Food-to-microorganism ratio is used at a wastewater treatment plant to control and manage the activated sludge process. It ensures that there is enough microorganisms in the aeration tank to remove the incoming organics (food) from the sewage. Without a correct ratio, there may not be enough microorganisms to remove the food from the influent, and could lead to an imbalance. By analyzing the food-to-microorganism of an aeration tank, the operator will know if more microorganisms need to be maintained in the system.
The established food-to-microorganism ratio for a conventional wastewater treatment plant is 0.5 to 1.0 pounds of COD per day / pound of MLVSS under aeration. If you’re using BOD to estimate the amount of incoming organics, then use 0.25 to 0.5 pounds of BOD per day / pound of MLVSS under aeration.
Note that COD is commonly used to estimate the amount of incoming organics, since the amount of COD can be obtained much quicker than BOD. Results for COD can be obtained in less than 4 hours, while the BOD will require a 5-day test to acquire the concentration. Many wastewater treatment plants cannot afford to wait for 5 days until an operational change is made. Typically, changes must be made as soon as possible. Therefore, the COD is more ideal in this case.
The microorganism concentration is estimated using the mixed liquor volatile suspended solids (MLVSS) concentration, since it is a measure of organisms and organic material. Do not use MLSS, since it measures both inorganic and organic compounds.
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