---
title: "Activated sludge and ASP treatment process"
subject: "Sanitary Engineering"
chapter: 7
key: "semester_6/sanitary_engineering:asp-sketch-treatment-process"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/asp-sketch-treatment-process
site: BCT Engineering
---

# Activated sludge and ASP treatment process

Sanitary Engineering · Chapter 7: Wastewater Treatment

- Asked in: 2071 Bhadra, 8 marks
- Video: [Activated Sludge Process (ASP) | Waste Water Engineering](https://www.youtube.com/watch?v=V8mb13pwTu8) — APSEd
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/asp-sketch-treatment-process
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7

## Question

What is meant by activated sludge? Describe with sketches the treatment process of wastewater by activated sludge process.

## Short answer

Activated sludge is a brown, flocculent microbial mass that adsorbs and oxidizes organic matter. Process: preliminary/primary treatment, aeration tank with return sludge and air supply (4-8 h HRT), secondary clarifier for settling, return sludge (25-50%), excess sludge wasting; BOD removal 85-95%.

## Answer

**Activated sludge** is the biologically active, brown, flocculent mass of bacteria, protozoa, fungi and other micro-organisms, together with organic solids, that is produced when sewage is aerated for some hours. It adsorbs and oxidises organic matter and settles readily; a portion is returned to the process (hence "activated").

#### Treatment process (conventional ASP)
```text
 Raw      +---------+  +-----+  +----------+  +----------+
 sewage ->| Screen  |->| PST |->| Aeration |->|Secondary |-> Effluent
          | + grit  |  |     |  | tank+air |  |clarifier |
          +---------+  +--+--+  +-----^----+  +----+-----+
                          |           | return     |
                    primary sludge    | sludge(RAS)|
                          |           +------------+
                          |                  | excess sludge
                          +--> thickener -> digester -> drying bed
```
1. **Preliminary and primary treatment:** screens remove floating matter, the grit chamber removes grit and the primary sedimentation tank (PST) removes settleable solids and about 30-35% of the BOD.
2. **Aeration tank:** the settled sewage is mixed with return activated sludge. Compressed air through diffusers (or surface aerators) supplies oxygen and keeps the flocs in suspension for 4-8 h (MLSS 2000-4000 mg/l). Bacteria oxidise the organic matter and build new cells.
3. **Secondary clarifier:** the mixed liquor flows in, the flocs settle in 2-3 h and the clear effluent (BOD 20-30 mg/l) overflows.
4. **Return sludge:** 25-50% of the settled sludge is pumped back to the aeration tank to maintain the MLSS.
5. **Excess sludge:** the surplus is wasted to thickening and digestion, then to drying beds, because it would otherwise raise the MLSS.
6. The BOD removal is 85-95%, and the effluent may be disinfected before discharge.

**Modifications:** tapered aeration, step aeration, contact stabilisation, extended aeration, oxidation ditch and sequencing batch reactor.

## Check yourself

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

1. What is activated sludge?
   A. The primary sludge from the PST
   B. A brown flocculent mass of micro-organisms and organic solids formed when sewage is aerated, able to adsorb, oxidise organics and settle well
   C. Dried sludge from drying beds
   D. Grit removed from sewage
   Answer: B. It is the biologically active floc produced in aerated sewage. (Key point: Definition)

2. What is the correct flow sequence?
   A. Screen/grit chamber, PST, aeration tank, secondary clarifier, effluent
   B. Aeration tank, PST, secondary clarifier, screens
   C. PST, screens, secondary clarifier, aeration tank
   D. Screens, aeration tank, grit chamber, PST
   Answer: A. Preliminary and primary treatment precede aeration, and the secondary clarifier follows it. (Key point: Order of units)

3. What does the aeration tank do?
   A. It settles sludge for 2-3 h
   B. It removes grit
   C. Settled sewage is mixed with return sludge and aerated 4-8 h so bacteria oxidise organics and build cells
   D. It disinfects the effluent
   Answer: C. Air from diffusers supplies oxygen and keeps flocs in suspension. (Key point: Aeration tank role)

4. What happens to the sludge from the secondary clarifier?
   A. All of it is wasted
   B. Part (25-50%) is returned to the aeration tank and the excess is wasted to thickening and digestion
   C. All of it is returned
   D. It is returned to the PST only
   Answer: B. Return keeps MLSS; surplus is wasted so MLSS does not keep rising. (Key point: Return and excess sludge)

## Related in Sanitary Engineering

- [Activated sludge: properties, mechanism](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aasp-properties-biochemical) — Sanitary Engineering, chapter 7
- [Is treatment necessary? Town wastewater 300 l/s](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Atreatment-needed-town-300-ls) — Sanitary Engineering, chapter 6
- [Degree of treatment, industrial wastewater](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adegree-treatment-industrial-waste) — Sanitary Engineering, chapter 6
- [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

## 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
- [Treatment process; sedimentation factors; coagulants](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Atreatment-impurities-coagulants) — Water Supply Engineering, chapter 6
- [Rainwater harvesting and its treatment](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-treatment) — Building Technology, chapter 12, asked 3×