The role of floc density in solid-liquid separation

Flocculation is a very important step in many solid-liquid separation processes and the structure of flocs can greatly influence the efficiency of separation. Flocs are typically fractal objects, for which the density decreases appreciably with increasing flee size, The open structure of large flocs can give much lower sedimentation rates than for more compact flocs of the same mass, because of increased drag, For similar reasons, cake filtration rates can be affected by flee density. Basic concepts of fractal aggregates are briefly reviewed, and the factors which influence free density considered. Some important practical consequences are discussed.