Potable water is safe but not pure

1. Potable water can contain dissolved salts and microorganisms, as long as their levels are low enough to be safe.
   1. It therefore contains more than one substance, so it is not pure in the chemical sense.
2. In the UK, most drinking water comes from fresh water in rivers, lakes and reservoirs, or from ground water stored in rocks.
3. Where fresh water is scarce, sea water or treated waste water can be used to increase the supply.
4. A growing population and more frequent droughts mean that more places need to use these other sources.

Four treatment stages make fresh water potable

1. Screening passes the water through a mesh that removes large objects such as twigs, leaves and litter.
2. In sedimentation tanks, the water is left to stand so that particles of sand and soil settle to the bottom.
   1. Chemicals such as aluminium sulfate can be added to make fine particles clump together and settle faster.
3. Filtration through beds of sand and gravel removes the remaining small insoluble particles.
   1. Filtration does not remove dissolved substances, because they pass through the filter with the water.
4. Chlorine is added to kill harmful microorganisms, although ozone or ultraviolet light can be used instead.

Distillation removes dissolved salts from sea water

1. Sea water contains about 35 g of dissolved salts, mainly sodium chloride, in every kilogram, which is far too salty to drink.
2. Sedimentation and filtration cannot remove these salts, because dissolved ions pass through filters.
3. In distillation, sea water is heated so that the water boils off as vapour and the salts stay behind.
   1. The vapour is cooled in a condenser, and the liquid water collected is pure.
4. Boiling large volumes of water needs a lot of energy, so distillation is expensive and is used mainly where fuel is cheap and fresh water is scarce, as in parts of the Middle East.
5. Distilled water tastes flat, so small amounts of minerals are added back before it is supplied.

Reverse osmosis holds back dissolved salts

1. Sea water is first filtered to remove sand and other insoluble particles.
2. It is then pumped at high pressure against a partially permeable membrane.
   1. The membrane lets water molecules through but stops most of the dissolved ions.
3. The very salty water left behind, called brine, is returned to the sea.
4. Reverse osmosis uses less energy than distillation, because the water does not have to be boiled.
   1. The pumps still need energy, and the membranes are expensive and need regular cleaning and replacement.

Treating waste water to supply potable water

1. Waste water from homes, farms and industry contains organic matter, harmful microorganisms and sometimes harmful chemicals.
2. Screening removes large objects and grit, as it does for fresh water.
3. Sedimentation then separates the waste into solid sludge, which settles, and a liquid layer above it.
4. Bacteria break down the organic matter left in the liquid, and further filtration removes fine particles.
5. The water is sterilised before it is returned to rivers or, after more treatment, reused as potable water.
6. Treating waste water to drinking standard takes more processing than treating fresh water, so it is used mainly where water is scarce.