- Aerobic Bacteria (microbes) use oxygen when growing
- More "food" = faster growth and higher OUR
- Mature or older biomass = slower growth and lower OUR
- More biological solids mean more endogenous respiration, so we introduce SOUR to account for different MLSS or MLVSS concentrations
Important for activated sludge – divide OUR by MLVSS in g – to adjust for background respiration by solids. Note if you use MLSS instead of MLVSS as solids. MLSS > MLVSS. Whatever you use just be consistent!
While there are "rules of thumb" for OUR, that does not replace routine testing of your specific system. Operations and sample points will make a difference in OUR. The test takes less than 10 minutes so take a few minutes and run the test as frequently as feasible.
Interpretation of Results
Remember we are using respiration rates to estimate where we are on the growth curve – works with microscopic exam, SV30/SVI, and other tests
Low OUR (< baseline average)
- Lower F/M (less BOD) or “older” biomass
- For industrial – can also indicate toxic shock where you actually killed viable microbes and have yet to enter log growth.
- Low OUR can indicate moving into endogenous phase of the growth curve
- Increased soluble BOD5 - microbes are growing faster which is also higher F/M and "young" biomass
- Very high OUR can indicate log phase of the growth curve