What is the difference between BOD and COD in wastewater analysis?

BOD (Biochemical Oxygen Demand) and COD (Chemical Oxygen Demand) measure organic pollution in water but differ in methodology: BOD measures oxygen consumed by microorganisms during biological degradation of organic matter over 5 days at 20C (BOD5). It represents the biodegradable organic content. Typical values for raw sewage are 200-400 mg/L. COD measures oxygen equivalent of organic matter chemically oxidized using strong oxidizing agents (potassium dichromate). It includes both biodegradable and non-biodegradable organics. Results available in 2-3 hours versus 5 days for BOD. COD is always higher than or equal to BOD. The BOD/COD ratio indicates biodegradability: >0.5 is easily biodegradable, <0.3 suggests recalcitrant compounds. Both are critical parameters for wastewater treatment design and monitoring discharge quality.