Unit 3: Nutrient Pollution - Sewage Treatment

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  • Sewage Treatment
    • Pre-Treatment (solid objects)
      • Screens trap floating suspended items
        • Incinerated to kill pathogens and reduce volume then put in landfill
          • Grit traps collect stones/road grit at the bottom of the water
          • Comminutors chop up  faecal solids - increase surface area
      • Grit traps collect stones/road grit at the bottom of the water
      • Comminutors chop up  faecal solids - increase surface area
      • Primary Treatment (organic solids)
        • Effluent allowed to stand and settle - faecal solids sink and are removed
        • Sludge Disposal (pathogens and foul smells)
          • Digested by anaerobic  bacteria for 4 weeks
          • Remaining solids settle
          • Produces methane and CO2 - fuel
          • Remaining sludge  disposed of in landfill (cheap), sea (banned), incinerated (expensive), fertiliser (difficult to apply)
          • Secondary Treatment (organic matter and bacteria)
            • Aerobic bacteria and large volumes of air mixed in to make process occur at constant rate
            • Bacteria allowed to settle and collected as activated sludge
            • Effluent usually discharged here
              • Tertiary Treatment (for sensitive or important areas)
                • Phosphates removed with iron sulfate
                • Bacteria removed using micro-strainers or chlorine

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