I think, Biological oxygen demand (BOD) is more realistic as it indicate amount of dissolved oxygen demanded by aerobic biological organisms to break down organic material present in a given water sample at certain temperature over a specific time period. It is often used as a surrogate of the degree of organic pollution of soil.

Chemical oxygen demand (COD) is less specific, since it measures everything that can be chemically oxidized, rather than just levels of biodegradable organic matter. COD is an important water quality parameter because, similar to BOD, it provides an index to asses the effect discharged waste water will have on the receiving environment. The COD test is often used as an alternate to BOD due to shorter length of testing time.

I agree with Dr J.C.Tarafdar

• Thank you  dear Dr. J.C.Tarafdar and Mr. Malek Hassanpour. So it's wrong to use COD to evaluate the removal efficiency for an aeration tank  (activated sludge process),of course for the removal of organic matter. 

Dr. Nabeel, higher COD levels mean a greater amount of oxidizable organic material in the sample, which will reduce the dissolved oxygen (DO) level. A reduction in DO can lead to anaerobic conditions which is deleterious to higher aquatic life form.

Yes Dr.J.C.Tarafdar, but you see most of our P.G. students do not differentiate between the real conditions related to bacteria in the BOD5 test from the chemical oxidation resulting from the COD test. If you refer to the aeration tank in the activated sludge process you will see that from a mass balance the organic matter is governed by BOD5 not COD. The oxygen requirements are goverened by the ultimate BOD and the recycled cells. Thank you for your precious information. 

Chemical Oxygen Demand is an important water quality parameter because, similar to BOD, it provides an index to assess the effect of discharged wastewater on the receiving environment. Higher COD levels mean a greater amount of oxidizable organic material in the sample, which will reduce dissolved oxygen (DO) levels. A reduction in DO can lead to anaerobic conditions, which is deleterious to higher aquatic life forms. The COD test is often used as an alternate to BOD due to shorter length of testing time.

I completely agree with you dear colleagues. Thank you for appreciation and precious information. 

I think COD can be more appropriate if the wastewater is from toxic industrial processes which thereby inhibit the action of Bacteria that BOD is supposed to measure. If the treatment process is Activated sludge then BOD is okay for measuring the efficiency of the plant

Thank you Hameed for your acknowledgement 

Dear Edwin Kimutai Kanda , even for toxic industrial wastewaters, bacteria can be acclimated . In my PhD thesis which is documented here I brought a bacterial seed from an aeration tank  (activated sludge process) in Baghdad/ Iraq then I began to acclimate this bacteria in a model aeration lagoon in the laboratory. It took about four weeks to acclimate this bacteria on an industrial wastewater containing emulsified oil which is very difficult to degrade . You can refer to (Industrial water pollution control) by Eckenfelder for this purpose and my PhD thesis. I also agree with you about the difficulty of performing BOD5 for a toxic wastewater. Best regards. 

Dear Nabeel,

COD and BOD are very important parameters that can not be neglected in determining waste water character. We can not reveal the true character of the waste water by only doing BOD or just COD analysis. Let's say the BOD value of waste water is zero. No one can say that this wastewater have not organic loadings. Similarly, no one can say that a wastewater with a high COD value can be biologically degraded. So both are the parameters that need to be measured. In addition, both are important parameters in terms of determining the wastewater that can cause biological accumulation in the environment.

OK dear Olcayto Keshinkan. I agree with you . Your opinion is the perfect explanation.

Dear Nabeel and respectful researchers

BOD, is an empirical measurement of the oxygen required by bacteria to decompose organic matter. Thus, the efficiency of a wastewater treatment plant can be determined by measuring the BOD concentration. By comparing the dissolved oxygen concentrations in sewage influent versus effluent, treatment plants can accurately gauge how much organic matter is being removed during the treatment process. Additionally, effluent water with a high BOD can negatively impact the natural body of water where it is discharged because dissolved oxygen is limiting in aquatic environments.

On the other hand, the COD test measures all organic carbon with the exception of certain aromatics (benzene, toluene, phenol, etc.) which are not completely oxidized in the reaction. COD is a chemically chelated/thermal oxidation reaction, and therefore, other reduced substances such as sulfides, sulfites, and ferrous iron will also be oxidized and reported as COD. NH3-N (ammonia) will NOT be oxidized as COD.

COD uses strong chemicals to oxidize organic matter. Generally, COD is preferred to BOD for process control measurements because results are more reproducible and are available in just two hours rather than five days. By the time you have the results from a five day test, the wastewater treatment plant conditions are no longer the same, so real time monitor and control can not be relied upon by the use of BOD. In this wastewater training you'll learn COD is a quick and easy measurement to get a snap in time picture of what is going on in the system, and with trending, long term predictions can be made and monitor and control of the process at the wastewater treatment plant can be optimized and controlled.

 For industrial samples, COD may be the only feasible test because of the presence of bacterial inhibitors or other chemical interferences, which would interfere with a BOD determination. COD testing also gives the fast measurements required in many treatment systems for informed decisions regarding process control adjustments. Many industrial and municipal laboratories find that parallel COD and BOD testing is beneficial because the COD test can be used to target a specific BOD range. The need for multiple BOD dilutions is minimized or eliminated.

Regards,

Ata

Right dear Ata Rafiee. For this my suggestion is to correlate BOD5 and COD readings with as much readings as possible  (not less than 45 observation). To calculate the expected BOD5 from the COD reading  (this is right for only domestic wastewater. Thank a lot for the precise analysis.