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.
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
