Topic 5: Water Treatment Processes

Question 1

Order of treatment processes

Answer 1

Sedimentation, screens, aeration, flocculation,filtration, activated carbon filters, sterilisation, ph control, fluoridation, ion exchange, reverse osmosis, distillation

Question 2

Sedimentation

Answer 2

Where the water is allowed to remain static, so suspended solids settle
E.g. silt

Question 3

Screens

Answer 3

Many items enter fluid flow which would cause problems later on, metal grills and meshes are used to remove litter and vegetation

Question 4

Aeration

Answer 4

Bubbles through water or aeration sprays are used to maintain a high dissolved oxygen content in the water
In anaerobic conditions, hydrogen sulphide is produced in the decomposition of dead organic Matter which leads to the water smelling of bard eggs
Aeration also removes toxic ions/metals because it makes them insoluble

Question 5

Why is flocculations/coagulation/clarification necessary?

Answer 5

Clay particles dont sink in sedimentation tanks because of the electrostatic charges on the surface cause them to repel

Question 6

Flocculations/coagulation/clarification

Answer 6

Flocculants e.g. aluminium sulphide are added
They are mixed quickly through then passed through a clarification tank allowing them to settle

Question 7

Filtration

Answer 7

Some treatment plants filter
To remove remaining suspended solids and bacteria
Slow flow through fine sand and gravel filters

Question 8

Activated carbon filters

Answer 8

Particles of activated carbon are used to remove organic fertilisers/pesticides because they absorb the activated carbons

Question 9

Sterilisation

Answer 9

Addition of chlorine and ozone to kill pathogens
Chlorine is most commonly used, but ozone can be used if the chemicals are likely to react with chlorine

Question 10

PH control

Answer 10

Sterilising water with chlorine can make it acidic
Passed through crushed lime to neutralise it

Question 11

Fluoridation

Answer 11

Fluorides are added to the water in some areas to improve the dental health of people who drink the water

Question 12

Ion exchange

Answer 12

Toxic ions are removed using ion exchange resins
The ions absorb onto polymer beads
This produces soft water

Question 13

Reverse osmosis

Answer 13

Seawater is the most abundant source but it has a high salt content
Desalination is expensive so only done if necessary
Saline water is passé through a polyamide tube at high pressure

Question 14

Distillation

Answer 14

Water is heated by boiling, or the pressure is reduced
Steam that is produced is collected and condensed