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MLVSS Calculator

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Activated sludge processMLVSS definitionMLVSS procedure for industryMLVSS calculation formulaHow to calculate MLVSS in a laboratoryFAQs

If you have to monitor an activated sludge process, you will surely find this mixed liquor volatile suspended solids (MLVSS) calculator helpful. As well as helping you find the industrial MLVSS for wastewater, it also allows you to quickly find it if you have a sample in a laboratory.

To help you better understand why calculating MLVSS is necessary, we will:

  • Give you the MLVSS definition in the context of a microbiological suspension in an aeration tank;
  • Walk you through the procedure of each MLVSS calculation formula so you can calculate MLVSS on your own; and
  • Explain the difference between MLSS and MLVSS in wastewater treatment.

Activated sludge process

As civilization increased in size and scope, one issue became more and more pressing: how to deal with waste, namely, human waste? While not the most pleasant of topics, when you have a city with millions of souls, this becomes an existential issue, as dumping it into a river or pit will likely cause the nearby sources of drinking water to become contaminated. This is what causes many of the cholera epidemics found throughout the world.

The issue of waste management got so bad in London that the summer of 1858 was known as 'The Great Stink', and led to the construction of the world's first modern sewer system, designed by Sir Joseph Bazalgette.

We have come a long way since then. Now, instead of dumping sewage straight into bodies of water, much of the developed world attempts to remove waste material from the water before reintroducing it into the water system.

Part of this system is the activated sludge process. Here, raw, untreated wastewater enters an aeration tank. It is then treated with aerobic microorganisms and the air is pumped through the vessel. The microorganisms digest much of the organic matter in the sewage, and the contents of the tank are moved to a clarifier-settler tank, where clean water can be extracted. Some of the sludge that is left behind is reintroduced back into the aerobic tank as activated sludge. This is where the microorganisms originally came from.

MLVSS definition

That brief description of modern wastewater treatment probably isn't enough for your keen inquisitive mind; you're probably still asking "What is MLVSS?!" As MLVSS stands for mixed liquor volatile suspended solids, it may be useful to go through what each of these terms means:

  • Mixed liquor – The name for the concoction of raw wastewater and activated sludge found in the aeration tank.
  • Volatile – Components of the liquor that are destroyed when exposed to temperatures of 550 °C. In this case, all volatile solids are assumed to be the bacteria that break down the sewage. This is the key difference between the MLSS and MLVSS definitions.
  • Suspended solids – The mass or concentration of solids in the liquor that do not dissolve but are instead dispersed throughout the liquid. The cut-off point between a suspended solid and a dissolved solid is usually taken to be 2 microns.

From this, we can derive the MLVSS definition: The mass or concentration of suspended bacteria within a combination of raw sewage and activated sludge. The more precise industrial definition of MLVSS for wastewater is:

"The concentration or mass of bacteria needed to break down a certain amount of organic material in sewage."

MLVSS procedure for industry

Before we get to the MLVSS calculation formula, it is worth discussing all of the terms that you need to accurately calculate the MLVSS of wastewater in an industrial setting.

  • Flow – The volume of mixed liquor that enters (or exits) the aeration tank per unit time. While this is usually given in MGD (millions of gallons per day) or as millions of m3 per day, you can input any value of flow into our tool for ease of calculation. It'll then do the conversion into MGD for you!
  • COD – Chemical oxygen demand. This is the amount of oxygen required to oxidize every atom or molecule in the mixed liquor that can be oxidized. COD is very closely related to BOD (biological oxygen demand), the concentration of oxygen needed to allow the aerobic bacteria to break down the organic matter – their "food". COD is preferred to BOD because BOD usually takes 5 days to determine, while we can find COD in around 2 hours. There are certain terms that you need to know that involve COD:
    • Average untreated in-fluent COD – The average COD of the mixed liquor entering the aeration tank.
    • Average primary treated COD – The average COD of the mixed liquor after being treated in the aeration tank.
    • Primary effluent COD – The COD of the system while it is in the aeration tank.
    • COD added to aeration – The amount of oxygen needed to be added to the aeration tank to meet the system's oxygen needs, given as mass per unit time.
  • Aeration volume - This is the volume of the aeration tank, in which part of the activated sludge process takes place. This requires four variables to calculate, or you can input the volume directly:
    • Length;
    • Width;
    • Sidewall depth; and
    • Free-board (the distance between the top of the liquor and the top of the sidewall).
    Ore use directly our tank volume calculator to speed up the task!
  • F/M ratio – Food to microorganism ratio. This is the amount of digestible organic matter entering the aeration tank to the amount of bacteria in the aeration tank. This ratio is usually between 0.3 and 0.6, and it will vary depending on the needs of the process: this will affect the growth of the bacteria population as you can learn at our generation time calculator.

MLVSS calculation formula

The tool actually contains several different formulas that allow you to calculate the MLVSS of wastewater, and so we will now go through each of the industrial MLVSS procedures one-by-one. The first converts flow in gallons per hour to flow in millions of gallons per day:

 ⁣FlowMGD=FlowGPH×241000000\!\scriptsize {\text{Flow}}_{\text{MGD}} = \frac{{\text{Flow}}_{\text{GPH}} \times 24}{1000000}


  • FlowMGD{\text{Flow}}_{\text{MGD}} – Flow in millions of gallons per day; and
  • FlowGPH{\text{Flow}}_{\text{GPH}} – Flow in gallons per hour.

If you wish, you can easily substitute m3 for gallons: if you need help with this conversion visit our volume converter.

The next determines the COD of the effluent:

 ⁣PrimaryeffluentCOD=Averageuntreatedin-fluentCODAverageprimarytreatedCOD\!\scriptsize {\begin{gather*}\text{Primary}\\ \text{effluent}\\ \text{COD}\end{gather*}} = {\begin{gather*}\text{Average}\\ \text{untreated}\\ \text{in-fluent} \text{COD}\end{gather*}} - {\begin{gather*}\text{Average}\\ \text{primary}\\ \text{treated} \text{COD}\end{gather*}}

What each of these variables means is described in the section above. The units of concentration used for each of them aren't important – all that matters is that they're the same.

The following formula uses the two previous MVLSS calculation formulae to find the mass of oxygen needed to meet the COD each day:

 ⁣CODadded toaeration=FlowMGD×PrimaryeffluentCOD×8.34\!\scriptsize \begin{gather*}\text{COD}\\ \text{added to}\\ \text{aeration}\end{gather*} = \text{Flow}_{\text{MGD}} \times {\begin{gather*}\text{Primary}\\ \text{effluent}\\ \text{COD}\end{gather*}} \times 8.34


  • Primary effluent COD{\text{Primary effluent COD}} – Given in mg per liter;
  • 8.348.34 – Conversion factor between millions of mg per liter and pounds per gallon; and
  • COD added to aeration{\text{COD added to aeration}} – Given in pounds per day.

Again, you can find the definition of these variables in the section above. Of note, one million mg per liter equals 8.34 pounds per gallon, which is why that conversion factor is used. If you are using other combinations of units for millions of volume per time and concentration, we recommend you use our calculator here and change the units as appropriately or find the correct conversion factor with our dedicated concentration calculator.

To find the value of MLVSS as a concentration, you will need to find the volume of the aeration tank (this is essential if you wish to find the concentration of MLVSS in wastewater treatment):

 ⁣V=l×w×(Side wall depthFree-board)\!\scriptsize V = l \times w \times (\text{Side wall depth} - \text{Free-board})


  • VV – Volume of the aeration tank;
  • ll – Length of the tank;
  • ww – Width of the tank;
  • Sidewall depth\text{Sidewall depth} – Total depth of the tank; and
  • Free-board\text{Free-board} – Distance between the top of the tank and the wastewater level.

You can use whatever units of length you like, as long as they are all the same (this will give you the units of volume in that unit cubed).

If we know the F/M ratio\text{F/M ratio}, we can now answer the question "How to find MLVSS?":

 ⁣MLVSS (mass)=COD added to aerationF/M ratio\!\scriptsize \text{MLVSS (mass)} = \frac{\text{COD added to aeration}}{\text{F/M ratio}}

The mass unit and time unit of both COD added to aeration\text{COD added to aeration} and F/M ratio\text{F/M ratio} must be the same for correct calculation of MLVSS. In the U.S.A., the units are usually lbs/day\text{lbs}/{\text{day}} and lbsday/lbs\text{lbs}\cdot\text{day}/\text{lbs}, respectively.

To calculate the MLVSS value as a concentration:

 ⁣MLVSS (conc.)=MLVSS (mass)V×8.34×106\!\scriptsize \text{MLVSS (conc.)} = \frac{\text{MLVSS (mass)}}{V \times 8.34 \times 10^{-6}}

How to calculate MLVSS in a laboratory

It is often easier to use the MLVSS calculation formula for laboratories than take numerous on-site readings. To do this, you must follow two MLVSS procedures: one to calculate the concentration of MLSS (mixed liquor suspended solids) in the liquor, and one to find the concentration of fixed solids. To find the MLSS concentration, you must take a volume of the wastewater, filter it, dry it thoroughly, and finally weight it:

 ⁣MLSS (conc.)=(Sample + filter weight)Filter weightVolume of sample\!\scriptsize \begin{align*} &\text{MLSS (conc.)} =\\\\ &\quad\frac{\text{(Sample + filter weight)} - \text{Filter weight}}{\text{Volume of sample}} \end{align*}

Regarding units, if you keep the weights the same, you won't have to deal with any pesky conversions!

Finding the fixed solids is very similar, but instead of thorough drying, you must heat the sample sufficiently to ensure conflagration:

 ⁣Fixed solids (conc.)=  (Ash + crucible weight)Crucible weightVolume of sample\!\scriptsize \begin{align*} &\text{Fixed solids (conc.)} =\\\\ &\ \ \frac{\text{(Ash + crucible weight)} - \text{Crucible weight}}{\text{Volume of sample}} \end{align*}

The same rules for units apply as above. The final step is then to subtract fixed solids from MLSS to get MLVSS:

 ⁣MLVSS(conc.)=MLSS (conc.)Fixedsolids (conc.)\!\scriptsize \begin{gather*}\text{MLVSS}\\ \text{(conc.)}\end{gather*} = \text{MLSS (conc.)} - \begin{gather*}\text{Fixed}\\ \text{solids (conc.)}\end{gather*}

🙋 We made a complementary tool for the MLVSS calculator: the wastewater calculator!


What is MLSS and MLVSS in wastewater treatment?

MLSS is the amount of solid material suspended in wastewater, while MLVSS is the amount of bacterial material suspended in wastewater. This is because MLSS is short for mixed liquor suspended solids, while MLVSS stands for mixed liquor volatile suspended solids.

How do I calculate MLVSS?

To calculate the MLVSS of industrial wastewater:

  1. Find the flow of the aeration tank in millions of gallons per day.
  2. Multiply that value by the COD of the primary effluent in milligrams per liter and 8.34.
  3. Divide the value from Step 2 by the food to microorganism ratio in pounds. This result is the MLVSS in pounds.

Is MLVSS the same as VSS?

All MLVSS is VSS, while not all VSS is MLVSS. This is because MLVSS are volatile suspended solids found specifically within mixed liquor, also known as wastewater or sewage.

What is the MLVSS when the COD is 3000 lb per day and the F/M ratio is 0.65?

In this instance, the MLVSS in pounds is 4615. This result is found by dividing the COD of the primary effluent by the food to microorganism ratio. If you divide this poundage by the volume of the wastewater in the aeration tank and 8.34, you'll get the MLVSS as a concentration

Mixed liquor suspended solid experiment

Fixed solids experiment

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