MLVSS Calculator
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Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
MLVSS = MLSS x (Volatile Fraction / 100); F/M = (Q x BOD) / (V x MLVSS)
MLVSS is calculated by multiplying the total Mixed Liquor Suspended Solids (MLSS) by the volatile fraction determined from lab ignition testing. The F/M ratio divides the daily organic load (flow x BOD concentration) by the total biomass in the aeration basin (volume x MLVSS concentration). Units must be consistent: flow and volume in same units, concentrations in mg/L.
Worked Examples
Example 1: Conventional Activated Sludge Plant
Problem:A plant has MLSS of 3,000 mg/L with 78% volatile fraction. The aeration basin is 750,000 gallons, flow is 2 MGD, and influent BOD is 180 mg/L. Calculate MLVSS and F/M ratio.
Solution:MLVSS = 3,000 x 0.78 = 2,340 mg/L\nF/M = (2,000,000 x 180) / (750,000 x 2,340)\nF/M = 360,000,000 / 1,755,000,000\nF/M = 0.205 day-1\nThis is within the conventional activated sludge range (0.2-0.5).
Result:MLVSS: 2,340 mg/L | F/M: 0.205 day-1 | Process: Conventional Activated Sludge
Example 2: Extended Aeration System
Problem:MLSS is 5,000 mg/L, 72% volatile. Aeration volume is 1,000,000 gallons, flow is 500,000 GPD, BOD is 150 mg/L.
Solution:MLVSS = 5,000 x 0.72 = 3,600 mg/L\nF/M = (500,000 x 150) / (1,000,000 x 3,600)\nF/M = 75,000,000 / 3,600,000,000\nF/M = 0.021 day-1\nThis is below the extended aeration range, indicating excess biomass.
Result:MLVSS: 3,600 mg/L | F/M: 0.021 day-1 | Process: Extended Aeration (consider wasting more sludge)
Frequently Asked Questions
What is MLVSS and why is it important?
MLVSS stands for Mixed Liquor Volatile Suspended Solids, which represents the organic (biological) portion of the total suspended solids in an activated sludge aeration basin. It is the primary measure of active biomass concentration in wastewater treatment. MLVSS is critical because it represents the living microorganisms that actually consume and break down organic pollutants (BOD). A typical MLVSS concentration ranges from 1,500-4,000 mg/L in conventional activated sludge systems. Operators use MLVSS to calculate the Food to Microorganism (F/M) ratio and Sludge Retention Time (SRT), both key process control parameters.
What is the difference between MLSS and MLVSS?
MLSS (Mixed Liquor Suspended Solids) measures the total suspended solids in the aeration basin, including both organic (volatile) and inorganic (fixed) components. MLVSS measures only the volatile (organic) fraction, which represents the active biological mass. The MLVSS/MLSS ratio typically ranges from 0.70 to 0.85 (70-85%). A ratio below 0.70 indicates high inorganic content, possibly from industrial inflows or old sludge. A ratio above 0.85 suggests young sludge or high organic loading. The ratio is determined by igniting dried solids at 550 degrees Celsius; the material lost is the volatile fraction.
How do you measure MLSS and MLVSS in the lab?
MLSS is measured by filtering a known volume of mixed liquor through a pre-weighed glass fiber filter (Standard Methods 2540D), drying at 103-105 degrees Celsius for at least one hour, cooling in a desiccator, and weighing. The weight gain represents total suspended solids. For MLVSS, the dried filter is then ignited at 550 degrees Celsius for 15-20 minutes in a muffle furnace, cooled, and reweighed. The weight lost during ignition is the volatile fraction. These tests should be performed at least daily for proper process control. Results are reported in mg/L.
What happens if MLVSS is too high or too low?
If MLVSS is too high (over 4,000 mg/L), the system may experience poor settling (bulking sludge), high oxygen demand that exceeds blower capacity, and high sludge blanket levels in the clarifier. The operator should increase sludge wasting (increase waste activated sludge flow). If MLVSS is too low (under 1,000 mg/L), there is insufficient biomass to treat the incoming load, resulting in poor effluent quality and high BOD/TSS in the discharge. The operator should decrease wasting and allow biomass to build up. Sudden drops in MLVSS may indicate toxic shock to the biomass requiring immediate investigation.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy