Particle aggregation in gravity thickening

The ability to economically thicken a sludge to the desired concentration and the accuracy with which continuous thickener performance can be predicted from batch settling data are governed by the flocculent nature of the suspension. A number of variables contribute to the flocculent nature of a suspension and the tendency for the formation of particle-particle attachments results in aggregate formation at low suspension concentrations and the formation of compressible sediments at high concentrations;An investigation was conducted in which the batch settling characteristics of a laboratory-prepared suspension were determined as a function of the magnitude of the flocculent nature, as governed by the dosage of a low-molecular-weight polyelectrolyte, and the intensity of mixing prior to settling. The results were quantified in terms of the characteristics of the aggregates which formed during settling as estimated by the application of a velocity-voidage correlation for the hindered settling of spherical particles;Two distinct modes of batch settling behavior were observed. At low concentrations, the suspensions underwent hindered settling as an assemblage of aggregates whose size and density were a function of the polyelectrolyte dosage. The intensity of mixing prior to settling had no direct effect on the aggregate properties. At intermediate and higher concentrations, the interaggregate porosity remained constant and the aggregate buoyant densities were inversely proportional to the aggregate diameters raised to some power--a diameter-density relationship that resulted from the basic aggregate structure and that was a function of the polyelectrolyte dosage and not of the intensity of pretreatment mixing;The role of aggregate formation in batch settling of flocculent suspensions was conceptually modeled and the effects of suspension concentration on the initial batch settling velocity were quantified for the two modes of settling. An expression was derived for constant interaggregate porosity settling in which the settling velocity was shown to be a function of the particle and fluid properties and the aggregate diameter-density relationship