Water Treatment for Different Sources Revision Notes | GCSE AQA Higher Chemistry

1. Why Water Treatment Matters

Not all water is ready to drink straight from the source. Ground water might contain bacteria, whilst seawater contains far too much salt for our bodies to handle. The treatment needed depends entirely on where the water comes from.

In water-scarce regions, communities face a crucial choice: treat contaminated ground water or remove salt from abundant seawater. Each path requires different technology, energy, and costs.

2. Ground Water vs Salty Water Treatment

Ground water and salty water need completely different treatment approaches. Think of it like cleaning two different types of mess - you wouldn't use the same method to remove mud as you would to remove paint.

Ground water treatment typically involves filtration through sand and gravel, followed by chlorination to kill microbes. The dissolved salt content is usually low enough that we don't need to remove it.

Salty water contains dissolved sodium chloride and other salts that must be physically separated from the water molecules. Simple filtration won't work - the salt particles are dissolved, not just suspended.

3. Desalination Methods

When fresh water supplies are limited, we turn to desalination. Two main methods dominate: distillation and reverse osmosis. Both work, but they use energy in very different ways.

Worked example: How Distillation Works

Describe the process of desalinating seawater by distillation

1. Seawater is heated to approximately 100°C using fossil fuels or solar energy — Only water molecules have enough energy to escape as steam
2. Pure water vapour rises, leaving dissolved salts in the original container — Salt has a much higher boiling point than water
3. Water vapour is cooled in a condenser, turning back into liquid water — This pure water is now safe to drink
4. Distilled water is collected whilst concentrated brine is disposed of — The remaining brine contains all the original salts in less water

Answer: Distillation produces pure water by using the different boiling points of water and salt

Reverse osmosis works like an incredibly fine filter. The membrane has pores just large enough for water molecules but too small for salt ions. High pressure forces water through whilst salt stays behind.

4. Energy Requirements

Here's the catch with desalination: it requires enormous amounts of energy. Whether you're boiling water or forcing it through membranes under pressure, you're fighting against nature's tendency for things to mix.

Distillation needs energy to heat water to approximately 100°C and convert it to steam. Reverse osmosis needs energy to create the high pressure that forces water through membranes. Both processes consume far more energy than treating ground water.

Worked example: Comparing Energy Costs

A desalination plant can use distillation or reverse osmosis. Compare the methods.

1. Energy per litre = $\frac{15 \text{ kWh}}{1000 \text{ litres}} = 0.015$ kWh/litre — Divide total energy by volume produced
2. Energy per litre = $\frac{4 \text{ kWh}}{1000 \text{ litres}} = 0.004$ kWh/litre — Same calculation for reverse osmosis
3. Distillation uses $\frac{0.015}{0.004} = 3.75$ times more energy — Reverse osmosis is more energy efficient

Answer: Distillation: 0.015 kWh/litre, Reverse osmosis: 0.004 kWh/litre

The high energy demand explains why desalination plants are often built in sunny, oil-rich countries. Solar energy can power the process during the day, whilst fossil fuels provide backup power.

5. Exam Technique and Common Mistakes

Water treatment questions often test your understanding of why different methods are needed for different water sources. The key is matching the treatment to the contamination type.

Common trap: Many students think all water treatment is the same. Ground water needs disinfection and basic filtration, whilst salty water needs complete salt removal - these require totally different approaches and energy inputs.

Exam precision note: In Water Treatment for Different Sources - Potable water - Using the Earth's resources and obtaining potable water, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

Water treatment depends on the source. Fresh water from rivers and reservoirs is filtered and sterilised, while seawater may need distillation or reverse osmosis. Distillation is energy intensive because water has to be heated and then condensed again, so it is usually reserved for places where fresh water is scarce.