The design of sedimentation basins should be based on experimental observations made upon the quantity of sediment removed at certain rates of flow and periods of retention in different types of basins. Hazen’s mathematical analysis is serviceable in making preliminary estimates and in checking the results. The shape of the tank, period of retention and rate of flow producing the most desirable results should be duplicated with the expectation of obtaining similar results or results but slightly modified from those obtained in the tests. This is the most satisfactory method of determining the proper period of retention.
233. Types of Sedimentation Basins.—A sedimentation basin is a tank for the removal of suspended matter either by quiescent settlement or by continuous flow at such a velocity and time of retention as to allow deposition of suspended matter.[[144]] The difference between sedimentation tanks and other forms of tank treatment is that no chemical or biological action is depended on for the successful operation of the tank. Sedimentation tanks may be divided into two classes, grit chambers and plain sedimentation basins.
A grit chamber is a chamber or enlarged channel in which the velocity of flow is so controlled that only heavy solids, such as grit and sand, are deposited while the lighter organic solids are carried forward in suspension. If the velocity of flow is more than about one foot per second, the tank is a grit chamber and below this velocity it is a plain sedimentation basin.
There are six general types of plain sedimentation basins:
1st. Rectangular flat-bottom tanks operated on the continuous-flow principle.
2nd. Rectangular flat-bottom tanks operated on the fill and draw principle.
3rd. Rectangular or circular hopper-bottom tanks operated on the continuous-flow principle, with horizontal flow.
4th. Rectangular or circular hopper-bottom tanks operated on the fill and draw principle, with horizontal flow.
5th. Rectangular or circular hopper-bottom tanks operated on the continuous-flow principle with vertical flow.
6th. Circular hopper-bottom tanks operated on the continuous-flow principle with radial flow.