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.
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.
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 20C is 9 mg/l.
Preparing Dilution Water
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.
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).