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About Oxygen Uptake Rates (OUR) and what those numbers mean

11/18/2019

 
Picture
OUR test in progress.
Oxygen Uptake Rate (OUR or sometimes DOUR) uses a dissolved oxygen probe with a BOD bottle to measure oxygen uptake by biomass. While you could purchase a highly automated, advanced respirometer, for wastewater operations we can do just fine with a standard BOD probe and bottle setup. 

Aerobic wastewater treatment microbes consue oxygen when growing. When you saturate a sample with oxygen and then cap the bottle with a DO probe, you measure the oxygen consumed by the bacteria. The observed oxygen uptake per minute gives your the OUR number. Here is a OUR test protocol - https://www.biologicalwasteexpert.com/useful-information.html.

Few bullet points on OUR/SOUR and what the numbers mean for operations.
  • OUR should be run frequently to get a baseline normal rate. This is very important, your normal OUR is specific to your system and MLSS concentrations. Using similar systems or rules-of-thumb are just not as good.
  • Make sure you calibrate your DO meter & probe - sometimes we get in a rush and don't check. This is where mistakes happen.
  • ​Higher respiration rates indicate more soluble organics (higher BOD5)
  • Low OUR indicates soluble BOD has been substantially removed and biomass is entering endogenous respiration. 
  • If the DO drops below 1 mg/L - the meter is not as accurate. Make sure you calculate based on DO from saturation down to a minimum of 1 mg/L.
  • For industrial waters, if you see a very low OUR & have normal/high COD influent - you may have toxicity. Be sure to check for changes in influent. Be aware of suddenly low OUR numbers.
  • If you change MLSS concentrations, convert OUR to SOUR which is OUR divided by MLSS (grams). Make sure you do the calculations the same every day - don't switch between MLSS and MLVSS for calculating SOUR.

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    Author

    Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater  facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.

    View my profile on LinkedIn

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