Enter your mixed-liquor MLSS result and the matching volatile or fixed solids result to calculate MLVSS concentration in mg/L.
This is the volatile portion of the reported MLSS. It is not interchangeable with total MLSS.
Check that the operation matches the form of the second value on the laboratory report.
Table of contents
How to use our MLVSS Calculator
- Under "Laboratory result available," choose the type of second result shown on the current mixed-liquor laboratory report.
- Enter the Mixed Liquor Suspended Solids result in "MLSS concentration (mg/L)."
- For "Use volatile solids percent," enter the reported whole percent. For example, enter 75 when the report says 75%, not 0.75.
- For "Use fixed solids concentration," enter the fixed-solids result in mg/L. It must come from the same mixed-liquor sample and concentration basis as MLSS.
- Select "Calculate." Read "MLVSS concentration" first, then check "Calculation used" to make sure it matches the report form you chose.
Definitions
MLSS: Mixed Liquor Suspended Solids. This is the total suspended-solids concentration in an aeration-basin mixed-liquor sample.
MLVSS: Mixed Liquor Volatile Suspended Solids. This is the volatile portion of MLSS, reported in mg/L.
Volatile solids: The portion of suspended solids measured as volatile during the laboratory solids test.
Fixed solids: The nonvolatile portion of the suspended-solids result. In the fixed-solids method, it is subtracted from MLSS.
mg/L: Milligrams per liter, a unit of concentration.
Common mistakes and quick fixes
Mistake: Entering 0.75 when the report says 75% volatile solids.
Fix: Enter 75. The "Volatile solids (percent)" field expects a whole percent, not a decimal fraction.
Mistake: Entering fixed solids greater than MLSS.
Fix: Check the units, sample identity, and copied numbers. Fixed solids are part of MLSS, so they cannot be greater than MLSS.
Mistake: Using total suspended solids from a non-mixed-liquor sample as MLSS.
Fix: Use the MLSS result from the sampled aeration-basin mixed liquor. Do not substitute a general TSS result unless the report identifies it as mixed liquor.
Limitations & Key Assumptions / Boundary Conditions
- The calculation depends on the accuracy of the laboratory results entered.
- For the fixed-solids method, MLSS and fixed solids must use the same mixed-liquor sample and concentration basis.
- The result is a concentration only. It does not assess aeration-basin performance, settling, sludge age, wasting rate, or permit compliance.
- MLVSS is the volatile portion of MLSS and is not interchangeable with total MLSS.
Methodology
Calculation
The selected report form determines the formula. If the report gives volatile solids as a percentage, the calculator multiplies MLSS by the entered percentage divided by 100.
MLVSS (mg/L) = MLSS (mg/L) x volatile solids (%) / 100
If the report gives fixed solids as a concentration, the calculator subtracts fixed solids from MLSS.
MLVSS (mg/L) = MLSS (mg/L) - fixed solids (mg/L)
Worked example
An MLSS result of 3,000 mg/L with 75% volatile solids gives:
MLVSS = 3,000 x 75 / 100 = 2,250.0 mg/L
The same MLVSS result occurs when MLSS is 3,000 mg/L and fixed solids are 750 mg/L:
MLVSS = 3,000 - 750 = 2,250.0 mg/L
The calculator accepts zero values, blocks negative concentrations and percentages outside 0% to 100%, and stops if fixed solids are greater than MLSS.