An increase in the optimum water temperature by industrial process (steel factories, electric power houses and atomic power plants) may be called as “Thermal Pollution.” Many industries generate their own power and use water to cool their generator.
This hot water is released into the system from where it was drawn, causing a warming trend of surface water. If the system is poorly flushed, a permanent increase in the temperature may result. However, if the water is released into the well flushed system, permanent increase in temperature does not occur.
Many organisms are killed instantly by the hot water resulting into a high mortality. It may bring other disturbance in the ecosystem. The egg of fish may hatch early or fail to hatch at all. It may change the diurnal and seasonal behaviour and metabolic responses of organisms. It may lead to unplanned migration of aquatic animals.
Macro-phytic population may also be changed. As temperature is an important limiting factor, serious changes may be brought about even by a slight increase in temperature in a population. For minimising thermal pollution, hot water should be cooled before release from factories and removal of forest canopies and irrigation return flows should be prohibited.
Causes or Sources of Thermal Pollution:
The various causes of thermal pollution are as follows:
(1) Coal-fired Power Plants:
Some thermal power plants use coal as fuel. Coal-fired power plants constitute the major source of the thermal pollution.
(2) Industrial Effluents:
Industries generating electricity require large amount of Cooling water for heat removal. Other industries like textile, paper, and pulp and sugar industry also release heat in water, but to a lesser extent.
(3) Nuclear Power Plants:
Nuclear power plants emit a large amount of unutilized heat and traces of toxic radio nuclear into nearby water streams. Emissions from nuclear reactors and processing installations are also responsible for increasing the temperature of water bodies.
(4) Hydro Electric Power:
Generation of hydro-electric power also results in negative thermal loading of water bodies.
(5) Domestic Sewage:
Domestic sewage is often discharged into rivers, lakes, canals or streams without waste treatment. The municipal water sewage normally has a higher temperature than receiving water. With the increase in temperature of the receiving water the dissolved oxygen content (DO) decreases and the demand of oxygen increases and anaerobic conditions occur.
Control of Thermal Pollution:
Control of thermal pollution is necessary as its detrimental effects on aquatic ecosystem may be detrimental in the future. Viable solutions to chronic thermal discharge into water bodies are as follows:
(1) Cooling Ponds:
Cooling ponds or reservoirs constitute the simplest method of controlling thermal discharges. Heated effluents on the surface of water in cooling ponds maximize dissipation of heat to the atmosphere and minimize the water area and volume. This is the simplest and cheapest method which cools the water to a considerable low temperature. However, the technique alone is less desirable and inefficient in terms of air-water contact.
(2) Cooling Towers:
Using water from water sources for cooling purposes, with subsequent return to the water body after passing through the condenser is termed as cooling process. In order to make the cooling process more effective, cooling towers are designed to control the temperature of water. In-fact, cooling towers are used to dissipate the recovered waste heat so as to eliminate the problems of thermal pollution.
(3) Artificial Lake:
Artificial lakes are man-made bodies of water which offer possible alternative to once through cooling. The heated effluents may be discharged into the lake at one end and the water for cooling purposes may be withdrawn from the other end. The heat is eventually dissipated through evaporation.
These lakes have to be rejuvenated continuously. A number of methods have been suggested and developed for converting the thermal effluents from power plants into useful heat resources for maximing the benefits.
Some of the potential physical applications for thermal discharge (rejected heat) of power plants are:
- Industrial and space heating.
- Biological applications such as soil warming.
- Fish culture, livestock shelters and for heating greenhouses.
Most of these potential physical applications are of colder regions or locations.