BOD can be higher than COD determined by dichromate oxidation in a few cases.

The most common is a sample with very degradable organic matter and high organic nitrogen or ammonium such as wastewater from fish processing. The reason is that chromate doesn't oxidise ammonium to nitrate but the inoculum for the BOD test can contain nitrifying bacteria which can consume oxygen by this reaction. Some BOD procedures use a nitrification inhibitor to avoid this. 

An effluent from a refinery could contain organic nitrogen or sulfur which might be biologically oxidisable but not oxidised by chromic acid in the COD procedure.

always COD is higher about 5 times than BOD because its comprises both Organic and inorganic Pollutants while BOD is only for Organic Pollutants.

regards

Dear brother,

Simply BOD indicates the biodegradable organic matter (OM) while COD represents both biodegradable and non-biodegradable OM in water. If all OM is your sample is biodegradable, so BOD will approximately equal COD. Therefore, COD should be higher than or equal BOD but never be lower than BOD.

Regards,

Ahmed

Basically it is not possible tpo have higher BOD in comparison with COD. If all pollution is biologically totally degradable, the BOD is equal to COD.

Best regards

Vit

Good answer Ahmed and Vit.

all the best

BOD can be higher than COD determined by dichromate oxidation in a few cases.

The most common is a sample with very degradable organic matter and high organic nitrogen or ammonium such as wastewater from fish processing. The reason is that chromate doesn't oxidise ammonium to nitrate but the inoculum for the BOD test can contain nitrifying bacteria which can consume oxygen by this reaction. Some BOD procedures use a nitrification inhibitor to avoid this. 

An effluent from a refinery could contain organic nitrogen or sulfur which might be biologically oxidisable but not oxidised by chromic acid in the COD procedure.

Thank you so much all of you for giving valuable suggestions.

I would add that if a nitrifying (non-inhibited in the assay) seed is used in the BOD measurement and significant ammonia is present relative to the concentration of biodegradable organics, the nitrogenous oxygen demand could be a significant fraction of measured BOD.  In this case, measured BOD can be higher than measured COD.  cBOD should never be higher than COD, as described above.  

Thank you so much Dr. Wilson.

Dear Dr. Gupta,

No, absolutely not possible, but vice versa

Professor Keskinkan, I think your answer is a bit short. Would you mind explaining why you disagree with the explanation of ammonia and organic nitrogen causing higher BOD than COD in certain samples given by Dr. Wilson and me.

I agreed with Prof Anderson and Christopher but in general COD is always higher than BOD.

Dr. Srivastava, I agree, my answer was intended as an addition to the observation that usually wastewater has higher COD than BOD as Dr. Hassaan and those that followed stated.

It would be most surprising to see a BOD higher than COD.

Just consider how the analysis is carried out.

In a refinery effluent the BOD will be low.

You may also refer to these two discussion on RG and I hope it will help you:

https://www.researchgate.net/post/What_is_the_relationship_between_the_COD_and_BOD_values_in_Waste_water

https://www.researchgate.net/post/We_have_a_few_cases_in_which_BOD_is_much_higher_than_COD_how_to_explain_this_type_of_scenario

they asked same question

all the best

Mohamed

Obviously a refinery effluent can be expected to be treated a a level there BOD is low in order to avoid hypoxia in environment the effluent are released into

However, the question is on the wastewater from a refinery that then received from the production can have a high BOD and occasionally contain ammonia or hetero organic molecules that gives a higher BOD than COD.

Dear Dr Andersen,

As far as I know, oxidation of S-2, SO3-2, NO2, NH3, Fe + 2, etc. with K2Cr2O7 causes the wastewater to be measured more than organic matter. For samples containing too much of these components, the COD value can be corrected stoichiometrically.  From here we understand that such components increase COD value. However, my knowledge may not be complete. I will search more, thank you.

Generally the COD procedure doesn't oxidise NH3/NH4+ but most wastewater treatment plants biomass do.

A refinery wastewater contains corrosion inhibitors, scaling agents, sulfide scavengers and S- and N-heteroatomic molecules from the crude oil that occasionally causes deviations between COD and BOD. 

Total organic carbon expressed as COD contains BOD, in other words BOD is a part of COD, so normally it should not be higher. Generally sometimes COD is too high because of some mineral compounds that can influence the reaction. What is the procedure of BOD in your laboratory (dillutions, OxiTop)? Do you use any inhibitor of nitrification (like ATU)?

Dear Gupta and Andersen,

First of all, the BOD experiment is carried out to measure the oxygen demand of carbonaceous substances, not to measure the oxygen requirement of nitrogenous substances. That's not our goal in BOD mesurements.

Please, first, let's look at the following explanation:

"Oxidation of reduced forms of nitrogen, such as ammonia and organic nitrogen, can be mediated by microorganisms and exert nitrogenous demand. Nitrogenous demand historically has been considered an interference in the determination of BOD, as clearly evidenced by the inclusion of ammonia in the dilution water. The interference from nitrogenous demand can now be prevented by an inhibitory chemical. If an inhibiting chemical is not used, the oxygen demand measured is the sum of carbonaceous and nitrogenous demands. Measurements that include nitrogenous demand generally are not useful for assessing the oxygen demand associated with organic material. Nitrogenous demand can be estimated directly from ammonia nitrogen" (Standard Methods 20th )

From this explanation we understand that if you do the experiment properly and take the necessary precautions for the interferences, the nitrogenous components can never interfere. So you will have a real BOD5 measurement. Because the BOD5 experiment only reveals the presence of carbonaceous organic substances in terms of the oxygen consumed. So, as I said at the before, if you measure COD and BOD5 in real terms, BOD5 never exceeds COD. It is important to remember that there are many BOD5 measurement methods. In all, inhibitors are used to prevent interference. If you are using the method correctly, the BOD5 parameter can not exceed COD.

Generally it is not possible to get higher BOD than COD. I prefer to do 3 days BOD at 27 degree  instead of 5 days BOD if you are doing conventional analysis and ensure water bubbles should not be present in the BOD bottles during three days of incubation.

BOD values can never be higher than COD values in refinery wastewater samples. You have to cross check the results of COD and BOD tests to eliminate the discrepancy

BOD values can never be higher than COD values in refinery wastewater samples. You have to check your analysis..

i suggested that you can repeat your experiment and do the same measurement in the same day also be careful that FAS is not stable and you should make  standrdization for it every day. 

regards

Good input by Mohamed Hassan

COD value always more than BOD. It is good to repeat the analysis after taking new sample.

I agree with Dr Andersen

This fact can be occurred when

1- we have Nitrogenous compounds in our sample 

2- and also can be happened that we have aromatic compounds like benzene    

3- permanganate be used instead dichromate 

4- also when chlorine be reduced in absence of HgSo4   

BOD is same or less than COD. If someone has got a result that BOD is greater than COD, it is interesting and novel. They should find out the reason for it.