In an aquatic system temperature is an important factor which affects physico-chemical parameters of water quality as well as plant, animal and microbial life considerably.
Aquatic life is usually well adjusted to diurnal and seasonal changes in temperatures which occur naturally. However, man-made changes in temperature of water often cause adverse changes which damage its utility and productivity.
Important Sources of Surplus Heat:
Important sources of surplus heat which is ultimately discharged into the environment are:
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1. Thermal or nuclear power plants.
2. Industrial effluents.
3. Sewage effluents and other waters.
4. Bio-chemical activity.
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The most important source of surplus heat are thermal or nuclear power generation plants which are responsible for contributing almost 70-80% of discharge of heated waters to aquatic bodies. For example, of the total amount of heat received by river Thames, England, 75% comes from Power plants, about 6% from industrial effluents, about 15% from sewage effluents and other heated waters and only 4% is contributed by biochemical activity within the system.
About 70% of the total heat produced in a thermal power station is discharged as waste heat. This becomes more significant when we look at the total amount of coal consumed for power generation which is almost 60% or more of the total global coal production of about 2730 million metric tons. Nuclear power plants discharge a little more heat than those of coal fired power plants, producing an equivalent amount of electricity.
A large fraction of this heat goes to the cooling waters drawn from some aquatic body and returned to the system as heated waters. In India about 262.7 million metric tons of coal was produced in the year 1994, out of which about 170 million metric tons were used to produce electricity.
Most of the sewage effluents and waste water, on an average, have temperatures 4°-8°C higher than the aquatic bodies in which they are discharged. Heated waters from thermal or nuclear power plants are, however, more injurious as they have temperatures about 10°-14°C higher than the temperatures of the receiving waters. The enormous volume of these heated waters causes significant changes in temperatures and temperature-induced changes in the aquatic system.