Physical characteristics of flocs—I. The floc density function and aluminium floc

Some characteristics of floc density were illuminated by experiments and model floc simulation by using clay-aluminium flocs. As an aluminium floc is a very fragile particle, the authors adopted an experimental method in which the settling velocity and diameter of a discrete floc were measured in a quiescent water column. Floc density was calculated by introducing the measured values and some constants such as water density and viscosity into a suitable settling velocity equation. In this study a modified Stokes equation was used. From the experimental results, the following conclusions were obtained. (1) Floc density decreases as floc size increases, and the floc diameter and the floc effective density (buoyant density = floc density — water density) have a straight line relationship on a logarithmic paper. This line is characterized by two constants, Kp and a, which show the slope of the line and the floc effective density of 1 cm diameter floc. The authors have designated this relationship as the floc density function. (2) ALT ratio (aluminium ion concentration dosed/suspended particle concentration) greatly affects the clay-aluminium floc density. As ALT ratio decreases, the floc density at the fixed size increases. Other effects of coagulation condition upon floc density function were also discussed. (3) The floc density function of coagulated colored water was studied. Iron, magnesium and calcium flocs with clay were also studied for reference purposes. (4) The floc density function was verified by the model floc simulation. Model flocs were produced by computer and the floc density function was derived by substituting the number of the primary particles contained in a floc and the model floc diameter into a mass balance equation of a floc particle.