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Potable Water

Worksheet Overview

QUESTION 1 of 10

Drinking water is vital for life.

Unfortunately, some water supplies are not safe to drink; they contain dissolved salts or microbes, which make people sick if they drink the water. Potable water is water that is safe to drink. This isn't totally pure (water with small concentrations of salts is better for us than chemically pure water), but it contains nothing harmful. So how do we make safe drinking water?

In the UK, we get our water from three main sources;

  • Surface water comes from rivers, lakes and reservoirs. These are fed by rain, so they can become limited during dry summers
  • Ground water comes from underground rocks, called aquifers. These store rain water below the ground surface. Wells are holes in the ground which let us collect ground water.
  • Waste water is water that has been used, so is dirty. We need to treat it to make it clean- either so it can be released back into rivers, or so we can use it again.

Making water potable

Ground water often doesn't need much treatment to be potable, but surface water and waste water need quite a lot of treatment before they are safe to drink. Either way, water normally goes through three stages of treatment before it reaches us.

  1. Filtration removes any large lumps from the water; twigs, rocks, bits of rubbish... Filtration is done in several stages. Early filtration uses a wire mesh with fairly large holes- this removes the largest lumps from the water. Later stages of filtration pass water through beds of gravel, then sand. These trap the smaller solid bits.
  2. The next stage is called sedimentation. By now, the water has no obvious lumps in it, but it will still look dirty. This is because there will still be small solid particles mixed up in the water. In the sedimentation stage, the water sits in large ponds, like this;

    The remaining solid particles sink to the bottom of the water. Adding iron sulfate or aluminium sulfate to the water helps with this, by making the small particles clump together. Now the water will look clean.
  3. The final stage is sterilisation. Filtration and sedimentation will not remove microbes from the water. We can kill microbes by bubbling chlorine or ozone gas through the water, or shining ultraviolet light on the water. 

Treating sewerage

If we have waste water, we have to work extra hard to ensure it is clean and safe. This is true whether we intend to reuse the water, or release it into a river.

  1. First, we filter the water to remove any large particles.
  2. Then, sedimentation takes place. Some particles sink to the bottom (called sludge), and others float on the top (called effluent). The water in the middle is reasonably clean.
  3. Bacteria are used to digest the sludge and effluent. Since the sludge is at the bottom of the water, sludge-digesting bacteria need to work anaerobically (without oxygen). The bacteria digesting the effluent are exposed to air, so digest aerobically (with oxygen)

Making sea water potable

If you've ever swallowed some sea water, you will know that it's not good to drink. There is such a high concentration of dissolved salt in it that sea water is not potable. Because the salts are dissolved in the water, filtration and sedimentation are not able to remove them. Removing the salt from sea water is called desalination. If we have to use desalination to make sea water potable, there are two ways of doing this.

One way is to distil the sea water. If we heat the water enough, pure water will evaporate, leaving the solid salt behind. We can collect the water vapour, condense it and make pure water. 

The other way to desalinate water is called reverse osmosis. In this, we force sea water through a very fine membrane. The water molecules pass through the membrane, but the salt ions do not.

Both these processes use a lot of energy. Distillation needs huge amounts of energy to evaporate water. Reverse osmosis needs smaller (but still quite large) amounts of energy to generate the pressure needed to force water through the membrane. Desalination is only used in places where there is not much rain, so that ground water and surface water are scarce.

 

What does potable mean?

Natural

Pure

Safe to drink

Environmentally friendly

Which of these substances are found in potable water?

Live Microbes

Dissolved salts

Solid particles

Dead microbes

Match these phrases with their meanings.

Column A

Column B

Ground water
Water in rivers, lakes and reservoirs
Surface water
Water in underground rocks (aquifers)
Waste water
Water which is dirty because it has been used

Mark this paragraph about water treatment. Tick the correct sentences.

The first stage in treatment is sedimentation.

In sedimentation, small particles settle at the bottom of the sedimentation pond.

In filtration, water passes through a wire mesh, then gravel, then sand.

Filtration removes large solid pieces from the water.

Finally, we sterilise the water by bubbling carbon dioxide through it.

Which of these processes can be used to sterilise water?

bubbling ozone through the water

shining infared onto the light

bubbling chlorine gas through water

shining ultraviolet onto the water

adding acid to the water

Use these words to complete this paragraph;

aerobic

anaerobic

effluent

sludge

bubbling ozone through the water

shining infared onto the light

bubbling chlorine gas through water

shining ultraviolet onto the water

adding acid to the water

Make a complete sentence describing the sedimentation process.

bubbling ozone through the water

shining infared onto the light

bubbling chlorine gas through water

shining ultraviolet onto the water

adding acid to the water

What is the name for removing salt from sea water?

Match up these half-sentences about desalination.

Column A

Column B

Sea water is not potable because
evaporating water, then recondensing the vapour.
Distillation works by
forcing seawater though a partially permeable memb...
Reverse osmosis works by
the concentration of salt is too high.
Distillation and reverse osmosis
both use large amounts of energy

Why is desalination not used in Britain? Tick all the relevant points.

Desalination plants are dangerous.

Ground water and surface water are abundant.

Desalination uses large amounts of energy.

Desalinated water is not potable.

  • Question 1

What does potable mean?

CORRECT ANSWER
Safe to drink
EDDIE SAYS
This distinction is important. Potable water isn't pure. In fact, when we make pure water by osmosis, it needs some substances added to make it safe to drink in large amounts.
  • Question 2

Which of these substances are found in potable water?

CORRECT ANSWER
Dissolved salts
Dead microbes
EDDIE SAYS
Sterilisation doesn't remove the microbes, though it does kill them so that they are not harmful.
  • Question 3

Match these phrases with their meanings.

CORRECT ANSWER

Column A

Column B

Ground water
Water in underground rocks (aquif...
Surface water
Water in rivers, lakes and reserv...
Waste water
Water which is dirty because it h...
EDDIE SAYS
These phrases matter, because different water sources need different amounts of treatment. Ground water generally needs the least treatment, waste water the most.
  • Question 4

Mark this paragraph about water treatment. Tick the correct sentences.

CORRECT ANSWER
In sedimentation, small particles settle at the bottom of the sedimentation pond.
In filtration, water passes through a wire mesh, then gravel, then sand.
Filtration removes large solid pieces from the water.
EDDIE SAYS
We do filtration before sedimentation; it's easier to remove the larger solid particles before the smaller ones. There are various ways of sterilising water, but carbon dioxide isn't one of them.
  • Question 5

Which of these processes can be used to sterilise water?

CORRECT ANSWER
bubbling ozone through the water
bubbling chlorine gas through water
shining ultraviolet onto the water
EDDIE SAYS
Chlorine is the commonest way of sterilising water, but ozone gas or ultraviolet will work as well.
  • Question 6

Use these words to complete this paragraph;

aerobic

anaerobic

effluent

sludge

CORRECT ANSWER
EDDIE SAYS
The key thing here is that aerobic is "with oxygen" and anaerobic is "without oxygen". At the bottom of the pond, there won't be much oxygen, so only anaerobic bacteria will thrive there.
  • Question 7

Make a complete sentence describing the sedimentation process.

CORRECT ANSWER
EDDIE SAYS
In sedimentation, we want the particles to sink to the bottom. Making them stick together helps this to happen. It's a bit like the way that dust can float in the air, but larger things can't.
  • Question 8

What is the name for removing salt from sea water?

CORRECT ANSWER
deasalination
EDDIE SAYS
"Saline" means "made of salt" in latin, so "desalination" is removing salt.
  • Question 9

Match up these half-sentences about desalination.

CORRECT ANSWER

Column A

Column B

Sea water is not potable because
the concentration of salt is too ...
Distillation works by
evaporating water, then recondens...
Reverse osmosis works by
forcing seawater though a partial...
Distillation and reverse osmosis
both use large amounts of energy
EDDIE SAYS
Check the introduction if you're not sure about these points. Most real desalination plants use reverse osmosis, but that still uses quite a lot of energy.
  • Question 10

Why is desalination not used in Britain? Tick all the relevant points.

CORRECT ANSWER
Ground water and surface water are abundant.
Desalination uses large amounts of energy.
EDDIE SAYS
Countries only use desalination if they have to; generally because there isn't much rain. Fortunately, dry countries have lots of potential to use solar energy to power desalination plants.
---- OR ----

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