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Understanding Biochemical Oxygen Demand (BOD)

David SmithEnvironmental Express Technical SpecialistLab MattersBiochemical Oxygen Demand (BOD) 1 Call or EXPRESSC opyright, Environmental Express, Inc., Lab Matters Biochemical Oxygen Demand (BOD)Introduction BOD, like COD, is not one definable particle. You cannot count BOD molecules. BOD is the amount of Oxygen consumed by decomposition of the sample during the incubation period. The intent is to measure what affect the sample will have on Oxygen available to living organisms in the waters into which the waste is discharged. If the BOD of a waste is high enough, the microbial population will quickly deoxygenate the water and render it unsuitable for other forms of marine life.

David Smith Environmental Express Technical Specialist Lab Matters Call 800.343.5319 or 843.881.6560 • www.environmentalexpress.com Biochemical Oxygen Demand (BOD

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Transcription of Understanding Biochemical Oxygen Demand (BOD)

1 David SmithEnvironmental Express Technical SpecialistLab MattersBiochemical Oxygen Demand (BOD) 1 Call or EXPRESSC opyright, Environmental Express, Inc., Lab Matters Biochemical Oxygen Demand (BOD)Introduction BOD, like COD, is not one definable particle. You cannot count BOD molecules. BOD is the amount of Oxygen consumed by decomposition of the sample during the incubation period. The intent is to measure what affect the sample will have on Oxygen available to living organisms in the waters into which the waste is discharged. If the BOD of a waste is high enough, the microbial population will quickly deoxygenate the water and render it unsuitable for other forms of marine life.

2 This can cause dead zones in a river or other body of water. There is an additional subset of BOD that is required in certain areas. This is referred to as carbonaceous BOD or CBOD. This measures along the same basic principle as BOD, except that an inhibitor is added to exclude the Oxygen consumption by nitrogen fixing bacteria. Sometimes BOD will be referred to as BOD5 or five day BOD. This number is related to the incubation period required for the standard analysis. Other versions of the test are possible and are distinguished by the proper numerical qualifier which is equal to the number of days of incubation. This can go all the way up to ultimate BOD; incubating the sample and reading the depletion until nothing else happens.

3 According to BOD legend, the source of the particular requirements for BOD incubation (5 days at 20 C) arise from the average temperature and flow time of the Thames river from London to the sea. An alternative theory, proposed by yours truly, is that the original BOD chemist, or possibly alchemist, went to a production of Shakespeare s MacBeth. During the scene with the witches and the cauldron he started commenting on how there wasn t any QC for the method the witches were following. His fellow viewers got annoyed and put the poor guy in the hospital where he remained for five days. Once he got back to his lab, instead of redoing his Oxygen Demand experiment with overnight values he wrote 5 days into his procedure and left it as is.

4 You decide which version is more likely. Approved Methods While there are a few different methods approved for BOD, one of them is used overwhelmingly by the analytical community, Standard Methods Summary A known volume of sample has its initial dissolved Oxygen content recorded. After a five day incubation period at 20 C, the sample is removed from the incubator and the final dissolved Oxygen content is taken. The BOD value is calculated from the depletion and the amount of sample You Should Know The actual procedure is so much more involved than the summary given above. The biggest challenge in the BOD test is that of time. The holding time for a BOD sample is 48 hours from collection.

5 The test itself requires a five day incubation period. Doing the math shows that by the time you find out if your test is valid, it is too late to do anything about potential problems. Therefore you must give the utmost care to each step of your procedure to avoid the possibility of having invalid data. Source water can be the trickiest part of beginning your BOD analysis. Your water will eventually be conditioned by the addition of certain trace nutrients so that the bacteria population can survive. Before this happens you need water that is free from any and all contaminants that could contribute to the Oxygen Demand of the samples. It also needs to be free of certain metal contaminants that could inhibit the microbial growth.

6 Simple distillation is not always suitable because volatile components and residue from certain metals can distill over with the water. Many labs have good results from a deionized water system. Special care must be paid to the maintenance of the tanks with these systems as organics can leach from the resin beds. Another David SmithEnvironmental Express Technical SpecialistLab MattersBiochemical Oxygen Demand (BOD) 2 Call or for the source water is to purchase it. Anything that is labeled steam distilled has gone through a specific process that further cleanses it from unwanted impurities. The ultimate determination of suitability of the water is if the Oxygen depletion after a 5-day incubation is less than mg/L.

7 Laboratories that are well versed in BOD analysis should be able to routinely produce blanks with a depletion of less than mg/L. For BOD to operate properly there must be a sufficient population of healthy bacteria in the bottle. Maintaining that bacterial population can be difficult for the average analyst, especially one in a commercial lab. Wastewater treatment plants will typically have a ready supply of bacteria, but it isn t always in the places one would expect to find it. The influent typically has a very high Oxygen Demand , but not necessarily a good population of bacteria. It tends to be variable with the fluctuations depending on time of day and weather.

8 Also, influents run the risk of being toxic in nature. The best source of seed material is found in the plant that treats the waste. Depending on the process and operating conditions in the plant, effluent from the primary or secondary treatment process will contain sufficient numbers of bacteria to inoculate the sample. Be sure to pull from a spot in the process stream ahead of the disinfection stage. If you do not work at a wastewater plant it may be possible to obtain a suitable amount of the effluent and keep a seed farm in your lab. You will need to pay special attention to the feeding and aeration of the stock, as well as its performance over time.

9 If a natural source of seed is unavailable to you will need to use a freeze dried seed. These seed materials will need to be rehydrated prior to use. Follow the manufacturer s instructions to prepare the seed. Generally, the volume of water used to rehydrate the seed can be increased if the seed concentration needs to be lowered or decreased if the seed concentration needs to rise. Always do a trial run before using any new seed source or new seed lot. This will inform you of the strength of that seed and will allow you to use the proper volume. The DO uptake attributable to the seed added to each bottle generally should be between and mg/L, (SM 5210B 5.)

10 D. 21st ed.) The glucose-glutamic acid check is the primary basis for establishing accuracy and precision of the BOD test and is the principal measure of seed quality and set-up procedure. (SM 5210B 6. b. 21st ed.) The requirement for the glucose-glutamic acid (GGA) check is a BOD result of 198 mg/L. Depending upon the version used you may have different requirements for arriving at the final value. The 21st edition requires 3 bottles being set up with GGA. The results for all three bottles are averaged together and the final average is what must be within the acceptance range. If results consistently fall outside of the acceptable limits you will need to evaluate possible sources of error.


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