Producing potable water from fresh water

Potable water is water that is safe to drink. When fresh water is available, it can be made potable by choosing a suitable source and treating the water to remove harmful material.

In the UK, rainwater forms fresh water with low levels of dissolved substances. It collects in the ground, lakes and rivers. These supplies can differ in quality and contamination, so a suitable source is selected before treatment.

Fresh water treatment

The selected water passes through filter beds, which remove solid particles and some suspended impurities. Filtration does not necessarily remove all microorganisms, so the filtered water is then sterilised to kill microorganisms that could cause disease. Sterilisation can use chlorine, ozone or ultraviolet light.

The aim is to make the water safe for drinking, not chemically pure. Potable water can still contain dissolved substances.

Potable water

Potable water is water that is safe to drink. It must contain sufficiently low levels of dissolved salts and microbes so that it does not harm health.

Potable water and pure water

Potable water is not necessarily pure water. In chemistry, pure water contains only $$\ce{H2O}$$ molecules and no dissolved substances. Potable water can contain dissolved minerals and gases, provided they are present at safe levels.

The distinction is that potable water is judged by its suitability and safety for drinking, not by whether it is chemically pure.

Desalination

When fresh water supplies are limited, salty water or sea water can be used to produce potable water: water that is safe to drink. First, much of the dissolved salt must be removed. This removal is called desalination.

Sea water contains far more dissolved salts than fresh water. Desalination therefore differs from the simpler treatment used for fresh water: it must remove the salt itself. Two desalination methods are distillation and membrane processes such as reverse osmosis.

Desalination by distillation

In distillation, salty water is heated so that water evaporates. Most of the dissolved salts remain behind. The water vapour is then cooled and condensed into liquid water, which is collected with much less salt than the original water.

Desalination by reverse osmosis

In reverse osmosis, pressure forces water through a membrane. The membrane allows water molecules through but blocks many dissolved ions and other substances. The water collected on the other side therefore contains much less dissolved salt.

Both distillation and reverse osmosis require large amounts of energy: distillation needs energy to evaporate water, while reverse osmosis needs energy to produce the pressure that forces water through the membrane.

Ease of obtaining potable water from different sources

Potable water is water that is safe to drink. It does not have to be pure, but harmful substances and microorganisms must be reduced to safe levels. The treatment needed depends on what is in the water before treatment begins.

Comparing water sources

Ground water is usually the easiest source to make potable. As water passes through soil and rock, it is naturally filtered, removing some particles and microorganisms. It may therefore need less treatment than other sources. However, it can still contain microbes or dissolved substances, including minerals picked up from rocks, so its suitability must be checked.

Waste water is harder to make potable because it can contain sewage, organic matter, microorganisms and other contaminants. These must be removed or reduced through substantial treatment before the water is safe to drink.

Salt water is generally the hardest source to use. In addition to treating any other impurities, the dissolved salts must be removed by desalination. Desalination requires a lot of energy and is expensive.

The ease of using a source therefore depends on both its starting contamination and the amount of treatment needed to make it safe.