Biochemical Oxygen Demand Questions

I receive e-mails from folks every day, asking about different water quality/water treatment/ environmental issues.  I guess they have learned where they can find MOST definitive answers!  (I suppose there are other explanations, too.)  Here’s the latest:

I am an applied science student from Hong Kong.  I saw your blog (about Biochemical Oxygen Demand.)  May I ask you some questions about BOD?

Q1:  What is/are the advantage(s) of using BOD?

Q2:  Why the BOD bottle should be sealed during incubation?

Q3:  Why the incubation should be taken place in the dark?

I can’t find the answers in the internet, so I want you to give me some hints & ideas to my questions. Thanks!


Hello Isabel:

Here are my best answers to your questions.

Q1: As we have stressed in our previous blogs on BOD, no other test (other than possibly Chemical Oxygen Demand, COD) measures such a wide range of environment-altering substances in the water.  You can find those earlier blogs by entering “Biochemical Oxygen Demand” in the Search window at the bottom of the page here at MOST.

Q2: The bottle must be sealed so that no additional oxygen enters the bottle during the test.  The test begins with a high level of dissolved oxygen (about 8 – 10 ppm), and this level decreases over the five days of the test period.  The more the DO level drops, the higher the BOD.  If additional oxygen entered the test bottle, we would deceive ourselves into believing that the water was cleaner than it truly is.

Q3: There are two main reasons why the test bottles are incubated in the dark.  The first is related to Q2.  If there is light, then algae will grow in the test bottle.  The algae will produce oxygen, once again misleading us as to the extent of the water’s cleanliness.  Secondly, light is injurious to many life forms.  Microorganisms that are present in the test bottle may be harmed by excess light, and thus would not be able to consume any oxygen — once again, skewing our results.

Back to COD.  The COD test measures mostly the same substances in water that the BOD test does.  However, it can be performed in a couple of hours instead of five days.  That is a huge advantage for the COD test.  There are some differences in the results, but many water agencies perform both routinely so as to develop a correlation for their water.  The COD gives them nearly instant results, which are usually confirmed five days later when the BOD test is concluded.

A drawback of COD is that toxic material in the water that could inhibit biological decomposition of waste would not be detected; however, in that instance the BOD test would also give us poor results: we would see a low BOD, but it would be caused by toxic material killing the microorganisms in our BOD test!


Good luck on your project, Isabel!  I hope you get a MOST excellent grade!

3 Responses to Biochemical Oxygen Demand Questions

  1. avatar Viral Mehta says:

    I am performing BOD experiments. I usually add seed solution in BOD water or Dilution water, is this correct to do? if not, then how much and how to add seed solution.
    Waiting for your response.

    • avatar Steve McLean says:

      Viral: I apologize for the late reply. I suspect you have already found an answer. It depends mostly on the sample matrix. If you are dealing with normal municipal waste, there are plenty of healthy microorganisms already present, so seeding is unnecessary. Matrices with low microbe counts or compromised by toxic agents will benefit from seeding. I haven’t done this myself, so I can’t help you with the details of the seeding procedure. If you find out, please share it with our MOST readers! And thank you for your question and your remarkable patience in waiting for my extremely late reply.

  2. avatar sanjib das says:

    Notes shared are very helpful.

Leave a Reply

Your email address will not be published. Required fields are marked *