potable water

Higher and foundation tier

Potable water is water that is safe to drink. In the UK we tend to take it for granted that when we turn on the tap fresh clean water will always be available, for many millions of people around the world this is not the case. Most of our water comes from surface water in rivers, lakes and reservoirs which are filled by rain water. However water from these sources is unlikely to be fit to drink due to high levels of bacteria and will need further treatment before it is fit to drink. Not all water sources are on the surface, there are many sources of underground water stored in aquifers. Once rain water soaks into the ground it will sink and can becomes trapped in permeable rocks, these are rocks which are porous and act a little like a sponge allowing the water to move through pores and holes on the rock. These underground aquifers can be huge and cover large areas of land. Water taken from aquifers via boreholes and wells can be quite pure as it is filtered by the rock and other layers, though it still needs to be disinfected with chlorine or another sterilising agent.

water aquifer

At the waterworks

Water from rivers and reservoirs may contain larger particles of mud and sand which may discolour the water. There may be larger objects such as twigs, branches and of course shopping trolleys and other rubbish as well as dead plants and animals! There will also be bacteria and other microbes in the water which could cause illness if someone drunk it. As the rainwater and then river water passes over soil and rocks soluble minerals will dissolve in the water. Many of these mineral such as calcium and magnesium ions can have health benefits but other especially from fertilisers in the soil can cause problems to aquatic life and also humans. The image below gives an indication of some of the processes that take place at the water works. At the waterworks the water is made safe to drink. water treatment

  1. screening- Here metal screens remove large pieces of rubbish and any organic matter such as leaves, twigs and branches.
  2. Filteration- next the water enters a tank with a course sand filter which traps and removes larger particles of mud and other matter.
  3. Sedimentation- next the water enters a large sedimentation tank. Here a coagulant such as aluminium sulfate is added. This will cause small suspended particles to clump together and settle as a thick sludge on the tank bottom. The diagram below explain how these coagulants work.
  4. using coagulation to clean water

    The suspended clay and silt particles are often covered in bacteria and other harmful microbes, after the removal of these particles the water is much clearer and cleaner.

  5. Filtration- next the water is passed through sand filters which gradually get finer and finer, this will remove any remaining small particles from the water.
  6. Sterilisation- chlorine or ozone are added to the water to kill any remaining bacteria which could pose a health risk. It is also possible to sterilize the water using ultraviolet radiation.
  7. pH correction- the pH of the water will be checked and adjusted as necessary to ensure it is within an acceptable range. If the water is too acidic it will be passed through a filter containing crushed limestone or if it is too alkaline then an acid will be added to ensure pH is within the range 6.5-8.5. It is important to realise that the water from the tap is not pure water, it is clean water. Tap water can have many dissolved mineral in it which are perfectly safe to drink such as calcium and magnesium ions.

Desalination

In many areas of the world there is not enough fresh water, particularly in the Middle East and North Africa, so how do people living in these hot dry areas get enough water to drink? The answer is they get their fresh water from the sea. Obviously you cannot drink seawater because it contains too much salt, but in a process called desalination the salt is removed from seawater to leave fresh water. There are two basic ways to desalinate seawater, these are distillation and reverse osmosis. Distillation you should already be familiar with, here water is simply heated till it boils. The steam produced then enters a Liebig condenser where it cools and condenses and turn back into water. The apparatus for simple distillation is shown below:

distillation of salt water

The water can be heated under reduced pressure, this will reduce its boiling point but this process is very energy intensive and is not economic as a method for producing large volumes of fresh water due to the costs involved. However for some countries such as Saudi Arabia and the UAE this is the only way they can obtain enough fresh water to meet their needs.

Reverse osmosis

Another method used to get fresh water from sea water is called reverse osmosis. Here a high pressure is used to force water molecules through tiny holes in a semi-permeable membrane. These holes will only allow water molecules to travel through and prevents ions which are dissolved in the water from travelling through.

reverse osmosis

One of the problems other than cost and the large energy requirements for reverse osmosis to occur is the fact that a concentrated salt solution called brine is left once some water has been removed from the sea water, essentially this process will increase the concentration of the salt in the sea water. This brine solution will then likely be pumped back into the ocean and is likely to harm the local marine environment.

Key Points

Practice questions

Check your understanding - Questions on potable water

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