Biological process calculator

Effluent Permeate BOD & COD Calculator

Estimate preliminary MBR permeate BOD₅ from soluble and particulate fractions, or use the alternative SRT-based kinetic equation to estimate permeate COD.

Preliminary mass-balance tool: In an MBR, membrane separation strongly controls the particulate portion of permeate BOD, while biological treatment is critical to residual soluble BOD. Enter representative, site-specific fractions and removal assumptions; this calculator does not replace pilot testing or calibrated process modelling.

Input parameters

Enter average BOD fractions and performance assumptions.

Average Influent Flow

m³/day

Influent Total BOD₅

mg/L as BOD₅

Influent Soluble-BOD Fraction

decimal fraction of total BOD₅; particulate fraction is calculated as 1 minus this value

Biological and membrane assumptions

Biological Soluble-BOD Removal

percent removal of the soluble-BOD fraction

Biological Particulate-BOD Removal / Conversion

percent removal of the particulate-BOD fraction before membrane separation

Membrane Retention of Residual Particulate BOD

percent physical retention of residual particulate BOD

Permeate BOD₅ Target

mg/L; used only for the target check

Calculated results

BOD concentrations are shown as mg/L BOD₅; loads use average influent flow.

Enter values and select Calculate to view the permeate-BOD estimate.

Alternative calculation method

Kinetic Permeate COD Estimate

Use the user-specified SRT-based equation as an alternative preliminary estimate of permeate COD: S = K_s × (1 + k_d) × SRT ÷ [SRT(μ_m − k_d) − 1].

Kinetic COD inputs

The values below are the typical assumptions requested for this method.

Solids Retention Time (SRT)

days; typical input shown as 15 days

Half-Saturation Constant (K_s)

g COD/m³; numerically equal to mg/L

Endogenous Decay Coefficient (k_d)

g VSS/(g VSS·day), equivalent to day⁻¹

Maximum Specific Growth Rate (μ_m)

g VSS/(g VSS·day), equivalent to day⁻¹

Kinetic COD result

Results follow the selected SRT kinetic equation.

Enter kinetic values and select Calculate COD.

Interpretation: This is an alternative kinetic estimate, not a conversion between BOD and COD and not a replacement for measured permeate COD. The denominator must remain positive for the formula to be valid.

Equations used

The calculation separates total influent BOD into soluble and particulate fractions. Biological removal is applied to both fractions; membrane retention is applied only to the residual particulate fraction.

Influent BOD load = Q × S₀ ÷ 1,000

Soluble BOD = S₀ × f_S

Particulate BOD = S₀ × (1 − f_S)

Residual soluble BOD = Soluble BOD × (1 − R_S)

Permeate particulate BOD = Residual particulate BOD × (1 − R_M)

Predicted permeate BOD = Residual soluble BOD + Permeate particulate BOD

Alternative kinetic COD equation

S = K_s × (1 + k_d) × SRT ÷ [SRT × (μ_m − k_d) − 1]

Scope and interpretation

MBR treatment combines biological treatment with membrane separation. The membrane acts as the solids-separation barrier, but soluble BOD performance depends primarily on the biological process and wastewater characteristics. This calculator deliberately exposes these distinct assumptions. U.S. Bureau of Reclamation MBR technical report [1] MBR review article [2]