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Sewage Treatment Process (STP) : Importance, Stages, and Environmental Impact

Sewage treatment is a crucial process that removes contaminants from wastewater, household sewage, and effluents. It involves physical, chemical, and biological methods to ensure that the treated water is safe for disposal or reuse. Proper sewage treatment helps protect the environment and reduces the demand for portable water in buildings.

Stages of Sewage Treatment

Sewage treatment typically involves three main stages: primary, secondary, and tertiary treatment.

1. Primary Treatment (Solid Separation)

  • Large solid particles and sediments are separated from the wastewater.
  • The solids settle at the bottom, while lighter materials like oils and grease float and are removed.

2. Secondary Treatment (Biological Process)

  • The remaining wastewater contains dissolved organic matter.
  • This is broken down using water-borne microorganisms that convert waste into a solid mass.
  • The biological solids are then neutralized and either disposed of or reused.

3. Tertiary Treatment (Final Purification & Disinfection)

  • The treated water undergoes chemical disinfection to remove harmful pathogens.
  • The final effluent can be:
  • Discharged into a stream, river, bay, lagoon, or wetland
  • Used for irrigation in golf courses, parks, or green areas

Key Parameters in Sewage Treatment

1. Dissolved Oxygen (DO) and Its Impact

  • In natural water bodies like rivers and lakes, organic waste consumes oxygen as it degrades.
  • If untreated sewage is released, it reduces dissolved oxygen, leading to fish and plant deaths.
  • Sewage Treatment Plants (STPs) supply enough oxygen for the breakdown of waste, preventing oxygen depletion in water bodies.

2. Biochemical Oxygen Demand (BOD)

  • BOD measures the amount of oxygen required to break down biodegradable organic matter in sewage.
  • The more decomposable material present, the higher the BOD value.
  • The general BOD range for raw sewage is 100-400 mg/L, with Indian cities usually observing lower values.

3. Chemical Oxygen Demand (COD)

  • COD measures the total oxygen required for chemical oxidation of organic and inorganic materials in sewage.
  • The COD of raw sewage is generally 200-700 mg/L, depending on location and waste type.

4. Turbidity (Clarity of Treated Water)

  • Treated water should have low turbidity (< 10 NTU2), meaning fewer suspended particles clouding the water.
  • Clear water prevents pipeline clogging and ensures better flow.
  • High turbidity in untreated water blocks sunlight in lakes and rivers, disrupting photosynthesis in aquatic plants and reducing oxygen levels.
  • This can kill aquatic life due to oxygen depletion.

Environmental Benefits of Sewage Treatment

  • Prevents water pollution by removing harmful waste before disposal.
  • Maintains dissolved oxygen levels in rivers and lakes, protecting aquatic ecosystems.
  • Reduces water demand by enabling the reuse of treated sewage for irrigation and landscaping.
  • Prevents sediment buildup in pipelines, reducing blockages and maintenance costs.

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