The gravitational force, acting downward. Then the resultant force acting on the particle will be zero, and particle quickly reaches velocity, which is the maximum attainable under the circumstances and which is called the terminal velocity. When a particle falls in a fluid under constant force, e.g. force of gravity, then particle accelerates for some time, but when the accelerating force equals the retarding force (Buoyant force and Drag force). When all the solid particles settle in zone D, then this is a critical stage and after this no settling takes place. With further passage of time, the height of the zone D will decrease due to the compression of the sludge on account of its own weight. Settling can be free settling or hindered settling. But most of the slurries settle under hindered conditions. A Supernatant Layer C Transition Zone B Slurry Zone D Concentration Sludge Zone.