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 dec.ny.gov

• 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 palmaison.com

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.