Potable Water

Potable water is water that has been processed and is safe for human consumption and daily use
The difference between pure water and potable water is that pure water is solely made up of H₂O molecules, whereas potable water may contain different substances, usually dissolved minerals and salts
Potable water should have the following characteristics:
- Have a pH between 6.5 and 8.5.
- The dissolved substances (e.g. salts) will be present is very small regulated quantities
- Be free of bacteria or potentially harmful microbes
Water is considered fresh when it is relatively free from dissolves substances e.g. rainwater
Water can collect in reservoirs, lakes and rivers and is known as surface water
In addition, it can collect in aquifers which are porous rocks that store water underground
This water is called groundwater

Water treatment

To obtain potable water, a suitable fresh water source is chosen
The origin of fresh water depends on the climate in the region in question
- In warmer areas, such as in the south-east of England, water primarily dries up before it can be collected so is found underground
Despite being relatively low in dissolved substances, there is still a process in place to ensure the fresh water is safe and suitable for use.
Two important steps in this process are:
- Filtration:
  - Debris such as pieces of soil and dirt, small pebbles, twigs, etc. are removed by a wire mesh screen
  - After this, other debris is filtered through sand beds and gravel
- Sterilisation:
  - Ultraviolet light or ozone can be used to sterilise water or alternatively chlorine gas is bubbled through the water
  - This removes any dangerous bacteria or microbes
Diagram showing the stages in the treatment of water

The three stages of treating water are sedimentation, filtration and chlorination

Where aquifers are not present and/or the collection of surface water is limited, the process of desalination must be used to provide potable water to the population
Desalination involves the treatment of seawater to remove the salt by distillation or reverse osmosis, a process that involves the use of membranes
When salt water is put through a semi-permeable membrane, only water molecules can pass through it. This happens as the membrane stops larger molecules and ions passing through
Desalination is an expensive process as it consumes large amounts of energy and is not ideal when producing large quantities of fresh water
This is used in regions with a very hot climate such as Saudi Arabia

The way in which potable water is prepared and delivered to a population depends largely on the local conditions of geology.