Water Works
How clean, safe drinking water is made: what potable water really means, how fresh water is filtered and sterilised, how salty sea water is desalinated, and how waste water is treated before it goes back to the environment.
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Making water safe to drink
Potable water is water that is safe to drink. Note that it is not the same as pure water: potable water still contains dissolved minerals, it is just safe. In the UK, fresh water from rain collected in reservoirs, rivers or underground is made potable by passing it through filter beds to remove solids, then sterilising it with chlorine, ozone or ultraviolet light to kill microbes. Where fresh water is scarce, salty sea water is desalinated, but that uses a lot of energy. Waste water must also be cleaned before it returns to rivers. This module covers all of these.
Water words
Learn these before you sort any step, and keep potable and pure clearly apart.
Potable against pure
These two are easy to confuse, and the exam often tests the difference.
How to make water safe
Keep two ideas straight. First, safe to drink is not the same as chemically pure: tap water carries harmless dissolved minerals. Second, know the treatment path. For fresh water, take it from a source, pass it through beds that trap solids, then kill the microbes with chlorine, ozone or ultraviolet light. Where only salty water is available, remove the salt, but note that this takes a great deal of energy. Match each step to what it removes rather than blurring them together.
Match the step to what it does
- choosing a fresh water source
- passing water through filter beds
- adding chlorine or using ultraviolet light
- desalinating sea water
- takes water from a reservoir, river or aquifer
- removes solid bits from the water
- kills the harmful microbes
- removes salt but uses a lot of energy
How does potable differ from pure?
A student says tap water is chemically pure. Why is this wrong?
- Tap water is potable, safe to drink, but still holds dissolved minerals.
- Potable and pure water are exactly the same thing.
- Because tap water is not safe to drink at all.
- Because tap water is full of salt like the sea.
Producing potable fresh water
Select the TWO steps used to make fresh water potable.
- Passing it through filter beds to remove solids
- Sterilising it to kill microbes
- Adding salt to the water
- Doing nothing and drinking it straight away
Count the steps left
Producing potable water from fresh water has 3 main steps: choose a source, filter, and sterilise. A student can already name 1 of them. Subtract to find how many steps are still to be learned. What is the answer?
Order the treatment
Put the steps of making fresh water potable in order, earliest first.
- Take water from a suitable source
- Pass it through filter beds to remove solids
- Sterilise it to kill microbes
- Supply the safe, potable water to homes
Complete the facts
Water that is safe to drink is called _____ water, and it still contains dissolved minerals. Fresh water is passed through _____ beds to remove solids, then treated to kill microbes, which is to _____ it. Removing salt from sea water is called _____.
Match the waste water step to what it removes
- screening
- sedimentation
- anaerobic digestion
- treating the effluent
- removes large solids and grit from the water
- lets the sludge settle out to the bottom
- breaks down the settled sludge without air
- removes the harmful microbes before release
Spot the true water facts
Tap the TWO statements that are true about water treatment.
- Potable water is safe to drink but not chemically pure
- Desalination removes salt but uses a lot of energy
- Tap water is completely pure with nothing dissolved
- Fresh water needs no treatment before drinking
Choose the method
Read each case and choose the best answer.
- A country with plenty of rainfall wants to supply safe tap water cheaply. Which approach fits best?
- A dry desert country by the sea has little fresh water. Which method must it rely on, accepting the cost?
- Used water from homes must be returned to a river safely. What must happen first?
Explain water treatment
Explain what potable water is and how fresh water, sea water and waste water are treated to make water safe.
- Explain the difference between potable water and pure water
- Explain how fresh water is made potable
- Explain what desalination is and one drawback of it
- Explain one step in treating waste water