Making sense of OUR tests

 We often run OUR tests and record the oxygen uptake of a sample as mg O2/L/hr.  The key is to make sense of the respiration rate.  First, unless you run a constant MLSS or MLVSS, you should divide the OUR by MLSS in grams to get the SOUR which helps account for background solids impact on respiration.  Now with the OUR or SOUR numbers, you need to think about what the test is telling you about microbial health.
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• Sudden increase in OUR is normally see with high soluble BOD (organic) loadings.  If you run ATP tests, you should see an increase in free ATP (ATP in solution) which indicates rapid cell division or a move  to the left on the growth curve.
  
  
• A slow increase in OUR numbers can be from changes in total biological solids which is why SOUR calculation is important.  I have also seen increases in VSS relative to MLSS create a change in OUR numbers.  Other factors can be "fat slduge" which is sludge with adsorbed insoluble organics or high EPS which is simply food in storage. 
  
  
• What about a sudden drop in OUR numbers.  Not often seen in domestic wastewater, shock or toxic loadings can cause a sudden drop in biological activity or respiration rates.  In this case, you will see a drop in OUR and other signs of stress such as deflocculation, increased turbidity, loss of indicator protozoa, and loss of nitrification.  If the shock was due to a short term spill, you will see the low OUR replaced by a rapid increase in OUR as the surviving bacteria begin to multiply in log growth.

I recommend daily running of OUR.  This test uses exisiting equipment with no reagents or excessive time requirements.  You can also use influente spiked OUR to predict toxicity or biomass inhibition.  Key thing is to run the test frequently so you know what the normal OUR is for the system and enable you to see any significant change in microbial activity.