Hydraulic retention time calculator explained
Hydraulic retention time (HRT) describes how long liquid spends inside a reactor, basin, or clarifier. This calculator multiplies the plan area {settling_area} by side water depth {water_depth} to obtain volume {volume}, then divides by influent flow {inlet_flow} to give HRT {hrt}. It makes it simple to see how depth or flow adjustments change residence time.
Use it when designing contact tanks, comparing equalization basins, or troubleshooting why a bioreactor is washing out.
How the conversion works
HRT follows the same mass-balance relationship as detention time:
If you know the target HRT and flow, solve backward for the required volume or depth. This is critical for activated sludge (4-8 h), sequencing batch reactors (6-24 h), and disinfection contact basins where minimum contact time is regulated.
Units and conversions
| Quantity | Units | Notes |
|---|---|---|
| Area | m, ft | Surface area of the basin. |
| Depth | m, ft | Effective water depth. |
| Volume | m, ft | Product of and . |
| Flow | m/h, MGD | Must match volume units. |
| HRT | h, min | Convert seconds to hours by dividing by 3600. |
Worked examples
- Activated sludge basin
Basin area , depth , influent flow .
Increase depth to 5 m to reach a 3.75 h HRT if required by process kinetics.
- Disinfection contact tank sizing
Regulations require a 30 min HRT at peak flow (0.35 m/s). Desired depth .
Lay out a 10 m by 11.5 m tank to meet contact time requirements.
Tips and pitfalls
- Always base HRT calculations on peak design flow so the system complies during storms.
- Account for baffling; poorly baffled tanks have effective HRTs smaller than the ideal value.
- Subtract volumes occupied by mixers, sludge blankets, or internal piping before computing .
- Pair HRT with surface overflow rate (SOR); both metrics control clarifier performance.