The oxygen uptake rate (OUR) is a parameter that can be used to evaluate the rate at which metabolic processes take place in activated sludge treatment processes with sludge in suspension. The main uses of the OUR test are to:
- Estimate the values of kinetic- and stoichiometric parameters
- Obtain data required to set up a mass balance of organic- or nitrogenous material
- Evaluate the sludge activity in terms of the maximum and endogenous substrate utilisation rate
- Determine the degree of sludge stabilisation after aerobic digestion
The principles involved in the execution of an OUR test are simple: the aeration of a mixed liquor batch is interrupted and the resulting decrease in the oxygen concentration is observed as a function of time. However, interpretation of the results is slightly more difficult, because what is measured is in fact the apparent OUR. Along with the metabolic activity, there are several other factors which influence the oxygen concentration and hence the measured value of the OUR. In This section the following subjects are discussed:
- Determination of the oxygen uptake rate
- Critical oxygen concentration
- Effect of reactor hydraulics and the absorption of atmospheric oxygen
- Relaxation effect of the oxygen sensor
(1) Determination of the oxygen uptake rate
Several experimental methods may be used to determine the OUR: (1) direct measurement in the reactor, (2) determination in a batch sample withdrawn from the reactor and (3) determination in a batch sample while influent and return sludge are fed to this sample at proportionally the same rate as to the aeration tank. All methods have their specific uses. Click here to download this section.
(2) Critical oxygen concentration
Oxygen consumption mainly occurs within the sludge flocs, so there is a tendency for the dissolved oxygen concentration to decrease from the periphery of the floc (where it is assumed to be equal to the bulk concentration) towards the floc centre. Depending on the applied bulk dissolved oxygen concentration, a zone without oxygen (either anoxic or anaerobic) may develop in the central region of the flocs. As no oxygen is consumed in this region, the effect will be a decrease in the measured overall OUR. In this section it will be demonstrated how to determine this critical oxygen concentration. Evidently, the OUR should be determined in the dissolved oxygen concentration range above the critical value. To download this section, click here.
(3) Effect of reactor hydraulics and the absorption of atmospheric oxygen
Apart from metabolic processes, the oxygen concentration may change because of other factors. In the case of a reactor with influent entering and mixed liquor leaving continuously, the rate of change of the dissolved oxygen concentration due to this hydraulic effect must be taken into consideration. Refer to Example A1.1. In the case of a low OUR, absorption of atmospheric oxygen may interfere with the determination of the OUR measurement. In Example A1.2 a method will be demonstrated to determine the value of the absorption constant. To download this section, click here.
(4) Relaxation effect
When the sensor of a dissolved oxygen meter is suddenly transferred from saturated water to water without dissolved oxygen, it can be observed that a relatively long period is required before the reading has adapted to the new situation and indicates the true dissolved oxygen concentration. This time period is called the relaxation period of the sensor and may be a problem for OUR measurements. A method is presented to determine the relaxation constant. Click here to download this section.