浓密机资料.ppt

SEDIMENTATION INTRODUCTION Sedimentation, or clarification, is the process of letting suspended material settle by gravity. Suspended material may be particles, such as clay or silts, originally present in the source water. More commonly, suspended material or floc is created from material in the water and the chemical used in coagulation or in other treatment processes, such as lime softening. INTRODUCTION Sedimentation is accomplished by decreasing the velocity of the water being treated to a pointbelow which the particles will no longer remain in suspension. When the velocity no longer supports the transport of the particles, gravity will remove them from the flow. FACTORS AFFECTING SEDIMENTATION PARTICLE SIZE The size and type of particles to be removed have a significant effect on the operation of the sedimentation tank. The shape of the particle also affects its settling characteristics. All particles tend to have a slight electrical charge. FACTORS AFFECTING SEDIMENTATION WATER TEMPERATURE When the temperature decreases, the rate of settling becomes slower. The result is that as the water cools, the detention time in the sedimentation tanks must increase. In most cases temperature does not have a significant effect on treatment. A water treatment plant has the highest flow demand in the summer when the temperatures are the highest and the settling rates the best. When the water is colder, the flow in the plant is at its lowest and, in most cases, the detention time in the plant is increased so the floc has time to settle out in the sedimentation basins. FACTORS AFFECTING SEDIMENTATION CURRENTS Several types of water currents may occur in the sedimentation basin: ? Density currents caused by the weight of the solids in the tank, the concentration of solids and temperature of the water in the tank. ? Eddy currents produced by the flow of the water coming into the tank and leaving the tank. The currents can be beneficial in that they promo