Environmental biotechnology (Anaerobic Digestion)

1. ANEROBIC DIGESTION

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 (CO2)
and methane (CH4) by the anaerobic microorganisms.
C6H12O6 → 3CO2 + 3CH4

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

10. 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.

11. 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.