---
title: "Suspended and attached growth processes"
subject: "Sanitary Engineering"
chapter: 7
key: "semester_6/sanitary_engineering:suspended-attached-growth"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/suspended-attached-growth
site: BCT Engineering
---

# Suspended and attached growth processes

Sanitary Engineering · Chapter 7: Wastewater Treatment

- Asked in: 2072 Asoj, 8 marks; 2070 Magh, 8 marks; 2069 Bhadra, 8 marks (asked 3 times)
- Video: [Suspended & Attached Growth Mechanism Story | Environmental Engineering | By Vijay Sir | AEC Plus](https://www.youtube.com/watch?v=noHlQ3Xxics) — AEC Plus
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/suspended-attached-growth
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7

## Question

What do you understand by suspended growth and attached growth process in wastewater treatment? Explain in detail the principles of biological wastewater treatment.

## Short answer

Suspended growth processes keep biomass in suspension via aeration, needing sludge recycle (e.g., activated sludge). Attached growth processes use a biofilm on fixed media (e.g., trickling filters) and are simpler and more shock-tolerant. Both rely on aerobic bacteria to oxidize organics into CO2, water, and new cells.

## Answer

Biological wastewater treatment uses micro-organisms (mainly bacteria) to convert dissolved and colloidal organic matter into carbon dioxide, water, new cells and stable end products. The organisms may be kept in suspension in the liquid or grown on a fixed surface.

#### Suspended growth process
The micro-organisms are kept **in suspension** in the wastewater by mixing or aeration, forming flocs. The mixed liquor is later settled and part of the biomass is recycled.
- Examples: activated sludge process (ASP), aerated lagoons, oxidation ditch, sequencing batch reactor (SBR), anaerobic digesters.
- Needs aeration/mixing energy, a clarifier and sludge return; more sensitive to shock loads; higher operating skill.
- Biomass is measured as MLSS/MLVSS.

#### Attached (fixed-film) growth process
The micro-organisms grow as a **biofilm on a fixed medium** (stones, plastic media, discs) over which wastewater flows.
- Examples: trickling filter, rotating biological contactor (RBC), submerged biofilters.
- Simple, low energy, stable with variable loads; needs a medium and a final clarifier to remove sloughed film; larger area.

| Point | Suspended growth | Attached growth |
|---|---|---|
| Biomass | Floc suspended in liquid | Film on medium |
| Example | ASP | Trickling filter |
| Energy | High (aeration) | Low |
| Shock-load tolerance | Poor | Better |
| Sludge recycle | Required | Not essential (only recirculation of effluent) |
| Operation | Skilled | Simple |

#### Principles of biological treatment
1. **Food and organisms:** organic matter in sewage is food (BOD) for bacteria. They need oxygen (aerobic), nutrients (N, P) and suitable pH (6.5-8.5) and temperature (20-35°C).
2. **Aerobic oxidation** (energy): $\text{organics} + O_2 \xrightarrow{\text{bacteria}} CO_2 + H_2O + \text{energy}$
3. **Synthesis** (growth): $\text{organics} + O_2 + N \rightarrow \text{new cells}$
4. **Endogenous respiration:** when food is short, cells oxidise their own mass: $\text{cells} + O_2 \rightarrow CO_2 + H_2O + NH_3$
5. Colloidal and suspended matter is first adsorbed on the floc/film, then hydrolysed and metabolised.
6. The cells are removed by settling as sludge, which leaves a clear effluent of low BOD.
7. **Anaerobic treatment** (no oxygen) converts organics to methane and CO₂, used for sludge digestion and strong wastes.
8. The process is controlled by the food-to-micro-organism ratio ($F/M$), mean cell residence time, oxygen supply and temperature.

## Check yourself

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

1. What defines a suspended growth process?
   A. Micro-organisms are kept in suspension as flocs by mixing/aeration, with sludge recycled
   B. Micro-organisms grow as a film on fixed media
   C. Only anaerobic bacteria are used
   D. No micro-organisms are used
   Answer: A. Suspended growth keeps biomass floating in the mixed liquor, e.g. activated sludge. (Key point: Suspended growth)

2. Which of these is an attached growth process?
   A. Aerated lagoon
   B. Trickling filter
   C. Oxidation ditch
   D. Activated sludge process
   Answer: B. Trickling filters and RBCs grow a biofilm on a fixed medium. (Key point: Attached growth)

3. Which statement correctly compares the two?
   A. Attached growth needs high aeration energy
   B. Suspended growth needs more energy and sludge return but is more shock sensitive
   C. Suspended growth needs no clarifier
   D. Attached growth is less stable to load variation
   Answer: B. ASP needs aeration and sludge return and is sensitive to shock loads; fixed-film is simpler and more stable. (Key point: Comparison)

4. What is endogenous respiration?
   A. Conversion of organics into new cells
   B. When food is short, cells oxidise their own mass to CO2, H2O and NH3
   C. Adsorption of colloids onto the floc
   D. Methane formation without oxygen
   Answer: B. Starved cells consume their own material, reducing sludge mass. (Key point: Endogenous respiration)

## Related in Sanitary Engineering

- [Biological treatment principles; grit vs sedimentation](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-vs-sedimentation-differences) — Sanitary Engineering, chapter 7
- [Degree of treatment, industrial wastewater](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adegree-treatment-industrial-waste) — Sanitary Engineering, chapter 6
- [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
- [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
- [Treatment processes, impurities removal, settlement](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Atreatment-impurities-sedimentation-factors) — Water Supply Engineering, chapter 6, asked 2×
- [Heat treatment processes of steel](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Aheat-treatment-of-steels) — Material Science, chapter 6
- [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