Water for the BOD
The water used to prepare the Dilution Water for the BOD test must be of good quality in order not to interfere with the test. Many regulations require that the dissolved oxygen (DO) depletion of dilution water blanks not exceed 0.2 mg/L over the 5-day test period. Various factors may contribute to blank failure, however it is known that water quality is a major contributor to such failures.
Various contaminants in the water may have an effect on BOD:
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.
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 testIn order to better understand the effect of water quality on BOD blanks, three types of water were evaluated:
- Tap water
- Deionized water (DI) obtained using ion-exchange resins
- High purity water produced by a Direct-Q 3 UV water purification system
For each water type, seven blanks were prepared every week for seven weeks. Disposable BOD bottles were used (made of PET with an inner amorphous carbon coating that prevents oxygen transport), as well as a luminescent dissolved oxygen probe (LBOD) from Hach Company (Loveland, CO).
The same three types of water (tap, deionized (DI), and Direct-Q 3 UV high purity water) were evaluated using either the standard glass BOD bottles or disposable PET bottles. Glass BOD bottles should be carefully washed, cleaned with acid and rinsed, while disposable PET bottles do not require washing.
The high purity water produced by the Direct-Q 3 UV water purification system is extremely low in organics and contains no inorganic substances toxic to bacteria. This study confirmed that the water produced by this system reliably yields low blank BOD values and good test reproducibility.
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