Fig. 62.—Pasteur
Water Filters.
271. Boiling Water.—In order to destroy the bacteria that may be in drinking water, it is not sufficient to heat the water or merely let it come to a boil. It has been found that if water is only partially sterilized and then cooled in the open air, the bacteria develop more rapidly than if the water had not been heated at all. It should boil vigorously five to ten minutes; cholera and typhoid bacteria succumb in five minutes or less. Care should be taken in cooling that the water is not exposed to dust particles from the air nor placed in open vessels in a dirty refrigerator. It should be kept in perfectly clean, tight-stoppered bottles. These bottles should be frequently scalded. Great reliance may be placed upon this method of water purification when properly carried out.
272. Filtration.—Among the most efficient forms of water filters are the Berkefeld and Pasteur. The Pasteur filter is made of unglazed porcelain, and the Berkefeld of fine infusorial earth (finely divided SiO2). Both are porous and allow a moderately rapid flow of water. The flow from the Berkefeld filter is more rapid than from the Pasteur. The mechanical impurities of the water are deposited upon the filtering surface, due to the attraction which the material has for particles in suspension. These particles usually are the sources of contamination and carry bacteria. When first used, filters are satisfactory, but unless carefully looked after they soon lose their ability to remove germs from the water and may increase the impurity by accumulation. Small faucet filters are made of porous stone, asbestos, charcoal, etc. Many of them are of no value whatever or are even worse than valueless. Filters should be frequently cleansed in boiling water or in steam under pressure. Unless this is done, the filters may become incubators for bacteria.
273. Distillation.—When an unquestionably pure water supply is desired, distillation should be resorted to. There are many forms of stills for domestic use which are easily manipulated and produce distilled water economically.[[93]] The mineral matter of water is in no way essential for any functional purpose, and hence its removal through distillation is not detrimental.
Fig. 63.—Water Still.
274. Chemical Purification.—Purification of water by the use of chemicals should not be attempted in the household or by inexperienced persons. When done under supervision of a chemist or bacteriologist, it may be of great value to a community. Turneaure and Russell,[[94]] in discussing the purification of water by addition of chemicals, state:
"There are a considerable number of chemical substances that may be added to water in order to purify it by carrying down the suspended matter as well as bacteria, by sedimentation. Such a process of purification is to be seen in the addition of alum, sulphate of iron, and calcium hydrate to water. Methods of this character are directly dependent upon the flocculating action of the chemical added, and the removal of the bacteria is accomplished by subsidence."
275. Ice.—The purity of the ice supply is also of much importance. While freezing reduces the number of organisms and lessens their vitality, it does not make an impure water absolutely wholesome. The way, too, in which ice is often handled and stored subjects it to contamination, and foods which are placed in direct contact with it mechanically absorb the impurities which it contains. For cooling water, ice should be placed around rather than in it. Diseases have frequently been traced to impure ice. The only absolutely pure ice is that made from distilled water.