Level 1: Oxygen Uptake Rate—An Introduction
Whenever I evaluate a biological treatment system there are two sets of tests I always run:
1) Oxygen uptake rate (OUR), and
2) Adenosine triphosphate (ATP) analysis.
When combined, the results of the OUR and ATP analyses can provide you with a very insightful picture of conditions in the bioreactor (biological treatment system) that other methods simply can't match. And if you are able to do so, adding a microscopic analysis of the mixed liquor suspended solids along with the evaluation of chemical oxygen demand (COD) or biochemical oxygen demand (BOD) data, you'll be in a position to generate a complete picture of plant operating conditions.
In this section we'll explore the fundamental use of the OUR test which allows us to make a quick, generalized, determination of the organic load being applied to the bioreactor and the rate at which the microorganisms are consuming (oxidizing) the organics in the wastewater. To learn more about ATP testing click here.
Level 1 OUR Test Description
I consider there to be three levels of oxygen uptake rate testing, with Level 1 being the essential, required, starting point for conducting any of the three levels. The graphic below summarizes how I define the three levels of OUR testing.
Conducting the Level 1 OUR Test
The OUR test is easily performed by recording a series of dissolved oxygen measurements in one minute increments over a 15 minute time period from a mixed liquor suspended solids (MLSS) sample collected from the discharge of a bioreactor. It should be noted that high organic loading conditions will result in oxygen depletion in the MLSS sample in less than 15 minutes.
The equipment required to conduct the oxygen uptake rate test consists of a handheld dissolved oxygen (DO) meter, a DO probe, a magnetic stirrer, a stir bar, and a 300 mL BOD bottle, as shown in the image below. (Details of this equipment, including pricing, can be found on my blog page entitled "Testing Equipment.")
Step-by-Step Level 1 OUR Testing
To determine the oxygen uptake rate you collect approximately 500 mLs of mixed liquor suspended solids (MLSS) as it is leaving the bioreactor. Promptly bring the MLSS to the lab to begin the test. The step-by-step procedure is as follows:
1. Pour the sample back and forth between two beakers (or any two containers) at least 10 times to fully aerate the MLSS before starting the test, as shown in the short video below. Though you are "falsely" elevating the dissolved oxygen (DO) concentration in the MLSS sample your goal here is to maximize the duration of the test, trying to go the full 15 minutes without running out of oxygen in the BOD bottle. You do not want the DO concentration in the MLSS to be a constraint. In spite of starting with the highest possible DO concentration, you may still run out of oxygen if the organic load entering the bioreactor is high.
2. With the magnetic stir bar already in a 300 mL BOD bottle, fill the BOD bottle to the point that it just begins to overflow.
3. Insert the dissolved oxygen (DO) probe into the BOD bottle, making sure you have a tight seal so that air cannot be introduced to the bottle during the test. With the BOD bottle completely filled, upon inserting the DO probe you will be assured that a airtight seal will be achieved.
4. Place the bottle on a magnetic stirrer and start mixing the sample. You need to keep the MLSS in suspension and it needs to be constantly moving past the DO probe in order to obtain accurate DO readings.
5. Start on the DO meter to begin the OUR test. 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. I use a rugged Hach HQ40D handheld meter (pictured below) which can be easily configured to record interval measurements to automate the DO recording step.
Interpretation of OUR Result(s)
The graph below, congested though it is, reveals quite a bit if we take some time to look at it closely. What it shows are the OUR results at a municipal wastewater plant in Florida from two consecutive days in May 2016. At time = 0, which means the MLSS samples had been collected and analyzed immediately, both OUR values are very high but the value measured on the 19th is 39.6% less than the value recorded approximately 24 hours earlier. This municipal plant receives large slug loads of leachate delivered in tanker trucks at different times of the day, throughout the day. So the significant change in the OUR from one day to the next is really not too surprising. The fact is, both OUR values indicate a heavy organic load being applied to the biological treatment system (high F:M ratio).
If we now compare the OUR results at time = 2, which means the MLSS has been aerating in the lab for two hours without any further addition of food, we can see that the higher initial OUR value of 49.3 mg Oxygen/L/hr recorded on 5/18 is reduced 82.8% to 8.5 two hours later. The sample from 5/19 is reduced 59.4% from an initial OUR of 29.8 to 8.5 mg Oxygen/L/hr after two hours. From this we can conclude that the nature of the organic loading on 5/18 was "easier" for the bacteria to oxidize than it was the next day, just 24 hours later.
The 39.7% increase in the 5/19 MLSS sample, where the OUR goes from 12.1 to 16.9 after seven hours of aeration without food, tells us the nature of the organics are much more complex than they were in the sample from the previous day. This may be an indication that the leachate quality is highly variable and its rate of discharge into the wastewater plant may need to be regulated. The graphic below is an attempt to portray the difference in the organic load between the two MLSS samples collected on two consecutive days.
When Biological Treatment Is Complete
The oxygen uptake rate will indicate whether or not biological treatment is complete. When all (or most) of the organics have been oxidized, which means all (or most) of the food has been consumed, the microorganisms will have reached endogenous respiration and you will have an OUR value typically <9.0 mg Oxygen/L/hr. An OUR value >9.0 mg Oxygen/L/hr indicates several possible conditions such as:
1) A slug load of organic material is making its way through the bioreactor at the time the MLSS sample was being collected;
2) There is insufficient biomass, typically measured as mixed liquor volatile suspended solids (MLVSS), relative to the applied organic load;
3) The bioreactor is too small for the applied organic load.
Note: I have selected the specific value of 9.0 mg Oxygen/L/hr somewhat arbitrarily, though based on having done a lot of OUR testing. In my experience this value is definitely the upper limit of endogenous respiration in a mixed liquor suspended solids sample. A "tighter" and perhaps more appropriate value indicating that biological treatment is complete would be <6.0 mg Oxygen/L/hr. I have repeatedly searched the literature for confirmation (or rejection) of my "arbitrary" range (<6.0 to 9.0 mg Oxygen/L/hr) for endogenous respiration but, so far, I have been unsuccessful in finding supporting documentation.
In the graph below, where the red dashed line is at 9.0 mg Oxygen/L/hr, you can see the OUR results for 12 different wastewater treatment plants measured during the time frame spanning 9/24/2013 to 8/31/2016. Of the 12 plants, only one had achieved endogenous respiration, with the other 11 plants reflecting OUR values indicative of high organic loading (or heavily loaded biological treatment systems).
To read about Level 2 OUR Testing please click here.
For an excellent paper on oxygen uptake rate I encourage you to download this PDF:
Oxygen Uptake Rate Measurements for Application at Wastewater Treatment Plants
by Marinette Hagman and Jes la Cour Jansen. Water and Environmental Engineering / Dep. of Chemical Engineering, Lund University.