The Capillary Suction Time has been used since the 1970s as a quick and reliable method for characterising sludge filterability and conditionability. It is sludge filterability that, predominantly, governs the output of nearly all the various types of dewatering equipment including: drying beds, belt press, vacuum filters, filter presses and centrifuges.

The capillary suction pressure generated by standard filter paper is used to ‘suck’ water from the sludge. The rate at which water permeates through the filter paper varies depending on the condition of the sludge and the filterability of the cake formed on the filter paper. The CST is obtained from two electrodes placed at a standard interval from the funnel. The time taken for the water front to pass between these two electrodes constitutes the CST.

The force generated by capillary suction is much greater than the hydrostatic head within the funnel, so the test is independent of the amount of sludge, as long as there is sufficient to generate the CST.

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History of the CST Sludge Filterability Measurement Apparatus >>>