Using resources 2 Flashcards
Potable and Waste water
What is potable water?
Water that is safe to drink.
How is potable water obtained?
Either it’s naturally safe or it has to be treated.
What is pure water?
Water only containing H₂O molecules.
Is potable water pure?
No because it can contain lots of other dissolved substances.
What should drinking water have for it to be considered ‘potable’.
Sufficiently low levels of dissolved salts and microbes (including bacteria).
A pH between 6.5 and 8.5.
What do the methods used to produce potable water depend on?
Available supplies of water
Local conditions
What does rain provide in the UK?
Water with low levels of dissolved substances (freshwater).
This collects in the ground, in
lakes, and in rivers.
Briefly describe how most potable water is produced.
An appropriate source of fresh water is chosen.
The water is passed through filter beds.
The water is sterilised.
List the sterilising agents used for potable water.
Chlorine
Ozone
Ultraviolet light
If supplies of fresh water are limited, where can we get water from?
Salty or sea water.
What is desalination?
The removal of salt from seawater using distillation or by processes that use membranes such as reverse osmosis.
Why is using freshwater rather than salt water preferable?
Desalination requires large amounts of energy.
How can rainwater collect?
As surface water (in lakes, rivers, and reservoirs).
As groundwater (in rocks called aquifers that trap water underground).
Why does most of the domestic water supply in warm areas e.g southeast of
England come from groundwater?
Surface water dries up too quickly
Filtration
A wire mesh screens out large items such as twigs.
Then, a gravel and sand bed is used to filter out any remaining insoluble solids.
Sterilisation
The water is sterilised to kill any harmful bacteria or microbes. This can be done by bubbling chlorine gas through it or by using ozone or ultraviolet light.
Why may seawater have to be used rather than freshwater to provide potable water?
In very dry countries there’s not enough surface or groundwater so potable water must be obtained from seawater.
Steps to desalinating saltwater using distillation:
-Test the pH of the water using a pH probe and meter
-Test the water for the presence of sodium chloride (the main salt in seawater)
-Distil the water by pouring it into a distillation apparatus (diagram?) and heating the flask. The water will boil forming steam, leaving any (dissolved) salts in the flask.
-The steam will condense back to liquid water in the condenser and can be collected as it runs out.
-Retest the distilled water for sodium chloride to ensure it has been removed.
-Retest the pH of the water to check that it’s neutral.
What must we do if the pH of the water is too high or low?
Neutralise it e.g. by titration.
Why do we use a pH meter rather than an indicator to test the pH of water?
It won’t contaminate the water.
How do we test for sodium ions?
Do a flame test on a small sample.
If sodium ions are present the flame will turn yellow.
How do we test for chloride ions?
Take a small sample of your water and add a few drops of dilute nitric acid, followed by a few drops of silver nitrate solution.
If chloride ions are present, a white precipitate will form.
Reverse osmosis
The salty water is passed through a membrane that only allows water molecules to pass through.
Ions and larger molecules are trapped by the membrane so separated from the water.
Why are desalination or reverse osmosis not practical for producing large quantities of fresh water?
The processes require loads of energy so are really expensive and hence not practical.
Industrial waste water may require what?
The removal of organic matter and harmful chemicals.
Sewage and agricultural waste water require what?
The removal of organic matter and harmful microbes.
Urban lifestyles and industrial processes produce large amounts of waste water that require what?
Treatment to ensure that we don’t pollute the natural environment.
Where does water from flushed toilets, sinks etc go?
Into the sewers and towards sewage treatment plans.
Why is the adding of additional chemicals e.g. fluoride to water controversial?
People aren’t given any choice as to whether they consume them or not.
How do agricultural systems produce waste water?
Nutrient run-off from fields
Slurry from animal farms
Why does sewage from domestic or agricultural sources need to be treated before it can be put back into freshwater sources (like rivers and lakes)?
To remove any organic matter and harmful microbes - which would pollute the freshwater sources and pose health risks.
Give an example of an industrial process resulting in waste water.
The Haber process.
What might industrial waste contain?
Organic matter
Harmful chemicals
So, industrial waste has to undergo additional stages of treatment before it is safe to release to the environment.
What does sewage treatment include?
- screening and grit removal
- sedimentation to produce sewage sludge and effluent
- anaerobic digestion of sewage sludge
- aerobic biological digestion of effluent.
1- Screening (before treatment of waste water)
Large insoluble solids like twigs, alongside grit, are removed from the sewage.
2- Sedimentation
The sewage is set aside to stand in a settlement tank and undergoes sedimentation, whereby heavier suspended solids sink to the bottom to produce sludge, and the lighter effluent floats to the top.
4- aneorobic digestion of sewage sludge
The sludge from the bottom of the settlement tank is removed and transferred into large tanks. It gets broken down here by bacteria in a process called anaerobic digestion.
3 - aerobic biological digestion
The effluent in the settlement tank is removed and treated by aerobic biological digestion, whereby air is pumped through the water to encourage aerobic bacteria to break down any organic matter in the water. This includes other microbes.
step 5 in treatment of sewage
When anaerobic respiration/the bacteria breaks down the organic matter in the sludge, this releases methane gas. The methane gas can be used as an energy source and the remaining digested waste can be used as a fertiliser.
step 6 in treatment of sewage
For waste water containing toxic substances, additional stages of treatment may involve adding chemicals (e.g. to precipitate metals), UV radiation, or using membranes.
Sewage treatment positive
Uses less energy than the desalination of salt water so could be used as an alternative in areas where there’s not much fresh water.
For example, Singapore is treating waste water and recycling it back into drinking supplies.
Sewage treatment negatives
Requires more processes than treating freshwater.
People don’t like the idea of drinking water that used to be sewage.
