Download Methods

Biogas production
Biogas production
1. Introduction:
Animal and agricultural wastes constitute a high proportion of
biomass and their utilization and recycling is important for
economical and environmental aspects. Anaerobic digestion one of the
most widely used processes for treating these wastes and represents an
attractive method for treating organic wastes for biogas production as
alternative energy sources.
Olive cake and some other residues such as cotton plant stalks and
tree trimmings represent poor agricultural residues that cannot be
used alone for ruminant nutrition, caused the presence of high
amounts of lignocellolosic materials which have low digestibility.
These raw materials have been given no specific application to date;
rather, their elimination, mostly by in situ burning, is the cause of
various environmental problems (pollution and fires). Wood of olive
trees is used to provide paper pulp. However, organic olive cake in
addition to animal wastes can be used as a source of fermentable
organic matter in biomass technologies for gas production [1].
The purpose of this experiment is to carry out an anaerobic
treatment of different proportions of mixtures of animal wastes and
olive oil wastes or any other plant wastes to obtain a certain volume of
combustible gas that could partly solve the energy demands of the
farm and to obtain an effluent with a lower polluting power and a
higher fertilizers value than the fresh waste.
2.Waste treatment:
Anaerobic digestion is usually the basic biological treatment process
for high organic strength wastewaters, since it results in limited
production of stabilized sludge compared to the conventional aerobic
treatment.
Anaerobic biogas production is an effective process for conversion of a
broad variety of biomass to methane to substitute natural gas and
medium calorific value gases. The process can be carried out in
relatively inexpensive and simple reactor design and operating
procedures.
1
Biogas production
Anaerobic digestion converts the carbonaceous matter into biogas
leaving stabilized slurry in a form suitable for reapplication to land as
fertilizer.
Common materials used for methane generation are often defined as
waste materials. i.e. crop residues, animal and urban wastes. Manures
and process waste waters have been extensively investigated as sources
of biogas.
2.1 Biological processes of anaerobic digestion:
Anaerobic digestion of organic material under methanogenic
conditions, is a complex process that can be divided into four steps:
 The first step is: hydrolysis where biopolymers are hydrolyzed
to form monomers, such as simple sugars, amino acids, longchain fatty acids and aromatic compounds.
 The second step is: acidogenesis where acidogenic bacteria
ferment these monomers to intermediate organic compounds
such as volatile fatty acids and alcohols with the simultaneous
production of carbon dioxide and hydrogen.
 The third step is: acetogenesis where acetogenic bacteria
metabolize these intermediate organic products, forming the
methanogenic substrates, acetate and hydrogen.
 The fourth step is: methanogenesis where the substrates
converted by methanogenic bacteria into methane and carbon
dioxide (biogas) [11].
2
Biogas production
2.1.1 Methanogenesis:
Methanogenesis or biomethanation is the formation of methane by a
group of microbes known as methanogens. Organisms capable of
producing methane have been identified only from the kingdom
Archaea, a group phylogenetically distinct from both eukaryotes and
bacteria, although many methanogenic organisms live in close
association with anaerobic bacteria.
The production of methane is an important and widespread form of
microbial metabolism. In most environments, it is the final step in the
decomposition of biomass.
2.1.2 Occurrence of methanogens
Organisms capable of methogensis are called methanogens. Microbes
performing Methanogenesis have no nucleus or membrane-bound
organelles (i.e. they are prokaryotes). Methanogens are considered to
be a very old group of organisms, being members of the
archaebacteria, also known as Archaea (depending on what taxonomic
system is being used).
Methanogens cannot exist in the presence of oxygen, so they are only
found in environments in which the oxygen has been depleted. Most
commonly these are environments experiencing the decay of organic
matter, such as wetland soils, the digestive tracts of animals, and
aquatic sediments. Methanogenesis also occurs in areas where oxygen
3
Biogas production
and decaying organic matter are both absent, such as the terrestrial
deep subsurface, deep-sea hydrothermal vents, and oil reservoirs.
Methanogenesis is the final step in the decay of organic matter.
During the decay process, electron acceptors (such as oxygen, ferric
iron, sulfate, nitrate, and manganese) become depleted, while
hydrogen (H2) and carbon dioxide accumulate. Light organics
produced by fermentation also accumulate. During advanced stages of
organic decay, all electron acceptors become depleted except carbon
dioxide. Carbon dioxide is a product of most catabolic processes, so it
is not depleted like other potential electron acceptors.
Only methanogenesis and fermentation can occur in the absence of
electron acceptors other than carbon. Fermentation only allows the
breakdown of larger organic compounds, and produces small organic
compounds. Methanogenesis effectively removes the semi-final
products of decay: hydrogen, small organics, and carbon dioxide.
Without methanogenesis, a great deal of carbon (in the form of
fermentation products) would accumulate in anaerobic environments.
Methanogenesis is useful to humanity. Through methanogenesis,
organic waste can be converted to useful methane "biogas."
Methanogenesis occurs in the guts of humans and other animals.
While methanogenesis is not believed to be necessary for human
digestion, it is required for the nutrition of ruminant animals, such as
cattle and goats. In the rumen (known incorrectly as the "second
stomach" possessed by some animals), anaerobic organisms (including
methanogens) digest cellulose into forms usable by the animal.
Without the microbes of the rumen, cattle cannot survive without
being fed a special diet.
Methanogens can also utilize methane as a substrate in conjunction
with the reduction of sulfate and nitrate.
2.2 By-products of anaerobic digestion
There are three principal by-products of anaerobic digestion.
Biogas, a gaseous mixture comprising mostly of methane and carbon
dioxide, but also containing a small amount hydrogen and
occasionally trace levels of hydrogen sulfide. Biogas can be burned to
produce electricity, usually with a reciprocating engine or
microturbine. The gas is often used in a cogeneration arrangement, to
generate electricity and use waste heat to warm the digesters or to
heat buildings. Excess electricity can be sold to electricity suppliers.
Electricity produced by anaerobic digesters is considered to be green
energy and may attract subsidies such as Renewables Obligation
Certificates.
4
Biogas production
Since the gas is not released directly into the atmosphere and the
carbon dioxide comes from an organic source with a short carbon
cycle biogas does not contribute to increasing atmospheric carbon
dioxide concentrations; because of this, it is considered to be an
environmentally friendly energy source. The production of biogas is
not a steady stream; it is highest during the middle of the reaction. In
the early stages of the reaction, little gas is produced because the
number of bacteria is still small. Toward the end of the reaction, only
the hardest to digest materials remain, leading to a decrease in the
amount of biogas produced [12].
2.2.1 Biogas typical composition range
The composition of biogas varies depending upon the origin of the
anaerobic digestion process. Landfill gas typically has methane
concentrations around 50%. Advanced waste treatment technologies
can produce biogas with 55-75%CH4.
Biogas composition
Matter
Methane, CH4
Carbon dioxide, CO2
Nitrogen, N2
Hydrogen, H2
Hydrogen sulphide, H2S
Oxygen, O2
%
50-75
25-50
0-10*
0-1
0-3
0-2*
The second by-product (acidogenic digestate) is a stable organic
material comprised largely of lignin and chitin, but also of a variety of
mineral components in a matrix of dead bacterial cells; some plastic
may be present. This resembles domestic compost and can be used as
compost or to make low grade building products such as fibreboard.
The third by-product is a liquid (methanogenic digestate) that is rich in
nutrients and can be an excellent fertilizer dependent on the quality of
the material being digested. If the digested materials include low levels
of toxic heavy metals or synthetic organic materials such as pesticides
or PCBs, the effect of digestion is to significantly concentrate such
materials in the digester liquor. In such cases further treatment will be
required in order to dispose of this liquid properly. In extreme cases,
the disposal costs and the environmental risks posed by such materials
can offset any environmental gains provided by the use of biogas. This
is a significant risk when treating sewage from industrialized
catchments.
5
Biogas production
Nearly all digestion plants have ancillary processes to treat and
manage all of the by-products. The gas stream is dried and sometimes
sweetened before storage and use. The sludge liquor mixture has to be
separated by one of a variety of ways, the most common of which is
filtration. Excess water is also sometimes treated in sequencing batch
reactors (SBR) for discharge into sewers or for irrigation.
3.Experimental set-up:
biogas
Graduated gas
stand
Cylinder
Water bath
bioreactors
3.1 Materials:
The equipment for anaerobic digestion is shown in the figure:
digesters of 1 liter capacity were used and kept in water bath at 30°C.
The volumes of the collected gas were measured through graduated
cylinders by the Marriott method.
Samples of the biogas produced were withdrawn by a gas injector
through syringe placed at the exit of the digesters and were tested by
their odor and using flam test.
3.2 Methods:
Animal wastes were collected from farms, where hay, straw and
concentrate constituted the feed ingredients. olive cake, which is a by
product of the olive industry, was collected from a local factory
employing a hydrolic press and utilizing water and high centrifugation
in extraction process, each waste was dried at room temperature for
few days separately and olive ground at 0.5 mm but animal waste at
2.5mm diameter.
6