Why does the BOD level in a stagnant water rises?

Question

Why does the BOD level in a stagnant water rises?

Answer 1

BOD = Biochemical Oxygen Demand.  This is a measure of the amount of oxygen that would be required to oxidize and/or metabolize all of the organic and chemical substances in that particular sample of water.  It was developed as a measure of the strength of sewage that was discharged to streams.  Sewage (or any other water with a high BOD) will cause the oxygen in receiving waters to be used up, or depleted, making it impossible for any organisms requiring oxygen to live there.  This includes almost all aquatic organisms, such as fish, aquatic plant life, etc.  The higher the BOD, the more oxygen will be used, and the greater the impact.  

The BOD level in stagnant water will rise largely because the stagnant water does not contain sufficient oxygen for any BOD present to be oxidixed or metabolized.  Therefore, the substances that cause high BOD will tend to accumulate and increase in concentration.  Stagnant water tends to have less oxygen than flowing water because there is less chance for mixing of the water and the surrounding air, which is where the oxygen comes from in the first place; for example, in a flowing stream you can often see air bubbles in the water, which gives the oxygen in the bubbles a greater chance to dissolve into the water.  In stagnant water there is much less of this mixing, so less oxygen.

Stagnant water also often stratifies, which means it forms layers which do not mix with one another.  This generally occurs when the warm water on the surface of a stagnant body of water "floats" on the colder water below.  If they do not mix, the cold water in the lower layers have no contact with the atmosphere and and as a result there will be little or no oxygen dissolved in those deeper, colder layers.  In this case, the BOD of the deeper, colder layers will also build up and the concentrations increase.  Again, as an example, if you have ever been swimming in a  lake in summertime, you can often feel a very sudden change in the water temperature as you dive down.  This is because you are crossing from the warm layer on top to the cold layer below.

Answer 2

I'm sorry, but I just wanted to say that that was quite interesting to read bernardo's answer. I just happen to see the question, or actually just popped out, and pleasantly surprised by Bernardo's thoroughness in the answer giving. Thanks for taking the time to explain Bernard! :) Take care to all!

Answer 3

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