Hi and welcome back! This week we'll talk
about
anaerobic digestion. Sometimes this is also called
biomethanation or sometimes just called
biogas treatment. I will probably often use
the abbreviation AD for anaerobic
digestion. For this module, I'm using this
book published by SANDEC as a main
resource. It's called "Anaerobic Digestion
of Biowaste in Developing Countries". You
can download this book on our website
and it's free of charge. You will
probably remember that in the past few
modules, we have covered composting as one
option for biowaste treatment. As you
now know, composting is a process of
controlled aerobic degradation, that means
with air or with oxygen. You see this
clearly here. This composting heap which
has a passive areating tunnel in the
center. In this module we'll talk about
anaerobic digestion, which is the
collection of processes were
micro-organisms break down biodegradable
matter in the absence of oxygen.
This is very common to many natural
environments, such as swamps or stomachs
of ruminants, cows. So, why is this
interesting for waste management? Because
in anaerobic digestion micro-organisms
produce biogas. That's a mixture of
methane and carbon dioxide. Methane is
really what is interesting, because it's
flammable and can be used as an energy
source. In addition, what we also get is a
nutritious digestate. Anaerobic digestion
is becoming very, very popular
for waste management. In Europe, many units
are being built and operated and even in
developing countries it's becoming very
interesting, especially from the point of
view of renewable energy resource. What
we are especially interested in, is using
this process in an engineered way with
control design, for instance in an air
proof reactor, in a digester to produce
biogas and digestate, as you can see on
this picture.
So, what are the key benefits of
anaerobic digestion? Of course, we're
producing renewable energy and that
means, we're reducing our dependency on
fossil fuels. We're also reducing
greenhouse gases. AD is pretty good
because it doesn't need much space. You
can actually build the reactors
underground. Of course, it also reduces
solid waste volumes and thus avoids
disposal costs. And it reduces pollution
from waste and pollution from fossil
fuels. Finally, of course, we can recover
value from waste, in the gas and the
nutrients. There are also a few drawbacks of AD,
especially when comparing to composting.
The process of the AD is more
sensitive, it's slower and it's less
energy intense. In fact, the energy is
contained in the methane, but that
means that there is also no heat
generation and that has implications for
hygenization. AD is also
technically more complex and therefore
also needs higher levels of skills and
investment. Now let's look at the
biochemical processes. Anaerobic
digestion happens in four steps. Although
these processes happened partly
simultaneously. The first step is
hydrolysis. It is the slowest of the four
degradation steps. Bacteria transforme
complex organic materials into liquefied
monomers and polymers. The
second step is acidogenesis. That's
where sugars and aminoacids are
converted. The third step is acetogenesis.
That is where the substances are
then transformed into hydrogen, carbon
dioxide and acetic acid. Finally, the last
fourth step is methanogenesis, where
the methanogenic bacteria convert
hydrogen and acid acetic into methane
gas and carbon dioxide. Typically the gas
mixture will also contain hydrogen
sulfide, that's the stuff that smells of
rotten eggs, but also nitrogen, oxygen and
hydrogen. In volume percent, methane
amounts to roughly about 60%
while CO2 is around 40%. Hydrogen
sulfide is usually lower than 2%.
Now, let us look at the parameters and
operational conditions and how these
influence the anaerobic digestion
process. Let's start with the feedstock,
also called the raw material or input
material. We can distinguish between the
solid content and the water content. The
dry matter is also what we call total
solids. Now, some of these total solids
will be biodegradable and some won't.
In this context, it is of course the
biodegradable organic fraction that is
relevant, and this we call volatile
solids. Levels of total solids and
volatile solids in waste differs
depending on the type of waste. In this
table you can see some examples
regarding total solids and volatile
solids of different waste feedstocks.
Depending on the type of waste you can
also expect different amounts of methane
yield, as shown in this table.
Interestingly, lignin, one of the main
wood constituents doesn't degrade under
anaerobic condition. So, this anaerobic
digestion is not really suited for
treating yard waste or woody waste.
You can see this here in the last row,
where methane yield of lignin rich
organic waste is quite low. An important
parameter in AD is the organic
loading rate OLR. This parameter
quantifies the substrate quantity per
reactor volume and
time. The unit is kilograms volatile
solids per cubic meter and day. A good
daily loading rate for unstirred
reactors is 2 or below. With a stirred
reactor this can be higher,
you can increase this up to a loading
rate of 8. The pH range for anaerobic
digestion is between 6.5 and 7.5 so
actually quite neutral. However, in the
acidic phases, the pH is rather lower,
whereas in the methogenic phase it is
somewhat higher. Now, when the loading
rate is too high,
acidogenic bacteria will cause
acidification of the reactor.
Methanogenic bacteria are rather more
sensitive to these conditions and will
thus be inhibited. To react to this
the loading rate should be reduced or
one can also add lime or sodium
hydroxide to increase the pH level.
Another factor influencing the AD
process is temperature. Temperatures
below 15 degrees Celsius are not ideal.
As then the organisms really slow down
their activity. Underground construction
or installation can buffer this
variation of temperature, but the
anaerobic process is most comfortable
in two temperature zones: the mesophilic
temperature zone between 30 and 40, and
the thermophilic temperature zone
between 45 and 60. Operation in the
mesophilic range is more stable and can
tolerate greater changes in parameters
and consumes less energy. However
mesophilic organisms are slower in
degrading and so you need to give them
more time.
Thermophilic organisms however are
faster but the system is more sensitive
to changes. Further parameters that
influences the process is hydraulic
retention time. That is
amount of time that the material stays in
the reactor. Ideal is a time between 10
and 40 days. The lower value is rather
for higher temperature in the
thermophilic range, because the process
is quicker. Now, here we are confronted
with an optimization process. If for given
inputs the reactor volume is small, then
the retention time is low, which means
we'll get little biogas yield as there
is little time for the process. If the
reactor volume is large, then
retention time increases and we get more
yield, but at the cost of having a large
reactor meaning more space needed and
higher investment costs. Another
parameter is the C/N ratio. We've
heard about this in the composting
module. Ideal is a value between 16 and
25. A higher value means limited supply of
nitrogen, that means food for the
bacteria and therefore less gas
production. A lower C/N ratio can cause
amonia accumulation which then
inhibits the anaerobic process. The last
parameter we'll look is the particle
size of the input material. Here,
small is beautiful. Particle sizes below
five centimeters are ideal. What that
does, it increases the surface area of
the material and allows the
microorganisms to faster degrade the
material. For operation that means, that
usually we chop up through a shredder
our input material, to make small
particles. So, let me summarize what we
covered in this module. We looked at
benefits and drawbacks, we looked at the
four stages of the biochemical process.
And then we looked at different
operational parameters: input material,
organic loading rates, pH, temperature,
hydraulic retention time, C/N ratio
and finally particle size.
In the next module, we will look at
different types of reactors and how to
operate them. So stay with me for the
next module!

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