Thermal pollution is a real and persistent problem in our modern society. Thermal Pollution may be defined as ‘the excessive raising or lowering of water temperature above or below normal seasonal ranges in streams, lakes, or estuaries or oceans as the result of discharge of hot or cold effluents into such water’. Thermal Pollution is harmful because its increase water temperature in streams, rivers, lakes, or occasionally, coastal ocean waters. Thermal pollution is caused by either dumping hot water from factories and power plants or removing trees and vegetation that shade streams, permitting sunlight to raise the temperature of these waters. Like other forms of water pollution, thermal pollution is widespread, affecting many lakes and vast numbers of streams and rivers in the United States and other parts of the world. A temperature increase as small as 1 or 2 Celsius degrees (about 2 to 4 Fahrenheit degrees) can kill native fish and plants or drive them out in favor of other species, often with undesirable effects.

Definition :-

Pollution, the idea of thermal pollution often doesn’t come to mind. People will first think of things like carbon emissions, personal pollution and waste, and a variety of other changing factors.
However, thermal pollution is a real and persistent problem in our modern society. In layman’s terms, thermal pollution is when an industry or other human-made organization takes in water from a natural source and either cools it down or heats it up. They then eject that water back into the natural resource, which changes the oxygen levels and can have disastrous effects on local ecosystems and communities.
Thermal pollution is defined as a sudden increase or decrease in temperature of a natural body of water, which may be ocean, lake, river or pond by human influence. This normally occurs when a plant or facility takes in water from a natural resource and puts it back with an altered temperature. Usually, these facilities use it as a cooling method for their machinery or to help better produce their products.
Plants that produce different products or wastewater facilities are often the culprits of this massive exodus of thermal pollution. In order to properly control and maintain thermal pollution, humans and governments have been taking many steps to effectively manage how plants are able to use the water. However, the effects are still lasting today.

Causes of Thermal Pollution: – It looks like thermal pollution occurs mainly due to industrial activities, there are geothermal sources as well. These are:

1. Heavy Industries :- Industries such as power, chemical, nuclear & manufacturing are the primary source of thermal pollution. These industries use water as a coolant and discharge it back at an elevated temperature that affects the ecosystem. Water bodies that are near the nuclear power plants get heated due to emissions from nuclear reactors. Warm water decreases the dissolved oxygen content that results in thermal shock causing adverse effects on aquatic life.

2. Domestic & Industrial Effluents :-

Household and industrial effluents that include drainage from hospitals and research institutions too are often discharged with minimum or no treatment. Normally sewage treatment plants operate at a higher temperature and when discharged in natural water bodies the effective temperature increases. Thus, domestic & industrial effluents are one of the main sources of thermal pollution.avement gets quite hot, which creates warm runoffs that get into the sewer systems and water bodies.

3. Runoff From Paved Surfaces :- Urban runoff discharged to surface waters from paved surfaces like roads and parking lots can make the water warmer. During summer seasons, the pavement gets quite hot, which creates warm runoffs that get into the sewer systems and water bodies.

4. Soil Erosion :- Soil erosion causes the top layer of soil to break and washed away into the natural water bodies. The level of water bodies rises due to consistent soil erosion and sedimentation, which makes it directly exposed to the sunlight. Massive removal of vegetation and trees speeds up the soil erosion process, thus contributing to thermal pollution.

5. Deforestation :- Trees and plants prevent sunlight from falling directly on lakes, ponds or rivers. When deforestation takes place, these water bodies are directly exposed to sunlight, thus absorbing more heat and raising its temperature. Deforestation is also the main cause of the higher concentrations of greenhouse gases, i.e. global warming in the atmosphere.

6. Geothermal Activities :-
Natural causes like volcanoes, geothermal vents can trigger warm lava to raise the temperature of water bodies. Lightening can also introduce a massive amount of heat into the oceans. This means that the overall temperature of the water source will rise.

Harmful Effects of Thermal Pollution :-

Specific ecological imbalances occur due to increased water temperature affecting the associated ecosystem. Here are some major effects of thermal pollution.

Reduction in Dissolved oxygen level :- The warm temperature reduces the levels of DO (Dissolved Oxygen) in water. The decrease in DO can create suffocation for plants and animals such as fish, amphibians and copepods, which may give rise to anaerobic conditions. Warmer water allows algae to flourish on the surface of the water, and over the long term, growing algae can decrease oxygen levels in the water.

Changes in water quality :- A high temperature of the water changes the physical and chemical properties of water. When the viscosity of the water decreases, the vapour pressure increases rapidly.

Increase in Toxicity :- The concentration of the pollutant is the factor in the increase in the temperature of water, due to which the poisoning in the water increases the toxicity. Which can increase the death rate of aquatic life.

Interference in Reproduction Rates :-Thermal pollution adversely affects the natural reproductive cycles of fishes and other aquatic organisms. Due to thermal pollution, the temperature gradient of water bodies gets disturbed and affects whole aquatic organisms. Due to thermal shock, mass killing of fishes and other aquatic organisms can occur when there is a very rapid water temperature change.

Control of Thermal Pollution :- There are a number of ways to minimize the harmful effects of Thermal Pollutions.

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.

Cooling Towers :- After using water from water sources for cooling purposes, it is subsequently returned to the water body after passing through the condenser, which is termed as the cooling process. Therefore cooling towers are designed to control the temperature of water to make the cooling process more effective. Cooling towers are mainly used to dissipate the recovered waste heat to eliminate the problems of thermal pollution.

Cogeneration (Combined heat and power; CHP) :- Cogeneration is the use of a heat engine or a power station to simultaneously generate both electricity and useful heat. All power plants emit a certain amount of heat during electricity generation. This can be released into the natural environment through cooling towers, flue gas, or by other means. By contrast CHP captures some or all of the by-product heat for heating purposes, either very close to the plant or as hot water for district heating with temperatures ranging from approximately 80 to 130 °C. This is also called Combined Heat and Power.

Artificial Lake :- Artificial lakes are man-made water bodies that offer a possible alternative. The heated effluents may be discharged into the lake at one end, and the water may be withdrawn from the other end for cooling purposes. The heat is eventually dissipated through evaporation. However, these lakes have to be rejuvenated continuously.

Spray Ponds :- In spray ponds, the water coming out from the condensers pass into the pond through water sprayer nozzles.
The fine water droplets dissipate into the atmosphere as its area is maximized and cooled rapidly.

Other Preventive Measures:- Water Recycling
Afforestation along with the riverbank
Stricter Rules & Regulations
Spreading Awareness
Use of alternative cooling agent ( fan coolers use air to cool).