Agriculture and Environmental Biotechnology
Nisreen Y. Badr
Objectives

Be able to carry out an anaerobic treatment of different proportions of mixtures of
animal 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.
I. Introduction
Biogas is the mixture (CH4 and CO2) produced when organic matter is decomposed by
anaerobic bacteria.
II. The process of Biogas Production:
The products of biological degradation of organic matter depends on the conditions
under which the degradation occur. If the degradation is aerobic, ammonia, carbon
dioxide and a lot of heat are produced. The leftover solid is known as compost and the
termed composting. On the contrary, if the decomposition is anaerobic biogas is
produced. The leftover sludge is used as fertilizer to fossil fuels. The process is known as
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, long-chain 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).
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Agriculture and Environmental Biotechnology
Nisreen Y. Badr
Simplified model:
II.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 of 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.
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)
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Agriculture and Environmental Biotechnology
Nisreen Y. Badr
become depleted, while hydrogen (H2) and carbon dioxide (CO2) 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 semifinal 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.
II.1.1 Example for production of Biogas under Anaerobic conditions:
Stage No.1
In the second stage of anaerobic digestion, acid- forming bacteria convert the soluble
organic matter or glucose into volatile acids- the organic acids that can cause odor
production from stored liquid manure. This can be shown simply as
Finally, methane- forming bacteria convert those volatile acids into biogas. The
formation of biogas can be represented as:
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Agriculture and Environmental Biotechnology
Nisreen Y. Badr
II.2 Factors affecting for the production of Biogas:
In biogas production, there are many influencing factors which are important in
controlling the speed and quality of the digestion. They are:
II.2.1 pH or Hydrogen Ion concentation or Acidity
The pH in a biogas digester is very important. Methane production proceeds quite
well as long as the pH is maintained between 6.6 and 7.6, with optimum range between
7.0 and 7.2.
II.2.2 Temperature
Operating temperature is another factor influencing digester efficiency. A digester
operate in three temperature ranges:
1. the low temperature, pscychrophilic bacteria range, (<35ºC)
2. the medium temperature, mesophilic bacteria, (29ºC- 40ºC)
3. the high temperature, thermophilic bacteria range, (50ºC- 55ºC)
The growth of the methanogenic bacteria is affected by the temperature inside the
digester which in turn is affected by atmospheric temperature at any given time. This the
optimum temperature for biogas production is 30ºC. Biogas production could be
increased up to 45ºC- 55ºC, but the biogas production could be hampered due to
destruction of enzymes at temperatures higher than 55ºC. Biogas could be produced at
temperatures of 12ºC- 18ºC but it could not be economically beneficial. Biogas
production stops at temperatures below 10ºC.
One way to overcome the problem of lower temperature is to dilute the daily
incoming waste material with preheated (solar heated) water. Or you can construct a
greenhouse or compost pile around the digester.
II.2.3 The ratio of carbon to nitrogen
The first requirement of the raw materials of biogas production is that they must
contain organic carbon and nitrogen in quantities that have a certain relationship to each
other.
II.2.4 Dilution
The anaerobic fermentation of organic matter proceeds best if the feeding material
contains 7 to 9% solid matter.
II.2.5 The starter
Many kinds of organic matter could be used to produce biogas. Some of the organic
matters that can be used are animal wastes like (dung), laves, stems, grass and husk.
The age and the amount of the starter play a very important role. The higher
percentage of starter, the better gas production.
II.3 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.
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Agriculture and Environmental Biotechnology
Nisreen Y. Badr
Biogas can be burned to produce electricity, usually with a reciprocating engine or
microturbine.
II.3.1 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 fiberboard.
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.
III. Basic design of the Digester:
Design differences mainly depend on the type of organic waste to be used as raw
material, the temperatures to be used in digestion and the materials available for
construction.
III.1 Continuous feeding (Mostly liquids)
In this type: gas production can be accelerated and made more consistent by
continuously feeding the digester with small amounts of waste daily. This will also
preserve the nitrogen level in the slurry for use as fertilizer.
For example, the complete anaerobic digestion of cow manure takes about 8 weeks at
normally warm temperatures. One third of the total biogas will be produced in the first
week, another quarter in the second week and the remainder of the biogas production will
be spread over the remaining 6 weeks.
III. 2 Batch feeding (Mostly solids)
In this type: biogas systems designed to digest solid vegetable waste alone. Since
plant solids will not flow through pipes, this type of digester is best used as a single batch
digester. The tank is opened, old slurry is removed for use as fertilizer and the new
charge is added. The tank is then resealed and ready for operation.
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Agriculture and Environmental Biotechnology
Nisreen Y. Badr
Dependent on the waste material and operating temperature, a batch digester will start
producing biogas after two to four weeks, slowly increase in production then drop off
after three or four months.
Most vegetable matter has a much higher carbon - nitrogen ratio than dung has, so
some nitrogen producers (preferably organic) must generally be added to the vegetable
matter, especially when batch digestion is used. Weight for weight, however, vegetable
matter produces about eight times as much biogas as manure, so the quantity required is
much smaller for the same biogas production. A mixture of dung and vegetable matter is
hence ideal in most ways, with a majority of vegetable matter to provide the biogas and
the valuable methane contained in it.
IV. Experimental set- up:
biogas
Biogas
graduated gas
stand
cylinder
bioreactors
water bath
IV.1 Materials and Methods:
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.
Not. 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 hydraulic 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.
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