What to Consider When You Actually Perform the OUR Test
You have two key concerns when you collect a mixed liquor suspended solids (MLSS) sample to run your initial oxygen uptake rate (OUR) test. The first concern is to perform the OUR test on an MLSS sample as soon after you have collected it as is reasonably practical. In contrast, what you do not want to do is collect your MLSS and then let it sit in the back of a truck for two hours in an enclosed (sealed) container while you make your rounds.That very first OUR test on a "fresh" mixed liquor sample is going to give an excellent idea of how complete treatment (chemical oxidation) is in your bioreactor. So perform the OUR test immediately after collecting the MLSS.
The second concern, and this is particularly true for heavily loaded biological treatment systems, is to make sure the MLSS has a high enough oxygen concentration before starting the OUR test to insure the test can run the full 15 minutes. Even with a fully oxygenated MLSS, in highly loaded systems you may find the oxygen depleted in the BOD bottle in less than 15 minutes. If you have a test that lasts at least five minutes you'll be able to generate an accurate enough OUR value that will produce a very high number indicating just how highly loaded the bioreactor is with organics.
A very simple technique I use to oxygenate the MLSS is to simply pour the sample back-and-forth between two beakers or containers to aerate the MLSS before pouring into the BOD bottle and inserting the probe to start the test. The video below shows this pour procedure. I always pour the MLSS back-and-forth 10 times minimum to fully oxygenate the sample. Doing this simple procedure gives you the best chance of completing the OUR test properly.