Water for BOD

Impact of Water

• Organics
  Biochemical oxygen demand is a measure of the quantity of oxygen used by microorganisms in the oxidation of organic matter. If organic matter is present in the dilution water, it may increase its oxygen demand. 
   
• Chlorine or other disinfectants
  Chlorine is often present in tap water in order to control microbial contamination. As chlorine would interfere with the microorganisms used in the BOD test, it should be removed from the water used for the dilutions and the blanks. The same is true of other commonly used tap water disinfectants (chloramine, etc.) 
   
• Heavy metals (copper, mercury, cadmium…)
  Water must be free of heavy metals toxic to microorganisms. Like for disinfectants, any compound which may inhibit the growth of microorganisms will have a deleterious effect on the BOD test.
   
• Bacteria
  While bacteria are a necessary component of the BOD test, it is best to minimize their levels in the dissolution water, as they may release organics during storage.
  
  Distilled water may be used to prepare dilution water; however, chlorine may co-distil with water and interfere with the BOD test. This water would require an additional sodium thiosulfate treatment. Distillation from alkaline permanganate is sometimes recommended, but this purification procedure is quite cumbersome as well. Deionized water, purified with ion exchange resins, may contain organics and microorganisms, thereby causing BOD blank failure. It is therefore not recommended for this test.
  
  Water purified with a combination of technologies, such as reverse-osmosis, ion exchange, activated carbon and ultra-violet photooxidation is extremely low in organics and contains no inorganic substances toxic to bacteria. It is best fitted for the preparation of BOD dilution water.

Examples illustrating the impact of water quality on the BOD test

• Tap water
• Deionized water (DI) obtained using ion-exchange resins
• High purity water produced by a Direct-Q 3 UV water purification system 

Figure 1: BOD blank averages and maximums for three water types. Disposable PET bottles and LBOD probe were used. Bars represent the average of 49 blanks, error bars represent standard deviations. Data courtesy of C. Fair, Hach Company (Loveland, CO).

Figure 2: BOD blank averages for glass and PET bottles Bars represent the average of 7 blanks, error bars represent standard deviations. Data courtesy of C. Fair, Hach Company (Loveland, CO).

More Information

• Get better BOD results –Three simple changes to the BOD test can virtually eliminate blank failure. Water Environment Laboratory Solutions (October/November 2008) 
• BOD Doctor: http://www.boddoctor.com/wiki/index.php?title=Main_Page 

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