HRT Calculator (Hydraulic Retention Time)
Nominal hydraulic retention time (HRT) estimates how long wastewater stays in a defined treatment volume, based on the volume and the flow through it.
Result
Hydraulic retention time
6 hours
Same time in days
0.25 days
At this flow, wastewater stays in this volume for about 6 hours on average. This is a nominal process reference, not a permit limit.
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The formula
HRT (hours) = V / Q × 24- V
- Tank volume (million gallons, MG)
- Q
- Influent flow rate (MGD)
Good to know
- Enter tank volume in million gallons (MG). For example, 250,000 gallons = 0.25 MG.
- Decide which zones count as the active volume and use the same basis every time you compare.
- Nominal HRT is a process reference, not a permit limit. Reference ranges vary by treatment configuration.
Read the full guide: Hydraulic Retention Time Wastewater: How to Calculate HRT
Frequently asked questions
How do I convert gallons to million gallons (MG)?
Divide gallons by 1,000,000. For example, 250,000 gallons is 0.25 MG.
Is HRT a permit limit?
No. Nominal HRT is a process reference, not a permit limit. Reference ranges vary by treatment configuration.
Which volume should I use?
Use the active treatment volume you are evaluating, and use the same basis every time you compare results.
Stop running this math by hand
ComplyFlo for wastewater plants can calculate and trend SRT, F/M, HRT, RAS, SVI and removal efficiency from your daily readings, next to your plant dashboard and DMR-ready reports.
See ComplyFlo for wastewater plantsFree checklist
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