Thermal Pollution

What is thermal pollution?

Thermal pollution is the degradation of water quality by any process that changes ambient water temperature.

Thermal pollution is usually associated with increases of water temperatures in a stream, lake, or ocean due to the discharge of heated water from industrial processes, such as the generation of electricity.

There are also situations in which the effects of colder-than-normal water temperatures may be observed. For example, the discharge of cold bottom water from deep-water reservoirs.

Sources of thermal pollution

The production of energy from a fuel source can be direct, such as the burning of wood in a fireplace to create heat, or by the conversion of heat energy into mechanical energy by the use of a heat engine. Thermal power generation (using oil, gas, peat, coal or nuclear fission to heat and steam to drive turbines) requires large volumes of cooling water to remove waste heat. The waste heat is transferred to cooling water obtained from local water bodies such as a river, lake, or ocean.

Heat engines can only convert 30 to 50 percent of the available input energy in the fuel source into mechanical energy, and the highest efficiencies are obtained when the input temperature is as high as possible and the sink temperature is as low as possible.

Water is a very efficient and economical sink for heat engines and it is commonly used in electrical generating stations.

Effects of thermal pollution on the marine environment

Natural sea surface temperatures vary widely; ranging from -2°C to 30°C in open oceans and -2°C to 43°C in coastal waters. Diurnal (day-night) fluctuations at the sea surface are rarely more than 1°C, but can be as much as 2°C in shallow seas.
Most thermal effluents are discharged into coastal waters, where physical and biological effects are likely to be greater. The temperatures of thermal discharges from power stations are typically 8-12°C higher than the natural ambient water temperature, though at some sites, particularly nuclear power stations, temperature rises can exceed 15°C. In some tropical coastal waters the temperature of the waste water has reached 42°C, though 35-38°C is more typical. The temperature fluctuations associated with this, is clearly far greater than natural temperature fluctuations.

Once discharged into the sea a typical thermal effluent will spread and form a three-dimensional layer with the temperature decreasing with distance from the outfall. This effect is known as a thermal plume. The behaviour and size of the plume depends on

  • the design and siting of the outfall,
  • the tidal currents,
  • the degree of exposure,
  • and the volume and temperature of the waste water.