Sewage treatment works at Morton Jaffray,Harare. Image credit

Sewage treatment works at Morton Jaffray,Harare. Image credit

ZIMSEC O Level Combined Science Notes: Centralised Sewage Treatment Works

  • Are used to collect and treat sewage waste from large industrial and residential areas
  • They treat large quantities of human waste
  • The raw sewage from flush toilets is collected by pipes and an intricate network of sewer tunnels
  • The waste contains pathogens, is toxic and has strong odours
  • This waste has to be processed properly
Centralised Sewage Treatment Process. Image credit

Centralised Sewage Treatment Process. Image credit

  • When the sewage gets to the treatment plants usually away from populated areas treatment begins
  • First the waste is physically separated
  • Solids and grit are removed at the protective bar screen which removes all coarse materials
  • The process is continued in the grit settling tank where water moves slows and almost comes to a rest
  • Solid sediments are left to all to the bottom
  • The liquid is left to pass through settling tanks
  • Here the suspended  solids precipitate to form sludge at the bottom
  • The sludge is tapped from the bottom and taken to a digester here
  • The sludge is later digested by anaerobic bacteria
  • This process forms methane gas
  • The biogas (methane and carbon dioxide) is burned to keep the digesters at optimum temperature
  • The sludge is broken down by this bacteria
  • Solid left overs are used as organic fertilizer
  • The liquid from the settling tanks is treated by aerobic bacteria which covert any ammonia to nitrites and nitrates
  • The waste though clear still contains solids
  • These solids are removed in the humus tank (final settling tank)
  • This liquid (rich in nitrates and phosphates) is used to irrigate farmlands
  • Examples of such farms include Honey Dew Farm
  • This liquid cannot be returned to rivers and dams as this will cause eutrophication
  • This is when algal bloom blocks sunlight for other algae which will die and is digested by bacteria
  • This uses up all the oxygen in water leading to the death of other water organisms including fish
  • The liquid waste can be treated using the active sludge method
Eutrophication at Lake Chivero. Image credit

Eutrophication at Lake Chivero. Image credit

Activated sludge

  • Effluent passes through the aeration tank
  • Air is pumped into the aeration tank to supply oxygen to microorganisms which decompose the sewage
  • Effluent is then passed on to the hums tank
  • Some of the sludge is pumped back to the aeration tank to supply incoming sewage with micro-organisms to speed up the decomposition process

Biological filters

  • Can be used instead of the activated sludge method
  • Here there are biological filters
  • Fluid from the sedimentation tanks is pumped and sprinkled over a bed of porous rocks covered by bacteria that decompose soluble waste
  • The fluid leaving through the bottom of the filter can be safely returned into water bodies e.g. rivers or dams
  • Sometimes the fluid is directly returned (recycled into the city’s water supply)

To access more topics go to the Combined Science Notes page.