Utility July 13, 2026 · 8 Min Read

MLVSS Calculator – Guide & Formulas

Calculate MLVSS from MLSS and volatile fraction. Determine F/M ratio, MLVSS/MLSS ratio, and biomass concentration for wastewater treatment plants.

Calculate MLVSS from MLSS concentration and volatile fraction. Determine F/M ratio, biomass concentration, and MLVSS/MLSS ratio for activated sludge wastewater treatment plants.

Key Takeaway

Use the free MLVSS Calculator to calculate mlvss from mlss and volatile fraction. determine f/m ratio, mlvss/mlss ratio, and biomass concentration for wastewater treatment plants. Get instant results with step-by-step explanations.

How to Use the MLVSS Calculator

  1. Enter the MLSS (Mixed Liquor Suspended Solids) concentration in mg/L.
  2. Input the volatile fraction (typically 0.60 to 0.85 for activated sludge).
  3. Optionally enter influent BOD, flow rate, and tank volume for F/M ratio calculation.
  4. Review MLVSS concentration, MLVSS/MLSS ratio, and food-to-microorganism ratio.

The Formula

MLVSS (mg/L) = MLSS (mg/L) × Volatile Fraction. F/M Ratio = (BOD × Q) / (MLVSS × V), where Q = flow rate (m³/day) and V = aeration tank volume (m³).

Variable Definitions

  • MLSS: Mixed Liquor Suspended Solids — total suspended solids in aeration tank (organic + inorganic)
  • MLVSS: Mixed Liquor Volatile Suspended Solids — volatile (organic/biological) portion representing active biomass
  • Volatile Fraction: Ratio of MLVSS to MLSS, typically 0.60–0.85, determined by lab ashing at 550°C
  • F/M: Food-to-Microorganism ratio — kg BOD per kg MLVSS per day
  • BOD: Biochemical Oxygen Demand of influent wastewater in mg/L
  • Q: Influent flow rate in m³/day or MGD
  • V: Aeration tank volume in m³ or MG

Calculating MLVSS and F/M Ratio

Determine MLVSS from MLSS of 3,000 mg/L with volatile fraction 0.75, and F/M ratio with BOD of 200 mg/L, flow 10,000 m³/day, tank volume 5,000 m³.

  1. Step 1: Identify inputs: MLSS = 3,000 mg/L, Volatile Fraction = 0.75.
  2. Step 2: Calculate MLVSS: 3,000 × 0.75 = 2,250 mg/L.
  3. Step 3: Calculate MLVSS/MLSS ratio: 2,250 / 3,000 = 0.75 (75%).
  4. Step 4: Calculate F/M ratio: (200 × 10,000) / (2,250 × 5,000) = 2,000,000 / 11,250,000 = 0.178 kg BOD/kg MLVSS/day.
  5. Step 5: The F/M ratio of 0.178 indicates a well-loaded activated sludge system suitable for BOD removal.

Financial Advisory Notice

This MLVSS Calculator provides informational estimates for wastewater treatment plant operators. Actual process parameters vary by plant design, influent characteristics, and operational conditions. Always verify with laboratory analysis.

Frequently Asked Questions

What is MLVSS and why is it important?

MLVSS (Mixed Liquor Volatile Suspended Solids) represents the active biological mass in an activated sludge system. It is a better indicator of treatment capacity than MLSS because it excludes inert inorganic solids that do not contribute to biological treatment.

What is a typical volatile fraction for activated sludge?

The volatile fraction for conventional activated sludge typically ranges from 0.60 to 0.85. A healthy system usually maintains a volatile fraction of 0.70 to 0.80. Values below 0.60 may indicate high inorganic loading or poor biological activity.

What F/M ratio is ideal for BOD removal?

For conventional activated sludge BOD removal, an F/M ratio of 0.2 to 0.6 kg BOD/kg MLVSS/day is typical. Lower F/M ratios (0.1–0.2) favor nitrification, while higher ratios increase treatment capacity but may reduce effluent quality.

How do I measure volatile fraction in the lab?

Volatile fraction is determined by drying a known volume of mixed liquor, weighing the total solids (MLSS), then igniting the sample in a muffle furnace at 550°C for at least 1 hour. The weight lost during ignition represents the volatile portion.

Can MLVSS be higher than MLSS?

No, MLVSS is always less than or equal to MLSS because MLVSS is the volatile (organic) subset of total suspended solids. If MLVSS appears higher than MLSS, there is likely a measurement error.