The measurement of OUR

To control the efficiency of a biological activated sludge treatment plant, the test for determining the Oxygen Uptake Rate is performed on a sample taken directly from the oxidation/nitrification basin. The classic method provides for the registration, at regular time intervals, of the consumption of dissolved oxygen by a sample of activated sludge, with known MLSS concentration and volume, previously brought to a rapid saturation with a forced ventilation system and kept constantly mixing (as schematically shown in figure 1). The time/concentration of oxygen pairs are then turned into a graph, and a descending, almost straight curve is obtained, whose slope represents the rate of consumption of oxygen by the biomass (see figure 2). The OUR value obtained in this way is generally expressed as mg O2/g SSV*h.

Some typical applications of the OUR test are listed below :

Test Use
Biological activity test Checking the degree of activity of the biomass in breaking down a certain organic substrate in relation to the endogenous OUR
Assessment of the degree of inhibition Determining the possible toxic effect of sewage containing potentially inhibitory substances by making use of the OUR test
Biodegradability test on special waste water Testing the behaviour of the activated sludge when fed with a compound, the effect of whose biomass is not known for certain; for example the acceptance of special waste water at the treatment plant
Characterisation of organic substrates Quantification of the organic substrate present in influent waste water, in order to determine the fraction of readily biodegradable COD of waste water for the integration of a carbonaceous substrate in a state of denitrification or biological dephosphating
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