AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Potable water: sterilising.
Sterilising is the final stage of producing potable water: killing or inactivating microorganisms so the water is safe to drink.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Sterilising is the final stage of producing potable water: killing or inactivating microorganisms so the water is safe to drink. Water is first screened to remove large debris, then filtered through beds of sand and gravel to remove smaller suspended particles, and finally sterilised. Sterilising does not remove dissolved substances, so it is not a purification step in the chemical sense. In the UK, chlorine is commonly added; ozone or ultraviolet light are alternatives. For example, chlorine added at around 0.5 mg per dm³ of water kills bacteria such as those causing cholera and typhoid. The method chosen must leave no harmful residue and must reach all of the water, so the dose and contact time matter.
What earns the marks
- Sterilising kills or inactivates microorganisms such as bacteria that cause disease, making water safe to drink.
- Sterilising is carried out after screening and filtration, not instead of them.
- Sterilising does not remove dissolved salts or other dissolved substances.
- Chlorine, ozone and ultraviolet light are all valid sterilising agents for potable water.
- Chlorine is the most common sterilising agent used in UK water treatment.
- Ozone and ultraviolet light kill microorganisms without adding a persistent chemical residual to the water.
Common mistakes and how to put them right
Thinking sterilising removes dissolved substances.
Put it right: It only kills microorganisms, so dissolved salts remain and the water is not chemically pure.
Confusing sterilising with filtration.
Put it right: Filtration removes insoluble particles, whereas sterilising targets microorganisms.
Believing boiling is used on a large scale for public water supplies.
Put it right: Boiling is used in emergencies or at home, while chlorine, ozone or ultraviolet light are used in treatment works.
Exam tips
- Link each treatment stage to what it removes: screening removes large objects, filtration removes small particles, sterilising kills microorganisms.
- Name at least two sterilising agents and state that they kill microorganisms rather than remove substances.
- Use the phrase 'safe to drink' when explaining the purpose of sterilising potable water.
Try the lesson
The start of the Potable water: sterilising. lesson, made from checked teaching notes for this specification point.
Have a go first
In the treatment of potable water, what is the purpose of sterilising?
Listen
Sterilising is the final stage of producing potable water: killing or inactivating microorganisms so the water is safe to drink. Water is first screened to remove large debris, then filtered through beds of sand and gravel to remove smaller suspended particles, and finally sterilised. Sterilising does not remove dissolved substances, so it is not a purification step in the chemical sense. In the UK, chlorine is commonly added; ozone or ultraviolet light are alternatives. For example, chlorine added at around 0.5 mg per dm³ of water kills bacteria such as those causing cholera and typhoid. The method chosen must leave no harmful residue and must reach all of the water, so the dose and contact time matter. A strong answer earns marks for these points. Sterilising kills or inactivates microorganisms such as bacteria that cause disease, making water safe to drink. Sterilising is carried out after screening and filtration, not instead of them. Sterilising does not remove dissolved salts or other dissolved substances. Chlorine, ozone and ultraviolet light are all valid sterilising agents for potable water. Exam tip: Link each treatment stage to what it removes: screening removes large objects, filtration removes small particles, sterilising kills microorganisms.
See it
Key words
Say what each word means before you turn the card.
Sterilising
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Chlorine
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Ozone
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Ultraviolet light
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See every definition
- Sterilising
- Killing or inactivating microorganisms so the water is safe to drink, carried out after screening and filtration.
- Chlorine
- A sterilising agent commonly added in the UK, killing bacteria such as those causing cholera and typhoid.
- Ozone
- An alternative sterilising agent that kills microorganisms without adding a persistent chemical residual to the water.
- Ultraviolet light
- An alternative sterilising agent that kills microorganisms without adding a persistent chemical residual to the water.
Label the diagram
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1Box 1
2Box 2
3Box 3
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