This is the easiest and quickest of the three OUR test levels we are going to talk about. All it takes to do this test is a mixed liquor suspended solids (MLSS) sample and the required OUR test equipment. You get your MLSS sample (typically the overflow from, not the influent to or front end of, the bioreactor), bring the MLSS to the lab, pour the sample back and forth to fully aerate the MLSS before starting the test, fill the 300 mL BOD sample bottle so that it overflows, making sure the magnetic stir bar is already in the bottle, insert the LDO probe making sure you have a tight seal so that air cannot be introduced to the bottle during the test, place the bottle on a magnetic stirrer, and hit start on the DO meter. If you have your DO meter configured to record the DO readings once every minute for the duration of the test, 15 minutes later you have the data you need to determine the oxygen uptake rate. Let’s discuss each of these steps in a little more detail.
Getting the MLSS sample is straight-forward enough. You will usually, but not always, collect this sample from the effluent end of the aeration basin or biological reactor (bioreactor). You are getting an overflow MLSS sample because, with Level 1 testing, you want to know how complete the treatment is, as the MLSS flows to the secondary clarifiers. Any exit point from the bioreactor is fine with the goal being to collect a sample that represents complete biological treatment. Always obtain this sample from the same location to minimize introducing another variable. You should collect at least 500 milliliters (mL) of sample since you are going to need 300 mLs for the BOD sample bottle you will use to insure an air-tight seal between the bottle and the DO probe. Once you’ve collected the MLSS sample, you want to get back to the lab and start the test as soon as possible. If, after having collected the sample, you get caught up doing something else and don’t get back to the lab an hour or so later, consider your MLSS sample to be no good. Dump it and go collect another sample.
Now that you are in the lab with a “fresh” MLSS sample, before starting the OUR test, make sure you pour the MLSS back-and-forth between two beakers to fully oxygenate the sample. You need the starting dissolved oxygen (DO) level to be as high as possible so that the test can run for a full 15 minutes without prematurely exhausting or consuming all of the oxygen in the MLSS/BOD bottle. I have found from experience that pouring the MLSS back-and-forth 10 times provides a sufficiently high DO concentration to start the test. The simple pour is shown in Figure 1. Simple to do, YES, but also this is critical so make sure you do perform this easily overlooked task.
Figure 1: To Aerate, Pour the MLSS Back-and-Forth 10 Times!!
As soon as you complete the aeration of the MLSS sample, pour the MLSS into a 300 mL BOD bottle with a magnetic stir bar in the bottom of the bottle. You need to fill the BOD right to where it just slightly overflows the bottle. Now insert the DO probe, making sure you displace some of the MLSS from the bottle and in doing so, make sure you also form a tight seal between the DO probe and the bottle. You do not want any air to enter the bottle during the test which will negate your results. For the Hach HQd meter shown in Figure 2, you hit the greenish button on the upper right and the DO concentration in the bottle will be recorded once every minute for 15 minutes, if you have set the meter up to record in this way. The particular meter shown is a Hach HQ30d. More information (model number, pricing, etc.) on the instruments shown in Figure 2 can be found at OUR Equipment.
Figure 2: OUR Test Set Up and Running
After 15 minutes, the data, depending on the DO meter you are using, can be copied over to a USB drive and then over to a computer so the actual oxygen uptake rate can be calculated. In Figure 3 you can see the actual minute-by-minute DO readings from an OUR test that has been entered into an Excel spreadsheet.
Figure 3: Excel Spreadsheet OUR Data
Worth noting in Figure 3 is that the oxygen is consumed in the BOD bottle after just 12 minutes. The full 15-min test does not go to completion because the oxygen demand is so high (23.1 mg O2/L/hr). When the bioreactor is overloaded with soluble and “desirable” or readily-oxidized food (COD or BOD) you will see this happen. This is not a desirable result because this is the oxygen demand in the MLSS as it is leaving the bioreactor on its way to the clarifier. At this point treatment is not complete, as it should be, and settling of the MLSS in the clarifier is going to be slower than normal. In addition, you are likely to see a higher COD in the clarifier overflow or effluent. Furthermore, you may be experiencing low dissolved oxygen concentrations in the bioreactor and an odorous environment.
When you do a Level 1 OUR test and get the result shown in Figure 3, you have good reason to move on to Level 2 OUR testing to determine how much additional aeration of the MLSS is required in order to reach endogenous. You’ll need to go get more MLSS and you will need to aerate the MLSS over a period of time as shown in Figure 4 where a sample of MLSS was aerated for 112 minutes with OUR tests run periodically.
Figure 4: Oxygen Runs Out in 12 Minutes
Figure 5 shows the Excel formulas used to calculate the correlation and the OUR values in units of mg/L of oxygen per minute and per hour. Click the following links for PDF documents that will provide you with more information on Excel’s built-in functions for RSQ, ABS, and Slope.
Figure 5: Excel Formula for OUR
The correlation calculation is used as a check on the validity of the OUR test data. In addition to using the RSQ function to calculate the correlation I graph the results of each test to visually confirm that I’m getting the expected linear reduction in the dissolved oxygen concentration as shown in Figure 6. If the correlation isn’t at least 90% I consider the OUR test to be suspect.
Figure 6: Excel RSQ Function Correlation Graph
In Figure 7 you can see a comparison of the initial or Level 1 OUR results from three different wastewater plants. The most significant point to notice in Figure 7 is that two of the three wastewater plants were not at an endogenous respiration rate, indicating that more aeration time was required in order to achieve “complete” treatment.
Figure 7: A Comparison of OUR Values from Different Petrochemical Wastewater Plants