---
title: "Types of impurities in water"
subject: "Water Supply Engineering"
chapter: 4
key: "semester_5/water_supply_engineering:types-of-impurities"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/types-of-impurities
site: BCT Engineering
---

# Types of impurities in water

Water Supply Engineering · Chapter 4: Quality of Water

- Asked in: 2080 Bhadra, 3 marks; 2079 Bhadra, 2 marks; 2075 Chaitra, 4 marks (asked 3 times)
- Video: [33. Impurities in Drinking water || Water supply Engineering](https://www.youtube.com/watch?v=YaDl8qR42Jo) — NITIANS ACADEMY
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/types-of-impurities
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-4

## Question

Define impurities in water and describe briefly the types of impurities present in water (according to their characteristics and state).

## Short answer

Impurities are substances in water other than H2O, classified by size as suspended (above 1 micron), colloidal (1 nm to 1 micron), and dissolved (below 1 nm). By characteristics they are physical, chemical, and biological, and also organic or inorganic. Removal methods include screening, sedimentation, coagulation, filtration, aeration, and special treatment.

## Answer

**Impurities** are the substances (physical, chemical or biological) present in water other than H2O molecules. Natural water picks up impurities from the atmosphere, the ground, and human and animal activity.

#### According to state (size)
| Type | Size | Examples | Effects | Removal |
|---|---|---|---|---|
| Suspended | Above 1 micron (0.001 mm) | Sand, silt, clay, algae, protozoa, bacteria | Turbidity, disease, silting | Screening, sedimentation, filtration |
| Colloidal | 1 nm to 1 micron | Fine clay, silica, colour-causing humic matter, viruses | Colour, turbidity, does not settle | Coagulation, flocculation, filtration |
| Dissolved | Below 1 nm (ionic or molecular) | Salts of Ca, Mg, Na, Fe, Mn; gases (CO2, H2S, O2); organic matter | Hardness, taste, odour, toxicity | Softening, aeration, ion exchange, adsorption, special treatment |

#### According to characteristics
- **Physical:** turbidity, colour, taste, odour, temperature.
- **Chemical:** pH, hardness, alkalinity, chloride, sulphate, iron, manganese, nitrate, fluoride, arsenic, dissolved gases, toxic metals.
- **Biological:** bacteria, viruses, protozoa, algae, helminths and other living organisms, some of which cause disease.

Impurities are also grouped as **organic** (plant, animal and sewage matter) and **inorganic** (minerals and salts).

## Check yourself

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

1. Impurities smaller than 1 nm, present as ions or molecules, are:
   A. suspended
   B. colloidal
   C. dissolved
   D. settleable
   Answer: C. Dissolved impurities are below 1 nm. (Key point: Size classes)

2. Colloidal impurities that do not settle by gravity are removed mainly by:
   A. coagulation and flocculation
   B. screening
   C. simple aeration
   D. boiling
   Answer: A. Their small size prevents settling, so coagulation is needed first. (Key point: Colloidal removal)

3. Hardness, pH and alkalinity belong to which characteristic group?
   A. Physical
   B. Chemical
   C. Biological
   D. Radiological
   Answer: B. Turbidity, colour, taste, odour are physical; bacteria, viruses are biological. (Key point: Characteristics)

4. Impurities are:
   A. only disease-causing bacteria
   B. only suspended solids
   C. only dissolved gases
   D. substances other than H2O molecules present in water
   Answer: D. They may be physical, chemical or biological. (Key point: Definition)

## Related in Water Supply Engineering

- [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×
- [Slow sand filter, 15000 present population](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Assf-c76) — Water Supply Engineering, chapter 6
- [Wholesome water and its requirements](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Awholesome-water-requirements) — Water Supply Engineering, chapter 1, asked 2×
- [Living organisms in water; health effects](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aliving-organisms-in-water) — Water Supply Engineering, chapter 4

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