Thermal Pollution

What do you think thermal pollution is? It's quite self-explanatory – it's unwanted heat. Like sound pollution, human activities produce large amounts of thermal pollution, and it can negatively impact ecosystems.

Thermal Pollution Thermal Pollution

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Table of contents

    Want to know more about where thermal pollution comes from? Warm up your brain and get reading!

    Thermal Pollution: Definition

    Let's begin with a definition.

    Thermal pollution is a rapid temperature increase in a body of water that affects water quality and aquatic life.

    The most common cause of thermal pollution is water coolant systems used by power plants or industrial manufacturers. Let's see how using water coolants can lead to thermal pollution.

    Types and Examples of Thermal Pollution

    Some thermal pollution is caused by natural processes, including volcanic eruptions, geothermal vents, and lightning strikes. However, most thermal pollution occurs as a by-product of human activities.

    Water Coolants

    Coolants are liquids used to prevent machines from overheating. Sometimes, they're used as lubricants on moving parts. The coolant is pumped through channels in a machine. As it passes through the channels, it absorbs excess heat.

    Power stations and manufacturing plants use water as a coolant – it's free and widely available. The water is drawn from a nearby source, used as a coolant, and then released back to its source at a much higher temperature.


    When domestic or industrial waste is dumped into bodies of water, the temperature increases rapidly, leading to thermal pollution.

    Sources of effluent (liquid) waste include desalination plants, domestic sewage, and nuclear plants.


    When deforestation takes place, previously shaded water bodies become directly exposed to sunlight. They absorb more heat and experience increased temperatures.

    Soil Erosion

    Deforestation is associated with soil erosion. Consistent soil erosion causes water bodies to rise, becoming more exposed to sunlight, and thus, temperature increase. Soil erosion of the bank can remove aquatic vegetation, increasing exposure to sunlight.

    Urban Runoff

    Urban areas are covered by impermeable surfaces, such as pavements, roads, and roofs. Water is unable to infiltrate into the soil, so flows along the ground as urban runoff until it reaches a body of water.

    Roads and pavements are typically dark-coloured, so retain heat during summer. Some of this heat is transferred to runoff, which then pollutes sewer systems and water bodies.

    Hydroelectric Power Stations

    Hydroelectric power stations dam water uphill, creating a reservoir. Water is forced through turbines on its way downhill, which power a generator. Electricity generation causes increased water temperature in the reservoirs.

    Consequences of Thermal Pollution

    The main consequence of thermal pollution is its effect on oxygen levels.

    Dissolved oxygen (DO) refers to the amount of oxygen available to aquatic organisms. Water temperature is negatively correlated with DO. This is because in warmer water, the water molecules have more energy and move faster, allowing oxygen to escape from the water. Think about a cold glass of water. This has more oxygen stored in it than a warm glass of water. This might make little sense because often hot or warm water has lots of bubbles. These bubbles are actually the oxygen escaping from the water. The more solubility, the more gas a liquid can contain meaning in hot water, dissolved oxygen is released.

    Freshwater has a higher DO concentration than saltwater. This is because saltwater has less space for oxygen molecules because of the sodium and chlorine ions it contains.

    Warm and salty water bodies have the lowest DO concentrations.

    Effects of Thermal Pollution on Aquatic Life

    Thermal pollution can devastate aquatic ecosystems. We will cover some of these effects below.


    Some aquatic organisms are highly sensitive to temperature changes, and cannot adapt, resulting in death by thermal shock.

    Alternatively, the significant temperature increase causes a decline in DO. A DO concentration of 5 mg/L is recommended for optimum fish health. Most species become distressed between 2-4 mg/L, and mortality typically occurs below 2 mg/L. Low DO results in suffocation for aquatic animals.

    But it's not just animals who are victims to thermal pollution. Widespread plant death is common, leading to a decline in photosynthesis, giving rise to anoxic conditions.

    Anoxia is the absence of oxygen.

    Algal Blooms and Anoxia

    Thermal pollution can also lead to anoxic conditions via algal blooms. Warmer water supports algae populations on the surface. Algae are fast-growing and quickly form blooms. These blooms prevent light from penetrating the water, stopping photosynthesis below the surface and eventually leading to anoxia.

    Thermal Pollution algal bloom anoxia death StudySmarterFig. 1 - This frog's habitat has become dominated by algae. There's so much that the water appears green, and light is unable to reach aquatic plants below. Soon the aquatic plants will be unable to photosynthesise, resulting in anoxic conditions. Pixabay

    Reproductive Issues

    Thermal pollution can cause reproductive problems in aquatic animals. High temperatures can trigger the release of immature eggs, or prevent normal egg development. Furthermore, the likelihood of birth defects in newborns is significantly higher in a water body affected by thermal pollution.

    Impacts on the Food Web

    A food web is a group of food chains linked together.

    Thermal pollution can alter the aquatic food web in a variety of ways.

    • Increasing water temperatures can cause mass death and species loss. This associated decline in biodiversity can disrupt the whole food web. Predators may be left without food, or prey populations may expand uncontrollably, overexploiting the habitat and its natural resources.

    • Surviving animal species are likely to migrate, leaving a gap in the food web.

    • Thermal pollution increases the metabolic rate of organisms. Consequently, organisms living in thermally polluted water bodies consume more food than usual. This impacts the food web's stability, and may alter species composition.

    Solutions to Thermal Pollution

    How can we prevent thermal pollution from affecting aquatic ecosystems?

    Cooling Towers

    If water coolants are returned to their source immediately, they will cause thermal pollution and affect aquatic ecosystems. Cooling towers act as a preventative measure, reducing the temperature of the water before it's returned to its source. Cooling towers work by maximising water's contact with the air. When the hot water meets the air, a small amount is evaporated. Latent heat is transferred from the water to the air, producing a cooling effect.

    Thermal Pollution cooling tower thermal pollution solutions StudySmarterFig. 2 - The two wide, concave structures are cooling towers. Unsplash

    Cooling Ponds

    Heated water is placed into a shallow reservoir with a large surface area, known as a cooling pond. The premise works the same as cooling towers – water coming into contact with air will evaporate, creating a cooling effect. Spray nozzles can be used to maximise the amount of water that comes into contact with the air.

    Cooling ponds are a simple and cheap method of water cooling, but they are inefficient.

    Man-made cooling ponds are known as artificial lakes. Used coolant is discharged into the lake at one end; heat is eventually dissipated through evaporation. Water is withdrawn at the other end for reuse.

    Water Recycling

    Sometimes, the used coolant is recycled. This saves energy because the water doesn't need to be heated up again! Uses for coolant include:

    • Heating (domestic or industrial)

    • Agriculture (soil warming and heating greenhouses)

    • Livestock shelters

    • Fish cultures

    I hope that this article has explained thermal pollution for you. The main cause of thermal pollution is water coolants used in power plants and industrial manufacturers. Thermal pollution is associated with a reduction in dissolved oxygen, affecting aquatic ecosystems.

    Thermal Pollution - Key takeaways

    • Thermal pollution is a rapid temperature increase in a body of water that affects water quality and aquatic life.
    • Causes of thermal pollution include coolant systems, waste disposal, deforestation, soil erosion, urban runoff, and hydroelectric power stations.
    • Thermal pollution is associated with a decrease in dissolved oxygen, which harms marine life.
    • Aquatic ecosystems affected by thermal pollution experience biodiversity loss, reproductive issues in animals, and disruption to the food web.
    • Thermal pollution can be minimised by the using of cooling towers and ponds, or by recycling hot water.

    1. Ruth Francis-Floyd, Dissolved Oxygen for Fish Production, University of Florida Institute of Food and Agricultural Sciences, 2003

    Frequently Asked Questions about Thermal Pollution

    What is thermal pollution, and what are its effects?

    Thermal pollution is a rapid temperature increase in a body of water that affects water quality and aquatic life. It causes a decline in the water's dissolved oxygen concentration, affecting aquatic life.

    What are some examples of thermal pollution?

    Thermal pollution can be caused by coolant systems, effluent waste, deforestation, soil erosion, urban runoff, and hydroelectricity generation.

    What is thermal pollution, and why do we care?

    Thermal pollution is temperature increase in a body of water caused by human activities. It can affect aquatic ecosystems, resulting in biodiversity loss.

    What is the greatest source of thermal pollution?

    The greatest source of thermal pollution is water coolant systems used by power plants or industrial manufacturers.

    How can we control thermal pollution?

    We can control thermal pollution by using cooling towers, cooling ponds, or recycling water.

    Test your knowledge with multiple choice flashcards

    What type of renewable energy resource is associated with thermal pollution?

    Water temperature is negatively correlated with dissolved oxygen.

    Fish mortality is likely at what concentration of dissolved oxygen?

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