GCSEChemistry: Combined ScienceAQARevision Notes10. Using Resources10.1 Global Reserves & Potable WaterWaste Water Treatment

Waste Water Treatment (AQA GCSE Chemistry: Combined Science)

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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 organic matter, harmful microbes, particulates and toxins 
  • 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

Examiner 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

What happens at a sewage treatment plant?

  • 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 sewage 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 sewage 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

Sewage treatment

Sewage treatment process involves many steps

Diagram showing the stages of waste water treatment

Removal of toxic substances

  • 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

Examiner Tip

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

You may be asked to compare how easy it is to obtain potable water from waste water, ground water and salt water. The treatment of waste water has been covered in these notes.

Depending on the source, ground water may need very little treatment, if any, to make it potable. Sometimes there are natural minerals, such as arsenic, in the water which make it unsafe to drink. These minerals can sometimes be removed by charcoal filters. Boiling will not remove the minerals, but it may kill harmful micro-organisms in ground water.

Obtaining potable water from salt water is called desalination and usually done by distillation or reverse osmosis (forcing water through a semi-permeable membrane that allows only small water molecules to pass through). Distillation is expensive as it requires a lot of energy to heat the water. Desalination is carried in areas that are very scare in rainfall or groundwater, such as some parts of the Middle East. 

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    Stewart

    Author: Stewart

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    Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.