Level 2 OUR

Level 2 or Time-to-Endogenous Respiration OUR Testing

Making periodic checks on the time-to-endogenous respiration is important for industrial wastewater treatment plants that experience highly variable organic loading rates. Let’s take a look at what we are actually trying to determine with the endogenous respiration rate and oxygen uptake rate (OUR) testing.

Endogenous Respiration

Endogenous respiration, also referred to as endogenous metabolism, is defined here as “a situation where living organisms oxidize some of their own cellular mass instead of new organic matter they adsorb or absorb from their environment” (California State University, Sacramento, Volume II). When endogenous respiration has been achieved, the microorganisms (primarily bacteria) have begun to use their own cellular mass because the food supply, the COD, has been exhausted. When treatment is complete in the biological reactor, you expect the bacteria to have reached a state of endogenous respiration. With industrial wastewater systems this is not always the case. When there is too much food (COD) it can be several hours or more before endogenous respiration is achieved. When this happens, treatment is incomplete and the COD concentration in the mixed liquor suspended solids (MLSS) leaving the biological reactor will be to high as illustrated in Figure 1. The great value of OUR testing is that it allows us to determine exactly how much more treatment time is required for the organic load entering the reactor.

Figure 1: Treatment is Incomplete and the COD Leaving the Reactor is Too High

Time to endogenous, hours—the OUR can be used to determine when an activated sludge has reached an endogenous state (when all the oxidizable substrate has been used up by the microorganisms). The time it takes the mixed liquor to reach an endogenous state should not exceed the detention time in the aeration basin, otherwise treatment will not be complete. The OUR measurement of activated sludge or the MLSS is a measure of the utilization rate of the readily biodegradable chemical oxygen demand (RBCOD) that remains in the wastewater. More RBCOD in the wastewater correlates directly with a higher oxygen consumption rate. The oxygen consumption rate falls to a minimum value only when all of the RBCOD has been consumed and this is the endogenous respiration rate.

In Figure 2 we can see a biological reactor that has been properly sized for the organic load. At the exit end of a plug-flow reactor the COD concentration has been reduced to a minimum value and the dissolved oxygen (DO) concentration has been steadily increasing. Even with more aeration time we would see only a minimal reduction in the COD concentration. The bacteria, having consumed the readily available food, have reached an endogenous state. When the bacteria are settled in the clarifier and returned to the aeration basin they will be hungry and will start oxidizing the organics in the wastewater immediately. This describes an optimized activated sludge treatment system.

Figure 2: Endogenous Respiration Achieved in Biological Reactor

In Figure 3 we see a very different situation compared to that shown in Figure 2. Here, the aeration time is not sufficient to allow for a complete reduction in the applied organic load. By the time the MLSS flows out of the biological reactor, oxidation is incomplete and the bacteria are still actively consuming organics. This is further reflected by the fact that the DO is also low at the exit end of the reactor. In this example, the oxygen uptake rate testing has determined that an additional three hours of aeration time is required to reach endogenous respiration, the point at which the COD concentration would be at its minimum value. Given the conditions shown in Figure 3, the wastewater plant could be described as being overloaded, undersized, or some combination of both. The bacteria flowing into the clarifier will continue to consume organics but will do so at a much slower rate due to the low dissolved oxygen concentration in the MLSS and poor contact between the bacteria and the food due to a lack of mixing. When the bacteria return to the aeration basin as return activated sludge (RAS) they will not be ready to immediately start consuming organics again. This describes a suboptimal activated sludge treatment system.

Figure 3: Treatment is Incomplete and Endogenous Respiration Was Not Achieved

We will step through a series of three graphs (Figures 4, 5, and 6), so we can see just how the data generated from OUR testing allows us to determine how much additional aeration time is required to achieve a relatively complete degree of treatment (COD reduction). In Figure 4, we have the initial or Level 1 OUR results for a wastewater system that is well-sized for the loading conditions it typically experiences. This plant was tested in November 2012 and again in January 2013. On both occasions the plant had achieved endogenous respiration by the time the MLSS was leaving the biological reactor. Repeated OUR testing at industrial wastewater plants has resulted in the determination that 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. In other words, below 9.0 (mg O2/L)/hr the OUR will decrease very slowly before it finally levels off.

Figure 4: Well Designed Biological Reactor Achieves Endogenous Respiration

Properly sized bioreactor

In Figure 5, a second wastewater plant has been added to the graph and this plant shows incomplete treatment as indicated by the high initial OUR value of almost 14 (mg O2/L)/hr (see note 1 on the graph in Figure 5). After an hour of additional aeration, as OUR testing continued, we see that we are getting close to endogenous respiration, achieved when the oxygen consumption falls below the somewhat arbitrarily chosen OUR of 9.0 (mg O2/L)/hr (see note 2 on the graph). It’s also interesting to see that the bacteria have been working hard to hydrolyze some difficult (refractory or complex) compounds that finally became available as food after nearly two hours in the reactor. This is shown by an increase in the OUR (see note 3 on the graph). This “uptick” or increase in the OUR often happens in chemical and petrochemical wastewater systems where numerous complex organic compounds are in the wastewater and have to be broken down to simpler compounds before they can be used by the bacteria for food.

When the OUR testing consistently shows that endogenous respiration is not being achieved by the time the MLSS exits the biological reactor, one of two options should be evaluated: 1) either the organic load to the plant needs to be reduced or 2) the capacity of the biological treatment system needs to be increased.

Figure 5: OUR of an Overloaded Wastewater Plant

Undersized bioreactor

In Figure 6, we have added OUR data for two more wastewater plants, both of which suffer from greater overloading than the plant we introduced in Figure 5. Here again, in Figure 6, we can see that refractory compounds had to be dealt with by the bacteria. And in Figure 6 we can also see that more than three hours of aeration have taken place before endogenous respiration was achieved for wastewater system “C” (the line with the purple triangle symbols). This particular plant is constantly struggling with both high flow and high COD conditions that result in frequent oxygen depletion in the biological reactor and odor complaints from the community. A second stage aeration basin is being added to the treatment process after OUR testing had repeatedly shown insufficient capacity based on a consistent inability to achieve endogenous respiration.

Figure 6: Significantly Overloaded Wastewater Plant

Overloaded wastewater plant

Once the time-to-endogenous respiration has been determined, the OUR data can be used to estimate the total oxygen consumption in the biological reactor. This is of particular concern for aeration tanks that do not consistently achieve endogenous respiration by the time the wastewater is leaving the tank. The graph in Figure 7 shows, in this case, a polynomial regression fit to OUR data generated from time t = 0 to t = 313 minutes. You can see from the graph that the OUR test was conducted five times.

The OUR at t = 0 minutes is 20.18 (mg O2/L)/hr, a high value indicating the treatment process is far from a state of endogenous respiration. Keep in mind this is the OUR value in the MLSS as it is leaving the biological reactor on its way to the secondary clarifier. The significance of this is that the oxygen demand in the reactor is extremely high at the exit end. In fact, as we will see in a moment, the oxygen demand associated with a very high COD loading exceeded the oxygen input capability of the aeration equipment which is surface aerators for this particular wastewater plant.

Figure 7: Regression Fit to OUR Data

Polynomial regression fit

Let’s calculate the oxygen consumption rate in the aeration tank for the OUR at t = 0 minutes where the OUR value is 20.18 (mg O2/L)/hr. The volume of the aeration tank at this plant is 5,077,220 gallons. And the surface aerators can produce 16,000 lb O2/day. We want to calculate the total oxygen consumption for a 24 hour period. With an OUR value of 20.18 (mg O2/L)/hr the total oxygen consumption in the reactor is 20,501 lb O2/day as shown in Equation 1. In other words, the oxygen consumption at the end of the aeration tank still exceeded the oxygen generation capability of the aerators by 4,501 pounds per day!!

Equation 1: Oxygen Consumption in MLSS Leaving the Biological Reactor


It is important to keep in mind that we are using the OUR value from the effluent end of the aeration tank. The OUR at the effluent end of the aeration tank is much lower than the OUR at the influent to the aeration tank. So, in effect, we are calculating the “minimum” total oxygen consumption of the biological reactor and this minimum value is still greater than the total quantity of oxygen the system can produce. The biological reactor can be described as either being overloaded, undersized for the applied COD, or some combination of both. As you can see in Figure 8, as the MLSS is leaving the bioreactor there continues to be an excessive oxygen demand, a high COD concentration, and almost no dissolved oxygen.

Figure 8: High Oxygen Demand, High COD Concentration, Low DO Concentration in the MLSS

At t = 31 minutes the OUR has dropped to 16.10 (mg O2/L)/hr. And at t = 103 minutes the OUR is 12.39 (mg O2/L)/hr. After 2.7 hours (t = 103 minutes) of aerating the MLSS, during which no COD was added to the sample, the OUR dropped to 9.2 (mg O2/L)/hr. But we know we need to get below 9 (mg O2/L)/hr to reach endogenous. Let’s calculate the oxygen consumption requirement for the bioreactor when the OUR value is 9 (mg O2/L)/hr. This is shown in Equation 2.

Equation 2: Oxygen Consumption in MLSS at Endogenous Respiration


When the OUR value is 9 (mg O2/L)/hr the oxygen requirement in the aeration tank is 9,143 lb O2/day, 6,857 below what the aerators are capable of producing. That is, as we approach the endogenous respiration rate we begin to have excess oxygen which increases the DO concentration in the reactor. This is illustrated in Figure 8 where you can see that a significant reduction in the COD concentration has occurred along with a significant increase in the dissolved oxygen concentration.

Figure 9: Low Oxygen Demand, Low COD Concentration, High DO Concentration in the MLSS

The OUR testing at this plant was stopped after 5.2 hours (t = 313 minutes) at which point the OUR value was 7.4 (mg O2/L)/hr which equates to an oxygen consumption rate of 7,518 pounds of oxygen per day, well below the oxygen generation capacity of 16,000 lb O2/day. One way we can interpret the OUR test results is to say that at least three more hours of aeration time is needed based on obtaining an OUR value of 9.2 (mg O2/L)/hr after 2.7 hours. In biological systems that don’t reach endogenous respiration oxygen generation capacity is often a constraint. These systems were designed for a lower organic load which inherently means they were sized with smaller oxygen generation systems (diffused air, mechanical surface aerators, etc.) than what are currently needed. Knowing oxygen consumption rates becomes very important so that estimates for how much more aeration is required to handle the applied COD load.

When the oxygen generation capability is insufficient it causes additional operating problems including the incomplete oxidation of organics, low or no dissolved oxygen, excessive growth of filamentous bacteria, and strong, offensive odors that can lead to complaints from residents. Level 2 OUR testing allows for a periodic check on actual operating conditions and the critical determination of whether or not endogenous respiration is consistently being achieved by the time the wastewater leaves the aeration basin.

In completing our discussion of OUR testing to determine when endogenous respiration has been reached, I’ve mentioned several times that the MLSS was being continuously aerated throughout the test period. That was meant literally, that a large sample (2 to 3 gallons) of MLSS was being aerated as shown in Figure 8. If you look closely at Figure 8 you should notice the four tubes running down into the bucket of MLSS (they are clear plastic so they’re a little hard to see). This sample is being aerated using an aquarium pump. The full setup used in the laboratory is shown in Figures 10, 11, and 12.

Figure 10: MLSS Being Aerated in Laboratory

The aquarium pump is a required piece of laboratory equipment for doing Level 2 (and Level 3) oxygen uptake rate testing. These pumps are inexpensive, costing approximately $30 at any pet store in the United States. I prefer the Top Fin air pumps sold by PetSmart. They offer several models with 1, 2, and 4 air outlets. I like the Air-8000 with four outlets, as shown in Figure 11, because it provides not only a lot of air, but good mixing in the 5-gallon pails I use. You will also need airline tubing and air stones. The tubing costs about $6 for 25-feet and a packet of six air stones costs $5.

Figure 11: Aquarium Pump Used to Keep the MLSS Aerated

Figure 12: LDO Probe, MLSS Sample, Magnetic Stirrer, DO Meter for OUR Testing

OUR instrument setup