Clarifier Surface Overflow Rate

Calculating surface overflow rate

The hydraulic loading of a clarifier is overflow flow divided by effective horizontal surface area. Flow in m³/h divided by area in m² gives m³/(m²·h). For example, 100 m³/h over 80 m² gives 1.25. Multiply by 24 to express the same rate per day.

Selecting an area

The second mode uses a design loading rate supplied by the user to calculate the required area. An equivalent diameter is shown for a single circular tank. This is not a mechanical tank design. Flow distribution, weir arrangement, solids loading and settling properties require separate assessment. The initial values are examples rather than design recommendations.

Matching the flow and area boundaries

Decide which stream and tanks are included before calculating. Using a flow from one operating condition and an area from another is misleading. If a parallel tank is unavailable, use only the effective area in service. Dividing total flow by one tank's area and dividing flow per tank by total area are different errors.

Suppose two operating tanks each have 50 m² of effective area. At a combined overflow flow of 150 m³/h, total area is 100 m² and loading is 150/100 = 1.5 m³/(m²·h). Its daily equivalent is 1.5 × 24 = 36 m³/(m²·d). If one tank closes while flow stays constant, loading becomes 150/50 = 3 m³/(m²·h). This increase does not specify a removal efficiency percentage.

For area sizing, the design loading is an independent input that must be selected from settling properties and applicable criteria. The calculator does not judge its suitability. The equivalent diameter represents the entire area as one circle. Do not apply it to every tank when dividing the area among several tanks.

Surface loading has the dimensions of velocity. Increasing depth does not change this ratio at unchanged flow and horizontal area. Assess effects on retention time and sludge storage separately. This preliminary calculation does not verify solids transport or actual flow distribution.