---
title: "Sludge treatment need; digester sketch"
subject: "Sanitary Engineering"
chapter: 8
key: "semester_6/sanitary_engineering:sludge-necessity-digester-sketch"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/sludge-necessity-digester-sketch
site: BCT Engineering
---

# Sludge treatment need; digester sketch

Sanitary Engineering · Chapter 8: Sludge Treatment and Disposal

- Asked in: 2068 Bhadra (old course), 2+6 marks
- Video: [Sludge Treatment Techniques 1](https://www.youtube.com/watch?v=2FqSKZA66Wk) — Sruthy S
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/sludge-necessity-digester-sketch
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-8

## Question

Explain the necessity of sludge treatment. Draw a sketch of sludge digester and explain its working.

## Short answer

Sludge treatment prevents nuisance, destroys pathogens, reduces volume and stabilises organics. A sludge digester is a closed tank with a gas cover; anaerobic bacteria decompose sludge in three stages: acid fermentation, acid regression and alkaline methane fermentation, producing 65% methane gas in 30-60 days.

## Answer

#### Necessity of sludge treatment
Sludge is the semi-solid residue (about 95-99% water) removed from screens, sedimentation tanks, filters and activated sludge units. Raw sludge is a highly putrescible mixture of organic matter, pathogens and heavy metals, so it must be treated before disposal.

- Prevent odour, flies and insect nuisance from putrescible sludge.
- Destroy pathogens and protect public health.
- Reduce the volume and make handling and disposal easier.
- Stabilise the organic matter and recover biogas.

#### Sludge digester (anaerobic, floating-cover type)
```text
        gas dome / floating cover
      gas out <-  ____|____
   sludge in ->  |  gas    |<- supernatant draw-off
   ~~~~~~~~~~~~~~|~~~~~~~~~|~~~~~~~~~ liquid level
   (heated)      | scum    |
   [heating coil] supernatant
                 | digesting sludge |
                 \  digested sludge /
                  \_______________/ slope 1:1 hopper
                     digested sludge out
```
**Construction:** a closed circular tank (diameter 6-30 m, depth 6-12 m), with a conical hopper bottom for sludge withdrawal, a fixed or floating gas-collecting cover (to avoid air entering), inlet for raw sludge, draw-off pipes at different levels for supernatant, a pipe for digested sludge, a gas outlet, a heating coil or heat exchanger to keep 35°C, and a mixer or recirculation pump (in high-rate type).

**Working (three stages):**
1. **Acid fermentation:** acid-forming bacteria break down the complex organic matter into volatile acids; pH falls to 5-6.
2. **Acid regression:** the acids are partly broken down, and the pH rises slowly.
3. **Alkaline (methane) fermentation:** methane bacteria convert the acids to CH₄ and CO₂ at pH 6.8-7.4. The gas (65% CH₄) collects under the cover and is used for heating.

Raw sludge is fed regularly; after 30-60 days (conventional rate), the stabilised sludge is withdrawn from the bottom to drying beds, the supernatant is drawn off and returned to the plant inlet.

## Check yourself

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

1. Which is a reason for sludge treatment?
   A. To increase pathogens
   B. To prevent odour, destroy pathogens and reduce volume
   C. To make sludge more putrescible
   D. To add water
   Answer: B. Treatment gives stable, safe sludge. (Key point: Necessity)

2. Why is the digester cover gas-tight (fixed or floating)?
   A. To keep air out and collect the gas
   B. To let air in
   C. To cool the sludge
   D. To stop supernatant draw-off
   Answer: A. Digestion is anaerobic; the gas is collected and used for heating. (Key point: Cover)

3. What temperature is maintained by the heating coil?
   A. 10°C
   B. 20°C
   C. 35°C
   D. 80°C
   Answer: C. 35°C is the optimum mesophilic temperature. (Key point: Heating)

4. What happens in the alkaline (methane) fermentation stage?
   A. Acids form and pH falls to 5-6
   B. Proteins form amines only
   C. Sludge is dried
   D. Methane bacteria convert acids to CH4 and CO2 at pH 6.8-7.4
   Answer: D. Gas production is highest and the sludge is stable. (Key point: Stage order)

## Related in Sanitary Engineering

- [Sludge treatment aims; digester volume (94%)](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-aims-digestion-volume-94) — Sanitary Engineering, chapter 8, asked 2×
- [Short note: Need of sludge treatment](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-need-examples) — Sanitary Engineering, chapter 8
- [Sludge treatment need; sand drying bed](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-treatment-necessity-drying-bed) — Sanitary Engineering, chapter 8
- [Digester volume for sludge, moisture 92%](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-digester-volume-92) — Sanitary Engineering, chapter 8, asked 2×
- [Sludge volume and digester, 4 MLD](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-volume-4mld-digester) — 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

## Related in other subjects

- [Municipal wastewater treatment plant](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Amunicipal-wastewater-treatment-plant) — Environment and Pollution Control, chapter 4