Specific Oxygen Uptake Rate

On January 29, 2015 I received a very nice email from an individual with the initials DP. That email, verbatim, is provided below. DP makes some excellent points and I want his comments shown. This kind of feedback is very much appreciated!

Email from DP:

“Hi Rick, you have done a lot of work on this web site and have a lot of good information. However, I happened to look at the page referencing SOUR and you have some incorrect information there.

SOUR is actually a calculation done on the OUR after you have measured the OUR at the sample point. And the OUR can be measured at any sample point and the SOUR calculated on the OUR measured on that sample point. OUR is measured as mg/L/Hr, which is a function of how many bugs are present among other things. To be able to compare my OUR to your OUR, we would have to have the same amount of bugs. Thus to correct for changing bug populations, the SOUR is calculated. Divide the OUR by the MLVSS you get mg/L/Hr per gram of VSS. Then I can compare my OUR to your OUR.

What you refer to as SOUR with the sample at the head end is actually the fed OUR. You can run an OUR at the head end and an OUR at the tail end. In each case when you divide by the grams of MLVSS, you get an SOUR for that sample point.

Congratulations on all this work and the web site. I have been in the business for 39 years now and like you still love it and still am learning. I admire the commitment you made to make this web site.”


I stated previously that the specific oxygen uptake rate (SOUR) test, in contrast to the oxygen uptake rate (OUR) test, is used to determine whether or not the applied organic loading is too high or inhibitory. And this is still true when the SOUR test result is used in conjunction with Table 1.

Table 1: Interpretation of SOUR Test Result

Specific oxygen uptake rate summary

But an SOUR can certainly be computed on any OUR test result, as shown in Equation 1, below, where the oxygen consumption rate is the OUR and the VSS is the mixed liquor volatile suspended solids (MLVSS) concentration. Personally, it has been my experience that an SOUR calculated on an MLSS sample collected from the effluent of a bioreactor is of little direct use because I’m more interested in the actual (absolute) oxygen uptake rate rather than a relative specific oxygen uptake rate. By that I mean I’m looking to see if the OUR leaving the bioreactor is at or near endogenous and I know it is when the actual OUR is ≤9.0 mg O2/L/hr regardless of the MLVSS concentration. And I can be even more certain that endogenous respiration, or complete treatment, has been achieved when the actual OUR leaving the bioreactor is ≤6.0 mg O2/L/hr. This is true for any biological treatment system so I don’t need to look at the SOUR.

But I do like to use the SOUR for testing what is going on at the influent end of the bioreactor, as shown in Figure 1, when I have a high organic loading (COD or BOD) and I see that the OUR leaving the bioreactor is high. In this situation, where the loading to the bioreactor can be highly variable, it becomes difficult, if not impossible, to make meaningful comparisons to loading rates from one wastewater system to another without normalizing the data using the MLVSS concentration.

For example, an OUR at the influent end of a bioreactor, the “fed OUR,” of 40 mg O2/L/hr, seems very high, but it is hard to know for certain. At one wastewater plant this may truly be a high OUR value while at another, perhaps not. This is where the benefit of computing the SOUR really becomes evident. If the MLVSS concentration at Plant A is 1,600 mg/L, the SOUR is 25.0 mg O2/g-MLVSS/hr, and, according to Table 1, this is high. But that same OUR, at Plant B with an MLVSS concentration of 2,400 mg/L, results in an SOUR of 16.7 mg O2/g-MLVSS/hr which is a “normal” SOUR value. So two operators, from two different plants, talking to one another about their perceived “high” fed OUR values of 40 mg O2/L/hr are actually talking about two very different loading conditions. In order to properly compare their loading conditions they have to compare SOUR values. In contrast, these same two operators, when talking to one another about their effluent OUR values of 6.0 mg O2/L/hr or so, can both be certain they have achieved complete treatment in their bioreactors.

Equation 1: Formula for Calculating the SOUR

SOUR Calculation

Figure 1: Sample Points for SOUR and OUR


Note in Figure 1 that a plug-flow bioreactor is portrayed. This flow configuration makes sample collection easy. It can be difficult to obtain a true “influent” MLSS sample in complete mix systems so you’ll want to evaluate your SOUR result carefully. Because MLVSS data is often not readily available, you can use MLSS data and a factor of 0.80 as follows: MLVSS = MLSS 0.80.