Thermal Pollution

A power plant boils water to make steam that spins a turbine, and that steam has to be cooled back into liquid afterward. The cheap way to cool it is to pull river water into the plant, run it past the hot condenser so it absorbs the waste heat, then pump it back out into the river downstream. No chemicals get added. The water that returns is the same water, just hotter. That is thermal pollution: a physical change in temperature, not a chemical contaminant.

Some plants skip the river return and use a cooling tower instead. Inside the tower the hot water is sprayed or trickled over a large surface while air flows through. Some of the water evaporates, and evaporation carries heat away as vapor, so the remaining water cools before it recirculates. This dumps waste heat into the atmosphere rather than concentrating it in the river, which is why cooling towers reduce thermal pollution even though they lose some water to evaporation.

The reason warm clean water still kills fish connects directly to the dissolved oxygen and dose-response ideas in this unit. Gas solubility drops as temperature rises, so warmer river water simply cannot hold as much dissolved oxygen as cold water. At the same time, heat speeds up fish metabolism, so their oxygen demand climbs. Supply falls while demand rises, and fish downstream of a thermal outfall can suffocate in water that passes every chemical purity test, which is why temperature itself counts as a pollutant alongside the toxins covered elsewhere in this unit.