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Details on Food to Microorganism Ratio F/M
Activated Sludge Systems


The F/M ratio is a process control number that helps you to determine the proper number of microorganisms for your system. To do this calculation, you will need the following information:
  • Influent Flow into your activated sludge system (Flow MGD)
  • Influent CBOD (mg/l) concentration into your aeration tank.
  • Mixed Liquor Volatile Suspended Solids Concentration (mg/l)
  • Volume (in gallons) of your aeration system

The term Food to Microorganism Ratio (F/M) is actually a measurement of the amount of incoming food ( Lbs of Influent CBOD) divided by the Lbs of Microorganisms in your system. Some calculations also include the volume of activated sludge in your clarifiers, the one demonstrated here does not. If you have an activated sludge system, you should determine your F/M ratio regularly.

To determine the amount of incoming food (F), you need to know the CBOD of the influent into your activated sludge(aeration) system. You also need to know the flow(MGD).  So to calculate the amount of food we do the following calculation:

F= Influent Flow (MGD) X  Influent CBOD Concentration (mg/l) X 8.34

To determine the volume of microorganisms (M), you need to know the volume of your aeration system and you need to know the concentration of Volatile Solids in your aeration system (MLVSS) or Mixed Liquor Volatile Suspended Solids. (Refer to the NPDES Lab Manual, the Solids Chapter for more info on MLVSS).  To calculate the microorganisms, we do the following calculation:

M= Aeration System Volume (in Millions of Gallons) X   MLVSS  X  8.34

To Calculate the Ratio:  F/M


Facility Flow = 1.2 MGD    Influent CBOD= 230 mg/l
1.2  X  230  X  8.34 = 2301.84 Lbs Coming In
F= 2301

Aeration System Volume 250,000 gal / 1,000,000 = 0.25MG
MLVSS = 2500mg/l
0.25  X 2500  X  8.34  = 5215 Lbs of Microorganisms under Aeration
M = 5212

2301/5212 = 0.44 F/M Ratio