NO, BOD is a fraction of COD

I agree with @Maurizio that BOD is normally a fraction of COD.

Nevertheless, the basis for the COD test is that nearly all organic compounds can be fully oxidized to carbon dioxide with a strong oxidizing agent (dicromate) under acidic conditions.Dichromate does not oxidize ammonia into nitrate (refer to the link below) as the case in BOD measurement. So, for sample rich with ammonia and very low organic carbon concentration there is a possibility of getting higher BOD value than COD.

http://en.wikipedia.org/wiki/Chemical_oxygen_demand

Not BOD is a fraction of COD, which can happen sometimes is that depending on the material or residue analyzed, these values are very close but by small errors in the analytical determination it overlap.

i agree with all of the answers; added to the material base which mainly originating by means of domestic wastes and with high in ammonia content which may leads to getting inverse results during the study. basically with night soil wastes, underground sewerage mixing zones, and higher ammonia content releasing industrial wastewater mixing areas. we may get the chance of experiencing the higher BOD levels.

all the very best

Yes, in some cases, you could find some observations that the analyzed BOD concentration is higher than the analyzed COD concentration.  It is fully depended upon the containing organic species.  Some organic species are bio-degradable but it is difficult to be oxidized by the used oxidant used in COD analysis.  For instance, if the wastewater contains TMAH, tetramethylazanium hydroxide, is the case.  This chemical is quite often to be used in the microelectronic industry.  These cases is normally to be found in industrial wastewater treatment applications.  In these cases, the TOC analysis normally could help the analyzer to understand the influent characteristics.  However, if the BOD value is higher than COD values, it just says that the root is caused by the used oxidants in the COD analysis but this wastewater is still bio-degradable.

COD is generally greater than BOD. However, high presence of microbiologically oxidisable chemicals such as ammonium could boost BOD readings.

As for water released from the bathroom, generally COD should be greater than BOD. However, this depends on the bathroom users' habits. If urination occurs during the use of bathroom this could cause high level of ammonium (urea in urine hydrolysed into ammonium) in the released water which in turn could result in high BOD. 

No, BOD is always less or equal to COD, since BOD gives you only the biodegradable organic content of a aquatic sample, while COD gives you the biodegradable and the non-biodegradable organic content, thus the total organic content.

I must disagree with my colleague Ioannis. Having reviewed a previous healthy discussion on COD vs BOD in Researchgate, there has been considerable misunderstanding about the theory behind COD and BOD analysis.

COD analysis (e.g. dichromate method) being an aggressive method should oxidise most organic carbon to CO2. The results are easily reproducible and the method yields quick results. However, there are limitations to COD tests; (a) interference by Cl- which can be alleviated by the addition of mercuric sulphate, (b) the inefficient oxidation of the NOCs (nitrogenous organic compounds). Some workers have tried to alleviate by the use of a TiO2 method, and (c) some workers claim simple aliphatic such as acetates are unable to oxidised by this method and (d) the inability to oxidise NH4/NH3.

On the other hand, BOD can only oxidise readily biodegradable organic-C. This is why most COD results are greater than that of BOD. However, unlike COD method BOD5 is capable of oxidising NH4 to NO3. This is important in case of water or wastewater having very high concentration of ammonium. Ammonium oxidation in the receiving environment (e.g. freshwater habitat) does demand dissolved oxygen hence it is important that any oxygen demand method capturing this process.

Some workers are only interested in the biological oxidation of organic-C oxidation and hence they apply nitrification inhibitors to BOD tests to inhibit ammonium being oxidised by bacteria. Such BOD is referred to as CBOD or Carbonaceous BOD.

Whichever method we choose, it has to fit the purpose.

COD method is relatively quick. However, do COD results represent what really occurs in the receiving environment? Would the more bacterially resistant organic-C (e.g. cellulose) be a threat to the receiving environment such as freshwater habitat?

With the above in mind many workers still in favour of BOD values. However, owing to 5-6 day waiting time many workers resort to COD methods. Some workers who are in favour of BOD values who cannot afford time resort to determining ratios between COD and BOD in an attempt to interpret the COD values into BOD values.

Unfortunately, the ratios will always be misleading because of a wide ranging effluent signatures, even within one type of effluent. If one takes treated sewage as an example, there are considerable differences in treatment methods (e.g. SBR, DAF, activated sludge, lagoons) hence widely differing effluent quality in terms of its organic-C and ammonium content and thus the corresponding COD or BOD values will be wide ranging.

So, getting back to my reference to urine in bathwater in my previous answer, since BOD method can capture the oxidation of ammonium process than the conventional COD method, potential oxygen demand of bathwater containing urine will be better captured by the BOD method than the conventional COD method.

This used to be one of my favorite questions in exams leading to overall failure, regardless of other anwers to questions, because it reflects the level of basic understanding in parameter assessment and evaluation. I will try to summarize what I think to be the right answer:

BOD and COD are collective parameters; they do not indicate a single parameter but an index value  - i.e. oxygen consumption – associated with an on-going reaction. In the case of BOD, this is the partial stoichiometry of heterotrophic substrate oxidation and nitrification.

However, the more important issue is why this index value is used and where it is used: In wastewaters and similarly in polluted surface waters BOD is/should be used to assess organic substrate/organic carbon, for the simple reason that unoxidized nitrogen – i.e. ammonia N, and organic N - can be separately and accurately measured by different techniques. Then the BOD test is conducted with a nitrification inhibitor and this way, it only represents organic substrate, only partially oxidized in five days. As such, BOD remains always lower that the corresponding COD.

In sources supplying consumption water, aside from the fact the nitrogenous compounds should be low and also measurable with precision by instrumental analysis, the organic carbon is also to low to be accurately detected, either by BOD and COD. Consequently, the correct parameter for water should be total organic carbon (TOC). Therefore, trying to compare BOD and COD for such waters becomes irrelevant and obselite.  

Dear Preethamol,

I also agree with Dr. Derin Orhon Answer. The correct parameter for water should be total organic carbon (TOC).

Dear Ms. Preethamol,

BOD express the total organic load that can be decomposed biologically in presence of oxygen (thus aerobically). COD express the total organic load that can be decomposed chemically (thus more drastically). Based on that, it derives that always BOD is less or equal to COD.

Regarding water disposed from bathroom:

(a) water (+waste) from toilet is completely decomposed biologically, so in that case BOD can be equal to COD

(b) water from the shower, due to the presence of detergents, is expected to have a BOD value less than the value of COD.

Hoping that my answer helped you

Best Regards

I found CBOD: 7 to 9 mg/l, COD: 3 to 5 mg/l, and TOC: 9 to 11 mg/l. Is there any explanation of this result?

In general, the COD value of a wastewater sample is greater than or equal to the BOD value; But there are cases where the BOD value can be higher than the COD value. One of these cases is when the wastewater contains high levels of organic nitrogen or sulfur compounds. These compounds can be biologically oxidized aerobically; But they are not oxidized by the dichromate reagent used in the COD test.

Another case where the BOD value can be higher than the COD value is when the wastewater contains nitrifying bacteria. These bacteria aerobically consume oxygen to convert ammonia into nitrate. If the wastewater contains high levels of ammonia, the nitrifying bacteria consume a significant amount of oxygen, which can lead to an increase in BOD relative to COD.

Some examples of wastewater that may contain high levels of organic nitrogen or sulfur compounds:

- Wastewater from meat processing factories

- Wastewater from tanneries

- Wastewater from oil refineries

- Wastewater from coal gas production plants

Wastewater from paper factories

- Wastewater from pharmaceutical factories

It is necessary to explain that CBOD value can never be higher than COD. In addition, if all pollutants are biologically degradable, BOD value will be equal to COD.

In general, the COD value of a wastewater sample is greater than or equal to the BOD value; But there are cases where the BOD value can be higher than the COD value. One of these cases is when the wastewater contains high levels of organic nitrogen or sulfur compounds. These compounds can be biologically oxidized aerobically; But they are not oxidized by the dichromate reagent used in the COD test.

Another case where the BOD value can be higher than the COD value is when the wastewater contains aerobic nitrifying bacteria. These bacteria use oxygen to convert ammonia into nitrate. If the wastewater contains high levels of ammonia, the nitrifying bacteria consume a significant amount of oxygen, which can lead to an increase in BOD relative to COD.

Some examples of wastewater that may contain high levels of organic nitrogen or sulfur compounds:

- Wastewater from meat processing factories

- Wastewater from tanneries

- Wastewater from oil refineries

- Wastewater from coal gas production plants

Wastewater from paper factories

- Wastewater from pharmaceutical factories

- Some wastewaters from food industries

It is necessary to explain that CBOD value can never be higher than COD. In addition, if all pollutants are biologically degradable, BOD value will be equal to COD.