Method of improving city water and an apparatus therefor

Abstract: The present invention provides a method of treating a water supply to alter its Langelier index and pH, the method comprising: (1) combining slaked lime with water to form a slaked lime aqueous solution and withdrawing undissolved matter therefrom as slurry; (2) mixing the withdrawn slurry with carbonic acid-containing water to produce a calcium hydrogen carbonate aqueous solution; and (3) adding the calcium hydrogen carbonate aqueous solution together with the slaked lime aqueous solution to the said water supply. An apparatus suitable for carrying out such a method is also provided.

Description: The present invention relates to a method of and an apparatus for improving the Langelier index and pH of city water for preventing corrosion of city water pipes. Conventionally, for preventing corrosion of water pipes, in other words, preventing red rust water, the caustic soda injection method, the soda ash injection method, the slaked lime injection method and the like are known, however, the slaked lime injection method recently draws attention. The reasons why slaked lime is often adopted in water purifying plants are as follows. (1) The slaked lime not only increases pH of water, but it also improves the Langelier index, (2) it has calcium which is said to also be good for health as its main component, and (3) a technique for injecting slaked lime not as powder but as an aqueous solution has been developed (for example, US Patent No 4 366 142). However, when the slaked lime is injected as an aqueous solution, since a greater or lesser amount of carbonic acid is dissolved in ordinary city water to be used for producing a slaked lime aqueous solution, it is impossible to avoid the formation of calcium carbonate during dissolution of the slaked lime. In addition, calcium carbonate is formed on the surfaces of undissolved slaked lime particles, so that depending on the carbonic acid content of the water used, a quantity of solid matter remaining as undissolved matter during production of the slaked lime aqueous solution reaches 10 or more percent of the raw material slaked lime. Thus, other than a problem of lowering the effective utilisation percentage of slaked lime, a problem occurs in treatment of non-utilised solid matter which is usually discarded. On the other hand, according to the water quantity standard, the pH of drinking water is determined to be 5.8 to 8.6, however, from a viewpoint of preventing corrosion of water pipes, that is prevention of red rust water, it is preferable that the pH of city water is higher, and it is desired to be at least pH 7, and to have a Langelier index of not less than -1.0, preferably not less than -0.5, more preferably -0.5 to 0.5. EP-A-0 380 299 discloses a method of modifying the Langelier index of water which involves dissolving carbon dioxide in the water before adding a saturated solution of slaked lime. JP-A-58-223481 discloses that slaked lime remaining undissolved in the slaked lime dissolving tank is taken out as a slurry and water is added to said slurry to dissolve the slaked lime in the slurry and then the solution thus obtained is injected in city water. Thus, the subject of the present invention is to provide complete utilisation of slaked lime, namely to utilise it without leaving any undissolved calcium and to provide a method of improving the pH and the Langelier index of city water and an apparatus for carrying it out. The Langelier index (LI) is an overall index of city water comprising the calcium hardness, pH and alkalinity, which is represented by the following equation: LI = pH - pHs = pH - 8.313 + log[Ca 2+ ] + log[A] - s wherein pH - measured pH value of water to be tested, pHs - theoretical pH value of water to be tested when CaCO₃ in the water maintains equilibrium according to the following equation - Ca⁺⁺ + HCO₃ ↔ CaCO₃ + H⁺, log[Ca²⁺] - logarithm of calcium ion concentration, log[A] - logarithm of overall alkalinity, s - correction value. The present invention provides a method of treating a water supply to alter its Langelier index and pH, the method comprising the steps of: preparing an aqueous solution of slaked lime by dissolving slaked lime in water in a dissolving vessel; withdrawing any undissolved matter therefrom as an aqueous slurry; preparing carbonic acid containing water of not less than 100 ppm carbonic acid by forcibly dissolving carbon dioxide in water; mixing and reacting the withdrawn slurry with the carbonic acid-containing water to produce an aqueous solution of calcium hydrogen carbonate; and adding the aqueous solution of calcium hydrogen carbonate together with the aqueous solution of slaked lime to said water supply. Further, the present invention also provides an apparatus suitable for use in treating a water supply to alter its Langelier index and pH, the apparatus comprising: a slaked lime dissolving vessel provided with an agitation mechanism for preparing a slaked lime aqueous solution; means for withdrawing any undissolved matter from the dissolving vessel as a slurry; a slurry vessel for storing the slurry; a line mixer for dissolving carbon dioxide in water to form carbonic acid containing water having not less than 100 ppm carbonic acid; a mixing and reacting vessel for mixing and reacting the slurry with the carbonic acid-containing water to produce a calcium hydrogen carbonate aqueous solution; and a water-receiving vessel for adding the slaked lime aqueous solution and the calcium hydrogen carbonate aqueous solution to city water; means for injecting the calcium hydrogen carbonate aqueous solution from the mixing and reacting vessel into the water-receiving vessel; means for transferring an aqueous solution from the dissolving vessel to the water receiving vessel. The present invention will be further described hereinafter, with reference to the following description of an exemplary embodiment and the accompanying drawings, in which: - Figure 1 is a flow sheet showing an embodiment of the improving method for city water according to the present invention. In Figure 1, 1 is an agitation mechanism, 2 is a slaked lime dissolving vessel, 6 is a slurry vessel, 8 is a mixing and reacting vessel, 9 is a line mixer and 10 is a water-receiving vessel. The present invention will be explained in detail hereinafter in accordance with Figure 1. The production of a slaked lime aqueous solution to be added to city water used for improving city water in the present invention is achieved as follows. A dissolving vessel 2 having an agitation mechanism 1, for example an ordinary stirrer as exemplified in US Patent No 4 366 142 is filled with water beforehand, and a predetermined amount of slaked lime is introduced, and then water is supplied from a water supply pipe 3 at a lower portion of the vessel with agitation at a low speed, thereby a substantially saturated aqueous solution of slaked lime is withdrawn from a withdrawing pipe 4 at an upper portion of the vessel. The obtained slaked lime aqueous solution is mainly used for adjustment of the pH of city water. The slaked lime aqueous solution for pH adjustment can be used for any one of the pre-alkali treatment, intermediate alkali treatment and post-alkali treatment during the water purifying treatment. Incidentally, the term "city water" used here means water subjected to ordinary water purifying treatment so as to be drinkable. As water to be used for the formation of the slaked lime aqueous solution, water subjected to ordinary purifying treatment in a water purification plant is used, however, it will inevitably contain more or less carbonic acid depending on the source thereof. Thus, a part of slaked lime becomes scarcely soluble calcium carbonate in the slaked lime dissolving vessel, and most of the calcium carbonate is formed on surfaces of slaked lime particles, so that the slaked lime covered with calcium carbonate also remains undissolved. Therefore, most of undissolved solid matter comprises calcium carbonate and slaked lime, an amount of which is 10% to approximately 20% of raw material slaked lime, depending on the carbonic acid content of the dissolving water. The undissolved solid is withdrawn from a slurry withdrawing pipe 5 at a lower portion of the dissolving vessel, which is first stored in a slurry vessel 6 and then quantitatively sent to a mixing and reacting vessel 8 by means of a pipe 7, for use in the production of calcium hydrogen carbonate, whereby almost all of it is dissolved. The calcium component used for improving city water is added in the form of calcium hydroxide for adjustment of pH, and for improving the Langelier index, it is added in the form of calcium hydrogen carbonate which does not change the pH as much. The ratio of addition of calcium hydroxide and calcium hydrogen carbonate is different, depending on the nature of the source water (Langelier index, pH, carbonic acid concentration and the like) and the desired Langelier index and pH, however, the amount of calcium hydrogen carbonate added is at least ¼, or often not less than ½ the amount of calcium hydroxide, so that even when the undissolved matter reaches 20% of the raw material slaked lime during production of the slaked lime aqueous solution, there is no problem in the method according to the present invention in which it is added in the form of calcium hydrogen carbonate. The undissolved solid in the slaked lime dissolving vessel 2 is withdrawn from the lower portion of the dissolving vessel 2 and is transferred to the slurry vessel 6 via the pipe 5, however the concentration of the slurry at this time, that is the amount of the slaked lime aqueous solution withdrawn together with the undissolved solid is determined in accordance with the ultimately desired Langelier index, the nature of the source water and the like. In many cases, this slurry is subsequently subjected to the production of calcium hydrogen carbonate in a concentration of 1 to 10%, preferably 2 to 5%, so that the concentration of the slurry withdrawn from the slaked lime dissolving vessel is preferably 1 to 10%. Indeed, it is also possible that the concentration of the slurry withdrawn from the slaked lime dissolving vessel is higher and then water is separately added to adjust to a concentration suitable for the subsequent reaction. The production of calcium hydrogen carbonate is performed by mixing and reacting the undissolved solid slurry in the dissolving vessel with carbonic acid-containing water, and a line mixer 8 is preferable as the mixing and reacting vessel 8, because it has an easy mixing and reacting property and the apparatus can be made compact. The carbonic acid-containing water to be used herein can be obtained by injecting carbon dioxide into city water and allowing it to dissolve. The carbonic acid content of this carbonic acid-containing water may be an amount sufficient for converting all calcium from the slurry vessel 6 simultaneously supplied to the reacting and mixing vessel 8 in the form of calcium hydrogen carbonate. Therefore, considering a calcium content in the slurry supplied to the reacting and mixing vessel 8, the carbonic acid concentration and the supply amount of carbonic acid-containing water are determined. The higher the carbonic acid concentration of the carbonic acid-containing water is, the better and a carbonic acid aqueous solution not less than 100 ppm, desirably not less than 300 ppm, more desirably not less than 400 ppm is used. For this purpose, it is preferable to dissolve carbon dioxide gas in water by force using the line mixer 9 under pressure, preferably under pressure of 2 to 3 kg/cm². A line mixer having a structure in which a porous plate or a filler is installed in a pipe passage is used. The calcium hydrogen carbonate aqueous solution obtained in the mixing and reacting vessel 8 is injected into city water in a water-receiving vessel 10 in order to improve the Langelier index of city water. It is necessary to make all calcium into calcium hydrogen carbonate in the mixing and reacting vessel 8 to allow free carbonic acid to remain as represented by the following formula, however, the excess carbonic acid makes the pH of water acid. Therefor, it is suitable also for the improvement in the Langelier index to use the substantially saturated aqueous solution of slaked lime obtained in the dissolving vessel 2 as the alkali for adjustment pH after injecting calcium hydrogen carbonate aqueous solution into city water. CaCO₃ + nCO₂ + H₂O → Ca(HCO₃)₂ + (n-1)CO₂ The substantially saturated aqueous solution of slaked lime obtained in the slaked lime dissolving vessel may be directly injected into the water-receiving vessel from the dissolving vessel, or may be first stored in a storing vessel. When slaked lime is made into an aqueous solution for use to prevent corrosion of water pipes, namely for preventing red rust water, 10 and several percent of slaked lime becomes undissolved calcium carbonate or slaked lime particles covered with calcium carbonate, which has previously been discarded, however, according to the present invention, almost all the undissolved calcium components can be dissolved by reacting with the carbonic acid-containing water, and further the calcium hydrogen carbonate thus obtained can be profitably utilised for improving the Langelier index. EXAMPLE An experiment was conducted using an apparatus as shown in Figure 1 consisting of a slaked lime dissolving vessel of 250 l, a slurry vessel of 75 l, a line mixer for dissolving carbon dioxide gas having a length of 1 m, a line mixer for producing calcium hydrogen carbonate having a length of 1 m, and a water-receiving vessel of 1 m³. City water, which was used for this experiment, had a calcium hardness of 41.3 ppm, an overall alkalinity of 60.4 ppm, a free carbonic acid concentration of 26.8 ppm, pH of 6.2 and a Langelier index of -2.3. 13 Kg of slaked lime was charged into the slaked lime dissolving vessel, and city water was supplied from a lower portion of the vessel to form a substantially saturated slaked lime aqueous solution (about 1600 ppm concentration), and when the obtained slaked lime aqueous solution was withdrawn from an upper portion of the vessel and used for improving the quality of the city water, the undissolved solid was about 2 kg at the time (after 46 hours) when a slaked lime concentration of water obtained from the slaked lime dissolving vessel lowered to 1000 ppm. This was withdrawn from the lower portion of the vessel to the slurry vessel as slurry having a concentration of about 3%. Incidentally, this undissolved solid was calcium carbonate and slaked lime covered with calcium carbonate. The city water was treated using this slurry and the slaked lime aqueous solution obtained from the slaked lime dissolving vessel. In a similar manner as described above, 13 kg of slaked lime was charged into the slaked lime dissolving vessel, and city water was supplied from the lower portion of the vessel in 168 kg/hr to continuously produce a slaked lime aqueous solution having a concentration of about 1600 ppm. On the other hand, when 88.6 kg/hr of carbon dioxide gas was supplied to the line mixer, carbonic acid-containing water having a free carbonic acid concentration of 564 ppm was formed under pressure of about 2 kg/m² and 1.4 kg/hr of slurry having a concentration of about 3% was supplied from the slurry vessel to the line mixer for forming calcium hydrogen carbonate, there was little solid content, and a calcium hydrogen carbonate aqueous solution of 1000 ppm was obtained. When 90 kg/hr of this calcium hydrogen carbonate aqueous solution and 169 kg/hr of the slaked lime aqueous solution obtained in the slaked lime dissolving vessel were continuously injected under agitation into the water-receiving vessel to which 10 m³/hr of city water was continuously supplied, the obtained water had a Langelier index of 0 and a pH of 8.

Claims: A method of treating a water supply to alter its Langelier index and pH, the method comprising the steps of: preparing an aqueous solution of slaked lime by dissolving slaked lime in water in a dissolving vessel (2); withdrawing any undissolved matter therefrom as an aqueous slurry; preparing carbonic acid containing water of not less than 100 ppm carbonic acid by forcibly dissolving carbon dioxide in water; mixing and reacting the withdrawn slurry with the carbonic acid-containing water to produce an aqueous solution of calcium hydrogen carbonate; and adding the aqueous solution of calcium hydrogen carbonate together with the aqueous solution of slaked lime to said water supply. A method according to claim 1, wherein the undissolved matter comprises calcium carbonate or calcium carbonate deposited on slaked lime particles. A method according to claim 1 or 2, in which the withdrawn slurry is mixed with carbonic acid-containing water in a line mixer. An apparatus suitable for use in treating a water supply to alter its Langelier index and pH, the apparatus comprising: a slaked lime dissolving vessel (2) provided with an agitation mechanism (3) for preparing a slaked lime aqueous solution; means (5) for withdrawing any undissolved matter from the dissolving vessel as a slurry; a slurry vessel (6) for storing the slurry; a line mixer (9) for dissolving carbon dioxide in water to form carbonic acid containing water having not less than 100 ppm carbonic acid; a mixing and reacting vessel (8) for mixing and reacting the slurry with the carbonic acid-containing water to produce a calcium hydrogen carbonate aqueous solution; and a water-receiving vessel (10) for adding the slaked lime aqueous solution and the calcium hydrogen carbonate aqueous solution to city water; means for injecting the calcium hydrogen carbonate aqueous solution from the mixing and reacting vessel (8) into the water-receiving vessel (10); means (4) for transferring an aqueous solution from the dissolving vessel (2) to the water receiving vessel (10).

Inventors: FUNAHASHI ISAO, YAMAMOTO KIWAMU, FUNAHASHI, ISAO, YAMAMOTO, KIWAMU
Applicants: KUREHA CHEMICAL IND CO LTD, KUREHA KAGAKU KOGYO KABUSHIKI KAISHA
Application year: 1992
Main classification: C23F15
IPC codes: C02F5, C02F1, C23F15