Anaerobic digestion

Environmental biotechnology (Anaerobic Digestion)

1. ANEROBIC DIGESTION by:pRiti verma

2. “ Anaerobic digestion (AD) is a microbiological process whereby organic matter is decomposed in the absence of oxygen. This process is common to many natural environments such as swamps or stomachs of ruminants. Using an engineered approach and controlled design, the AD process is applied to process organic biodegradable matter in airproof reactor tanks, commonly named digesters, to produce biogas. Various groups of microorganisms are involved in the anaerobic degradation process which generates two main products: energy- rich biogas and a nutritious digestate” (Vögeli et al, 2014). Anaerobic Digestion

3. Process of anaerobic digestion The four key stages of anaerobic digestion involve Hydrolysis,  Acidogenesis,  Acetogenesis  Methanogenesis The overall process can be described by the chemical reaction, where organic material such as glucose is biochemically digested into carbon dioxide (CO 2 ) and methane (CH 4 ) by the anaerobic microorganisms. C 6 H 12 O 6  → 3CO 2  + 3CH 4

4. Key Stages of anaerobic digestion Hydrolysis Bacteria transform complex organic materials into liquified polymers and monomers. Acidogenesis Acidogenic bacteria convert monomers of sugars and amino acids into acids + C2 H6 O + CH3 CO2 - + H2 + CO2 + NH3 (indirect) Acetogenesis BOD & COD reduced, pH decreased - long-chain and volatile fatty acids and alcohols transformed to H+ + CO2 + CH3 COO- Methanogenesis CH3 COO- + H+ -> CH4 + CO2 Anaerobic Digestion / biomethanation / biomethanisation

5. The digestion process begins with bacterial hydrolysis of the input materials. Insoluble organic polymers, such as carbohydrates, are broken down to soluble derivatives that become available for other bacteria.  Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. In Acetogenesis, bacteria convert these resulting organic acids into  acetic acid , along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens convert these products to methane and carbon dioxide. The methanogenic archaea populations play an indispensable role in anaerobic wastewater treatments.

6. Benefits of anaerobic Digestion Anaerobic digestion is widely used as a source of renewable energy. The process produces a biogas, consisting of methane, carbon dioxide, and traces of other 'contaminant' gases. This biogas can be used directly as fuel, in combined heat and power gas engines or upgraded to natural gas-quality biomethane. The nutrient-rich digestate also produced can be used as fertilizer.

7. Operational temperature levels for anaerobic digesters The two conventional operational temperature levels for anaerobic digesters determine the species of methanogens in the digesters: Mesophilic  digestion  takes place optimally around 30 to 38 °C, or at ambient temperatures between 20 and 45 °C, where mesophiles are the primary microorganisms present. Thermophilic  digestion  takes place optimally around 49 to 57 °C, or at elevated temperatures up to 70 °C, where thermophiles are the primary microorganisms present.

8. Categories of digesters Digesters can be divided into three categories: Passive Systems : Biogas recovery is added to an existing treatment component. Low Rate Systems : Manure flowing through the digester is the main source of methane-forming microorganisms. High Rate Systems : Methane-forming microorganisms are trapped in the digester to increase efficiency.

9. Anaerobic Digestion of High solid waste There are two primary high-solids technologies: Plug flow Dry fermentation. Plug flow allows for more contamination, with thick materials like rock passing through. continuous and most efficient method to produce biogas. requires less preprocessing and the material coming out has more solid matter so it’s good for composting. Dry fermentation is a batch process done in a large vault where bacteria-containing liquid is pumped in, saturating the waste. 75 percent solids can be going in.

10. low solid and high solid anaerobic digesters Both plug flow and dry fermentation produce biogas in about 21 days. both low-solids and high-solids ADs will have a significant role in managing the spectrum of waste streams into the future, converting volatile organics into biogas, projects Philip. Low-solids ADs will focus on processing feedstocks with low non-digestible content and slurries generated from contaminated organics that are easy to separate from their packaging and co-mingled non- digestibles . High-solids ADs, because of their ability to accept material with a lot of difficult to remove non- digestibles will  be more and more popular with cities and waste haulers.

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