JP4522534B2 - Water purification method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a water purification method for performing coagulation sedimentation treatment and non-corrosion of water. Specifically, it is an improved method of a water purification treatment method for injecting a slaked lime aqueous solution and carbon dioxide gas, and is provided with a slaked lime aqueous solution production apparatus and a carbon dioxide gas injection apparatus in a water purification treatment facility, and the slaked lime aqueous solution production apparatus The present invention relates to a water purification treatment method that simultaneously achieves high-efficiency coagulation sedimentation treatment and non-corrosion of water by injecting undissolved substances and carbon dioxide gas from appropriate portions of the water purification process.
[0002]
[Prior art]
Conventionally, in a surface water treatment plant, a coagulation-precipitation / filtration process in which a coagulant is added to the water to be treated (hereinafter referred to as raw water) is indispensable. As coagulants, polyaluminum chloride (PAC) and sulfate bands are used. Is used. In recent years, the amount of flocculant used tends to increase with the deterioration of surface water quality, especially in order to increase the flocculation efficiency when the raw water introduced into the landing well is at low temperature, low turbidity, or low alkalinity. It is necessary to increase the injection rate of the agent. As a result, the amount of flocculant used is increased, the amount of sludge generated is increased, the flocculant is mixed into the filtered water, the cost of the flocculant is increased, and the corrosiveness of the purified water is increased due to the increase in chloride and sulfate ions derived from the flocculant. The problem of increase arises.
Further, when the pH of the raw water introduced into the landing well is high, an acid such as sulfuric acid is injected to lower the pH to perform an agglomeration treatment, and then an alkaline agent is injected to obtain an appropriate pH value. However, in free carbonic acid and water with low alkalinity, the pH buffering property is small, so increasing the injection amount of the alkaline agent will greatly increase the pH and easily exceed the upper limit value (8.6 in Japan). Therefore, it is difficult to inject a large amount of slaked lime aqueous solution, and it is impossible to increase the Langeria index, which is an index for determining the corrosion of water, to improve noncorrosive water.
Furthermore, there is concern that aluminum will cause Alzheimer's disease.
For these reasons, studies are being made to reduce the amount of purified water sludge generated and chemical costs, and to reduce the injection rate of PAC and sulfuric acid bands.
[0003]
On the other hand, a technique for injecting a slaked lime aqueous solution for non-corrosion of water is also becoming widespread with the advent of an apparatus for continuously producing a slaked lime aqueous solution disclosed in, for example, Japanese Patent No. 1459175. In the apparatus disclosed in this publication, there is a problem that undissolved substances that are difficult to dissolve remain in an aqueous solution having a pH of 12 or more, but the undissolved substances are treated in water maintained at a pH of 10 or less and then introduced into the landing well. This method is solved (Japanese Patent No. 141605). Japanese Patent No. 2048482 discloses that a slaked lime aqueous solution and carbon dioxide gas are used in combination to increase the Langeria index and make water non-corrosive.
Furthermore, Japanese Patent Laid-Open No. 6-83566 proposes that the concentration of the hardly soluble floc produced during the production of the slaked lime aqueous solution is increased, and this floc is introduced into the mixing pond and used as an alkali agent. However, it does not improve the coagulation sedimentation. There is a need for a method that achieves high-efficiency coagulation precipitation and non-corrosion of water simultaneously.
[0004]
[Problems to be solved by the invention]
In view of the current situation, the present inventors have studied a method for simultaneously solving high-efficiency coagulation sedimentation and non-corrosion of water, and as a result, improved and developed a conventionally known slaked lime carbon dioxide combined injection method. That is, by injecting undissolved matter, slaked lime solution and carbon dioxide gas from the slaked lime dissolving apparatus into appropriate portions of the water purification treatment process according to the water quality, the undissolved matter is a core function of aggregation in the coagulation sedimentation operation. In addition to the slaked lime solution and carbon dioxide gas normally used for water quality improvement, and the undissolved material coming out of the slaked lime dissolving equipment, it is necessary to use additional chemicals such as coagulant aids. Thus, the present inventors have found that agglomeration and precipitation can be effectively carried out and completed the present invention.
[0005]
[Means for Solving the Problems]
The present invention provides a slaked lime aqueous solution production device and a carbon dioxide gas injection device in a water purification treatment facility including a landing well, a mixing pond, a sedimentation basin, a filtration pond and a distribution pond, and adjusts the water quality by injecting the slaked lime aqueous solution and carbon dioxide gas. In water purification treatment, the pH, alkalinity, calcium hardness, etc. of the water to be treated are measured to determine the Langeria index, pH 7.5-8.3, alkalinity 60-80 mg / l, Langeria index, which is a preferred water quality for corrosion prevention Determine the injection rate of slaked lime and carbon dioxide gas in the amount of -1 to 0, and in the sedimentation basin of the water purification treatment facility or in the previous process, undissolved slurry, carbon dioxide gas and slaked lime aqueous solution coming out of the slaked lime solution manufacturing apparatus Adjust the pH to 6.7 to 7.3 and adjust the calcium bicarbonate concentration to 80 to 160 mg / l, increase the pH buffering property, and then inject the flocculant to perform aggregation, precipitation, and filtration. Further, remove the water from the filtered water. By injecting ash solution and / or carbon dioxide, a water treatment method characterized by performing high efficiency flocculation and non-corrosive of water at the same time.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a slaked lime aqueous solution production device and a carbon dioxide gas injection device in a water purification treatment facility including a landing well, a mixing well, a sedimentation basin, a filtration pond, and a water distribution basin. Slaked lime aqueous solution, undissolved slurry and carbon dioxide gas from the apparatus are injected according to the water quality.
The undissolved slurry in the present invention refers to a slurry containing undissolved material remaining without being dissolved in a slaked lime aqueous solution having a pH of 11 or more in a slaked lime aqueous solution manufacturing apparatus. The solid in the undissolved slurry is mainly calcium carbonate and contains impurities of undissolved slaked lime and other raw material slaked lime contained in the calcium carbonate.
As the slaked lime dissolving device, for example, a device as described in Japanese Patent No. 1459175, which introduces water from the lower part of the stirring tank and continuously takes out the slaked lime aqueous solution from the upper part can be used. Undissolved slurry is removed from the bottom of the tank after the introduction of water is stopped.
[0007]
In carrying out the present invention, prior to the water purification treatment, the pH, alkalinity, calcium hardness and the like of raw water are measured in advance to obtain the Langeria index, and pH 7.5 ~ 8.3, The injection rate of slaked lime and carbon dioxide gas in an amount that makes the alkalinity 60-80 mg / l and Langeria index -1-0 is determined.
[0008]
The injection rate of the carbon dioxide gas and the slaked lime aqueous solution adjusts the pH of the water during the coagulation operation using part or all of the predetermined amount as described above. For example, when the pH of the raw water is higher than a predetermined pH, carbon dioxide is injected, and when the pH is low, a slaked lime aqueous solution is injected to adjust the pH to the predetermined pH. This pH adjustment may be performed after the addition of the flocculant, but may also be performed by injecting carbon dioxide gas and / or slaked lime aqueous solution before the addition of the flocculant in anticipation of the degree of pH decrease due to the addition of the flocculant. it can. In addition, the calcium bicarbonate concentration of water during the coagulation operation is adjusted to be in the range of 80 to 160 mg / l. When the concentration of calcium bicarbonate in raw water is lower than 80 mg / l, the buffering capacity is small, and even if the pH at the time of adding the flocculant is adjusted within the predetermined range, the contained floating substances can be sufficiently settled and separated. Can not. The calcium bicarbonate concentration is adjusted using slaked lime and carbon dioxide.
A typical case will be described below.
The amount of the undissolved material corresponding to the properties of the raw water is preferably injected into the landing well, and then injected with carbon dioxide and slaked lime aqueous solution to cause coagulation and precipitation, ie, pH 6.7 to 7.3, calcium bicarbonate The concentration is adjusted to 80 to 160 mg / l, preferably pH 6.8 to 7.2, and the calcium bicarbonate concentration is adjusted to 90 to 120 mg / l. Thus, by adjusting using an insoluble matter, it becomes possible to form a heavy floc without using a coagulant auxiliary agent at the time of coagulation sedimentation.
When the raw water has a low pH and low alkalinity, first use a slaked lime aqueous solution and, if necessary, carbon dioxide, and if the raw water has a high pH, first, carbon dioxide and, if necessary, slaked lime. The aqueous solution may be used to adjust the properties close to those preferable for the aggregation reaction as much as possible, and then the undissolved material and the flocculant may be added. After adding the insoluble matter and the flocculant, the pH and calcium bicarbonate concentration may be further adjusted. In some cases, the use of a coagulant aid or the like is not excluded.
Undissolved material is preferably injected in a 2-3% slurry. Carbon dioxide is injected as a whole or a part of the necessary amount for adjusting the Langelia index determined in advance to -1 to 0. The carbon dioxide gas inlet is provided at a position deeper than the appropriate depth of 2 m depending on the size of the landing well or the mixing well.
After injecting undissolved material into the landing well, carbon dioxide gas is injected into the landing well or the mixing well, and the pH is adjusted to 6.7 to 7.3. When the pH after the treatment is as low as 6.7 or less, slaked lime aqueous solution is injected to adjust the pH to be within the above range. When the slaked lime aqueous solution is required, when taking out the undissolved material from the slaked lime dissolving tank, it can be taken out as a slurry together with a required amount of the slaked lime aqueous solution and added to the landing well. If the pH after the treatment is higher than 7.7, the amount of carbon dioxide injection is increased.
[0009]
The water adjusted in this way has a large buffering capacity, and when a flocculant such as PAC is added, the undissolved material added previously performs the core function of aggregation, forming a heavy floc and increasing its sedimentation rate. Become. As a result, the settling speed of the generated floc is high and efficient processing becomes possible. Moreover, since the addition of the conventionally used flocculating aid is not necessary and most of the added undissolved slaked lime and calcium carbonate are dissolved, the amount of solids to be separated is significantly reduced. .
[0010]
[Example 1]
The raw water was a highly corrosive surface water with a water temperature of 15 ° C., pH 6.7, alkalinity 5 mg / l, calcium hardness 5 mg / l, Langeria index −3.8, and turbidity 50 °. As the water quality improvement target value as pH 7.8, alkalinity 70 mg / l, calcium hardness 70 mg / l, Langeria index -0.5 as purified water with low corrosiveness, the required injection rate of slaked lime and carbon dioxide gas was determined by experiment. 48 mg / l and 57 mg / l.
The total amount of slaked lime required to achieve the water quality improvement target value for raw water of 20 m ³ / day was calculated as 1 kg / day, and the amount of carbon dioxide was calculated as 1.2 kg / day.
Diameter 0.22 m, height 1.5 m, was charged with slaked lime 1.2kg to cylindrical vertical low speed stirrer with dissolver volume 0.06 m ^3, and water from the bottom of the dissolution tank pH12 or more overflow liquid in the intermediate vessel I saved it. When 80% of the slaked lime is dissolved and overflows to the intermediate tank, the water supply to the dissolution tank is stopped and the undissolved matter remaining in the lower part of the dissolution tank after being allowed to settle for 1 hour is put into an undissolved slurry tank. Extracted. The concentration of slaked lime obtained in the intermediate tank was approximately 1500 mg / l. The undissolved slurry had a pH of 12.0 and a slurry concentration of 2.9%, and the components of the undissolved material were calcium carbonate 57%, slaked lime 40%, silicon dioxide 2%, and aluminum oxide 0.2%.
[0011]
The water purification process was performed as follows by the water treatment flow shown in FIG. The raw water 20m ³ / day was taken into the landing well and the undissolved slurry was injected at 0.3 l / h, resulting in a pH of 10.0. When 57 mg / l of carbon dioxide was injected into this mixed water and then 19.3 l / h of 1500 mg / l of slaked lime aqueous solution, pH 7.1, alkalinity 59 mg / l, calcium hardness 60 mg / l, calcium bicarbonate concentration It became 98 mg / l water. PAC was injected at a dose of 25 mg / l to perform aggregation precipitation. The turbidity of the sediment supernatant was 3 degrees, and the efficiency of coagulation sedimentation was 94%. The precipitate supernatant was subjected to sand filtration to obtain filtrate having a turbidity of 0.1 degree. When 5 l / h of slaked lime solution was injected into the filtered water, non-corrosive water having a pH of 7.8, an alkalinity of 67 mg / l, a calcium hardness of 72 mg / l and a Langeria index of -0.5 was obtained.
[0012]
[Comparative example]
Using the same raw water, slaked lime aqueous solution and PAC as in the examples, a batch experiment of coagulation precipitation was performed. Since injection of slaked lime aqueous solution alone does not provide pH buffering to raw water, injection of a small amount of slaked lime, 1.8 mg / l, raises the pH to 8.6, and the water quality at this time is 7 mg / l for alkalinity and 7 mg / l for calcium hardness. It was impossible to set the Langeria index to -0.5 at a pH of 7.8, an alkalinity of 70 mg / l, a calcium hardness of 7 mg / l, which is a preferable water quality value for clean water with low corrosivity, and a Langeria index of -1.6. . Next, the slaked lime injection rate required to set the pH for the coagulation treatment to the same 7.1 as in the example was determined to be 0.8 mg / l.
According to the water treatment flow shown in Fig. 1, 0.5 mg / l of slaked lime aqueous solution of 1500 mg / l is injected into raw water 20 m ³ / day, and PAC is injected 25 mg / l into this, Aggregation precipitation was performed. The turbidity of the precipitate supernatant was as high as 8 degrees, and when this was subjected to sand filtration, the turbidity became 0.5 degrees of filtered water. When the quality of purified water obtained by injecting slaked lime aqueous solution to pH 7.8 into this filtered water was measured, it was found to be non-corrosive water with a calcium hardness of 11 mg / l, alkalinity of 7 mg / l, and Langerian index -2.3. In addition, the coagulation precipitation efficiency was 79%, which was lower than that of the example.
[0013]
[Example 2]
The raw water temperature is 28 ° C. It was surface water with a pH of 9.4, an alkalinity of 57.1 mg / l, a calcium hardness of 58.5 mg / l, a Langeria index of 1.3, and a turbidity of 30 degrees.
Water quality improvement target values as purified water were set at pH 7.8, alkalinity 65 mg / l, calcium hardness 58.5 mg / l, Langeria index -0.2, and the required injection rates of slaked lime and carbon dioxide were determined experimentally, 10 mg / l each. l, 16.1 mg / l.
It was calculated as 0.02 kg / day of total slaked lime and 0.32 kg / day of carbon dioxide required to achieve the above water quality improvement target values for raw water of 20 m ³ / day. Diameter 0.22 m, height 1.5 m, was charged with slaked lime 1.2kg to cylindrical vertical low speed stirrer with dissolver volume 0.06 m ^3, and water from the bottom of the dissolution tank pH12 or more overflow liquid in the intermediate vessel I saved it. When 80% of the slaked lime is dissolved and overflows to the intermediate tank, the water supply to the dissolution tank is stopped and the undissolved matter remaining in the lower part of the dissolution tank after being allowed to settle for 1 hour is put into an undissolved slurry tank. Extracted. The concentration of slaked lime obtained in the intermediate tank was approximately 1600 mg / l. The undissolved slurry had a pH of 12.1 and a slurry concentration of 2.9%.
The raw water 20m ³ / day was treated as follows by the water treatment flow of FIG. That is, first, raw water was taken into the landing well, and undissolved slurry was injected here to 0.06 l / h and carbon dioxide gas to 16.1 mg / l, pH 7.2, alkalinity 58.7 mg / l, calcium hardness It was 60.7 mg / l and the Langeria index -1.0. Subsequently, about 1600 mg / l of slaked lime solution was injected as 3 l / h as a pre-alkali, and PAC was injected at 36 mg / l, pH 7.3, alkalinity 60.5 mg / l, calcium hardness 67.9 mg / l, Langeria index -0.8 It was. When this water was coagulated, the turbidity of the sediment supernatant was 4 ° C., and the coagulation efficiency was about 87%. When this precipitated supernatant was subjected to sand filtration, the turbidity of the filtrate was 0.1 degree. When 2 l / h of slaked lime solution was injected into the filtrate, non-corrosive water having a pH of 7.8, an alkalinity of 65 mg / l, a calcium hardness of 71.9 mg / l, and a Langeria index of -0.2 was obtained.
[0014]
【The invention's effect】
According to the method of the present invention, regardless of whether the raw water is low-temperature, low-turbidity water, low-alkaline water, or pH-high water quality, a heavy floc can be formed without using a coagulant aid when coagulating and sedimenting. Therefore, coagulation and precipitation can be performed effectively, and non-corrosive water can be easily obtained.
[Brief description of the drawings]
FIG. 1 is a flow showing an example of water treatment according to the present invention.

Claims (1)

In water purification treatment that adjusts water quality by injecting slaked lime aqueous solution and carbon dioxide gas into water purification treatment facilities including landing wells, mixing wells, sedimentation basins, filtration ponds and distribution ponds, and injecting slaked lime aqueous solution and carbon dioxide gas, The pH, alkalinity, calcium hardness and the like of the water to be treated are measured to obtain the Langeria index, and the water quality is preferably 7.5 to 8.3, alkalinity 60 to 80 mg / l, Langeria index-1 Determine the injection rate of slaked lime and carbon dioxide gas in an amount of ˜0, and inject the undissolved slurry obtained from the slaked lime aqueous solution manufacturing apparatus into the sedimentation basin or the previous step and based on the determined injection rate A slaked lime aqueous solution obtained separately from the undissolved slurry from the slaked lime aqueous solution production apparatus, and carbon dioxide gas obtained from the carbon dioxide gas injection device were injected to adjust the pH to 6.7 to 7.3, The concentration of lucium was adjusted to 80 to 160 mg / l, flocculation, precipitation, and filtration were carried out in the presence of a flocculant, and slaked lime aqueous solution and / or carbon dioxide gas was injected into the filtered water based on the determined injection rate. 8.3, A water purification method for simultaneously performing high-efficiency coagulation sedimentation treatment and water non-corrosion, having an alkalinity of 60 to 80 mg / l and a Langeria index of −1 to 0.

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