St Ignatius College - Ignatian Yearbook (San Francisco, CA)

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28 IGNATIAN ing the vigor. Again deep wells are likely to furnish water filled with dissolved mineral matter that renders it unfit for many household uses. This is hard water. As a supply of some twenty-five gallons a day for each individual must be provided and an equal amount is needed for animals and manufactures, engineers are seldom able to find an available supply of wholesome water large enough to fill the reservoirs of cities. In the Middle States rivers must be tapped yet all of them are polluted at some time with infectious matter. On the Eastern, and especially the Western, seaboard mountain streams are often available and the problem of purification is not so necessary nor expensive as when the rivers are concerned. The methods in use depend upon subsidence of debris, filtration of impurities or chemical treatment for the success- ful removal of dangerous substances, be they gases or or- ganisms or minerals. Polluted rivers will be purified in time by aquatic animals or plants and to a certain extent by the dissolved oxygen taken from the air. Living organisms which produce dis- ease are killed by direct exposure to sunlight. By one or all of these means of purification river water contaminated with sewage is rendered wholesome in the course of time. Thus the Mississippi, carrying the outfiow of the Chicago river, is purified before it reaches St. Louis-some 250 miles away. Filtering Plants. Filtering plants always have subsidence basins into which the water from the intake pours. VVater in motion will carry heavy debris with ease, as the carrying capacity increases enormously in proportion to the velocity of flow, but once arrested the debris falls by gravity and hence subsidence is both a cheap and effective process for the removal of undesirable material. The effect of light 5' ,. -yi

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Ghz Iiuritiratinn nf Brinking Mater ATER and air are indispensable for the sus- taining of the life process in animals. The limit of human endurance of conditions M D if h - - W ic prevent the consumption of water is probably about ten days. A daily allow- L!-Q ance is absolutely needed both in food and as drink. Hence the plentiful supply of this necessary chem- ical compound that was rated so long as an element. Na- ture is never prodigal but of water there is an abundance. I XJJ Y' ll tn 5? Yet our water supplies never furnish absolutely pure liquid. All the sources we dep-end upon derive their supplies directly or indirectly from the condensed moisture of the atmosphere. Rain or melted snow collects in lakes or passes to underground caverns to form wells. The following enu- meration of sources embraces all those we depend upon for drinking water: Rain water, surface water C such as ponds and riversj, ground water ffrorn rain held by the sub-soill. and deep collections called artesian water. By passing through the air or percolating through the ground all this water is contaminated, more or less, by sus- pended or dissolved material. Hence a supply of water may be rated as wholesome, or suspicious or dangerous accord- ing to the debris it carries. Spring water and that from deep wells is usually of good quality and is very palatable. Water from unpolluted streams and clgan stored rain water has not an attractive Havor. Surface water that has poured over cultivated land is always suspicious even if it has an agreeable taste. Rivers carrying sewageuj-and shallow wells are to be rated as dangerous until an analysis has proved the contrary. Polluted water may carry living organisms which will produce disease in the human body or it may contain sub- stances which will dispose a person to infection by depress- x . ff, -.

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PURIFICATION OF DRINKING WATER 29 and oxygen is considerable in a shallow basin. Twenty- four hours is sufficient time for keeping the raw water in these reservoirs if filtration is to follow. The filters are made of sand, charcoal, spongy iron, unglazed porcelain and like substances which will strain out particles from the passing water, which are too large to fioat through the interstices of the filterls medium. And debris that might pass will settle in the microscopic cavities between the grains of sand, or in the interstices of the charcoal. A gelatinous mat will form on the sand after some time which adds ,greatly to the efficiency of the filter. This gelatinous material is due to bacterial growth and produces the best effect when it descends from the surface into the body of the filtering layer of sand. Then silt and organisms are entrapped by its sticky layers as well as prevented from floating through unfilled spaces. Moreover, there is some oxidation of dissolved organic matter within the body of the filter. A well constructed layer of sand, allowed to age until the mat grows, will remove almost all the bacteria from raw water. Such filters are at least a quarter of an acre in area and are from three to four feet thick. The sand rests upon an additional foot of gravel of decreasing size, as it ascends from the drains. The grains of sand are about as large as the crystals of granulated sugar. The uniformity coefHcient and the effective size of the grains of sand are matters of detail which are never overlooked. Rapid Filtration. A bed of one acre will filter 3,000,000 gallons a day. This amount is considered too small and a method of rapid filtration has been devised. Nevertheless, the slow process is in use in the older cities of good size. It originated in England in 1830 and has been adopted ex- tensively on the Continent. The rapid filtration allows from four to twelve hours for sedimentation after the raw water is pumped from