Specific Oxygen Uptake Rate (SOUR)

Reference:

Standard Methods; 2710 A, 2710 B

Scope and Application

The Specific Oxygen Uptake Rate (SOUR), also known as the oxygen consumption or respiration rate, is defined as the milligram of oxygen consumed per gram of volatile suspended solids (VSS) per hour. This quick test has many advantages; rapid measure of influent organic load and biodegradability, indication of the presence of toxic or inhibitory wastes, degree of stability and condition of a sample, and calculation of oxygen demand rates at various points in the aeration basin.

This test was originally developed as a plant control parameter. SOUR is now also used as an alternative test method to meet the vector attraction reduction requirement imposed by 40 CFR Part 503 standards for the use or disposal of sewage sludge. This requirement reduces the potential of spreading infectious disease agents by vectors (e.g. insects, rodents, and birds). SOUR - 503 is defined as milligram of oxygen consumed per gram of total solids (TS) per hour.

Apparatus

Reagents

The same reagents used to perform the Dissolved Oxygen test may be needed to calibrate the oxygen monitoring apparatus.

Storage / Preservation

The samples collected for SOUR testing should be analyzed as soon as possible. Realizing that the test will probably not be performed in the field and that some time will be required to transport the sample to the laboratory, delays will cause the value obtained to be lower than the actual value. If conditions cause the testing to be delayed, it may be better to obtain a new sample that can be analyzed without delay.

Raw Data Sheet Format

The following must be recorded on the data sheet:

Quality Control

When performing replicate determinations, keep analysis temperature constant for best precision

Procedure

Calibration of Oxygen Reading Apparatus

  1. Calibrate the oxygen electrode and meter according to the manufacturer’s calibration procedure. Generally, calibrate membrane electrodes using water saturated air or samples of known DO concentration (as determined by Winkler method).
  2. OR

  3. Calibrate the manometric or respirometric device according to manufacturer’s instructions.

Preparation of the Sample

Collect enough sample to both fill a 300 ml BOD bottle and run a VSS test. Leave a minimum of one-inch headspace in the sample collection bottle. Shake sample to incorporate oxygen into the sample. Begin SOUR test immediately after collection of the sample.

Quickly make a composite of several grab samples, collecting enough sample to both fill a 300 ml BOD bottle and run a TS test. Leave a minimum of one-inch headspace in the sample collection bottle. Shake sample to incorporate oxygen into the sample or bubble air or oxygen through sample. Begin SOUR test immediately after the collection of the sample.

Measure the SOUR of the sludge at the temperature at which the aerobic digestion is occurring in the treatment works. If a magnetic stirplate is used, it must not change the temperature of the sample during the test. Insulated stirplates are available. If this type is not on hand, insulate the stirplate top with a piece of Styrofoam or other insulating material.

 

Measurement of Oxygen Consumption Rate

  1. Fill sample container to overflowing with a representative sample.
  2. Begin monitoring of the dissolved oxygen.
    1. If an oxygen sensing electrode is used, immediately insert it into the BOD bottle containing a magnetic stirring device and sample. Displace enough sample with the probe containing the electrode to fill the flared top of the BOD bottle in order to isolate its contents from the atmosphere. Stir sample. Adequate mixing is essential!
    2. If a manometric or respirometric device is used, follow the manufacturer's instructions for startup.
  3. After the meter reading has stabilized, record initial DO or manometric or respirometric reading, and start timing device. Record appropriate DO, manometric or respirometric data at least once per minute, depending on rate of consumption.
  4. Record data over a 15 minute period or until DO levels drop below 1 mg/L, whichever occurs first. (The oxygen electrode may not be accurate at DO levels below 1 mg/L.)
  5. Low DO (<2.00 mg/L at the start of the test) may limit oxygen uptake by the sample. This phenomenon will be indicated by a decreasing rate of oxygen consumption as the test progresses. Reject such data as being unrepresentative of oxygen-consumption rate and repeat the test beginning with a sample having a higher initial DO level.

Calculations

If an oxygen electrode is used, plot observed readings (mg/L DO) versus time (minutes) on graph paper and determine the slope of the line of best fit. The slope is the oxygen consumption rate in milligrams per liter per minute.

If a manometric or respirometric device is used, refer to manufacturer’s instructions for calculating the oxygen consumption rate.

Calculate specific oxygen rate in milligrams per liter per gram per hour as follows:

 

Parameter Testing

A sample is collected for parameter testing at a treatment plant. The oxygen level of the sample is monitored at 30-second intervals. After 10 minutes, the dissolved oxygen level was below 1.00 mg/L. The oxygen depletion of each reading averaged 0.40 mg/L. The volatile suspended solids was analyzed and reported as 3500 mg/L. What is the SOUR?

First, note that the dissolved oxygen level was recorded every 30 seconds, therefore the average needs to be multiplied by 2 to get the oxygen consumption rate in the proper units.

The most useful SOUR values will be determined by individual treatment plants for their own use, however, the following may be a helpful guideline:

 

>20

High

 

This may indicate that there are not enough solids for the BOD loading.

12 – 20

Normal

 

This range will usually produce a good BOD removal and a sludge that settles well in the final clarifier.

<12

Low

 

This may indicate that there are too many solids or there has been a toxic occurrence.

 

 

503 Testing

Notice that total solids are used rather than volatile solids. This is consistent with the 503 regulations. The rational for this is because all the sludge solids may degrade and exert an oxygen demand, not just the volatile portion.

It may be necessary to make dilutions of the sample in order to get a linear slope for the oxygen consumption rate. Use distilled water to make any necessary dilutions to ensure that no organic material is added in the dilution process. Use the undiluted sample to determine total solids and back calculate to correct if a dilution was used for oxygen consumption testing.

A sample is collected for 503 testing at a treatment facility. The sample is collected from an anaerobic digester. Due to the low oxygen levels, 500 mL of the sample is diluted to 1 L using distilled water. The oxygen level of the sample is monitored at 15-second intervals. After 14 minutes, the dissolved oxygen level was below 1.00 mg/L. The oxygen depletion of each reading averaged 0.12 mg/L. The total solids was analyzed and reported as 4.8 %. What is the SOUR?

 

Temperature Correction

In order to meet the SOUR criteria for 40 CFR 503, the SOUR must be less than or equal to 1.5 (mg/g)/hour of total solids (dry weight) at 20° C. This level is used to discriminate between adequately stabilized and poorly stabilized sludge.

The SOUR of the sludge must be measured at the temperature at which the aerobic digestion is occurring and corrected to 20° C by the following equation:

SOUR20 = SOURT×Q (20-T)

Where Q is a temperature dependent variable:

Q = 1.05 above 20° C

Q = 1.07 below 20° C

 

This correction may be applied only if the temperature of the sludge is between 10° C and 30° C

 

If the digested sludge used in the above example was

 

SOUR Analysis; the Tips for Techs version