Instead of the water passing slowly through the sand, it is forced through in mechanical filtration. The sand is contained in cylinders or tanks and the water is forced through either as the result of gravity and weight of the water above or by mechanical means. Before it is passed through the sand in the mechanical filter, it is treated chemically to coagulate and precipitate the suspended impurities. This process is not only mechanical in the sense that it is accomplished by means of certain machinery, but the action on the water is mechanical. There is no bacteriological action which is so essential as this mechanical action in the process of water purification. The mechanical method of water filtration has not been found to be as satisfactory as the slow sand filter process, although very large amounts of water may be filtered in much less time. One great disadvantage of this method is that the filters require such frequent cleaning and in this respect are very expensive to maintain. =Chemical purification= is not considered by some hygienists as a satisfactory method for purifying large water supplies. There are many objections to this method. Even though the water is freed from the impurities it has received in its natural course, it is laden with chemicals that may be as injurious to health as the impurities themselves. The chemicals used are: Chlorine, bromine, copper sulphate, metallic iron, ozone and sodium bisulphate. Swimming Pools In connection with the subject of water a word relative to the sanitation of swimming pools will not be out of place. There are many municipal swimming pools in operation and while they are intended to improve the sanitary conditions, they may at the same time become a great menace to hygiene if they are not properly conducted. No individual should be allowed to enter a swimming pool without first having taken a shower bath with a liberal use of soap. No one should be allowed in the pool who is infected with skin dis-eases of any kind, ulcers, running sores, conjunctivitis, or venereal dis-eases. The water should be properly filtered when it enters the pool and there should be a constant process of disinfection. There should be not only right construction, but also fit maintenance of the pool. The swimmer should be properly instructed relative to swimming pool sanitation. SEWAGE AND REFUSE DISPOSAL Methods of Sewage Disposal One of the greatest problems the hygienists and sanitary engineers have to solve is the disposal of sewage and refuse without contaminating the water supply and in other ways rendering the environment unhealthful. The methods used for the immediate removal of sewage may be either the dry method or the water method. By far the more common one in towns and cities is the water carriage system. In small towns, however, the more prevalent way is to provide a house some distance from the dwelling which contains a privy vault.