MLVSS Calculator

Mixed liquor volatile suspended solids, abbreviated MLVSS, is an operational measure used in biological wastewater treatment. It estimates the volatile portion of suspended material in mixed liquor from an aeration basin. That portion is often used as a practical indicator of biomass, although it can also include other volatile organic material. Operators track MLVSS alongside dissolved oxygen, sludge age, food-to-microorganism ratio, settling performance, and loading data to understand how an activated-sludge process is behaving. This calculator expects gravimetric masses from the same measured sample. Total suspended solids, or TSS, represents the dried residue retained by the test method. Non-volatile suspended solids, or NVSS, represents the ash remaining after ignition. Subtracting NVSS from TSS gives the volatile suspended-solids mass. Dividing that mass by the sample volume in liters produces MLVSS in milligrams per liter. If volume is entered in milliliters, the calculator divides it by 1,000 before performing the concentration calculation. For a sample containing 120 mg TSS and 40 mg NVSS in 50 mL, the volatile mass is 80 mg. The sample volume is 0.05 L, so 80 divided by 0.05 gives 1,600 mg/L MLVSS. The volatile fraction is calculated from the two measured masses: 80 divided by 120 times 100, or 66.67 percent. This percentage is distinct from the concentration and should not be calculated by dividing mg/L by the TSS mass. Reliable results depend on representative sampling and consistent laboratory technique. Mix the sample appropriately, use clean and conditioned filters or dishes, follow the applicable drying and ignition temperatures, cool samples in a desiccator, and record masses with adequate precision. TSS and NVSS must refer to the same sample volume and mass basis. An NVSS value greater than TSS is physically inconsistent and usually indicates an entry, weighing, labeling, or test-procedure error. MLVSS is useful for trending, but it is not a direct live-cell count. Inert organic solids can raise the result, while changes in influent composition can alter the relationship between MLVSS and active biomass. Compare measurements over time and interpret them with plant-specific operating targets. Unexpected shifts should prompt review of sampling, laboratory quality control, aeration, wasting, recycle flows, toxicity, and influent loading before process changes are made.