Water Pollution by Sewage

Sewage is a one of the major causes of water pollution. When untreated sewage gets released into rivers or seas:

• Microorganisms decompose organic matter in the sewage.
• During the decomposition process, microorganisms use oxygen dissolved in the water for aerobic respiration.
• There will be less dissolved oxygen left for organisms living in the water.
• As a result, aquatic organisms will die.

Frequently asked in IGCSE Biology: 

Eutrophication by Leached Minerals from Fertilisers

1. Fertilisers used to enhance crop yield are rich in nitrates and phosphates. The problem occurs when these nutrients are washed off the land into rivers or lakes by rainwater. This is called leaching.
2. The excess nutrients cause aquatic plants to flourish: the growth of algae, duckweed and other plants is increased. This is called eutrophication.
3. Increased algae growth causes algae bloom. Algae bloom blocks sunlight from getting through to other plants. Plants will not be able to photosynthesize, so plants will die, and oxygen level in the water drops.
4. Decomposers decompose dead material. They use oxygen for aerobic respiration, so oxygen level further becomes depleted.
5. Eventually, oxygen level will become so low that no plants and animals can survive in the water.

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