Settling tanks

Highly indicative calculation of retention time, surface overflow and weir loading for secondary clarifiers and primary settling tanks, plus solids surface loading.

Clarifier and primary settling hydraulics

This tool provides a very rough check of retention time, surface overflow rate and weir loading for one secondary clarifier or primary settling tank. A second section estimates solids surface loading (kg/m²/h). Results are indicative only and primarily intended for quick sanity checks, not for detailed final design.

Average daily flow Q24 [m³/day]

Design maximum flow Qmax [m³/day]

Water surface area A [m²]

Tank volume V [m³]

Total weir length L [m]

Tank type (for recommended ranges)

Solids surface loading (indicative)

Influent flow to clarifier Q_in [m³/day]

Return sludge flow Q_R [m³/day]

Influent solids concentration X_in [g/L]

Surface area for solids loading check [m²]

Results are for quick checks only. For borderline designs or unusual configurations, always perform a more detailed hydraulic analysis and follow the relevant standards and design guidelines.

Hydraulic parameters of the tank

Retention time for Q24 [h]: –

Retention time for Qmax [h]: –

Surface overflow for Q24 [m³/m²/h]: –

Surface overflow for Qmax [m³/m²/h]: –

Weir loading for Qmax [m³/m/h]: –

recommended retention time Qmax ≈ 1 h; recommended retention time Q24 ≈ 2.0–3.0 h; recommended surface overflow Qmax ≈ 1.8–2.4 m³/m²/h; recommended surface overflow Q24 ≈ 0.7–1.4 m³/m²/h

Solids surface loading

from influent only (no return sludge) [kg/m²/h]: –

from influent + return sludge [kg/m²/h]: –

Indicative guide: secondary clarifiers are often kept below about 0.1–0.3 kg/m²/h; primary settlers typically below roughly 0.5–1.0 kg/m²/h (depending on wastewater and operation).

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