This document discusses anaerobic digestion from a microbiological perspective. It begins by refreshing some key terms related to anaerobic digestion processes. It then compares aerobic decomposition and anaerobic digestion, noting the different bacteria involved and end products. The document outlines the main stages of the anaerobic digestion process and factors that can affect it, such as temperature, organic loading, pH, and inhibitory substances. It also briefly discusses related processes like silaging and composting. Finally, it provides an overview of how a lab-scale continuous stirred-tank reactor can be used to simulate large-scale anaerobic digestion conditions.

1. Microbiological perspective of
Anaerobic Digestion

2. Biological systems
An Engineer :
1 + 1 = 2 !
For Biologists :
1 + 1 = Most of the times 2, but can vary from 1 t o 3
No. of occurrence
Value of
1+1
1 32
Variation due to biological
complexity, environmental
factors and uncertainty of
measurement.

3. Outline
• Refreshing some known facts
• Aerobic decomposition v/s Anaerobic digestion
• Anaerobic digestion process
– Stages involved
– Factors affecting
• Silaging
• Composting
• Working of Lab Scale CSTR Model: Simulating Large scale
model

4. Refreshing some known
facts
• TS / DM : Total Solids / Dry matter
• VS / oTS / oDM: Volatile Solids / organic total solids / Organic
Dry matter
Moisture VS Ash
TS
• HRT : Hydraulic retention time
• OLR : Organic loading rate
Amount of VS added per unit of digester volume
• VFA :Volatile fatty acids
• TIC : Total inorganic carbon
• TOC NPK : Total organic carbon Nitrogen, Phosphorus and
Potassium
• Biogas yield : Biogas produced per kg of VS of the substrate

5. Aerobic Decomposition
v/s Anaerobic degradation
Aerobic Decomposition
• Bacteria involved are
generally called Aerobes
or Aerobic bacteria
• Requires O2 growth.
• Final oxidation product is
CO2
• Converts more carbon
from organic material to
Bacterial biomass
Anaerobic Degradation
• Bacteria involved are
called as Anaerobes or
anaerobic bacteria.
• Can not grow in presence
of O2.
• Final degradation product
is CH4.
• Converts more carbon to
CH4 than to Bacterial
biomass.

6. Anaerobic digestion process

7. Anaerobic digestion process
Factors affecting :
• Temperature
• Organic load
• pH
• TIC /VFA ration
• Ammonia content
• H2S content
Environmental factor
Bacterial
activity
Biological bell curve

8. Silaging
• A storage procedure used in western world for Feed storage
• Also anaerobic degradation but without methanogens.
• Sugars get degraded anaerobically, produces acids like lactic
acid and acetic acid.
• pH of the feedstock drops up to 3-4.
• Bacterial activity reduced so no further loss of biomass.
• Acids produced are still in system and can be used in digestion
for methane production.
• Additives like microbial cultures and acetic acid can be used
for enhancement of silaging.

9. Composting
• Sludge from anaerobic degradation is always considered as
very good fertilizer.
• EX :
Feedstock Methane + CO2 + Sludge
(10)C(1)N (4)CH4 + (3)CO2 + (3)C(1)N
• C:N of 10:1 is converted to C:N ratio of 3:1.
• We feel necessity to dry it further for achieving higher TS also
allow for aerobic degradation to remove smell.
• Additives like Potash, Chemical NPK or value addition like
Vermi composting or added biofertilizer shall generate more
revenues.

10. Working of Lab Scale CSTR
Model: Simulating Large
scale conditions
Design of the Lab scale CSTR Model
Input of crushed
feedstock
mixture with
known pH and
adjusted TS of
17% to have
35dayHRT
Output with less
TS checked for pH,
TIC/VFA and
Ammonia
Biogas collected
volume
measured and
analyzed
Stirrer assembly
Digester with 3L
working volume

11. Thank you !