How is Biochemical Oxygen Demand Performed

How is Biochemical Oxygen Demand Performed

	How is Biochemical Oxygen Demand Performed When testing for BOD, the first step is to collect equal amounts of water from the research waterway and dilute each of the samples (usually 2 or 3 equal-sized containers) with exactly the same amount of distilled water. The fact that many samples require dilution stems from the fact that the BOD can be higher than the available level of oxygen found in water. For example, blood discharged horn a slaughterhouse can have a BOD of literally millions of milligrams per litre (e.g. BOD = 2,000,000 mg/l), which obviously would require considerable dilution before running a standard BOD.
By comparison, the BOD found in raw sewage is about 200 to 250 mg/l, which would require far less dilution. Since the saturation level for the dilution water will only be 8 mg/l or so, the raw sewage would have to be diluted from 40 to 50 times to make sure it did not deplete all of the oxygen in the BOD bottle during the 5 day period i.e. one has to take care that at BOD5, residual oxygen must be present in order to calculate the difference in DO between BOD5 and BODI bottles.(e.g. diluting a 200 mg/l BOD solution forty-fold would drop the BOD down to 5 mg/I, which would then result in a residual DO after 5 days of about 3 mg/l). Next, one of the samples, BOD1, is tested for dissolved oxygen content. The remaining samples are placed in foil (to keep out all light) and placed in a safe dark area where they will stay undisturbed for five days (BOD5), to seven days (BOD7), depending upon the test and time available. Then the dissolved oxygen test for BOD5 or BOD7 is subtracted from the first dissolved oxygen test for BOD1, to give the result of the Biochemical Oxygen Demand test.
Most laboratories are equipped with special BOD machines that utilise computer software to make all necessary calculations based on dilution factors, seed control, and incubation time. Otherwise standard methods provide formulae for using the DO, dilution factors, and seed control results to calculate the BOD. The decrease in the DO in a BOD bottle over a period of 5 days provides a measure of the respiration of the microorganisms present because there is no interchange with other sources of oxygen.
The rate of oxygen consumption varies with temperature, and at a constant temperature, the total amount of oxygen consumed varies with time. The DO of the diluted water should be close to saturation before the dilutions are made because the respiration of some aerobic microorganisms is inhibited if the DO drops below 1.0 mg/l during the BOD test thus giving a false negative result. The residual DO in the BOD bottle after 5 days of incubation should be at least 2.0 mg/I. Since the dilution water is seeded with organisms capable of assimilating the organic matter of the waste water, there may be a small depletion of DO in 5 days even if no waste water is added. To account for this, control bottles containing only dilution water are incubated with the BOD5 test bottles and a correction is made for the DO depletion in the controls. It is important that the dilution water does not contain any constituents which are inhibitory to the microorganisms which exert the carbonaceous oxygen demand. Hence a control must be prepared from pure organic compounds usually glucose and glutamic acid.
	For measurements of the BOD5 of industrial waste waters, the dilution water must be seeded with microorganisms which are capable of degrading the waste organic compounds. The seed may be obtained from a biological treatment process which has been applied to the waste water or from sediment downstream from the point at which the effluent is discharged into the receiving waters. If a diluted sample of waste water is incubated under the proper conditions and the oxygen consumption is recorded every day, the rate of oxygen consumption is high initially but decreases continuously over time, approaching zero finally.

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