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The Sewage treatment plants works in four different stages of treatment. They are preliminary, primary, secondary and tertiary.

  1. Preliminary treatment: This process involves removal of floating materials (papers, leaves, rags) inorganic solids (sand, grit) and oily substances (grease, fats, oils). The equipments used for carrying out preliminary treatment include screeners, skimming tanks and grit chambers.
  2. Primary treatment: This step involves sedimentation or settling. This is aimed at removal of fine suspended organic solids that cannot be removed by preliminary treatment.
  3. Secondary or Biological treatment: The removal of fine suspended and dissolved organic material present in the sewage comprises the secondary treatment. This process involves the use of microorganisms (bacteria, algae, protozoa, fungi) that decompose the unstable organic matter to stable inorganic forms. This process can be carried out by using aerobic or anaerobic biological units
  4. Tertiary or advanced treatment: This is the final step of sewage treatment. It is sometimes required for the removal of suspended and dissolved substances, after the completion of primary and secondary treatments. This step basically involves removal of suspended dissolved solids, phosphorous, nitrogen and pathogenic substances. 
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