MLVSS Calculator for Wastewater Treatment

Mixed Liquor Volatile Suspended Solids (MLVSS) is the organic, biodegradable portion of the suspended solids in an activated sludge aeration tank. It is used as a stand-in measurement for the amount of active microorganisms treating the wastewater. This calculator finds MLVSS from lab data using either the VSS percentage method or the FSS (Fixed Suspended Solids) method.

What is MLVSS?

Wastewater treatment plants that use the activated sludge process rely on microorganisms to break down organic pollutants. The mixture of water, microorganisms, and solids in the aeration tank is called mixed liquor. Some of the suspended solids in that mixture are organic material, mostly living and dead microorganisms. This organic fraction is the volatile suspended solids, and when measured in the mixed liquor it is called MLVSS.

The rest of the suspended solids is inorganic material such as grit, sand, and minerals. This inorganic fraction is called fixed suspended solids (FSS), because it does not burn off when heated. Together, MLVSS and FSS make up Mixed Liquor Suspended Solids (MLSS), the total suspended solids concentration in the tank.

$\text{MLSS} = \text{MLVSS} + \text{MLFSS}$

Operators track MLVSS because it approximates the size of the biological workforce doing the treatment. A steady MLVSS level generally means stable, predictable treatment. A sudden drop can signal toxic influent, excess sludge wasting, or a storm washing biomass out of the tank.

How to calculate MLVSS

There are two common ways to calculate MLVSS, and both should give the same answer if the lab measurements are accurate.

VSS percentage method

Some labs report VSS as a percentage of TSS (Total Suspended Solids). In that case:

$\text{MLVSS} = \text{TSS} \times \frac{\text{VSS\%}}{100}$

• MLVSS = Mixed Liquor Volatile Suspended Solids (mg/L)
• TSS = Total Suspended Solids (mg/L)
• VSS% = the share of suspended solids that is volatile, usually 65-85% in a conventional activated sludge system

FSS method

Other labs report the fixed (inorganic) fraction directly. In that case, MLVSS is found by subtraction:

$\text{MLVSS} = \text{TSS} - \text{FSS}$

Because VSS and FSS always add up to TSS, this formula gives the same result as the percentage method whenever the underlying measurements agree. If the two methods give noticeably different answers for the same sample, the lab data or sample handling should be checked.

Worked examples

Example 1: VSS percentage method. An operator measures TSS of 3,500 mg/L in the aeration tank, with a VSS percentage of 75%.

$\text{MLVSS} = 3{,}500 \times \frac{75}{100} = 2{,}625 \text{ mg/L}$

Example 2: FSS method. The same tank is also measured directly: TSS is 3,500 mg/L and FSS is 875 mg/L.

$\text{MLVSS} = 3{,}500 - 875 = 2{,}625 \text{ mg/L}$

Both methods agree, as expected, since 875 mg/L of FSS is exactly 25% of 3,500 mg/L of TSS.

How to use this calculator

1. Enter the Total Suspended Solids (TSS) value in mg/L.
2. Choose a calculation method: "Using VSS Percentage" or "Using Fixed Suspended Solids (FSS)".
3. Enter the second value: either the VSS percentage (0-100%) or the FSS value in mg/L.
4. Read the MLVSS result in mg/L, shown with the formula and the numbers used.

The calculator checks that TSS is zero or greater, that the VSS percentage is between 0 and 100, and that FSS does not exceed TSS, since FSS is part of TSS. If a check fails, it shows a message explaining what to fix.

The MLVSS/MLSS ratio

In a conventional activated sludge plant, the MLVSS/MLSS ratio usually falls between 0.65 and 0.85, meaning 65-85% of the suspended solids are organic. A ratio drifting below that range can point to inert solids building up or extra inorganic material entering from an industrial source. A ratio near the top of the range usually means the biomass is mostly organic and active.

Why MLVSS matters for process control

MLVSS feeds into several calculations operators use every day:

• Food-to-Microorganism (F/M) ratio: compares the organic load coming in to the amount of biomass available to treat it.
• Solids Retention Time (SRT), also called sludge age: how long biomass stays in the system, which affects treatment stability and the ability to remove ammonia (nitrification).
• Sludge production: more biomass generally means more sludge to waste and dispose of.
• Oxygen demand: the biomass needs oxygen to work, so aeration energy use tracks roughly with MLVSS.

Standard laboratory methods for MLVSS are described in Standard Methods for the Examination of Water and Wastewater (Method 2540E) and in EPA Method 160.4.

How to calculate the F/M ratio

$\text{F/M ratio} = \frac{\text{Influent BOD load (kg/day)}}{\text{MLVSS mass in the tank (kg)}}$

Example. A plant has these values:

• Influent flow = 10,000 m³/day
• Influent BOD = 250 mg/L
• Aeration tank volume = 2,000 m³
• MLVSS = 2,500 mg/L

Because 1 mg/L in a flow measured in m³/day equals 1 gram of mass per day, the load in kg/day is found by multiplying and dividing by 1,000:

• Influent BOD load = 10,000 m³/day × 250 mg/L ÷ 1,000 = 2,500 kg/day
• MLVSS mass = 2,000 m³ × 2,500 mg/L ÷ 1,000 = 5,000 kg
• F/M ratio = 2,500 kg/day ÷ 5,000 kg = 0.5 per day

A conventional activated sludge plant typically runs with an F/M ratio between 0.2 and 0.6 per day. A ratio that is too high can cause filamentous bulking, where thread-like bacteria make the sludge settle poorly. A ratio that is too low wastes energy aerating biomass that has little food left to consume.

Typical MLVSS operating ranges

Membrane bioreactors can run at much higher MLVSS because they separate solids with membranes instead of relying on gravity settling in a clarifier.

How MLVSS is measured in a laboratory

The standard procedure has two heating steps:

1. TSS step. A known volume of mixed liquor is filtered through a pre-weighed glass fiber filter, dried at 103-105°C for at least an hour, cooled, and weighed. The weight gained is the TSS (or MLSS, when the sample is mixed liquor).
2. Ignition step. The same filter is heated in a furnace at 550°C for 15-20 minutes. The organic material burns off, leaving inorganic ash. The filter is cooled and weighed again.
3. Result. The weight lost during ignition is the volatile, organic portion, which is the MLVSS.

Furnace temperature matters. Above about 575°C, some inorganic carbonates start to break down and give a falsely high volatile reading. Below about 525°C, organic material may not fully burn off. Most labs target 550°C.

Alternatives to MLVSS

MLVSS is widely used because it is simple and reliable, but other methods measure biomass more specifically:

• ATP (adenosine triphosphate) measures a molecule found only in living cells, so it counts active biomass without dead material. It needs a dedicated analyzer.
• Respirometry measures the rate at which biomass consumes oxygen, which shows biological activity directly. It is useful for testing whether influent is toxic to the biomass.
• DNA quantification and FISH (Fluorescence In Situ Hybridization) identify and count specific microorganisms. These are mainly research tools, useful for tracking down the cause of filamentous bulking.

These methods give more detail but cost more and take more time, which is why MLVSS remains the standard for routine process control.

Frequently asked questions

What is the difference between MLSS and MLVSS? MLSS is the total suspended solids in the aeration tank, including both organic and inorganic material. MLVSS is just the organic part. MLSS equals MLVSS plus MLFSS (Mixed Liquor Fixed Suspended Solids).

What is a typical MLVSS/MLSS ratio? In a conventional activated sludge plant, it is usually 0.65 to 0.85 (65-85%). A lower ratio suggests more inorganic material in the mix.

How does MLVSS affect the F/M ratio? MLVSS is the denominator in the F/M ratio. A higher MLVSS, for the same organic load, gives a lower F/M ratio. A lower MLVSS gives a higher F/M ratio, which can promote poor-settling filamentous bacteria if it climbs too high.

What causes MLVSS to drop suddenly? Common causes include excess sludge wasting, toxic material in the influent killing biomass, heavy rain flushing biomass out of the tank, and low food availability during periods of light loading, such as holidays.

Can MLVSS be too high? Yes. High MLVSS raises the oxygen demand and therefore the aeration energy cost, can overload the secondary clarifier with solids, and can limit oxygen from reaching the center of dense floc particles.

How soon after sampling should MLVSS be measured? Analysis should start within about 2 hours of sampling. If that is not possible, the sample should be refrigerated at 4°C and analyzed within 24 hours.

References

1. Water Environment Federation. Operation of Water Resource Recovery Facilities, 7th Edition. McGraw-Hill Education, 2018.
2. Metcalf & Eddy. Wastewater Engineering: Treatment and Resource Recovery, 5th Edition. McGraw-Hill Education, 2014.
3. American Public Health Association, American Water Works Association, & Water Environment Federation. Standard Methods for the Examination of Water and Wastewater, 23rd Edition, 2017.
4. U.S. Environmental Protection Agency. Wastewater Technology Fact Sheet: Activated Sludge Process. EPA 832-F-00-016.