Purifying Water by Action of Ultra-Violet Light
It is highly important that a good drinking water shall be free from microbes, because many disease germs, such as the bacteria of typhoid, cholera, etc., are transmitted by water. A single person has caused infection to springs with typhoid and thus caused epidemics in which many people died. Hence modern hygienics demands, nrst of all, sound drinking water for public distribution as well as for private use. Many springs deliver good, wholesome water, very low in their microbic contents, particularly in bacterium coli. How can these sources deliver pure water if they are supplied with water from rivers or effluents heavily charged with the refuse of towns and villages? Simply by following the methods of nature. Nature employs two means for purification. The first is the action of sun or daylight on the microbes floating in the water. It has long been known that the sun has so strong a bactericidal effect that rivers are purified some miles below the entrance of city effluent, so that the water contains scarcely more microbes than before entering the city. The second means employed by nature for purifying water is filtration over natural sand beds, such as river alluvials, thereby mechanically freeing the water of suspended matter and of germs.
Until recently only the latter process has been artificially employed. For nearly eighty years water plants have been provided with sand filter beds so that the water which they deliver is nearly always physically good. In other words, it is free from suspended matter. Bacteriologically, however, the water is not always up to the standard which, modern hygienics demand. For instance. Miquel, the famous French water hygienist, demands for water of the very best finality a microbic content not higher than ten per cubic centimeter. And the number of bacterium coli must not exceed ten per thousand cubic centimeters. To obtain such results the first one of the above mentioned natural methods of killing microbes is now practically applied; namely, exposing water to a strong light. The strong bactericidal or sterilizing action of the sunlight is due to the ultra-violet rays in the solar spectrum. Hence, we must choose a light which is extremely rich in these ultra-violet rays. We must create a miniature “artificial sun.” In the quartz mercury vapor lamp, which closely followed the fundamental work done by P. C. Hewitt in illuminating mercury vapor lamps, such a source of ultra-violet rays has been found. Purification, or sterilization of water by ultra-violet rays has become increasingly important within the last three years. The principles which form the basis of the water sterilization by mercury quartz lamps are as follows:
First. Water, being easily penetrated by ultraviolet rays, may be exposed in a thick layer to the rays. But the water must be free from floating matter. Usually it must be first filtered.
Second. The apparatus must be so arranged that the ultra-violet rays are produced with a minimum of electric energy; also the ultra-violet ray lamps must have an economical life.
Third. The apparatus must be so constructed that the maximum amount of ultra-violet light enters the water. In other words, as little light as possible should be lost.
Fourth. The apparatus must be so constructed that the water passes under the influence of the strong light several times. This is effected either by leading the water over and over again to the same lamp, or by passing it successively into the luminous zone of several lamps. Besides, the apparatus must be so constructed that the water, while under the influence of the light, is stirred up so as to turn over and over microscopic suspended matter which might have escaped filtration and which might screen microbes from the light.
A typical microbe sterilizer is to be found in one ot the suburbs of Kouen, France. For over two years this installation has supplied water sterilized by ultra-violet rays. Since the introduction of the system not a single typhoid case has been known among the subscribers, whereas the percentage of typhoid cases in neighboring districts is appreciable. The most recent types of sterilizing apparatus are characterized chiefly by the fact that they use the light in a more economical way than any apparatus hitherto constructed. The principles involved are such that the construction ot very large sterilizing units is possible. The apparatus consists of a tank or long canal through which the water flows. Baffle plates are interposed to stir up the water as it flows. Into the walls of this tank are inserted at intervals of several feet quartz tubes fitted water-tight to vertical plates constituting parts of the walls of the sterilizing tank. These quartz tubes are about two inches in diameter and eight inches long, and are closed at the end. The vertical plates are provided with external metal boxes which carry the lamp supports. The lamps are not unlike a pistol in shape and are characterized by a very narrow U-shaped luminous tube, of such length and diameter that it nearly fills the two-inch quartz tubes. All the light produced by these “pistol” lamps therefore enters the water through these quartz tubes, whereas the lamp itself is protected from contact with the water by these same tubes. This is of importance, because the production of ultraviolet rays in quartz mercury vapor lamps is more economical the higher the temperature of the tube (the lamps usually run at a temperature of about 800 deg. Cent.). It is, therefore, necessary to prevent the lamps from touching the water, which would cool them down to such an extent that the production of ultra-violet rays would be very low and uneconomical. The sterilizing canal is equipped with a maximum of twelve such lamps, each using 1,500 watts. One of the installations has been running for eight months and sterilizes the water in a municipal plant at the rate of 1,700,000 gallons per day. Most of the time only a few of the lamps are illuminated. In larger water plants, several such canals may be operated in parallel. If the unit is small fewer lamps, even only a single one, may be employed and the sterilizing tank reduced correspondingly in size. In the latter case the baffle plates must be so arranged that the water is led several times near the single source of light. The small apparatus illustrated in the accompanying engraving has been used for military work as a field sterilizer. The complete equipment in this case, mounted on a gun carriage, consists of a gasoline motor generator set, water pump, filter, and the ultra-violet ray sterilizer. A similar outfit has supplied water to a field hospital during the French-Morocco campaign, where it has been in constant use for nearly two years for supplying water for drinking and surgical purposes. The report by the surgeon in charge states that there has been no case of hospital infection or typhoid during all this time, proving thereby the utility of this sterilizing apparatus It is evident that the application of ultra-violet rays in water purification has developed into an industry which promises to do away with the disastrous effect of the many maladies which are spread by the water.—Scientific American.