🧫 MLVSS Calculator
Calculate the Food-to-Microorganism (F/M) ratio for an activated sludge process using influent BOD, flow rate, aeration tank volume, and MLVSS concentration.
What MLVSS and the F/M ratio measure
Mixed Liquor Volatile Suspended Solids (MLVSS) is the organic (living and biodegradable) fraction of the suspended solids in an activated sludge aeration tank, and it serves as a proxy for the concentration of active microorganisms treating the wastewater. Operators use MLVSS together with the incoming organic load to calculate the Food-to-Microorganism (F/M) ratio, a key indicator of whether an activated sludge system is properly balanced.
How the F/M ratio is calculated
The standard formula is F/M = BOD loading (lbs/day) ÷ MLVSS mass in the aeration tank (lbs). BOD loading is found by multiplying influent BOD (mg/L) by flow rate (MGD) by the standard conversion factor 8.34 (lbs per gallon per mg/L). MLVSS mass is found the same way, multiplying MLVSS concentration (mg/L) by aeration tank volume (in million gallons) by 8.34.
A typical conventional activated sludge plant targets an F/M ratio around 0.2 to 0.5 lb BOD per day per lb MLVSS, though acceptable ranges vary by treatment process type (extended aeration, contact stabilization, high-rate, etc.). This calculator provides a general reference figure for operator training and planning purposes; always follow your facility’s specific operating permit and process guidelines for actual plant operation.