---
title: "Aerobic and anaerobic digestion"
subject: "Sanitary Engineering"
chapter: 8
key: "semester_6/sanitary_engineering:aerobic-anaerobic-digestion"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/aerobic-anaerobic-digestion
site: BCT Engineering
---

# Aerobic and anaerobic digestion

Sanitary Engineering · Chapter 8: Sludge Treatment and Disposal

- Asked in: 2070 Bhadra, 8 marks; 2080 Chaitra, 4 marks (asked 2 times)
- Video: [Aerobic Digestion and Anaerobic Digestion](https://www.youtube.com/watch?v=p5c_qz4gAps) — chrvoje_engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/aerobic-anaerobic-digestion
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-8

## Question

Briefly describe the aerobic and anaerobic sludge digestion processes. Explain the effect of temperature and pH in digestion process.

## Short answer

Aerobic digestion aerates sludge for 10-20 days, reducing volatile solids 35-50% via endogenous respiration; advantages: simple, low capital, no odor; disadvantages: high power, no methane, poor dewatering. Anaerobic digestion in closed heated tanks destroys 40-60% volatile solids, produces biogas (65% CH4, 22 MJ/m3). Temperature: mesophilic 30-38°C (optimum 35°C, 20-30 days), thermophilic 50-57°C (faster but less stable); digestion time doubles per 10°C drop. pH: methane formers need 6.8-7.4; below 6.2 digestion stops.

## Answer

#### Aerobic digestion
Sludge (usually waste activated sludge or mixed primary and secondary sludge) is aerated for 10-20 days (at 15-20°C) in an open tank, in the same way as in the activated sludge process. Without fresh food the aerobic bacteria oxidise their own cell mass (endogenous respiration):
$$
C_5H_7NO_2+5O_2\rightarrow5CO_2+2H_2O+NH_3+\text{energy}
$$
It reduces volatile solids by 35-50%.
- Advantages: simple operation, low capital cost, no odour problems, a supernatant of lower BOD, good for small plants.
- Disadvantages: high power for aeration, no methane gas is produced, and the sludge dewaters less easily.

#### Anaerobic digestion
Sludge is held in a closed, heated tank without air, with decomposition in three stages by two groups of bacteria. **Acid-forming bacteria** convert complex organics to volatile acids, and then **methane-forming bacteria** convert the acids to CH₄ and CO₂:
$$
\text{organics}\rightarrow\text{volatile acids}\rightarrow CH_4+CO_2+H_2S
$$
It is the common method for primary and mixed sludge in medium and large plants: 40-60% volatile solids are destroyed, and the biogas (65% CH₄, calorific value about 22 MJ/m³) is used for heating and power. The digested sludge is stable, inoffensive and can be dried on beds and used as manure.

| Point | Aerobic | Anaerobic |
|---|---|---|
| Air | Required | Absent |
| Detention | 10-20 days | 20-60 days |
| Gas | None | Methane gas |
| Power | High | Low (gas is a by-product) |
| Cost | Low capital, high running | High capital, low running |

#### Effect of temperature and pH
- **Temperature:** the rate of digestion increases with temperature. Anaerobic digestion works at the mesophilic range 30-38°C (optimum about 35°C, digestion time about 20-30 days) and the thermophilic range 50-57°C (faster, 10-15 days, but less stable). Below 20°C digestion becomes very slow (60 days or more); the digestion time about doubles for each 10°C fall. Sudden changes of more than 1°C per day upset the methane bacteria, so digesters are heated and insulated.
- **pH:** acid formers tolerate pH 5-6 but methane formers need 6.8-7.4. If volatile acids accumulate, pH falls below 6.2 and digestion stops ("sour" digester, foul odour, little gas). The pH is corrected by reducing the feed rate and adding lime or bicarbonate to maintain alkalinity of 1500-5000 mg/l.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In aerobic digestion, volatile solids decrease mainly because
   A. methane bacteria form gas
   B. bacteria oxidise their own cell mass by endogenous respiration
   C. algae consume the solids
   D. lime dissolves the solids
   Answer: B. Without fresh food, aerobic bacteria oxidise their cells to CO2, H2O and NH3. (Key point: Aerobic digestion principle)

2. In anaerobic digestion, what do methane-forming bacteria do?
   A. Convert volatile acids to CH4 and CO2
   B. Convert complex organics to acids
   C. Oxidise sludge using air
   D. Produce oxygen
   Answer: A. Acid formers make volatile acids, which methane formers convert to methane and CO2. (Key point: Anaerobic stages)

3. Which is true of aerobic versus anaerobic digestion?
   A. Aerobic gives methane, anaerobic does not
   B. Aerobic has low power need
   C. Anaerobic needs air supply
   D. Aerobic has low capital cost but high running power; anaerobic gives biogas
   Answer: D. Aerobic needs aeration power and produces no gas; anaerobic produces methane and has low running cost. (Key point: Comparison)

4. What happens when volatile acids accumulate and pH drops below 6.2?
   A. Digestion speeds up
   B. Digestion stops (sour digester) with foul odour and little gas
   C. More methane is produced
   D. The sludge turns to ash
   Answer: B. Methane formers are inhibited; correct by reducing feed and adding lime or bicarbonate. (Key point: Effect of pH)

## Related in Sanitary Engineering

- [Sludge digestion: stages and factors](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-digestion-stages-factors) — Sanitary Engineering, chapter 8, asked 3×
- [Sludge volume before and after digestion](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-volume-before-after-600kg) — Sanitary Engineering, chapter 8
- [Sludge digestion tank, 6.5 MLD](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-digestion-tank-6p5mld) — Sanitary Engineering, chapter 8
- [Sludge digestion tank, 812.5 m³ sed. tank](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-digestion-tank-812) — Sanitary Engineering, chapter 8
- [Activated sludge and ASP treatment process](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aasp-sketch-treatment-process) — Sanitary Engineering, chapter 7
- [Temperature effect on BOD; 5-day BOD](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Atemperature-effect-decomposition) — Sanitary Engineering, chapter 5

## Related in other subjects

- [Organic pollution and aerobic/anaerobic decomposition](https://www.bctengineer.com/question.md?key=semester_7%2Ftechnology_environment_and_society%3Aorganic-pollution-decomposition) — Technology, Environment and Society, chapter 5
- [Break point chlorination; pH and temperature effects](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Abpc-ph-temperature-hocl) — Water Supply Engineering, chapter 6, asked 2×
- [Propionic acid buffer pH; effect of HCl](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Apropionic-buffer-hcl) — Engineering Chemistry, chapter 1
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