Detention Time Calculator — Hydraulic Retention Time (HRT)
Enter a tank volume and volumetric flow rate to calculate the hydraulic detention time — the average time a unit of liquid spends inside the reactor or basin.
m³
Flow rate unit
Very long — typical for anaerobic digestion
- 1
Flow rate (m³/h)
100 ÷ 24 = 4.1667Convert daily flow to hourly so volume and flow share the same time unit. - 2
Detention time (h)
500 ÷ 4.1667 = 120
How does this calculator work?
Hydraulic detention time = V / Q, where V is tank volume (m³) and Q is volumetric flow rate (m³/h or m³/d). Enter volume and flow to get HDT in hours, days and minutes. Typical design values: 2–4 h for clarifiers, 4–8 h for aerobic biological treatment, days to weeks for anaerobic digestion.
Formula
How this is calculated
Hydraulic detention time (HDT), also called hydraulic retention time (HRT) or hydraulic residence time, is the theoretical time a parcel of liquid spends inside a treatment vessel. It is calculated as the tank volume divided by the volumetric flow rate: HDT = V / Q. If flow is given in m³/day, the formula gives detention in days; dividing by 24 converts to hours.
In water and wastewater treatment, HDT is a key design parameter because biological and chemical processes need a minimum contact time to achieve the required removal efficiency. Activated-sludge aeration tanks are typically designed for 4–8 hours HDT; clarifiers for 2–4 hours; anaerobic digesters for several days. An HDT that is too short means the liquid exits before the treatment process completes; too long wastes capital in oversized tankage and can cause process upsets like septicity in aerobic systems.
Note that HDT is a theoretical mean value assuming ideal plug-flow or completely-mixed conditions. Real tanks have short-circuiting, dead zones and dispersion that reduce effective contact time. A dispersion analysis or tracer test gives the actual distribution of residence times in a physical vessel.
Frequently asked questions
HDT (or HRT) is the average time liquid stays inside a treatment tank, calculated as tank volume divided by the volumetric flow rate. A 500 m³ tank with 100 m³/day inflow has an HDT of 5 days (120 hours).
It depends on the process: rapid-mix chambers ≈ 1–5 minutes; flocculation basins ≈ 20–40 minutes; settling/clarification ≈ 2–4 hours; activated-sludge aeration ≈ 4–8 hours; anaerobic digesters ≈ 15–30 days. These are typical design ranges — site-specific conditions and permit requirements govern the actual design.
HDT (hydraulic detention time) is based on liquid flow only; SRT (solids retention time or sludge age) is based on the biological solids in the system. In most biological processes SRT > HDT because solids are recycled (e.g. return activated sludge) to maintain biomass even as liquid passes through quickly.
Also known as
TG we-Calculate Editorial Team. (2026). Detention Time Calculator — Hydraulic Retention Time (HRT) [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/detention-time-calculator
TG we-Calculate Editorial Team. "Detention Time Calculator — Hydraulic Retention Time (HRT)." TG we-Calculate. 2026. https://we-calculate.com/calculator/detention-time-calculator.
TG we-Calculate Editorial Team, "Detention Time Calculator — Hydraulic Retention Time (HRT)," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/detention-time-calculator
@misc{wecalculate_detention_time_calculator, title = {Detention Time Calculator — Hydraulic Retention Time (HRT)}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/detention-time-calculator}}, year = {2026}, note = {TG we-Calculate} }
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