BOD note "Detection limit raised based on sample volume used for analysis"

Modified on: Mon, 10 Jan, 2022 at 7:59 PM


The note "Detection limit raised based on sample volume used for analysis." doesn't mean that we didn't have enough volume, rather it means that the volume of sample used from the bottle dictates the dilution factor that was used to calculate the final result. 
When setting up BOD tests, the lab uses historical values, the appearance/smell of the sample, and the initial DO reading to make an educated guess on if a dilution is required and if it is, how much of a dilution should be performed. Multiple dilutions are set up and the best result is reported. 
BOD values can sometimes be reported either over-range or under-range when the dilution used is incorrect despite our best efforts. 

Here is a brief summary of our BOD method:
Biochemical oxygen demand (BOD) is an empirical test used to measure the amount of biodegradable organic material in a sample of water. This is done by adding microbial organisms to an aliquot of sample and measuring, by probe or titration, the dissolved oxygen (DO) present before and after a 5 day ± 6 hour incubation period. The difference in the amount of DO is the result of the biochemical activity of the microbes consuming the organic material of the sample, which also provides an estimation of amount of biochemical demand. BOD can be attributed to the degradation of organic material (carbonaceous demand), the oxidation of inorganic materials such as sulfides and ferrous iron, and the oxidation of reduced forms of nitrogen (nitrogenous demand). This method has BOD as the measurement of all components. Inhibited BOD (BODI) inhibits the nitrogenous demand, measuring only the carbonaceous demand; often referred to as carbonaceous BOD (CBOD).
 
Sample aliquots are transferred into BOD bottles and are seeded with microbial organisms. The bottles are then filled with dilution water and the initial DO is measured.  Bottles are then stoppered, creating an air-tight environment, and incubated for 5 days ± 6 hours at 20 ± 1°C and a final DO reading is taken. The difference between the two DO readings, corrected for seed and volume of sample used, is BOD.





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