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

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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

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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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30 IGNATIAN the intake. If it is turbid, coagulants are added, these form a math on the sand even without germ growth. The settling basins are deep and baffles are numerous. The layer of very coarse sand is but thirty inches thick and rests upon a bed of gravel laid on the porous pipes which act as drains. Fifty times the volume of water will pass through this filter than can be delivered by the older type. It costs New Orleans S600 a day to maintain, clean and air the filtering beds which deliver 40,000,000 gallons of good water from the turbid Mississippi. Chemical Treatment. Chemical treatment is needed to reduce the hardness of water,-a condition due to dissolved Calcium salts or like minerals. Lime is used in the Clarke process with success, on such water as is drawn, for example, from the chalk beds of England. Alum will be decomposed in impure water and yield a bulky jelly-like precipitate which falls slowly to the bottom. Hence this chemical is used to remove suspended and dissolved impurities and even coloring matter. One grain of Aluminum sulphate fthe Alumj added to a gal- lon of water will, after agitation and settling for twenty- four hours, render foul water so clean that it will remain clear for some time. This chemical is not a violent nor a cumulative poison, but it impairs the flavor of the treated water. Ozone has been used to disinfect water quickly. That an army might be provided with safe drinking Water special apparatus to apply ozone has been built for use in the iield. Iron sulphate and other chemicals have been used with advantage to render unwholesome water Ht for consumption. The most recent and by far the most effi- cient and convenient chemical treatment has been intro- duced by Vincent and Gaillard very recently. They use