3 Flocculation- The joining of the particles so that they will form larger settable particles is called flocculation. The larger formed particles are called floc. In flocculation coagulants are used but the resultant floc is settled out rather than filtered through sand filters. The chosen coagulant and the raw water are slowly mixed or water which previously coagulated is directly taken in a large tank called a flocculation basin (Chamber). Unlike a rapid mix tank, the flocculation paddles turn very slowly to minimize turbulence. The principle involved is to allow as many particles to contact other particles as possible generating large and robust floc particles. Generally, the retention time of a flocculation basin is at least 30 minutes with speeds between 0.5 feet and 1.5 feet per minute (15 to 45 cm / minute).
4 Sedimentation/Clarification/Settling -Water exiting the flocculation basin (chamber) enters the sedimentation basin, also called a clarification or settling basin (chamber). It is a large tank with slow flow, allowing floc to settle to the bottom. The sedimentation basin is best located close to the flocculation basin so the transit between does not permit settlement or floc break up. Sedimentation basins can be in the shape of a rectangle, where water flows from end to end, or circular where flow is from the center outward. Sedimentation basin outflow is typically over a weir so only a thin top layer-furthest from the sediment-exits. The amount of floc that settles out of the water is dependent on the time the water spends in the basin and the depth of the basin. The retention time of the water must therefore be balanced against the cost of a larger basin. The minimum clarifier retention time should be normally 4 hours. A deep basin will allow more floc to settle out than a shallow basin. This is because large particles settle faster than smaller ones, so large particles bump into and integrate smaller particles as they settle. In effect, large particles sweep vertically though the basin and clean out smaller particles on their way to the bottom. As particles settle to the bottom of the basin a layer of sludge is formed on the floor of the tank. This layer of sludge must be removed and treated. The amount of sludge that is generated is significant, often 3%-5% of the total volume of water that is treated. The tank may be equipped with mechanical cleaning devices (called as bridge units in conventional clarifiers) that continually clean the bottom of the tank or incase of sedimentation tanks, without such cleaning devices, the tank can be taken out of service when the bottom needs to be cleaned.
Now a day new type of settlers called as Tube Settlers are used. Tube settlers offer an inexpensive method of upgrading existing water treatment plant clarifiers and sedimentation basins to improve performance. They can also reduce the tankage/footprint required in new installations or improve the performance of existing settling basins by reducing the solids loading on downstream filters. Made of lightweight PVC, tube settlers can be easily supported with minimal structures that often incorporate the effluent trough supports. They are available in a variety of module sizes and tube lengths to fit any tank geometry, with custom design and engineering offered by the manufacturer.