Yamuna & water
What is BOD?
Biochemical oxygen demand is the amount of oxygen that micro-organisms will consume while breaking down the organic matter in a water sample, measured over five days at 20°C and reported in milligrams per litre. It is the standard proxy for organic pollution — mostly sewage. India's bathing-water criterion is less than 3.0 mg/L. Delhi's Yamuna stations report maxima between 46 and 72 mg/L.
A measurement that takes five days
BOD is not read off an instrument. It is the difference between two readings taken five days apart.
The test is deliberately simple. A sample is sealed, its dissolved oxygen measured, and it is incubated in the dark at 20°C for five days. The oxygen is measured again. The difference is the BOD — how much oxygen the biology in that sample needed. The five-day convention is written BOD₅ and it is what Indian criteria are set against.
It measures demand, not a substance. Sewage, food waste, dairy effluent and slaughterhouse discharge all produce a high BOD; so does a dead algal bloom. What they have in common is organic carbon that bacteria will consume, and the oxygen consumed is taken out of the river.
That is why BOD and dissolved oxygen are two readings of one process. A stretch with BOD in the tens of milligrams per litre will not hold oxygen, because the demand exceeds anything the surface can resupply.
- BOD₅
- The five-day test at 20°C. The convention behind every Indian BOD criterion.
- COD
- Chemical oxygen demand — a chemical oxidation test that measures everything oxidisable, not just what bacteria will eat. Always higher than BOD, and not interchangeable with it.
- Organic load
- Concentration multiplied by flow. Two stretches with the same BOD can carry very different loads.
What the law asks for, and what is measured
The criterion is a single figure. The measurements below are CPCB's own published maxima for 2025.
BOD, bathing water
< 3.0 mg/L
PWQC, Environment (Protection) Rules 1986
Yamunotri, at source — maximum
1 mg/L
CPCB NWMP 2025
Palla, entering Delhi — maximum
6 mg/L
CPCB NWMP 2025
ITO Bridge — maximum
70 mg/L
CPCB NWMP 2025
Okhla, after the Shahdara drain — maximum
72 mg/L
CPCB NWMP 2025
The criterion is under 3.0. The last figure in that table is over twenty times it. Both numbers are published by the same agency in the same document, which is the whole reason a reading is worth printing against its limit rather than on its own.
How to read a BOD figure
It is a five-day laboratory result, and it inherits every limitation of one.
A high BOD is evidence of organic pollution and is not evidence of what produced it. Domestic sewage and food-industry effluent are indistinguishable in this number.
Grab samples are points in time. CPCB publishes a minimum and a maximum per station per year rather than a mean, which is more honest than an average would be — but neither figure tells you how often the river was at either.
Concentration is not load. A stretch where most of the flow has been abstracted can show a very high BOD while carrying less total organic matter than a larger, cleaner-looking river downstream. On the Delhi Yamuna both are bad, but the distinction matters when comparing cities.
What this number cannot tell you
- It cannot tell you about anything inorganic — metals, nutrients, salts and pharmaceuticals are invisible to it.
- It cannot be compared with COD. They measure different things and the ratio between them varies by effluent.
- It cannot be measured continuously. There is no live BOD feed anywhere, for anyone: the test takes five days.
- A single maximum does not describe a year. It describes the worst sample taken.
Where this comes from
Every figure above is read out of one of these, at the address printed beside it.
- Water quality data of river Yamuna, 2025 Central Pollution Control Board, National Water Quality Monitoring Programme · Every station figure above
- Primary Water Quality Criteria for bathing waters Central Pollution Control Board · The BOD criterion and the designated-best-use classes
Reuse freely — CC BY 4.0. The grant covers this page. Each source above keeps its own terms, which is why every one of them is named.
Next
The reading this explains, the explanations next to it, and the one place to stand in it.
compared with other rivers? A national ranking of 47 rivers — and the coverage hole that stops it seeing the worst stretch of this one. Stand in it Yamuna Yatra A river that reads 1 mg/L at its source and 72 downstream of a drain, in one continuous journey.