Water Treatment

Question 1

Objectives

Answer 1

• Remove suspended matter
• Disinfect the water
• Remove other chemicals harmful to health
• Reduce corrosive properties
• Minimise the material passing through

Question 2

Corrosivity

Answer 2

Affected by pH - low pH is corrosive to metals, high pH reduces effectiveness of chlorine - aim 7.5-7.9
And affected by carbonate chemistry

Question 3

Hardness

Answer 3

Sufficient levels of calcium and magnesium can cause limescale

Question 4

Stages of Water Treatment:

Answer 4

1. Storage
2. Screening
3. Chemical
4. Settlement
5. Filtration
6. Disinfection by Chlorination
7. Sludge Treatment and Disposal

Question 5

1. Storage

Answer 5

This is used to:
- reduce bacteria numbers
- reduce turbidity
- reduce nutrients and soluble metals
However, issues include blue-green algae in Autumn or stratification

Question 6

2. Screening

Answer 6

Screens at 2-4 mm to prevent gross solids such as leaves and debris. Can be a micro strainer (revolving steel drum mesh - 20-60 microns)

Question 7

Coagulation

Answer 7

Chemical conditioning to assist in the removal of non settleable fine particles, algae, colloids, colour

Question 8

Coagulant

Answer 8

Chemical substance added to water to precipitate suspended solids in water.Minute particulate matter is encouraged to collect together to facilitate their removal

Question 9

Floc

Answer 9

Precipitate from chemical coagulation is called floc.

Question 10

Alum

Answer 10

Commonly used coagulant aluminium suphate

Question 11

Flash Mixer

Answer 11

Chamber in which coagulants are stirred into raw water

Question 12

Coagulant aid

Answer 12

A substance which when added to water, intensifies the settling action

Question 13

Focculation

Answer 13

The process of gently stirring a chemical coagulant in water to promote the collection together of dissolved irons, colloids and other suspended matter.

Question 14

Clarification

Answer 14

To remove heavy, settleable solids/silts. Is the treatment process in which minute suspended solids are removed. Water is conducted at a controlled rate through a tank allowing the particles to settle or become suspended in a ‘blanket’ suspension.

Question 15

4. Settlement

Answer 15

Occurs in a settling tank (clarifier, sedimentation basin, settling basin) is where settlement occurs.

Question 16

Dissolved air flotation

Answer 16

Removes non-settleable material, algae as a variant of clarification

Question 17

Flotation Tank

Answer 17

1. Water saturated with air is injected through special nozzles causes dispersed mass of tiny air bubbles.
2. These attach to and induce suspended matter to rise to the top in a flotation tank
3. Sludge is removed from the top of the surface with algae

Question 18

Slow Sand Filters

Answer 18

They combine the removal or particulate matter and to some degree the removal of bacteria and other organic impurities.
They are efficient with no chemical addition, reduces turbidity, coliform. However, is labour intensive to wash the sand and provides poor algae and colour removal.

Question 19

Rapid Gravity Filtration

Answer 19

A bed of filter sand with supporting gravel, overlies a series of collection pipes fitted with special slotted nozzles. These work at high rates and can be backwashed to dislodge the accumulated dirt.

Question 20

Pressure Filters

Answer 20

Same concept as rapid gravity but controlled by pressure either horizontally or vertically. This is much more compact.

Question 21

Disinfection

Answer 21

the final stage of treatment in which any remaining bacteria, pathogens are killed off.

Question 22

6. Disinfection by chlorination

Answer 22

Typically marginal chlorination - approx. 0.3 mg/l for 30 minutes
Else superchlorination to kill all bacteria - 1-4 mg/l for 30 minutes excess chlorine is then removed by the additions of sulphur dioxide

Question 23

7. Sludge Treatment and Disposal

Answer 23

Sludge collected needs to be disposed of. It can be transported to the nearest STW. On large works de-watering and volume reduce techniques then can be sent to landfill.

Question 24

Groundwater Treatment

Answer 24

Typically only disinfection, but may include iron, manganese, pesticide, nitrate, solvent, crypto or arsenic removal.

Question 25

Further works: Ultrafiltration

Answer 25

Can be used for the removal of viruses and crypto

Question 26

Further works: Nanofiltration

Answer 26

Can be used for colour removal

Question 27

Further works: Reverse osmosis

Answer 27

Used for desalination