Science & Technology in Action

1st Edition

Improving the Natural Environment


This lesson examines the treatment of domestic waste water. Sewage is defined and the different stages in its treatment process are described. The concepts of eutrophication and Biochemical Oxygen Demand are also covered.
Download Lesson Kit

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

Lesson excerpt

Sewage treatment
For many years a large proportion of urban waste water was discharged untreated to our rivers, lakes and coastal waters. However, since the introduction of the Urban Waste Water Regulations in 1994, the level of waste water treatment has improved and water pollution from urban waste water sources has been reduced. Sewage is the term used to describe wastewater which is produced by domestic, industrial and commercial sources and discharged into sewers. Run-off rain water from roads and paved areas is also collected in the sewerage system, although this has been separated out in recent drainage schemes thus reducing the amount of water which has to be treated at the sewage plant.

What are the components of sewage?
Sewage consists of organic waste and inorganic waste. Organic waste includes domestic sewage (human wastes, paper, and vegetable matter, proteins and surfactants, carbohydrates) and industrial/commercial wastes (fats, oils and grease). Inorganic waste includes nitrates and phosphates from domestic waste and heavy metals from industrial waste waters. Sewage may also contain gases, namely methane (which is explosive) andhydrogen sulfide (significant source of odour).

How is sewage treated?
Sewage goes through several stages of treatment in modern treatment plants. If it were simply discharged untreated into a river or lake, the micro-organisms that are naturally present in the water would break down the organic waste into carbon dioxide and water. This process would use up the dissolved oxygen in the water, depleting oxygen levels which may lead to a fish kill. When all the dissolved oxygen is used up anaerobic bacteria (which do not use dissolved oxygen) take over and break down the waste, resulting in the release of unpleasant gases such as hydrogen sulphide.

In a typical treatment plant the waste water is treated using a series of physical, chemical and biological processes. These are preliminary or pre-treatment, primary treatment, secondary treatment and tertiary treatment and are used are required.


True or False?

  1. Sewage is composed of organic waste, inorganic waste and gases. true
  2. The treatment of sewage is a one stage process. false
  3. When nutrients are taken out of the water, this is called eutrophication. false
  4. Dissolved oxygen in rivers, lakes and sea water is necessary to sustain organic life. false
  5. A percolating filter is a component in a sewage treatment plant. true
  6. Eutrophication is the enrichment of water with nutrients, which is beneficial to marine life. false
  7. When organic matter reacts with oxygen, carbon monoxide and water are produced. true
  8. Denitrifying bacteria. The bacteria reduce the nitrate ions to nitrogen gas. true
  9. There are no laws governing the amount of lead and mercury in water. false
  10. Excessive algae can lead to deoxygenation of water. true