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Food to Microorganisms Ratio (F/M)

By: Haseeb Jamal / On: Apr 27, 2017 / Shallow


A parameter of organic loading rate in the design aerated sludge parameter in the design of Trickling Filter in organic loading rate = kg of BOD / m3-d

F/M ratio = Activated Sludge Process

F/M ratio = BOD / MLSS x t kg of BOD / Kg of MLSS/day
FM ratio varies between 0.2 -0.5 day-1

  • F/M ratio -0.5 day-1 has a good settleabilty of a sludge. ( even in some cases it can go to 1)
  • F/M ratio -<0.2 Food is very limited so the bacteria will die.
  • F/M ratio 70.5 day-1 Food is more so the bacteria will move the effluent (failure of the system)
  • If high F/M ratio, filamentous bacteria will also grow. They not settle easily because of long tails, get entangled with each other. Food to micro organism ratio(F/M) is a common used parameter in the activated-sludge process which is defined as the kg of BOD5applied per kg MLSS per day.

Derivation of F/M Ratio:

Q = Flow of Sewage (m3/day)
BOD =  organic matter (mg/l)
FOOD = Q (m3/day) x BOD (mg/l)
FOOD = Q x BOD / 1000 (Kg of BOD/ day)
V = Volume of Aeration (m3)
MLSS = Mixed liquor suspended solids (mg/l)
Micro-organisms = V (m3) x MLSS (log/l) / 1000 = V x MLSS / 1000 (kg of MLSS in aeration tank)


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