Potable Water and Waste Water Treatment

Everything humans need for warmth, shelter, food and transport comes from the Earth's resources. Natural resources, supplemented by agriculture, provide our food, timber, clothing and fuels. Chemistry helps by improving agricultural and industrial processes so we get more from the same materials, and by developing new ways to use resources.

Resources fall into two groups. A finite resource is used up faster than nature can replace it, so supplies run out. A renewable resource is formed (or grown) at the same rate as, or faster than, it is used, so it can be replenished.

Type	Meaning	Examples
Finite	Used up faster than it is replaced; supplies run out	Crude oil, natural gas, coal, metal ores
Renewable	Replaced at least as fast as it is used	Timber from replanted forests, fresh water from rain

Sustainable development means meeting the needs of people today without stopping future generations from meeting their own needs. Using resources carefully, recycling materials and finding renewable alternatives all support sustainable development.

Many products once taken only from nature are now supplemented or replaced by agricultural and synthetic ones: rubber can be grown as latex or made synthetically, and cotton (natural) sits alongside synthetic fibres such as polyester.

Potable water is water that is safe to drink. This is not the same as chemically pure water, and the exam tests this distinction directly.

Potable water has low levels of dissolved salts and low levels of microbes (bacteria and other microorganisms), so it will not make you ill. It still contains dissolved substances, so to a chemist it is not pure.

Chemically pure water contains only water molecules, H₂O, and nothing else dissolved in it. Pure water boils at exactly 100°C at atmospheric pressure; water with dissolved substances boils at a slightly higher temperature.

Potable = safe to drink, low in salts and microbes. Pure = only H₂O molecules. Tap water is potable but not pure.

The method used to produce potable water depends on the supplies available and on local conditions. A country with plenty of rain, such as the UK, treats fresh water; a dry country with little rain may have to treat sea water instead, which takes much more energy.

The UK has plenty of rain, which provides fresh water with low salt levels in rivers, lakes and reservoirs (ground and surface water). This makes producing potable water relatively straightforward.

Most potable water in the UK is produced by three steps:

Step	What happens	Why it is done
1. Choose a source	Select an appropriate source of fresh water	Fresh water has low dissolved-salt levels, so it needs the least treatment
2. Filtration	Pass the water through filter beds (layers of sand and gravel)	Removes insoluble solids such as grit, leaves and larger particles
3. Sterilising	Treat with a sterilising agent	Kills microbes so the water is safe to drink

The sterilising agent can be one of three things the specification names:

• Chlorine — a gas added to the water
• Ozone (O₃)
• Ultraviolet (UV) light

Filtration removes solids; it does not kill microbes. Sterilising kills microbes; it does not remove solids. Both steps are needed, and they do different jobs.

Because the source is fresh water, no salts need to be removed, so the process uses far less energy than treating sea water.

Where fresh water is in short supply, potable water is produced from salty water or sea water by desalination (removing the dissolved salts). Two methods are used.

Distillation boils the salty water. The water evaporates, leaving the dissolved salts behind; the water vapour is then cooled and condensed back to liquid water, which is now free of salt.

Reverse osmosis forces the salty water at high pressure through a partially permeable membrane (a membrane with tiny holes). Water molecules pass through, but the larger dissolved salt ions are held back, so the water coming out the other side has had its salts removed.

Method	How it works	Main drawback
Distillation	Boil water, condense the vapour, leave salts behind	Boiling needs large amounts of energy
Reverse osmosis	Force water through a partially permeable membrane	High pressure needs large amounts of energy; membranes need replacing

Both desalination methods require large amounts of energy, which makes them expensive. This is why desalination is used mainly in dry countries with little fresh water, not in the UK.

Homes, farms and industry produce huge amounts of waste water that must be treated before it can safely return to rivers and the sea. Different waste water needs different treatment.

• Sewage and agricultural waste water need the removal of organic matter and harmful microbes.
• Industrial waste water may need the removal of organic matter and harmful chemicals as well.

Sewage treatment follows a set sequence:

Stage	What happens
1. Screening and grit removal	Large solids (rags, sticks) and grit are removed
2. Sedimentation	The water is left to settle; solids sink to form sludge, and the liquid on top is the effluent
3. Anaerobic digestion	The sludge is digested by bacteria without oxygen (anaerobically)
4. Aerobic treatment	The effluent is treated by bacteria with air (aerobically) to break down organic matter and microbes

The word "anaerobic" means without oxygen; "aerobic" means with oxygen. Sludge is digested anaerobically, while effluent is treated aerobically by microbes that need air.

Sedimentation splits sewage into two streams: sludge (solid, treated anaerobically) and effluent (liquid, treated aerobically). Keep the two treatments the right way round.

The specification asks you to comment on how easy it is to obtain potable water from different starting points. The key idea is that the more treatment a source needs, the more energy it costs.

Source	Treatment needed	Relative ease / energy cost
Fresh (ground) water	Filtration + sterilising	Easiest; lowest energy
Waste water	Screening, sedimentation, anaerobic + aerobic treatment	Harder; needs more processing
Salty / sea water	Desalination (distillation or reverse osmosis)	Hardest; highest energy

Fresh ground water needs only filtration and sterilising, so it is the cheapest and easiest source. Waste water needs several treatment stages. Salty water needs desalination, which uses large amounts of energy, so it is the most expensive and difficult source.

Where fresh water is available, treating it is far cheaper than desalinating sea water. This is why countries choose sea water only when fresh water runs short.

This required practical tests two things: analysing water samples from different sources, and purifying a sample by distillation.

Part 1 — analysing the samples. You test water from several sources (for example distilled water, sea water and pond water).

• pH: dip universal indicator paper or a pH probe into each sample and record the pH. Pure and neutral water is pH 7; other samples may differ.
• Dissolved solids: measure the mass of an empty evaporating basin. Add a known volume of the water sample, then gently evaporate the water so the dissolved solids are left behind. Reweigh the dry basin. The increase in mass is the mass of dissolved solids in that volume.

$\text{mass of dissolved solids}=\text{mass of basin}+\text{residue}-\text{mass of empty basin}$

Sea water leaves the most residue (most dissolved salt); distilled water leaves almost none.

Part 2 — purification by distillation. Heat a salty water sample in a flask. The water boils and turns to vapour; the dissolved salts stay behind because they do not evaporate. The vapour passes into a condenser, where cooling water turns it back to liquid, collected as pure distillate.

• Independent variable: the water sample (which source).
• Dependent variables: pH and mass of dissolved solids.
• Control variables: volume of sample used, and using the same balance and method each time.

Test the distillate the same way you tested the original sample: it should have pH near 7 and leave no residue, proving the distillation removed the dissolved solids.

1. Saying potable water is pure

Potable water is safe to drink but still contains dissolved substances, so it is not chemically pure. Only water containing nothing but H₂O molecules is pure.

2. Confusing filtration with sterilising

Filtration through filter beds removes insoluble solids. It does not kill microbes. Sterilising with chlorine, ozone or UV light kills microbes but does not remove solids. Both steps are needed for different reasons.

3. Forgetting that desalination is energy-intensive

The examiner wants you to know that distillation and reverse osmosis both use large amounts of energy. That is the reason fresh water is preferred wherever it is available.

4. Mixing up sludge and effluent treatment

Sedimentation gives sludge (solid) and effluent (liquid). Sludge is digested anaerobically (without oxygen); effluent is treated aerobically (with air). Swapping these loses the mark.

5. Not naming three sterilising agents

The three the specification lists are chlorine, ozone and ultraviolet light. Learn all three so you can answer if a question rules one out.