Would are the causes of a High BOD Blank and how can they be avoided?

Document ID

Document ID TE177

Published Date

Published Date 08/20/2019
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Question
Would are the causes of a High BOD Blank and how can they be avoided?
Summary
High BOD blanks
Answer
When testing Biochemical Oxygen Demand (BOD), it is necessary to use high quality water when preparing the dilution water. The quality of the dilution water is checked by running a dilution water blank along with the samples. If the dissolved oxygen uptake of the dilution water blank exceeds 0.2 mg/L after 5 days, the water should not be used.

The most common cause of a high BOD blank, is contamination. Contamination may come from the following sources:
  • The Source of Water
  • The Dilution Water Buffer
  • Dirty BOD Bottles
  • Dirty Carboy
  • Dirty Pipettes or Graduated Cylinders
  • Dirty Probe (if measuring with a Dissolved Oxygen Probe instead of a Winkler Titration)
  • Dirty Spigots, Fittings, Tubing
  • Aeration Stones
Outside of contamination, supersaturation of the blank or an incorrect dissolved oxygen measurement could also be responsible for a High Blank.

The following practices are recommended for obtaining good BOD blanks:
  • Use recyclable PET BOD bottles (achieves about 0.04 mg/L lower blanks on average).
  • Use air calibration instead of winkler titration (achieves about 0.09 mg/L lower blanks on average).
  • Use a water polisher instead of deionized or distilled water (achieves about 0.03 mg/L lower blanks on average).
  • Add BOD buffering reagents as late as possible and rinse the carboy that contained them thoroughly.
  • "Seat" the stoppers carefully, twisting while stoppering them to ensure that no air remains in the bottle.
  • Perform all suggested maintenance on your DO probe at the manufacturer-recommended frequency.

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