Factors affecting biogas yield

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[HCO 3 /dissolved CO 2 ]

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 NH 4 -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 NH 4 - N given the pre -requisite that the uncharged ammonia (NH 3 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 I ndia : 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 : m 3 / kg of volatile solids 1 kg of cow dung – 0.04 m 3 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 CO 2 + Ca (OH) 2 = CaCO 3 + H 2 O

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.