CHLORINE FOR WATER WORKS USE MADE FROM COMMON SALT
Electrolytic Apparatus Which Enable the Water Departments to Make Own Chlorine Gas—Also Controls the Water Treatment
THE following paper treats of the production of chlorine direct for water works use by means of the electrolytic cell, and describes some interesting tests as to the effect of the chlorine upon pipes and parts of machinery it might come in contact with:
Chlorine, although one of the most widely distributed elements known to chemists, is never found in the free condition in nature. It exists in enormous quantities in combination with sodium, potassium, magnesium and calcium. In combination with sodium, as sodium chloride, or common salt, it occurs in practically inexhaustible quantities.
Scheele, a Swedish chemist, discovered chlorine in 1774, but was unaware of the elemental nature of the new substance. Sir Humphrey Davy investigated and found chlorine to be an element in 1810.
Chlorine was first used as a bleach and later as a disinfectant, but until the middle of the last century, disinfection was regarded as a process that prevented putrefaction. The thought of micro-organisms was not associated with the process.
Spontaneous generation had been the explanation of the work, now attributed to bacteria until 1862, when Pasteur proved the possibility of preparing sterile culture media and demonstrated the manner in which they could be protected from contamination. The big step in the study of bacteria did not occur until twenty years later, in 1882, when, Koch developed a solid culture media which permitted disinfectants and antiseptics to be studied quantitatively with a greater degree of accuracy than had been possible previously.
Use of Chlorine in Water Purification
Chlorine, in various forms, had been used in treating sewage for many years before its value as an agent in water purification was appreciated. As an agent in water purification, it has only been used since about 1905, but in the comparatively short time it has been in use we have all been convinced beyond doubt as to its value in the prevention of the spread of disease by water.
Use of the Electrolytic Cell
As early as 1893, the electrolytic cell was in use, but the earlier type of cell was utilized, mainly, for the purpose of making hypochlorites. A current was passed through a saturated salt solution causing the salt or sodium chlorine to break down into sodium and chlorine, the sodium thereafter reacting with water to form sodium hydrate, which then combined with the chlorine to form sodium hypochlorite. This reaction was not complete— it was found that only half of the chlorine first formed combined with sodium hydrate for the formation of hypochlorite, and the other half reforming sodium chloride. The secondary reactions thus set up resulted in such low efficiency of the cell that it was not practical to use it. The reason for the secondary reactions taking place was because of the fact that there was no provision made for separating the chlorine and sodium after they were once torn apart by the current. The old process was called the “non-diaphragm” process, while the new is a diaphragm process. The diaphragm process is the method by which the chlorine and sodium can be separated. once they are torn apart, and is the principle of operation of the Williams Electrolytic Cell.
The Williams Cell consists of a concrete box, rectangular in shape, its depth being its smallest dimension. The positive electrode is made up of graphite pins, suspended front the top of the cell, while the negative electrode is formed by an iron wire screen at the bottom of the cell. The iron wire screen is separated from the brine in the cell by an asbestos diaphragm. Brine is then automatically fed to the cell through a small funnel at the top. When the cell is filled with brine and the current turned on, chlorine, as gas. is liberated at the positive electrode, while the sodium is liberated at the negative electrode. The chlorine is drawn away from the cell as it is liberated and the caustic soda drops from the under side of the cell into a catch basin, thus preventing their contact and secondary reactions.
The Control of the Cell Simple
The quantity of chlorine produced depends entirely upon the amount of current passed through the brine, which current is regulated by a rheostat. An ammeter indicating the amount of current, whereby the exact quantity of chlorine desired can be produced, is used. The writer has found the control of the cell to be very easy and entirely practical. The cell requires approximately 15 amperes of current, at not less than 2.28 volts, flowing for 24 hours, to produce one pound of chlorine. The writer has found that it takes 4 lbs. of salt to the pound of chlorine produced. Hard rubber tubing, graphite, iron wire, concrete and asbestos are the materials used in the construction of the cell, which are cheap elements to replace, should replacement be necessary. The only replacement that has been necssary at the Topeka Filtration plant consists in replacing the asbestos diaphragm about every two months, which change can be readily made in about fifteen minutes time.
Use of Caustic Soda as Water Softener
The caustic soda formed by the cell can, if desired, be piped into the raw water, and its value thus utilized, due to the fact that caustic soda is a water softener.
Earle Bernard Phelps, in his Water Supply Paper, No. 229, Government bulletin, dealing with the “Disinfection of Sewage and Sew’age Filter Effluents,” states that he found that . chlorine attacks the organic matter and the bacteria simultaneously, but that its effect on the former is a direct function of its concentration, while its germicidal effect does not bear such exact relation.” If this is true for sewage, it would also be true for water. This important fact is availed of in the functioning of the Williams cell, by mixing the chlorine with air before its application to the water, thus decreasing its concentration and thereby retarding its action towards the organic matter in the water and causing a longer contact period with the bacteria, which is most important, because, as Mr. Phelps says, “. . . disinfectant action is a function of the chlorine concentration and of the time of contact.”
Description of the Test
At various times questions have come up as to the effect of chlorine on the iron surfaces it comes in contact with; as to the caustic soda produced causing damage to workmen as a result of its caustic action producing burns; as to the liability of the cells exploding. The following is the report of a test made by the Kansas City Testing Laboratory concerning the effect that chlorine applied in the manner as applied in the Williams cell process, might have upon pipes and parts of machinery it might come in contact with.
Two test rods each 1/2 inch in diameter, were introduced into the Side of a water main to a depth of about 6 inches. The water in this main flowed continuously in one direction and the chlorine was introduced into this main at a point between the two test rods. By this arrangement both test rods were subjected to exactly the same conditions except that rod No. 1 was immersed in the water before chlorination, while _rod No. 2 was immersed in the water after chlorination. The rods were of Bessemer steel.
The chlorine treatment used in this test was that of the regular chlorinating operation used at the Independence, Missouri, pumping station, and was made at that plant during the regular run for a period of two months.
Weight of rod No. 1, before test, 144,0740 Grams.
Weight of rod No. 2, before test, 143,7345 Grams.
After One Month’s Exposure
Rod No. 1 weighed 144,1820 Grams, a gain of 0.1080 Grams or 0.074%
Rod No. 2 weighed 144.1745 Grams, a gain of 0.4400 Grams, or 0.31%
After Two Months’ Exposure
Rod No. 1 weighed 144.0722 Grams, a loss of 0.1098 Grams, or 0.01%
Rod No. 2 weighed 144.2272 Grams, a gain of 0.0527 Grams, or 0.036%
Some Results of the Test
The gain in all these cases was due to the formation of a coating of iron oxide on the exposed surfaces of both test rods. It will lie noted that roil No. 2, which was subjected to chlorinated water, gained most in weight during the first month. This, of course, was due to the formation of a greater amount of oxide. A point of great importance is to he noted from the fact that this rod No. 2 gained only 0527 grams during the second month’s exposure. This shows that the coating of iron oxide was acting as a protective coating and that, with a little more exposure, the iron oxide would cease to form or, at least, would form no faster than one could expect from the natural disintegration of the pipe. This coating was strongly adherent.
Rod No. 1 showed a loss during the second month’s exposure. This probably resulted from the dropping off of some of the iron oxide which had formed on it. This would leave the surface more exposed and oxidation would takeplace easily on such exposed surface. The oxide on this roil was not so firmly adherent as that on rod No. 2, and formed in spots, for the greater part, rather than over the entire surface. This report was signed by J. G. Hawthorne, manager of the Kansas City Testing Laboratory. It disposes of the question that chlorine acts corrosively on the iron pipes.
That any damage might happen to workmen because of the caustic burns, the writer fails to see the possibility thereof. The caustic soda is not handled in any way by the workmen and it seems unlikely and almost an impossibility that they would come in contact with it. Moreover, the caustic is so diluted that it would not exert any detrimental action.
That the cell is likely to explode is something the writer can not figure out. but can say he has operated the cell now about a year and nothing like an explosion has yet taken place. The possibility of explosion appears to the writer as a mere myth.
(Excerpts from paper read before the annual convention of the Iowa Section, American Water Works Association.)