Capillary Suction Time (CST) Test: Developments in testing methodology and reliability of results

Abstract

The dewatering of wastewater sludge (slurry) is a routine operation at wastewater treatment plants, and the results of dewaterability tests underpin the selection of dewatering processes. The two most commonly applied dewaterability tests for this purpose are the capillary suction time (CST) test and the specific resistance to filtration (SRF) test. The aim of this research was to develop improved methods of estimating sludge dewaterability by modifying the components and procedures used in the standard CST test, and by exploring the causes of the high variability that confounds the interpretation of the CST test results. The applications of this research were to recommend alternative methodologies that would help to improve the accuracy and precision of the standard CST test device and procedures, and ideally reduce operational and consumable costs. Multi-factorial experiments were designed to test the capillary suction times and the specific resistances to filtration of natural sludges and also of a synthetic medium which was formulated to simulate the properties of natural sludges. The applicability of altering the funnel geometry of the CST device, and the use of several alternative types of filter paper was evaluated. The applicability of incorporating stirring activity to eliminate or a least reduce sedimentation, and of adding a sealant at the bottom of the funnel, to eliminate or at least minimize unwanted filtrate leakage between the edge of the funnel and the filter paper, were studied. Experiments were performed to analyze the effects of temperature on the properties of sludges and the results of CST tests. Improved methods of estimating sludge dewaterability were developed by modifying the components and procedures used in the standard CST test, and by exploring the causes of variability in the test results. Stable synthetic sludges were successfully formulated to simulate the properties of natural sludges for experimental purposes. A rectangular funnel significantly reduced the variability and the time taken to conduct the CST test, relative to a circular funnel, particularly when testing heavy sludges. Whatman 17 chr (the most commonly used anisotropic filter paper) did not produce the most consistent CST test results in the shortest time. It is recommended that isotropic filter papers could be used, to lower the cost, reduce the test time, and improve the test precision. The addition of a sealant to the CST test apparatus also reduced the variability in the test results. No significant effects were found when a stirrer was added to the apparatus. The best line of fit to estimate filterability was defined by loge Y = β0 + β1 √x where Y = the mean CST value (s); β0 = the intercept (the predicted mean CST (s) when the distance4 between electrodes of the CST device is zero); β1 = the filterability (s/m2); X = the distance4 (m) between the electrodes of the CST device. Non-linear relationships were found between the CST test times and the temperature, associated with a complexity of effects of the temperature on sludge viscosity, filterability, settleability, desorptivity, and flocculation behaviour. It is recommended that the temperature should be recorded and controlled during the conduction of CST tests. SRF test results were predicted from the results of CST tests by the empirical model loge SRF = 46.128 – 1.346 T + 0.035 T2 + 13.760 F/TSS where SRF is the specific resistance to filtration (m/kg); T is the temperature (ºC); F is the filterability (loge s/m2) and TSS is the total suspended solids concentration (g/l).