When we talk about pollution, thermal pollution rarely comes to our minds. However, with the innumerable thermal power plants and industries mushrooming everywhere, thermal pollution is a real and persistent problem of current times.

Thermal pollution in the broadest sense can be defined as the abrupt change in ambient temperature of a natural water body by any human induced processes. The increase or decrease of water temperature degrades the quality of water and makes it unfit for consumption or the life-systems of aquatic organisms. It could happen in lakes, rivers, oceans or even ponds.

What are the sources of thermal pollution?

The major sources of thermal pollution are heated waste water produced from production and manufacturing plants – coal fired thermal power plants, natural gas plants, nuclear plants, textiles, paper and pulp industries, etc. These units utilize a huge amount of water as a ‘cooling agent’ in lowering the temperature of machinery such as generators and heat engines. Thus, water acts as an efficient sink for heat waste. The heated water is then released back to the source which is either a river or an ocean in most cases causing a disturbance in the thermal equilibrium.

Removal of vegetation or deforestation is another unpopular but significant source of thermal pollution. Vegetation, particularly trees, keeps the temperature of the water cool by providing shades to ponds, lakes, and rivers etc. In the absence of trees, the water bodies are exposed to more sunlight and absorb heat, which raises the normal temperature of the water. Also, soil erosion leads to a raise in the levels of water and shallowing of lakes and rivers which could increase sunlight concentration and warming of water.

Natural processes such as volcanic eruption or geothermal activities under the ocean or land could also increase thermal pollution. The lava (molten rocks) could lead to a sharp rise in the temperature of water.

Ecological impacts of thermal pollution

Thermal pollution is still widely seen as an insignificant problem when compared to other kinds of pollutions such as carbon emissions or solid waste. Many have contested the importance of production plants and industries over the potential ecological threat posed by thermal pollution.

The most vital requirement for survival of aquatic animals is the ambient temperature. A slight deviation from this temperature could bring a detrimental effect on the living organisms and their surrounding ecology. The following are a few important ecological impacts of thermal pollution-

• Decrease in dissolved oxygen – Oxygen, like in the air is equally essential for aquatic life. The warmer the water the lesser is the rate at which oxygen is dissolved in it. Therefore, a slight change in temperature of water could lead to an anaerobic or oxygen deficient condition thus sabotaging aquatic life. Warm water also increases the rate of organic component decomposition which is an oxygen consuming process thus contributing to depletion of dissolved oxygen.
• Most of the aquatic organisms have evolved in such a way that the enzyme systems are functional in a very narrow range of temperature called ‘stenothermic organisms’. A small decrease or increase of temperature may bring a ‘thermal shock’ which sometimes can lead to mass killings of fishes, plants, insects etc (Pollution Issues, 2017). Organisms such as mollusc, sponges, and crustaceans are eliminated easily at temperatures above 37° C (Techgape, 2017).
• On the other hand, there may be sudden increase in metabolism of certain species and impetuous rise in numbers. This disrupts the stability of food chains and alters the species composition in the environment. Thermal pollution is also known to have an adverse affect on the reproductive system of aquatic animals (Conserve Energy Future, 2017).
• Abrupt mass migration of species is another severe problem associated with thermal pollution. The change in temperature may force species to migrate to another habitat creating a hollow food web.
• Corals are highly susceptible to temperature anomaly therefore thermal pollution from run-offs and effluents causes bleaching e.i the expulsion of its symbiont algae and subsequent death of corals.

As an example, it is estimated that, about 46 million adult fish, 2.4 billion eggs, larvae and young fish are killed annually by the effluents from the First Energy Bay Shore Plant on the Maumee River shoreline near Toledo, Ohio. Also, the Monroe Power Plant in Michigan reportedly kills more than 25 million fish each year (Sourcewatch.org, 2017).

Control of Thermal Pollution

A few scientific techniques that could be employed by any industrial units for controlling thermal pollution are – creation of cooling ponds, artificial lakes, and cooling towers. Cooling ponds is one of the simplest and cheapest methods where the heated waste water is stored in a pond before releasing it into the mainstream waterways. It gives enough time to dissipate the heat into the atmosphere through the process of evaporation. Likewise, cooling towers are installed in chemical processing power plants, steel mills, and other manufacturing processes where cooling of the effluent is required before discharge. It involves upward recirculation of a cascade of heated waste water through the towers into the air by evaporative processes. However, these methods are found to be effectively operating in small scale industries limited to the developed nations. Also, change in local meteorological conditions is a concern in cooling towers as large amounts of water vapour enters the atmosphere.

The role of government in controlling thermal pollution is very crucial in developing countries such as India concerning legal and abatement policy frameworks for wastewater treatment. This could be realised through stringent regulations and constant monitoring of effluent discharges from different industrial sectors.