The document discusses anaerobic digestion, which is the decomposition of organic matter by microorganisms in the absence of oxygen. It occurs in four stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The document outlines the stages and factors that affect the anaerobic digestion process, such as temperature, pH, nutrients, mixing, and seeding. Anaerobic digestion produces methane gas and reduces volatile solids in sludge while advantages include using the biogas as fuel and easier dewatering of the treated sludge. However, it also has disadvantages like needing constant supervision and being difficult to control.

1. Anaerobic Digester
Suspend Growth Anaerobic Digester
• A digester is a large
container in which
substances are treated with
heat or enzymes to promote
decomposition or to extract
essential components
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2. Anaerobic Digestion
• Anaerobic digestion is a naturally occurring process
of decomposition and decay
• Here, the organic matter is broken down to simpler
chemical compounds under anaerobic conditions
• Anaerobic digestions is
– Digestion of Organic Material
– In absence of Oxygen
– Forming end products of Methane and Carbon dioxide
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3. Stages in Anaerobic Digestion
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4. Hydrolysis
• Large Organic Compounds are converted into simple
monomeric compounds
• It is accomplished through extracellular enzymes
• Example:
– Cellulose gets converted to starch using cellulases
– Casein gets converted to amino acids using Proteases
– Triglyceride gets converted to fatty acids using lipases
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5. Acidogenesis
• The soluble monomeric compounds formed by
hydrolysis undergo fermentation
• The products obtained due to acidogenesis are
propionic acid and butyric acid and which are
intermediaries and others such as acetic acid, ethanol,
hydrogen and carbon dioxide.
• Example of bacteria that bring about fermentation are
Lactobacillus sp.
• In this stage due to acid production the pH falls down
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6. Acetogenesis
• Some of the acetate is produced through mixed acid
fermentation
• Rest of acetate is produced through secondary
fermentation of products obtained in previous stage
• Acetate formation is important for formation of
methane
• Example of acetogenic bacteria: Acetobacter sp.
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7. Methanogenesis
• This is the last metabolic stage in anaerobic digestion
• Methane is formed either from acetate or carbon
dioxide and hydrogen
• The bacteria responsible for carrying out
methanogenesis are called methogens and are
classified as
– Litotrophic or Hydrogenotrophic i.e. they produce methane
from CO2 and H2
– Acetotrophs or Acetoclastic, these convert acetate to
methane
– Methylotrophs, convert methanol to methane
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8. Factors Affecting sludge digestion
Process
• Temperature
– Most methane-forming bacteria are active in two
temperature ranges.
– These ranges are the mesophilic range from 30 to
35°C and
– The thermophilic range from50 to 60°C.
– At temperatures between 40°C and 50°C, ethane-
forming bacteria are inhibited.
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9. Factors Affecting sludge digestion
Process
• pH value
– When the digester pH is 7.2 or lower, the presence of NH4
+
is favoured.
– When the digester pH is greater than 7.2, the presence of
NH3 is favoured.
– Dissolved ammonia gas or NH3is toxic to bacteria,
especially methane forming bacteria.
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10. Factors Affecting sludge digestion
Process
• Nutrients
– Macronutrients
• The two main macronutrients are nitrogen and phosphorous
– Micronutrients
• Methane-forming bacteria possess several unique enzyme systems
• The need for several micronutrients, especially cobalt, iron, nickel,
and sulfide, is critical
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11. Factors Affecting sludge digestion
Process
• Mixing
– Mixing enhances the digestion process by distributing
bacteria, substrate, and nutrients throughout the digester as
well as equalizing temperature.
– Methane-forming bacteria are very sensitive to rapid
mixing. If rapid mixing continuously washes out methane-
forming bacteria in the effluent
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12. Factors Affecting sludge digestion
Process
• Seeding
– To seed an anaerobic digester with an adequate population
of facultative anaerobes and anaerobes including methane-
forming bacteria, a ratio of 1 :10 of secondary sludge to
primary sludge may be used
– Without seeding it may take a few months to get the tank
operational
– Proper seeding will help attain quick balance of reactions
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13. Advantages of Anaerobic Digestors
• It produces methane and carbon dioxide which can be
collected and used as fuel
• Reduces the volatile content of sludge
• Liquid is more easily dewatered as liquid and solids
form separate layers
• Reduction in number of pathogens
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14. Disadvantages
• Needs constant supervision
• Formation of explosive gases
• Not easily controllable
• Maintenance and cleaning is difficult
• It is a slow process
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