If 2.0 mg/L D.O. is enough for nitrification, why do I need to maintain 3.0+ D.O. to keep my system removing ammonia?

As you go deeper into a sludge granule or floc, you have decreasing oxygen residuals with even true anaerobic conditions deep in the biofilm.

All the literature says that you get over 90% efficiency from AOB/NOB when you run a D.O. residual of 2.0 mg/L. Yet I often encounter systems that have to keep D.O. above 3+ mg/L to maintain stable AOB/NOB populations. The utility costs associated with the extra D.O. are reason enough to investigate why running high D.O. is needed. To fully understand what is going on, we have used Microbial Community Analysis (MCA) which is the total microbial census inside the aeration tank using 16s sequencing. 

Here is what we found:

• D.O. readings come from meters mounted to tank walls or dropped in by an operator. This is the D.O. in the water phase - remember the bacteria in activated sludge other systems are designed to be in the floc or biofilm.
• Oxygen transfer across the biofilm results in zones that move from oxygen rich to even anaerobic at the center of floc. This is how we find Clostridia  and other anaerobic bacteria in highly aerobic systems.
  
• While AOB/NOB tend to grow on the outside layer of the biofilm where oxygen is most abundant, the cells are still subject to biofilm oxygen transfer across extracellular polymer (EPS) materials that bind the floc.
  
• There is a relationship between D.O. required and the amount of biological solids maintained in the system. Higher MLSS/MLVSS needs more oxygen to keep a greater percentage of the floc oxygen rich.
  
• Ability of oxygen to penetrate into floc is specific to system. Variation in aeration type, EPS makeup, sludge age, inorganics, temperature, and other factors impact oxygen transfer into the floc.
  

So, the real reason for needing a higher than textbook D.O. residual in the aeration tank has to do with how we measure D.O. in the water phase. We cannot really measure D.O. inside the floc or biofilm. We can learn about D.O. and biochemical processes in the biofilm/floc by using MCA and qPCR testing which shows the actual % of obligate aerobes vs anaerobic cultures inside the MLSS.