Study of flowthrough couette flocculators—I. Design for uniform and tapered flocculation

Abstract

Based on published hydrodynamic data for Couette apparatuses, two flocculators were designed consisting of co-axial cylinders. The annular gap between the cylinders constituted the flocculation chamber, in one case of uniform width, and in the other tapering from a narrow to a wider gap by a taper on the inner cylinder. For best hydrodynamic stability the outer cylinder only rotated, creating a velocity gradient across the annular gap. A novel feature of the apparatuses were inflow and outflow connections allowing flocculation to be studied on a flowthrough basis, which is more realistic in water treatment, rather than as a batch process. In this flowthrough condition the tapered flocculator reduced the velocity gradient in the direction of flow reducing the shear on flocs as they grew in size. Visual studies of the flow stability, using dye, of the two flocculators used both in batch and continuous flow processes, revealed the presence of currents along the cylinder boundaries, attributed to end effects. Residence time distribution tests, using lithium tracer, indicated some short circuiting varying with velocity gradient in the tapered flocculator, but independent of velocity gradient in the uniform flocculator.