Today, I want to talk about using the oxygen uptake test done daily in most facilities as a way to estimate influent toxicity. Does it work in all places, all the time..... no. But, It can prove useful in many systems that have periodic "suspicious" influent conditions.
The spiked DOUR simply consists of a set amount of MLSS from the aeration tank to which you add a set amount of influent or waste stream to be tested. You can allow to react with the biomass for 2 - 4 hours using an aquarium air stone or you can do the test immediately. Then, you saturate the sample with oxygen as in a typical DOUR by shaking. Once saturated with oxygen, you use a BOD bottle & DO probe to record oxygen uptake by the biomass + sample. Once the DO is depleted to <1.0 mg/L, you convert the uptake rate to mg O2/L/hour.
An increase in DOUR in the spiked versus the normal MLSS and is expected. The red flag here is if the spiked DOUR pulls down the O2 extremely fast - due to chemical oxygen demand. Or, if the respiration is reduced significantly which indicates toxicity.
Key to the spiked DOUR being an effective operational tool is running the test frequently to get a correlation between the spiked DOUR to system operational numbers. Another important issue is whether to allow the sample to aerate for a time period before running the DOUR tests.
I have used spiked DOUR to test equalized waste streams in industrial pretreatment and pulp & paper wastewater. With frequently tested samples, it proved useful in predicting high strength or toxic influent which allowed for operators to slow the influent flow and prevent system wide upset.