MLVSS Calculator

How to use this MLVSS calculator

What this calculator does

This calculator estimates mixed liquor volatile suspended solids, or MLVSS, using either a laboratory gravimetric test or an operating food-to-microorganism calculation. In laboratory mode, it separates total suspended residue into fixed ash and volatile material. In industrial mode, it estimates the MLVSS mass required by a selected F/M ratio and converts that mass to an aeration-basin concentration. The result is a process calculation, not a substitute for an approved sampling plan, calibrated balance, replicate analysis, or permit-required laboratory method.

When to use it

Use the laboratory method when you have dried filter and ignited-ash weights from a mixed-liquor sample. Use the industrial method for preliminary activated-sludge loading checks, operator worksheets, process troubleshooting, or comparing a target biomass inventory with the COD load applied to an aeration basin. The U.S. EPA describes activated sludge as a suspended-growth biological treatment process and provides broader operational context in its municipal wastewater resources.

How to calculate

1. Select Method. Choose laboratory gravimetric data or the industrial F/M method.
2. Enter every required value in the units printed beside the field. Use a period as the decimal separator; commas and scientific notation are rejected to avoid ambiguous entries.
3. Read the large MLVSS concentration result and the supporting cards. The results update as you type.
4. Review Calculation breakdown to see the mass differences and conversions used.
5. Select Download Excel to export the current valid model, or Reset to restore the documented starting example.

Input guide

Method is required and changes the entire model. In laboratory mode, Sample volume is a positive number in milliliters; 100 mL is a typical calculation example, and a larger volume lowers the reported concentration for the same recovered mass. Weight of filter paper and Weight of filter paper + residue are gram measurements. The second must not be lower than the tare; confusing milligrams with grams creates a thousand-fold error. Weight of crucible and Weight of crucible + ash are also grams, and the ash-plus-crucible value must not be below the crucible tare.

In industrial mode, Flow is million gallons per day and must be nonnegative. Untreated influent COD and Primary-treated COD are concentrations in mg/L; treated COD cannot exceed influent COD in this simplified removal model. Food-to-microorganism ratio must be positive and is expressed as pounds of applied COD per pound of MLVSS per day. A lower selected ratio requires more biomass for the same load. Aeration volume is the actual liquid volume in million gallons and must be positive; a larger volume lowers the calculated MLVSS concentration while leaving required MLVSS mass unchanged.

Output guide

In laboratory mode, MLVSS concentration is the volatile fraction in mg/L. MLSS concentration is total dried suspended residue per sample volume, while Fixed solids is the ash remaining after ignition. Volatile share of MLSS is MLVSS divided by MLSS; a zero MLSS result produces 0% rather than a division error. Volatile residue mass is the gram difference between dried residue and fixed ash. A negative MLVSS would indicate inconsistent measurements and is rejected.

In industrial mode, the primary result is again MLVSS concentration in mg/L. The supporting results become Primary effluent COD, COD load to aeration, Required MLVSS mass, and F/M ratio. COD load uses the conventional 8.34 conversion factor for MGD and mg/L. These values are estimates driven by average loading assumptions, not direct laboratory observations.

Worked example

For a 100 mL sample, suppose the filter paper weighs 1.000 g and the dried filter plus residue weighs 1.250 g. The dried solids mass is 0.250 g, so MLSS is 0.250 × 1,000,000 ÷ 100 = 2,500 mg/L. If the crucible weighs 30.000 g and the crucible plus ash weighs 30.075 g, fixed solids mass is 0.075 g and fixed solids concentration is 750 mg/L. Therefore MLVSS is 2,500 – 750 = 1,750 mg/L, and the volatile share is 1,750 ÷ 2,500 × 100 = 70.00%. These values match the starting calculator state and exported workbook.

Learn more

Standard gravimetric solids work depends on representative sampling, controlled drying and ignition, desiccator cooling, and repeated weighing to constant mass. For regulatory context, consult the EPA's Clean Water Act analytical methods program and the agency's wastewater effluent-guideline resources. For activated-sludge operating concepts, an EPA technical document on characterizing the activated sludge process discusses control parameters used in aeration systems.