# Calculating the Hydraulic Retention Time (HRT) or t (tau)

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## Calculate the Hydraulic Retention Time

Below you can find the typical formulas, coefficients and empirical data used to establish the basic design of a complete mix suspended growth biological sewage wastewater process.

To use this calculator please fill in the input value data of your sewer wastewater and as a result some basic plant design and effluent BOD data will follow out of the calculations. If the normal sewer water characteristics are unknown, please press the "typical data" button for an auto fill of the input fields.

Documentation
This formula will establish the Hydrolic Retention Time of an municipal wastewater treatment plant:

Where:
Characteristics of Primary Sedimentation Effluent influent soluble substrate concentration (bsCOD) S0 = BOD or bsCOD g/m3 X0,i = g/m3 or mg/l XT = g/m3 or mg/l SRT = d k = g COD/g∙d Y = g VSS/g COD used kd = g VSS/g VSS∙d Ks = g COD/m3 fd = g VSS/g VSS t = d
 t = Hydraulic Retention Time (HRT) [d] SRT = Sedimentation Retention Time [d] XT = total MLVSS in aeration tank (MLVSS) [g VSS/m3] Y = biomass yield [g VSS/g COD used] S0 = influent soluble substrate concentration (bsCOD) [g bsCOD/m3] S = effluent soluble substrate concentration (bsCOD) [g bsCOD/m3] fd = fraction of biomass that remains as cell debris [g VSS/g VSS] kd = endogenous decay coefficient [g VSS/g VSS∙d] Xo,i = nbVSS concentration in influent [g nbVSS/m3]