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

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.