MLVSS Calculator — Mixed Liquor Volatile Suspended Solids
Calculate Mixed Liquor Volatile Suspended Solids (MLVSS) from MLSS and the volatile (organic) fraction, then compute the Food-to-Microorganism (F/M) ratio, hydraulic retention time (HRT) and organic loading rate for an activated-sludge aeration basin.
mg/L
m³/day
mg/L
m³
MLVSS = MLSS × Volatile fraction (VSS/TSS)
- 1
Volatile (organic) fraction
0.75 × 100 = 75 %Organic fraction of MLSS — the active biological mass in the basin. - 2
MLVSS
3,000 × 0.75 = 2,250
How does this calculator work?
MLVSS (mg/L) = MLSS × VSS/TSS fraction (typically 0.70–0.80). For a 10,000 m³/day plant with 200 mg/L BOD in a 5,000 m³ basin at MLVSS 2,250 mg/L: F/M ≈ 0.178 kg BOD/(kg VSS·d) and HRT ≈ 12 h — conventional activated sludge. Adjust MLSS and volatile fraction to your measured values.
Formula
How this is calculated
In activated-sludge wastewater treatment, the aeration basin contains a suspension of microorganisms called mixed liquor. Mixed Liquor Suspended Solids (MLSS, mg/L) is the total concentration of particles; Mixed Liquor Volatile Suspended Solids (MLVSS, mg/L) is the combustible (organic/biological) fraction — the fraction that burns off at 550 °C in the laboratory. MLVSS ≈ MLSS × f_v, where f_v is the VSS/TSS ratio. For municipal wastewater, f_v is typically 0.70–0.80; for industrial effluents it may differ significantly.
MLVSS is used as a proxy for the active biomass in the reactor — the microorganisms that consume the organic load. The Food-to-Microorganism (F/M) ratio, also written F:M, quantifies how hard the biomass is working: F/M = BOD loading rate (kg BOD/day) ÷ VSS mass in basin (kg VSS). Low F/M (0.05–0.15) characterises extended aeration with highly stabilised effluent; conventional plants run 0.2–0.4; high-rate processes may reach 0.4–1.0. Very low F/M can cause bulking sludge; very high F/M produces poorly settled, dispersed growth.
The Hydraulic Retention Time (HRT = V ÷ Q) is the average time wastewater spends in the basin. The Organic Loading Rate (OLR = Q × BOD ÷ V ÷ 1000) normalises BOD load to basin volume. All calculations assume steady-state conditions and a completely mixed reactor; in practice, plug-flow or sequential batch reactors behave differently. Densities, settling characteristics and temperature effects are not included.
Frequently asked questions
Conventional activated-sludge plants typically operate at MLVSS 1,500–3,000 mg/L (MLSS 2,000–4,000 mg/L with f_v ≈ 0.75). Extended-aeration plants (oxidation ditches) may use MLVSS up to 4,000–6,000 mg/L at very low F/M ratios.
MLSS includes inert inorganic solids (sand, clay, precipitates) that do not consume oxygen or BOD. MLVSS measures only the organic (volatile) fraction — a better approximation of the active microbial biomass responsible for treatment. F/M and SRT calculations based on MLVSS are more meaningful biologically.
A mixed-liquor sample is filtered through a glass-fibre filter, dried at 105 °C to give TSS (total suspended solids = MLSS), then ignited at 550 °C for 2 hours. The mass lost on ignition is the volatile fraction (MLVSS). The volatile fraction f_v = MLVSS ÷ MLSS, typically 0.65–0.85 for domestic wastewater.
Also known as
TG we-Calculate Editorial Team. (2026). MLVSS Calculator — Mixed Liquor Volatile Suspended Solids [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/mlvss-calculator
TG we-Calculate Editorial Team. "MLVSS Calculator — Mixed Liquor Volatile Suspended Solids." TG we-Calculate. 2026. https://we-calculate.com/calculator/mlvss-calculator.
TG we-Calculate Editorial Team, "MLVSS Calculator — Mixed Liquor Volatile Suspended Solids," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/mlvss-calculator
@misc{wecalculate_mlvss_calculator, title = {MLVSS Calculator — Mixed Liquor Volatile Suspended Solids}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/mlvss-calculator}}, year = {2026}, note = {TG we-Calculate} }
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