Successful dewatering of sludge is considered to be one of the most significant challenges associated with sludge management, as well as being the most costly process in treatment plants (Jin et al., 2004). The dewaterability of sludge is fundamentally determined by the chemical composition and physical configuration of the flocs or solid particles that make up the sludge (Verrelli et al., 2009). In treatment plants, a number of process stages are employed to treat water to remove contaminants. Zhan et al. (2011) identified coagulation as one of the key elements within the treatment process, whilst research conducted by Diaz et al. (2011) and Verrelli et al. (2009) highlighted the importance of coagulation in influencing both the production and the dewaterability of sludge.
The coagulation process produces purified water and sludge flocs as a by-product (Byun et al., 2005; Gray, 2005; Diaz et al., 2011). In this process, small contaminants, which have a diameter of less than 1µm, attach themselves to one another to produce agglomerations, which can be easier removed than fine particles from water. Sludge properties such as volume, strength, size and dewaterability influence the methods of dewatering and sludge disposal (Razi and Molla, 2007; Yu et al, 2020).
Dewaterability testing is concerned with determining the ease with which water is released from sludge (Sanin et al., 2011). Capillary suction time (CST) and specific resistance to filtration (SRF) are widely accepted measurements of sludge dewaterability properties (Chen et al., 2004; Dentel and Dursun, 2009). The CST measurement was devised by Baskerville and Gale (1968). A CST value is obtained from two electrodes placed at a standard interval from the funnel. Sludge is exposed to an area at the centre of the CST filter paper and the filtrate from the sludge is absorbed by the CST paper. The time is recorded for the filtrate to travel between the two electrodes. The lower the CST value, the easier it is for the sludge to be filtered or dewatered (Besra et al., 2000).
The CST test can be used to examine the impact of different rapid mixing velocities (Sawalha and Scholz, 2012) and mixers on sludge dewaterability (Dentel et al., 2000). It is a valuable tool for characterizing biosolids pre-treatment for dewatering (Mayer, 2008). A practical use for CST is the determination of filterability after the addition of a coagulant aid (Scholz, 2005). The CST apparatus provides a simple, rapid and inexpensive method to measure sludge dewaterability (Scholz, 2005, 2006). The test can be performed at any location by a person with little training, because it does not require an external source of pressure or suction, and the automated CST test device is portable and easy to use. Baskerville and Gale (1968), Sawalha and Scholz (2012) and Fitria et al (2022) observed that the results of CST tests were sensitive to variations in temperature. The results tend to reduce with higher temperatures, which is probably due to the increase in filtrate viscosity with increasing temperature.
An alternative to the CST test is the SRF test, which utilizes a Buchner funnel apparatus with a vacuum port and filter paper. The CST and SRF results usually correlate well with each other (Scholz, 2005) and, for the same sludge sample, the CST and SRF values may show significant correlations (Sawalha and Scholz, 2010).
The SRF test, however, is more difficult to execute, is time consuming, and expensive. No specific standard device to measure SRF is available (Ayol and Dentel, 2005; Li et al., 2005; Teoh et al., 2006; Yukseler et al., 2007). Furthermore, the SRF values vary with pressure, area of filter paper, solid concentration and liquid viscosity (Sanin, 2011).
According to various researchers (Tebbut, 1998; Wu et al., 2003; Li et al., 2005; Pollice et al., 2007, 2008; Yuan et al., 2011; Sawalha and Scholz, 2012; Arimieari & Ademiluyi, 2018), the results of the CST and SRF tests correlate well with each other. For example, Wu et al. (2003) and Li et al. (2005) assessed different mixing intensities and pH values, respectively. Pollice et al. (2007) researched various mixed liquor suspended solids and mixing times. Yuan et al. (2011) researched extracellular polymeric substances and disintegration degrees. It follows that the SRF value can be predicted from the CST test results using regression equations based on historical data (Sawalha and Scholz, 2010). Thus, both the CST and SRF test devices are used in this research as a means of quantifying sludge dewaterability. The CST test is much easier and quicker than the SRF measurement (Tebbut, 1998). The CST is often preferred because it is easy to use in purification plants, results are obtained quickly, it is less expensive than SRF, and it has a standardized test procedure. The SRF test is often used as a verification tool for the CST results.
Despite a common belief that dewaterability indicators can be related to each other, some investigations indicated no correlations between CST and SRF for example: Chang et al. (2001) assessed six different water samples. Lee and Liu (2001), and Buyukkamaci and Kucukselek (2007) researched different polymers. Khongnakorn et al. (2007) looked at different mixing times.
The objectives were to assess relationships including correlations between CST and SRF in response to varying process variables such as rapid mixing velocities, mixer shapes, coagulants and temperatures. Furthermore, this article also assesses relationships between individual dewaterability indicators and various process variables.