Wastewater enters the basin and very fine particles in the water are separated by means of gravity. The water must be in the basin long enough for the desired particle size to be removed. Smaller particles require longer periods for removal and thus larger basins. In some basins a flocculant may be added to help smaller particles stick together and form larger particles. Stokes law can be used to calculate the size of a settling basin needed in order to remove a desired particle size.  Stokes law gives a settling velocity determining an effective settling basin depth; so solids removal depends upon effective settling basin surface area, while the depth component of settling basin volume remains important for storage of settled solids.
Translation of required settling time surface area to settling basin geometry requires consideration of short circuiting and turbulence induced by wind, bottom scour, or inlet and overflow design.  Settling basin geometry is important because effective time of settling within the basin will be the time a volume of water spends in non-turbulent conditions before reaching the settling basin overflow.  Median time is always less than the mean time calculated by dividing available volume by anticipated flow.  The median time of passage through a short, wide settling basin may be significantly less than the median time of passage through a long, narrow settling basin.  Settling basins with overflow structures near the entrance points may hold a large volume of stagnant water while newly admitted water rapidly reaches the overflow point before settling can occur.  Effective surface area for settling seldom extends perpendicularly more than a tenth the distance of a flow line from basin entrance to overflow unless baffles are installed.  Effective surface area and geometry may change as accumulating sediment fills part of the originally constructed volume.  Short cut channels may rapidly form through heavier sediment accumulations near the entrance to the settling basin.  Flow through shallow portions of the settling basin may cause turbulence resuspending sediment from the bottom of the basin.  Two feet has been recommended as a minimum settling basin depth to avoid bottom scour.