May I know what are the reasons contributed to the higher COD at the effluent of anoxic biological treatment as compared to the influent COD? (I am using mixed bacterial culture here) I suspect that it is due to the formation of aromatic amine ( a kind of recalcitrant intermediate metabolite formed after anoxic decolorization, break down of azo bond), however, most of the journal articles did not suffer this problem. The dye-containing wastewater consists of sole diazo dye and some trace element in very low ppm. I am using COD titrimetric method to evaluate the COD of the anoxic effluent COD.
I dont know realy strange
Did you try to repeate your experiment:
it should be lower in the effluent due to the degredation processes of the parent compound of the dye
but may be other compound are formed such as you said aromatic amine or compined compound with metals but, eve this couldnot make your effluent higher than the influent.
I suggested that repeate the test and becareful when the color changed after digestion of the COD and during the titration with FAS, cause one drop could make your effluent higher at the formula for calculation. also, try to make standradaization for FAS every day.
let me know the results.
regards
try to use another method for COD measurement. I suggest COD cuvette test (HACH LANGE LTD.).
As strong as the conditions are, the COD test still does not completely oxidize all organic compounds. Thus it may miss some of the potential oxygen demand - from some ring compounds, for example. Biological transformation may then convert the compound to one that is captured in the COD test, leading to an apparent COD increase. If you are working with a specific known compound, perhaps you can compare its measured COD to its theoretical oxygen demand (assume any N becomes ammonium), and see if that accounts for the difference. Good luck!
Peter
Uausally, anoxic biological treatment does not suffers from this problem. However, in special cases it can occur. During COD tests, not all the organic compounds are oxidized, while during anoxic biological treatment, some dysoxidizable macromolecules which cannot be measured by COD degrade into small molecules which can be tested by COD. Therefore, I recommended that you can evaluate the organic content by TOC rather than COD.
One addition: COD may also increase as a result of dying bacterial cells: they decompose and release DOC, which in turn increases COD (since you usually filter samples before analysis, nearly all intact cells are beeing withhold, so their carbon does not contribute to the COD). So if your conditions are unfavourable for your organisms (toxic, suffocating, starving), their cell debris will increase your COD.
If you are sure about the method procedure and filtration before testing your samples, I have the same idea with Mr. Peter F Strom. Good luck!
Sometimes the materials used in the COD titration method are expired or incorrectly storage, in which it affects the reaction roles of such materials. Thus, you should repeat the procedure and compare the titration results with other COD measuring methods such as; Fast digestion - Spectrophotometric method (HJ/T 399-2007). However, I agree with the suggestion of Mr. Peter.
Regards…
Considers a possible influence of the oxygen of the nitrates (or other inorganic source) in the analysis. In the anoxic treatment could have been eliminated.
Is the anoxic zone reducing nitrates? If so, I would be very surprised that additional COD from cell death/degradation and any additional COD from the biological oxidation of recalcitrant species would exceed the influent COD. Is the anoxic zone becoming anaerobic?
how many sample results do you have?
@Simon Collett: The initial information was a litttle vague. From the question: "at the effluent of anoxic biological treatment (...) I am using mixed bacterial culture here". So if you incubate aerobic organisms (mixed culture? e.g. activated sludge from wastewater treatment process) under anoxic/anaerobic conditions for too long (e.g. in batch experiments, you will usually see it after 24 hours already), they will suffocate -> die -> disintegrate -> increase DOC -> increase COD. Seen it multiple times.
Same will occure under starving conditions, although it will usually take more time.
Hello Sandro Castronovo . Your answer really fit with my question as my filtration process also shows increase in COD. but is there any journal i can refer to that prove this statement?
Hello Muhammad Burhanuddin Bahrodin , no unfortunately I haven't read any paper discussing this topic. I mentioned rising DOC in a paper very shortly if this is of any help: "Only a small offset was observed between the delta DOC and the nominal DOC which could be mainly explained by a slight increase of the delta DOC during the first 10 d of incubation due to a stronger increase of the natural background DOC in the spiked samples in comparison to the unspiked control samples." (doi: 10.1016/j.watres.2016.11.041) I haven't done a systematic analysis to prove my interpretation that the COD (DOC) results from desintegrated cells, since it was of no particular relevance for my work, but to me it is obvious. I also saw an increase of NH4 in a lot of my experiments, especially in anoxic and anaerobic batches, which makes perfect sense and fits to my assumption (see "nitrogen cycle").
Dear Sandro Castronovo , thank you for the answer. Really appreciate it and really help my concern. Thank you so much
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