Capillary suction time measurement

Characterization of sludges by CST measurement


Since the introduction in the 60’s, organic flocculants have a high market share as conditioning agents in sludge dewatering. Polyelectrolytes are not only used in the dewatering with centrifuges or belt filter presses, in which the process necessarily requires the addition of organic flocculants, but also increasingly in sludge dewatering with chamber filter presses. Users of polymeric flocculants are often confronted with the problem of determining the ideal product from a large selection.


To determine the effectiveness of polymer flocculants, the CST test has proven to be a suitable method for carrying out laboratory tests for the following sludge dewatering processes:

Dewatering machine:

  • Decanter centrifuge
  • Filter press
  • Band filter
  • Vacuum filter


CST value


CST = capillary suction time

The CST unit measures the time (caused by applying sludge) a water front needs to travel a distance defined by 2 data points on a special filter paper. This time is a measure for the ability of the sludge to release water. The higher the CST value, the stronger the water is bound to the solid particles contained in the sludge. A slurry with a high CST value can only be dewatered successfully after pretreatment, for example, using polyelectrolytes.


Properties of the CST unit at a glance

  • Conformity to DIN EN 14701
  • Compact design
  • Display of the CST-measured value up to 1/100 seconds
  • Optical control of the spread of the water front through circular reference markings on the measuring board
  • Acoustic signal at the end of the measurement process
  • Robust design for daily on-site use
  • Energy-saving construction in CMOS technology ensures long battery life
  • AC power use via special adapter possible
  • Practical packaging in a sturdy carrying case