Potable and Waste Water

Clean, safe drinking water is something we take for granted, but producing it — and dealing safely with the dirty water we throw away — is a major global challenge. The water that comes out of the tap is potable: safe to drink, but not chemically pure. Where fresh water is scarce, drinking water has to be made from sea water at much greater cost. And the waste water (sewage) we produce must be treated before it can be returned to rivers and the sea. This lesson, part of Topic C6 of OCR Gateway Science A, explains the difference between potable and pure water, how potable water is produced from fresh water and from salt water, and how waste water is treated.

By the end of this lesson you should be able to define potable water and distinguish it from pure water, describe how potable water is produced from fresh water, describe desalination and explain why it is expensive, and describe the stages of sewage treatment.

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Potable Water Is Not Pure Water

Potable water is water that is safe to drink. It contains only low levels of dissolved salts and low levels of microbes (microorganisms), so it will not make you ill. Importantly, potable water is not the same as pure water:

• Pure water (in the chemical sense) contains only $\text{H}_2\text{O}$H2​O — nothing dissolved in it.
• Potable water is safe to drink but still contains small amounts of dissolved substances (such as harmless minerals).

So tap water is potable but not pure: it has dissolved minerals in it, which is perfectly safe (and often gives water its taste). To produce potable water, we need to reduce the dissolved salts and microbes to safe levels — not remove everything.

Exam Tip: Be precise: potable = safe to drink (low dissolved salts and microbes); pure = only $\text{H}_2\text{O}$H2​O. Saying "potable water is pure water" is a common and easily avoided error.

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Producing Potable Water from Fresh Water

In a country like the UK, which has plenty of rain, potable water is usually produced from fresh water (water with low levels of dissolved salts) from sources such as rivers, lakes and aquifers (underground rock that stores water). The steps are:

flowchart LR
  A["Choose a source\n(river, lake or aquifer)"] --> B["Filter\nto remove solids"]
  B --> C["Sterilise\nto kill microbes"]
  C --> D["Potable water\n(safe to drink)"]

1. Choose an appropriate source of fresh water (river, lake or aquifer).
2. Filter the water to remove insoluble solids such as grit, leaves and other debris (often by passing it through filter beds of sand and gravel).
3. Sterilise the water to kill microbes, making it safe to drink. Sterilising can be done using:
   • chlorine (a gas added to the water), or
   • ozone, or
   • ultraviolet (UV) light.

The two key stages are filtration (to remove solids) and sterilisation (to kill microbes) — and it is important not to confuse them. Filtering removes bits of solid; it does not kill microbes. Sterilising kills the microbes.

Exam Tip: Two stages for fresh water: filter (removes solids) then sterilise (kills microbes). Name a steriliser — chlorine, ozone or UV. Don't say filtering kills microbes — it does not.

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Desalination: Potable Water from Salt Water

In countries where there is little fresh water (for example very dry regions), potable water has to be produced from sea water or salt water. Removing the dissolved salts from salt water is called desalination. There are two main methods:

• Distillation: the salt water is boiled to produce steam (the dissolved salts are left behind because they do not evaporate), and the steam is then condensed back into pure liquid water.
• Reverse osmosis: the salt water is forced at high pressure through a partially permeable membrane that lets water molecules through but holds back the dissolved salts.

The big drawback of both methods is that they require a lot of energy — boiling water or pumping it at high pressure is expensive — so desalination is costly and is generally used only where fresh water is genuinely scarce.

Method	How it works	Drawback
Distillation	Boil → condense the steam	Lots of energy to boil the water
Reverse osmosis	High pressure through a membrane	Lots of energy to pump at high pressure

Exam Tip: Desalination (distillation or reverse osmosis) makes potable water from salt/sea water but uses a lot of energy, so it is expensive. That energy cost is the key reason it is only used where fresh water is scarce.

Worked Example — Choosing the right treatment

Country A has plenty of rivers and lakes; Country B is a desert with little fresh water but a long coastline. Suggest how each country should produce potable water, and explain why Country B's method costs more.

Step 1 — for Country A: it has fresh water, so it should filter the water (to remove solids) and then sterilise it (chlorine, ozone or UV) to kill microbes. This is relatively cheap.

Step 2 — for Country B: it has little fresh water but plenty of sea water, so it must use desalination (distillation or reverse osmosis) to remove the dissolved salts.

Step 3 — explain the cost: desalination requires a lot of energy (to boil the water or to pump it at high pressure), so Country B's method is more expensive than simply filtering and sterilising fresh water.

Answer: Country A: filter then sterilise fresh water; Country B: desalinate sea water — which costs more because of the large amount of energy required.

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Required Practical: Analysing and Purifying Water

A required practical for this topic is the analysis and purification of water. In it you:

• test water samples — for example, measure the pH with universal indicator or a pH meter, and find the amount of dissolved solids by evaporating a measured volume of the water to dryness and weighing the solid left behind (a higher mass of residue means more dissolved solids);
• purify a water sample by distillation — heat the water so it evaporates, then condense the steam to collect pure water, leaving the dissolved solids behind in the flask.

Distilled water gives no residue when evaporated (because it is pure), whereas tap water or salty water leaves a solid residue, showing it contained dissolved solids. This practical demonstrates both how to test water and how distillation purifies it.

Exam Tip: In the water practical, evaporating to dryness and weighing the residue measures the dissolved solids, and distillation produces pure water. Pure (distilled) water leaves no residue; tap or salt water leaves a solid residue.

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Why Water Treatment Matters

It is easy to forget how important these processes are. Water that has not been treated can carry microorganisms that cause diseases such as cholera and typhoid, and untreated water may also contain harmful dissolved substances and solid debris. Producing potable water — by filtering out solids and sterilising to kill microbes — is therefore one of the most important public-health measures there is, preventing the spread of water-borne disease. In many parts of the world a reliable supply of safe drinking water is still a serious challenge, which is why this is a genuine global issue and not just a laboratory exercise.