Water Fit to Drink
Introduction


Did you know that over two thirds of the mass of your body is made up from water? It's not surprising that we are advised to drink between six and eight glasses of water each day!

We take for granted that the water piped to our homes will be clean and safe to drink. But what happens to the water to make it fit for us to drink? This unit shows the journey of water from a reservoir to your tap at home.

From reservoir to tap
We collect water from rivers and streams in large reservoirs. Look at Fig.1 below to see the processes it goes through.

Figure 1.   Summary of water treatment.
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Now try the questions below:

Which substance is added in the sedimentation tank?
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Which particles are coagulated by the substance added in the sedimentation tank?
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What process happens after sedimentation in the treatment of water?
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Why is chlorine added to the water?
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Stages in the treatment of water
Step 1 – aeration

The water is stirred up with air to dissolve plenty of oxygen into the water (for example, by letting it fall down a series of steps). This gets rid of iron(III) ions from the water. They
precipitate
A precipitate is an insoluble solid formed when two solutions react together.
precipitate
out of solution as iron(III) hydroxide.

If iron(III) ions remain in the water supply and precipitate out at home:

  • clothes get rusty stains on them after washing;

  • tea tastes bitter and has an inky colour;

  • vegetables cooked in the water turn brown.

Figure 2.   Aeration prevents iron(III) hydroxide precipitating out of your water at home.
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What is the formula of an iron(III)
ion
An ion is a charged particle made when an atom (or group of atoms) loses or gains one or more electrons, e.g. Na+ and Cl.
ion
?
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What is the formula of iron(III) hydroxide? (The charge on the hydroxide ion is 1−.)
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We can think of iron(III) hydroxide as a form of …
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Step 2 – sedimentation

In the sedimentation tanks, solid particles are allowed to settle at the bottom of the tank. However, tiny insoluble clay particles stay suspended in the water. The clay particles carry a negative charge so they repel each other and stay dispersed through the water. So water companies add aluminium sulfate, containing the highly charged Al3+ ions.

The negatively charged clay particles are attracted to the Al3+ ions, forming larger clumps which become heavy enough to sink to the bottom of the tank. We say that the clay particles coagulate, and the aluminium sulfate is called a coagulant.

At this stage, lime is also added to neutralize acidity in the water.

Which three of the following words describe the mixture of clay in water?
  • Aerosol
    Sol
    Colloid
    Emulsion
    Suspension
    Filtrate
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What is the formula of aluminium sulfate? (The charge on a sulfate ion is 2−.)
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Decide whether the following statements about lime are true or false.
  • Lime is an alkaline substance.
    Lime is an acidic substance.
    The chemical name for lime is calcium hydroxide.
    The chemical name for lime is calcium carbonate.
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Step 3 – filtration

After sedimentation, the water is filtered through layers of sand and gravel to make sure all the solid particles have been removed. These filter beds also contain a slurry of carbon which gets rid of unwanted smells and tastes from the water.

Step 4 – chemical purification

After the treatments above, the water looks good enough to drink. However, it may still contain potentially harmful bacteria. Chlorine (Cl2) is added to the water to kill them.

Then the water company adds sulfur dioxide (SO2) to remove the excess chlorine. The sulfur dioxide reduces chlorine molecules to chloride ions (Cl-). Just enough sulfur dioxide is added to leave a little chlorine in your tap-water.

When a chlorine
molecule
A molecule is a group of two or more atoms bonded together.
molecule
is reduced to chloride ions, each chlorine
atom
An atom is the smallest particle of an element that can still be defined as that element.
atom
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Which substance acts as a
reducing agent
A reducing agent is an element or compound that removes oxygen from a compound (or provides electrons in a reaction).
reducing agent
in the removal of chlorine from water?
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Why do water companies leave a little chlorine in the water on the way from the storage reservoir to your home?
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Some water companies also add fluoride to their water supplies to protect people's teeth, especially children's, from decay. However, too much fluoride can cause health problems (such as mottling on the teeth or bone disease). Since fluoride protection is now available from many toothpastes, many people believe that adding fluoride to everyone's water is unnecessary and may pose a health risk.

Summary


Water is made fit to drink by processes involving physical separation and chemical reactions.

These are summarized below:

Figure 3.   Water treatment.
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Exercises
1. Match each description of a stage in the treatment of water with the correct name.
  • Allowing solid particles to settle out of the water
    Removing solid particles by letting water seep through sand and gravel
    Agitating water to mix in oxygen gas
    Killing bacteria in the water
    Adding lime to remove acidity from the water
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2. Which substance is used to remove unpleasant smells and tastes from water?
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3. Which particles does aluminium sulfate remove from water?
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4. Which ions in the aluminium sulfate make it suitable for the use outlined in the previous question?
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5. Aluminium sulfate is called a …
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6. Which ions are removed from water by aeration?
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Well done!
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