According to my experience it is not possible. However, some researchers show that to see the residual COD possible when you are treating recalsitrant and xsenobiotic pollutants by microorganisms. These kind of residuals may becoming degredable form in AOP treatment. For example, lignin can not be degradaded completely in biological treatment. If you introduce these kind of effluents to the AOP process, may be COD concentration increase temporarily. After a certain time COD then decrease with the continue of the AOP processes. But I'm not sure. I will search it.

Thanks Olcayto, actually im using textile wastewater after biological treatment. the COD of raw water about 120ppm, after treating the water for 12hr the COD was 145ppm and in some sample (different treatment method) the color turned green and i had to dilute the sample. im using closed fluxing titration method. I couldn't find any explanation except the one you posted but no reference.

i have seen similar results for electrochemical oxdation of olive mill effluents. Check the  article:  The effect of solids on the electrochemical treatment of olive mill effluents

which process is used?

Chloride or hydrogen peroxide?

thank you Zacharias, Im using H2O2, and I think its interfering of H2O2 in COD test. I haven't make sure yet.

Dear Ahmad,

You reported that you are using biological treatment method for textile effluent before an AOP process. Are you sure that all of organisms removed from effluent? I thought that may be residual microorganisms go on to the reproducing the new cell in your AOP process. So, COD concentration may be increasing. I don't know which method you are using. If your AOP process going on with a carbon electrode in an electrolise process, this kind of electrode may give additional COD to the water. I will follow this question.

It's most likely that the responsible is the H2O2. I have see similar behavior  in a diploma thesis about azo dye & Fenton. process You can try to compute the interference by constructing something like a curve for known solutions of hydrogen peroxide (i.e 5 -10-50 100 mg/L) in ultrapure water (X) and measure the COD (Y) in order to obtain a linear equation. (if you can't quenching the H2O2 without  altering the cod)

Check also https://www.researchgate.net/publication/223915596_Interference_of_hydrogen_peroxide_on_the_standard_cod_test  

Article Interference of hydrogen peroxide on the standard COD test

Good suggestion, also I will try to measure the residual of H2O2 in samples after treatment, then we can know how much exactly the H2O2 has added to the COD!!

The COD may  increase and then decrease when applying AOPs for the POPs treatment. Generally, the COD was determined by oxidation of reductive substances (including low molecular weight organics which are easily oxidized) by K2Cr2O7 or KMnO4. However, the POPs are not easily oxidized by  K2Cr2O7 or KMnO4., whereas the POPs canbe relatively easily oxidized by free radicals generated via AOPs to form  low molecular weight organics which are easily oxidized by K2Cr2O7 or KMnO4. Therefore, when you  treat wastewater  polluted by POPs, such as textile effluent (including a lot of dyes and dyeing and finishing auxiliaries which are POPs), with AOPs, the intermediates of POPs, i.e. the low molecular weight organics, may be firstly formed by oxidation of free radicals, and then the low molecular weight organics degraded by free radicals. As such,  it  is possible that the COD can increase and decrease when applying AOPs treatment.

If H2O2 has been used in  a treatment I prefer to remove it before the COD analysis by treatment with catalase rather than analyzing the hydrogen peroxide and subtracting the calculated effect on COD.

The predicted effect of H2O2 on COD appears in some of the sample types I have worked with not to follow the støkiometric reaction or even to be somewhat non reproducible.

Thanks Dr. Henrik for your suggestion, can we use MnO2 to remove the H2O2 instead of catalase?

I found MnO2 can remove effectively the H2O2 residual and can be easily filtered out of the solution.

Hydrogen peroxide -- as mentioned by others -- if present in solution will contribute to COD.  A mg/L of H2O2 increases COD by about 0.393 mg/L ; this value was obtained employing Milli-Q water spiked with varying concentrations of H2O2; you may do your own measurements employing distilled and deionized water or Milli-Q water, etc.

Addition of catalase also introduces error in COD measurement that has to be accounted for; this can be done by the following relation.

CODCOR = CODS+C - CODC

Where CODS+C is COD of water sample after catalase addition and CODC is COD of Milli-Q water with the same amount of catalase.

Thanks Dr Ali, The problem now even we using the MnO2 to remove the residual of H2O2, but we still can see higher COD in some samples comparing with the raw water.

Hello Ahmad,

This could be because some of MnO2 is ending up in your COD vial and hence the higher COD. Have you tried correcting COD contribution from H2O2 using the correction value provided earlier?   

Did not say that it cannot be used; one has to be careful not transfer any MnO2 into the COD vial; that is all. Correction for H2O2 contribution to COD is simple and conveniet.

Dear Dr. Ali,

We filtrate the sample before testing the COD, and to make sure the MnO2 is not interfering, we will test it with the standard solution of COD. Also we can do the correction of h2o2 contribution as you suggested. I found some reference using MnO2, here is one: Comparison of various advanced oxidation processes

and chemical treatment methods for COD and color

removal from a polyester and acetate fiber dyeing effluent

We found that adjusting pH 10-11 and heat overnight can be an economic way to remove the residual of H2O2.