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Friday, October 31, 2008


• Biogas can be defined as a gas produced by the biological breakdown of organic matter in anaerobic conditions

• Biogas originates from organic material and is a type of biofuel
• The other method of producing biogas is wood gas which is created by gasification of wood or other biomass

• Depending on where it is produced, biogas can also be called swamp, marsh, landfill or digester gas

• Biogas contains,
• Methane -50-75%
• Carbon dioxide -25-50%
• Nitrogen -0-10%
• Hydrogen -0-1%
• Hydrogen sulfide -0-3%
• Oxygen -0-2%

• Biogas plants
• Materials used in biogas production
• The materials commonly used in biogas production are,

» Human & animal waste
» Crop residue
» Agro industrial waste
» Biomass combined with water (sewage)
» Energy crops- Maize, sunflower, wheat, grass

• Methods of biogas production
• Mainly biogas is produced using anaerobic digestion
• There are various methods & reactor types used to produce biogas
• But the production process has common steps though the methods are different
• stages of biogas production
• This natural, biological process takes place in three stages:
 Hydrolysis :insoluble solids are broken down to monomers
 Acidogenesis:monomers are converted to VFA
 Methanogenesis:acids are converted to bio gas
• Different types of reactors
Type 1

• Influent is added to the reactor, and mixing is accomplished with gas mixers, mechanical
mixers, or recirculation pumps

• High installation and operating costs, high hydraulic retention times are required to achieve an acceptable level of degradation, signifying large reactor sizes

• Type 2

• The biomass forms sludge granules, producing a sludge bed which is completely retained in the reactor

• Allows for higher-strength wastewaters to be treated

• requires a significant amount of time, causing longer start-up times when compared to other reactors

• A diagram of type 2 reactor
• Contact Digester

• Retains biomass by separating and concentrating the solids in a separate reactor, returning these solids to the influent

• Solids separation can be achieved with gravity separators, solids thickeners, centrifuges, gravity belts, and membranes
• Sewage treatment
• Sludges from the primary and secondary treatment settling tanks are collected into an anaerobic digester

• Sludges contain cellulose, proteins, lipid and other insoluble polymers

• Anaerobic bacteria digest the sludge to methane and carbon dioxide

• Sewage treatment plant
• Using a biodigester
• Biodigester is a system that promotes decomposition of organic matter.
• It produces biogas, generated through the process of anaerobic digestion.

• Factors Affecting Biogas Production
 Biogas yield is measured as
• m3 gas/kg volatile solids
• The maximum possible gas yield on complete digestion of carbohydrates (starch, cellulose, glucose) would be 0.8 m3kg.

• for fatty acids this value it is about 1.5 m3kg.

• proteins it is about 0.9 m3/kg.
This depends on,

• Type of waste

• Temperature during digester operation,

• Retention time (the period of time a given sample of waste/substrate stays in the digester/ fermenter before it flows out)

• Presence of toxic materials

• pH
• Stirring
• Carbon – Nitrogen Ratio

• Type of waste

-Livestock & poultry wastes
-Crop residues
-Paper wastes
-Aquatic weeds
-Algae & seaweeds

• The yellow circles represents evolving biogas bubbles and the brown circles represent sludge granules. The upward liquid flow and uplifting action of gas bubbles causes the partial fluidization of the sludge bed and hydraulic mixing. The background is a scanning electron micrograph inside a sludge granule cultivated on sucrose-containing wastewater, show a diverse population of microorganisms.
• Temperature
• The choice of the temperature to be used is influenced by climatic considerations.

• Optimum process stability temperature should be carefully regulated within the a narrow range of operating temperature.

• Mesophilic fermentation at about 35°c gives the maximum gas yields,

• Thermophilic bacteria give best yields around 55°c.

• For sewage sludge, the gas yield at 20°c may be only 80% of that at 35°c.

• Retention time
• At high temperature bio – digestion occurs faster, reducing the time requirement.

• A normal period for the digestion of dung be two to four weeks.

• Gas yields increase with retention time since a greater proportion of the organic matter will be digested.
• Amount of OM vs retention time

• Toxic Materials
• Wastes & biodegradable residue are often accompanied by a variety of pollutants that could inhibit anaerobic digestion.

• Toxic components may include ammonia, SO42-, antibiotics, etc.

• Common toxic substances are

soluble salts of,
• Salts of
* Sodium
* Potassium
* Calcium
* Magnesium may be stimulatory or toxic in action.
• Pesticides & synthetic detergents may also be troublesome to the process.
• pH
• Low pH inhibit the growth of the methanogenic bacteria.
• A successful pH range for anaerobic digestion is 6.0 – 8.0.
• Efficient digestion occurs at a pH near neutrality.
• Low pH may be remedied by dilution or by the addition of lime.
• Carbon – Nitrogen Ratio
• The bacteria responsible for the anaerobic process require both elements
• They consume carbon roughly 30 times faster than nitrogen
• A carbon – nitrogen ratio of about 30:1 is ideal for the raw material fed into a biogas plant
• Stirring
• Stirring the slurry in a digester is always advantageous, if not essential

• If not stirred, the slurry will tend to settle out and form a hard scum on the surface, which will prevent release of the biogas

• Problem is much greater with vegetable waste than with manure, which will tend to remain in suspension

• Advantages of biogas
• provide better & cheaper fuel
• produce quality manure to improve soil fertility
• provide an effective way for sanitary disposal of human excreta
• use as a smokeless domestic fuel
• helps in generation of production employment
• Disadvantages of biogas
• Yields are lower due to the dilute nature of substrates
• The process is not very attractive economically on a large industrial scale
• New technologies cannot be used to enhance the efficiency of the process

• The process can be improved only by optimizing the environmental conditions of the anaerobic digestion
• Biogas contains some gases as impurities
• Uses of biogas
• electricity production
• Cooking
• water heating
• can replace compressed natural gas for use in vehicles

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