potable water and water treatment

Cards (13)

  • potable water
    water that has been treated or is naturally safe to drink - the water is not pure however, as it contains many dissolved minerals and salts
  • pure water
    pure water is solely made up of H2O molecules
  • how is potable water produced?
    1. an appropriate source of fresh water is chosen
    2. the water is then passed through filter beds to remove insoluble particles
    3. sterilise the water to kill harmful bacteria or microbes by bubbling chlorine gas through it or using ozone/ultraviolet light
  • sources of fresh water
    lakes, ponds, wetlands, streams, rivers, ground water, glaciers, drainage basins
  • desalination
    the removal of salt from seawater to make it usable for drinking and farming
  • distillation of water

    1. test the pH of the water using a pH meter - if the pH is too high or too low, it needs to be neutralised via a titration, but with a pH meter instead of an indicator
    2. test the water for the presence of sodium chloride - to test for sodium ions, use a flame test on a small sample, and to test for chloride ions, take another sample and add a few drop of dilute HNO3 then a few drops of silver nitrate solution
    3. to distil the water, pour the salty water into a distillation apparatus. heat the flask from below. the water will boil and form steam, leaving any dissolved salts in the flask. the steam will condense back to liquid water in the condenser & can be collected as it runs out
    4. retest the distilled water for sodium chloride to check that is has been removed. also, retest the pH of the water with a pH meter to check that it's between 6.5 and 8.5
  • reverse osmosis of water
    salty sea water is passed through a membrane that only allows water molecules to pass through, with the result of the ions and larger molecules being trapped by the membrane and so separated from the water
  • distillation and reverse osmosis disadvantages
    - very expensive
    - lots of energy is used
    - not practical for producing large quantities of fresh water
  • why may chemicals be added to water supplies?
    to treat and purify water, examples being chlorine for disinfection and fluoride for dental health - however this is controversial as people are not given any choice over whether they consume it or not
  • waste waster sources

    - domestic uses such as water from having a bath, using the toilet and washing-up goes into sewers after being flushed down the drain
    - agricultural systems produce a lot of waste water including nutrient run-off from fields and slurry from animal farms
    - industrial processes, such as the Haber process
  • what must be done to waste water?
    it must be treated to remove any organic matter and harmful microbes before it can be put back into fresh water sources like rivers of lakes - otherwise it would pollute them and pose health risks. industrial waste water can also contain harmful chemicals, which mean sit must undergo additional stages of treatment
  • sewage treatment
    1. screening and grit removal before the water is treated
    2. the water then stands in a settlement tank and undergoes sedimentation to produce sewage sludge (heavier suspended that solids sink to the bottom) and effluent (lighter and floats to the top)
    3. the sludge from the bottom of the settlement tank is removed and transferred into large tanks where it undergoes anaerobic digestion, which involves bacteria breaking down the organic matter in the sludge, releasing methane in the process. the methane gas can be used as an energy source and the remaining digested water can be used as a fertiliser
    4. effluent goes through aerobic biologic treatment, which uses oxygen and bacteria, to break down organic pollutants and nutrients in wastewater, converting them into stable forms like carbon dioxide and water
    5. waster water containing toxic substances requires additional stages of treatment, such as adding chemicals (e.g. to precipitate metals), UV radiation or using membranes
  • why may sewage treatment be used as an alternative for desalination of salt water?
    because it uses less energy, despite requiring more processes