Carbonaceous biochemical oxygen demand or CBOD info Chemical Man

Carbonaceous biochemical oxygen demand or CBOD is a method defined test measured by the depletion of dissolved oxygen by biological organisms in a body of water in which the contribution from nitrogenous bacteria has been suppressed. CBOD is a method defined parameter is widely used as an indication of the pollutant removal from wastewater. It is listed as a conventional pollutant in the U.S. Clean Water Act.

Method summary

Bring the sample to ambient room temperature. If pH of sample is <6.5 or >7.5 neutralize the sample to approximately a pH of 7.0 using either sulfuric acid or sodium hydroxide. Aliquots of the neutralized sample are transferred to 300 mL CBOD bottles. These CBOD samples must be at concentrations that will deplete by at least 2 mg/L dissolved oxygen (DO) and have at least 1 mg/L DO left after five days of incubation. Therefore, make enough dilutions (minimum of 3) of the prepared sample to bracket the predicted CBOD.

The minimum aliquot volume transferred to a 300 mL CBOD bottle will be 3 mL as set by Standard Methods. If a smaller volume is needed to meet the DO depletion requirements, then you must make dilutions to the sample. Add approximately 0.1 g of Nitrification Inhibitor (2-chloro-6-(trichloro-methyl) pyridine) to each 300mL CBOD bottle before adding CBOD dilution water. If the sample is being prepared as a seeded sample, add enough prepared seed to the sample to achieve acceptable dissolved oxygen depletion. Add CBOD Dilution water to each CBOD sample bottle so as to completely fill the bottle with no air spaces or bubbles when the stopper is placed in the bottle.

Place the dissolved oxygen probe in the bottle and allow the dissolved oxygen meter to come to equilibrium. Allow the meter to come to equilibrium prior to accepting dissolved oxygen value. Record the DO of the sample, stopper the bottle, add DI water to the water seal if needed, cap the water seal, and incubate for 5 days at 20 °C ± 1 °C. Exclude light to avoid growth of algae in the bottles during incubation.

Upon completion of the 5-day incubation± 6 hours, record the DO of the depleted samples with a calibrated DO meter. Allow the meter to come to equilibrium prior to accepting dissolved oxygen value. Calculate the CBODs from the formula below. Only bottles, including seed controls, giving a minimum DO depletion of 2.0 mg/L and a residual DO of at least 1.0 mg/L after 5 days of incubation are considered to produce valid data, because at least 2.0 mg oxygen uptake per L is required to give a meaningful measure of oxygen uptake and at least 1.0 mg/L must remain throughout the test to ensure that insufficient DO does not affect the rate of oxidation of waste constituents.

Regulatory use

In order to reduce a wastewater plants BOD5 values to meet regulatory compliance requirements, some plant operators try to suppress nitrification when they are not required to meet ammonia limits. This practice usually results in increased effluent toxicity and oxygen demand on the receiving waters. Therefore, to eliminate this situation and because the BOD5 test is not reflective of effluent quality under nitrifying conditions, the wastewater plant should:

    Perform parallel CBOD5 and BOD5 tests to indicate whether there is a problem with BOD5 compliance due to nitrification in the BOD5 test results and that the CBOD5 is not directly correlated with the BOD5 test results, and
    Baseline wastewater plant influent and effluent ammonia, nitrite and nitrate data (same frequency and duration as the parallel CBOD5 and BOD5 data) have been provided to perform mass balances for nitrification inhibition.

The results of these analysis can show that CBOD5 should be utilized for regulatory compliance with wastewater discharge requirements.