is a renewable energy source with many different
production pathways and various excellent opportunities
typically refers to a gas produced by the anaerobic
digestion or fermentation of organic matter
including manure, sewage sludge, municipal solid
waste, biodegradable waste, energy crops or
any other biodegradable feedstock. Biogas is
comprised primarily of methane and carbon dioxide.
advantage of biogas is the waste reduction potential.
Biogas production by anaerobic digestion is
popular for treating biodegradable waste because
valuable fuel can be produced while destroying
disease-causing pathogens and reducing the volume
of disposed waste products.
burns more cleanly than coal, and emits less
carbon dioxide per unit of energy. The carbon
in biogas was recently extracted from the atmosphere
by photosynthetic plants. Releasing it back
into the atmosphere adds less total atmospheric
carbon than burning fossil fuels.
biogas production kills two birds with one stone:
it reduces waste and produces energy. In addition,
the residues from the digestation process can
be used as high quality fertilizer. This closes
the nutrient cycle.
biogas is a perfect energy source including
- The Feedstock
production for biogas is very diverse, ranging
from livestock waste, manure, to harvest surplus.
Also, wastewater sludge, municipal solid wastes
and organic wastes from households can be used
as feedstock. Recently, dedicated energy crops
are more and more used as feedstock source for
biogas production. Finally, biogas can be collected
from landfill sites.
One main advantage of methane production is
the ability to use so-called “wet
biomass” as feedstock source.
Wet biomass can not be used for the production
of other biofuels such as biodiesel or biomethane.
Examples for wet biomass are sewage sludge,
manure from dairy and swine farms as well as
residues from food processing. They all are
characterized by moisture contents of more than
The use of waste materials
is not only excellent suitability for biogas
production it also creates some additional benefits.
Thus, it contributes to reduce animal wastes
and odors. Digestion effectively eliminates
environmental hazards, such as overproduction
of liquid manure. Therefore biogas production
is an excellent way for livestock farmers to
comply with increasing national and European
regulations of animal wastes. In addition it
destroys disease-causing pathogens existing
in waste materials. Nevertheless, using animal
feedstock can be critical as well. For instance
anaerobic degradation of poultry excrements
with high contents of organic nitrogen produce
high concentrations of ammonium. Furthermore,
new economical and ecological solutions for
the treatment of animal by-products are required
due to the BSE-crisis. However, it is often
the combination of environmental, economical
and legal reasons that motivates farmers to
use digester technology for waste treatment.
Apart from waste materials, suitable feedstock
also includes dedicated energy crops.
The suitability of energy crops for biogas production
was received through improvements in the fermentation
process. The main disadvantage of energy crops
when compared to waste materials is their need
for additional agricultural land. Nevertheless,
energy crops for biogas production have several
advantages which make them very promising for
the future. One main advantage is the production
of considerably high yields of energy crops
even when they are cultivated extensively. Chemical
fertilizers and pesticides are not required
or only in small amounts. Damaged and uneatable
harvests resulting from unfavorable growing
and weather conditions, as well as from pest
contaminations are suitable for biogas production,
too. In addition, cultivations do not have to
become fully ripe, since the whole plant can
be used for biogas production. Harvests do not
have to be dried.
- The Fermentation Process
is produced by means of anaerobic digestion.
Organic matter is broken down by microbiological
activity and in the absence of air. Symbiotic
groups of bacteria have different functions
at different stages of the digestion process
in order to break down complex organic materials.
are four basic types of microorganisms involved.
Hydrolytic bacteria break down complex organic
wastes into sugars and amino acids. Fermentative
bacteria then convert those products into organic
acids. Acidogenic microorganisms convert the
acids into hydrogen, carbon dioxide and acetate.
Finally, the methanogenic bacteria produce biogas
from acetic acid, hydrogen and carbon dioxide.
Bacteria are sensitive to temperature, which
plays an important role in the digestion process.
In order to promote bacterial activity, temperatures
of at least 20° C are required. Generally,
higher temperatures shorten processing time
and reduce the required volume of the digester
tank by 25 % to 40 %. Regarding the temperature,
bacteria of anaerobic digestion can be divided
into psychrophile (25 °C), mesophile (32
– 38 °C) and termophile (42 –
55 °C) bacteria. The choice of the process
temperature depends on the feedstock and of
the utilized digester type. Thus, digesters
have to be heated in colder climates in order
to encourage the bacteria to carry out their
Digestion time ranges from a couple of weeks
to a couple of months depending on feedstock
and digester type as well as on the digestion
- The Procuction Technology
technology for biogas production is the digestion
of feedstock in specially designed digesters.
These must be strong enough to withstand the
buildup of pressure and must provide anaerobic
conditions for the bacteria inside.
Today, there are many different technologies
and digester types available. Generally, the
size of biogas plants can vary from a small
household system to large commercial plants
of several thousand cubic meters. Digester size
also influences logistics and vice versa. Therefore,
they are often built near the source of the
content of substrate influences the design and
type of digester. One of the most common classifications
regarding the water content of the substrate
is the classification into wet digestion which
is fed with dry mass contents lower than 15
% and into dry digestion which is fed with dry
mass content between 20 and 40 %. Wet digestion
usually is applied to manure and sewage sludge,
whereas dry digestion is often applied to the
fermentation of energy crops.
digesters can be classified by the number of
process steps. Single-stage and two-stage digesters
are the most common technologies today. Single-stage
digesters are characterized by no special separation
of different process steps (hydrolysis, acidification,
methanisation). All process steps are conducted
in one single digester.
Digesters can be also classified regarding filling
procedure and filling interval. Digester types
include following technologies:
Batch type: the digester is filled at once,
the feedstock digests and subsequently the whole
system is emptied
• Continuously expanding type: firstly,
the digester is filled up to 1/3, then it is
continuously filled until it is full and finally
the digester is emptied
• Continuously flow type: the digester
is initially filled completely, then the feedstock
is continuously added and digested material
is continuously removed
• Pug flow type: the feedstock is added
regularly at one end and overflows the other
• Contact type: this is a continuous type,
but a support medium is provided for the bacteria
- The Characteristics
of digestion is Biogas, a combination of methane
and carbon dioxide, typically in the ratio of
6:4 (55-80 % methane). In addition, there are
small quantities of hydrogen sulfide and other
composition of biogas
- The Use
containing methane is a valuable product of
anaerobic digestion which can be utilised in
the production of renewable energy. Biogas can
be used for generating electricity and heat.
It can also be burned directly for cooking,
heating, lighting and process heat. Furthermore,
even pilot fuel cells installations are operated
with by biogas.
biogas can be cleaned and upgraded to natural
gas quality. The final product is biomethane,
which has methane content between 95 and 100%.
This biomethane is suitable for all natural
gas applications: for instance, it can be fed
into the natural gas grid or used for transport