Using Oxygen Uptake Rate tests for waste stream screening

The dissolved oxygen uptake rate, commonly referred to as DOUR or OUR, is simply the amount of oxygen consumed by either chemical or biological activity over a set period of time. This means the OUR tests have many similarities to the respirometer tests which have the ability to feed more oxygen into the samples and take multiple time-series measurements. The difference being the OUR test uses a stirred BOD type bottle and oxygen probe whereas the respirometer is a much more complex and expensive single use device. If want to review the basics of performing a OUR test, I detailed it fully in a previous post (DOUR Procedure).

What do changes in OUR mean?
OUR usually remains in a steady range so we become concerned when the OUR is significantly higher or lower than normal. A high OUR means there is increased soluble organics increasing microbial respiration or a chemical oxygen demand such as sulfide has created a chemical oxygen demand. A low OUR, can mean either very low organic loading rates where the biomass has moved further into decline phase growth or a toxic shock event that causes a biomass die-off. Examples of toxic shock include - phenol, cyanides, solvents, or biocides.

Can I screen influent for toxicity with a DOUR test?
Yes, you can screen influent to some extent with a DOUR. It requires taking standard influent and adding the correct percentage of the new waste stream. Add this to mixed liquor at the appropriate percentage. For a control, use the existing influent and mixed liquor at the same percentages. Add an aquarium air stone and allow to aerate/mix for at least 4 hours up to 12 hours. Run a standard DOUR and test different dilutions between influents and mixed liquor to get a uptake rate profile. (contact me if you have questions on using OUR as a toxicity screen).