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.

Waste

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

Deforestation

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.

Effects of Thermal Pollution on Aquatic Life

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

Death

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.

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.

Fig. 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

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.

Fig. 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.

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