How Does Water Treatment Work? GCSE Chemistry Step-by-Step

Why does water need treatment?

Raw water drawn from rivers, reservoirs, or underground aquifers contains a mixture of dissolved minerals, suspended solids, micro-organisms, and potentially harmful chemicals. Rainwater picks up dissolved gases (including carbon dioxide, forming carbonic acid) and mineral ions as it percolates through rock. River water can carry agricultural run-off containing nitrates and pesticides, sewage, and industrial pollutants.

The goal of treatment is to produce water that is:

• Safe — free from pathogens and toxic chemicals
• Palatable — clear, odourless, and pleasant to taste
• Chemically acceptable — meeting legal limits for dissolved substances

How does water treatment work step by step?

1. Screening. Raw water passes through metal screens or grids that remove large debris such as leaves, twigs, and litter. This protects equipment in later stages from physical damage.

2. Sedimentation (settling). Water is held in large settlement tanks where the flow rate slows dramatically. Gravity causes suspended particles — fine sand, silt, and clay — to sink to the bottom as sediment. Coagulants such as aluminium sulfate (alum) are often added; they cause tiny particles to clump together (flocculate) into larger aggregates called floc, which settle far more quickly than individual particles.

3. Filtration. Water passes slowly through beds of sand and gravel. These filter out remaining fine particles and some micro-organisms. The sand layer removes particles as small as a few micrometres. Some treatment works also use activated carbon filters, which adsorb dissolved organic compounds and improve taste and odour.

4. Chlorination. Chlorine (or chlorine dioxide) is added in precisely controlled quantities. Chlorine kills bacteria, viruses, and other pathogens that have survived previous stages. It also provides a residual disinfectant effect throughout the distribution network, protecting water from recontamination in pipes. The dose must be high enough to be effective but low enough to avoid producing harmful disinfection by-products.

5. pH adjustment. Lime (calcium hydroxide) or sodium hydroxide may be added to raise the pH of slightly acidic water to between 7 and 8. This reduces corrosion of metal pipes and is required by law in the UK.

6. Fluoridation (where applicable). In some parts of England, sodium fluoride or fluorosilicic acid is added at approximately 1 mg per litre. Fluoride reduces tooth decay, particularly in children. This is a public health measure that is mandatory in some regions and controversial in others.

7. Final testing and distribution. Treated water is tested for microbial and chemical quality against Drinking Water Inspectorate standards before entering the distribution network.

What is the difference between drinking water and distilled water?

Distilled water is produced by boiling water and condensing the steam, leaving dissolved salts and ions behind. It is not suitable for routine drinking in large quantities because it lacks the minerals the body needs and can, in theory, leach minerals from the body if consumed exclusively.

What is the connection between hard water and water treatment?

Hard water contains dissolved calcium ions (Ca²⁺) and magnesium ions (Mg²⁺), picked up as water passes through limestone (calcium carbonate) and dolomite rocks. Hard water is safe to drink and may even have mild health benefits, but it causes several practical problems:

• Scale (limescale): Temporary hardness is caused by dissolved calcium hydrogencarbonate. When hard water is heated, it decomposes back to calcium carbonate, which precipitates as limescale in kettles, boilers, and heating systems, reducing efficiency.
• Soap scum: Hard water reacts with soap to form an insoluble precipitate (scum) rather than a lather. More soap is needed, increasing cost.

Removing hardness:

• Temporary hardness can be removed by boiling, which drives off CO₂ and deposits CaCO₃.
• Permanent hardness (caused by calcium sulfate) cannot be removed by boiling. It requires ion exchange (water softeners replace Ca²⁺ and Mg²⁺ with Na⁺) or the addition of sodium carbonate (washing soda), which precipitates calcium carbonate.

Some water treatment works use ion exchange softening as part of the treatment process in hard-water areas.

How is the quality of treated water tested?

Water companies carry out routine tests including:

• Microbial testing: Coliform bacteria counts to detect faecal contamination
• Chemical testing: Levels of nitrates, pesticides, heavy metals, and disinfection by-products
• pH and turbidity: Clarity and acidity measurements
• Chlorine residual: To ensure adequate disinfectant remains in the network

Results must meet standards set by the Drinking Water Inspectorate (DWI) and are published annually.

Frequently asked questions

Q: Why is chlorine used rather than just boiling the water?

Boiling kills pathogens effectively at the treatment works, but it provides no ongoing protection once water enters the distribution network. Pipes can harbour bacteria, and joints and storage tanks can allow contamination. Chlorine remains dissolved in the water throughout the network, providing continuous disinfection. Boiling water at a treatment works on the scale of millions of litres per day would also be prohibitively expensive.

Q: Is fluoride in drinking water safe?

At the concentrations used in the UK (around 1 mg per litre), fluoride is considered safe by the World Health Organization, Public Health England, and the UK government. It occurs naturally in many water supplies at similar or higher concentrations. High concentrations — above 1.5 mg/L — can cause dental fluorosis (mottling of teeth) and, at very high levels, skeletal fluorosis, but these levels are far above those used in public supplies.

Q: What is the difference between hard and soft water?

Hard water contains high concentrations of dissolved calcium and magnesium ions, typically picked up from limestone rock. Soft water contains low concentrations of these ions. You can feel the difference: soft water lathers easily with soap, while hard water forms a scum. Rainwater is naturally soft; it becomes hard after percolating through chalky or limestone ground.

Q: Why are coagulants added during sedimentation?

Very fine particles such as clay and colloidal material carry negative surface charges that keep them suspended by mutual repulsion — they resist settling under gravity alone. Aluminium sulfate (alum) dissolves to produce positively charged aluminium ions and aluminium hydroxide, which neutralise the surface charges and cause the particles to clump together (flocculate) into larger aggregates that settle quickly. Without coagulation, the water would remain turbid despite prolonged settling time.

Q: Could salt water be treated this way to make drinking water?

No. The stages described above — sedimentation, filtration, and chlorination — do not remove dissolved ions such as sodium chloride. To desalinate seawater you need either reverse osmosis (forcing water under high pressure through a semi-permeable membrane) or distillation. Both processes are far more energy-intensive than conventional treatment, which is why desalination is mainly used in water-scarce regions such as the Middle East.

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