---
title: "Biological treatment principles; grit vs sedimentation"
subject: "Sanitary Engineering"
chapter: 7
key: "semester_6/sanitary_engineering:grit-vs-sedimentation-differences"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/grit-vs-sedimentation-differences
site: BCT Engineering
---

# Biological treatment principles; grit vs sedimentation

Sanitary Engineering · Chapter 7: Wastewater Treatment

- Asked in: 2079 Chaitra, 4+4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/grit-vs-sedimentation-differences
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7

## Question

Explain briefly the principles of biological wastewater treatment. Enlist the differences between grit chamber and sedimentation tank.

## Short answer

Biological treatment uses microorganisms (mainly bacteria) to oxidize organic matter (BOD) and produce new cells under aerobic conditions, controlled by F\/ M, sludge age and oxygen. Grit chambers remove heavy inorganic grit (specific gravity2.65, size >0.15 mm) with high flow velocity (0.2–0.3 m\/ s) and short detention (30–60 s), whereas sedimentation tanks settle fine organic solids (gravity ~1.2) at very low velocity (~0.005 m\/ s) over2–3 hours.

## Answer

#### Principles of biological wastewater treatment

Biological treatment removes dissolved and colloidal organic matter by the metabolic action of micro-organisms, mainly bacteria.

- **Food and organisms:** the organic matter (BOD) of sewage is the food for the bacteria. They need oxygen (aerobic process), nutrients (N and P), pH of 6.5-8.5 and temperature of 20-35°C.
- **Oxidation (energy):** $\text{organics} + O_2 \rightarrow CO_2 + H_2O + \text{energy}$
- **Synthesis (growth):** $\text{organics} + O_2 + N \rightarrow \text{new cells}$
- **Endogenous respiration:** when food runs short the cells oxidise their own mass, $\text{cells} + O_2 \rightarrow CO_2 + H_2O + NH_3$.
- The new cells are removed as sludge by settling, leaving a clear effluent with low BOD.
- **Anaerobic treatment** (no oxygen) gives methane and CO₂ and is used for sludge digestion and strong wastes.
- The process is operated as **suspended growth** (activated sludge) or **attached growth** (trickling filter, RBC), controlled by the $F/M$ ratio, sludge age, oxygen supply and temperature.

#### Differences between grit chamber and sedimentation tank

| Point | Grit chamber | Sedimentation tank |
|---|---|---|
| Purpose | Removes heavy inorganic grit (sand, gravel, cinders) | Removes settleable organic and fine suspended solids |
| Particle size / $G$ | About 0.15 mm and above, $G\approx 2.65$ | Fine particles, $G\approx 1.2$ or less |
| Position | Before the sedimentation tank (after screens) | After the grit chamber |
| Flow velocity | About 0.2-0.3 m/s, kept constant | Very low, about 0.3 m/min (about 0.005 m/s) |
| Detention time | 30-60 seconds | 2-3 hours (primary) |
| Size | Small, narrow, long channel | Large rectangular or circular tank |
| Velocity control | Needed (proportional weir or Parshall flume) | Not critical |
| Sludge | Dry, inert grit; removed by hand or scraper | Wet putrescible sludge (about 95% water) needing digestion |
| Surface loading | High, 800-1200 m³/m²/day | Low, 25-40 m³/m²/day |

## Check yourself

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

1. In biological treatment, what acts as food for the bacteria?
   A. Dissolved oxygen
   B. Settled grit
   C. The organic matter (BOD) of the sewage
   D. Chlorine
   Answer: C. Micro-organisms feed on the organic matter, measured as BOD, and oxidise it. (Key point: Food for bacteria)

2. What is the difference between oxidation and synthesis in aerobic treatment?
   A. Oxidation makes new cells; synthesis gives CO2 and energy
   B. Oxidation breaks organics to CO2, water and energy; synthesis builds new cells
   C. Both only produce methane
   D. Oxidation occurs without oxygen
   Answer: B. Part of the organics is oxidised for energy, and part is used with N to form new cells. (Key point: Oxidation versus synthesis)

3. Which comparison between grit chamber and sedimentation tank is correct?
   A. Grit chamber: 2-3 h detention; sedimentation tank: 30-60 s
   B. Both have a flow velocity of 0.2-0.3 m/s
   C. Grit chamber is placed after the sedimentation tank
   D. Grit chamber: about 0.2-0.3 m/s and 30-60 s; sedimentation tank: very low velocity and 2-3 h
   Answer: D. A grit chamber is fast and short, so organics stay in suspension; a sedimentation tank is slow and long to settle fine organic solids. (Key point: Flow velocity and detention contrast)

4. How does the sludge differ between the two units?
   A. Grit is dry inert material; sedimentation sludge is wet, putrescible and needs digestion
   B. Both give dry inert sludge
   C. Grit is putrescible; sedimentation sludge is inert sand
   D. Grit sludge is about 95% water
   Answer: A. Grit is heavy inorganic matter, while primary sludge is organic and about 95% water. (Key point: Nature of sludge)

## Related in Sanitary Engineering

- [Skimming tank; grit vs sedimentation tank](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Askimming-tank-differences) — Sanitary Engineering, chapter 7
- [Grit chamber for 190 l/s](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-190-ls) — Sanitary Engineering, chapter 7, asked 2×
- [Rectangular grit chamber for 10 MLD](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-10mld-0p2mm) — Sanitary Engineering, chapter 7
- [Grit chamber for 180 l/s, SOR 1.5](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-180-ls-sor-1p5) — Sanitary Engineering, chapter 7
- [Grit chamber: purpose, construction, design](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-chamber-purpose-design) — Sanitary Engineering, chapter 7, asked 4×
- [Grit chamber for 60 MLD, 0.23 mm](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-60mld-0p23) — Sanitary Engineering, chapter 7, asked 2×

## Related in other subjects

- [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
- [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
- [Rectangular sedimentation tank, 9 million L peak demand](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-c75) — Water Supply Engineering, chapter 6
- [Rectangular sedimentation tank, 20000 population, 0.4 mm/s](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-s73) — Water Supply Engineering, chapter 6