Thermal Pollution

What is thermal pollution?

The release of excess heat into the environment, changing the natural temperature range and balance.

What is the main source of thermal pollution?

Hot water in steam turbine power stations that use cold water from a lake, river, or the sea to condense steam

The coolant water is returned to its source at a higher temperature, increasing the temperature of this water body and harming the ecosystem

What are the effects of thermal pollution?

- Killing organisms as they are living in conditions outside their range of tolerance

- Deoxygenation of water

Aquatic organisms ROT

•Aquatic organisms can be killed by high temperatures, because proteins (like enzymes) are denatured outside of the range of tolerance.

•Only thermophilic organisms (organisms that can survive above 40C) can survive in these conditions

Deoxygenation of Water

- The maximum amount of oxygen that can be dissolved in water is controlled by temperature.

- So, the higher the temperature, the lower the concentration of dissolved oxygen.

Why is the deoxygenation of water bad?

Some aquatic organisms are often already near the lower end of their range of tolerance for dissolved oxygen. An increase in temperature causes further deoxygenation, leading to the death of these sensitive organisms.

Statistics about deoxygenation

- Low oxygen levels are rarely a problem in the atmosphere, because each litre of air contains about 210cm³ of oxygen

- Deoxygenation is much more likely to be a problem in water because at 20C, a litre of water holds a maximum of 6cm³ of dissolved oxygen, which is less than 3% of the amount found in air

How to control thermal pollution?

Temperature reduction using cooling towers.

What do cooling towers do?

They cool power station effluent water so that it does not cause deoxygenation when returned to a water body

How do cooling towers cool effluent water?

1. A shower floor sprays the hot water onto the lower part of a tall hollow tower.

2. The air in the tower absorbs the heat from the water and rises up the tower, escaping into the open atmosphere

3. This causes an updraught which draws more air through the shower spray, aiding cooling

Statistics to know about power stations

A large 2000MW power station uses 200 million litres of cooling water each hour which is returned at a temperature 5 to 8°C higher than its original temperature.

The shower mixes the water with air, so it is usually saturated with dissolved oxygen. Without this aeration, discharged hot water could harm ecosystems by raising local water temperatures and lower dissolved oxygen levels