Waste Water Treatment (Triple)AQA GCSE Chemistry: Revision notes

Potable water is water that is safe to drink because it has sufficiently low levels of dissolved salts and microbes.

Importantly, potable water is not the same as pure water: pure water contains only H2O with nothing dissolved in it, whereas potable water can still contain low levels of dissolved substances — as long as they are safe.

In the UK, where fresh water is generally available, potable water is produced by:

1. Choosing an appropriate source of fresh water (e.g. a lake, river or reservoir)
2. Passing the water through filter beds to remove solid particles
3. Sterilising the water using chlorine, ozone or ultraviolet light, to kill microbes

Each of these steps has a purpose: filtering removes physical debris, and sterilising removes the biological risk from microbes that filtering alone cannot.

When supplies of fresh water are limited, desalination of salty or sea water can be used instead. This can be done by:

• Distillation — evaporating the water and condensing the vapour, leaving the dissolved salts behind
• Reverse osmosis — forcing water through a membrane that lets water molecules through but not dissolved salts

Desalination is generally more energy-intensive than treating fresh water, which is why it is mainly used where fresh water sources are limited.

Waste water (sewage) treatment involves several stages:

1. Screening and grit removal — removes large solid objects and grit
2. Sedimentation — solids settle out, producing sewage sludge (bottom) and effluent (the liquid on top)
3. Anaerobic digestion of the sewage sludge — breaks it down without oxygen
4. Aerobic biological treatment of the effluent — breaks down remaining organic matter using oxygen

Obtaining potable water from waste water requires more extensive treatment than from ground water, and is generally more involved than desalinating salt water in terms of the biological contamination that must be removed.