I tried to measure BOD with the APHA standard method, however in some cases I found BOD values higher than COD values.
that certainly is a paradox...even in the case of using the old school system of quantitative analysis...COD>BOD...if it persist, try using the quantitative system to get a clearer feel of a close value.
Chemical oxygen demand (COD) is a measure of the total quantity of oxygen required to oxidize all organic material into carbon dioxide and water. does not differentiate between biologically available and inert organic matter, and COD values are always
greater than BOD values.
COD includes both biodegradbale and non-biodegradable substances whereas BOD contains only bio-degradable.
the APHA method is good. Maybe it is the measurement of COD that there would be problem
check the products used can be there some that are obsolete.
Did you denitrify? Not all BOD is organic carbon - remember, it is a measure of the oxygen demand, not the organic carbon. There are multiple noncarbonaceous species that will give you an appreciable BOD. Also, did you satisfy the IBOD? If not, you may be looking at erroneously high BOD values based on an initial IBOD (excess sulfites, for example).
COD > BOD is a very simplistic relationship, and only valid under certain circumstances. In anything other than heavily treated waters this relationship is very complex.
Agreed Mark. COD is not always greater than BOD. Waste containing high ammount of ammonia and reduced nitrogen compounds can be oxidized by microbes in BOD test, unless you inhibit nitrification. Dichoromate used in standard COD test cannot oxidize ammonia . Therefore, under such conditions BOD could exceed COD.
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