Air calculation should be based on BOD. For domestic sewage this would be about 40 m3/kgBOD to 55m3/kgBOD (or higher). This ratio is dependant on a number of factors including the oxygen transfer efficiency of the air diffusers, nitrification, sludge age and sewage temperature. Designers commonly normally use SOTR and AOTR.

The BOD/COD ratio would typically be between 0.4 and 0.5.

In practice, either COD or BOD can be used for estimating oxygen requirements and then corresponding air requirement. The COD is easy to use because 1 mg COD oxidized, 1 mg oxygen is required. You do not need to establish a ratio between BOD and COD. The bCOD is the more proper parameter than total COD for estimating oxygen requirements. However, this value, often, may not be available. If this is the case, total COD may be used in estimating oxygen demand. However, you need to consider other key parameters when you estbalish your oxygen requirements such as operating SRT, endogenous decay, etc.). The commercially available models can quickly estimate oxygen requirements.

Sandip

Go for 1 kg/O2/Gk COD irrespective of BOD/COD Ratio or the  BOD value.The reason is BOD estimatiion could be misleading , as it is depending upon the seeding and the dilution used and so many other parametres.

Using O2 /air based on COD avoids the error when designing the plant, it takes care of all that can be there BOD/ UBOD/bBOD/bCOD/COD. 

Be practical !

Dr Pimparkar

Oxygen requirement must be calculate on the eliminated biodegradable COD (bCOD) base, specially when there is a significant percentage of slowly biodegradable COD