The SOUR is simply the DOUR divided by the MLSS or MLVSS in grams - just make sure you use the same divisor each time as the oxygen uptake rate tests rely on trends rather than a single data point. As the biomass reaches a more "mature" stage - stationary or decline phase growth (all depends on system type), the SOUR decreases. Lower oxygen uptake rate values show the cells do not have excess soluble organics and are not rapidly dividing. However, if you see a high COD and still have a low OUR - it could indicate toxicity or lag phase growth. Neither condition makes for good effluent quality.
For OUR to be a powerful tool, it must be frequently run. Depending upon fluctuations in influent quality or makeup - OUR should be run anywhere from every shift to weekly. The more often it is run, the better the information provided by the test.
By running OURs, a facility can replace the use of expensive equipment such as respirometers or high cost reagent tests for daily monitoring. So remember that while the OUR test is simple and not the latest in technology, it is very useful in monitoring biomass activity.