• In its broadest definition thermal pollution is the degradation of water quality due to processess that changes the water temperature. Hot water from industries and heat from other human activities into any water body is considered to be an important thermal pollutant, along with cold water from different sources such as reservoirs. Coolant from industries such as power plants discharged using once-through (OT) technique into water bodies is a major cause of thermal pollution leading to sudden change in oxygen and consequent deaths of organisms in water. In OT cooling water is passed through condensers for absorption of heat for one or two times. Disruptions in the aquatic ecosystems due to sudden increase or decrease in water temperature is also known as thermal shock.

Sources of Thermal Pollution

• Waste heat is a by-product in electricity production in power plants (which accounts for roughly 80% of usage of water in all industries). As per one study, in fossil fuel-based power plants 45% of energy is wasted through cooling water, where as it is 60% in nuclear fuel-based power plants. 
• Deforestation lead to increase in interaction of sunlight with water bodies leading to increase in temperature. Runoff water over concrete and roads pick up temperature and become warm. So, more urbanization means more concrete surface for warming and warmer water mixing into water bodies. 
• Temperature of sewage often remains higher due to bacterial activities. This warm water from habitations is source of thermal pollution to nearby aquatic ecosystems. Water stored for a long time becomes colder which when adds into the aquatic ecosystems may cause a thermal shock as cold temperature. Change in temperature of water may also happen due to activities such as road building and logging. Thermal pollution may happen naturally also due to volcanic activity, geothermal vents, lava flows, and lightening.

Impact of Thermal Pollution

Negative impacts of Thermal Pollution: 

• Increase in temperature may lead to deaths of aquatic organisms due to thermal shock. Temperature rise may also make them vulnerable to diseases, lead to physical damage of body parts, affects metabolic rates, prevent migration, or changes in behavior of foraging and spawning.
• Increase in temperature may change river’s capacity to assimilate wastes, change algal composition or lead to algal bloom, causing eutrophication.
• Increase in temperature may also lead to depletion of oxygen affecting aerobic decomposition, and increased bacterial activity, and thus disrupts nutrient cycling. 
• Increase in temperature may lead to higher metabolism which accelerate the nutrient uptake by many organisms and disrupts composition of species. 
• Temperature may also raise toxicity of water such as lethal effect of potassium cyanide increases two fold with 10 degree rise, and facilitate pathogens.
• Similarly, discharge of colder water affects productivity of aquatic system which changes structure and function of the system. This happens mainly because fish and invertebrate populations are mainly affected especially at egg and larval stage.

Positive impacts of Thermal Pollution:

• Heating may lead to melting ice in harbors. In some cases, higher temperature may favourably change aquatic ecosystem and its functions (also known as thermal enrichment)

Preventive measures

• Preventive measures while discharging hot water which can be taken care of include; limit to maximum discharge allowed, differential temperature limits for different areas within the aquatic body such as deeper part of river or estuaries, or limits during different seasons.
• Regulations may also be put on temperature range for different fish species, structural design for outlets, and flow rates of discharge.
• Cold water from reservoir may be regulated by changing design of outlet. 
• Development of thermal plumes is observed in many instances which can be modeled mathematically leading to forecasting of thermal contours in river space. 
• Thermal pollution can be prevented by structural measures such as application of different types of cooling towers such as wet type, dry type, and fluid type which cools water using evaporation before releasing. 
• Spraying can be done to lower the temperature, or artificial lake can be used for dual purpose of cooling and intake. 
• Some non structural measures involve awareness, application of technology, planting more trees, recycling and cogeneration. 
• In cogeneration (or combined heat and power – CHP) plants recover the heat instead of cooling and dissipating in atmosphere. The recovered heat can be used for cooling houses and even towns. Cogeneration plants are also known as trigeneration plants because of heat, cold, and electricity. Although cogeneration has certain advantages such as increase in efficiency of fuels, GHG mitigation, etc. it has some disadvantages also, such as high cost, a match is required between demand and supply, and complexities in rules and regulations.
• Excess heat can be utilized in heating soil, and rearing fishes and livestock.