potable/safe drinking water and water treatment

Question 1

What is the difference between potable water and pure water?

Answer 1

potable water (safe drinking water)
- water with low levels of 
  dissolved salts such as 
  sodium chloride
- water with low levels of 
  microbes such as bacteria 

pure water
- contains no dissolved
substances at all

Question 2

where is the water used to produce potable water found and why do we use it?

Answer 2

- rain water 
  is collected in the ground 
  aquifers (which can sometimes be polluted eg fertilisers from farms) and in lakes, rivers 
  and reservoirs 
- we use it because it has 
  low levels of dissolved 
  substances

Question 3

Describe the steps used to produce potable water from fresh water
(this is the easiest method of producing potable water)

Answer 3

• find a good source of fresh
  water eg a river

- then pass the water 
  through filter beds to 
  remove materials such as 
  leaves and suspended 
  particles 

• the water is then sterilised
  to kill microbes
  ( in the uk we use chlorine)

Question 4

Describe the steps used to produce potable water from sea water.

Answer 4

• sea water has very high
  levels of dissolved minerals
  so potable water is
  produced by desalination
• one way of carrying out
  desalination is to use
  distillation
• another way is to pass
  water through membranes
  which is called reverse
  osmosis

- both distillation and 
  reverse osmosis reduce 
  the amount of dissolved 
  minerals in the water but 
  both processes require 
  very large amounts of 
  energy which makes the 
  very expensive

Question 5

what is waste water and what do we do with it

Answer 5

the water that is used by humans eg toilets, showers, washing clothes and agriculture

it contains large amounts of organic molecules (eg from urine and faeces)
it also contains harmful microorganisms such as bacteria

waste water must be carefully treated before being released back into the environment

Question 6

Describe how waste water is treated

Answer 6

• in the first stage the sewage is screened by passing through a mesh which removes solids and pieces of grit
• the sewage is allowed to settle in large sedimentation tanks which produces liquid effluent and a semi-sold sludge which sinks
• the sludge is then takes away and digested by anaerobic bacteria
  in the absence of oxygen these bacteria produce biogas which can be burned for electricity
• the digested sludge can then be used as fertilisers for farming
• the liquid effluent contains large amounts of organic molecules and harmful microorganisms which need to be reduced before the water is returned to the environment so air is bubbled through the liquid effluent to allow aerobic bacteria to multiply
• after this stage the liquid effluent can be safely discharged into near by rivers or the sea

Question 7

how is water from making paper and chemicals treated

Answer 7

all harmful chemicals need to be removed

the water then enters general sewage treatment

Question 8

what happens to treated sewage

Answer 8

in some parts of the world it is used directly to produce potable water

but this takes many purification steps so this is only done in places where water is scarce