Which of the following is a process for sludge treatment that involves the removal of water by gravity?

The correct answer is thickening. Thickening is a process used in sludge treatment that primarily focuses on the removal of water through gravity. During thickening, the sludge is allowed to settle, enabling the denser solids to form a concentrated layer at the bottom of a tank while the water (supernatant) is removed from the top. This reduction of water content is essential to improve the efficiency of subsequent treatment processes and reduce the volume of sludge that needs to be handled and disposed of.

Thickening typically employs gravity-driven processes, making it an effective initial step to enhance solid concentrations. It contrasts with dewatering, which implies further mechanical operations to significantly reduce moisture content to a level suitable for handling or disposal. In the case of conditioning and stabilization, these are primarily focused on altering the chemical and physical characteristics of the sludge rather than primarily dealing with gravity-based water removal.