text: Although sedimentation might occur in tanks of other shapes, removal of accumulated solids is easiest with conveyor belts in rectangular tanks or with scrapers rotating around the central axis of circular tanks. Settling basins and clarifiers should be designed based on the settling velocity (v) of the smallest particle to be theoretically 100% removed. The overflow rate is defined as:
  :Overflow rate (v ) = Flow of water (Q (m/s)) /(Surface area of settling basin (A(m))
  In many countries this value is named as surface loading in m/h per m. Overflow rate is often used for flow over an edge (for example a weir) in the unit m/h per m.
  The unit of overflow rate is usually meters (or feet) per second, a velocity. Any particle with settling velocity (v) greater than the overflow rate will settle out, while other particles will settle in the ratio v/v.
  There are recommendations on the overflow rates for each design that ideally take into account the change in particle size as the solids move through the operation:
  * Quiescent zones:  per second
  * Full-flow basins:  per second
  * Off-line basins:  per second
  However, factors such as flow surges, wind shear, scour, and turbulence reduce the effectiveness of settling. To compensate for these less than ideal conditions, it is recommended doubling the area calculated by the previous equation.
  It is also important to equalize flow distribution at each point across the cross-section of the basin. Poor inlet and outlet designs can produce extremely poor flow characteristics for sedimentation.
  Settling basins and clarifiers can be designed as long rectangles (Figure 1.a), that are hydraulically more stable and easier to control for large volumes. Circular clarifiers (Fig. 1.b) work as a common thickener (without the usage of rakes), or as upflow tanks (Fig. 1.c).
  Sedimentation efficiency does not depend on the tank depth. If the forward velocity is low enough so that the settled material does not re-suspend from the tank floor, the area is still the main parameter when designing a settling basin or clarifier, taking care that the depth is not too low.