INTERPRETATION OF DE-WATERABILITY MEASUREMENTS BY CAPILLARY SUCTION TIME (CST)

The specific resistence to filtration can be described by an equation that relates pressure difference, filtrate viscosity, cake mass deposited per unit filtration volume, volume of filtrate, and time. The CST is the time period during which a certain volume of filtrate from the sludge is sucked into the filter paper under the sludge head and capillary suction pressure produced by the interaction of the filtrate with the fibrous matrix of the filter paper. The filtration rate is dependent on the properties of the particular grade of paper, the filtrability of the sludge and a number of other factors. By experimental CST measurements of a number of suspensions it was found that mainly mineral suspensions (coagulation sludges, lime sludges, stabilized waste sludges) do not show a relationship between the specific CST and the concentration of total suspended solids (TSS). Biological and raw sludges show a variable degree of CSTs dependence on TSS whereby this degree is dependent both on the colloidal organic matter in the sludge and on the particle size.