I will start with something that puzzeled me for years. In the lab, ammonia oxidizing bacteria (AOB or nitrifiers) have excellent activity at a D.O. of 2.0. Moving up from 2.0 mg/L DO to say 4.0 mg/L gives a slight boost in growth rates, but is not worth the energy cost required to increase the oxygen residual. Meanwhile, I have seen working activated sludge systems where a D.O. residual below 3.0 mg/L results in a loss of nitrification. So what is going on?
The answer came after we started running molecular tests that look directly at the microbial makeup of MLSS. Running these tests, we found obligate anaerobic bacteria growing inside high D.O. basins. We then did extensive work on AOB/NOB inhbition with various influents and MLSS from different systems. Here is what we found:
The answer came after we started running molecular tests that look directly at the microbial makeup of MLSS. Running these tests, we found obligate anaerobic bacteria growing inside high D.O. basins. We then did extensive work on AOB/NOB inhbition with various influents and MLSS from different systems. Here is what we found:
- D.O. meter placement
D.O. meters read oxygen residual in the water phase at the probe/water interface. D.O. residuals are subject to probe placement and mixing. Is your D.O. meter giving you a true average residual or D.O. you do manual checks at other locations to confirm readings? - Impact of MLSS concentrations
The aeration basin is not a homogenous mixture. We have water phase and solids. For oxygen to get inside the floc, it must pass through EPS and move from the water column into the biofilm. Larger floc or higher biosolids mean lower oxygen residuals inside the floc compred to the water on the exterior. - Floc EPS levels
Floc includes insolube organics, biopolymers, and inorganics. Increasing amounts of non-living materials result in lower oxygen transfer efficiency. Remember that high EPS levels can reduce how much oxygen moves into the floc. Check your EPS using India Ink for an easy on-site EPS test.
- Check D.O. residual manually to confirm in basin meter numbers
- Monitor D.O. meter numbers and see where you get needed activity from obligate aerobes (AOB/NOB). You can also look at denitrification or phosphate release/uptake that require anoxic and anaerobic zones.
- Run Microbial Community Analysis (MCA) which is a molecular test that identifies all bacteria present. The MCA tells you what is growing and functioning in the system.
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