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Secondary treatment:
Secondary treatment is the second phase of wastewater treatment, used to remove the biodegradable dissolved organic matter. The process is designed to degrade the biological content of the waste. It is performed in the presence of oxygen through biological oxidation by aerobic microorganisms. The step mixes the waste with bacteria and oxygen.
Aerator which pumps air so that the aerobic bacteria grows
Oxygen helps the bacteria to digest the pollutants easier and faster. Secondary treatment is also called biological treatment as it degrades the biological content of waste through biological agents' use (aerobic microorganisms).
The microorganism is separated from the treated wastewater through sedimentation. Once the sediments of biological solids are separated, the remaining liquid is discharged for tertiary treatment.
Tertiary treatment:
The tertiary treatment is also called the advanced treatment. It is the final step of sewage treatment. The tertiary treatment aims to raise water quality to the industrial and domestic standards or meet specific requirements.
The tertiary treatment involves the removal of inorganic constituents like nitrogen, phosphorous and microorganisms. This ensures that the water is safe for drinking purposes.
Physio-chemical treatment of wastewater focuses on the separation of colloidal particles. This is done through the use of chemicals called coagulants and flocculants. The fine colloidal particles in sewage water are precipitated by adding chemical coagulants like alum or ferric sulphate.
Tertiary treatment
A flow chart on wastewater recycling stages is depicted below:
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Water recycling steps
Uses of Recycled Water:
  • Agriculture
  • Cooling water for power plants and oil refineries
  • Public parks
  • Construction activities
  • Dust control
  • Landscape
  • Toilet flushing.