water treatment Flashcards
What are the criteria that must be fulfilled before water is supplied to houses?
It must be:
attractive looking (i.e. colourless)
odourless
safe to drink (no active bacteria present)
must be fluoridated
What corporation carries out water treatment in Ireland?
The Environmental Protection Agency (EPA)
What are the necessary steps of water treatment in order?
- screening
- flocculation
- settlement
- filtration
- chlorination
- fluoridation
- pH adjustment
What happens in the first step of water treatment (screening)?
screening:
the water is passed through grided screens/wire mesh to remove debris (floating debris such as twigs and plastic bags)
What happens in the stage of flocculation? (stage 2)
flocculation:
a flocculating agent such as aluminium sulfate { Al2(SO4)3 }, is added to cause any suspended particles to coagulate together so that they can be removed
what does flocculation mean?
coagulation
How can very small suspended particles in the water form larger particles?
By adding certain chemicals to water, ‘flocs’ are produced
what flocculation agent is used in step 2?
ammonium sulfate
define flocculation
flocculation is the coming together (coagulating) of small suspended particles in water
What does excess aluminum sulfate in water do?
it affects the taste of water and causes corrosion of pipes
What happens after flocculation? (step three in other words)
settlement (sedimentation):
the water passes into settlement tanks so that the coagulated suspended particles can settle on the bottom of the tank
What happens to clear water in sediment tanks?
the water flows in at the bottom of these tanks and rises up slowly to the surface where the clear water is allowed to overflow (decant) into channels
What is the 4th step of water treatment? explain this process?
filtration:
the water passes through filter beds (sand and gravel) and any remaining suspended particles can be removed.
Is the water clear after passing through the filter beds in step 4 of water treatment (filtration)?
Yes, it is now clear but is not safe to drink as it may contain harmful bacteria
Explain the 5th step of water treatment. (after filtration)
- chlorination:
chlorine is added in small quantities, usually as sodium hypochlorite (NaClO) to sterilise the water, as it may contain harmful bacteria as ‘free chlorine’ is an oxidising agent and will oxidise the bacterial enzymes
how is chlorine added to water/in what form?
sodium hypochlorite
what does ‘free chlorine’ do to bacteria?
‘free chlorine’ is an oxidising agent and will oxidise the bacterial enzymes
is free chlorine an oxidising or reducing agent?
oxidising agent
what are the three methods/steps that remove suspended solids from water?
flocculation
sedimentation
filtration
Explain the 6th step of water treatment
Fluoridation:
hexafluorosilicic acid (H2SiF6) or sodium fluoride (NaF) is added (small quantities - 1ppm) to prevent tooth decay
How does the presence of fluoride ions in water help prevent tooth decay?
it does this by strengthening the enamel of teeth
What is the 7th step of water treatment?
- pH adjustment
Explain the 7th step of water adjustment
pH adjustment:
If the pH of the water is too high then dilute sulfuric acid is added.
If the pH is too low then calcium hydroxide is added.
The pH of drinking water should be in the range of 7-9.
An acidic pH would lead to corrosion of pipes.
What must be added to water if the pH is too high?
dilute sulfuric acid is added
What must be added to water if the pH is too low?
calcium hydroxide is added
What would an acidic pH in water do?
An acidic pH would lead to corrosion of pipes
What would happen if too much calcium hydroxide (lime) is added to water?
the amount must be controlled as adding too much could affect the taste of the water
Answer the following questions in relation to the chemical aluminium sulfate
What is its purpose in water treatment?
What are the problems caused by adding excess?
- coagulation of small suspended particles
- taste of water is affected.
Corrosion of pipes
Answer the following questions in relation to the chemical chlorine
What is its purpose in water treatment?
What are the problems caused by adding excess?
- sterilise water (kill harmful micro-organisms)
- taste and smell of water affected
Answer the following questions in relation to the chemical fluorine compound (e.g. sodium fluoride or hexofluorosilicic)
What is its purpose in water treatment?
What are the problems caused by adding excess?
- reduce tooth decay
- staining of teeth
Answer the following questions in relation to the chemical calcium hydroxide
What is its purpose in water treatment?
What are the problems caused by adding excess?
- raise pH
- hardness of water
Answer the following questions in relation to the chemical sodium carbonate
What is its purpose in water treatment?
What are the problems caused by adding excess?
- soften water
- taste of water affected
Answer the following questions in relation to the chemical sulfuric acid
What is its purpose in water treatment?
What are the problems caused by adding excess?
- lower pH
- corrosion of pipes
