Water Treatment

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CognitoWater treatment and desalination

Key facts & methods

Making water potable (UK — fresh water source)

Potable water is safe to drink. It is NOT pure (contains dissolved minerals — essential for taste and health).
Step 1: Sedimentation — water held in reservoir/settlement tank. Large particles settle.
Step 2: Filtration — water filtered through sand and gravel beds. Removes smaller particles.
Step 3: Chlorination — small amounts of chlorine added. Kills bacteria and other microbes.
UV treatment is an alternative to chlorination.
pH adjustment (lime) may also be used.

Desalination (removing salt from sea water)

Used in water-scarce regions. Two main methods:
Distillation: sea water heated → water evaporates → condensed → collected. Salt left behind. Very high energy cost.
Reverse osmosis: water forced through semi-permeable membrane under high pressure. Water molecules pass through; salt ions are too large.
Both methods are more expensive and energy-intensive than treating fresh water.
Used in Middle East, parts of Australia, California.

Waste water treatment

Sewage treatment works:
Screening: large solids removed by metal grids.
Sedimentation: settled sludge removed. Water enters biological treatment.
Biological treatment: microorganisms break down organic matter in aerated tanks.
Treated water tested and released into rivers. Must meet strict legal standards.
Sludge: dried and used as agricultural fertiliser or for biogas production.

Exam questions — 3 questions · 9 marks · AQA 8462 style

1 markTreatment stepAQA 8462 style

Which treatment step kills bacteria in drinking water?

A Sedimentation

B Filtration through sand and gravel

C Chlorination

D Reverse osmosis

1 markPotable vs pureAQA 8462 style

Why is potable water NOT the same as pure water?

A Potable water contains harmful microbes

B Pure water is unsafe to drink

C Potable water contains dissolved minerals and is safe to drink; pure water contains only H2O

D Potable water is treated with chemicals that make it impure

4 marksCompare desalination methodsAQA 8462 style

Compare distillation and reverse osmosis as methods of desalination. Include how each method works and one advantage and disadvantage of each. (4 marks)

Hint: For each method: explain how it works (1 sentence). Then one advantage and one disadvantage. Be specific.

+40 XP

Making water potable (UK — fresh water source)

Desalination (removing salt from sea water)

Waste water treatment

Module complete! 🎉