---
title: "Activated sludge: properties, mechanism"
subject: "Sanitary Engineering"
chapter: 7
key: "semester_6/sanitary_engineering:asp-properties-biochemical"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/asp-properties-biochemical
site: BCT Engineering
---

# Activated sludge: properties, mechanism

Sanitary Engineering · Chapter 7: Wastewater Treatment

- Asked in: 2076 Bhadra, 8 marks
- Video: [(Part-1)Activated Sludge Process|Working of Activated Sludge Process](https://www.youtube.com/watch?v=7btREr5DcPw) — ISHA MISHRA
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/asp-properties-biochemical
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7

## Question

What is meant by activated sludge? What are its properties? Describe with sketch the biochemical mechanism of the activated sludge process.

## Short answer

Activated sludge is a brown, flocculent microbial mass (MLSS 2000-4000 mg/L, SVI 50-150 mL/g). Biochemical mechanism: adsorption of organics on flocs, oxidation to CO2/H2O, synthesis of new cells, endogenous respiration, nitrification at long sludge age, and flocculation/settling in clarifier.

## Answer

**Activated sludge** is the flocculent, brown, microbial mass formed in aerated sewage; it consists of bacteria, protozoa, fungi, rotifers and organic and inorganic solids, and is capable of adsorbing and oxidising organic matter in sewage. A part of it is returned to the aeration tank to keep the process active.

#### Properties of activated sludge
- **Colour and odour:** healthy sludge is brown to chocolate and has an earthy odour; black colour and foul smell show septic (oxygen-deficient) conditions.
- **Floc form:** gelatinous, sticky floc, 50-200 μm in size, which settles well.
- **Concentration:** MLSS 2000-4000 mg/l in the tank; return sludge 8000-12000 mg/l (about 0.5-1% solids).
- **Sludge volume index (SVI):** $SVI=\dfrac{\text{volume of settled sludge (ml/l)}\times1000}{MLSS\ (\text{mg/l})}$; good sludge has SVI 50-150 ml/g; above 150 is bulking sludge.
- **Specific gravity:** slightly above 1 (about 1.005-1.02), so it settles slowly but compacts.
- **Moisture:** about 99% water in the aeration tank and 98-99% in the secondary underflow.
- **Biological activity:** mostly aerobic bacteria (*Zoogloea ramigera*, *Pseudomonas*, *Flavobacterium*), with ciliated protozoa which graze free bacteria and make the effluent clear.
- **Volatile fraction:** MLVSS is 70-80% of MLSS.

#### Biochemical mechanism
```text
  Organic matter + O2 + nutrients
        |        (bacterial floc)
        v
  1. Adsorption on floc  ---- (first 15-30 min)
  2. Oxidation  -> CO2 + H2O + energy
  3. Synthesis  -> new cells (more sludge)
  4. Endogenous respiration -> cells + O2 -> CO2 + H2O + NH3
  5. Flocculation -> settling in clarifier -> RAS / waste
```
1. **Adsorption (biosorption):** colloidal and suspended organic matter is quickly adsorbed on the floc surface. Soluble matter is taken up through cell walls with the help of exo-enzymes.
2. **Oxidation:** $\text{organics}+O_2\rightarrow CO_2+H_2O+\text{energy}$ (about one-third to one-half of the BOD removed).
3. **Synthesis:** $\text{organics}+O_2+NH_3\rightarrow C_5H_7NO_2\ (\text{new cells})$ with energy from step 2.
4. **Endogenous respiration:** $C_5H_7NO_2+5O_2\rightarrow5CO_2+2H_2O+NH_3+\text{energy}$, reducing the sludge quantity.
5. **Nitrification** (at long sludge age): $NH_4^+\rightarrow NO_2^-\rightarrow NO_3^-$.
6. **Flocculation and settling:** the bacteria secrete a polysaccharide slime that binds the cells into flocs, which settle in the clarifier. Settled sludge is returned to the aeration tank and the excess is wasted.

## Check yourself

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

1. What is activated sludge?
   A. Settled inorganic grit removed from the grit chamber
   B. A brown flocculent microbial mass in aerated sewage that adsorbs and oxidises organic matter
   C. Digested sludge dried on sand beds
   D. Chemical precipitate formed after adding alum
   Answer: B. It is the brown floc of bacteria, protozoa and other organisms formed in aerated sewage; part is returned to keep the process active. (Key point: Meaning of activated sludge)

2. A good activated sludge has which SVI, and what does a value above 150 ml/g indicate?
   A. SVI 50-150 ml/g; above 150 is bulking sludge
   B. SVI 150-300 ml/g; above 150 is ideal sludge
   C. SVI 5-15 ml/g; above 150 is dense sludge
   D. SVI 50-150 ml/g; above 150 is septic sludge
   Answer: A. SVI between 50 and 150 ml/g means good settling; above 150 the sludge is bulking and settles poorly. (Key point: Sludge volume index)

3. Which step of the biochemical mechanism reduces the quantity of sludge by bacteria oxidising their own cell mass?
   A. Adsorption
   B. Synthesis
   C. Flocculation
   D. Endogenous respiration
   Answer: D. In endogenous respiration cells (C5H7NO2) are oxidised to CO2, H2O and NH3, so sludge quantity falls. (Key point: Oxidation, synthesis and endogenous respiration)

4. What happens in the first 15-30 minutes of the process?
   A. Colloidal and suspended organic matter is quickly adsorbed on the floc surface
   B. Ammonia is oxidised to nitrate
   C. All the organic matter is converted to CO2 and water
   D. The sludge is wasted from the clarifier
   Answer: A. Biosorption on the floc comes first; oxidation and synthesis follow. (Key point: Adsorption stage)

## Related in Sanitary Engineering

- [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