---
title: "Short note: Need of sludge treatment"
subject: "Sanitary Engineering"
chapter: 8
key: "semester_6/sanitary_engineering:sludge-need-examples"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/sludge-need-examples
site: BCT Engineering
---

# Short note: Need of sludge treatment

Sanitary Engineering · Chapter 8: Sludge Treatment and Disposal

- Asked in: 2080 Chaitra, 4 marks
- Video: [How Wastewater Plants Turn Sludge Into Biosolids (Step-by-Step)](https://www.youtube.com/watch?v=7mWjFNYY0_E) — The Wastewater Enthusiast
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/sludge-need-examples
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-8

## Question

Write a short note on need of sludge treatment with examples.

## Short answer

Sludge treatment is needed to control odor/nuisance (e.g., untreated sludge putrefies, digestion makes it inoffensive), destroy pathogens (e.g., raw sludge on vegetables transmits worms), reduce volume (e.g., 100 m3 at 5% solids to 25 m3 at 20%), stabilize organics (anaerobic digestion converts half VS to gas), recover energy (biogas for heating/power) and resources (dried sludge as soil conditioner), and protect environment from oxygen depletion and sludge banks.

## Answer

Sludge is the semi-solid material (95-99% water) that settles out in the primary and secondary sedimentation tanks, or is removed from screens and filters, in a sewage treatment plant. It is putrescible and contains pathogens, so it needs treatment before disposal.

#### Need of sludge treatment
1. **Control of odour and nuisance:** fresh sludge from a primary tank putrefies within hours, giving hydrogen sulphide and other foul gases and attracting flies. *Example:* untreated sludge dumped near a settlement gives a smell nuisance; digestion makes it inoffensive.
2. **Pathogen destruction:** raw sludge from domestic sewage carries bacteria, helminth eggs and viruses. *Example:* using raw sludge as manure on vegetables can transmit worm infection; digestion and drying kill most of them.
3. **Volume reduction:** the sludge from a plant is about 0.25-0.5% of the sewage volume but contains 95% water. Thickening and dewatering reduce the volume from, say, 100 m³ at 5% solids to 25 m³ at 20%, which lowers the cost of transport.
4. **Stabilisation of organic matter:** anaerobic digestion converts about half of the volatile solids to gas, leaving inert humus-like matter.
5. **Energy and resource recovery:** digester gas (about 65% methane) is used for heating and power; dried sludge is used as soil conditioner.
6. **Environmental protection:** untreated sludge discharged into a river creates sludge banks and oxygen depletion.

## Check yourself

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

1. Why must fresh primary sludge be treated quickly?
   A. It is too dry
   B. It contains no organic matter
   C. It putrefies within hours giving H2S, foul gas and flies
   D. It is too cold
   Answer: C. Raw sludge is putrescible and causes nuisance. (Key point: Odour and nuisance)

2. What risk comes from using raw sludge as manure on vegetables?
   A. Transmission of worm infection through helminth eggs and pathogens
   B. Excess oxygen
   C. Soil becomes too acidic only
   D. No risk
   Answer: A. Raw sludge carries bacteria, helminth eggs and viruses; digestion and drying kill most. (Key point: Pathogens)

3. In the example, 100 m3 at 5% solids becomes 25 m3 at
   A. 10% solids
   B. 15% solids
   C. 25% solids
   D. 20% solids
   Answer: D. Solids are constant: 100 x 5 = 25 x 20, so 20% solids. (Key point: Volume reduction)

4. What is recovered from sludge treatment?
   A. Chlorine gas
   B. Digester gas (about 65% methane) and dried sludge for soil conditioning
   C. Fresh drinking water directly
   D. Sand
   Answer: B. Biogas is used for heating and power. (Key point: Resource recovery)

## Related in Sanitary Engineering

- [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
- [Sludge treatment need; digester sketch](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-necessity-digester-sketch) — Sanitary Engineering, chapter 8
- [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×
- [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
- [Methods of sludge treatment and functions](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-treatment-methods-functions) — 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×

## 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