Self-purification of waters - how a river copes on its own
A river or a lake is not passive in the face of pollution - it has a natural capacity for self-purification, that is, the gradual removal or transformation of substances that reach it. Understanding these processes helps you interpret results and appreciate why protecting water is not only “do not pollute” but also “do not overload”.
Three groups of processes
- Physical - dilution, sedimentation (the settling of suspension), sorption, aeration on riffles.
- Chemical - oxidation and other reactions transforming pollutants.
- Biological - the decomposition of organic matter by micro-organisms and the uptake of nutrients by algae and plants. This is usually the most important mechanism.
What it looks like along a watercourse
Self-purification is a process stretched out in space: what the measurements show depends on how far below the load the station stands. The classic description is the self-purification zones with the oxygen sag curve:
| Zone below the discharge | What dominates | Oxygen | What the measurements show |
|---|---|---|---|
| Dilution | mixing of the load, settling of suspension | starts to fall | jump in conductivity and turbidity, rise in BOD₅ |
| Active decomposition | bacterial decomposition of organic matter | minimum - the deficit is greatest | lowest oxygen, high BOD₅, ammonium |
| Recovery | aeration already exceeds consumption; ammonium oxidises to nitrate | rises | oxygen returns, ammonium falls in favour of nitrate |
| Clean water | a state close to the reach upstream | full saturation | indicators return to catchment background |
⚠️ The oxygen minimum does not lie at the point of discharge but below it - sometimes a few, sometimes more than a dozen kilometres further, depending on flow and temperature. A station set just below a discharge can therefore show water in a better state than a station half a day’s flow away. This is the most common interpretation error when planning measurement points.
What it depends on
The efficiency of self-purification rises when the water is well oxygenated (the decomposition of matter uses up oxygen!), warmer (up to certain limits, it speeds up biological processes) and when there is enough time (flow, the length of the reach). That is why regulated, oxygen-poor watercourses cope worse.
The limits of this capacity
This is crucial: self-purification has a limited capacity. Under too large a load - intensive decomposition exhausts the oxygen, deficits and an oxygen crash appear, and an excess of nutrients drives eutrophication. In other words: water will “clean up” after moderate pollution, but overloaded - it will collapse.
Why know this
- Interpretation: an improvement in indicators downstream can be the effect of self-purification, not the absence of a pollution source.
- Protection: the point is not to exceed the natural capacity of the waters.
Sources
- The oxygen sag curve (the oxygen deficit below an organic load as the balance of consumption and reaeration) - the classic model of Streeter and Phelps (1925), still the basis for describing self-purification in sanitary engineering textbooks.
- Self-purification zones and the communities of organisms that accompany them are described by the saprobic system (Kolkwitz and Marsson, 1909) - in today’s monitoring replaced by benthic macroinvertebrate indices, but the zoning itself has remained.
In practice
Self-purification is best seen in a series of measurements along a watercourse (above and below a source) and over time. In LimnoLog you can run several stations in one project and compare how, for example, the oxygen or the load changes between points.
See it in the LimnoLog app
Explore LimnoLog features