Biochemical Oxygen Demand question (BOD) (1 Viewer)

[Giant Lobster]

Im confused about the methods used to measure BODs in a sample of water. In conq they say "it is measured by adding nutrient to the sample and incubating it at 20 degrees in a sealed air-free container in teh dark for 5 days..." but it mentions that you add nutrients. First of all, what is a BOD? My current understanding is that it is the oxygen needed to decompose / react with all of the 'stuff' in the water, and thus is a measurement of how much oxygen the 'stuff' in the water can absorb. If this is correct, then wouldn't adding nutrients be stupid? its like, ur measuring the stuff u added...

Further down the page, conq then goes "Samples with high BOD such as untreated or partially treated sewage would quickly use up all the dissolved oxygen... For these samples a suitable micro-organism as well as nutrient is added before incubation" so by that it implies BOD depends neither on the nutrient or micro-organism content in water... Have I missed the correct definition of the BOD? conceptual error? argh... so gay.

 

[izi]

from what i remember... BOD is the amount of dissolved oxygen needed by aerobic bacteria to break down the organic waste

 

[jims]

yes, i agree that its stupid. but then again, i aint doing to well at chem.
anyway, in pathways, it doesnt say anything about adding nutrients or mircobes, just put in dark room at 20 degrees c for 5 days.

 

[Giant Lobster]

hmm well exactly, if BOD is the amount of O2 needed for 'aerobic' bacteria to break down the stuff in the water, then adding nutrients to it then measuring how much oxygen is consumed is exactly like putting more firewood into a flame and measuring the oxygen consumed. Also, by adding micro-organisms to it as well, its like giving an army reinforcements, adding a few more cities for them to raze, and measuring how much destruction they can deal... And no, my analogies are only incorrect if the given definition of BOD is incorrect.

is it just me or is BOD all screwed up?

 

[Akira_Tikira]

BOD is the amount of oxygen that is consumed by the aerobic organism. It measure the amount of organic materials in the water. As we know, the more organic materials the more micro-organism to feed on these resources.

What we did was to take a sample of water, use an oxygen sensor to measure dissolved oxygen before hand. Put it in a dark cupboard to prevent photosynthesis and wait for 5 days and measure it again. I think there is no units, BOD is a comparative though of thing.

Hope it helps