Specific Oxygen Uptake Rate (SOUR)
This laboratory activity explores the SpecificOxygen Uptake Rate analysis (SOUR) and its use in measuring the
metabolic activity of organisms in aquatic systems.
Microorganisms use oxygen as they consume food in an aerobic
aquatic system. The rate at which they use oxygen is an
indicator of the biological activity of the system and is called
the Oxygen Uptake Rate (OUR). High oxygen uptake rates indicate high
biological activity; low oxygen uptake rates indicate low
In biological waste treatment facilities, oxygen uptake rates are
used to monitor performance of process units. The analysis is
based on a series of dissolved oxygen (DO) measurements taken on
a sample over a period of time. The test is most valuable for
plant operations when combined with volatile suspended solids
data. Combining oxygen uptake and volatile suspended solids data
yields a value called the Specific Uptake Rate (SOUR).
Specific Uptake Rates (SOUR) describe the amount of oxygen used
by the microorganisms to consume one gram of food and is reported
as mg/L of oxygen used per gram of organic material per hour.
The specific uptake rate is valuable when comparing one aquatic
system with another or if a single system is to be charted over
time. The performance of one aeration basin can be compared with
another or the biological activity in a stream can be studied and
compared both above and below a waste outfall. Furthermore,
toxic or high organic loads can often be detected before severe
deterioration of effluent quality occurs. Changes in the SOUR on
effluent samples will indicate changes in loading.
Biological waste treatment in the activated sludge process is
based on the ability of the microorganisms to utilize dissolved
oxygen in breaking down soluble organic substances.
The oxygen uptake test is a means of measuring the respiration
rate of the organisms in the activated sludge process. Since it
measures the oxygen used in the process, it is a useful tool in
the evaluation of process performance, aeration equipment and
biodegradability of the waste. So that comparisons can be made
between various plants, it is usually expressed as the SOUR
(specific uptake rate); i.e. the amount of oxygen in mg utilized
by one gram of the volatile suspended solids in the activated
sludge, in on hour.
- Dissolved Oxygen Meter with probe adapted to BOD bottle
- 300 ml BOD bottle
- Magnetic stirrer and magnetic stirring bar
- Collect sample of mixed liquor suspended solid (MLSS) from
local wastewater treatment facility.
- Continuously aerate sample prior to initiation of step 5 of
- Calibrate the DO probes.
- Fill a 300-mL BOD bottle with an MLSS sample. Immediately
after filling, place a glass stopper in the bottle.
- Place the BOD bottle on stirrer with magnetic stir bar.
- Place DO probe in the bottle making sure the stopper provides
a good seal and switch on the magnetic stirrer.
- Measure DO as quickly as possible; record the DO concentration
in the bottle as a function of time. Continue to record the
DO concentration in the bottle every 30 seconds for
approximately 10-15 minutes (it may take longer). The actual
time required depends on the rate of oxygen depletion. Allow
sufficient time to get at least 1 mg/L DO difference between
start and finish of the test. Be sure to record both the DO
measurement and the time. The meter reads the DO
concentration in mg/L.
- At the completion of the experiment, dump aqueous content from
BOD bottle back into the appropriately marked container. Wash
BOD bottle and repeat experiment with the other sample.
- Construct a plot of DO vs. time. You should have two curves,
one for each sample.
- Fit the best straight lines through points.
- The slope can now be calculated: two points in time are
selected through which the line passes. By subtracting the
lower DO value from the higher and dividing the answer by the
time interval selected, the slope is obtained.
- Calculate Oxygen Uptake Rate for curves representing both
bottles A and B. The absolute value of the slope is the oxygen consumption rate
in milligrams per liter per minute (mg/L / min). A minimum of five data points in the linear portion
of the DO versus time curve is required to calculate the oxygen uptake rate.
- Calculate the Specific Oxygen Uptake Rate. The specific
oxygen uptake rate (SOUR) can now be calculated. Assume the
volatile suspended solids (VSS) concentration (VSS is normally
obtained from routine daily solids analysis) is 2400 mg/L.