This level of oxygen uptake rate (OUR) testing is used to evaluate the impact of individual waste streams. It can be used to evaluate the oxygen consumption of a proposed waste stream or it can be used to evaluate the oxygen demand from several existing streams to determine which are inhibitory and which exert a high oxygen demand. A key to doing this testing is that you first make certain the microorganisms (or mixed liquor suspended solids) have reached a state of endogenous respiration. Your goal is to evaluate the impact of one or more samples so you need to start your testing with the bacteria hungry, though well-oxygenated, and ready to start consuming organics as soon as a potential source of food is introduced.
The way you go about doing this testing is to generate OUR data using several different sample volumes. Once you’ve confirmed that endogenous respiration has been reached (When the OUR drops below 9.0 (mg O2/L)/hr it can be assumed that endogenous respiration either has been achieved or will soon be achieved. If the OUR is below 6.0 (mg O2/L)/hr you can begin testing immediately.), you run at least three sample volumes using, for example, 1 mL, 3 mL, and 5 mL of test sample added to the 300 mL BOD bottle which is then filled to just overflowing with MLSS as pictured in Figure 1. A magnetic stir bar needs to be in the BOD bottle because the sample needs to be well-mixed during the 15-minute OUR test.
Figure 1: Level 3 OUR Sample Volumes
After generating OUR data using three or more sample volumes, you produce a simple bar graph such as the one shown in Figure 2. The sample being tested was from an “off-spec” tank and there was concern about possible inhibition or toxicity. But these are good results and the interpretation of this graph is straight-forward. Because the OUR is increasing with each increase in sample volume we know the sample is not inhibitory. In fact, the more of this sample we feed to the bacteria, the more they like it. So we would want to be careful in introducing this waste stream at some limiting flow rate to be certain we don’t exceed the oxygen input capability of the aeration system. For example, if the tank was pumped down at a rate of 314 gallons per minute (gpm) it would place an additional oxygen demand that would require 1,612 pounds of oxygen per day, in addition to the wastewater already being treated.
Figure 2: Level 3 OUR Test Results
Figure 3 shows the possible outcomes from testing different samples. An increasing OUR is the best result though an OUR that stays constant with increasing sample volume also indicates no inhibition from the waste stream. Where you need to be careful in trying to treat a particular waste stream is when you have a decreasing OUR with increasing sample volume. When you see that you know you have an inhibitory wastewater that will need to be introduced slowly so the microorganisms can acclimate to it. And if the OUR drops to zero you know you’ve got a toxic waste that is going to need some sort of physical or chemical treatment, such as the addition of hydrogen peroxide, before it can be introduced to an activated sludge system.
Figure 3: Interpretation of OUR Results for Different Sample Volumes
You can see a direct interpretation of OUR testing using three different sample volumes and two waste streams in Figure 4. This graph shows the results from testing the wastewater leaving an equalization tank and entering the SASTU or Staged Activated Sludge Treatment Unit. This petrochemical plant has an “off-spec” tank called 20D3 and there was concern about potential toxicity in the wastewater stored there. The graph shows two important aspects regarding the tank 20D3 waste: 1) It is not inhibitory at all (the bacteria actually found this waste to be desirable) and 2) This is a very “strong” waste in terms of the oxygen demand it will exert on the bacteria so it will need to be fed into the bioreactor at a very low rate.
Figure 4: Interpretation of OUR Results for Different Sample Volumes
Another type of testing at Level 3 is to compare several different wastewater sources within a plant to determine those streams that have the highest oxygen demand and, therefore, need to be controlled so as not to exhaust the oxygen in the biological reactor. In addition, this testing provides a comparison of one stream to another to evaluate which one or more streams may be harder to oxidize due to inhibitory or refractory compounds in the wastewater. An actual comparison of eight different wastewater sources is shown in Figure 5. The largest wastewater flows come from the desalters and wastewater from these process units has the highest oxygen uptake rates which tells us the bugs or microorganisms really like this food source. The large difference between desalter #1 and desalter #2 is due to the fact that different crude oils were being processed at the time of sample collection.
At this refinery Crude Tank 1 is the tank that usually receives “opportunity crude oil” which is an oil that is often more difficult to process. These crudes can have much higher concentrations of phenol, sulfide, heavy metals, amines, and solvents so the bugs find the wastewater generated during the processing of opportunity crudes harder to utilize as a food source. And that is certainly reflected in Figure 5 where the lowest oxygen uptake rate was the water drain (bottom sediment and water or BS&W) from the crude storage tank. So the value of OUR testing is that it informs us as to how fast we should drain a crude tank. We don’t have to worry too much about the flow rates out of Crude Tanks 3 and 4 but we would definitely want to throttle the drain valve out of Crude Tank 1 to make sure we don’t overwhelm the biological reactor with a wastewater that the bacteria won’t be able to process very quickly.
Figure 5: OUR Comparison of Different Waste Streams
Oxygen uptake rate testing is one of the most effective and insightful techniques available to the wastewater plant operator when it comes to determining the impact a given waste stream will have on the wastewater system. But the testing just discussed does require a lot of time. The testing itself is easy and with the right instrument the OUR test runs itself, recording the dissolved oxygen measurement every minute for 15 minutes. Then you have got to rinse your BOD bottle and DO probe, pour your MLSS back-and-forth 10 times!! to fully aerate or oxygenate the sample, and start your next test. When you have the number of samples you see in Figure 6 be prepared to spend a very long day in the lab. And that’s just to generate the data you need. Once you have the data, you need to do the analysis that produces a graph like that shown in Figure 5, above. I’m not trying to discourage you; I just want you to be prepared for a substantial time investment.
Figure 6: Lots of Samples Means Long Hours in the Lab
There are several documents available that provide formal test guidelines for doing toxicity or inhibition testing using the OUR method. The Organization for Economic Cooperation and Development (OECD) provides one very well-known document, which you can download here as a PDF file, called the “OECD Guidelines for the Testing of Chemicals.”
If you are serious about doing OUR testing, and you want to make sure that your results can be reproduced, then you should also consider purchasing the test procedure from the ASTM International standards organization for approximately $41.00. Their document, which is an excellent source of information, is shown in Figure 6. From the ASTM website, the D5120 standard is described as follows: “The objectives of the respiration inhibition tests may be defined by the interests of the user, but the test method is designed primarily for examination of the inhibition response with operating microbial systems such as an activated sludge process treating domestic or industrial wastes.” You can get to this document by clicking here to go to the ASTM website or on the Figure 7 graphic itself.
Figure 7: ASTM Standard for OUR Testing