waste water treatment

    Cards (11)

    • humans use a great deal of water - only a small percentage is for drinking
    • uses of water
      • drinking
      • personal hygiene e.g in baths and showers
      • flushing toilets
      • washing clothes
      • agriculture
      • industry e.g in making paper and chemicals
    • waste water contains a very large amount of organic molecules e.g from urine and faeces, also contains harmful microorganisms e.g bacteria
    • waste water must be carefully treated before being released back into environment
    • treatment pt. 1
      1. sewage screened by passing through a mesh - removes solids and pieces of grit
      2. sewage left to settle in large sedimentation tanks - produces liquid effluent and semi-solid sludge which sinks
      3. sludge taken away - digested by anaerobic bacteria
      4. in absence of oxygen, bacteria produce biogas - burned for electricity
      5. digested sludge can be used as fertilisers for farming
    • treatment pt. 2
      1. liquid effluent contains large amounts of organic molecules and harmful microorganisms - both need to be reduced before water returned to environment
      2. air bubbled through liquid effluent - allows aerobic bacteria to multiply
      3. in presence of oxygen, aerobic bacteria digest organic molecules and harmful microorganisms
      4. liquid effluent safely discharged into nearby rivers or the sea
    • when water is used in industry e.g to make paper and chemicals - harmful chemicals first need to be removed - water can safely enter general sewage treatment
    • in some parts of world, treated sewage used directly to produce potable water
    • easiest way to produce potable water is to use ground water from aquifers - usually safe to drink once it has been treated with chlorine - aquifers can sometimes be polluted e.g with fertilisers from farms - water from aquifers needs to be tested carefully
    • can make potable water directly from waste water e.g sewage - takes many purification steps - only done in places where water is scarce
    • salt water needs to be desalinated to produce potable water - desalination requires a lot of energy and is expensive