Factors affecting Biogas Production: There are several factors such as biogas potential of feedstock, inoculums, nature of substrate, pH, temperature, loading rate, hydraulic retention time (HRT), C:N ratio, volatile fatty acids (VFA), inhibitory substances, etc.

1. Lecture - 3
Analysis of factors affecting biogas
yield

2. Factors
• Total solids content
• pH
• Temperature
• C/N ratio
• Retention time
• Uniform Feeding & Type
• Loading rate
• Seeding
• Mixing / Agitation
• Toxicity
• Pressure - low
• Design of the plant

3. Total Solids Content

4. pH
• Acetate and fatty acids : lower the pH of digester liquor
- ion bicarbonate equilibria of the carbon dioxide in the digester exert
substantial resistance to pH change
- buffer capacity or buffer intensity : quantified by the amount of
strong acid (or base) added to the solution in order to bring about a
change in pH
• Presence of bicarbonate helps prevent : low pH
pH = 6.3 + log[HCO3/dissolved CO2]

5. Temperature
• Cryophiles (Psychrophiles)
- Occupies the area of cryophilic digestive regime
- 12°C and 24°C
• Mesophiles
- Occupies the area of mesophilic digestive regime
- 22°C and 40°C
• Thermophiles
- Occupies the area of thermophilic digestive regime
- 50°C and 60°C

6. Temperature
Rate of reaction doubles for every 10°C rise in
temperature up to an optimum and then
declines rapidly

7. C/N ratio
• Carbon & Nitrogen : nutrients for efficient anaerobic digestion
• Carbon : energy source
• Nitrogen: building the cell structures
• C:N ratio = 25-30:1
• If N concentration is too high (>1,700 mg/l of NH4-N)
• pH is high
- Growth of bacteria is inhibited due to toxicity caused by
high levels of (uncharged) ammonia
• Methanogens : adapt to 5,000 - 7,000 mg/l of NH4 - N given
the pre -requisite that the uncharged ammonia (NH3
controlled by pH) level does not exceed 200 - 300 mg/l

8. Retention time
• Average time spent by the input slurry inside the digester
before it come out
• 20 – 120 days – depends on design & operating temperature
• In India : 40 -50 days
• Cold climates : 100 days
LR = k (TS / HRT)
k = 7

9. Loading Rate
• Waste material fed / unit volume of digester capacity
• Gas production : m3 / kg of volatile solids
• 1 kg of cow dung – 0.04 m3 of biogas
• Based on gas requirement
• Uniform feeding
• Under feeding & Over feeding

10. Seeding
• Introduction of enriched anaerobic bacteria
- for start up
• Anaerobic bacteria proceeds
- acid forming bacteria : abundant
- methane forming bacteria : lesser
• Improper mix of feedstock : change in pH
- excessive formation of volatile fatty acids
CO2 + Ca (OH)2 = CaCO3 + H2O

11. Mixing / Agitation
• Mixing devices
• Feeding daily
• Effective and rapid digestion with much decreased retention time
• Solid sub-layers undergo improved digestion with significant
utilization
• Completely destroys the undesired pathogens as well as the seeds
of weeds
• Liquid isolation from solid fractions is highly feasible

12. • Removing metabolites (gas removal)
• Bringing fresh material in contact with bacteria
• Reducing scum formation and sedimentation
• Preventing temperature gradients in the digester
• Avoiding the formation of blind spots (short cuts)
• Excessive stirring disturbs the symbiotic relationship between
the different bacteria species

13. Toxicity
• Volatile acid concentration reaches a value of 200 ppm
• Ammonical nitrogen concentration exceeds a value of 1500 ppm
• Ammonia, volatile acids, and heavy metals
• Nitrogen, phosphorus, magnesium, sodium, potassium, manganese,
calcium and cobalt
• Inhibition indicators
a. Reduction in methane yield over time
- indicated by two or more consecutive decreases of more
than 10% in daily yield at a constant loading rate
b. Increase in volatile acids concentration over time
- generally occurring when the total volatile acids (expressed
as acetic acid) exceed the normal range of about 250 to 500 milligrams
per liter

14. • Retarded microbial activity
- low quality gas with low methane content
• Reasons
- excessive acid formation : presence of high level
acid forming bacteria
- high acid contents in feed
- delays in addition of fresh feeds
- delayed withdrawal of digested sludge
- improper feed composition

15. Design of the Plant
KCIV Biogas Plant Deenabandhu Biogas Plant

16. Reference
• Mittal, K.M. 1996. Biogas system: principles and
applications. New age international (P) Ltd., New
Delhi. P:No: 151-177.