Hydraulic Detention Time (HRT)
Theoretical time water spends in a tank, basin, or contact chamber. Computed as volume divided by flow. Includes surface overflow rate output for sedimentation tanks and clarifiers.
Defaults: 500 kgal tank, 2 MGD flow, 2000 ft² surface area = typical primary clarifier. HRT does not equal real residence time — short-circuiting reduces effective contact to T10 < HRT.
HRT vs T10 vs SRT
HRT is the theoretical residence time and assumes plug-flow with no short-circuiting. Real tanks have dead zones where some flow bypasses quickly. T10 — the time at which 10% of a tracer pulse has passed through — is always less than HRT and is the conservative number for disinfection compliance.
SRT (solids retention time, sludge age) is different from HRT in activated sludge: water passes through quickly while solids are retained much longer because of clarifier underflow recycle. Typical activated sludge: HRT 4–8 hr, SRT 5–15 days.
Typical HRTs by process
- Grit chamber (aerated): 3–5 minutes
- Primary clarifier: 1.5–2.5 hours
- Activated sludge basin: 4–8 hours (conventional) up to 36 hours (extended aeration)
- Secondary clarifier: 2–3 hours
- Chlorine contact chamber: 30 minutes (T10)
- Anaerobic digester: 15–30 days
- Stormwater wet pond: 24–72 hours target for water-quality volume
- Drinking-water flocculation: 20–30 minutes
- Drinking-water sedimentation: 2–4 hours
Surface overflow rate as a clarifier criterion
In a sedimentation tank, the controlling design parameter is not HRT but surface overflow rate (OFR = Q/A). Particles with settling velocity ≥ OFR are removed regardless of basin depth (Hazen 1904). HRT only matters insofar as it provides enough time for hindered settling and zone settling effects in concentrated suspensions.
Typical OFR ranges:
- Primary clarifier: 600–1200 gpd/ft² (24–49 m³/m²/d)
- Secondary clarifier (BOD removal): 400–800 gpd/ft² (16–33 m³/m²/d)
- Secondary clarifier (nitrification): 300–600 gpd/ft² (12–24 m³/m²/d)
- Tertiary filter: 2–5 gpm/ft² (5–12 m/h)
Reference: Metcalf & Eddy / AECOM (2014). Wastewater Engineering: Treatment and Resource Recovery, 5th ed. Hazen, A. (1904). "On Sedimentation." Trans. ASCE, 53, 45-71.