---
title: "Point and non-point sources; waste water treatment"
subject: "Engineering Chemistry"
chapter: 3
key: "semester_2/engineering_chemistry:point-nonpoint-waste-water"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/point-nonpoint-waste-water
site: BCT Engineering
---

# Point and non-point sources; waste water treatment

Engineering Chemistry · Chapter 3: Environmental Chemistry

- Asked in: 2074 Bhadra, 2+3 marks
- Video: [𝙋𝙤𝙞𝙣𝙩 & 𝙉𝙤𝙣 𝙋𝙤𝙞𝙣𝙩 𝙎𝙤𝙪𝙧𝙘𝙚𝙨 𝙤𝙛 𝙒𝙖𝙩𝙚𝙧 𝙋𝙤𝙡𝙡𝙪𝙩𝙞𝙤𝙣 ! 𝙐𝙂𝘾 𝙉𝙀𝙏 𝙋𝙖𝙥𝙚𝙧 1 𝙗𝙮 𝙎𝙝𝙞𝙫𝙖 𝙎𝙞𝙧](https://www.youtube.com/watch?v=kFkxuW9V0Fg) — Achievers Adda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/point-nonpoint-waste-water
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-3

## Question

What do you mean by point and non point sources of water pollutants? Give an account of primary and secondary processes of waste water treatment to control water pollution.

## Short answer

Point sources discharge at a single point (e.g., factory pipe) and are treatable; non-point sources are diffuse (e.g., agricultural runoff) and hard to control. Primary treatment physically settles solids; secondary treatment uses aerobic microbes to oxidise organic matter, removing 85–95% of BOD.

## Answer

#### Point and non-point sources
- **Point sources** discharge pollutants at a single, identifiable location, usually through a pipe or drain, so they can be monitored and treated. Examples: factory outlets, sewage outfalls, power plant cooling water.
- **Non-point sources** are diffuse, with no single discharge point; pollution enters over a wide area, usually by runoff, and is hard to control. Examples: agricultural runoff (fertilisers, pesticides), urban storm water, atmospheric deposition, soil erosion.

#### Waste water treatment
```text
Waste water → Screening → Grit → PRIMARY → SECONDARY → Disinfection → Discharge
              (preliminary)       settling   biological   (chlorine)
```
**Primary treatment (physical)**
1. Screening and grit removal remove large objects and sand.
2. In the sedimentation tank (clarifier) suspended solids settle as sludge. Alum or lime may be added as coagulant. This removes about 60 % of suspended solids and 30 % of BOD.

**Secondary treatment (biological)**
1. The water from primary treatment is treated with aerobic microorganisms that oxidise dissolved organic matter to CO₂, water and new cells.
2. Methods: **activated sludge process** (air is blown into the tank containing microbe-rich sludge) or **trickling filter** (waste water sprinkled over a bed of stones coated with microbial film).
3. The secondary clarifier then settles the microbial flocs; part of the sludge is returned. BOD removal is 85 to 95 %.
4. Sludge is digested anaerobically in a digester to give biogas (CH₄).

The effluent is then disinfected (chlorine or UV) and discharged. Tertiary treatment (nutrient removal, filtration, activated carbon) may follow when water of high quality is needed.

## Check yourself

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

1. Which is a point source?
   A. A factory outlet or sewage outfall
   B. Agricultural runoff
   C. Urban storm water
   D. Atmospheric deposition
   Answer: A. Point sources discharge at one identifiable location; non-point sources are diffuse. (Key point: Point source)

2. Why are non-point sources hard to control?
   A. They always contain radioactive material
   B. Pollution enters diffusely over a wide area with no single discharge point
   C. They are treated in clarifiers
   D. They are always hotter
   Answer: B. No single outlet exists, so they can't be monitored and treated easily. (Key point: Non-point)

3. What does primary treatment do?
   A. Aerobic microbes oxidise organic matter
   B. Removes all BOD
   C. Physical removal by screening, grit removal and sedimentation of suspended solids (about 60% SS, 30% BOD)
   D. Adds chlorine only
   Answer: C. Primary treatment is physical; biological oxidation is secondary. (Key point: Primary treatment)

4. How does secondary treatment work?
   A. Anaerobic bacteria remove metals
   B. Alum precipitates all organic matter
   C. Screens remove dissolved matter
   D. Aerobic microorganisms oxidise dissolved organic matter (activated sludge or trickling filter), 85 to 95% BOD removal
   Answer: D. Aerobic microbes convert organic matter to CO2, water and new cells. (Key point: Secondary treatment)

## Related in Engineering Chemistry

- [Air pollution, pollutants, sources and remedies](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aair-pollution-sources-remedies) — Engineering Chemistry, chapter 3
- [Water pollution, pollutants and adverse effects](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awater-pollutants-effects) — Engineering Chemistry, chapter 3
- [Water pollutants, sources; oxides making water acidic](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awater-pollutants-acidic) — Engineering Chemistry, chapter 3
- [Water pollutants, harmful effects, control measures](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awater-pollutants-effects-measures) — Engineering Chemistry, chapter 3
- [Sources, effects and remedies of water pollution](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awater-pollution-sources) — Engineering Chemistry, chapter 3, asked 2×
- [Sources and impacts of water pollution](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awater-sources-impacts) — Engineering Chemistry, chapter 3

## Related in other subjects

- [Water pollution: sources and effects](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Awater-pollution-sources-effects) — Environment and Pollution Control, chapter 4
- [Water pollution: sources, causes and impacts](https://www.bctengineer.com/question.md?key=semester_7%2Ftechnology_environment_and_society%3Awater-pollution-sources-impacts) — Technology, Environment and Society, chapter 5
- [Sources and types of water pollution](https://www.bctengineer.com/question.md?key=semester_7%2Ftechnology_environment_and_society%3Awater-pollution-sources-types) — Technology, Environment and Society, chapter 5
- [Classification and sources of air pollutants](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Aair-pollutants-classification-sources) — Environment and Pollution Control, chapter 1