Water Treatment and Potable Water

Potable Water

Potable water is water that is safe to drink. It is not pure water (pure water contains only H₂O molecules) — it contains low levels of dissolved substances that are not harmful to health.

Potable water must:

• Have sufficiently low levels of dissolved salts and other substances
• Contain no harmful microbes (bacteria, viruses, parasites)
• Be pleasant to drink (clear, no bad taste or smell)

Sources of Fresh Water in the UK

In the UK, most potable water comes from fresh water sources:

• Rivers
• Lakes and reservoirs
• Groundwater (aquifers — underground rock that stores water)

Fresh water is a renewable resource (replenished by the water cycle), but only a small fraction (about 1%) of the Earth's water is fresh and accessible.

Water Treatment Process (UK)

Treating fresh water to make it potable involves several stages:

1. Screening and Sedimentation

• Screening: Water passes through wire mesh screens to remove large debris (leaves, sticks, litter)
• Sedimentation: Water is held in large tanks where heavier particles settle to the bottom under gravity. Chemicals called flocculants may be added to help fine particles clump together and settle faster

2. Filtration

• Water passes through sand beds (sand filters) to remove fine particles
• Gravel and sand trap remaining suspended solids and some microorganisms
• Some treatment works use activated carbon filters to remove organic compounds, pesticides and improve taste/smell

3. Sterilisation (Disinfection)

The water must be sterilised to kill any remaining harmful microorganisms:

Method	How it works
Chlorine	Small amounts of chlorine gas or sodium hypochlorite are added; kills bacteria and viruses; residual chlorine stays in the water to prevent recontamination in pipes
Ozone	Ozone gas (O₃) is bubbled through the water; very effective disinfectant; breaks down to oxygen
Ultraviolet light	UV radiation kills microorganisms by damaging their DNA; leaves no chemical residue

Chlorination is the most common method in the UK. The amount of chlorine added is carefully controlled — too little would not kill all microbes, but the low levels used are safe for drinking.

Desalination

In some parts of the world (e.g. Middle East, Australia, and some islands), there is not enough fresh water. Sea water or brackish water (slightly salty) must be treated by desalination — removing the dissolved salts.

Methods of Desalination

Distillation:

1. Sea water is heated to boiling

2. Water evaporates, leaving dissolved salts behind

3. Steam is cooled and condensed to collect pure water

This produces very pure water but requires a large amount of energy to heat the water, making it very expensive.

Reverse osmosis:

1. Sea water is forced through a special membrane at high pressure

2. The membrane has tiny pores that allow water molecules through but block dissolved salt ions

3. Pure water collects on the other side

This requires less energy than distillation but still requires significant energy to maintain high pressure. The membranes also need regular replacement.

Why Desalination Is Not Widely Used in the UK

• The UK has sufficient rainfall and fresh water sources
• Desalination is expensive due to the energy required
• The infrastructure (pipelines, membranes) has high capital costs
• It is only economically viable where fresh water is very scarce

Required Practical: Water Purification

You may be asked about testing and purifying water in the lab.

Testing Water Purity

• pH test — use universal indicator or pH meter (potable water should be close to neutral, pH 6.5–8.5)
• Dissolved solids — evaporate a known volume of water to dryness and weigh the residue
• Presence of specific ions — use chemical tests (e.g. silver nitrate for chloride ions, barium chloride for sulfate ions)

Simple Distillation in the Lab

1. Place the water sample in a round-bottomed flask

2. Heat to boiling

3. Steam travels through a condenser (cooled by cold water flowing around the outside)

4. Pure water condenses and drips into a collection flask

5. Dissolved solids remain in the original flask

This produces pure water but uses a lot of energy. It is useful for separating water from dissolved solids or for obtaining a small sample of pure water for analysis.

Waste Water Treatment

Used water (sewage and industrial waste) must be treated before it can be released back into the environment. Sewage treatment involves:

1. Screening — remove large solids (rags, sticks)

2. Primary treatment — sedimentation produces sludge (settles) and liquid effluent

3. Secondary (biological) treatment — aerobic bacteria break down organic matter in the effluent; sludge is digested anaerobically (producing biogas — mainly methane)

4. Final treatment — effluent may be filtered and sterilised before release into rivers

Industrial waste water may require additional treatment to remove harmful chemicals (e.g. heavy metals, organic solvents) before entering the sewage system.

Exam Tips

• Potable water is NOT pure water — it contains dissolved substances at safe levels
• Know the three steps: sedimentation → filtration → sterilisation
• Be able to compare chlorination, ozone and UV as sterilisation methods
• Desalination is energy-intensive — explain why using distillation or reverse osmosis
• In the required practical, simple distillation is used to purify water in the lab
• Waste water treatment involves both physical (screening, sedimentation) and biological (bacterial digestion) stages