Science & Technology in Action

2nd Edition

Treating Our Water


This lesson looks at the need for clean water and describes the contaminants that can be present. It then describes the processes involved in the purifi cation of water and the tests that are carried out before the water is ready for public distribution.

Contains the full lesson along with a supporting toolkit, including teachers’ notes.

Lesson excerpt

Polluted water can contain many disease-causing organisms such as bacteria, viruses and parasites. Examples of such diseases are cholera, typhoid fever, hepatitis and dysentery. Major sources of pollutants are plant nutrients (phosphates and nitrates) and sediment. Sediment does not directly affect the quality of the water but it can help to transport harmful substances such as pesticides. This lesson describes the process of water treatment and describes the different stages involved.

To be suitable for consumption, water must fulfil certain criteria. It must be free from any micro-organisms and parasites that could be a danger to health and must meet specified water quality standards. The colour, odour and taste must also be acceptable to consumers. Most public water supplies must be fluoridated. In order to ensure the quality of our drinking water, the water must be treated and the quality must be constantly monitored to ensure that contaminated water is not consumed. 

Where does our water come from?
Approximately 80% of our drinking water is sourced from surface water such as rivers, lakes and reservoirs. The other 20% of water comes from underground sources such as springs and wells.

Where is water stored?
Surface water is usually stored in large reservoirs before it is taken into a water treatment plant. Before it enters the treatment plant the water is often stored in large holding tanks or ponds to allow any suspended solids to settle. This also allows water to bebiologically degraded by micro-organisms. At the same time theUV radiation in sunlight destroys some bacteria. However, if water is stored for too long it can become polluted by the atmosphere, by bird droppings, or by algal growth .

Quiz questions

  1. The main source of water in Ireland comes from underground wells. false
  2. Lime is added to purify water. false
  3. Untreated water is stored in settlement tanks. false
  4. Elemental chlorine can be used in water treatment. true
  5. Chlorine is added to give water a pleasant taste. false
  6. Bacteria are killed by adding chlorine. false
  7. Flocculation is the coagulation of small particles into larger ones. true
  8. The ideal pH for water is 9. false
  9. Fluorine is added to make our bones stronger. false
  10. Water quality checks are carried out continuously. true