AQA GCSE Chemistry

Revision Notes

10.1.4 Waste Water Treatment

Sources of Waste Water

  • Water is used on a daily basis in a domestic environment. For example, washing-up dishes, showers and baths and cooking
  • When you run water down a drain, it passes through sewers and then finally to sewage treatment plants
  • Agricultural waste from animal farms and nutrient run-off which is collected from fields produces an abundance of waste water
  • Both domestic and agricultural sewage needs to be processed to remove particulates, toxins and any harmful bacteria and microbes
  • This can then be safely returned to freshwater sources i.e. lakes and rivers
  • If this process did not take place, it could potentially pose health risks for the population
  • Waste water that is produced by the Haber process and other industrial processes needs to be gathered and treated appropriately
  • Harmful chemicals and organic matter are present in industrial waste
  • This therefore means that additional treatment has to be in place to ensure it is safe for the environment

Exam Tip

Waste water, sometimes called ‘grey water’, cannot be allowed to run untreated as it contains dangerous substances and toxins and is detrimental to the environment and health.

Sewage Treatment

  • Screening & Grit Removal
    • The first stage of treatment removes large materials such as plastic bags and twigs and grit by screening
  • Sedimentation
    • Sedimentation comes next which occurs in a settlement tank. The water is allowed to stand still in the tank while heavier solids sink to the bottom creating a sludge, whilst lighter matter which is also known as effluent, floats to the top
  • Aerobic Digestion
    • The effluent is removed and treated by biological aerobic digestion
    • This involves pumping air into the water to encourage the breakdown of organic matter and other microbes by aerobic bacteria
  • Anaerobic Digestion
    • Anaerobic digestion is then used to break down the sludge from the bottom of the settlement tank. It is firstly removed and placed in large tanks where bacteria break it down
    • Anaerobic digestion releases methane gas as a by product from the organic matter in the sludge. Methane gas is used as a source of energy and the leftover, digested waste as a fertiliser

Diagram showing the stages of waste water treatment

  • When there are toxic substances within waste water, it is important to use additional phases of treatment
  • This can include using membranes, adding additional chemicals e.g. to precipitate metals out of solution, and also U.V. radiation
  • Sewage water is often treated in areas where there is little freshwater available
  • Though this process is longer than processing and preparing freshwater, it uses less energy than the desalination of salt water

Exam Tip

Waste water comes from many different sources so the process of waste water treatment is complex and involves various stages.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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