Primary Clarifier Sizing Calculator

Inputs

Unit System

Switching units keeps your entered numbers.

Design Basis

Use peak for conservative sizing checks.

Average Flow (m³/d)

Typical municipal plants use m³/d.

Peak Factor

Only applied when design basis is peak.

Surface Overflow Rate (m³/m²·d)

Area = flow ÷ overflow rate.

Weir Loading Rate (m³/m·d)

Checks effluent weir capacity.

Side Water Depth (m)

Used for volume and detention time.

Number of Clarifiers

Flow is divided across units equally.

Safety Factor

Applied to required surface area.

Minimum Diameter (m)

If set, computed diameter will not be smaller.

Maximum Diameter (m)

If set, computed diameter will not be larger.

Target Detention Time (hr)

Adds a sizing suggestion if the target is unmet.

Reset

Formula Used

Core sizing uses surface overflow and weir loading checks.

Design flow: Qd = Qavg or Qd = Qavg × PF
Surface area: A = (Qd / SOR) × SF
Area per unit: Au = A / N
Circular diameter: D = √(4Au/π)
Volume: Vu = Au × SWD
Detention time: t = (Vu × 24) / Qu hours
Weir length: Lw = πD
Weir loading: WLRach = Qu / Lw

How to Use

Select metric or imperial units based on your project.
Enter average flow and choose average or peak basis.
Provide overflow rate and weir loading target values.
Set depth, number of units, and a safety factor.
Optionally add diameter limits or detention time target.
Click Calculate to view results above the form.
Use CSV or PDF buttons to export your calculation sheet.

Example Data Table

Scenario	Avg Flow	Basis	SOR	WLR	SWD	N	SF	Result Diameter (per unit)	Detention Time
Municipal Example	20,000 m³/d	Peak (PF 2.5)	40 m³/m²·d	250 m³/m·d	3.5 m	2	1.10	~18.7 m	~1.9 hr
Conservative Check	35,000 m³/d	Peak (PF 3.0)	35 m³/m²·d	200 m³/m·d	4.0 m	3	1.15	~20.1 m	~2.3 hr

Example outputs are approximate and depend on rounding.

Design Inputs That Control Footprint

Primary clarifier sizing starts with the selected design flow, usually average or peak. The surface overflow rate sets the required plan area by dividing flow by the rate and applying a safety factor. Multiple basins split the flow and reduce individual footprints, improving redundancy, control, and maintenance flexibility. Include anticipated temperature, solids loading, and sludge removal strategy during detailed design verification later.

Overflow Rate Check And What It Means

Surface overflow rate is a hydraulic loading surrogate that influences capture of settleable solids. If the achieved value is higher than the target, the tank area is undersized for the chosen basis. Adding units, increasing diameter, or adjusting peak assumptions reduces the achieved overflow rate. Document the check basis so comparisons remain consistent.

Weir Loading And Effluent Stability

The peripheral weir length increases with diameter, so larger tanks typically reduce weir loading for the same unit flow. Keeping weir loading within the selected limit helps maintain uniform effluent withdrawal, reduces short circuiting near launders, and supports stable downstream disinfection and filtration performance. If launder layouts differ, confirm effective weir length from the detail.

Depth, Volume, And Detention Time

Side water depth converts plan area into volume and helps estimate detention time. Detention time is computed as basin volume divided by unit flow, converted to hours. Adequate detention supports settling and buffers short hydraulic spikes. If detention is below the target, the calculator suggests a larger diameter or greater depth so hydraulic conditions better support settling.

Using Results For Practical Layout

After calculating diameter, confirm geometric constraints such as minimum and maximum diameters, site clearances, and future expansion. Review achieved overflow and weir loading side by side, then document assumptions, basis selection, and safety factor. When performance margins are small, run both average and peak cases for comparison. Coordinate inlet dissipation, sludge collection mechanism selection, and scum baffle geometry with the final diameter. Export results to track revisions through submittals, stakeholder reviews, and future construction coordination meetings.

FAQs

1) What does the safety factor change?

The safety factor increases required surface area, producing a larger diameter. It adds conservatism for uncertainty in flows, upstream solids, or future load growth, while keeping the same design overflow and weir criteria.

2) Should I size on average or peak flow?

Use average flow to understand typical loading and compare options. Use peak flow when wet weather, diurnal surges, or permitting checks govern. Many teams run both to see the margin and sensitivity.

3) Why can weir loading fail when overflow rate passes?

Overflow rate depends on plan area, while weir loading depends on outlet weir length. A small diameter can still have adequate area if multiple basins exist, yet the per-unit weir length remains too short.

4) How is detention time used here?

The calculator estimates detention time from volume and unit flow as a hydraulic screening check. It supports comparing alternatives, but final design should also consider inlet hydraulics, baffling, sludge blanket control, and temperature effects.

5) Can I apply diameter limits for site constraints?

Yes. Minimum and maximum diameter inputs force the computed diameter to respect limits. If the maximum limit drives the result smaller, review the achieved overflow and weir loading values to confirm the design remains acceptable.

6) What should I attach to a submittal package?

Include inputs, basis selection, assumptions, and the exported results table. Add a brief narrative on overflow and weir checks, plus references to the project criteria and any vendor or standard details used.