Engineering calculators · Hydraulics
Hydraulic retention time and tank sizing calculator
Calculate the hydraulic retention time of a tank from its volume and the flow through it, or the volume you need for a target retention time. The calculator reports HRT at average and peak flow, allows for dead volume and several tanks in series or parallel, and converts the volume into tank dimensions: diameter for a circular tank or length and width for a rectangular one, with freeboard added to the wall height. A reference table of typical retention times lets you check the result against the usual design range for the process.
Inputs
Results
- Aeration tank (conventional activated sludge): HRT at average flow is 5.40 h, within the typical 4.00 h to 8.00 h.
- At peak flow the HRT falls to 2.16 h, below the typical minimum. Check performance at peak as well as average.
- Freeboard of 0.5 m is within the usual 0.3 to 0.6 m allowance.
HRT across the flow range
| Flow, × average | Flow, m³/h | Flow, L/s | HRT, h | HRT, min | Surface loading, m/h |
|---|---|---|---|---|---|
| 0.5 | 50.0 | 13.9 | 10.8 | 648 | 0.375 |
| 0.75 | 75.0 | 20.8 | 7.20 | 432 | 0.563 |
| 1 | 100 | 27.8 | 5.40 | 324 | 0.750 |
| 1.5 | 150 | 41.7 | 3.60 | 216 | 1.13 |
| 2 | 200 | 55.6 | 2.70 | 162 | 1.50 |
| 2.5 | 250 | 69.4 | 2.16 | 130 | 1.88 |
| 3 | 300 | 83.3 | 1.80 | 108 | 2.25 |
Typical hydraulic retention times
| Process | Typical HRT | Basis | Source |
|---|---|---|---|
| Grit chamber (aerated) | 2 min to 5 min | at peak flow | [me] |
| Primary settlement tank | 1.5 h to 2.5 h | at average flow | [me] |
| Aeration tank (conventional activated sludge) | 4.0 h to 8.0 h | at average flow | [me] |
| Aeration tank (extended aeration) | 18 h to 36 h | at average flow | [me] |
| Final (secondary) clarifier | 2.0 h to 4.0 h | at average flow | [me] |
| Flow equalisation tank | 4.0 h to 12 h | of average daily flow | [wef] |
| Chlorine contact tank | 30 min to 1.0 h | at peak flow (CT basis) | [epa] |
| Flocculation tank | 20 min to 45 min | at design flow | [me] |
| Rapid mix (coagulant) | 0 min to 1 min | at design flow | [me] |
| Anaerobic digester (mesophilic) | 15 d to 25 d | sludge retention | [me] |
How the calculation works
- Effective volume. Veff = V × (1 − dead fraction). In the target-HRT mode, Veff = HRTtarget × Qavg and the liquid volume is Veff ÷ (1 − dead fraction).
- Retention time. HRT = Veff ÷ Q at average and at peak flow [M&E]. Flow is converted to m³/h first, so HRT is in hours (× 60 for minutes).
- Several tanks. Liquid volume per tank = V ÷ n. In parallel each tank takes Q ÷ n; in series each takes Q. Total HRT is the same either way.
- Dimensions. Surface area per tank A = Vtank ÷ depth. Circular: D = √(4A ÷ π). Rectangular with ratio r = L:W: W = √(A ÷ r), L = r × W. Wall height = liquid depth + freeboard.
- Surface loading = flow per tank ÷ A, in m³/m²/d (equal to m/d) and m/h, which is the overflow rate used to size settlement tanks [WEF].
Worked example with the default inputs
Average flow 100 m³/h, peak 250 m³/h, liquid volume 600 m³ in two parallel tanks with 10% dead volume. Effective volume = 600 × 0.9 = 540 m³. HRT at average = 540 ÷ 100 = 5.4 h (324 min); at peak = 540 ÷ 250 = 2.16 h. Each tank holds 300 m³ and at 4.5 m depth has an area of 66.7 m², giving a diameter of √(4 × 66.7 ÷ π) = 9.2 m. Each tank takes 50 m³/h, so the surface loading is 50 × 24 ÷ 66.7 = 18 m³/m²/d. With 0.5 m freeboard the wall height is 5.0 m.
What this cannot tell you. HRT is a nominal figure. It does not describe mixing, short-circuiting or the residence time distribution, which need tracer tests or CFD. It is not a process design on its own: aeration tanks are sized on sludge age and oxygen demand, settlement tanks on surface and solids loading, and contact tanks on the CT value with a baffling factor. Use the typical ranges as a sanity check against the design guide your project follows, and confirm depth and freeboard against BS EN 12255 and the structural design.
Sources
- Metcalf & Eddy / AECOM (2014) Wastewater Engineering: Treatment and Resource Recovery, 5th ed. (typical detention times for grit chambers, primary and secondary clarifiers, aeration tanks, chlorine contact tanks)
- Water Environment Federation, Manual of Practice No. 8: Design of Water Resource Recovery Facilities, 6th ed. (unit process design criteria)
- BS EN 12255 series, Wastewater treatment plants (part 4 primary settlement, part 6 activated sludge, part 10 safety and part 16 physical filtration)
- US EPA (1999) Wastewater Technology Fact Sheet: Chlorine Disinfection (contact time of at least 30 minutes at peak flow)
Frequently asked questions
What is hydraulic retention time?
Hydraulic retention time (HRT), also called detention time, is the average time water spends in a tank: HRT = volume ÷ flow. It is a nominal figure because real tanks short-circuit, so the effective HRT is lower. Dead volume in this calculator allows for that.
Should I calculate HRT at average or peak flow?
Both. Biological reactors and settlement tanks are usually sized on average flow and then checked at peak, because the peak HRT governs settling and washout. Grit chambers and chlorine contact tanks are sized on peak flow directly, since they must perform during the highest flow. The calculator reports both and the process check uses the basis given in the reference table.
How do tanks in series differ from tanks in parallel?
In parallel each tank takes a share of the flow, so the HRT in each tank equals the HRT of the whole volume. In series the whole flow passes through each tank in turn, so each tank has a shorter HRT but the total is the same. The total HRT is identical in both cases; what changes is the flow per tank and the hydraulic behaviour, which is closer to plug flow in series.
What is the difference between HRT and sludge age?
HRT is how long the liquid stays. Sludge age, or solids retention time (SRT), is how long the biomass stays, and in activated sludge it is much longer than the HRT because sludge is recycled. A plant with 6 hours HRT may have a sludge age of 10 to 20 days. Use the activated sludge calculator for SRT.