Find the hydraulic retention time of a tank or basin from its volume and the flow through it, plus two companion clarifier-design parameters — the surface overflow rate and the weir loading rate. These three numbers govern whether a sedimentation or biological tank will perform.
Hydraulic retention time (HRT) is the average time a parcel of water spends inside a tank — and it is the single most important design parameter for sedimentation basins, equalisation tanks, and biological reactors alike. This calculator computes HRT from volume and flow, and adds the two surface-loading parameters that govern clarifier performance: the surface overflow rate and the weir loading rate.
HRT (also called detention time or θ) is simply θ = V / Q: the tank volume divided by the volumetric flow rate. It is the mean time a water molecule resides in the tank. In a sedimentation basin, the HRT must be long enough for target particles to settle out before the water exits. In a biological reactor, HRT controls how long microorganisms have to consume the organic substrate. Too short and treatment is incomplete; too long wastes tank volume and can cause septicity. Watch your units — V/Q in m³ ÷ (m³/day) gives days; multiply by 24 for hours.
The surface overflow rate (SOR), also called the surface loading rate or overflow velocity, is SOR = Q / A, the flow divided by the basin surface area. It has units of m³/m²·day, which simplify to m/day — it is effectively the upward velocity the water must rise. Any particle whose settling velocity exceeds the SOR will be captured regardless of where it enters; SOR is therefore the controlling design criterion for clarifiers, more fundamental than HRT for settling. Typical values: primary clarifiers 30–50 m/day, secondary (activated sludge) clarifiers 16–32 m/day at average flow.
The weir loading (or weir overflow) rate is Q divided by the total length of effluent weir, in m³/m·day. It measures how concentrated the outflow is along the weir. If the weir loading is too high, the local upward velocity near the weir lifts settled or settling solids back into the effluent, degrading clarity. Design guidance typically keeps weir loading below about 125–250 m³/m·day for clarifiers; plants add launder troughs or multiple weirs to increase weir length and lower the loading.
For primary clarifiers, target HRT 1.5–2.5 hours, SOR 30–50 m/day, weir loading under ~250 m³/m·day. For secondary clarifiers following activated sludge, HRT 2–4 hours and SOR 16–32 m/day at average flow (lower because the biological floc settles more slowly). Equalisation and aeration tanks have much longer HRTs — aeration basins commonly run 4–8 hours. Always check both average and peak-flow conditions, since peak flow shortens HRT and raises both surface and weir loading.
HRT (hydraulic retention time) is how long the water stays in the tank, V/Q. SRT (solids retention time, or mean cell residence time) is how long the biomass stays in the system, which in activated sludge is decoupled from HRT by recycling sludge. SRT controls the microbial population and treatment performance; HRT controls contact time and tank size. They are independent design levers.
Ideal sedimentation theory shows that whether a particle is captured depends on its settling velocity versus the overflow rate Q/A, not on detention time. Two tanks with the same HRT but different depths have different overflow rates and remove different particle sizes. That is why clarifiers are designed primarily on SOR, with HRT and depth as secondary checks.
Primary clarifiers are typically designed for 30–50 m³/m²·day (m/day) at average flow, and up to ~80–120 at peak. Secondary clarifiers after activated sludge use lower rates, roughly 16–32 m/day at average flow, because biological floc settles more slowly than primary solids. Local standards (e.g., Ten States Standards) give the governing values.
Excessive weir loading creates a strong local upflow near the weir that resuspends settling solids and carries them over into the effluent, raising turbidity and TSS. Keeping weir loading within recommended limits (commonly under ~125–250 m³/m·day) preserves a quiescent zone at the outlet. Adding more weir length — launders, finger troughs, or multiple weirs — is the standard fix.
HRT is inversely proportional to flow, so a peak flow of twice the average halves the HRT and doubles both the surface overflow rate and the weir loading rate. Because settling and clarification degrade at high loading, clarifiers are checked at the peak hourly flow as well as the average — the peak condition often governs the design.