Practice in the Use of Hypochlorite of Lime
In discussing the treatment of public water supplies with hypochlorites, it is not necessary to enter into the question of the efficiency of the process. That is a matter of common consent. The use of the material has extended over a sufficient length of time and under enough different conditions to make it possible to draw certain general conclusions as to the advisability of different devices in its application. The active agent—hypochlorous acid—may be obtained either from hypochlorite of lime or soda. The use of hypochlorite of soda, while preferable on account of greater solubility and ease in handling, is not practical at the present time on account of the higher cost the lowest figure being about 6.7 cents per pound of available chlorine. Hypochlorite of lime, ordinarily known as bleaching powder, may be obtained from a number of firms, among which are the Castner Electrolytic Alkali Company, of Niagara Falls, N. Y.; the Pennsylvania Salt Company, having a factory at Wyandotte, Mich., and the How Chemical Company, at Midland, Mich. For water purification use it is most satisfactorily obtained in drums containing either 100, 350 or 750 pounds, the material containing from 30 to 35 per cent, available chlorine. It may be kept in drums after opening for periods as long as a week except at times of extreme humidity. The successful application of any chemical to water depends upon the uniformity of application and thoroughness of mixing. It has been possible in the case of lime and iron at the St. Louis plant to apply these chemicals to the water in a powdered form, so that a great many of the problems of application, such as store of solution and the like, are eliminated. The application of calcium hypochlorite in this manner docs not, at least at the present time, seem to be wise. In the first place, the quantity to he use in any place is so much less than the ordinary coagulants that devices which would be acceptable for use in their case would not satisfy the needs of the hypochlorite. There is the further possibility that the moisture absorbing capacity of the hypochlorite would, on account of its tendency to ball up, seriously interfere with its regularity of application if the direct method were tried. It is possible when the flow’ of water to be treated is relatively uniform, changing only on twelve or twenty-fourhour periods, to make use of some such method as has been successfully used at the Indianapolis plant during the past few months. At this place it is the practice to divide the total amount to be fed during the twenty-four hours into batches and place these at the regular hourly intervals in a small dissolving tank of galvanized iron holding about 5 cubic feet of water. This tank is so arranged that the incoming water flows from the bottom of the tank up through the hypochlorite, being constantly stirred by means of a small propeller wheel actuated by’ a water motor and finally flowing out near the top of the tank through a 1-inch pipe into a larger equalizing tank. In the earlier experimental work it was attempted to so regulate the flow of water and application of chemical in the primary dissolving box so that it would not be necessary to use an equalizer. It was, however, found upon test under the best conditions possible of application that the variations between charges resulted in doses ranging from 50 per cent, of the intended amount to 150 per cent. The equalizing tank— also of galvanized iron is so arranged that the suspension of hypochlorite in the water is carried to the bottom, where the suspended matter settles out and the active solution displaces in the tank, flowing out at the opposite end from the point of application. This is so arranged as to give about one hour and a half standing in the equalizer and has reduced the variations in application to less than 2 per cent. The necessary features of a device such as this are (1) a small water motor, costing about $5 complete; (2) one small galvanized iron tank, 18×30, costing about 53; (3) a larger galvanized iron tank of 350 gallons capacity, costing approximately $10, and (4) piping connections. A device of this character will not entail the employment of any additional labor, attention being needed only once an hour, when not more than three or four minutes’ time is taken. In the case of filter plants where the flow of water is irregular, depending upon variations in the filtration or pumpage, such a method of application would not be advisable. Some such device as was installed by Mr. Brewster at the Seymour, Ind., plant may be used. Enough solution is made to treat one day’s supply stored in a small tank connected to the suction main by pipe line and valve, the flow into the main is varied by the draft of the pumps. It is also possible to make application of the sterilizing agent more accurately under these conditions by making up a solution sufficient for about one day’s time, storing this in the tank, connecting the tank to a small orifice box which will maintain a constant bead over a small opening of some sort and having this opening either controlled by a slide valve or a pinch cock closing over rubber tubing in such a way that the pump strokes will control the opening and closing of the valve or rubber tube. In this way it is possible to regulate the strength of the solution so that the required amount per stroke of the pump will be fed from the orifice box and directly as the pump strokes increase or decrease in rapidity the rate of opening and closing of the valve or tube connection will be changed. It is the author’s opinion that in any plant handling over 1,000,000 gallons of water per day either of these methods is not to be commended except as experimental—that is, until the operators of the plant or those in charge of the operation are sufficiently satisfied regarding the efficiency of the hypochlorite treatment to warrant the installation of permanent devices. Without question the best method of application of hypochlorite solution is by means of solution tanks and orifice boxes, which may or may not be connected to pumps so as to make a variable rate of flow. The controlling feature in the making up of the calcium hypochlorite solution is the amount of calcium oxide present. Analyses indicate an average amount of 45 per cent, calcium oxide is practically insoluble; so that to make up a solution without leaving any sludge would limit the strength to the solubility of the calcium oxide. Some such device as is in use at the Boonton or Montreal plant, where the solution tanks are connected to dissolving boxes so arranged that the entire charge is thrown into the dissolving box, the active agent leached out and run over into the storage tank and the inert sludge drawn away into the sewers, Is perhaps the best method of preparing the solution. The strength of the hypochlorite solution with this qualification is adjusted to the demand rather than to the solubility of the material. The small amounts used make it possible to operate with solutions of ½’ or 1 per cent, strength, the best practice being to make up such an amount of material as will last for twentyfour hours. As to the storage of hypochlorite solutions, the best tank to he used seems to he concrete. Any material, such as iron or galvanized iron, is destroyed more or leas rapidly, galvanized iron being pitted pretty thoroughly within two or three months. Use of wooden tanks has been accompanied in several places by some rather disagreeable experiences, the apparent tendency of the hypochlorite solution being to leach out certain of the extractives from the wood, which upon the application of the solution to the water, produce disagreeable tastes. Concrete tanks have been successfully used at Boonton, N. J., since the fall of 1908 without any particular deterioration. The stirring of the hypochlorite solution is to be done with a certain degree of caution. In the first place, when the material is being dissolved in the dissolving box it is not wise to stir the material so rapidly that any amount of the insoluble lime will be carried over into the storage tank. This, of course, necessitates that plenty of time he taken. The pipe line leading from the dissolving box to the storage tank is preferably carried almost to the bottom of the storage tank so that the material will not he to much exposed to the air. If a 1 per cent, to ‘i per cent, solution is used a great deal of stirring is not necessary during the period of application. It is not well advised to stir with air. Mechanical stirrers, operating at slow rates, are best adapted to this purpose, and certainly need not be used except to keep the solution relatively uniform during the time of emptying the tank. The experience at Boonton, N. j., has been that it is possible to keep the solution stored for two or three days without any appreciable loss. The difficulty in making perfect joints with lead-lined pipe, and the extreme rapidity with which all copper alloys are attacked by hypochlorite solutions, seem to make it just as advisable to make the piping connections in wrought or cast iron, it being expected to renew these from time to time. The only particular suggestion to be made regarding orifice boxes is one which, as a matter of personal opinion, is true regarding orifice boxes for use with any chemical solution, and that is the placing of the orifice opening on the side instead of the bottom of the tank. It has been customary in practically all cases to place orifices in the bottom of the tank, covered with about 1 foot head of the solution to be applied. Practically every solution used in water treatment carries along with it more or less insoluble matter, such as lint and strings from storage sacks, or pieces of wood from barrels, and in the case of iron sulphate and hypochlorites, insoluble matter from the chemical itself, which deposits on the bottom of the tank and in a great many cases shuts off the flow through the orifices without any knowledge of it being gained by the attendant. On this account it seems better in all cases to build an orifice box so that the solution will flow out through an opening in the side, over which a constant head of solution is kept, and preferably so arranged that there is at least as much room below the orifice for settling inside the tank as there is head above it. The amount of hypochlorite that may be used seems to depend to a certain degree upon the hardness of the water, the taste of chlorine being apparent in soft water much more quickly than in relatively hard water. If there is any oxidizahlc matter in the water, such as ferrous carbonate or various organic compounds, it is necessary that enough hypochlorite be used to oxidize these before -disinfecting action is obtained. It is, however, not clear what proportion of the organic matter may remain unoxidized and sterilization he effected. This must he developed according to local conditions. Ordinary conditions seem to require the use of about one part per 1,000,000, or 8 1/3 pounds of the dry material, the available chlorine content of which is 30 per cent, per 1,000,000 gallons. As to the application, one of the essentials is the thoroughness of the mixing. In one installation where hypochlorite has been used, the practice was, for a time, to allow the solution to flow into the main body of water at the top of a conduit. Although there was a considerable distance of flow before the water reached any consumer, it appeared that the friction in passing through the conduit did not produce sufficient stirring to adequately mix the solution with the water. As a result there was a taste in portions of the city using the supply, while in other portions there was no taste whatever. Either the use of a grid or very thorough stirring with a portion of the water to be treated is necessary in order that the mixture may he sufficient. In regard to the place of application there are several factors which must be taken into consideration—the speed of reaction being one. At the Boonton plant it was found that all of the action was completed within a flow of perhaps a quarter of a mile, judged to take place within fifteen or twenty minutes. Experiments which have been conducted at Indianapolis recently indicate—with water at 1 degree Centigrade (where the reactions would be most delayed)—that the results are accomplished in ten to fifteen minutes, that obtained at Harrisburg indicated that practically all of the sterilizing action was complete within a point about 15 feet from the point of application in the raw water pipe. Certain possibilities, however, as to delayed reactions would apparently make a period such as this latter too brief. Apparently all of the reactions are completed within one-half hour under zero temperature conditions. With this in mind, it seems permissible to apply the hypochlorite at some such place as will leave at least this much time before the water reaches the first consumer. The use of the material in connection with filters is subject to certain other conditions which must not be ignored. If, as in the case of the Bubbly Creek water at Chicago, where there is such a proportion of organic matter that serious growths would be produced in the sand layer, it is without question advisable to apply the hypochlorite before filtration. In the ordinary mechanical tilter, where the use is not constant, it is not altogether clear that it should be applied before filtration. Should the application be made long enough before the water reaches the titters to insure the completion of all sterilizing action the application of such sterile water to the filter may under certain conditions encourage growths within the sand layer. If the reactions are not complete and the sterilizing goes on while the water is passing through the tilter, there will be necessarily a certain portion of time when the ordinary growths in the sand layer will be dislodged and passed on into the filtered water with an increase in bacterial content. This will occur until the sand layer is thoroughly sterilized. Following this, as long as the use of hypochlorite is continued it is possible to produce a filtered water of very low bacterial content, if not sterile. In connection with, slow sand filters it is advisable to make the application after filtration. There has been during recent yeafs a considerable tendency on the part of those operating slow’ sand filters to introduce such pretreatment as would make it possible to operate the filter units with a water of practically constant character, such a supply being obtained either by the use of preliminary coagulation and sedimentation or prefiltration. Although this has resulted in the obtaining of great efficiency in point of quantity of water produced and ordinarily more satisfactory bacterial efficiency, there are times, particularly when the water being filtered is at the .freezing point, that the bacterial reduction is considerably reduced. It would appear from these results that the removal of so large an amount of organic and suspended matter from the raw water results in a deficiency of “Schmutzdecke” forming material, so that when the conditions of bacterial growth are unfavorable, such as is the case at zero temperatures, the ordinary results of slow sand filtration are not easily attained. The pretreatment of a slow sand filter influent with such material as hypochlorite of lime, removing from the water practically all of the bacterial life, would increase the above-mentioned difficulties. Since there are no deleterious products left in the water after hypochlorite treatment and no advantage to be gained by subsequent filtration, it is advisable to make the application after the water has passed the filters. Like every other device or operation which has been developed in connection with filtration processes, there is a certain amount of ignorant public sentiment to be expected, and more or less foolhardy schemes may be put into use which will result in complaint on the part of consumers. It must be conceded, however, that the efficiency of the process is so great and its application subject to so few possibilities of error that its use in connection with water purification as a secondary safeguard will be eventually accepted with universal consent.