The permissible limits of BOD and COD for STP

 The permissible limits of BOD (Biochemical Oxygen Demand) and COD (Chemical Oxygen Demand) for sewage treatment plant (STP) effluents are determined by local environmental regulations. These limits vary based on the discharge destination or reuse application, such as irrigation, industrial use, or release into surface water bodies.

---

Typical Permissible Limits for Treated Effluent

Parameter	Discharge to Surface Water Bodies	Reuse for Irrigation	Reuse in Industrial Applications
BOD	≤ 20–30 mg/L	≤ 10–20 mg/L	≤ 10 mg/L
COD	≤ 250 mg/L	≤ 50–100 mg/L	≤ 50 mg/L

---

Factors Affecting Permissible Limits

1. Discharge Destination:
   • Surface Water Bodies: Typically require BOD ≤ 20–30 mg/L and COD ≤ 250 mg/L to minimize environmental impact.
   • Marine Outfalls: Slightly higher limits may be allowed due to greater dilution capacity.
2. Reuse Applications:
   • Agricultural/Irrigation: Lower limits are set to protect soil and crops from organic and chemical buildup.
   • Industrial Use: Requires stricter control for sensitive applications, like cooling or process water.
3. Local Regulations:
   • Limits vary by country and regulatory authority, such as EPA (USA), CPCB (India), or EU Directives.

---

Why These Limits Are Important

1. Protect Aquatic Life:
   • High BOD and COD levels deplete dissolved oxygen, harming aquatic ecosystems.
2. Prevent Water Pollution:
   • Reduces organic and chemical pollution in receiving water bodies.
3. Enable Safe Reuse:
   • Ensures treated water is safe for agricultural or industrial applications.

Regular monitoring and compliance with these standards are essential for sustainable wastewater management and environmental protection.