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Biochemical Oxygen Demand (BOD). CE 370 - Lab. Introduction. The Biochemical Oxygen Demand (BOD) test measures the oxygen consumed by microorganisms in decomposing organic pollutants of the wastewater.

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Biochemical Oxygen Demand (BOD)

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Biochemical Oxygen Demand (BOD)

CE 370 - Lab


The Biochemical Oxygen Demand (BOD) test measures the oxygen consumed by microorganisms in decomposing organic pollutants of the wastewater.

The BOD test is performed by incubating a sealed wastewater sample for the standard 5-day period, then determining the change in dissolved oxygen content.

The bottle size, incubation temperature, and incubation period are all specified.

Most wastewaters contain more oxygen demanding materials than the amount of DO available in air-saturated water. Therefore, it is necessary to dilute the sample before incubation to bring the oxygen demand and supply into appropriate balance.

Because bacterial growth requires nutrients such as nitrogen, phosphorous, and trace metals, these are added to the dilution water, which is buffered to ensure that the pH of the incubated sample remains in a range suitable for bacterial growth.

Complete stabilization of a sample may require a period of incubation too long for practical purposes; therefore, 5-day period has been accepted as the standard incubation period.

BOD is often used as a quality parameter to assess the extent of organic pollutants in municipal wastewaters.

BOD Measurement

Winkler Method (to measure oxygen concentration)

Dilute sample with dilution water. Why?

Because wastewater usually have high BOD and it is not possible to measure more than 9 mg/l of BOD in this test due to the fact that the solubility of O2 in water at 20C is  9 mg/l.

Preparing Dilution Water

Prepare Samples

  • Add X ml of sample and fill with dilution water to the 300 ml level.

  • Therefore, the fraction of the sample (p ) = X/300

Chemicals added

Phosphate buffer to maintain favorable pH condition (pH  7) and provide phosphorus as a nutrient.

MgSO4, CaCl2, FeCl3to provide minerals required by microorganisms for their growth.

How to measure O2?

Concept: O2 measurement depends on the fact that O2 oxidizes Mn++ under alkaline conditions:

Mn++ + 2OH + ½O2 MnO2 + H2O

MnO2 is capable of oxidizing iodide (I( to free iodine (I2) under acidic conditions:

MnO2 + 2 I + 4H+ Mn++ + I2 + H2O

The amount of free iodine released is equivalent to the dissolved oxygen.

Some wastewaters may not contain microorganisms, therefore addition of microorganisms is required (seeding).


  • P = Decimal fraction of the sample = 6/300

  • If seeding is used, it is usually added to dilution water.

  • Vss = % seed  vol. of diluted water = 0.2%  (300-6)

  • Vsc = % seed  vol. of diluted water = 0.2%  (300)

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