Oxygen Uptake Rate (OUR) is an important wastewater control parameter for activated sludge process carried out using dissolved oxygen analyzer that measures the amount of oxygen used up by the microorganisms expressed in unit time of mg/L (ppm) per hour. Basically by knowing the OUR, it should tell you the information that reflects on how active your microbes are in your aeration tank and whether they are consuming the oxygen that you provide for them to biodegrade the organic matter in the incoming influent.
OUR is usually converted to Specific Oxygen Uptake Rate (SOUR) so that this will give you a more accurate representation based on the concentration of microorganisms in your sludge culture. SOUR can be obtained by simply dividing your OUR with MLVSS (refer to this topic on explanation of MLSS and SVI). Normally based on experience by wastewater operators and data collection gathered over the years, the optimum range of SOUR is usually within 8 – 20. The value is indicated and measured in terms of unit of mg of oxygen consumed per liter in an hour based on per milligram of MLVSS.
Measuring the oxygen uptake rate and then the specific value is something which is considered a very straightforward approach carried out in a simple test that can be completed in less than an hour. Usually by using a dissolved oxygen analyzers and then plot all the results into a graph and then apply some simple mathematical calculation, you should get the value in no time. What you will need are:
- DO probe with an oxygen-sensitive electrode and measuring meter complete with a stirrer at the tip
- A stopwatch
- 300ml BOD bottle
Preparation of Sample:
- Collect at least 500ml of the wastewater sample from the aeration tank into a sampling jar until it is full and then immediately close the lid to prevent further oxygen-water gas exchange
- Return to lab and pour the sample into the BOD bottle and leave at least minimum one inch of headspace in the bottle
- Dip the DO analyzer and wait until the reading has stabilized and then record the initial value. At the same time start your timer.
- Record the reading taken every 1 minute and conclude the test after 15 readings has been taken
A very low DO value (less than 2.0 mg/L) indicated at the beginning of the test will provide a data not accurate or representative of the actual oxygen uptake rate. You will need to repeat the test with a sample having higher initial DO level.
Let’s look at the graph derived from a simple test to measure the SOUR value.
OUR= (6.37-3.79)/14 * 60 = 11.057 mg/L O2 per hour
SOUR = 11.057/MLVSS
Both SOUR and OUR can tell you a lot of information for you to analyze and better manage your wastewater treatment process. If let’s say your SOUR is above the recommended optimal range, that basically means that your aeration system F/M ratio is on the uptrend which translate to too much food (BOD load) and too few microorganisms (represented by MLVSS). This also gives you an indication that your sludge is basically too young (which relates to the sludge age) and potentially if this condition is not addressed, floating suspended solids that do not settle fast enough will cause carry over of the particles together with your discharge effluent.
Similarly the reserve condition is what happens when SOUR drops below the 8 recommended limit which means that there is now insufficient food to support the microbes’ growth and then cause the sludge to settle too quickly and lead to pinpoint floc effect. Thus any wastewater treatment plant should have at least dissolved oxygen measuring kit, best using a DO meter and also have an online transmitter or probe to give an indication on whether your aeration tank has sufficient oxygen to biodegrade organic compound. With that you will obtain your SOUR reading that tells you how well or how efficient your bacteria is to convert and breakdown the waste.