How this calculator works
Nominal hydraulic retention time is working volume divided by volumetric throughput. A straight-sided tank can use settling area times liquid depth. This steady-state ratio is an average hydraulic quantity: short-circuiting, dead zones and mixing produce a distribution of actual residence times. It does not predict treatment efficiency, settling performance or solids retention time. Volume and flow are converted to cubic metres and cubic metres per second before division.
Formula
HRT = V/Q; V = Q x HRT; for a straight-sided tank V = A h. Rearranging gives A = Q x HRT/h and h = Q x HRT/A.
Worked example
A 120 m³ tank receiving 10 m³/h has a nominal HRT of 12 h. A flow of 0.01 m³/s held for 3600 s requires 36 m³; over 20 m² that means 1.8 m depth.
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Frequently asked questions
Which volume should I enter?
Use the active liquid volume at operating level, excluding freeboard and displaced space.
Is hydraulic retention the same as sludge age?
No. Solids may be retained or recycled independently of the water flow.
Can I use changing flow?
A single average flow gives a planning estimate. Time-varying inflow or volume requires a dynamic balance.