U2- T8

Question 1

What must be done before treating water and why?

Answer 1

Before water is treated, its quality must be assessed to ensure that the appropriate methods are used to purify it.

Question 2

Explain what are screens and for what are they used ?

Answer 2

Screens are metals grills or meshes. They are used to remove floating vegetation, plastic and paper that would clog later processes.
Screens prevent large solids like branches and plastic bags from entering the water treatment plant.

Question 3

Explain what happens while sedimentation

Answer 3

The water is allowed to remain static to enable suspended solids such as silt to settle

Question 4

Explain what is aeration

Answer 4

It’s the process of bringing water and air into close contact in order to remove dissolved gases such as carbon dioxide, and to oxidize dissolved metals such as iron. It can also be used to remove volatile organic chemicals in the water. Aeration is often the first major process of treatment plant. Bubbles of air or water sprays are used to aerate the water and ensure a high dissolved oxygen content. Water sources that are anaerobic contain hydrogen sulfide from the decay of organic matter (smell: bad eggs)

Question 5

For what are used particles of activated carbon?

Answer 5

They are used to remove organic chemicals such as pesticides and some substances that cause bad tastes

Question 6

Explain the process of sterilisation

Answer 6

The addition of chlorine, ozone or exposure to ultraviolet light is used to sterilise the water and kill the pathogens. Chlorine is the most common method of sterilisation but if water is from peaty sources then organic matter ma be present, which could react with chlorine and produce toxic substances such as trihalomethanes that are carcinogenic. Ozone also helps to break down pesticides.

Question 7

What does osmosis mean?

Answer 7

It is the diffusion of water through a partially permeable membrane from an area of high water potential to an area of low water potential

Question 8

For what fluorides are added to water in some areas?

Answer 8

To improve the dental health of people that drink the water

Question 9

What happens during reverse osmosis?

Answer 9

During reverse osmosis the saline water is filtered at very high pressure through a partially permeable membrane of very small polyamide tubes

Question 10

What makes desalinisation of water very expensive?

Answer 10

A lot of energy is needed to produce the very high pressures, which makes desalinated seawater very expensive

Question 11

Explain the process of distillation

Answer 11

Water is boiled by heating and reducing the pressure. The steam that is produced is condensed and collected

Question 12

Which chemicals is sometimes added to water to improve the resistance of tooth enamel to decay?

Answer 12

Fluorine

Question 13

Describe 3 methods by which harmful bacterias may be killed to produce potable water

Answer 13

Chlorine
Ozone
Ultraviolet light

Question 14

Explain the disadvantages of the use of chlorine to kill bacterias in water

Answer 14

1. It has a distinctive taste and smell
2. The production of chlorine is energy intensive
   3 it may convert organic material into harmful chemical

Question 15

Explain the advantages of the use of chlorine to kill bacterias in water

Answer 15

It is very effective and can kill microorganisme over the whole time from initial treatment, through the pipes and and to the out flow from taps

Question 16

Explain the disadvantages of the use of ozone to kill bacterias in water

Answer 16

Energy is needed for the process of oxidizing sulphur and iron salts in water so it is more costly and cannot be carried out if there is a power failure.
Ozone can also form some unpleasant by-products such as bromates which are harmful to human health

Question 17

Explain the advantages of the use of ozone to kill bacterias in water

Answer 17

It has no bad smell and is relatively quickly converted back to oxygen and can oxidize sulphur and iron salts in water

Question 18

Explain the disadvantages of the use of UV light to kill bacterias in water

Answer 18

It’s quiete expensive, and doesn’t work very well if the water is turbid or colored. The main disadvantage however is that it has no residual effect, so any bacteria picked up during distribution would not be killed –> it is therefore more useful for private supplies where chlorination or ozone treatment would be difficult to administer.

Question 19

Why do we treat water?

Answer 19

To make it potable quality

Question 20

What does the treatment of freshwater involves?

Answer 20

Sedimentation, flocculation, filtration, activated carbon filtration, aeration, chlorination/ozonation/ UV treatment and fluoridation

Question 21

Name 2 méthodes for desalination of water

Answer 21

Distillation or reverse osmosis, both of which require large amounts of energy

Question 22

Name 5 types of reasons for which demand on water may vary

Answer 22

1. Population size
2. Living standards
3. Attitudes to water use
4. Agricultural practices
5. amount and type of industry