Chemistry
Hydraulic Retention Time Calculator
Work out the average time wastewater spends in a reactor, and compare it with the solids retention time.
Your inputs
Change any value to update the result instantly.
Hydraulic retention time
2 days
In hours
48 hours
Daily throughput
600 m³/day
Solids retention time
Enter the sludge figures to get the SRT
Difference
HRT only — the water residence time
HRT is about the water; SRT is about the microorganisms. Biological treatment is designed around both, and they are deliberately different in an activated-sludge plant.
What this calculator does
Gives the hydraulic retention time of a reactor and, optionally, the solids retention time for comparison.
What your result means
HRT is how long the water stays; SRT is how long the biomass stays, and the two are deliberately different.
Formula and working
HRT = V / Q
- 1
Collect the inputs
Reactor volume = 1,200 m³; Influent flow = 600 m³/day; Sludge in the reactor (0 to skip SRT) = 0 kg; Sludge wasted per day = 0 kg/day
- 2
Apply the formula
HRT = V / Q
- 3
Result
Hydraulic retention time = 2 days
Frequently asked questions
Symbols and assumptions
- HRT
- Hydraulic retention time
- SRT
- Solids retention time
- V
- Reactor volume
- Q
- Influent flow
- • Steady influent flow.
- • Sludge figures are on a dry-mass basis.
Sources and review
Reviewed 2026-09-22 · Next review 2027-09-22
- • HRT alone does not describe biological performance; activated-sludge design depends on both HRT and SRT.
Important limitation
This is an educational estimate based only on the values and formula shown. Verify current rates, rules, units, and professional standards before relying on the result. It is not personalized financial or investment advice.
Report a mistake or request an improvement
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