Solid/liquid separation by sedimentation

The analysis of batch sedimentation tests performed for the purpose of continuous sedimentation vessel design is described. Conventional design techniques employ the concept of settling flux with, under certain conditions, a constitutive equation linking solid concentration and pressure in a unique and time-independent manner. Experimental studies employing measurement of local concentration and the liquid pressure gradient during sedimentation were used to determine the limits of the conventional design techniques. A 'maximum fluidized concentration' is defined which is the value above which it is difficult to maintain stable flux conditions and is the limit of applicability of conventional design methods based on settling flux. It is shown that the suspended solids contribute towards the liquid pressure gradient and, therefore, to the buoyancy experienced by the settling solids. However, during settlement the liquid pressure gradient reverts to the hydrostatic gradient alone; hence the buoyancy effect is a function of sedimentation time. Additional considerations also suggest that a unique relation between concentration and pressure should only be used as a constituent term in a time-dependent consolidation model. Current research effort includes a suitable method of linking time-dependent consolidation theory, under conditions of extremely low applied pressure, and sedimentation flux analysis.