INTRODUCTION Sedimentation is the processes of letting suspended material settle by gravity. Suspended material may be particles, such as clay or silts, originally present in the source water. The purpose of sedimentation is to enhance the filtration process by removing particulates. Sedimentation is the process by which suspended particles are removed from the water by means of gravity or separation. For example, sand and silt can be carried in suspension in river water and on reaching the seabed deposited by sedimentation. PRINCIPLE of SEDIMENTATION Particles whose density is higher than that of the solvent sink (sediment), and particles that are lighter than it floats to the top. The greater the difference in density, the faster they move. Sedimentation may depend to objects of various sizes, ranging from large rocks in flowing water, to suspensions of dust and pollen particles, to cellular suspensions, to solutions of single molecules such as proteins and peptides. Even small molecules supply a sufficiently strong force to produce significant sedimentation. TYPES OF SEDIMENTATION There are four types: lithogenous, hydrogenous, biogenies and cosmogenous. 1.Lithogenous sediments come from land via rivers, ice, wind and other processes. 2.Hydrogenous sediments come from chemical reactions in the water. 3.Biogenies sediments come from organisms like plankton when their exoskeletons break down. 4.Cosmogenous sediments come from outer space and have extraterrestrial sources, such as the remains from the impact of large bodies of space material like comets and asteroids. This type of sediment is rarely found by scientists. Factors affecting Sedimentation. Particle size, density and concentration, and fluid viscosity, other factors affect the sedimentation rate. These include particle shape and orientation, convection currents in the surrounding fluid, and chemical pretreatment of the feed suspension. Alum increases the rate of sedimentation because alum being an external agent causes the particles present in the container to colloid together and form heavier particles, as heavier particles sink in water these particles settle down therefore increasing the speed of sedimentation. There is a general increase in the rate of sedimentation with temperature. A 20°C rise in temperature typically doubles the rate of sedimentation. Water treatment Process The sedimentation process is used to reduce particle concentration in the water. The advantage of sedimentation is that it minimizes the need for coagulation and flocculation. Chemicals are needed for coagulation and flocculation, but improved sedimentation controls the need for additional chemicals. Functions A sedimentation tank allows suspended particles to settle out of water or wastewater as it flows slowly through the tank, thereby providing some degree of purification. A layer of accumulated solids, called sludge, forms at the bottom of the tank and is periodically removed.