Maarg Manthan · Topic 11.3

Measurement of Water Pollution

Environment & Ecology › Water Resources and Water Pollution · Topic 11.3

Water pollution is measured through physical, chemical and biological indicators. The key chemical measures are dissolved oxygen (DO), the oxygen dissolved in water that fish and other aquatic life need, with clean water usually above 5 to 6 mg per litre; biochemical oxygen demand (BOD), the oxygen consumed by micro-organisms to break down organic matter in water over five days at 20 degrees Celsius, where a higher value means more organic pollution; and chemical oxygen demand (COD), the oxygen needed to chemically oxidise all matter, usually higher than BOD. Other indicators are pH, turbidity, total dissolved solids (TDS), total suspended solids, conductivity, hardness, nitrates and phosphates, heavy metals and pesticides, and coliform bacteria, including faecal coliform and E. coli, which signal contamination by human or animal waste. The Central Pollution Control Board classifies surface waters into designated best use classes A to E by DO, BOD, pH and coliform count, and India runs the National Water Quality Monitoring Programme.

5 min read · UPSC Prelims · Environment & Ecology · Water Resources and Water Pollution

At a Glance

DO Dissolved oxygen, higher is better
BOD Oxygen used in 5 days at 20 C
COD Chemical oxygen need
Bacteria marker Coliform
Classes A to E
Exam link UPSC Prelims, Environment & Ecology: Water Resources and Water Pollution

Where Does It Fit?

This topic follows the sources of water pollution and explains how pollution is measured. These indicators are used in the next topics on harmful effects, eutrophication and contaminants. Previous topic: Sources and Causes of Water Pollution. Next topic: Biomagnification Ecosystem Fish kills Infant risk Blue baby syndrome…">Harmful Effects of Water Pollution.

Which Indicators Measure Water Quality?

Indicator	Meaning and use
Dissolved oxygen (DO)	Oxygen dissolved in water; clean water has high DO; low DO harms fish; falls with warmth and organic pollution
Biochemical oxygen demand (BOD)	Oxygen used by microbes to decompose organic matter in 5 days at 20 degrees C; high BOD means heavy organic pollution
Chemical oxygen demand (COD)	Oxygen needed to oxidise both biodegradable and non-biodegradable matter chemically; COD is always higher than or equal to BOD
pH	Acidity or alkalinity; most natural waters are around 6.5 to 8.5
Turbidity	Cloudiness from suspended particles, measured with a turbidity meter or Secchi disc
Total dissolved solids (TDS)	Dissolved salts and minerals; high TDS affects taste and use
Coliform bacteria	Indicator organisms such as faecal coliform and E. coli that show faecal contamination

How Are Water Bodies Classified in India?

Class A: drinking water source without conventional treatment but with disinfection; DO 6 mg per litre or more, BOD 2 or less, total coliform 50 MPN per 100 ml or less
Class B: outdoor bathing; DO 5 or more, BOD 3 or less, total coliform 500 or less
Class C: drinking source with conventional treatment and disinfection; DO 4 or more, BOD 3 or less, coliform 5,000 or less
Class D: wildlife and fisheries; DO 4 or more, pH 6.5 to 8.5
Class E: irrigation, industrial cooling and controlled waste disposal
Water Quality Index: combines several parameters into a single score from good to very poor
Monitoring: National Water Quality Monitoring Programme by CPCB and states
Real-time stations: online monitoring on major rivers and industrial outlets

What Else Should You Remember?

Saturation of DO: water at 25 degrees C can hold roughly 8 mg per litre, and less in warm water
BOD test: a water sample is incubated in the dark for five days at 20 degrees C, and oxygen loss is measured
BOD limits: Indian standard of 3 mg per litre or less for bathing; sewage can exceed 200 mg per litre
COD test: uses a strong oxidising chemical such as potassium dichromate and takes a few hours
BOD COD ratio: a high BOD to COD ratio means mostly biodegradable waste
Coliform: most probable number per 100 ml expresses the count
Faecal coliform: shows contamination from warm-blooded animals
Total suspended solids: particles in water that can be filtered out
Conductivity: a quick measure of dissolved salts
Hardness: calcium and magnesium content
Alkalinity: buffering capacity of water against pH change
Nitrate: BIS limit of 45 mg per litre in drinking water
Fluoride: BIS acceptable limit of 1.0 mg per litre and permissible 1.5
Heavy metals: measured by atomic absorption and similar methods
Biological indicators: presence of sensitive insects such as mayfly and stonefly larvae indicates clean water, while sludge worms and bloodworms indicate heavy pollution
Indicator species: organisms whose presence or absence shows water quality
Secchi disc: a black and white disc lowered to measure water clarity
BIS 10500: Indian standard for drinking water quality
WHO guidelines: global guidance on drinking water quality
Remote sensing: satellites track chlorophyll, turbidity and algal blooms
Biomonitoring: uses living organisms to track water health over time
CPCB reports: list polluted river stretches by BOD priority classes
Link to previous: see Sources and Causes of Water Pollution
Link to next: see Harmful Effects of Water Pollution
Link to indicator species: see Endemic Species, Keystone Species, Indicator Species, Invasive Species
Caution: limits differ by standard and year, so quote with the standard name
Exam hint: remember BOD at five days and 20 C and that high BOD means more pollution

Why Does It Matter for the Exam?

Higher is better: DO
Higher is worse: BOD, COD
Bacterial marker: Coliform
Current affairs link: River water quality reports
Common errors: treating high BOD as good water

As an analytical point, no single number captures water quality. Using DO, BOD, coliform and toxic contaminants together gives a fair picture, and real-time monitoring helps enforcement more than annual reports.

Why Is This Topic Important?

Oxygen DO, BOD
Germs Coliform
Class A to E

What Came Next?

The next topic covers the harmful effects of water pollution. Next topic: Harmful Effects of Water Pollution.

Key People and Terms

DO Dissolved oxygen
BOD Biochemical oxygen demand
COD Chemical oxygen demand
TDS Total dissolved solids
MPN Most probable number
WQI Water Quality Index

Exam Corner

Points to Remember

BOD: 5 days at 20 C.
High BOD = more pollution.
High DO = cleaner water.
COD is at least BOD.
CPCB classes: A to E.

Do Not Confuse With

BOD and COD Biological versus chemical oxygen demand
DO and BOD Available oxygen versus oxygen needed
Class A and Class C Disinfection only versus conventional treatment

Memory Hook

DO up is good, BOD up is bad, coliform tells germs.

Mains Angle

Questions ask for indicators and their meaning. These points help.

Indicators DO, BOD
Classes A to E
Use Monitoring

A Question You May Face

An original practice question, not a past paper question.

“Explain the indicators used to measure water pollution. How are surface waters classified in India?”

How to Answer

Indicators DO, BOD.
Meaning High or low.
Classes A to E.
Use Monitor.

GS Relevance

Prelims: BOD, DO, COD, coliform, classes. GS Paper 3: pollution monitoring and standards.

Frequently Asked Questions

What is BOD in water quality?

Biochemical oxygen demand is the amount of dissolved oxygen needed by micro-organisms to break down organic matter in a water sample, measured over five days at 20 degrees Celsius. A higher BOD means more organic pollution, such as sewage, and less oxygen left for fish, while clean water has a low BOD.

What is the difference between BOD and COD?

BOD measures oxygen used by living microbes to decompose biodegradable matter over five days. COD measures the oxygen needed to chemically oxidise all organic matter and some inorganic matter, using strong chemicals, in a few hours. COD is normally higher than or equal to BOD, since it also counts non-biodegradable matter.

What does dissolved oxygen tell us?

Dissolved oxygen shows how much oxygen is available in water for aquatic life. Healthy waters usually have DO above 5 to 6 milligrams per litre. Low DO signals pollution by organic matter, warm water or algal growth, and can cause fish kills when it falls below about 3 to 4 milligrams per litre.

Why are coliform bacteria used as indicators?

Coliform bacteria, especially faecal coliform and E. coli, live in the intestines of humans and animals, so their presence shows that water has been contaminated by faeces and may hold disease-causing germs. Testing for coliform is cheaper and easier than testing for each pathogen, so it is a standard measure.

How does the CPCB classify surface water?

The Central Pollution Control Board classifies surface water into classes A to E by its designated best use: A for drinking after disinfection, B for outdoor bathing, C for drinking after conventional treatment, D for wildlife and fisheries, and E for irrigation, industrial cooling and waste disposal, with limits for DO, BOD, pH and coliform.

PYQ Practice — Statement Analysis

1 A higher BOD indicates greater organic pollution.

True

More oxygen is used.

2 BOD is measured over 10 days.

False

Over 5 days at 20 C.

3 Coliform bacteria indicate faecal contamination.