Mesophilic digestion ek anaerobic digestion process hai jo medium temperature range me hota hai.
🔹 Definition:
Jab organic waste (jaise sewage sludge, food waste, animal manure) ko anaerobic condition me 30°C – 38°C temperature range par digest kiya jata hai, to ise Mesophilic digestion kehte hain.
🔹 Key Points:
Microorganisms ka optimum growth is range me hota hai.
Sabse zyada commonly use kiya jata hai waste treatment plants me.
Energy requirement kam hoti hai (compare to thermophilic digestion).
Gas production steady hota hai (mainly methane aur CO₂).
Stability aur reliability zyada hoti hai.
🔹 Comparison:
Mesophilic digestion (30–38°C): Stable, low cost, common.
Thermophilic digestion (50–60°C): Faster digestion, more gas, but costly aur unstable.
👉 Simple shabdon me: Mesophilic digestion ek biological process hai jisme bacteria medium temperature (30–38°C) par organic waste ko todkar methane gas banate hain.
📝 MCQ Practice Set on Mesophilic Digestion
Q1. Mesophilic digestion generally occurs at which temperature range?
(a) 15–25°C
(b) 30–38°C
(c) 45–55°C
(d) 55–65°C
✅ Answer: (b) 30–38°C
Explanation: Mesophilic bacteria optimum growth medium temperature 30–38°C par karte hain.
Q2. The main gas produced in mesophilic anaerobic digestion is:
(a) Oxygen
(b) Methane
(c) Nitrogen
(d) Ozone
✅ Answer: (b) Methane
Explanation: Anaerobic digestion ka main by-product methane (biogas) hota hai.
Q3. Which group of microorganisms dominate mesophilic digestion?
(a) Mesophiles
(b) Thermophiles
(c) Psychrophiles
(d) Halophiles
✅ Answer: (a) Mesophiles
Q4. Mesophilic digestion is commonly used in:
(a) Biogas plants
(b) Sewage treatment plants
(c) Food waste treatment
(d) All of the above
✅ Answer: (d) All of the above
Q5. Mesophilic digestion compared to thermophilic digestion is:
(a) More stable but slower
(b) Less stable but faster
(c) More energy-intensive
(d) None of these
✅ Answer: (a) More stable but slower
Explanation: Thermophilic digestion fast hai but unstable; mesophilic digestion stable hai but relatively slow.
Q6. Retention time in mesophilic digesters is usually:
(a) 5–7 days
(b) 15–30 days
(c) 40–60 days
(d) 1–2 days
✅ Answer: (b) 15–30 days
Q7. Which factor is NOT critical for mesophilic digestion?
(a) Temperature
(b) pH level
(c) Oxygen concentration
(d) Volatile fatty acids concentration
✅ Answer: (c) Oxygen concentration
Explanation: Anaerobic process me oxygen bilkul nahi hona chahiye, isliye iska control critical parameter nahi hota.
Q8. Optimum pH range for mesophilic anaerobic digestion is:
(a) 3.5 – 4.5
(b) 5.5 – 6.0
(c) 6.8 – 7.2
(d) 8.5 – 9.5
✅ Answer: (c) 6.8 – 7.2
Q9. In mesophilic digestion, the methanogens responsible for methane formation are:
(a) Aerobic bacteria
(b) Archaea
(c) Cyanobacteria
(d) Actinomycetes
✅ Answer: (b) Archaea
Q10. Which is a major advantage of mesophilic digestion?
(a) Faster digestion rate
(b) Higher pathogen removal
(c) Low energy requirement and stable process
(d) Requires high heating cost
✅ Answer: (c) Low energy requirement and stable process
Q11. Mesophilic digestion is most widely adopted because:
(a) It requires high heating
(b) It is less stable
(c) It is energy-efficient and reliable
(d) It removes pathogens completely
✅ Answer: (c) It is energy-efficient and reliable
Q12. Methane production in mesophilic digesters is mainly due to:
(a) Hydrolytic bacteria
(b) Acidogenic bacteria
(c) Methanogenic archaea
(d) Sulfate-reducing bacteria
✅ Answer: (c) Methanogenic archaea
Q13. The solids retention time (SRT) in mesophilic anaerobic digestion usually ranges between:
(a) 2–5 days
(b) 10–20 days
(c) 15–30 days
(d) 40–60 days
✅ Answer: (c) 15–30 days
Q14. Which of the following is NOT a stage in anaerobic digestion?
(a) Hydrolysis
(b) Acidogenesis
(c) Methanogenesis
(d) Nitrification
✅ Answer: (d) Nitrification
Q15. Mesophilic digestion produces biogas having methane content of about:
(a) 20–30%
(b) 40–50%
(c) 55–65%
(d) 80–90%
✅ Answer: (c) 55–65%
Q16. The digested sludge from mesophilic digesters is:
(a) Rich in pathogens
(b) Stabilized and safer for disposal
(c) Highly toxic
(d) Radioactive
✅ Answer: (b) Stabilized and safer for disposal
Q17. The main drawback of mesophilic digestion is:
(a) Low stability
(b) Low methane yield per unit time
(c) Very high energy requirement
(d) Incomplete organic breakdown
✅ Answer: (b) Low methane yield per unit time
Q18. In mesophilic digestion, alkalinity is maintained to buffer against:
(a) Nitrite accumulation
(b) Ammonia loss
(c) Volatile fatty acid accumulation
(d) Oxygen depletion
✅ Answer: (c) Volatile fatty acid accumulation
Q19. Which of the following organic materials can be treated using mesophilic digestion?
(a) Sewage sludge
(b) Animal manure
(c) Food waste
(d) All of the above
✅ Answer: (d) All of the above
Q20. The loading rate of mesophilic anaerobic digesters is usually expressed in:
(a) kg BOD/m³·day
(b) kg COD/m³·day
(c) kg VS/m³·day (Volatile solids)
(d) None of these
✅ Answer: (c) kg VS/m³·day
Q21. Mesophilic digestion requires mixing of sludge mainly to:
(a) Provide oxygen to microbes
(b) Prevent scum and improve contact of microbes with substrate
(c) Increase sludge density
(d) Remove toxic gases
✅ Answer: (b) Prevent scum and improve contact of microbes with substrate
Q22. Optimum C/N ratio for mesophilic anaerobic digestion is:
(a) 5–10
(b) 20–30
(c) 35–40
(d) 50–60
✅ Answer: (b) 20–30
Q23. In mesophilic digestion, which gas is considered an impurity in biogas?
(a) Methane
(b) Carbon dioxide
(c) Oxygen
(d) Hydrogen
✅ Answer: (b) Carbon dioxide
Q24. Pathogen reduction in mesophilic digestion compared to thermophilic digestion is:
(a) Higher
(b) Lower
(c) Equal
(d) Negligible difference
✅ Answer: (b) Lower
Q25. Mesophilic digestion works best at moisture content of feedstock around:
(a) 10–20%
(b) 30–40%
(c) 80–90%
(d) Completely dry
✅ Answer: (c) 80–90%
Q26. A sudden drop in pH during mesophilic digestion usually indicates:
(a) Increase in alkalinity
(b) Accumulation of volatile fatty acids
(c) Rise in methane yield
(d) Proper stabilization
✅ Answer: (b) Accumulation of volatile fatty acids
Q27. In mesophilic digestion, high ammonia concentration can:
(a) Increase methane production
(b) Inhibit methanogens
(c) Stabilize pH
(d) Improve sludge settling
✅ Answer: (b) Inhibit methanogens
Q28. The calorific value of methane-rich biogas produced in mesophilic digestion is approximately:
(a) 5 MJ/m³
(b) 10 MJ/m³
(c) 20–23 MJ/m³
(d) 40 MJ/m³
✅ Answer: (c) 20–23 MJ/m³
Q29. In mesophilic digestion, higher solids loading than design capacity leads to:
(a) Improved methane yield
(b) Acidic conditions and process failure
(c) Lower sludge volume
(d) No effect
✅ Answer: (b) Acidic conditions and process failure
Q30. Mesophilic digestion is preferred in developing countries mainly because:
(a) Requires sophisticated technology
(b) High energy requirement
(c) Low cost and simplicity
(d) High pathogen kill
✅ Answer: (c) Low cost and simplicity
Q31. Methane-forming organisms in mesophilic digestion are strictly:
(a) Aerobic
(b) Facultative anaerobes
(c) Obligate anaerobes
(d) Facultative aerobes
✅ Answer: (c) Obligate anaerobes
Q32. Which stage of anaerobic digestion is most sensitive to temperature changes in mesophilic digestion?
(a) Hydrolysis
(b) Acidogenesis
(c) Acetogenesis
(d) Methanogenesis
✅ Answer: (d) Methanogenesis
Q33. In mesophilic digestion, foaming in digesters occurs mainly due to:
(a) Excessive protein and grease in feed
(b) Low alkalinity
(c) High oxygen content
(d) Low temperature
✅ Answer: (a) Excessive protein and grease in feed
Q34. The typical gas composition of biogas in mesophilic digestion is:
(a) 55–65% CH₄, 30–40% CO₂, traces of H₂S
(b) 70% CH₄, 20% CO₂, 10% O₂
(c) 90% CO₂, 10% CH₄
(d) Pure methane
✅ Answer: (a)
Q35. The energy recovery from mesophilic digestion depends mainly on:
(a) Solids concentration of feed
(b) Retention time
(c) Temperature stability
(d) All of the above
✅ Answer: (d) All of the above
Q36. Which of the following is a co-product of mesophilic digestion apart from biogas?
(a) Digestate (stabilized sludge)
(b) Pure ethanol
(c) Oxygen gas
(d) Sulfuric acid
✅ Answer: (a) Digestate (stabilized sludge)
Q37. For mesophilic digestion of sewage sludge, typical volatile solids reduction achieved is:
(a) 10–15%
(b) 20–30%
(c) 40–60%
(d) 80–90%
✅ Answer: (c) 40–60%
Q38. Mesophilic digestion performance is monitored using which parameter?
(a) Alkalinity ratio (VFA/Alkalinity)
(b) Dissolved oxygen concentration
(c) Nitrate levels
(d) Chloride levels
✅ Answer: (a)
Q39. Failure of mesophilic digestion system is usually indicated by:
(a) High methane yield
(b) Rapid pH drop below 6.5
(c) Increase in alkalinity
(d) Constant gas production
✅ Answer: (b)
Q40. In wastewater treatment, mesophilic digestion is primarily used for:
(a) Grit removal
(b) Sludge stabilization and energy recovery
(c) Disinfection only
(d) Heavy metal removal
✅ Answer: (b)
Bahut badiya 👍
📘 Summary Notes: Mesophilic Digestion
🔹 Definition
Mesophilic digestion ek anaerobic digestion process hai jisme 30–38°C temperature par microorganisms organic waste ko todte hain aur biogas (CH₄ + CO₂) banate hain.
🔹 Key Features
Temperature Range: 30–38°C
pH Range: 6.8 – 7.2
SRT (Solids Retention Time): 15–30 days
Methane Content in Biogas: 55–65%
By-products: Biogas + Digestate (stabilized sludge)
🔹 Microbes Involved:
Hydrolytic bacteria
Acidogenic bacteria
Acetogenic bacteria
Methanogenic archaea (strict anaerobes)
🔹 Stages of Mesophilic Digestion
1. Hydrolysis
Complex organics (proteins, fats, carbohydrates) → simpler compounds (amino acids, sugars, fatty acids).
2. Acidogenesis
Simple compounds → organic acids (VFAs), alcohols, H₂, CO₂.
3. Acetogenesis
VFAs + alcohols → acetic acid, H₂, CO₂.
4. Methanogenesis
Acetic acid + H₂ + CO₂ → Methane (CH₄) + CO₂
Done by methanogenic archaea.
🔹 Advantages
Stable & reliable
Low energy requirement
Commonly used in sewage treatment plants and biogas plants
Produces useful renewable energy (biogas)
🔹 Limitations
Slower than thermophilic digestion
Less pathogen removal
Sensitive to pH drop (due to VFA accumulation)
📊 Flow Diagram: Mesophilic Anaerobic Digestion
Organic Waste (Sludge, Manure, Food Waste)
│
▼
[Hydrolysis]
Complex organics → Sugars, Amino acids, Fatty acids
│
▼
[Acidogenesis]
Sugars & acids → VFAs, Alcohols, H₂, CO₂
│
▼
[Acetogenesis]
VFAs & alcohols → Acetic acid, H₂, CO₂
│
▼
[Methanogenesis]
Acetic acid + H₂ + CO₂ → CH₄ + CO₂ (Biogas)
│
▼
✅ Biogas (55–65% CH₄)
✅ Digestate (stabilized sludge)
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