Compute mixed liquor volatile suspended solids (MLVSS) from MLSS and the volatile fraction, plus basin inventory.
MLVSS is an operational measurement: the volatile fraction depends on the ignition test conditions, and volatile solids are only a proxy for active biomass, not a direct count of living organisms.
MLVSS (mg/L)
How to use this tool
Mixed liquor suspended solids in mg/L from your solids test.
The percentage of MLSS lost on ignition (typically about 70–80% for municipal sludge).
Aeration basin volume in m³ to convert concentration into total MLVSS inventory.
MLVSS, inert solids and total MLVSS mass in the basin are shown.
Formula & worked example
MLSS 3000 mg/L at 75% volatile in a 500 m³ basin: MLVSS 2250 mg/L, inert solids 750 mg/L, inventory 2250 × 500 / 1000 = 1125 kg.
Limitations — read this before you rely on it
Volatile solids are a proxy for biomass, not a live-cell count — dead cells and organic debris also volatilise. Ignition conditions and sampling method affect the measured fraction.
Frequently asked questions
MLSS counts all suspended solids; MLVSS counts only the volatile (organic) part. Operators track MLVSS because it better reflects the biomass doing the treatment.
Often roughly 1500–3000 mg/L in municipal activated sludge, at a volatile fraction near 70–80%. Industrial wastes vary widely.
Dry a sample, weigh it (MLSS), ignite it at about 550 °C, and weigh again. The weight lost is the volatile fraction.
Food-to-microorganism (F/M) ratio and sludge age calculations use MLVSS mass, so inventory links the basin concentration to process control.
No — it is a fraction of MLSS. A result above 100% signals a measurement or data-entry error.