:SEDIMENTATION is effected by either bringing water to rest or passing it very slowly through reservoirs or basins. The first is called the intermittent and the second the continuous system of sediinentation. The force of gravity carries down some of the clay and silt which compose turbidity. The degree of el:u•ilication effected depends upon the size and specific gravity of the particles and the length of time afforded for sedimentation. Some waters are notably im proved in a few hours; others are still turbid after a number of days. Partly through the action of gravity, and prtly through entangle ment with the other suspended matter, a consid erable percentage of bacterial reduction is ef fected by sedimentation. Where bacterial purl fivation, rather than clarification, is the object, sedimentation alone is insufficient, but it may be a great aid to filtration. Storage reservoirs af ford more Or less sedimentation and bacterial reduction. depending upon both their size as com pared With the daily draught and the character of the water. The design and construction of set tling reservoirs does not differ from that of other reservoirs, except in the relative shallow ness of the former, the arrangement of the inlets and outlets, and the provisions made for re moving the deposits of from their bottom.
In eontinuous-tlow basins having a number of compartments the water generally passes from one section to another in a thin sheet over a long weir. This insures the drawing off of the upper and clearest layer, minimizes the disturbance of water in the successive basins, and may provide aeration of value. If aeration is dosired and the levels permit it there may be a series of steps between each weir and the next basin. Two or more compartments are usually necessary, so one may be at rest while the other is being filled: but where the constant-flow• system is used for water containing but little sediment one basin may do. Provision must be made for drawing the water down as quietly as possible to a level well above the deposit of sediment, so as to avoid disturbing the latter. The bottoms of settling reservoirs should slope to some common point, where connection should be made with a pipe for flushing out the mud when the reservoir is eleaned. The mud is often loosened by means of water under pressure, thrown from a hose, much as in hydraulic mining.