What Is Detention Time?

Detention time (also called Hydraulic Retention Time, HRT, or Hydraulic Residence Time) answers one question: on average, how long does a unit volume of water spend inside a particular tank or basin? It's the ratio of the tank's volume to the flow rate passing through it.

Detention Time Formula

DT = Volume ÷ Flow Rate

Units must match. Volume in gallons ÷ Flow in GPD = DT in days. Convert to hours (× 24) or minutes (× 1,440)

The critical rule: units must cancel correctly. If your volume is in gallons and your flow is in gallons per day (GPD), then DT comes out in days. Multiply by 24 to get hours, or by 1,440 to get minutes. Common conversions on the exam:

Volume in gallons ÷ GPD = days → × 24 = hours → × 60 = minutes
1 MGD = 1,000,000 GPD
Volume in cubic feet × 7.48 = volume in gallons

Live Detention Time Calculator

⚙ Detention Time Calculator

Tank Shape

Length (ft)

Width (ft)

Depth (ft)

Diameter (ft)

Depth (ft)

Flow Rate

Flow Unit

Result

Volume Formulas Reference

Rectangle / Box Tank

V = L × W × D × 7.48

Length × Width × Depth (all in feet) × 7.48 gal/ft³ = gallons

Circle / Cylinder (Clarifier, Tank)

V = 0.785 × D² × Depth × 7.48

0.785 × Diameter² × Depth (all in feet) × 7.48 = gallons

Cone (Sludge Hopper)

V = 0.333 × 0.785 × D² × H × 7.48

1/3 of the cylinder volume. H = height of cone in feet

Three Worked Examples

Example 1 - Rectangular Clarifier

A rectangular clarifier is 60 ft long, 20 ft wide, and 12 ft deep. The plant flow is 1.5 MGD. What is the detention time in hours?

Volume (ft³) = 60 × 20 × 12 = 14,400 ft³
Volume (gal) = 14,400 × 7.48 = 107,712 gallons
Flow (GPD) = 1.5 MGD × 1,000,000 = 1,500,000 GPD
DT (days) = 107,712 ÷ 1,500,000 = 0.07181 days
DT (hours) = 0.07181 × 24 = 1.72 hours

Example 2 - Circular Tank

A circular clarifier has a 40 ft diameter and is 15 ft deep. Flow is 0.75 MGD. What is the detention time?

Volume (ft³) = 0.785 × 40² × 15 = 0.785 × 1,600 × 15 = 18,840 ft³
Volume (gal) = 18,840 × 7.48 = 140,923 gallons
Flow (GPD) = 0.75 × 1,000,000 = 750,000 GPD
DT (days) = 140,923 ÷ 750,000 = 0.1879 days
DT (hours) = 0.1879 × 24 = 4.51 hours

Example 3 - Reverse: Solve for Volume

A contact tank needs a minimum detention time of 4 hours with a plant flow of 2 MGD. What tank volume (in gallons) is required?

Rearranged: Volume = DT × Flow Rate
DT = 4 hours = 4/24 days = 0.1667 days
Flow (GPD) = 2 × 1,000,000 = 2,000,000 GPD
Volume = 0.1667 × 2,000,000 = 333,400 gallons
In cubic feet: 333,400 ÷ 7.48 = 44,572 ft³

Why Detention Time Matters

Detention time isn't just an exam concept - it's a critical design and operational parameter that determines whether your treatment process actually works.

Sedimentation

Settling basins typically require 2-4 hours detention time to allow suspended particles to settle out by gravity. Too short and particles carry through to filtration, overloading it.

Disinfection (CT)

CT values (Concentration × Time) require a minimum contact time for disinfection credit. A 4-hour DT at 0.5 mg/L residual gives CT = 120 min × 0.5 = 60 mg·min/L.

Coagulation / Floc

Flash mixing needs only 30-60 seconds. Flocculation basins need 20-40 minutes. Knowing these design targets helps you answer "what's wrong with this process?" questions.

The take-away: if detention time is too short, treatment is inadequate. If it's too long in the wrong process (like aerobic digestion), you can create other problems. The exam will test whether you understand these relationships, not just the arithmetic.

Detention Time Questions? Ask Randy.

Randy can walk you through detention time problems step by step, explain the design criteria for different processes, and quiz you on the types of problems that appear on your exam level.

Heather Heltzinger

Licensed Class C Water & Wastewater Operator | Founder, Renaissance Labs LLC

23+ years of experience in SCADA systems and treatment plant management. CO-100 Top 100 Small Business winner. Built RandyAI to give operators the study tools she wished she had.

Time Unit Trap: Calculate detention time in hours when comparing to treatment standards (contact time for disinfection = hours, not minutes). A 2-minute result might be 120 minutes OR 2 hours depending on flow units. Read carefully what the question is asking for.

Practice Problem

Problem

A clarifier basin holds 75,000 gallons. If water flows through at 150 GPM, what is the detention time in hours?

Detention Time (HRT) Calculations