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.
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.
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
Another great problem is the disposal of refuse such as ashes, garbage, and in the larger cities, street sweepings. It is estimated by Whipple that in the larger cities like New York the refuse will be in round numbers a ton per capita per year. This is an enormous amount of material and the disposal of it becomes a great problem. This is not only of hygienic importance, but also one of economical significance.
The collection of waste material is objectionable from the social viewpoint; also an accumulation of ashes and dust will irritate the eyes and respiratory tract, while piles of garbage and other filth are conducive to the breeding of flies and mosquitoes. Such collections result also in the development of ill-smelling odors and unsightly streets and alleys.
There are different methods used to dispose of this waste material. It may be collected and burned. This method is called incineration. Or the refuse may be collected separately and disposed of. When it is to be disposed of separately there are usually city ordinances which require the householder to keep the garbage and ashes separate and they are then collected by the city in separate wagons. In most cities tall garbage cans are required and the garbage is drained and wrapped in paper, the ashes are placed in cans or containers. They are used for filling in low lands while the garbage is taken away and buried or burned.
For the collection of garbage specially constructed wagons or carts are used. The garbage must be collected frequently and at regular intervals before it becomes decomposed and gives off objectionable odors. The wagons must be properly cleaned for the same reason. In the removal of ashes and other refuse, care should be taken not to create a dust and the wagons should not leak so as to litter the streets and alleys.