COD and BOD5 ratio
COD is always higher than BOD5.
The ratio of COD/BOD for municipal wastewater is 1.5 to 2 times and corresponds to a normal degradation of the effluent.
Put simply, BOD5 indicates the biodegradable organic material, while COD indicates all organic material, including the biodegradable content. Therefore BOD5 is included in COD, so COD is higher.
Conclusion: for domestic effluent COD/BOD5 = 2.6
For explanation, see below:
-Ultimate BOD/BOD5 = 1.46
- Biodegradable COD/BOD5 = 1.7
-soluble refractory COD = 4 to 10%
- refractory particulate COD = 15-25%
- total refractory COD = 20 to 35%
COD/BOD5 = 1.7/0.8 = 2.1
COD/BOD5 = 1.7/0.65 = 2.6 “quality of domestic wastewater produced by small communities - Applies to treatment systems processing less than 2,000 PE.”
I have come across the results of analyses in which BOD5 is higher than COD. However, this was due to an error when the sampling was performed (samples from two different bottles in terms of concentration) or a reversal when writing up the report. Hoping this mitigates any concern should the situation arise because, as explained above, it is actually impossible.