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 chemical or alum) are added during the treatment process to enhance the removal of TSS and BOD. The removal efficiency for TSS and BOD by this process are in 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 in the settled sewage is decomposed by micro-organisms in the biological treatment process. Treated effluent from this process meets with 30mg/L TSS and 20mg/L BOD standard.
Tertiary Treatment - highest level of treatment to polish the effluent from secondary treatment process. This process comprises a combination of physical and biological processes with the objective of removing nutrients and any remaining suspended solids in the sewage.