MLVSS Calculator (mixed liquor volatile suspended solids from filter weights or MLSS)

Understanding the MLVSS calculator — concept and principle

This calculator finds mixed liquor volatile suspended solids (MLVSS) in activated sludge, either from laboratory filter weights after drying and ignition or from MLSS and a known volatile fraction. It also gives MLSS, the fixed (inorganic) solids, the volatile share and the mass held in the aeration tank. Wastewater operators use it for process control.

Drying gives all the solids; burning removes the organic part, and what burns off is the biomass estimate.

How it is calculated

MLSS (mg/L) = (dried weight − tare) × 10⁶ ÷ sample volume (mL)

MLVSS (mg/L) = (dried weight − ignited weight) × 10⁶ ÷ sample volume (mL)

Fixed solids = MLSS − MLVSS

Mass in the tank (kg) = MLVSS (mg/L) × volume (m³) ÷ 1,000

Weights in grams; drying at 103–105 °C, ignition at 550 °C. Alternatively MLVSS = MLSS × VSS/TSS.

The principle

Filtering a sample traps the suspended solids. Drying removes the water, so the weight gain gives total suspended solids. Igniting the filter at 550 °C burns away organic matter, and the weight lost is the volatile part. Because living and dead microorganisms are mostly organic, MLVSS is used as a rough measure of the active biomass in the aeration tank, for example in the food-to-microorganism ratio and sludge age.

Worked example

Default inputs: tare 1.2500 g, dried 1.2650 g, ignited 1.2538 g, sample 5 mL, tank 1,000 m³.

MLSS = (1.2650 − 1.2500) × 10⁶ ÷ 5 = 3,000 mg/L.
MLVSS = (1.2650 − 1.2538) × 10⁶ ÷ 5 = 2,240 mg/L.
Fixed solids = 3,000 − 2,240 = 760 mg/L.
Volatile share = 2,240 ÷ 3,000 = 74.7%.
In the tank: 2,240 × 1,000 ÷ 1,000 = 2,240 kg.

Each point is the calculator re-run with only “Sample volume filtered (mL)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Sample volume filtered (mL)	MLVSS (mg/L)
1.25	8,960
2.5	4,480
3.75	2,987
5	2,240
6.25	1,792
7.5	1,493
10	1,120

How to read the result

A volatile share around three quarters, as here, is common for activated sludge, but plant-specific trends matter more than one number. A falling volatile share can indicate grit or inorganic build-up. Tracking MLVSS over time helps keep the right amount of biomass for the incoming load.

Common mistakes

Not drying or igniting to constant weight.
Weighing less precisely than 0.1 mg with small samples.
Using too small a sample volume, which magnifies weighing errors.
Confusing MLSS (total) with MLVSS (volatile).

Keep in mind: Process decisions should follow your plant's procedures and the analytical methods your permit requires.

Frequently asked questions

What is the difference between MLSS and MLVSS?

MLSS is all suspended solids; MLVSS is the part that burns off at 550 °C, mostly organic.

Why use MLVSS for biomass?

Microorganisms are mostly organic, so the volatile fraction tracks them better than total solids.

Why ignite at 550 °C?

That temperature burns off organic matter while leaving most mineral solids.

How precise must the balance be?

Weigh to 0.1 mg, because the solids on a small filter weigh only milligrams.

Sources: University of Maryland Center for Environmental Science, NASL SOP "Determination of Total Suspended Solids (TSS) and Total Volatile Solids (TVS)" (Rev. 2023-1) · Standard Methods for the Examination of Water and Wastewater, methods 2540 D and 2540 E

Checked 9 Nov 2026. TSS and volatile-solids calculations (weight gain after drying, weight loss on ignition, × 1000 ÷ litres filtered): University of Maryland Center for Environmental Science, NASL SOP “Determination of Total Suspended Solids (TSS) and Total Volatile Solids (TVS)” (Rev. 2023-1), §11.3. Drying 103–105 °C and ignition 550 °C as in Standard Methods 2540 D/E — standard values.

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