MLVSS Calculator

Mixed Liquor Volatile Suspended Solids represent the organic fraction of Mixed Liquor Suspended Solids (MLSS), which essentially correlates to the living biological population (bacteria and protozoa) in an activated sludge system. Depending on your available lab data, MLVSS can be determined using multiple standard engineering formulas.

When evaluating based on process loading, the system calculates total volatile solids mass using the chemical oxygen demand (COD) and the target Food-to-Microorganism (F/M) ratio: MLVSS_lbs = COD_lb/day / F/M. To find the concentration in milligrams per liter (mg/L), the total mass is divided by the aeration tank volume in million gallons, adjusted with the conversion factor of 8.34 lb/gal: MLVSS = MLVSS_lbs / (Aeration Vol_mg * 8.34). Alternatively, if direct laboratory filtration data is available, MLVSS is found by subtracting the inert fixed solids from the total MLSS concentration: MLVSS = MLSS - Fixed Solids.

Let us walk through a practical scenario for a municipal wastewater facility. Suppose your plant operates with an incoming organic loading of COD equal to 3,000 lb/day, and you want to maintain a target Food-to-Microorganism (F/M) ratio of 0.65 to ensure efficient biological digestion.

First, determine the required MLVSS weight in pounds: MLVSS_lbs = 3,000 / 0.65 = 4,615.38 lbs. Next, assume your aeration basin volume is 0.50 million gallons (MG). To find the MLVSS concentration in mg/L, divide the total weight by the volume and the conversion factor of 8.34: MLVSS = 4,615.38 / (0.50 * 8.34) = 4,615.38 / 4.17 = 1,106.8 mg/L. This target concentration informs your sludge wasting and recycling schedules.

Maintain consistent sampling times; diurnal flow variations can significantly alter influent COD and active biomass concentrations. Regularly calibrate your analytical balances and muffle furnaces when measuring volatile versus fixed solids to prevent volatile loss errors. Always monitor the MLVSS-to-MLSS ratio; a healthy municipal activated sludge system typically maintains an MLVSS/MLSS ratio between 0.70 and 0.85, indicating a robust active biological population versus inert mineral buildup.