The discharge of warm water into a river is usually called a thermal pollution. It occurs when an industry removes water from a source, uses the water for cooling purposes and then returns the heated water to
its source. Power plants heat water to convert it into steam, to drive the turbines that generate electricity. For
efficient functioning of the steam turbines, the steam is condensed into water after it leaves the turbines. This
condensation is done by aking water from a water body to absorb the heat. This heated water, which is at least 15oC higher than the normal is discharged back into the water body

EFFECTS:- The warmer temperature decreases the solubility of oxygen and
increases the metabolism of fish. This changes the ecological balance of the river. Within certain limits thermal additions can promote the growth of
certain fish and the fish catch may be high in the vicinity of a power plant.
However sudden changes in temperature caused by periodic plant shutdowns both planned and unintentional can change result in death of these fish that are acclimatized to living in warmer waters. Tropical marine animals are generally unable to withstand a temperature increase of 2 to 30C and
most sponges, mollusks and crustaceans are eliminated at temperatures above 370C. This results in a change in tthe diversity of fauna as only those species that can live in warmer water survive

CONTROL MEASURES:-Thermal pollution can be controlled by passing the heated water through a cooling pond or a cooling tower after it leaves the condenser. The heat is dissipated into the air and the water can then be
discharged into the river or pumped back to the plant for reuse as cooling water.There are several ways in which thermal pollution can be reduced. One method is to construct a large shallow pond. Hot water is pumped into one end of the pond and
cooler water is removed from the other end. The heat gets dissipated from the pond into the atmosphere. A
second method is to use a cooling tower. These structures take up less land area than the ponds. Here
most of the heat transfer occurs through evaporation Here warm waters coming from the condenser is
sprayed downward over vertical sheets or baffles where the water flows in thin films. Cool air enters
the tower through the water inlet that encircles the base of the tower and rises upwards causing evaporative
cooling. A natural draft is maintained because of the density difference between the cool air outside and the warmer air inside the tower. The waste heat is dissipated into the atmosphere about 100 m above the base of the tower. The cooled water is collected at the floor of the tower and recycled back to the power plant condensers. The disadvantage in both
these methods is however that large amounts of water are lost by evaporation.