By far the more desirable method of separating gross particles from water is the sand method of filtration in use in many cities. In this country extensive sand filtration plants have been installed for the use of the City of Philadelphia, Washington, and many other cities. It is not the purpose in this manual to go into any description of the methods of building sand filters. The principle of the sand filtration rests upon the fact that where muddy water or water polluted with gross materials is brought upon a bed of sand so arranged as to allow free percolation from the lower surface the water may be secured in the effluent perfectly bright when the filter is properly working. Such sand filters do not give a clear filtrate when first put in use because the sand particles are too coarse and allow some of the silt to pass. There is gradually formed upon the upper surface a layer of silt or mud which fills up the grosser pores of the sand and finally becomes thick enough to retain all of the suspended particles, even without previous coagulation. When the filter is thus covered with its mud covering the water in the effluent is perfectly clear. The sand bed remains in operation until the mud layer becomes so thick that the water ceases to percolate. The mud with the adhering sand is then removed mechanically, washed if necessary to recover the sand, which is replaced upon the bed, when it is again ready for use. It is evident that a sand filter of this kind must cover a large area where the water supply is for a populous community. Many acres of filter surface are necessary for a city of 100,000 inhabitants. The original construction of such a sand filter is extremely costly, but when once finished its operation is comparatively economical.