Inputs
- HRT –
- As days –
- As minutes –
Required Volume for Target HRT
Computed from V = HRT × Q.
Example Scenarios
| # | Unit / Tank | Volume | Flow | HRT (h) | HRT (d) |
|---|---|---|---|---|---|
| 1 | Aeration Tank | 1,500 m³ | 4,500 m³/d | – | – |
| 2 | Primary Clarifier | 2,000 m³ | 85 m³/h | – | – |
| 3 | Anaerobic Digester | 3,500 m³ | 1.2 MGD | – | – |
| 4 | Contact Tank | 180 m³ | 2,200 L/min | – | – |
| 5 | Equalization Basin | 750,000 L | 900 m³/h | – | – |
Formula Used
Hydraulic retention time is the theoretical average time a parcel of water remains in a reactor or tank:
HRT = V / Q
- V = working volume of the tank.
- Q = influent flow rate (consistent units).
In this tool, internal conversions normalize units to m³ and m³/h. Outputs are presented as hours, days, and minutes. For target HRT, required volume is V = HRT × Q.
Note: Real systems may deviate due to short-circuiting, dead zones, baffling, or stratification. Consider using a dispersion or tanks-in-series model when accuracy is critical.
How to Use This Calculator
- Enter the reactor volume and select its unit.
- Enter the flow rate and select the relevant unit.
- Optionally provide a target HRT to compute required volume.
- Click Calculate to see results, or tweak inputs for instant recalculation.
- Use Download CSV and Download PDF to export results and the example table.
Tip: For peaking conditions, evaluate with peak-hour flow to ensure compliance under worst-case loading.
FAQs
1) What typical HRT ranges are used for common units?
Primary clarifiers often target 1–2 hours; aeration basins can vary from 4–12 hours (conventional) to 12–24 hours (extended aeration); anaerobic digesters often use several days. Check local design standards.
2) How is HRT different from solids retention time (SRT)?
HRT is based on liquid residence time; SRT (or sludge age) is the average time solids remain in the system and depends on wasting and biomass inventory. They are conceptually independent.
3) Should I use average or peak flow?
Use average flow to assess typical operation and peak flow to test worst-case performance and check compliance during high-load events.
4) How do multiple tanks in series affect results?
Total HRT equals the sum of volumes divided by flow, but baffling or tanks-in-series can reduce short-circuiting and better approach plug flow behavior.
5) Do I need to correct for dead zones?
Yes, effective volume may be less than geometric volume due to dead zones. Use tracer studies or computational models to estimate effective volume for critical applications.
6) Can I include recycle streams?
Yes. If internal recycles materially change the net influent flow to the reactor, include them in Q to reflect the hydraulic loading the tank experiences.
7) What about variable flow over the day?
Evaluate multiple scenarios or use diurnal profiles. The example table lets you compare cases; design checks commonly include average, peak hour, and low-flow conditions.