Where sewage is removed by the water method and carried through sewers to the waterways, such as rivers and lakes, the great problem which the sanitary engineers have to solve is how to dispose of the sewage and not pollute the water supply of the city. When there is a sufficient amount of water to insure proper dilution sewage may be emptied directly into it without treatment. But where there is a limited amount of water, as in a small stream, and a very large amount of sewage to be disposed of, the water may become overloaded and thus the supply of other towns become polluted. When sewage is disposed of in the ocean, oyster beds may become contaminated as well as the bathing beaches. Sewage Purification It is considered cheaper and at the same time satisfactory from a hygienic standpoint to depend more upon proper filtration than upon any other method of purification. There are many different methods used for sewage purification. Sewage may be screened. This frees it from the larger particles. It is then placed in tanks and by a slow process of sedimentation relieved of a large amount of the suspended matter that passes through the screens. After these processes it may then be carried to fields for subsurface irrigation. It is carried through porous tile pipes ten to eighteen inches below the surface and two feet to three feet apart. In this way the sewage seeps through into a sandy soil. An acre will take care of the sewage from 150 to 250 people a day. This method can not be used at all in clay soil. The crude sewage may be applied to the surface of the land and carried by means of ditches, but this is not considered satisfactory. Sand filters are also used, but they require frequent cleaning and renewing. According to Price the most desirable method of sewage disposal is known as the septic method. In this method the sewage is sorted and the action that follows is bacteriological. The bacteria present act in the absence of air or oxygen. The sewage thus undergoes a process of decomposition, fermentation and purification. The next step is to pass the sewage that has come from the septic tank through open sand filters very slowly. These sand filters must be properly cleaned and renewed at regular intervals and must at no time be overloaded. This makes the effluent from the filter beds entirely free from germs and harmful matter. It is not necessary for us to consider the construction of these septic tanks. There are any number of different types, but the results are supposed to be the same. The construction of such tanks is in the province of sanitary engineering and not in that of practical hygiene. Price states that the water which comes from the filter beds after the sewage has first been through the septic tank is free from injurious substances, is capable of supporting life and that even the drinking of it has been known not to be followed by injurious results. Refuse Disposal