a. Process. Sediment is material carried in a fluid by reason of motion of the fluid. The purpose of primary clarification is to separate floating and setttleable solids from the wastewater. The objective is to clarify or settle out the sediment when the motion of the fluid lessens or stops. When fresh domestic wastewater stands quiescent or flows very slowly, a considerable portion of the suspended organic matter and the solid mineral matter remaining after grit removal settles rapidly. Floating solids may be skimmed off the top. The accumulation of nonfiltrable residues, which is called sludge, settles to the bottom of the tank or basin and is removed from the bottom of the tank. This sludge is still quite fluid, containing 95 percent or more of water and 4 to 5 percent (40,000 to 50,000 ppm) of solids. Most of the residue is highly putrescible organic materials. The sludge must be removed frequently to prevent septic conditions in the basin and to maintain the designed basin volume. Removal of the settleable organic sediment markedly reduces the biochemical oxygen demand of the remaining waste stream. Treatment disposal of sludge is discussed in Section II.