Capillary Suction Time, CST (Method 2710G, APHA, 1998)

CST method was developed to measure the dewatering properties of activated sludge. This method is essentially measuring how quickly the activated sludge wet a filter paper. When sludge contact with filter paper, water from the sludge starts to wet the paper due to capillary suction phenomena of the spaces among the hydrophilic fibers that consist of the filter paper. Once the water in the sludge-paper interface is absorbed by paper, the sludge that contacts directly with paper becomes compact and acts as a barrier for further water loss to the paper. If macromolecules and fine particles are abundant in the sludge, the compact sludge formed in the interface would act as more efficient barrier.

According to the Standard Method, sludge is poured in the test cell reservoir located in the center as shown in Fig. 1 (top). As soon as sludge contacts with a filter paper in the bottom, it starts to wet the filter paper and the water proceeds radially as shown in the diagram (bottom). The time required for water to proceed from r1 to r2, which is called CST, is measured using a conductivity sensor. CST can be normalized by dividing the reading by MLSS (g/L) in order to reduce the effect of MLSS in a narrow MLSS range.

This is a convenient quick test method, but the direct correlation with membrane fouling is questionable. The CST test conditions are quite different from actual filtration condition, where crossflow exists that can discourage particle deposition and TMP induces cake layer compaction.

CST.ht9Fig. 1. Apparatus to measure capillary suction time (APHA, 1998)

   CST can be used in MBR to approximately estimate the membrane fouling potential of mixed liquor. As shown in Table 1, the normalized CST decreases when MCRT increases. (note: MCRT is same as SRT in MBR) As normalized CST decreases, TMP increasing rate (or membrane fouling rate) also decreased in the same experiment.

Table 1. Biomass concentration, CST, and zeta-potential of mixed liquor at different MCRT (or SRT) (NG, 2006)


© Seong Hoon Yoon