The MLVSS calculator above converts a mixed liquor suspended solids concentration and its volatile fraction into mixed liquor volatile suspended solids, then uses that figure to derive the three numbers an activated sludge basin is usually described by: the solids inventory held in the basin, the food-to-microorganism ratio, and the solids retention time. It works in either US customary or metric units and shows the volatile and fixed split as a bar.
Arb Digital publishes this as an arithmetic and education tool for operators in training, engineering students and anyone reading a plant's daily sheet. Every formula it uses is standard operator mathematics. What the resulting numbers mean for a particular plant, and what should be done about them, is a matter for the plant's own operating strategy, its engineer and the permit issued by the regulator with jurisdiction.
What This MLVSS Calculator Does
The first calculation is simple: MLVSS is MLSS multiplied by the volatile fraction. The rest follows from converting concentrations into masses. A concentration in milligrams per litre becomes a mass once you multiply it by a volume, and every one of the derived figures — inventory, F/M, SRT — is a ratio of masses or a mass divided by a mass flow.
The tool reports the MLVSS concentration as the headline, then the mass of volatile solids actually held in the aeration basin, the F/M ratio in pounds of BOD per pound of MLVSS per day, the solids retention time in days, and the daily BOD load arriving at the basin. The bars show how much of the mixed liquor is volatile and how much is fixed, which is a useful sanity check: a volatile fraction that drifts is telling you something about what is coming into the plant.
Boundary worth stating: this page is the aeration basin's solids inventory. Plant-level organic and hydraulic loading — what arrives at the works and how it is distributed — is the job of the wastewater loading calculator, and how long liquid spends in a given tank is the job of the detention time calculator. The three are designed to sit alongside each other without repeating one another.
How to Use It
- Pick your unit system. US customary uses million gallons and pounds; metric uses cubic metres and kilograms. The ratios come out identical either way, which is a useful check that the conversion factors are right.
- Enter MLSS and the volatile fraction from the laboratory sheet. The volatile fraction is normally reported as a percentage of the total suspended solids.
- Enter the basin volume and the flow reaching it, along with the BOD concentration in that flow.
- Enter the wasting figures — waste sludge flow and its solids concentration — plus the final effluent suspended solids, so the SRT accounts for both routes solids leave by.
- Read F/M and SRT together. They are two views of the same inventory and they move in opposite directions; looking at one alone hides half the picture.
The Formulas and How They're Calculated
MLVSS = MLSS × volatile fraction. With 3,000 mg/L MLSS at 78 percent volatile, MLVSS is 2,340 mg/L.
Mass in the basin = concentration × volume × conversion factor. In US units the factor is 8.34, the weight in pounds of a gallon of water, so 2,340 mg/L in a 1.0 million gallon basin is 2,340 × 1.0 × 8.34 = 19,515.6 pounds of volatile solids. In metric the factor is 0.001, so the same concentration in a 3,785 m³ basin is 2,340 × 3,785 ÷ 1,000 = 8,857 kilograms.
F/M = BOD load per day ÷ MLVSS in the basin. A flow of 2.0 MGD at 180 mg/L BOD is 180 × 2.0 × 8.34 = 3,002.4 pounds of BOD a day. Divided by 19,515.6 pounds of MLVSS that gives an F/M of 0.15 per day.
The 8.34 factor and the mass-balance approach behind all of this are standard operator mathematics; the EPA's Math for Wastewater Operators training material covers the same ground for flow, volume and concentration conversions.
SRT = solids inventory ÷ solids leaving per day. Solids retention time is conventionally taken on total suspended solids rather than volatile, so the inventory is 3,000 × 1.0 × 8.34 = 25,020 pounds. Wasting 0.02 MGD at 8,000 mg/L removes 8,000 × 0.02 × 8.34 = 1,334.4 pounds a day, and 15 mg/L over the weir in the remaining 1.98 MGD removes another 247.7 pounds. Dividing 25,020 by 1,582.1 gives an SRT of about 15.8 days.
Why MLVSS Rather Than MLSS
Suspended solids in mixed liquor are a mixture of biological cells, non-living organic material and inert mineral matter. Only the first of those is doing the treatment. The volatile suspended solids determination separates the organic portion from the inert one by burning the dried residue and measuring what is lost, and the loss on ignition is taken as the organic share. That is why F/M is properly expressed against MLVSS: dividing food by a figure that includes grit and silt would understate the loading on the organisms that are actually consuming it.
It is still only a proxy. Volatile solids include dead cells, cell debris and any non-degradable organic material the plant happens to be receiving, none of which are metabolising anything. A works receiving a strong industrial organic load can carry a volatile fraction that overstates its active biomass considerably. The determination itself is defined in Standard Methods under the total, fixed and volatile solids procedures, and the laboratory doing the work is the authority on how its own numbers should be read.
How F/M and SRT Relate
The two describe the same inventory from opposite ends. F/M asks how much food each unit of biomass is being offered each day; SRT asks how long an average solid particle stays in the system before it is removed. Increase the inventory by wasting less and F/M falls while SRT rises; waste more heavily and F/M rises while SRT falls. Because the two are so tightly coupled, quoting one without the other tells only half the story.
SRT is the more fundamental of the pair, because it sets which organisms can persist. Any organism that cannot double faster than solids are being removed is washed out of the system, which is why the retention time governs whether slow-growing populations establish at all. That is a design and operating consideration for the plant's engineer, and the target for any specific works comes from its process design and its discharge permit — not from a general figure quoted online.
What These Numbers Do Not Tell You
None of these figures is a verdict. They are indicators calculated from a handful of laboratory results, and each of those results carries sampling and analytical error. A grab sample taken at the wrong moment in a diurnal flow cycle can move the calculated F/M substantially without anything at the plant having changed. Settleability, dissolved oxygen, temperature, nutrient balance and microscopic examination all sit alongside these numbers, and none of them appear here.
Discharge quality itself is governed by permit. In the United States, the EPA's Secondary Treatment Standards set minimum technology-based requirements for publicly owned treatment works in terms of five-day biochemical oxygen demand, total suspended solids and pH, with the numeric limits carried in the regulation itself and in each plant's own permit. That is a statement of where the requirements live, in one jurisdiction, and not a target this page is proposing.
Sampling Errors That Move the Answer Most
The waste sludge concentration is the single most influential input on SRT, and it is also the hardest to sample well. Solids concentration in a wasting line varies over the course of a wasting cycle, and a sample drawn at the start of a draw is not the same as one drawn at the end. A twenty percent error there moves the SRT by roughly twenty percent, straight through.
The second is basin volume. Nominal design volume and actual working volume diverge as soon as a basin is partly out of service, a weir is set differently or solids accumulate in a corner. Because volume appears in both the inventory and the SRT, an out-of-date figure biases everything downstream of it. If you are converting between volume units to check a figure, our unit converter handles the arithmetic, and the percentage calculator is useful for working the volatile fraction back and forth.
Arb Digital publishes hundreds of free, no-signup tools covering water and wastewater, laboratory maths, engineering and everyday arithmetic — no accounts, no stored data.
Browse All Free Tools Contact Arb DigitalCommon Mistakes to Avoid
- Using raw influent BOD when primary clarifiers are in service. The basin only sees the primary effluent, and using the raw figure inflates F/M.
- Calculating F/M against MLSS instead of MLVSS, which mixes inert mineral solids into a ratio that is meant to be about biomass.
- Leaving effluent solids out of the SRT denominator. Solids over the weir leave the system just as surely as solids that are wasted.
- Using design basin volume rather than the volume actually in service when a basin or a pass has been taken offline.
- Reading a single day's figures as a trend. These indicators are noisy day to day and only mean something as a moving average.
Related Free Tools From Arb Digital
For plant-level organic and hydraulic loading, use the wastewater loading calculator; for tank residence times, the detention time calculator. On the laboratory side, the chemical oxygen demand calculator and the total dissolved solids calculator cover neighbouring determinations, and the log reduction calculator handles count ratios. For unit work there is the unit converter. Browse the full free online tools hub for the rest.
Frequently Asked Questions
Multiply the mixed liquor suspended solids concentration by the volatile fraction. At 3,000 mg/L MLSS and a volatile fraction of 78 percent, MLVSS is 2,340 mg/L. The volatile fraction comes from igniting the dried solids residue and measuring the loss.
MLSS is every suspended solid in the mixed liquor, including inert mineral matter. MLVSS is only the organic portion, measured as loss on ignition, and is used as a proxy for biomass because inert grit and silt do no treatment.
Divide the BOD load arriving at the basin each day by the mass of MLVSS held in the basin. With 3,002 pounds of BOD a day against 19,516 pounds of MLVSS, the F/M ratio is about 0.15 per day.
It is the weight in pounds of one gallon of water. Multiplying a concentration in mg/L by a volume in million gallons and then by 8.34 converts the concentration into pounds, which is what makes the mass ratios work in US customary units.
Conventionally on total suspended solids, because SRT tracks the residence of all solids in the system rather than only the organic share. F/M is the figure that uses MLVSS.
That is not a question a general calculator can answer. Operating targets follow from the process design, the wastewater characteristics and the discharge permit for that specific works, and they belong to the plant's engineer and the regulator with jurisdiction.
No. It measures organic suspended solids, which include dead cells, cell debris and any non-degradable organic material present. It is a widely used approximation of biomass, not a count of viable organisms.
This calculator applies standard operator formulas to figures you supply and does not assess, certify or verify the performance of any treatment works. Operating targets and discharge requirements are set by the plant's process design, its engineer, and the permit issued by the water authority or environmental regulator with jurisdiction, on data from an accredited laboratory.
