MLVSS Calculator

Calculate percent volatile solids from MLSS and MLVSS, or back-calculate MLVSS from a target percent VS, for activated sludge process control.

💧 MLVSS Calculator
mg/L
mg/L
mg/L
%
% Volatile solids
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MLFSS
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Step-by-step working
MLVSS
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MLFSS
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Step-by-step working

💧 What is the MLVSS Calculator?

MLVSS (Mixed Liquor Volatile Suspended Solids) is a core process control measurement in the activated sludge wastewater treatment process. This calculator finds percent volatile solids from a measured MLSS (Mixed Liquor Suspended Solids) and MLVSS pair, or back-calculates the MLVSS you would expect at a given MLSS and target percent VS, along with MLFSS, the complementary fixed (inorganic) solids fraction.

Wastewater treatment plant operators use the %VS ratio to monitor the character of their activated sludge biomass day to day, it feeds directly into other process control numbers like the food-to-microorganism (F/M) ratio and sludge retention time calculations, which assume MLVSS approximates the living microbial mass doing the actual treatment work.

It's important to understand that MLVSS itself is a laboratory measurement, found by drying a mixed liquor sample to get MLSS, then igniting that same dried sample at 550 degrees Celsius in a muffle furnace and measuring the weight lost, per Standard Methods 2540 E. This calculator does not replace that lab procedure, it performs the arithmetic between MLSS, MLVSS, and %VS once you already have lab results.

This calculator is useful for wastewater treatment plant operators, environmental engineering students, and process control staff who need a quick, correct %VS or back-calculated MLVSS figure without redoing the arithmetic by hand every shift.

📐 Formula

%VS  =  (MLVSS ÷ MLSS) × 100
MLSS = Mixed Liquor Suspended Solids, mg/L
MLVSS = Mixed Liquor Volatile Suspended Solids, mg/L (per Standard Methods 2540 E, ignition at 550°C)
MLFSS = MLSS − MLVSS (fixed/inorganic fraction)
Example: MLSS=3,000 mg/L, MLVSS=2,250 mg/L: %VS = 75%, MLFSS = 750 mg/L.

📖 How to Use This Calculator

Steps

1
Choose From Values or From Ratio - From Values computes %VS from a measured MLSS and MLVSS. From Ratio back-calculates MLVSS from MLSS and a target %VS.
2
Enter your MLSS and MLVSS, or MLSS and %VS - In mg/L (and % for the ratio input).
3
Read the result - See percent volatile solids or MLVSS, plus MLFSS and a note on whether the ratio is in the typical healthy range.

💡 Example Calculations

Example 1 - Typical municipal activated sludge

1
MLSS = 3,000 mg/L, MLVSS = 2,250 mg/L
2
%VS = (2,250 ÷ 3,000) × 100 = 75%
3
MLFSS = 3,000 − 2,250 = 750 mg/L
%VS = 75% (MLFSS = 750 mg/L)
Try this example →

Example 2 - Lower MLSS basin

1
MLSS = 2,500 mg/L, MLVSS = 1,875 mg/L
2
%VS = (1,875 ÷ 2,500) × 100 = 75%
3
MLFSS = 2,500 − 1,875 = 625 mg/L
%VS = 75% (MLFSS = 625 mg/L)
Try this example →

Example 3 - Back-calculating MLVSS from a target ratio

1
MLSS = 4,000 mg/L, target %VS = 78%
2
MLVSS = 4,000 × (78 ÷ 100) = 3,120 mg/L
3
MLFSS = 4,000 − 3,120 = 880 mg/L
MLVSS = 3,120 mg/L (MLFSS = 880 mg/L)
Try this example →

❓ Frequently Asked Questions

What is MLVSS in wastewater treatment?+
MLVSS stands for Mixed Liquor Volatile Suspended Solids, the fraction of suspended solids in the aeration basin's mixed liquor that burns off (volatilizes) when a dried sample is ignited at 550 degrees Celsius. It approximates the living microbial biomass in the activated sludge process.
What is the difference between MLSS and MLVSS?+
MLSS (Mixed Liquor Suspended Solids) is the total suspended solids in the mixed liquor, both organic and inorganic. MLVSS is only the organic (volatile) portion of MLSS that ignites away at 550 degrees Celsius, representing mostly living and dead microbial biomass.
What is the formula for percent volatile solids?+
Percent volatile solids = (MLVSS divided by MLSS) times 100. For example, an MLSS of 3,000 mg/L with an MLVSS of 2,250 mg/L gives 75% volatile solids.
What is a normal MLVSS/MLSS ratio for activated sludge?+
Municipal activated sludge plants typically run a %VS (MLVSS/MLSS) ratio of about 70-80%, per common wastewater engineering references. Industrial waste streams or plants with significant grit or inorganic loading can run outside this range.
What is MLFSS?+
MLFSS (Mixed Liquor Fixed Suspended Solids) is the inorganic, non-volatile remainder of MLSS, calculated as MLSS minus MLVSS. It represents the ash, grit, and other inert mineral solids left behind after ignition at 550 degrees Celsius.
How is MLVSS actually measured in the lab?+
A known volume of mixed liquor is filtered, dried at 103-105 degrees Celsius to find MLSS, then the same dried sample is ignited in a muffle furnace at 550 degrees Celsius (Standard Methods 2540 E), the weight lost during ignition, divided by the sample volume, is MLVSS.
Why does %VS matter for activated sludge process control?+
MLVSS is used as a practical stand-in for active microbial biomass in calculations like the food-to-microorganism (F/M) ratio and sludge retention time, a stable %VS ratio suggests consistent biomass character, while a sudden drop can signal an inorganic solids problem entering the system.
Can I use this calculator without running a lab test?+
You need at least two of the three values, MLSS, MLVSS, and %VS, from an actual lab measurement or a known target, this calculator only performs the arithmetic between them, it cannot substitute for the required ignition test.
What units should MLSS and MLVSS be entered in?+
Enter both in milligrams per liter (mg/L), the standard unit for suspended solids concentration in wastewater treatment, and make sure both readings come from the same sample.
Can MLVSS ever be greater than MLSS?+
No, MLVSS is by definition a subset of MLSS, since it is the organic portion of the total suspended solids. If a measured MLVSS exceeds MLSS, it indicates a lab or sampling error, this calculator flags that combination as invalid.
Tags: Mlvss Calculator Mixed Liquor Volatile Suspended Solids Mlss Mlvss Ratio Activated Sludge Process Control Calculator Formula Free Online

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What is MLVSS in wastewater treatment?

MLVSS stands for Mixed Liquor Volatile Suspended Solids, the fraction of suspended solids in the aeration basin's mixed liquor that burns off (volatilizes) when a dried sample is ignited at 550 degrees Celsius. It approximates the living microbial biomass in the activated sludge process.

What is the difference between MLSS and MLVSS?

MLSS (Mixed Liquor Suspended Solids) is the total suspended solids in the mixed liquor, both organic and inorganic. MLVSS is only the organic (volatile) portion of MLSS that ignites away at 550 degrees Celsius, representing mostly living and dead microbial biomass.

What is the formula for percent volatile solids?

Percent volatile solids = (MLVSS divided by MLSS) times 100. For example, an MLSS of 3,000 mg/L with an MLVSS of 2,250 mg/L gives 75% volatile solids.

What is a normal MLVSS/MLSS ratio for activated sludge?

Municipal activated sludge plants typically run a %VS (MLVSS/MLSS) ratio of about 70-80%, per common wastewater engineering references. Industrial waste streams or plants with significant grit or inorganic loading can run outside this range.

What is MLFSS?

MLFSS (Mixed Liquor Fixed Suspended Solids) is the inorganic, non-volatile remainder of MLSS, calculated as MLSS minus MLVSS. It represents the ash, grit, and other inert mineral solids left behind after ignition at 550 degrees Celsius.

How is MLVSS actually measured in the lab?

A known volume of mixed liquor is filtered, dried at 103-105 degrees Celsius to find MLSS, then the same dried sample is ignited in a muffle furnace at 550 degrees Celsius (Standard Methods 2540 E), the weight lost during ignition, divided by the sample volume, is MLVSS.

Why does %VS matter for activated sludge process control?

MLVSS is used as a practical stand-in for active microbial biomass in calculations like the food-to-microorganism (F/M) ratio and sludge retention time, a stable %VS ratio suggests consistent biomass character, while a sudden drop can signal an inorganic solids problem entering the system.

Can I use this calculator without running a lab test?

You need at least two of the three values, MLSS, MLVSS, and %VS, from an actual lab measurement or a known target, this calculator only performs the arithmetic between them, it cannot substitute for the required ignition test.

What units should MLSS and MLVSS be entered in?

Enter both in milligrams per liter (mg/L), the standard unit for suspended solids concentration in wastewater treatment, and make sure both readings come from the same sample.

Can MLVSS ever be greater than MLSS?

No, MLVSS is by definition a subset of MLSS, since it is the organic portion of the total suspended solids. If a measured MLVSS exceeds MLSS, it indicates a lab or sampling error, this calculator flags that combination as invalid.

📌 Quick Tips

💡MLSS and MLVSS are both lab measurements, per Standard Methods 2540 D and 2540 E, MLVSS is found by igniting a dried MLSS sample at 550 degrees Celsius and measuring the weight lost, this calculator does not replace that lab test, it works with results you already measured.
💡A healthy municipal activated sludge process typically runs a %VS (MLVSS/MLSS) ratio of roughly 70-80%, a ratio trending noticeably lower can signal inert grit or inorganic solids accumulating in the mixed liquor.
💡MLVSS approximates the biomass (living microorganism) fraction of the mixed liquor, it is the number most process control calculations, like food-to-microorganism ratio and sludge age, actually want.
💡Use From Ratio mode when you want to plan around a target %VS, for example estimating what MLVSS reading to expect once your MLSS stabilizes at a certain level.
💡Always keep MLSS and MLVSS in the same units (mg/L) and from the same sample, mixing readings from different sample times will distort the calculated ratio.