Calculate hydraulic detention time (HDT/HRT) and solids retention time (SRT) for wastewater treatment tanks, clarifiers, and aeration basins. Supports US gallons, MGD, cubic meters, and liters — and solves for volume, flow rate, or detention time.
✓ Verified: Ten States Standards & EPA Design Manual for Wastewater Facilities — April 2026
Enter a valid tank volume.
Enter a valid flow rate.
Enter target detention time in hours.
Enter flow rate in m³/day.
Enter volume in m³.
Enter required detention time in hours.
Enter total volatile suspended solids (kg).
Enter daily solids wasting rate.
Enter to calculate SRT/HRT ratio
Hydraulic Detention Time
—
💡
Was this calculator helpful?
✓ Thanks for your feedback!
Sources & Methodology
🛡️Design criteria from Ten States Standards, EPA Municipal Wastewater Treatment Design Manual, and Metcalf & Eddy Wastewater Engineering.
📋
Ten States Standards — Recommended Standards for Wastewater Facilities (2004)
Primary US design reference specifying minimum detention times for all treatment processes. glc.org/tenstate
📖
EPA Design Manual for Municipal Wastewater Treatment
US EPA reference providing design parameters including HRT, SRT, and hydraulic loading rates. epa.gov
📕
Metcalf & Eddy: Wastewater Engineering, 5th Ed.
Standard engineering textbook covering HDT, SRT, activated sludge design, and all major wastewater treatment calculations.
Key Formulas:
HDT (hr) = Volume (L) / Flow (L/hr)
Volume needed (m³) = HDT (hr) × Flow (m³/day) / 24
Max Flow (m³/day) = Volume (m³) × 24 / HDT (hr)
SRT (days) = Mass VSS in system (kg) / VSS wasted per day (kg/day)
HDT = V/Q | SRT = X_system / X_waste
Example: V = 500,000 gal, Q = 1 MGD: HDT = 500,000/1,000,000 = 0.5 days = 12 hours
SRT: 5,000 kg VSS in system, wasting 500 kg/day: SRT = 5000/500 = 10 days
Hydraulic Detention Time: The Core Wastewater Design Parameter
Hydraulic detention time (HDT), also called hydraulic retention time (HRT) or hydraulic residence time, is the most important parameter in designing tanks and reactors for water and wastewater treatment. It defines the average time a parcel of water spends inside a treatment unit — and therefore how long it is exposed to settling, biological oxidation, disinfection, or chemical reaction.
The formula is simply HDT = Volume / Flow Rate. However, unit consistency is critical. Volume and flow must share the same volume unit. A common error is dividing 500,000 gallons by 1 MGD and forgetting that MGD means 1,000,000 gallons per day — yielding 0.5 days (12 hours) not 500,000 days.
Typical Detention Times by Treatment Process
Treatment Unit
Typical HDT
Authority
Rapid / Flash mix
30 sec – 2 min
Ten States Standards
Flocculation basin
20 – 30 min
Ten States Standards
Primary clarifier
1.5 – 2.5 hr (peak)
Ten States §73
Conventional activated sludge
4 – 8 hours
Metcalf & Eddy
Extended aeration
18 – 36 hours
EPA Design Manual
Secondary clarifier
1.5 – 2.5 hr (peak)
Ten States §73
Aerobic digester
16 – 18 days min
40 CFR Part 503
Anaerobic digester
15 – 30 days
Ten States §74
Chlorine contact chamber
15 – 45 min
EPA CT Tables
HRT vs SRT: The Critical Distinction
In a simple tank without recycle, HRT and SRT are identical. But in activated sludge systems with return sludge (RAS), they are completely decoupled. HRT may be 6 hours while SRT is 10–20 days, because biomass is recycled while liquid passes through quickly. This distinction is critical: different microorganisms need different minimum SRTs to grow. Heterotrophs need 3–5 days, nitrifiers need 8–12 days at 20°C, and anaerobic methanogens need 15–30 days.
💡 SRT Controls Biology: Increasing SRT enables nitrification, increases MLSS, reduces excess sludge, but increases oxygen demand. Decreasing SRT produces more sludge faster, but risks losing nitrification and causing bulking. SRT is the primary biological control lever in activated sludge operation.
Frequently Asked Questions
Detention time (HDT/HRT) is the average time liquid spends in a treatment tank: HDT = Volume / Flow Rate. It determines treatment exposure time — longer HDT allows more settling, biological reaction, or disinfection. It is the most fundamental wastewater treatment design parameter.
HDT = Volume / Flow Rate in consistent units. Example: 500,000 gal tank, 1 MGD flow: HDT = 500,000 / 1,000,000 = 0.5 days = 12 hours. Ensure units match: both gallons, both m³, etc.
Primary clarifiers require 1.5 to 2.5 hours at peak flow and 2 to 4 hours at average flow. Ten States Standards require minimum 2 hours at peak hourly flow. Secondary clarifiers use similar 1.5 to 2.5 hour ranges at peak flow.
SRT = Total VSS mass in system (kg) / VSS wasted per day (kg/day). SRT controls which microorganisms survive. Heterotrophs need SRT of 3-5 days; nitrifiers need 8-12 days at 20°C; methanogens need 15-30 days. SRT is the primary operational control variable in activated sludge.
HRT = time liquid stays in reactor (V/Q). SRT = time biological solids stay in system (mass/waste rate). In activated sludge with return sludge, SRT (10-20 days) far exceeds HRT (4-8 hours) because biomass is recycled. Without recycle, HRT = SRT.
Conventional activated sludge: 4-8 hours HRT, 5-15 days SRT. Extended aeration: 18-36 hours HRT, 20-30 days SRT. Oxidation ditch: 18-30 hours HRT. Higher HRT and SRT improve treatment but require larger tanks and more energy.
Longer HDT improves removal up to a point. In clarifiers, more settling occurs. In biological reactors, microorganisms have more contact time with waste. However doubling HDT does not double removal — optimal HDT balances performance with capital cost and energy.
CT = disinfectant concentration (mg/L) x detention time (minutes). EPA requires minimum CT values for Giardia and Cryptosporidium inactivation under SWTR. Longer detention allows lower disinfectant doses for the same CT. Chlorine contact chambers are designed for 15-45 minutes HDT.
Hours for short processes (clarifiers 1.5-4 hr, aeration tanks 4-8 hr). Days for biological processes (SRT 5-15 days, digesters 15-30 days). Volume and flow must be in consistent units. Common pairs: m³ / (m³/hr) = hours; million gal / MGD = days.
Clearwell minimum detention depends on CT requirements under EPA SWTR. Many systems require 30 minutes to several hours. Baffling efficiency (T10/T) typically ranges from 0.3 (unbaffled) to 1.0 (ideal plug flow), significantly affecting effective CT.