Processes to purify water

  • Why is a clean water supply so important?
  • How can water be cleaned?
  • How is water cleaned by municipalities to ensure that we have clean water in our homes?

Clean water

Have you ever seen a sign like the one in the picture? This sign is a warning that the water is not clean and humans should not drink it as it could be dangerous to your health. If it is unsafe for consumption, it is also unsafe for swimming.

A warning against drinking polluted water

What does 'clean water' mean to you?

Clean water is water that is free of pollutants.

Can you remember what pollutants are? Write it down below.

Pollutants are things that do not belong in the water. Insoluble substances (oil, rubbish and waste), soluble substances (chemicals and poisons) and germs are all pollutants.

We have also learned that nature has special methods of cleaning polluted water. Water purification happens in special natural environments called wetlands. Wetlands are very efficient natural 'water treatment' facilities, but they work slowly. Humans, animals and plants need fresh water every day, and for this reason we have to clean our dirty water so that it can be reused. Cleaning dirty water is what this section is all about.

Why do we need clean water?

Why is it so important for humans, plants and animals to have clean water?

  • Our bodies are contain a lot of water. In fact you are mostly water! We need water every day because we lose water everyday with the waste our bodies produce. The water we drink must be clean to prevent us from becoming sick or being poisoned.
  • Plants and animals need clean water too, so that they can grow and be healthy. We need healthy plants and animals in our environment because they provide an ecological balance and food.
  • Water is also used for fun. Water sports are a very popular recreational activity and include things like swimming, surfing and waterskiing. We want clean water in our seas and lakes so we can enjoy being in the water without becoming ill.

How can water be cleaned?

Imagine you are in a place without clean water. The only water around is a muddy stream. How can the dirty water be made clean enough to drink? When thinking about how to purify water, we need to consider what it is that we need to do to separate the contaminating particles from the water. The muddy water is actually a mixture of sand and water, and possibly other contaminants.

Do you remember in the beginning of the term we looked at different ways to separate out mixtures? What are some of these ways?

Filtering/filtration, hand sorting, sieving, evaporation, distilling/distillation, decanting, boiling

Which methods do you think would be useful to separate the large impurities from the dirty water?

Filtering, sieving (possibly hand sorting)

Which methods would be useful to remove the soluble impurities from the dirty water?

Evaporation, boiling, distilling

Let's investigate some different ways to clean water at home or at school.

Cleaning water in a still


  • clean, dry small pot/container
  • muddy water
  • big, deep cooking pot
  • ruler
  • 1 marble or clean stone
  • plastic cling wrap


  1. Pour the muddy water into the big pot until it is about 5 cm deep (you can use the ruler to measure the depth of the water).
  2. Place the smaller pot/container inside the bigger pot.

The rim of the smaller pot/container must be lower than the rim of the bigger pot so that the water which condenses on the cling wrap has space to run down to the centre where the stone/marble is weighing it down and drip into the smaller pot/container.

  1. Cover the bigger pot with plastic cling wrap.
  2. Place a marble or stone in the middle of the plastic wrap so that it is above the small pot/container inside the pot. You have now made a still for purifying water.
  3. Leave the still out in the sun for a day.

You should continue with the activity after the still has been out in the sun for some time.

  1. Look carefully at the still and answer the questions below.
  2. If you have a phone with a camera, you could take pictures of the still and show them to your class.


What do you notice about the inside of the smaller container? Is it wet or dry?

The small container is wet on the inside.

What do you notice about the cling wrap? Is it wet or dry?

The cling wrap is wet.

Write a paragraph to explain how the water got inside the yoghurt tub. You can use words from the following list, but you can also add your own words:

energy, sun, evaporate, water, water vapour, condense, drip, clean

Learners may write:

Energy from the sun caused the water in the pot to evaporate. Since the pot was sealed with cling wrap, the water vapour condensed against the cling wrap. The marble made the cling wrap dip in the middle. The condensed water slid down and dripped into the smaller container. The water inside the cup was clean.

Why is there no dirt inside the small container/pot?

Dirt does not evaporate.

Where was the dirt at the end of the experiment?

The dirt stayed in the big pot.

Make a poster of the process. You can draw pictures or use the photos you took of the experiment.

Can you remember what method we used to remove sand from water in the activity: Mixing a solid and a liquid in the chapter Mixtures?

We filtered the water and sand through a paper towel to remove the sand.

Filtering is a good way of removing insoluble substances from water. Large pieces of insoluble substances can also be sieved out of the water.

Design, make and evaluate a filter

This is the first time that learners are doing a Technology project this year, in Gr. 6, so you will probably need to remind them of the Design Process which is followed in Technology Projects. The first part is identifying the need and in this project a scenario has been set where the Thunderbolt Kids are planning a camping trip and have invited the Gr. 6 class to come with them. However, there is no clean drinking water at the campsite they want to go to and they cannot carry enough water for everyone for the whole trip. Use this scenario to construct the need to design a filter to take with them on the camping trip.

We follow the pattern IDMEC in the Design Process.

Istands for Investigating the problem which some people have, investigating existing products, and investigating concepts and skills that you will need to solve the problem, i.e. purifying water. Encourage learners to investigate filtering methods such as using sand, cotton wool and charcoal.

Dstands for Designing. That means using what you have learnt from investigations to think of good ways to design a filter.

Mstands for Making. When you make your filter, you use the materials and tools to make the filter according to the design. Notice that most children design with their hands, not only with pencil and paper. As they work with materials they get more ideas, and their design improves. So we should think of designing and making as more or less the same stage of a project.

Estands for Evaluating. After you have made your filter to solve the problem, you have to ask, does it work? Could we do a better one? During this phase, take all the learners outside and get them to hang their filters up and test them out by pouring a cup of muddy water through. NB. Make sure learners DO NOT drink the water. Rather compare the water before and after filtering to see if it has been purified. Prepare a big bucket of muddy water which you can scoop cups of water out of to purify. That way you will have some water left over to compare the filtered water to, to see if the learners' filters did purify the water.

Cstands for Communicating. You must show other people how you decided on your solution to the problem. You need to write and draw your ideas. (The learners should be drawing and writing all through the project. Do not leave the writing until the end, because they find it boring at that stage. When they are getting new ideas they often enjoy writing because they are writing about their own ideas; this is a great strength of Technology in school. A Technology project gives the learners reasons for reading and reasons for writing. As a result, we can address the literacy problem through the subject of Natural Science and Technology.)

The Thunderbolt Kids are planning a camping trip and they have invited you and your classmates to come with them. They have all the equipment such as tents, sleeping bags and mats. The only problem is that there is no clean drinking water at the campsite. For such a large group, the Thunderbolt Kids cannot take enough bottles of clean water. So, Tom has decided that the group should build a filter to purify the muddy water from the nearby stream. But, they do not know how! We need to help them design and build a filter to purify the water on the camping trip.


A Design brief for a Technology project is a short statement of what you are going to make, why you are going to make it and what you are going to make it for.

Write a short statement where you state what you are going to be designing and why.

Your filter has the following specifications:

  • The filter must be able to filter a cup of muddy water and the water which comes out must be cleaner than the water that went in.
  • The filter must be able to hang up, for example from the branch of a tree.
  • You must be able to use it outside.

Your filter has the following constraints:

  • You cannot make the filter at home, you must make it in class.
  • You are confined to using as many recyclable materials as possible, such as 2 litre plastic bottles, plastic bags, etc.


We now need to do some more investigating about how to purify water. We saw how to purify water using a still and evaporation. But what are the ways to purify water using a filter. Do some research on the internet and in books and answer the following questions.

  1. What types of filters can you make?

  2. What materials do these filters make use of?

  3. What is the purpose of each of the materials used to filter? For example, are some materials there for filtering large particles and some for filtering small particles? If so, which one is which?


Now start designing your filter! Answer these questions before you start drawing your design.

  1. What kind of filter are you going to design?

  2. What size and shape will your filter be?

  3. What recyclable materials could you use?

  4. How are you going to hang it up, for example from the branch of a tree?

  5. How will you collect the water that has been filtered?

Use the space below to draw a design for your filter. Remember to label the different parts and show which materials you are going to use. Leave some space to do a second drawing as you might come up with a better design as you are making your filter and see ways to improve it.


Now that you have a design, it is time to make your filter according to the specifications and constraints. Once you have all made your filters, you need to test them out to see if they work.

Do the testing outside. Show the class how your filter works and pour a cup of muddy water through the filter and collect the water which comes out.

DO NOT DRINK THE WATER as you do not know if you have completely purified the water. To further purify the water from the filters, you can also do a solar distillation using the still you made in the last activity to remove solids, and finally followed by boiling.


After you have made your filter, you have to ask, does it work and could you do a better one?

  1. Compare the water before and after filtering. Was the water cleaner after filtering?

  2. Which impurities did your filter remove - big or small or both?

  3. How much filtered water were you able to collect from your filter? Was it the same amount that you poured in?

  4. Did your filter leak anywhere? If so, how could you prevent it from leaking?

  5. Do you think you could further purify the water you filtered by passing it through your filter again? Try it out and see if this makes a difference.

  6. How could you improve your design?

  7. What further steps could you take to purify the water which comes out of your filter.

You could do a distillation using the still made in the previous activity, and finally followed by a boiling step.


The last part in the Design Process is to communicate what you designed and made to others so that they can also learn about what you did, and learn from you. Write a paragraph below where you tell Tom about the filter that you designed and made to take on the camping trip to purify the muddy water in the stream. Tell Tom what you found that worked and what did not work and anything that you would change.

How is water cleaned by municipalities?

Do you have running water in your home? If you have, you are very fortunate, because many South Africans do not.

Clean water coming out of the tap

South African municipal water is generally clean and fresh, and safe to drink. How does it get that way? This section tells the story of how water is treated.

The water from a tap does not start out clean and fresh. It may come from a river or a dam, or it may even be waste water that was used by a community or in a factory.

The process of cleaning the water is called water treatment, and the place where it happens is called a water treatment plant.

So what exactly happens at a typical water treatment plant?

There are five steps (or processes) in the treatment of water. The five processes commonly used to treat water are screening, sedimentation, aeration, settling, and disinfection.

Let us look at them in turn.


The raw water that arrives at the plant may contain dirt, fish, rubbish, plants and even sewage.

These things are screened out as the water flows into the plant. This means that the water passes through a screen (which is very much like a large sieve), and the solid matter stays behind on the screen.

After the screening step the water is still dirty, but the large pieces of rubbish have been removed.


During this step the raw water is allowed to stand in a large tank called a settlingtank.

What happens to the dirt when muddy water stands for a long time?

It sinks to the bottom.

When the raw water stands in the settling tank, the medium-sized pieces of solid matter (called sludge) sink to the bottom of the tank.

The raw water at the top is still dirty but now it only contains small pieces of solid matter. The bits of solid matter left are small enough for small organisms (such as bacteria) to eat. This is what happens in the next step of the treatment process.


The raw water now flows into a special tank that contains bacteria. These are useful bacteria, because they help to break down the last little bits of solid matter as well as breaking down any natural soluble pollutants.

Since bacteria need oxygen to stay alive and healthy, air is bubbled through the water. This process is called aeration, because the name comes from the word 'air'.


Next, the water flows through a special filter made of layers of sand and gravel, just like the one you designed, only much bigger. The gravel layer of the filter is about 30 cm deep and the sand layer is about 1 m deep! The filtering step removes any remaining particles and most of the bacteria left in the water.

After this step the water is clear, but some germs and bacteria from STEP 3 may still be in the water. Remember that germs and bacteria are small enough to pass between the gaps between sand and gravel.


During disinfection, chemicals are added to the water to kill any surviving germs.

  • Clean water is important for people, plants and animals.
  • Water can be cleaned by processes such as sieving, filtering, settling, decanting, boiling and adding chemicals to kill germs.
  • The water we use in our homes is cleaned before and after we use it.

What does it mean to purify water?

It means to clean water; to remove pollutants from the water.

What is clean water?

Clean water does not contain any pollutants.

Why do humans, animals and plants need clean water? Write a paragraph where you describe some of these needs.

Learner dependent answer. Water is necessary for life. We need clean drinking water for metabolic processes in our bodies and to replace water which is lost due to urination and excretion. We use water for washing and cooking and need clean water so that we do not become ill. Many animals make their homes/shelters in and around water and need clean water so that they do not become ill.

If you were not sure about the water that came out of the tap and you had a kettle, what would you do to the water to purify it? Why do you think this method of purifying works?

You could boil the water. This would kill any living microorganisms or bacteria in the water.

The woman in the picture is drinking water from the edge of a dam. What possible threats could she face from drinking this water without doing anything to purify it?

The water could contain disease-causing bacteria such as cholera, hookworm, bilharzia, typhoid, and it could contain chemicals or fertilisers from surrounding farmlands, etc.

Name the 5 steps in the water treatment process.

The five steps are screening, sedimentation, aeration, settling, and disinfection.

Do you think it is important to conserve water? Why do you think so? Write a paragraph to justify your answer.

Learner dependent answer.