=Filtration= is by far the most practicable method of water purification. There are many different methods used. The two most used are known as the natural and mechanical. Under what is known as the natural methods, the sand filter seems to have precedent. The efficiency of this filter depends upon the material used, the velocity of the water through the filter and the care, such as proper cleaning which the filter receives. As a matter of information the student should become somewhat familiar with the general construction of a sand filter. The size of the filter, that is, the area it covers, will be governed by the amount of water to be filtered per day, which in turn will depend upon the demands. The greatest objection to a slow sand filter is that a very large tract of land is required to provide sufficient surface for the percolation of the water. The bottom and sides of the filter-beds are water-tight and are constructed of cement with proper drains to carry off the filtered water. A layer of very coarse gravel is placed on the bottom of the filter, then on top of this is a layer of fine gravel. This is then covered with fine sharp sand about three feet deep. Ordinarily the entire filter medium is from four to six feet deep. On top of this layer of fine sand a thin scum is formed which increases the efficiency of the filter, its action being biological. As this scum accumulates it becomes necessary to remove it. This is done by special devices. It is necessary to remove only a small portion of the sand, for it will be clean immediately below the surface. This process is continued until the layer of sand has decreased in depth to about twelve inches when it is replenished with new, or with polluted sand which has been removed and thoroughly washed with filtered water. The filter is first filled by admitting water from beneath for the purpose of forcing the air out. The depth of water is kept about three feet above the sand. The rate of filtration is controlled and kept at a velocity of about two inches per hour. At this rate a filter of one acre area will provide about 3,000,000 gallons per day. A first class slow sand filter should free the water of 99% of the bacteria. Hygienists claim that filtered water should not contain more than 100 bacteria to a cubic centimeter. Water from a new filter or one that has just been cleaned should be tested before it is used. By this method one can determine whether the desired results are being obtained. It may be necessary to store the water in reservoirs before it is passed through the sand filter for the purpose of sedimentation. This will rid the water of the small particles of sediment which is contained in the turbid water. If turbid water is passed through a sand filter the smallest particles will pass on through with it. This causes a tendency to fill the filter with clay and other substances.