Potable Water

Potable water is water that is safe for humans to drink. For water to be drinkable, it must have low levels of dissolved salts and microbes, such as bacteria.

  • Chemicals can also be added to the water. For example, most water supplies contain some fluoride, which reduces tooth decay.

It’s important to remember that pure water and potable water are not the same. Pure water does not contain any dissolved substances, while potable water almost always contains small amounts of impurities.

Producing Potable Water

In the UK, most potable water comes from rainwater collected in lakes, rivers, and underground aquifers (rocks that trap water underground).

Although it’s easier to produce potable water from fresh water, seawater can also be treated. However, it requires a large amount of energy to remove the salt content from seawater. Salt dissolves in water very easily, forming strong bonds that are hard to break.

Producing potable water from fresh water involves two main stages:

1. Filtration – Water is passed through filter beds to remove larger objects such as twigs, pebbles, gravel, and sand. This stage removes insoluble particles.

2. Sterilisation– The water is treated to kill any remaining microbes. In the UK, chlorine is used to sterilise potable water. Chlorine gas is bubbled through the purified water to kill harmful bacteria and other microbes. However, in some parts of the world, other methods such as ozone or ultraviolet (UV) light are used for sterilisation.

Desalination

In the UK, there is a large amount of fresh water, which contains low levels of dissolved substances. However, many places use seawater to make potable water, through a process called desalination.

Desalination reduces the levels of dissolved substances to make the water potable. Two ways to carry out desalination are distillation and reverse osmosis.

Distillation

The distillation of seawater involves boiling the seawater to produce steam, which is pure water. After this, the steam is cooled and condensed, while the salt and other impurities are left behind.

Here’s a method to perform distillation on a smaller scale, suitable for a laboratory setting:

1. Pour a sample of seawater into a distillation flask.

2. Attach the condenser to the flask and connect it to a water tap using rubber tubing.

3. Circulate cold water through the condenser to maintain a cool temperature.

4. Gently heat the flask, causing the liquid with the lowest boiling point (water) to evaporate first at 100°C.

5. The water vapour travels through the condenser, cooling and condensing into liquid form, which is collected in a separate conical flask.

6. The remaining liquid in the distillation flask reduces, leaving salt behind.

Labelled distillation apparatus for the purification of sea water.

Disadvantages of using distillation for purifying water on a large scale:

  • High energy consumption due to the need for continuous boiling, which can be costly
  • Difficulty in disposing of the remaining salty wastewater

Reverse osmosis

Reverse osmosis is another method for purifying water, which relies on applying high pressure to force seawater through a semi-permeable membrane. Only water molecules can pass through the membrane, as the tiny pores prevent larger molecules and ions from passing through.

Salt water being converted to fresh water through the process of reverse osmosis.

Disadvantages of using reverse osmosis for purifying water on a large scale:

  • Produces a large amount of wastewater
  • The semi-permeable membranes used in the process are expensive

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