4.7 Water Pollution and Its Sources

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Describe the sources of water pollution.
  2. Give specific examples of water pollution sources, including:
    • Domestic waste (sewage from urban and rural settlements)
    • Industrial processes
    • Agricultural practices

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What Is Water Pollution?

Water pollution happens when harmful substances enter a body of water — such as a river, lake, ocean, or groundwater — and make it dirty or dangerous. Polluted water can harm living things that drink it, swim in it, or live in it. It can also make water unsafe for humans to use for drinking, cooking, or washing.

Water can be polluted from many different places and activities. These are called sources of water pollution. There are three main types of sources you need to know:

  1. Domestic waste (including sewage)
  2. Industrial processes
  3. Agricultural practices

Source 1: Domestic Waste and Sewage

Domestic waste means the waste that comes from people's homes and communities — the rubbish, dirty water, and human waste that everyday life produces.

Sewage is one of the most important types of domestic waste. It is the mixture of human faeces (solid body waste), urine, and wastewater that flows out of toilets, sinks, and drains in homes, schools, and other buildings.

From Urban Settlements (Towns and Cities)

  • Urban settlements are towns and cities where large numbers of people live close together.
  • Cities produce enormous amounts of sewage every day because of the high population.
  • In many cities, sewage is collected through underground pipes called a sewer system and taken to a sewage treatment plant, where it is cleaned before being released into rivers or the sea.
  • However, when the sewer system is overloaded (e.g. during heavy rain), or if the treatment plant does not work properly, untreated sewage can flow directly into rivers and coastal waters.
  • Domestic waste also includes household chemicals such as bleach, paint, and medicines that are poured down drains. These chemicals can pass through treatment plants and enter waterways.
  • Litter and solid rubbish from cities can also be washed into drains and end up in rivers and oceans.

From Rural Settlements (Villages and Countryside Communities)

  • Rural settlements are villages and small communities in the countryside, away from cities.
  • Many rural areas do not have proper sewage systems or treatment plants.
  • Human waste is often stored in a septic tank (an underground tank buried in the ground). If this tank leaks or overflows, sewage can seep (leak slowly) into the soil and reach underground water supplies called groundwater.
  • In some very rural areas, people may have no sewage system at all, and waste is disposed of near rivers or open ground, which pollutes nearby water sources directly.
  • This is a major problem in many developing countries, where sewage infrastructure is limited or absent.

Why Is Sewage Pollution Harmful?

  • Sewage contains bacteria and viruses (tiny, invisible germs) that cause serious diseases such as cholera, typhoid, and dysentery. If people drink or swim in sewage-polluted water, they can become very ill.
  • Sewage is rich in nutrients — especially nitrates and phosphates. These come from human waste and household detergents (washing products). When these nutrients enter rivers and lakes, they cause a process called eutrophication (explained in Key Terms below), which destroys aquatic (water) life.

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