Power plants- thermal and nuclear, chemical and other industries use lot of water (about 30% of all abstracted water) for cooling purposes and the used hot water is discharged into rivers, streams or oceans. The waste heat from the boilers and heating processes increases the temperature of the cooling water. Discharge of hot water may increase the temperature of the receiving water by 10 to 15°C above the ambient water temperature. This is thermal pollution. Increase in water temperature decreases dissolved oxygen in water which adversely affects aquatic life. Unlike terrestrial ecosystems, the temperature of water bodies remain steady and does not change very much. Accordingly, aquatic organisms are adopted to a uniform steady temperature of environment and any fluctuation in water temperature severely affects aquatic plants and animals. Hence discharge of hot water from power plants adversely affects aquatic organisms. Aquatic plants and animals in the warm tropical water live dangerously close to their upper limit of temperature, particularly during the warm summer months. It requires only a slight deviation from this limit to cause a thermal stress to these organisms.
Discharge of hot water in water body affects feeding in fishes, increases their metabolism and affects their growth. Their swimming efficiently declines. Running away from predators or chasing prey becomes difficult. Their resistance to diseases and parasites decreases. Due to thermal pollution biological diversity is reduced. One of the best methods of reducing thermal pollution is to store the hot water in cooling ponds, allow the water to cool before releasing into any receiving water body.
How do power plants pollute water?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → By discharging hot water
Power plants use a lot of water for cooling, and "the used hot water is discharged into rivers, streams or oceans". This raises the temperature of the receiving water by 10 to 15°C: the passage calls it thermal pollution.
Why the Other Options are Incorrect?
Option 1: By killing aquatic bodies — Incorrect. Result, not cause: harm to aquatic life follows from the pollution; it is not the way the plants pollute the water.
Option 2: By cooling boilers — Incorrect. Wrong link: cooling is the purpose for which the water is used; the pollution comes when the heated water is let out.
Option 4: By using a lot of water — Incorrect. Wrong focus: using a large amount of water does not pollute it; discharging it hot does.