i carried out BOD and COD of same water sample . BOD and COD are respectively 7 and 52 mg /L . I am confused because i know COD is twice time of BOD. any procedure about ensure ?
in domestic wastewater sample COD *0.66 = BOD but this value is different for industrial waste
in samples at low pH less than two this relation can't takes place
in milk or meat waste samples may be found that BOD is equal or slightly greater than COD and so on
Well,
Wastewater and especially industrial wastewater COD/BOD reach more than 3.
Sincerely
There is no general rule about the relation between COD and BOD. Generally it is expected that COD is higher than BOD. Your result can be correct, though it is unusual.
The BOD and COD results depends on the oxidisable matter in the wastewater.
E.g. if the sample only contain acetate the COD and BOD might be the same. Or the water might contain a non-biodegradable polymer like effluent from a paint factory there BOD is sometimes less than 1 % of COD.
To test your procedures for COD and BOD analysis you can make standard addition. Increase the oxidisable matter concentration of your sample by adding a defined chemical that gives an exact increase. The chemical to add should be described in the standard method you use (e.g. glucose glutamic acid for BOD, phthalic acid for COD).
You can also determine total suspended solids TSS of the sample and it's results slightly greater than BOD result in your case about 10 or 11
In municipal wastewater the ratio of COD to BOD is 2.2 to 2.5 depending upon the wastewater strength and its contributing source. Ratio of 4 is common when trade waste/industrial effluent gets into stream. In your case, since the ratio is high, there is no science to say that it should not be beyond the typical ratios. Perhaps, best would be to check the point source and also the values quoted are low when compared to typical values. I believe the sample refers to 'drinking or potable water' and not 'sewage'.
Look for potential interference's with the COD test as well, chlorides as well as ammonia in the presence of chlorides can exhibit high bias with the COD. There may be toxicity affecting the BOD. What method are you using to perform the COD analysis?
Ok, that has a detection limit around 5-10 mg/L. If the 2:1 holds up COD "should" be around 15 mg/L. In my opinion, that is really pushing the accuracy limits for this test. Run a couple more tests to verify?
To Bayan's point, if you are dealing with Oil & Gas wastewater I have seen COD:BOD at >7:1. That's when we use the PACT process for recalcitrant (non-biodegradable) COD adsorption and removal, especially before UF and RO to extend membrane life.
Hope this helps!
Best method to check the accuracy of COD experimental results are by conducting analysis for 3 different dilutions for single sample. If the results for all dilutions are closely distributed (say