Preliminary Treatment / Screening - includes screening and removal of grit. Solids larger than 6 mm in diameter are removed from the sewage. 
Primary Treatment - includes screening, removal of grit and a sedimentation process. Solid waste and settleable suspended solids are removed from the sewage. Removal efficiency for Total Suspended Solids (TSS) is about 70% and Biochemical Oxygen Demand (BOD) is about 30% by this process.

Chemically Enhanced Primary Treatment – chemicals (e.g., ferric salt or alum, often with polyelectrolyte) are added during the treatment process to enhance the removal of TSS and BOD. The removal efficiencies for TSS and BOD by this process are in the region of  80% and 60% respectively.

Secondary Treatment - the sewage is purified by means of a biological treatment process after the primary treatment. The organic matter and nutrient in the settled sewage are decomposed by micro-organisms in the biological treatment process. Treated effluent from this process meets 30mg/L TSS and 20mg/L BOD standard.
Tertiary Treatment - the highest level of treatment for polishing the treated effluent from secondary treatment process. This process typically comprises a combination of physical and biological processes with the objective of further removing nutrients and  suspended solids in the sewage.

Type of Sewage Treatment Facilities

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