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Sewage Treatment Plant (STP) : A Comprehensive Guide (Including Bar Screen Chamber and Oil & Grease/Grit Trap)

Introduction
Sewage treatment is an essential process designed to remove contaminants from wastewater, household sewage, and industrial effluents. The primary goal is to ensure that the treated water is safe for disposal into the environment or suitable for reuse in various applications. This process involves physical, chemical, and biological methods that collectively help maintain environmental balance and reduce the strain on potable water sources.
Proper sewage treatment is crucial to:
  • Prevent water pollution
  • Protect human health
  • Conserve natural resources by reusing treated wastewater
  • Reduce the environmental impact of urbanization and industrialization
Stages of Sewage Treatment
Sewage treatment consists of three main stages: primary, secondary, and tertiary treatment. Each stage plays a critical role in ensuring the removal of solids, organic matter, and pathogens.
1. Primary Treatment (Solid Separation)
  • Primary treatment is the initial phase where large solids and sediments are separated from the wastewater.
  • The process involves screening and sedimentation to remove floating debris, sand, and grease.
  • Heavy solids settle at the bottom, forming sludge, while lighter materials like oils and grease float to the surface and are skimmed off.
  • This stage reduces suspended solids and organic matter, making the wastewater easier to treat in the next stages.
2. Secondary Treatment (Biological Process)
  • Secondary treatment focuses on breaking down dissolved organic matter using biological processes.
  • The remaining wastewater contains dissolved organic compounds, which are biodegraded by microorganisms.
  • Bacteria and other microbes feed on organic pollutants, converting them into carbon dioxide, water, and energy.
  • Biological solids generated in this stage are either reused in treatment plants or safely disposed of.
  • Common methods include activated sludge systems, trickling filters, and biofilms to enhance microbial activity.
3. Tertiary Treatment (Final Purification & Disinfection)
  • Tertiary treatment ensures that the water is free from harmful contaminants before disposal or reuse.
  • The treated water undergoes chemical or physical disinfection methods like chlorination, UV radiation, or ozonation to kill harmful pathogens.
  • The final effluent can be safely discharged into natural water bodies like rivers, wetlands, or oceans.
  • Treated water can also be reused for irrigation in public parks, golf courses, and green landscapes.
Key Parameters in Sewage Treatment
To ensure efficiency, sewage treatment plants (STPs) monitor specific parameters that define water quality and treatment effectiveness.
1. Dissolved Oxygen (DO) and Its Impact
  • DO levels in natural water bodies influence the survival of aquatic organisms.
  • Organic waste in untreated sewage consumes oxygen, reducing DO levels and causing fish and plant deaths.
  • STPs maintain sufficient DO levels to support microbial breakdown and prevent oxygen depletion in rivers and lakes.
2. Biochemical Oxygen Demand (BOD)
  • BOD measures the amount of oxygen required for microbes to decompose organic matter.
  • High BOD values indicate excessive organic waste, which can deplete oxygen levels in water bodies.
  • Typical BOD values for raw sewage range from 100-400 mg/L, while treated sewage should have lower values for safe disposal.
3. Chemical Oxygen Demand (COD)
  • COD quantifies the oxygen required to chemically oxidize organic and inorganic pollutants.
  • Raw sewage COD levels range between 200-700 mg/L, depending on waste composition and location.
4. Turbidity (Water Clarity)
  • Turbidity measures suspended particles in treated water.
  • Treated water should have a turbidity level of < 10 NTU to ensure proper flow and prevent pipeline clogging.
  • High turbidity in natural waters disrupts photosynthesis, affecting aquatic life and oxygen levels.
Environmental Benefits of Sewage Treatment
Effective sewage treatment has significant environmental advantages:
  • Prevents water pollution by removing contaminants before disposal.
  • Maintains oxygen levels in rivers and lakes, protecting fish and aquatic plants.
  • Reduces water demand through the reuse of treated water for irrigation and landscaping.
  • Prevents sediment buildup in pipelines, reducing blockages and maintenance costs.
Sewage Treatment Plant (STP) Process

1. Bar Screen Chamber
  • The function of the bar screen is to prevent entry of solid particles above a certain size; such as plastic cups, paper dishes, polythene bags into the STP. 
  • The screening is achieved by placing a screen made out of vertical bars, placed across the sewage flow.
  • The gaps between the bars may vary between 10 and 25 mm.
  • Larger STPs may have two screens: A coarse bar screen with larger gaps between bars, followed by a fine bar screen with smaller gaps between bars. 
  • In smaller STPs, a single fine bar screen may be adequate.
1 Inlet pipe for the STP.
2 Debris 
3 Muck accumulates 
4 Grill. Must be cleaned regularly to avoid
a build-up of debris (2) and muck (3).
5 Screened sewage. 
6 Outlet pipe (goes to the Equalization Tank)
7 Platform with weep holes. The STP operator stands here to rake the debris (2).

Operation and Maintenance Considerations
  • Regularly clean the bar screen to prevent clogging.
  • Prevent solid waste from escaping the screen.
  • Replace corroded screens immediately to maintain efficiency.
2. Oil and Grease/Grit Trap
  • The grease and grit trap is installed at kitchen or canteen discharge points to remove fats, oils, and solid matter.
  • Wastewater flows into the trap, where:
    • Fats and grease float to the top and are periodically removed.
    • Heavier solids settle at the bottom and are cleaned out regularly.
    • A baffle plate prevents grease from reaching the outlet, ensuring clean water flows out.
Operation and Maintenance Considerations
  • Regularly clean the grease trap to maintain efficiency.
  • Remove floating grease and settled solids to prevent clogging and contamination.
Conclusion
Sewage treatment is a critical process that ensures environmental sustainability by treating wastewater before disposal or reuse. Understanding the stages of treatment, key parameters, and plant operations helps in maintaining an efficient and safe sewage management system. Proper maintenance of STPs not only enhances operational efficiency but also contributes to water conservation, pollution control, and ecological balance.

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