The relationship is different for each type of wastewater. It also influences the type of analysis used for COD and the acclimatization of microorganisms for BOD.
COD represents all oxidizable materials and BOD 5 represents the proportion of biodegradable organic materials. The difference between COD and BOD 5 is the organic matter load, which is low or not biodegradable. CALCULATION: COD: BOD 5 = Ratio. For domestic wastewater the ratio is 1.5 to 2. This corresponds to easy biodegradation. It can reach 2.5 to 3 without very noticeable disadvantage. Important: COD will always be higher than BOD 5 (unless special conditions apply).