There is no straightforward relation between COD and BOD, as it depends on the nature of your organic pollutants and on the oxidizing agent you are using. The idea behind COD is to oxidize "all" organic matter. This means that COD could be expected to be higher than BOD as a total oxidizing of organic matter through biological activity would require a very long time which is not practicable on BOD tests (5day BOD is the standard). How much more COD than BOD can be expected? That is more complicated, but as much as 2 times is not that rare of case.

Hi Ashutosh,

Please check this researchgate link.

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

There is no direct relation between chemical Oxygen demand and Biochemical Oxygen demand. Both are independent parameters. They vary river to river and time to time based on several contaminants factors. However in any particular sample it is possible to have more COD than BOD.

 Dear Ashutosh,

COD represents all oxidizable materials and BOD represents the share of biodegradable organic matter. The difference between the COD and BOD represents the organic load little or not biodegradable.

For domestic wastewater the ratio is 1.5 to 2. This corresponds to an easy biodegradation. It can reach 2.5 to 3 without very sensitive problem. The COD is always higher than the BOD (except special conditions).

With my best regards

Prof. Bachir ACHOUR

Thank you Mr. Achoour for your valuable response. Sir, I am having samples from a river and my COD values are varying with BOD sometimes it 5 times of BOD and sometimes its 2 to 3 times. I am a Little bit confused whether my data is correct or not although I get some papers they have reported this factor up to 10 times also for the same river. pls. comment in this regard.....

Dear Ashutosh,

Relation COD/BOD about 2 is very common for plain large rivers; about 3-5 is a characteristic of black waters from marshes watershads with color of 50 - 150 degrees Pt/Co. If ration is 10, then it may be very clean mountin river water, where contents of biodegradable organics are too small. In your case which actual values of COD and BOD do you have?  

Thank you Mr. Timur for your response. Actually the river is having Supersaturation DO  around 8.5 to 11 on temperature range about 32 to 39. samples are just downstream of a Industrial city. I think that is the reason for getting higher COD but BOD value is not as much because of DO value. my 3 Days BOD values ranging from 5-10 and COD values around 40-55

 Ashutosh, commonly we operate with BOD 5, but you talk about BOD 3 (?). Moreover, industrial wastes have a lot of hard oxidazed (non-biodegradable) dissilved organic matters!!! So far, your situation is typical for water with wastes of domestic+industrial origin. 

COD is always higher than DBO5. It measures the organic compunds (DBO) plus the inorganic( if that  thing exists in the sample).

Look at this interesing website:

http://waterfacts.net/Treatment/Activated_Sludge/BOD-COD-TOC/COD_Ratio_Examples/cod_ratio_examples.html

http://waterfacts.net/Treatment/Activated_Sludge/BOD-COD-TOC/bod-cod-toc.html

In the COD all oxidable chemicals are included, such as for example ammonia. So there is no simple interpretation of COD/BOD results. they are different parameters. Where are your samples? Are there any WWTP discharge? Are there any untretaed sewage discharge?

Dear Mishra,

The amount of COD is depend on the condition of river if there is wastewater i could be high, but if the rive is clean it must be low amount, but the amount of COD is always higher than BOD. BOD in clean river is around of 3 mg/l.

Usually, COD is Higher than BOD

I agree with the previous comments.

BUT COD, using dichromate does not oxidize ammonium.BOD determines the fraction that is oxidized by the bacteria (in the sample or added as a seed) within 5 days, often about 2/3 of all biochemically oxidizable organic matter ( "BODtot" 21 days). However, as all commentators write COD oxidizes about 90 % of all organic matter in a normal water sample.

I would say that a natural clean water should have a BOD of 1–2 mg/l (swedish experience.

thank you, Anders, for your preciuos comment. Ammonia can be oxidised by dichromate only at high potassium dichromate concentration (0.25 N) and in presence of chloride.

see Effect of Ammonia on COD Analysis

Byung R. Kim

Journal (Water Pollution Control Federation)

Vol. 61, No. 5 (May, 1989), pp. 614-617