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Tank Volume, Flow & Hydraulic Retention Time

Estimate nominal hydraulic retention time from working liquid volume and steady flow, with area-depth volume calculation.

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

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Formula and method

A working liquid volume of 100 m³ at 20 m³/h gives nominal hydraulic retention time 5 hours. An area of 50 m² at depth 2 m gives the same volume.

Nominal retention time = working liquid volume / flow; volume = area × depth

Worked example

Enter these known values and leave the other values blank.

Horizontal liquid area
50 m²
Uniform liquid depth
2 m
Steady volumetric inlet flow
20 m³/hr
Working liquid volume
100 m³
Nominal hydraulic retention time
5 hrs

Assumptions and limitations

  • Working volume, flow, area and depth are positive. Area times depth assumes a uniform cross section and does not account for internal displacement.
  • The volume/flow result is a nominal turnover time. Short-circuiting, dead zones and mixing can produce a distribution of actual fluid residence times.
  • Use a consistent operating basis for volume and steady flow. The result does not establish treatment performance, pollutant removal or solids retention time.

Common questions

Should I use the tank’s total shell volume?

Use its operating liquid volume after accounting for fill level and displaced volume. Empty headspace does not contribute.

Is hydraulic retention the same as sludge age?

No. Hydraulic retention concerns liquid volume and flow. Solids retention depends on the solids inventory and removal rate.

References

Bookify replaces the captured rounded US gallon with the exact 3.785411784-liter definition. Volumetric flow rates use exact US or imperial fluid-volume definitions and exact seconds per minute, hour and day. US gallons per minute use exactly 0.003785411784 cubic meters per gallon and 60 seconds per minute.

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