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How It Works
The mean residence time tells you how long, on average, a fluid element stays inside a reactor. For an ideal continuous-flow stirred-tank reactor (CSTR), it is simply the tank volume divided by the volumetric flow rate. This single number drives sizing decisions for chlorine contact chambers, aeration basins, and chemical reaction vessels.
Rearrange the same equation to find the required reactor volume for a target detention time, or the allowable flow rate for an existing tank.
Example Problem
A chlorine contact tank has a volume of 120 m³ and treats water at 0.04 m³/s. What is the mean residence time?
- tR = V / Q = 120 / 0.04 = 3,000 s (50 min)
This exceeds the typical 30-minute minimum for disinfection, confirming the tank is adequately sized.
Frequently Asked Questions
What is residence time in water treatment?
Residence time (also called detention time or hydraulic retention time) is the average time water stays in a treatment unit. It directly affects the degree of reaction -- longer times generally mean more complete treatment.
What is the difference between a CSTR and a plug-flow reactor?
In a CSTR, the contents are perfectly mixed so every element has the same composition. In a plug-flow reactor, fluid moves through in orderly fashion with no back-mixing. Real reactors fall somewhere between these two ideals.
How do you size a reactor for a target residence time?
Multiply the desired residence time by the design flow rate: V = tR × Q. For example, a 30-minute (1,800 s) target at 0.05 m³/s requires a 90 m³ reactor.
Related Calculators
- Microorganism Disinfection Calculator -- determine the CT value needed for pathogen inactivation.
- Mixing Design Calculator -- calculate power requirements for reactor mixing.
- Activated Sludge Calculator -- evaluate biological reactor performance through SVI.
- Flocculation Calculator — design flocculation basins that precede reactor contact chambers.
- BOD Calculator — measure organic loading entering the reactor.
- Volume Unit Converter — convert reactor volumes between liters, gallons, and cubic meters.