Biochemical oxygen demand (BOD) is the amount of dissolved oxygen
needed by aerobic biological organisms in a body of water to break down
organic material present in a given water sample at certain temperature
over a specific time period.
The BOD value is most commonly expressed in milligrams of oxygen consumed per litre of sample during 5 days of incubation at 20 °C and is often used as a robust surrogate of the degree of organic pollution of water.
Uses
The BOD test is used primarily to measure waste loads to treatment plants, determine plant efficiency (in terms of BOD removal), and control plant processes. It is also used to determine the effects of discharges on receiving waters. A major disadvantage of the BOD test is the amount of time required to obtain the results.
In many biological treatment plants the facility effluent contains large numbers of nitrifying organisms which are developed during the treatment process. These organisms can exert an oxygen demand as they convert nitrogenous compounds (ammonia and organic nitrogen) to more stable forms (nitrates). In some instances it might be an advantage to be able to measure both the oxygen demand exerted by organic (carbonaceous) compounds and nitrogenous compounds. To accomplish this the nitrifying organisms can be inhibited from using oxygen during the period of the test by the addition of a nitrification inhibitor to the samples.
Description of BOD Test Procedure
A sample is pipetted into a BOD bottle containing dilution water. The dissolved oxygen content is determined and recorded and the bottle is incubated in the dark for 5 days at 20°C. At the end of five days, the final dissolved oxygen content is determined and the difference between the initial D.O. is calculated and used to calculate the biochemical oxygen demand of the sample.
Determination of Sample Size
The BOD test must rely on a measurable depletion of dissolved oxygen over a specified period of time. Because most samples of wastewater will have a BOD higher than the amount of oxygen available in the BOD bottle during the incubation period, the samples must be diluted. This dilution is done by adding the dilution water to the sample in the BOD bottle. If the sample is not diluted, the biological activity of the microorganisms will use up the D.O. in the BOD bottle before the five-day incubation time is up. If the final D.O. is zero, the BOD cannot be determined because there is no way of telling at what point during the five days the D.O. reached zero.
One of the most difficult steps in the BOD procedure is deciding how much sample to place in the BOD bottles for incubation. Some plants have influent and effluent BOD's that do not vary greatly over time, while others fluctuate greatly from day to day. In all cases, several different dilutions of each sample should be prepared to obtain the desired dissolved oxygen depletions.
If you have a general range for the BOD of your sample, you can estimate the dilutions needed to insure that at least one dilution will meet the criteria for most valid BOD results. The following procedure can be used to calculate volumes for sample dilution from the estimated BOD.
If there is no
evidence of toxic effects, average the results from all dilutions which meet
the minimum oxygen depletion and minimum residual DO requirements. A toxic effect is indicated when BOD
increases significantly as the sample dilution increases. This is often referred to as “sliding
BODs”. Helpful Hints #17 (March, 1991)
provides additional guidance for reporting results.
BOD testing in Winkler Method
Sampling
Good sampling is necessary if laboratory testing is to be accurate. The sample must be representative. It must be collected properly and handled carefully. No matter how accurate the actual testing is, if the sample is not representative, the results of the test can not be accurate and can lead to poor plant performance.
Types of Samples
Samples used for the BOD test can be either grab or composite. The type of sample taken will depend on the monitoring requirements, plant operations and testing and sample storage facilities for each plant. Samples should be the type which best fits the capabilities of and requirements for each individual plant. Whichever type is used, samples should be taken at a point where they will be well mixed and proportional to the amount of the flow.
Sample Preservation
Samples for BOD analysis may change greatly during handling and storage. Testing should be begun as quickly as possible. To reduce the changes in those samples which must be held, keep the samples at or below 4°C. Do not allow samples to freeze. Samples may be kept for no more than 48 hours before beginning the BOD test.
Sample Containers
Special sampling devices and storage containers are not necessary for collecting samples for BOD determinations. Sampling devices should be able to be used to collect samples from well-mixed areas of tanks and/or pipes, made of resistant materials that will not rust or corrode, capable of taking samples that are proportional to the plant's flow and can be easily cleaned (including acid cleaning).
The BOD value is most commonly expressed in milligrams of oxygen consumed per litre of sample during 5 days of incubation at 20 °C and is often used as a robust surrogate of the degree of organic pollution of water.
Uses
The BOD test is used primarily to measure waste loads to treatment plants, determine plant efficiency (in terms of BOD removal), and control plant processes. It is also used to determine the effects of discharges on receiving waters. A major disadvantage of the BOD test is the amount of time required to obtain the results.
In many biological treatment plants the facility effluent contains large numbers of nitrifying organisms which are developed during the treatment process. These organisms can exert an oxygen demand as they convert nitrogenous compounds (ammonia and organic nitrogen) to more stable forms (nitrates). In some instances it might be an advantage to be able to measure both the oxygen demand exerted by organic (carbonaceous) compounds and nitrogenous compounds. To accomplish this the nitrifying organisms can be inhibited from using oxygen during the period of the test by the addition of a nitrification inhibitor to the samples.
Description of BOD Test Procedure
A sample is pipetted into a BOD bottle containing dilution water. The dissolved oxygen content is determined and recorded and the bottle is incubated in the dark for 5 days at 20°C. At the end of five days, the final dissolved oxygen content is determined and the difference between the initial D.O. is calculated and used to calculate the biochemical oxygen demand of the sample.
Apparatus:
1. 300
ml BOD bottles
2. 2 - 5 liter glass bottle with siphon. Avoid using detergents to clean these
bottles. Periodically clean with bleach
water.
3. 20
±
1°C
incubator
4. DO
meter
5. Buret
Determination of Sample Size
The BOD test must rely on a measurable depletion of dissolved oxygen over a specified period of time. Because most samples of wastewater will have a BOD higher than the amount of oxygen available in the BOD bottle during the incubation period, the samples must be diluted. This dilution is done by adding the dilution water to the sample in the BOD bottle. If the sample is not diluted, the biological activity of the microorganisms will use up the D.O. in the BOD bottle before the five-day incubation time is up. If the final D.O. is zero, the BOD cannot be determined because there is no way of telling at what point during the five days the D.O. reached zero.
One of the most difficult steps in the BOD procedure is deciding how much sample to place in the BOD bottles for incubation. Some plants have influent and effluent BOD's that do not vary greatly over time, while others fluctuate greatly from day to day. In all cases, several different dilutions of each sample should be prepared to obtain the desired dissolved oxygen depletions.
If you have a general range for the BOD of your sample, you can estimate the dilutions needed to insure that at least one dilution will meet the criteria for most valid BOD results. The following procedure can be used to calculate volumes for sample dilution from the estimated BOD.
Calculations:
In
general, BOD5 values less than 2.0 mg DO/L should be reported on
DMRs as non-detects ( i.e., < LOD).
Using the data recorded for un-seeded samples:
BOD mg/l = (Initial DO - DO5)
x Dilution Factor
Dilution Factor = Bottle Volume (300 ml)
Sample
Volume
Notes: 1. Blank BOD5. If the DO depletion in the dilution water
blanks exceeds 0.2 mg/l, the results of the test are questionable. The results of samples which used the same
dilution water the "high" blank should be qualified on the DMR. Do not subtract blank values from sample
results.
2. Dissolved
oxygen values of the samples. Only
dilutions with DO depletions of at least 2 mg/L, and DO5 of at least
1 mg/L may be used to calculate the sample's BOD.
3.
Average results from sample dilutions to calculate final sample BOD5.
4.
See page BOD - 9 for calculations appropriate for seeded samples
Sampling
Good sampling is necessary if laboratory testing is to be accurate. The sample must be representative. It must be collected properly and handled carefully. No matter how accurate the actual testing is, if the sample is not representative, the results of the test can not be accurate and can lead to poor plant performance.
Types of Samples
Samples used for the BOD test can be either grab or composite. The type of sample taken will depend on the monitoring requirements, plant operations and testing and sample storage facilities for each plant. Samples should be the type which best fits the capabilities of and requirements for each individual plant. Whichever type is used, samples should be taken at a point where they will be well mixed and proportional to the amount of the flow.
Sample Preservation
Samples for BOD analysis may change greatly during handling and storage. Testing should be begun as quickly as possible. To reduce the changes in those samples which must be held, keep the samples at or below 4°C. Do not allow samples to freeze. Samples may be kept for no more than 48 hours before beginning the BOD test.
Sample Containers
Special sampling devices and storage containers are not necessary for collecting samples for BOD determinations. Sampling devices should be able to be used to collect samples from well-mixed areas of tanks and/or pipes, made of resistant materials that will not rust or corrode, capable of taking samples that are proportional to the plant's flow and can be easily cleaned (including acid cleaning).
Example:
Suppose the estimated BOD of an influent sample is 400 mg/L and assume the D.O of saturated dilution water is 8.0 mg/L. Since the criteria for most valid results states that the D.O. depletion at the end of 5 days incubation should be at least 2.0 mg/L and the residual D.O. at least 1.0 mg/L, the formulas to calculate the minimum and maximum estimated dilutions are:
Since the BOD value is only an estimate, and BOD bottles do not always have a volume of exactly 300 mL, several bottles with different volumes of sample are set up to insure that test requirements are met. For the examples above, four bottles would be used with 1 mL, 3 mL, 4 mL, and 6 mL, and the results averaged for the final BOD.
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