Biochemical Oxygen Demand (BOD)

The most widely used parameter of organic pollution applied to both wastetwater and surface water is the 5-day BOD (BOD5). This determination involves the measurement of the dissolved oxygen used by microorganism in the biochemical oxidation of organic matter. Despite the widespread use of the BOD test, it has a number of limitations. It is hoped that, through the continued efforts of workers in the field , one of the measures of organic content, or perhaps a new measure, will ultimately be used in its place. Why, then, if the test suffers from serious limitations, is further space devoted to it in the text? The reason is that BOD test results are now used (1) to determine the approximate quantity of oxygen that will be required to biologically stabilize the organic matter present, (2) to determine the size of waste-treatment facilities, (3) to measure the effeciency of some treatment processes, and (4) to determine compliance with wastewater discharge permits. Because it is likely that th BOD test will continue to be used for some time, it is important to know the details of the test and its limitations.

1/Basic for BOD test.

-If sufficientoxygen available, the aerobic biological decomposition of an organic waste will continue until all of the waste is consumed. Three more or less distinct activities occur. First, a portion of the waste is oxidized to end products to obtain energy for cell maintenance and the synthesis of new cell tissue.

-Simultaneously, some of the waste is converted into new cell tissue using part of the energy released during oxidation. Finally, when the organic matter is used up, the new cells begin to consume their own cell tissue to obtain energy for cell maintenance. This third process is called endogenous respiration. Using the term COHNS (which represents the elements carbon, oxygen, hydrogen, nitrogen, and sulfur) to represent the organic waste and the term C5H7NO2 [first proposed by Hoover and Porges (1952)] to represent cell tissue, the three processes are defined by the following generalized chemical reactions:

+Oxidation:

COHNS + O2 + bacteria → CO2 + H2O + NH3 + other end products + energy

+Synthesis:

COHNS + O2 + bacteria + energy → C5H7O2N (New cell tissue)

+Endogeneous respiration:

C5H7O2N + 5O2 → 5CO2 + NH3 + 2H2O

If only the oxidation of organic carbon that is present in the waste is considered, the ultimate BOD is the oxygen required to complete the three reactions given above. This oxygen demand is known as the ultimate carbonaceous or first-stage BOD, and is usually denoted as BOD.

2/BOD test procedure

In the standard BOD test, a small sample of the wastewater to be tested is placed in a BOD bottle (volume = 300 ml). The bottle is then filled with dilution water saturated in oxygen and containing the nutrients required for biological growth. To ensure that meaningful results are obtained, the sample must be suitaby diluted with a specially prepared dilution water so that adequate nutrients and oxygen will be available during the incubation period. Normally, several dilutions are prepared to cover the complete range of possible values. Before the bottles is stoppered, the oxygen concentration in the bottle is measured.

-After the bottle is in cubated for 5 day at 20oC, the dissolved oxygen concentration is measured again. The BOD of the sample is the difference in the dissolved oxygen concentration values, expressed in milligrams per liter, divided by the decimal fraction of sample used. The computed BOD value is known as the 5-day, 20oC biochemical oxygen demand. When testing waters with low concentrations of microorganism, a seeded BOD test is conducted. The organisms contained in the effluent from primary sedimentation facilities are used commonly as the seed for the BOD test. Seed organisms can also be obtained commercially. When the sample contains a large population of microorganisms (e.g., untreated wastewater), seeding is not necessary.

-The standard incubation period usually 5 days at 20oC, but other lengths of time and temperatures can be used. Longer time periods (typically 7 days), which correspond to work schedules, are often used, especially in small plants where the laboratory staff is not available on the weekends. The tenperature, however, should be constant throughout the test. The 20oC tenperature used is an average value for slow-moving streams is temperate climates and is easily duplicated in a incubator. Different results would be obtianed at different temperature, because biochemical reaction rates are temperature-dependent. After incubation, the dissolved oxygen of the sample is measured and the BOD is calculated using Equations below:

*When the dilution water is not seeded:

BOD, mg/L = (D1-D2)/P

*When the dilution water is seeded:

BOD, mg/L = [(D1-D2)-(B1-B2)f]/P

where D1 = dissolved oxygen of diluted sample immediately after preparation, mg/L

D2 = dissolved oxygen of diluted sample after 5-day incubation at 20oC, mg/L

B1 = dissolved oxygen of seed control before incubation, mg/L

B2 = dissolved oxygen of seed control after incubation, mg/L

f = fraction of seeded dilution water volume in sample to volume of seeded dilution water in seed control

P = fraction of wastewater sample volume to total combined volume

**Note: Biochemical oxidation theoretically takes an infinite time to go to completion because the rate of oxidation is assumed to be proportional to the amount of organic matter remaining. Within a 20-day period, the oxidation of the caronaceous organic matter is about 95 to 99 percent complete, and in the 5-day period used for the BOD test, oxidation is from 60 to 70 percent complete.