Question of the Day: BOD & COD

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• Industrial Wastewater Treatment
• Industrial Water Treatment
• Industrial Water Treatment

9 Answers

1. Please find an article about the differences here. Alternatively you can also watch the 3 min video tutorial: BOD explained - The water quality indicator

2.   The BOD and COD relationship is dependent upon each individual wastewater.  An analysis of the wastewater and the effleunt may give a better picture of how to intrpret the BOD and COD results and understand their relationship.

   Most people know what BOD and COD mean.  However, do not always assume that BOD means BOD5 since there are modications to the BOD analysis.  For example respirometry can give extented length BOD such as a BOD10, BOD15, BOD 30.  If there is a high amount of oxidiable material, the test may be decreased to a BOD3. There is also the CBOD (carbonaceous) analysis that uses an nitrification inhibitor.  

   Rubin - really like your comment because that is truly what COD and BOD can do for design engineers in the real world.  To add to his comment, operations may look at the BOD:COD ratio as a way of managing the health of the WWTP.  

   I am going to disagree with some of what some people stated below because a lot of the assumptions are based upon generalizations of the BOD5 (five day) analysis of muni wastewater.  These assumptions do not alway hold true for BOD testing with different lengths of time or for industrial wastewater.

   Jacque is incorrect in stating BOD is organic and COD is organic + inorganic.  If it is the CBOD5 where a nitrification inhibitor is added, then he is generalization is assumed correct for muni wastewater that have normal ranges of nigrogen containing constituents.  If there is high nigrogen containing constituents, nitrification may start in the first 5 days.  If respirometry is performed without the inhibitor for a longer period say BOD30, nitrification will occur and that can include inorganic material. 

   Second,  I have seen industrial wastewater that contains inorganics and low soluble organic material.  The inorganics can be readily oxidized by the bugs quicker than the organics.  It all depends upon what the bugs are acclimated to.  Think of an upset condition in a refinery API separator that results in FOG going to a WWTP.  The complex oils will most likely go though the system, cause a lot of foaming, and may inhibit/kill the bugs.  The BOD5 of the wastewater can show uptake of oxygen even though there is inhibition.  Analysis of the wastewater compared to the effluent may show that sulfides are still being oxidized to sulfate while FOG is passing through the system.

   Sean comments is correct for general muni wastewater.  (Sean, you are the man when it comes to wastewater and I loved working with you at USPT).  Be very careful when it comes to industrial wastewater where compounds like simple organic acids are present.  The C1 - C5 organics acid do not always fully oxidize in the COD test where bacteria will consume this quite readily.  You may find the BOD:COD ratio near or above 1.   

   Bascially the relationship of BOD and COD all depends upon each individual wastewater.  

    

   1 Comment

   1. I have a question:  In general, what is people's experience with plant sanitizers affecting the biology of the system and the BOD measurement of an industrial wastewater?  Could a BOD level be affected if the plant's wastewater sample is pulled with high amounts of sanitizers in the stream?  What if we are talking not residuals, but actual concentrations?  What would happen to the BOD measurement of the stream?  Would it show a lower value in the stream if no sanitizers existed?  What method of BOD measurement should be used if this is a suspicion?

3. The ratio of BOD to COD gives you the orientation of the engineering that should be applied to the treatment of a determined effluent.That is, if the treatment is only Biological, Physical-chemical and Biological or only chemical.

4. BOD is simply the portion of COD that is biodegradable.  BOD is based on a five day test for oxygen uptake by biomass (activated sludge mixed liquor) as it consumes that portion of the COD.  BOD is a small circl right now the bigger COD circle.  Helps to visualize it, I think.  Sean

5. I agree with John Steel,

   BOD = Biochemical Oxigen Demand -> required oxigen to oxidize biochemical content in wastewater

   COD = Chemical Oxigen Demand -> required oxigen to oxidize chemical and biological (as chemical) content

   always COD > BOD ml equivalent

6. COD is the inorganic and organic content in wastewater, BOD is the organic part, therefore BOD is included in the COD count.

   1 Comment

   1. absolutely NO, review a trust water engineering book or author like Metcalf

7. The  key difference  between BOD and COD is that  the BOD is the oxygen demand of microorganisms to oxidize organic matter in the water under aerobic conditions while the COD is the oxygen demand to oxidize all the pollutants organic and inorganic in the water chemically .

   High BOD can be a result due to domestic sewage, petroleum residues and wastes of animals and crops.

   COD always high in comparison to BOD since COD is a measurement of a complete breakdown of pollutants, unlike BOD in which biochemical breakdown is often not complete as a chemical breakdown.

   COD test utilizes a strong oxidizing reagent to oxidize materials in water while BOD test does not use a chemical reagent instead it allows microorganisms to act on the organic matter.

    

8. The main thing they have totally in common is the demand for dissolved oxygen.  One is chemical, the other biological.  Both can be gotten rid of with proper treatment.

9. COD includes the BOD so the lab can test for it right away. The BOD takes 5 days for digestion, the  discharge water can be out of compliance for the whole time rolling up a huge fine! However if the COD is in compliance the BOD will test OK after 5 days