The History of the CST Sludge Filterability Measurement Apparatus

In the early 1960's at the Water Pollution Research Laboratory which had very foresightedly been commissioned by the U.K government to provide a sound scientific foundation and understanding of the processes used in the treatment of domestic and industrial waste waters, Chris Baskerville was part of the small team researching into methods then used for the disposal of sludges, which was then predominately either by drying bed or storage lagoon. Now it was because some sewage sludges were found to drain very readily on drying beds, whilst other sludges drained hardly at all, that a quantitative measurement of sludge filterability was developed, termed the Specific Resistance to Filtration.

This was an extremely tedious measurement to undertake, necessitating the use of controlled partial vacuum being applied to the sludge undergoing filtration and recording manually the decline in rate of the volume of filtrate collected.

Noticing that a fast filtering sludge when poured onto the surface of some filter paper caused the peripheral filter paper to become rapidly saturated with adsorbed filtrate as compared with that of a slow filtering sludge, led to the development and patenting of the Capillary Suction Time (CST) Filterability Apparatus. The CST simplistically being the time to collect a unit volume of filtrate of a sludge undergoing filtration in a standard sized (CST) funnel, when placed upon a standard grade of chromatography paper- (Whatman No 17).

Following the construction of a prototype which established the viability of the measurement of the CST for determining the filterability of sludges, Triton Electronics rapidly developed a commercial version of the apparatus initially using their Centisecond Timer for the timing mechanism before producing a range of different models, which have now been sold in their thousands world wide over the past 40 years.

As it is an automatic measurement that requires little expertise or training to enable a precise measurement of filterability to be recorded it is ideally suited for use wherever sludge dewatering is undertaken, which in the sewage treatment industry can be by, filter press, vacuum filter or centrifuge. Moreover as sewage sludges almost invariably require the addition of chemical conditioners to improve their dewatering characteristics the use of the CST apparatus enables an optimum dose of conditioning chemical to be utilised and it permits the identification of the cause of inadequate performance, such as the excessive shearing forces occurring within worn pumps.

It has been used extensively in the polymer chemical production industry to verify that the polymer produced is an effective sludge conditioner and it is also used in the oil drilling industry to determine the exact characteristics of the mud suspensions required to lubricate the diamond drilling boring tool.

It is probably fair to state that the CST sludge research team at the Water Pollution Research Laboratory under the direction of Robert Gale and assisted by many Chemical Engineering University sandwich course students, brought more in benefits to Great Britain Ltd than the total expenditure of all the Laboratory over the whole of its existence.

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