EngiMetric Arena / Design Challenge
What circular clarifier diameter does the SOR require?
Competency Design and sizing primary Quantitative reasoning supporting
What you should take away: Size clarifier plan area from the surface overflow rate, apply a safety factor and convert the area to an equivalent circular diameter.
The situation
What is happening
A clarifier receives 180 m3/h. The permissible surface overflow rate is 1.8 m/h, the settling depth is 3 m, and the design applies a 1.15 safety factor. What equivalent circular diameter is required?
Given
| Quantity | Value | Unit | Note |
|---|---|---|---|
| Design flow | 180 | m3/h | Clarifier influent flow. |
| Surface overflow rate | 1.8 | m/h | Permissible settling criterion. |
| Settling depth | 3 | m | Water depth in the settling zone. |
| Safety factor | 1.15 | - | Applied to the required area. |
Assumptions this challenge makes
- The surface overflow rate is the governing settling criterion for this design.
- The basin is circular and the reported diameter is an equivalent diameter for the design area.
- Solids loading and weir loading are not supplied, so they are not checked.
Your call
Your answer
Hints
- Required area is flow divided by surface overflow rate.
- Apply the 1.15 safety factor to the area before converting to a diameter.
- For a circle, D = sqrt(4A / pi). Keep the result in metres.
Your answer Engine result
Where that comes from
Each limit, checked
Why
- The required area is 180 / 1.8 = 100 m2.
- With the 1.15 safety factor, the design area is 100 x 1.15 = 115 m2.
- The equivalent circular diameter is sqrt(4 x 115 / pi) = 12.10 m.
- The basin volume at 3 m depth is 345 m3, giving a nominal detention time of about 115 minutes. That is a geometry consequence, not a removal prediction.
- The calculation does not check solids loading or weir loading because neither was supplied. A basin sized on SOR alone can still be overloaded on solids.
What this challenge does not establish
- The surface overflow rate is an input; it must be selected for the actual floc and process.
- No solids-load or weir-load check is performed without the required inputs.
- The equivalent diameter is for a circular basin; a rectangular basin has different geometry and weir behaviour.
- The calculation does not predict effluent quality or solids removal.