THE PURIFICATION OF WATER

THE PURIFICATION OF WATER

Specially written for FIRE AND WATER ENGINEERING

In a paper by Professor John M. Ordway on the subject of “The purification of water” he states that even the long enduring turbidity of the Mississippi, caused by fine clay, can be purified by a soft porous earthenware filter burned at a moderate heat. Even an ordinary clay flower pot with an inner surface of 148 square inches yields 300 cubic centimetres in twenty-four hours. The color of the water, caused by humic acid is “probably brought into solution by an exceedingly small amount of potash or soda, furnished by the general decomposition of the rock particles of the soil. It seems to be harmless, though it gives the water an unpleasant appearance.” The artesian water of some cities in Louisiana is very highly colored, and contains about thirty-seven parts of sodium carbonate to 100,000, while decayed vegetable matter abounds in the depths of the Mississippi delta. It can be “completely decolorised in a few minutes by adding bibasic or tribasic aluminum chloride. Humic acid has a stronger affinity for alumina than for soda, and, therefore, a flocculent precipitate of aluminum humate is formed and rapidly subsides.” To decolorise brown waters the “highly basic chloride of aluminum is much better than the ordinary sulphate, as it acts more quickly, and, the needed alumina being combined with only one-half or one third as much acid, much less alkali in the water will suffice to take up this acid and turn over the alumina to the undisputed possession of the humic acid.” Some waters experimented on were “cleared by half a drop, a drop, or, in one or two instances, two drops of a thirteen per cent, solution of the tribasic chloride to a litre of water. When the color is very faint, it takes several hours for the precipitate to gather and settle.” Iron may be present as ferrous carbonate or sulphate. Although present, the water is sometimes “perfectly clear and colorless as it comes from the ground, but in a few minutes it becomes opaline, and after long exposure to the air it deposits the iron as insoluble ferric oxide.” To purify it and fit it for bathing purposes the simplest way is to “shower the crude water through the air in a very much broken fall, and then filter it through sand.” Or it may be pumped into very large cisterns, and, after adding very small quantities of lime and aluminum sulphate, let air be blown through it for an hour or more. Within a few hours it settles perfectly and permanently clear. When iron is not obstinately held in solution by organic matter, the change to insoluble peroxide may readily be brought about by forcing air through the water or by a much retarded dropping of the water through the air. The addition of a little lime or soda facilitates the oxidation. Such an addition is particularly needed when ferrous sulphate is present, since this salt is changed very slowly by the air; but, when the stronger acid is taken up by an alkali, there is formed ferrous oxide, which is very greedy of oxygen.” The most common and troublesome fault in hard water is the presence of lime and magnesia salts, which form sticky oleostearates of calcium and magnesium, owing to the presence of the soap when washing The waters of the great lakes, the St. Lawrence, and most of the Western and Southern streams are unpleasantly hard. In that of the Mississippi analysis shows an “average of 6.95 parts of calcium carbonate and 1.9 of magnesium carbonate in 100,000 parts of the settled water. The maximum amount of the two parts together was 13.66, and the minimum. 6.86.” Sodium is “present only as sulphate and chloride. The mud, also, contains some of the same substances.” After getting rid of the mud. the idea was to eliminate the soapkilling impurities. A ready means seems to be afforded by “Clark’s soap test, which consists in adding to 100 centimetres of the treated water a standard solution of soap, a little at a time, till, on thorough shaking, a permanent foam remains on the surface. According to the French mode of reckoning, which is preferable to the English or German, the number of degrees of hardness is supposed to represent the number of grammes of calcium carbonate in 100,000 c. c. of the water. The method does not admit of great precision, but it serves well for comparison.” The possible ranges of hardness were arrived at by Prof. Ordway. That of the Peabody river, of New Hampshire, rising in the highest of the White mountains among the gravitic formation was less than 1 degree, as was that of Colorado Springs from the Rocky mountains. That of Boston (Chestnut Hill reservoir) and that of Quebec was 1.2 degrees. Buffalo’s from lake Erie was 7.6; of the St. Lawrence, at Montreal, 8.2; at St. Louis, 9; at Denver, 9.5. Among the Green mountains, in Vermont, in a mica clay formation, well water in a high ridge of Randolph Centre the hardness was 21 degrees; another well some rods away tested 15 degrees; a copious spring, about 200 rods from the latter and some 300 or 400 feet lower, showed 9.5 degrees. Last November, in working with the Mississippi water, so as to have a stock of uniform composition and free from the interference of suspended clay, about seventy gallons of the raw water were run into a galvanised iron tank, and cleared by treatment with bibasic chloride of aluminum and iron—the water at that time containing about the maximum of dissolved impurities. “Two or more litres of clarified water were taken for each trial. Of the precipitants the limewater contained one gramme of lime in about 800. Of the others normal solutions were made, like those used in volumetric analysis—that is, the soda salts, for instance, had twenty-three parts of sodium to a litre. “As to lime: Water containing calcium and magnesium carbonates held in solution by carbonic acid may be partially purified by treating it with just enough limewater or milk of lime to combine with the excess of carbonic acid. The lime added is all precipitated, and, with it, a part of the carbonates originally present. Lime being a stronger base than magnesia, it may be expected to decompose the magnesium salts and set the base free; of the calcium compounds it can affect only the bicarbonate.” In the clarified Mississippi water one-fifth of the calcium carbonate had been changed to chloride by the coagulant used. About three-fifths of the remaining carbonate and one-sixth of the magnesia were thrown down by an optimum of 50 c. c. of limewater to a litre reduced the hardness from 7.5 degrees to 4.5. “So lime makes an improvement, but does not carry the softening quite far enough. Lime acts slowly, but the precipitation is completed in twenty-four hours.” Soda ash is a very suitable purifier where calcium sulphate is much in evidence. It decomposes gypsum and leaves in solution harmless sodium sulphate, the calcium sulphate being precipitated. Sodium carbonate has also an effect on calcium bicarbonate, as it has a strong affinity for carbonic acid, and, when put into the clarified water, although it took some time, in a day or two a granular coating formed on the sides and bottom of the vessel. The maximum effect was produced by 6 c. c. of the normal soda solution to a litre, which precipitated most of the lime and reduced the hardness from 10 degrees to 4.5. It is slow in action, and sometimes lacking in efficiency. Trisodic phosphate, a salt composed of caustic soda combined with the ordinary disodic phosphate, is a better, though a dearer worker than the carbonate. It precipitates both lime and magnesia phosphates, which settle readily. In the clarified November water of the Mississippi 6 c. c. of the normal solution threw down nearly three-fourths of the two carbonates, and brought the hardness down to 34 degrees; from the softer water of April, it took out nearly all the lime and magnesia, and lowered the hardness to 1.6 degrees. Caustic soda Prof. Ordway calls the most effective of all the single purifiers. It forms a slightly gelatinous precipitate, which is deposited in a few hours. The maximum effect on the clarified water was produced by 6 c. c. to a litre, which removed nearly all the lime and magnesia and reduced the hardness to less than 1 degree; with 5 c. c. it was reduced to 1.8 degrees—soft enough for any use. With raw river water almost as good results were produced. The precipitate has some clarifying power as well, “but the gelatinous calcium and magnesium phosphates take a stronger hold on the fine clay, and, hence, the caustic operates more quickly when it has some trisodic phosphate mixed with it.” The turbid water, with 4 c. c. of caustic soda and 2 c. c. of the phosphate added to every litre, deposited the mud very quickly and became quite clear in less than twenty-four hours. Its hardness was then only 1.8 degrees. Aluminum and ferric salts, however, far exceeded this alkaline coagulant for mere clarification, because much more of the latter is required, and, if there is any organic matter present, a little of it is left in solution. Both may be advantageously used together for cleaning and softening by putting in the soda mixture first, and after a while the aluminum or ferric salt, and leaving the treated water for twenty-four hours to settle. Aluminate of sodium, a cheap stuff, also does the work, but is too slow, and the same is to be said of double oxalate of sodium and aluminum, which, besides, is too costly. In such exceedingly weak solutions as the water under consideration the salts are more or less dissociated into their constituents. and, to counteract this, there must be an excess, in some cases, of the basic, in others, of the acid ingredient. In analytical work it was found that 1.5 c. c. of the normal acid to the litre threw down about half the lime and reduced the hardness from 7.5 degrees to 3.7 degrees, while 3 c. c. increased the precipitate one-fifth, but was so much in excess as to carry the hardness up to 16 degrees. “Among the reagents used, only the lime, the alumina, the phosphoric and the oxalic acids are removed with the precipitates formed. The alkali of the sodium compound remains in solution as bicarbonate, carbonate and a little caustic. For most uses this small quantity of soda is unobjectionable, it being equivalent to not over three parts of sodium carbonate in 10,000 or eighteen grains in a gallon. The alkalinity can be reduced by neutralising one-third or one-half with any acid, after the water is settled and drawn off. But this complicates matters too much. Our (New Orleans) artesian well water contains nearly four parts of sodium in 10,000, and this is certainly very good for steam boilers at least.” For steam use at least artesian well water (better, if possible) should be used. Failing that, where lime or magnesia are chief impurities they can both be eliminated at a moderate cost. For boiler incrustation caustic soda and lime in the case of waters charged with the earthy carbonates arc the best; sodium carbonate, for those containing the sulphates or chlorides; and trisodic phosphate for such as are turbid. The obnoxious substances should be removed before the water goes to the boiler. The most feasible apparatus is one in which the “water, after receiving the precipitant, is forced upwards in a slow current against a series of deflectors, which are expected to turn aside the precipitate, and let the clear water remain at perfect rest for several hours,” preferably in two or more tanks of sufficient size to furnish a full supply, when used alternately. Such an appa ratus as the following can be used for work on a moderate scale. Procure a galvanised iron cylinder two feet in diameter and three feet deep, with a conical bottom nine inches deep. About three inches above the outlet of the inverted cone is soldered to the sides a brass cross-bar perforated to receive the pivot of a three-quarter-inch vertical shaft, passing at the top through the fixed middle piece of the wooden cover, a crank turning horizontally being placed a little above. Just above the lower brass bar there is fastened a piece of two-inch by three-inch joist, cut so as to form two propeller blades. A little brisk turning of the crank mixes thoroughly the raw water and the chemicals. The clear water is drawn off by a faucet close to the bottom, and the mud can be run out by a cock at the apex of the cone. The cylinder not being filled higher than to within two inches of the top, a yield of about thirtythree inches or sixty-five gallons may be reckoned on for one operation. When the river is only moderately turbid, the mud need be disposed of only after the third or fourth filling. A galvanised iron tank three feet in diameter and three feet deep, simply locked and soldered and wired at top would probably be strong enough, and would give 145 gallons at a time. In very large apparatus the stirring would be done better by blowing in air at the bottom. For washing or cooking good rain water is far better than the filtered or clarified river water. But the true rain water is not that gathered from the city roofs, but from among the granite hills. Rain water in private cisterns is often of over three degrees of hardness, and contains sulphate of calcium, derived, no doubt, from the fine coal ashes and soot carried up the chimney by the draught and deposited on and washed into the cisterns from the rough slated roof. “Still it causes no harsh feeling in washing with soap. We may. perhaps, consider 4 degrees the limit below which water begins to be passably soft.”

THE PURIFICATION OF WATER.

THE PURIFICATION OF WATER.

Supt.S. A. Charles

The Record of Eighteen Years’ Experience of Filtration and Aeration.*

By the word “Purification” I mean not merely filtration, but also aeration and Hushing of mains, etc. My experience has been at Lexington, Ky., in a warm climate, and where the ordinary modes of purification will not answer. Let me give a history. Our first reservoirs comprised about 120 acres of water, with a capacity of 360,000,000 gallons, but it was all impounded water, collected by building a dam across the valley and collecting the rainfall. In our warm climate, however, I found that, while we had the water, we had also the odor connected with it which arose from the decomposition of algae and other vegetable matter. My first attempt to remove the algae was by means of boxes of sponge, through which the water was passed, and thereby got it a kind of a coarse straining or filtering. This, of course was better than nothing, but I cannot say it was very much better. About that time mechanical filters were beginning to be used, and the question in my mind was whether they could be made available. There was a long discussion at each waterworks convention about the respective merits of the filterbed system, and the mechanical filter system. The filterbed system was out of the question with us. We did not have the land or the money to build them with, neither was the sand necessary for the use of them available. The question, therefore, was practically which of the mechanical filters was the best for our use. The pressure filter advocates and the gravity filter advocates were each anxious to sell us, but we finally decided on the gravity filter system, and as yet we have seen no reason to regret our choice.

I think that much of the efficiency of the mechanical filter system depends on the proper admixture of the coagulant. In most of the systems that I have seen it is largely a matter of guesswork and chance, and is left to the judgment of the filter man. We have a little simple device with us by which we are enabled with the utmost accuracy to inject either one grain or a millionth of a grain of coagulant per gallon. It is done in a very simple manner, by having an auxiliary pump (without going into details), which is driven by the main nump and goes faster or slower as the main pump does. Tts relative capacity to the main pump may be one-thousandth of the main pump, and it is very easy to see that, if this auxiliary pump injects clear water, no coagulant at all is injected, but it may also inject a solution of any desired strength, whether it is one grain or fifty per the gallon.

But, with us, at least, filtration and the proper coagulant and the strength of the coagulant arc not the only necessaries. While these may filter out all solids, there yet remains with us and probably in most southern climates, an odor, due to the decomposition of algae and vegetable matter. This odor, being due to a gas and being more volatile than the water itself, cannot be filtered out. The only wav to remove it is to aerate the water and expose it to streams of air. We accomplish this by delivering the water to the filters through a pipe or tube perforated with about 1,600 minute holes, through which it falls in a spray to the filter below. We have also tried the effect of an air-tube underneath this spray, filled with minute holes and supplied by an aircompressor—the intention being to blow sprays of air through the spravs of water; but we found the air-compressor expensive to run in practice and of but little, if anv benefit, because the simple falling of the spray of water through the air to the filter gave all the desired result in a much cheaper way.

So far. we have dealt only with filtration and aeration, but there still remains something to be done with us. We found that some few minute germs of algae in process of time would escane our filter, find lodgment in the pipes, and would there germinate and, in turn, decompose and give out unpleasant odors to the customers. This last trouble we have entirely remedied by flushing. I am aware that flushing is not a new process at all. but I doubt whether it has yet been done as thoroughly and on as systematic a plan as ours. It is evident that, when flushing is onlv done through the hydrants, and the opening in the hydrant is three feet above the main, that flushing will not be as thorough as if the opening or exit were directly from the main itself. We have, therefore, put in what we call washouts at the lowest points in our svstem. and each night in the week we flush some portion of our line directly through these washouts, aiming to flush the entire system thoroughly at least once a week, oftener, if necessary. To do this wc have a man stationed at the washout and assistants in other parts of the city, who, under his directions by telephone, open or close the proper valves to direct the main current to any line whatever out to the final exit, thus thoroughly washing and scouring that line.

1 think 1 have explained our system. 1 do not care to go into theory. With us it has been an accomplished fact, and we believe we are now furnishing better water under adverse circumstances at less cost than any other waterworks we know of.

*Paper read at the annual convention of the Central States Waterworks association, Indianapolis. September, 1902