Many water purification methods require pretreatments such as coagulation and flocculation.
A coagulant is added to the source water in traditional coagulation-flocculation-sedimentation to produce an attraction between the suspended particles. Slowly stirring the fluid causes the particles to clump together forming “flocs.” The water is then pumped into a silent sedimentation basin, where the solids will settle out.
The suspended particles are likewise coagulated and flocculated in dissolved air flotation systems, but instead of sedimentation, pressured air bubbles drive them to the water surface, where they may be skimmed off.
A point-of-use flocculation-chlorination system has been developed, particularly for underdeveloped nations. To purify the water, it employs small packets of chemicals and inexpensive equipment like buckets and a cloth filter.
Finally, lime softening is a technique for “softening” water, or removing calcium and magnesium mineral ions from it. The stuff that settles out in this situation is dissolved salts, not suspended silt.