Dear Osim Aquatar, the explanation has no relation to Cattle Wastes. But by the definition BOD is the biochemical oxygen demand which is related to the biological oxidation of the organic matter (inside the Bacteria) while COD is the chemical oxygen demand for the organic + inorganic matter. So logically COD is higher than BOD.
A large portion of COD is produced by suspended matter. It is more logical if the sample is flattened and the soluble COD should be determinate.
I would just add that what is Biodegradable is not necessary the same as what can be oxidised by bacteria, it is well known that Anaerobic treatment of manure will reach higher removal rate - than we can do with Aerobic treatment alone. Further I will just remind that as long as we have not defined BOD as i.e BOD5 or BOD7 at i.e. 20°C for the incubation and whether adapted or not adapted sludge is used it is still very difficult to know how much of COD can be degraded by aerobic biology - here you will need to test over longer time i.e. 20 - 30 days as it is done in the Zahn Wellen test (http://www.oecd-ilibrary.org/environment/test-no-302b-inherent-biodegradability-zahn-wellens-evpa-test_9789264070387-en) were the goal is to find the aerobic non-degradable COD fraction.
I agree Mr. Soerensen' s answer, it is difficult to measure BOD a long time. In cattle waste there is a high amount of cellulosic material hardly degraded by microorganisms (when compared with pigs manure for ex.).
COD is always greater than BOD no matter what the waste is... BOD is the amount of Oxygen consumed during the degradation of organic as well as inorganic materials while BOD is the amount of Oxygen required to degrade organic matter only.
A large portion of COD is produced by suspended matter. It is more logical if the sample is flattened and the soluble COD should be determinate.
I agree Mr.Soerensen's answer. Please read the discussions in the text book authored by Metcalf and Eddy for more information
With fermentation the BOD5, 7 or 14 will eventually increase asymptotally (coming close to the COD value), especially with high temperature pretreatment. This is somewhat the same as Sorenson's answer but from a different starting point. In simple terms, the fermented the recalcitrant (e.g., hemicellulose and cellulose) substrates are oxidized to lower chain molecules that can be oxidized in the lesser time frames of the BOD test protocols.
When read your question I had in my mind just the answer which Mr. Amin gave you. Moreover, beside the answer coming from BOD and COD definitions, this result is very logical while cattle manure or any manure is practically an already fermented product which is resulted from digestion, so as a very wide microbiota from the intestinal tract already consumed a big part of the organic compounds. This makes BOD be lower than COD in lab analysis.
Dear Osim,
Composition of waste generated by digestion system of cattle is not always fully digested rather it contains part of lignin ( covering of seeds & part of wood etc.) which escape from degradation by intestinal juices. This material could be oxidized by strong chemicals Like chromates etc. that is why when you test COD, it contribute to COD but within 5 day BOD test this material could not decomposed by bacteria leading to show lower BOD value.
hope this help you
regards
dinkar
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