Hydraulic Retention Time Calculator
Calculate how long water or wastewater remains in a tank.
How Is Hydraulic Retention Time Calculated?
A hydraulic retention time calculator estimates the theoretical time water or wastewater remains in a tank based on the effective tank volume and the flow rate through the tank.
The tank volume is first converted into cubic metres and the flow rate is converted into cubic metres per day. The hydraulic retention time is then calculated by dividing the tank volume by the flow rate.
HRT (days) = Tank Volume (m³) ÷ Flow Rate (m³/day)
How Is HRT Converted Into Hours and Minutes?
The calculated hydraulic retention time is first obtained in days when tank volume is expressed in cubic metres and flow rate in cubic metres per day. The result is then converted into hours and minutes.
HRT (minutes) = HRT (hours) × 60
What Units Can Be Used for HRT Calculation?
Tank volume can be entered in cubic metres, litres, kilolitres, cubic feet or US gallons. Flow rate can be entered in cubic metres per day, KLD, MLD, cubic metres per hour, litres per second, litres per minute or US gallons per minute.
The calculator converts the selected units to a common basis before calculating the hydraulic retention time. This allows different combinations of volume and flow units to be used in the same calculation.
How Do Tank Volume and Flow Rate Affect HRT?
Hydraulic retention time increases when tank volume increases while the flow rate remains unchanged. Conversely, increasing the flow rate reduces the calculated retention time for the same tank volume.
For example, doubling the tank volume at the same flow rate doubles the theoretical HRT. Doubling the flow rate at the same tank volume reduces the theoretical HRT by half.
Hydraulic Retention Time in Water and Wastewater Treatment
HRT is commonly used as a hydraulic parameter for water and wastewater treatment units, including tanks, equalization basins, biological treatment units, sedimentation units and other liquid-holding processes.
The appropriate HRT depends on the treatment process, hydraulic loading, wastewater characteristics and other process-specific design conditions. HRT alone does not determine treatment performance.
What Is Theoretical Hydraulic Retention Time?
The calculated HRT represents a theoretical hydraulic retention time based on tank volume and flow rate. It assumes that the stated volume and flow rate adequately represent the liquid passing through the process unit.
Actual hydraulic behaviour can differ because of dead zones, short-circuiting, mixing conditions, variable flow, sludge accumulation and differences between total tank volume and effective liquid volume.
Why Is Effective Tank Volume Important?
HRT should be based on the effective liquid volume available to the process rather than automatically using the total physical volume of a tank.
Where part of a tank volume is occupied by sludge, equipment or other materials, the effective liquid volume may be lower than the total geometric volume. The appropriate volume should therefore reflect the actual operating condition of the process unit.
What Does the Hydraulic Retention Time Calculator Estimate?
The calculator estimates theoretical hydraulic retention time from the entered tank volume and flow rate. Results are provided in hours, days and minutes after converting the input units to a common basis.
The calculated value is a hydraulic estimate and should not be treated as a process-specific treatment design criterion. Final design should consider the applicable treatment process, loading conditions, effective volume, hydraulic characteristics and relevant engineering design criteria.