Artificial Methods of Water Purification
The only way to obtain chemically pure water is by distillation. However, since this method is very slow and expensive, it is not at all practicable for the purification of even a private water supply, and since a chemically pure water is not required for drinking or other domestic purposes it would be entirely unnecessary to provide it. Again, distilled water is not desirable for drinking purposes since its taste is insipid until it is aerated.
When only a small amount of water is required it may be freed of organic poisonous substances by boiling. This will not, however, render water that contains lead and other stable chemical substances injurious to health safe for domestic use.
Sand Filter.
The dissolved gases are given off by boiling and this leaves the water with a flat taste which may be improved by aerating; this may be done by stirring or by placing the water in a jar and shaking it.
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.
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.