unit.
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Anaerobic Digestion
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Ir. P. Reumerman
Tel +31 53 486 11 98
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