Sedimentation is employed to remove suspended solids from a waste stream, either as a primary clarifier to reduce solids loading to the secondary treatment system, as the principal physical treatment device, or as a final clarifier to meet effluent discharge requirements. Settler design entails applying theoretical considerations, factored by many years of wastewater and water treatment operating experience. Discrete settling involves removing unhindered, nonflocculating, and unchanging solids traveling at a constant velocity, the settling rate being a function of the liquid and solid properties. Scouring is a major concern where high-level sludge inventories are subjected to excessive forward velocities, upsetting the solids, resuspending them, and potentially resulting in high carry over. Flocculent settling involves removing agglomerating solids, caused by solids frequently in contact, or the presence of flocculating agents. Zone settling involves removing solids from highly concentrated suspensions with considerable solids contact, resulting in hindered settling and a distinct mass zone formed.