Applying Shallow Depth Sedimentation Theory

T removal of particulate matter by sedimentation is one of the most widely used processes in the field of water and waste treatment. Despite the importance of this process, design criteria for its application established more than SO years ago are still in widespread use. Settling-basin design continues to conform to previous practice, with little consideration for the principles involved or the characteristics of the particles to be settled. be completely removed if false bottoms or trays were inserted at intervals h. Without such trays, a basin with a length much greater than Lo would be required to capture these particles. It is apparent from Fig. 1 that as the interval h is reduced, the size of the basin required to remove a given percentage of the incoming settleable material decreases. As the flocculent materials normally encountered in water and sewage treatment are not discrete particles, howDescription of Theory ever, they can flocculate and agglomA description of the settling paths erate further within the settling basin. of discrete particles in an ideal, rectThus, settling velocities may be rnaangular basin is useful in understandterially improved by floc growth within ing settling phenomena. In such an the basin. Inlet and outlet conditions, ideal basin, as defined by Camp,' the turbulence, and density currents cause paths of all discrete particles will be the settling paths in conventional straight lines, and all particles with settling basins to differ from the hythe same settling velocity will move in pothetic path in an ideal basin. parallel paths. The settling pattern The above theory for an ideal basin shown in Fig. 1 would be the same for demonstrates that the removal of settleall longitudinal sections. All partiable material is a function of the overcles having settling velocities VB flow rate and basin depth and is indegreater than Vo will fall through the pendent of the detention period. This entire depth, ho, and be removed. The theory was proposed in 1904 by Haportion of particles with settling vezen 2 when he pointed out that the loci ties VB less than u; which will be proportion of sediment removed in a removed is equal to the ratio of the settling basin is primarily a function