In some water bodies of Central México with solonchak soil type, We have been recorded high values of chemical oxygen demand in a rate of several times the values of biochemistry oxygen demand.
Dear Jacinto,
It might be attributed to the nature of the soil which I believe is composed of derivatives of silica and alumina (oxides and etc). Further, such soil lacks organic compounds which generally increase the BOD levels.
Hoping this will be helpful,
Rafik
Dear Rafik
Efectively, both, Water and sediment values of aluminum were high.
Thanks for your opinion.
Dear Marcelo
We are using the APHA method (potassium dichromate). It is posible that interference is happening. However, no in all study sites we have this diferences in the rate of BOD/COD. Thanks.
COD values will remain higher in comparison to BOD, because free salts present in saline water form complexes with impurities resulting in deviation in value. Use newer methods like Total organic content analyzer for better accuracy.
I think u should quantitatively determine Chloride in your sample first. then u modify the reagent accordingly.
- Chlorides are quantitatively oxidized by dichromate and represent a positive interference. Mercuric sulfate is added to the digestion flask to complex the chlorides, thereby effectively eliminating the interference on all but brine and estuarine samples.
- In the method of APHA mercuric sulfate mixed with digestion mixture of the COD reagent to remove the interference of Cl- ion by precipitating as different species like HgCl2, HgCl42-, etc. Alternatively you can use this thumb rule: 0.5 gm HgSO4 when added to a single vial of COD can remove maximum 8000 ppm Cl- interference
I am attaching a document which was previously added to research gate by some other researcher, you can get help from this also.
The first issue to consider is the reliability of analytical protocols.i.e. chloride interference with COD (the document provided by Ms.Dutta may be quite useful) ; or BOD assessment in saline water.
Once this issue is settled, if you still have high COD/BOD ratio, this may be attributed to the presence of slowly biodegradable or even, non degradable organics in the lake water, such as humid acids, etc. or the presence of inhibitors/toxicants which actually impair the biological reactions in the BOD test. Check for both!..
In both cases, please abandon the BOD test!...,which usually brings more problems than useful information. Try the TOC/COD couple and/or a well defined biodegradation test preferably using respirometry.
Thank you Derin. I will review methods for BOD in saline water and other organics.
Hello,
Could you please mention the high value of COD in numbers. If it is really high in a natural water body, it is a serious issue. No water organisms like fish cannot survive in the system. Otherwise if there is natural living habitats in the system, your calculated value is wrong. It may be due to wrong method or due to interferences. As the other researcher suggested, it is mostly due to high chloride interferences.
"Chemical oxygen demand (COD) does not differentiate between biologically available and inert organic matter, and it is a measure of the total quantity of oxygen required to oxidize all organic material into carbon dioxide and water. COD values are always greater than BOD values, but COD measurements can be made in a few hours while BOD measurements take five days."
https://www.google.com/search?q=chemical+oxygen+demand+in+lakes+values&source=lnms&sa=X&ved=0ahUKEwiPp5Wy6ubTAhWJ2SYKHcvUDd0Q_AUIBSgA&biw=1280&bih=802&dpr=1#
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