Food-to-Microorganism is a way to manage the activated sludge process at a wastewater treatment plant. It’s a common concept that is tested on the wastewater treatment operator exam, so make sure that you understand it before you take your exam.
In short, it calculates the ratio of organics (BOD or COD) to the amount of microorganisms in the aeration tank. Note that the BOD data of the sewage takes 5 days to acquire in the lab, so COD is often used, since it can be obtained much quicker.
The following practice problem is for the entry-level operator, so it’s a fairly basic one. It will help you get acclimated to using the food-to-microorganism formula.
To estimate the amount of microorganisms in the aeration tank, use MLVSS concentration instead of MLSS. If the problem statement gives you the MLSS concentration, you’ll need to calculate the MLVSS concentration using the information that they give you. In the following example, the problem statement mentions that MLVSS is 80% of MLSS. Using this information, you will be able to calculate the MLVSS concentration and apply it in the F/M formula.
Practice Problem:
Wastewater with a COD of 160 mg/L flows into a 0.5-MG aeration tank at a flowrate of 4.5 MGD. If the MLSS concentration is 2,780 mg/L, then find the F/M ratio. Assume that the MLVSS concentration is 80% of the MLSS.
Known:
COD = 160 mg/L
Volume of aeration tank = 0.5 MG
Flowrate = 4.5 MGD
MLSS = 2,780 mg/L
MLVSS = 0.8 x MLSS = 2,224 mg/L
Solve for:
F/M ratio = ? lb COD/day per lb of MLVSS
Step 1:
Use the Food-to-Microorganism formula to solve the problem.
F/M ratio = $latex \frac{COD \;entering \;aeration \;tank \;(lbs/day)}{MLVSS \;in\; aeration \;tank\; (lbs)}&s=2$
Step 2:
Use the lbs/day formula to calculate the COD entering the aeration tank for the top of the equation. Meanwhile, use the lbs formula to calculate the MLVSS in aeration tank for the bottom of the equation.
F/M ratio = $latex \frac{160mg/L \;\times \;4.5MGD \;\times \;8.34\;lbs/gal}{2,224mg/L \;\times \;0.5MG\; \times\;8.34\;lbs/gal}&s=2$
F/M ratio = 0.65 lb COD/day per lb of MLVSS