Potable water

• Give 3 methods used to sterilise potable water

  - UV light

  - Ozone

  - chlorine gas.
• What is potable water?

  Water that has been treated or is naturally safe for humans to drink
• What are 2 key features of potable water?

  • pH between 6.5 and 8.5
  • Levels of dissolved salts aren't too high
• What is surface water?

  Water in lakes, rivers and reservoirs
• What is ground water?

  Water trapped in rocks underground
• What are the rocks that trap water underground called?

  Aquifers
• What are the 3 stages of treating water?

  • Filtration- wire mesh screens out large twigs etc
  • Gravel and sand beds filter out any other solids
  • Srerilisation - bubble chlorine or ozone through or use ultraviolet light
• Why is water sterilised?

  To kill any harmful bacteria or microbes
• What is the process of treating sea water called?

  Desalination
• Why might sea water have to be used and treated?

  In dry countries there may not be enough surface or ground water
• How can you desalinate sea water?

  - Distillation

  - Membranes.
• How can membranes desalinate sea water?

  • Salty water 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 does domestic and agricultural waste water need to be treated?

  To remove any organic matter and harmful microbes before it can be put back into freshwater sources like rivers or lakes
• What happens to sewage at the start of the treatment process?

  Screened = removing any large bits of material like twigs or plastic bags and grit
• What is the second stage of the sewage treatment process?

  Sedimentation
• What is sedimentation?

  • Sewage stands in a settlement tank
  • Heavier solids sink to the bottom to produce sludge
  • Lighter effluent floats on the top
• What is the third stage in the sewage treatment process?

  Aerobic digestion
• What substance goes to do aerobic digestion in the sewage treatment process?

  Effluent
• What happens during aerobic digestion?

  • Effluent removed from settlement tank
  • Air pumped through water
  • Encourages aerobic bacteria to break down any organic matter including microbes
• Which substance goes for anaerobic digestion in the sewage treatment process?

  Sludge
• What is the 4th stage of the sewage treatment process?

  Anaerobic digestion
• What is an anaerobic digestion in the sewage treatment process?

  • Sludge removed from bottom of settlement tank
  • Transferred into large tanks
  • Broken down by bacteria
• What gas is released during anaerobic digestion and what can it be used for?

  • Methane
  • Energy source - natural gas
• What can the remaining digested waste after anaerobic digestion be used as?

  Fertiliser
• If the waste water contains chemicals (industrial waste) what additonal stages may there be in the sewage treatment process?

  • Adding chemicals
  • UV radiation
  • using membranes