When asked about F/M most consultants and engineers open a standard engineering text and give an answer from standard design numbers. I often will give a number from Metccalf & Eddy but indicate that this is just a starting point and influent makeup, temperature, pH, treatment targets, and a big "other factors" can require modifications on the number. For your reference here are a few of the target numbers:
- Coventional AS Plug Flow - 0.2 - 0.4 (kg BOD/kg MLVSS*d)
- Complete Mix - 0.2 - 0.6
- Extended Aeration - 0.04 - 0.1 (this being the most common in industrial & food processing)
In the dairy case that I first mentioned, I suggested that they look at their current F/M and check SV30, microscopic exam, and oxygen uptake rates to get a full picture of the system's biological state.
The easiest to interpret may be the microscopic exam. Use the following chart to determine how indicator protozoa, floc formation, and F/M relate. Note extended aeration operated at the far right side of the ideal F/M ratio section - this uses a lower F/M and higher mean cell residence time (MCRT) to both reduce solids yield and nutrient (N,P) control in systems with nutrient permits.