The effect of global velocity gradient on the character and filterability of aggregates formed during the coagulation/flocculation process

This paper describes the influence of the global velocity gradient G on the properties of aggregates formed during the coagulation/flocculation process. The methods of image and fractal analysis were used to determine aggregate size and structure, respectively. The influence of these aggregate properties on separation using depth filtration is also described. Experiments were conducted in a pilot plant operation. The suspension was formed in a flow mixing tank with global velocity gradients ranging from 28.4-307.2 s(-1) and ferric sulphate used as a coagulant. Filtration velocities were 3 and 6 m h(-1). Predictably, it was shown that the aggregate size decreased with increasing global velocity gradient G. Furthermore it was demonstrated that, with increasing G, the aggregates became more compact and regular (the D-2 fractal dimension increased) and the suspension became more homogeneous in size. The aggregates with the smallest diameter and highest D-2 fractal dimension displayed the best filterability, i.e. penetrated throughout the full depth of the filter bed and generated a minimum pressure drop.