COD in Sewage Treatment Plants

 

COD in Sewage Treatment Plants (STPs)

COD (Chemical Oxygen Demand) is a critical parameter in wastewater treatment that measures the total amount of oxygen required to chemically oxidize both biodegradable and non-biodegradable organic and inorganic matter present in the water. It is used to assess the pollution load and treatment efficiency in Sewage Treatment Plants (STPs).

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Role of COD in STPs

1. Measuring Pollution Load:
   • COD provides a comprehensive measure of all oxidizable pollutants in wastewater, unlike BOD, which measures only biodegradable organic matter.
2. Designing Treatment Processes:
   • COD levels help determine the treatment capacity and technology required for efficient wastewater management.
3. Monitoring Treatment Efficiency:
   • COD reduction between the influent (incoming sewage) and effluent (treated water) indicates how well the STP is performing.

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Stages in STPs and COD Reduction

1. Preliminary Treatment:
   • Removes large debris and grit, but has minimal impact on COD.
2. Primary Treatment:
   • Removes settleable solids, slightly reducing COD (approximately 20–30%).
3. Secondary Treatment:
   • Biological processes, such as Activated Sludge Process (ASP), MBBR, or SBR, degrade biodegradable components, significantly reducing COD (60–90%).
4. Tertiary Treatment (if applied):
   • Advanced treatment methods like filtration, chemical oxidation, or activated carbon adsorption further reduce COD to meet stringent discharge or reuse standards.

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COD Standards for STP Effluent

The permissible COD levels in treated effluent depend on local environmental regulations and the intended use of treated water. Common limits include:

• Discharge to surface water bodies: ≤ 250 mg/L
• Reuse for irrigation or industrial purposes: ≤ 100 mg/L
• Advanced reuse applications (e.g., drinking water): ≤ 10 mg/L

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Difference Between COD and BOD

Parameter	COD	BOD
Measures	Total oxidizable matter (biodegradable and non-biodegradable)	Biodegradable organic matter only
Test Duration	2–3 hours	Typically 5 days
Usage	Comprehensive pollution assessment	Biological treatment design and monitoring
Typical Levels in Influent	Higher than BOD (e.g., 500–1500 mg/L)	Lower than COD (e.g., 200–600 mg/L)

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Importance of COD Monitoring

1. Comprehensive Pollution Assessment:
   • COD includes all oxidizable matter, providing a broader measure of pollution compared to BOD.
2. Early Detection of Non-Biodegradable Pollutants:
   • Identifies pollutants that may not be addressed through biological treatment.
3. Process Optimization:
   • Helps operators adjust processes to optimize the removal of both biodegradable and non-biodegradable components.

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Conclusion

COD is a key parameter in wastewater treatment, reflecting the overall pollution load in sewage. Monitoring and reducing COD in STPs ensure compliance with discharge regulations, protect the environment, and enable safe wastewater reuse for various purposes.