**7. Sludges, Slurries and Wastewater**

The problem of movement, transportation, and *handleability* of slurries and sludges is a major preoccupation for many industries. Their causes are many and various. The character of the particulates, particularly clays, and their size distribution have an important influence. Moisture has a complex relationship with handleability: too much and a slurry must be contained, too little and a sludge can be sticky and difficult to move. The study of flow problems from hoppers has a long history [14] and since the 1950s it has gathered pace as a subject of interest to industry and academia. The Jenike Cell [15–17] was devised to measure the shear forces in particulate materials.

Industrial slurries are generally thickened to a sludge of water content of about 25% *v/v*. The applied stress at which a sludge distorts or moves is termed the *yield stress*. A shear vane tester (**Figure 3**) is a widely-used and readily-available device for measurements of shear stress in cohesive sludges [18]. The vanes are distributed in a cruciform shape around a spindle which can be rotated. Immersed in a sludge the

**Figure 3.** *Vane Tester.*

*Introductory Chapter: Slurry Technology – New Advances DOI: http://dx.doi.org/10.5772/intechopen.109473*

spindle is rotated at between 5 and 12 degrees per minute until the sludge fails and the vane rotates in equilibrium with the RSS or "Residual Shear Stress" [19]. The insertion of the vanes, and the rotation of them, has been suspected of changing the structure of a sludge and causing inaccuracies in measurement [20–22]. The controversy stems from the observation of viscous deformation before the yield point [23, 24]. This "wall slip" phenomenon is fairly important in geo-structures (harbor walls for example) and is not shown by vane tests. Wall slip is explored in later pages of this volume.

Despite controversies over certain critical measurements, the vane tester continues to be the measurement of choice for the shear strength and handleability of sludges.

Dewatering in municipal wastewater treatment is an important field of study. Some reasons for sludge dewatering are as follows.

1.Reclamation of water.


Dewatering can be carried out in many ways, for example, in warm countries lagoons can allow the sun to evaporate the water content. Periodically, industrial machines can then be employed to carry off the dried sediment. Large plate presses can be employed to reduce the moisture content of sludge to cakes of concentrations of about 20% *v/v*. Plate press installations are costly to build and equip, but the running costs are relatively low. An alternative to plate presses are belt filter presses which are less expensive to install, but with relatively high running costs due to the additives which are needed [25].
