The capillary suction time (CST) test provides a quantitative measure of the rate of water release from a sludge and has the advantage of being simple to run. The test consists of allowing sludge water to filter into and penetrate a filter paper by capillary force. Although the CST works well for the majority of sludges, it is not a useful test for measuring the filterability of highly flocculated sludges or sludges conditioned by other methods such as freeze-thaw. The escape of water from the sludge in such cases is too fast and the rate-limiting step becomes the movement of water through the filter paper instead of the release of water from the sludge. There is a need to improve the CST apparatus so it can be used to measure the degree of filterability for super flocculated sludges as well. Such a device is described in this paper. It consists of first draining the free water from the well-flocculated sludge and only then allowing the flocculated sludge to come in contact with the filter paper.The results of our study show that the filterability of super-flocculated sludges can be measured with acceptable precision using the modified CST apparatus.

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