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Detention Time Calculator - Hydraulic Retention Time with Compliance Check

Calculate theoretical and effective detention time for any basin, tank, or lagoon

Free hydraulic detention time calculator for water and wastewater treatment operators. Enter basin dimensions or volume and flow rate to get theoretical detention time, effective detention time with dead zone correction, and compliance status against regulatory minimums. Supports rectangular tanks, circular clarifiers, and lagoons with built-in design criteria for primary clarification, secondary clarification, aeration, chlorine contact, equalization, digesters, and lagoons.

Pro Tip: The detention time on paper is almost never what you get in reality. Short-circuiting, dead zones, and inlet turbulence can reduce effective contact time by 15-40%. T10 (the time for 10% of the water to pass through) is typically 30-70% of theoretical detention time depending on basin geometry and baffling.

Calculate CT values using your contact time

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Detention Time Calculator
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How It Works

  1. Choose Your Input Method

    Enter basin volume directly in gallons or cubic feet, or provide dimensions (length × width × depth for rectangular, or diameter × depth for circular) and the calculator converts to volume.

  2. Enter Flow Rate

    Input your flow in MGD, GPM, or GPD. Use average daily flow for normal operation or peak flow to check worst-case detention time.

  3. Select Process Type

    Choose from primary clarification, secondary clarification, aeration basin, chlorine contact chamber, equalization, digesters, or lagoons. Each has built-in regulatory minimum ranges.

  4. Adjust Dead Zone Factor

    Default is 15% for well-baffled basins. Increase to 25-40% for poorly baffled or aging basins. The effective detention time accounts for short-circuiting.

Built For

  • Operators checking compliance after flow increases
  • Engineers verifying basin sizing during design reviews
  • Inspectors evaluating chlorine contact time adequacy
  • Operators planning for seasonal peak flow impacts on detention time
  • New operators understanding theoretical vs. effective retention time

Frequently Asked Questions

Detention time = Volume / Flow rate. Convert both to consistent units: Volume in gallons divided by Flow in gallons per day gives detention time in days. Multiply by 24 to get hours. For example, a 500,000-gallon basin at 1 MGD flow: 500,000 / 1,000,000 = 0.5 days = 12 hours.
Theoretical detention time assumes all water stays in the basin for the calculated period - perfect plug flow. Effective detention time accounts for dead zones, short-circuiting, and poor flow distribution. Multiply theoretical by (1 minus your dead zone factor). A basin with 20% dead zones at 12 hours theoretical has 9.6 hours effective detention time.
Most states require a minimum of 15-30 minutes of contact time at peak flow for chlorine disinfection. The key number is T10 - the time for 10% of the water to pass through - which is what you use for CT calculations. T10 is always less than theoretical detention time.
Insufficient detention time means treatment processes cannot complete. Clarifiers with short detention time produce high-TSS effluent. Aeration basins without enough time cannot fully oxidize BOD. Contact chambers without adequate time fail CT disinfection requirements. Every process has a minimum time requirement set by physics and regulation.
Disclaimer: This calculator provides estimates for planning and educational purposes. Actual detention time depends on basin geometry, baffling, flow distribution, and operating conditions. Always verify with tracer studies or T10 measurements for regulatory compliance. Follow your state permit requirements.

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