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.

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



Now try the questions below:

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.
  
  

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.

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.



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: