Anaerobic Digester

Microbiological Aspects

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 1

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 2

3. Stages in Anaerobic Digestion 3

4. Hydrolysis Large Organic Compounds are converted into simple monomeric compounds It is accomplished through extracellular enzymes Example: C ellulose gets converted to starch using cellulases Casein gets converted to amino acids using Proteases Triglyceride gets converted to fatty acids using lipases 4

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 5

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. 6

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 7

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 . 8

9. Factors Affecting sludge digestion Process pH value When the digester pH is 7.2 or lower, the presence of NH 4 + is favoured. When the digester pH is greater than 7.2, the presence of NH 3 is favoured. Dissolved ammonia gas or NH 3 is toxic to bacteria, especially methane forming bacteria. 9

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 10

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 11

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 12

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 13

14. Disadvantages Needs constant supervision Formation of explosive gases Not easily controllable Maintenance and cleaning is difficult It is a slow process 14