Biological process calculator

Biomass Concentration Calculator

Estimate steady-state MLVSS and MLSS concentration from removed BOD, SRT, HRT, yield, endogenous decay, and the selected MLVSS/MLSS fraction.

Preliminary steady-state mass balance: This calculator estimates biomass associated with removed BOD. It does not include influent inert solids, endogenous-residue accumulation, chemical solids, or measured effluent solids losses; use plant data and a calibrated process model for final design.

Input parameters

Enter design flow, loading, and kinetic assumptions.

Average Influent Flow

m³/day

Hydraulic Retention Time (HRT)

hours; used to calculate bioreactor volume

Solids Retention Time (SRT)

days; selected average biological solids residence time

Organic loading

Influent Total BOD₅

mg/L as BOD₅

BOD Removal

percent of influent BOD assumed removed biologically

Biokinetic and solids assumptions

True Biomass Yield (Y)

g VSS formed per g BOD removed

Endogenous Decay Coefficient (k_d)

day⁻¹

MLVSS / MLSS Fraction

decimal fraction used to convert estimated MLVSS to MLSS

Calculated results

Concentrations are reported as mg/L; biomass values are VSS-based unless labelled MLSS.

Enter values and select Calculate to view the steady-state biomass estimate.

Equations used

The estimate starts with removed BOD, applies an observed-yield adjustment for endogenous decay at the selected SRT, and converts the biomass inventory into concentration using the HRT-derived reactor volume.

V = Q × HRT ÷ 24

BOD removed = Q × S₀ × R_BOD ÷ 1,000

Y_obs = Y ÷ (1 + k_d × SRT)

P_X,VSS = BOD removed × Y_obs

MLVSS inventory = P_X,VSS × SRT

MLVSS = MLVSS inventory × 1,000 ÷ V

MLSS = MLVSS ÷ f_MLVSS/MLSS

F/M = BOD removed ÷ MLVSS inventory

Scope and interpretation

HRT, SRT, food-to-microorganism ratio, sludge yield, and biomass are interrelated operating variables in activated sludge and MBR systems. Longer SRT can support higher solids concentration and lower F/M, but practical solids concentration is also constrained by wastewater characteristics, oxygen transfer, membrane fouling, viscosity, and wasting strategy. MBR biological-process guidance [1] Activated-sludge biokinetics training [2]