MLVSS Calculator

MLVSS (Mixed Liquor Volatile Suspended Solids) is the portion of the mixed liquor suspended solids that burns off at 550°C — essentially the living biomass plus biodegradable organics in the aeration tank. Operators track it because it represents the active microorganisms doing the treatment work.

MLVSS = MLSS × volatile fraction. For example, an MLSS of 2,100 mg/L at 75% volatile gives an MLVSS of 1,575 mg/L. The volatile fraction of municipal activated sludge is typically 0.70–0.80.

MLSS is the total suspended solids (fixed + volatile). MLVSS is only the volatile (organic) portion. The F/M ratio and other process calculations use MLVSS because the fixed, inert fraction does not consume BOD.

F/M = (flow × influent BOD) ÷ (aeration volume × MLVSS), in consistent units. Example: 3.5 MGD × 175 mg/L BOD with 1.27 MG volume and 1,575 mg/L MLVSS gives F/M ≈ 0.31 lb BOD/day per lb MLVSS.

Conventional activated sludge runs at F/M 0.2–0.5, extended aeration at 0.05–0.15, and high-rate systems up to 1.0. Below 0.05 risks filamentous bulking and pin floc; above 1.0 the plant is organically overloaded.

MCRT = (aeration volume × MLSS) ÷ (WAS flow × WAS TSS + effluent flow × effluent TSS), in consistent units. Example: 4.0 MG at 3,000 mg/L MLSS, wasting 0.05 MGD at 8,000 mg/L plus effluent losses, gives MCRT ≈ 25.1 days.

3 to 15 days is typical for conventional activated sludge. Under 3 days produces young, poorly settling sludge with turbid effluent; above 15–30 days favors nitrification but can produce pin floc.

SVI = (30-minute settled volume in mL/L × 1,000) ÷ MLSS in mg/L, giving mL/g. Example: 300 mL/L settled sludge with 3,000 mg/L MLSS gives SVI = 100 mL/g.

SVI measures settleability. Below 100 mL/g is excellent, 100–120 good, 120–150 fair, and above 150 indicates filamentous bulking. A rising SVI trend warns of a settling problem before the clarifier fails.

Pounds = volume in million gallons × concentration in mg/L × 8.34. Example: 1.27 MG at 2,100 mg/L MLSS contains about 22,243 lb of solids.

US units (MGD, gallons, MG, ft³, lb) and metric units (MLD, m³/day, m³, ML, kg). F/M, MCRT and SVI results are identical in either system because the formulas are unit-consistent.

Raising MLVSS lowers the F/M ratio, which improves BOD removal and effluent quality up to a point. It is controlled by reducing sludge wasting — but too much biomass overloads the secondary clarifier's solids loading rate.