My BOD results show that there was no dissolved oxygen depletion. What is the reason?

Document ID

Document ID TE888

Version

Version 2.0

Status

Status Published

Published Date

Published Date 27/05/2017
Question
My BOD results show that there was no dissolved oxygen depletion. What is the reason?
Summary
No dissolved oxygen depletion in BOD
Answer
Potential Causes:
  • No bacteria to deplete the oxygen.
  • No food to deplete - the sample has no BOD.
  • The dissolved oxygen meter is not functioning.
Recommendations:
  • Certain samples are not suitable for BOD analysis. If there are large amounts of toxic metals in the sample, COD is the only way to be certain of its oxygen demand.
  • Check the sample for chlorine. Any residual chlorine will cause zero depletion. If the sample has ever been disinfected, it must be seeded. Ensure that the pH of the sample is 6.5-8.5.GGA must be seeded.
  • Verify that the dissolved oxygen meter is measuring correctly in a zero-DO solution. If it measures below 0.1 mg/L then it is working correctly. If it measures above 0.1 mg/L then re-calibrate using a zero-DO solution.
To create a zero-DO solution: Measure 150 mL of Water, Deionized, 4 L (catalog number 27256) into a 250-mL beaker. Add a magnetic stir bar and place beaker atop a stir plate. Add 0.25 g Sodium Sulfite, Anhydrous 454 g (catalog number 19501H) or the contents of one Silica 3 Reagent Powder Pillows, pk/100 (catalog number 27169) to the water. Stir to dissolve the reagent. Add 0.1 mL of Cobalt Standard Solution, 1000 mg/L as Co (NIST), 100 mL (catalog number 2150342) to the beaker. Continue stirring. Place the probe in the stirring sample for at least 10 minutes.The solution is ready and will remain steady at zero-DO for at least 30 minutes.