Gravity sedimentation is a process of solid-liquid separation that separates a feed sus­pension into a slurry of higher concentration and an effluent of substantially clear liquid. It is the most common concentration process for sludge treatment at wastewater treatment plants. To remove particles that have reasonable settling velocity from a suspension, gravity sedimentation under free or hindering settling is satisfactory. However, to remove fine par­ticles with a diameter of a few microns and for practicable operation, flocculation should be induced to form larger particles that possess a reasonable settling velocity. The thickened underflow of sludge is withdrawn from the bottom of the tank; the effluent or supernatant overflows a weir and is pumped back to the inlet of the treatment plant.

Gravity sedimentation is used for algae separation where the clarity of the overflow is of primary importance and algal feeds suspension is usually dilute (Mohn and Soeder, 1978; Mohn, 1980; Eisenberg et al., 1981; Venkataraman, 1980; Sukenik and She1ef, 1984) or where a thickening of the underflow and the algae feed slurry is usually more concentrated (Mohn, 1980).