JPH10296019A - Method for pretreating raw water - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for making raw water containing a suspension limpid more efficiently than by a conventional method, as a pretreating method. SOLUTION: This method for pretreating raw water is to add an acid to the raw water containing a suspension to set the pH value within the range of 3-8.5 and further add chitosan as a natural polymer flocculant so that the concentration of the flocculant falls within the range of 0.02-0.5 in terms of mg/l as a unit, and further, filter the raw water. Consequently, it is possible to make the filtrate highly lim. The pretreatment is made more efficient by carrying out the filtration process using a particulate filter medium with an effective diameter of 200-1000 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pretreatment method for clarifying raw material water containing suspended substances, for example, concentration, desalination, separation, purification by electrodialysis, reverse osmosis, or the like. And the fields of treatment of industrial water, clean water, waste water, etc.

[0002]

BACKGROUND OF THE INVENTION One of the problems in the field of electrodialysis and reverse osmosis is membrane fouling. The raw material water contains organic or inorganic colloidal substances and suspended substances (hereinafter referred to as suspended substances). It is considered that the suspended matter in the raw material water causes contamination of the membrane, and a pretreatment method capable of removing as much as possible is desired. Conventionally, various methods have been adopted as a method for removing suspended substances in raw water. For example, as a filtering material, sand, a particulate filtering material such as anthracite, a method of filtering using a ceramic or synthetic resin filter, a flocculant is added to form a floc of a suspended substance, and the filtration, sedimentation separation or There are a method of flotation separation, a method of filtering with a filtering material having an adsorption ability such as activated carbon, and the like.

[0003] The following is a description of some specific examples. In the field where the electrodialysis method is used, there is production of salt by ion exchange membrane electrodialysis (hereinafter referred to as salt production). In the ion exchange membrane electrodialysis apparatus used for salt production, an anion exchange membrane and a cation exchange membrane are alternately arranged, and a plastic spacer having a mesh shape as thin as 1 mm or less is inserted between the membranes to maintain a thin gap. The structure is such that a uniform flow of raw material water can be maintained. Due to such a special structure, clarification of seawater is important in order to minimize contamination of the ion exchange membrane and blockage of the passage between the membranes. The pretreatment method currently used in the field of salt production is often a gravity or pressure filtration method using a particulate filter medium such as sand having an effective diameter of 200 μm to 1000 μm.

However, a recent ion exchange membrane electrodialysis apparatus used for salt production is to make the gap between the ion exchange membranes thinner, reduce the electric resistance, and reduce the production cost. There has been a strong demand for saving energy by operating the apparatus continuously for a long period of time. However, it is difficult to obtain raw material water that satisfies this requirement by the conventional pretreatment method. It has become necessary to further reduce suspended materials to obtain clearer raw water. Next, there is desalination of seawater as a field using the reverse osmosis method. Two modules of a spiral type and a hollow fiber type are widely used in a reverse osmosis membrane desalination apparatus for removing fresh water by removing inorganic salts and organic substances in seawater. The spiral element is wound around a water collecting tube around a reverse osmosis membrane and a spacer, and the hollow fiber type element is formed by weaving a bundle of hollow fibers and bonding the ends. In any module, the surface of the reverse osmosis membrane has a dense structure, which can be called a kind of filtration.If there is a suspended substance in the raw water, it is deposited on the membrane and reduces the permeability of the water. It is indispensable to supply raw material water of clear and stable quality by pretreatment. In addition, in the industrial water field, the required water quality varies greatly depending on the type of industry and the intended use.However, suspended solids contained in raw material water are deposited on pipes, manufacturing equipment, boilers, etc. Cause various obstacles when used for product processing.
In the field of industrial water, clarification of raw water is also required. As described above, the field of use of the present invention requires a pretreatment method that can efficiently and highly clarify.

[0005] Some of the proposed methods are described below. In the method described in Japanese Patent No. 1042319, an inorganic flocculant is used as a flocculant in the presence of the following amount of flocculant in which floc formation is observed in salt water having an SS concentration of 5 ppm or less including suspended substances. In this method, any of the organic polymer flocculants is added without any particular limitation, followed by sand filtration. However, as described in the present invention, in order to clarify the raw material water, from the knowledge that the type and concentration of the coagulant greatly affects the quality of the filtered water and the stable operation of the filter,
It can be said that it is extremely difficult to obtain raw material water required by the field targeted by the present invention simply by performing sand filtration with the concentration of the flocculant specified. If a synthetic resin filter, membrane filtration, etc. are used, the treated water can be clarified, but it is not suitable for applications in which large-scale treatment is performed efficiently. There is also a method of clarifying by filtration after sedimentation separation and floating separation, but there is a problem that the equipment becomes large.

[0006]

SUMMARY OF THE INVENTION The present invention provides a method for efficiently removing and clarifying suspended substances in raw water for pretreatment such as electrodialysis, reverse osmosis, and water treatment. provide.

[0007]

In order to pre-treat raw material water, the pH of the raw material solution is adjusted, and then chitosan is added so as to have a concentration within the above range, followed by filtration. It has been found that the water quality is remarkably clarified as compared with the conventional method, and the pressure loss of the filtration layer can be suppressed by making the filtration layer more available in volume. It is a major feature of the present invention that such characteristics can be further utilized by using filtration using a granular filter medium. Suspended substances are widely contained in raw material water with a particle size of μm. It is said that ordinary filtration, for example, sand filtration, can remove particles having a particle size of several tens of μm, but the raw material water required by the reverse osmosis method or the electrodialysis method is water quality obtained by such a method. However, it is not enough, and for the reasons described in the above-mentioned conventional technology, more severe water quality is required. What is the quality of filtered water that can satisfy such conditions, for example, an ion exchange membrane electrodialysis method, which is one of industrial applications, will be described. After two-stage filtration using sand filtration, one of the pretreatment methods for seawater in the ion-exchange membrane electrodialysis method, the solution was further filtered through a 10 μm polypropylene thread-wound cartridge filter, and supplied to the ion-exchange membrane electrodialysis device. When the change with time of the pressure loss at the inlet was evaluated, it was found that the clogging between the ion exchange membranes was several times smaller than that in the case where the filtration was performed by the conventional pretreatment method, and stable operation was possible for a long period of time. Therefore, the water quality was evaluated by three methods of measuring the FI value, which was evaluated by the turbidity meter manufactured by Hach Co. and the multisizer manufactured by Coulter Counter Co., Ltd., and the membrane filter having a pore diameter of 8 μm, and evaluated for the obstructiveness. As a result, the water quality was 0.12 (NTU) or less, the number of particles was 500 (particles / ml) or less, and the FI value was 2.5 or less. This water quality is several times lower than the water quality obtained by the conventional pretreatment method.

In the conventional pretreatment method, water cannot be clarified to this level of water quality at all, and it is difficult to perform a large amount of filtration. By using the pretreatment method of the present invention, such severe water quality can be satisfied, and a large-scale and efficient treatment can be performed. Hereinafter, the present invention will be described in detail. The function required of the flocculant was originally to increase the floc size and strength. However, if this function is directly applied to the field of use of the present invention, due to excessive occlusion of the filter layer,
The filtration process itself becomes unstable, and it is difficult to simultaneously satisfy the two requirements of clarification of filtered water and efficient filtration. Therefore, if the type of the flocculant and the concentration range of the flocculant found in the present invention are set to the range, the floc of the suspended substance in the raw material water can be filtered while maintaining the appropriate size and strength, and the conventional method can be used. Instead of such filtration of the surface layer, it can be effectively used volumetrically up to the inside of the filtration layer, and clarification of filtered water and reduction of pressure loss of the filtration layer can be achieved at the same time. The present invention is a preprocessing method based on such a concept.

The coagulant includes an inorganic coagulant, an organic polymer coagulant and a natural polymer coagulant. In the present invention, natural polymer agglomerates obtained by treating chitin contained in crustaceans such as crabs were evaluated based on the cohesiveness of suspended substances in raw material water and compatibility with various filtration methods. Chitosan, an agent, was used. In an acidic solution, chitosan is positively charged by dissociation of an amino group contained in a polymer, and exhibits an action as a cationic flocculant. In the present invention, by adjusting the pH of the raw material water from 3 to 8.5, such a function of the chitosan as a coagulant is effectively used, and the pH of the raw material water is adjusted to reduce the cationicity of chitosan. It has a major feature that it can respond to changes in the quality of raw water by changing it. Chitosan used in the present invention is a combination of a cationic organic polymer flocculant, an anionic organic polymer flocculant, a nonionic organic polymer flocculant, and other natural polymer flocculants or inorganic flocculants. It may be used, or used in combination with an oxidizing substance such as sodium hypochlorite used for the purpose of preventing biological adhesion of a water pipe for raw material water.

The chitosan to be added to the raw material water needs to be adjusted according to the filtration method, in the case of a granular filter material, the effective diameter of the filter material, the nature and concentration of the suspended substance in the raw material water, and the like.
An appropriate concentration may be selected within the concentration range specified in the present invention. If the concentration exceeds this range, surface filtration is performed instead of volume filtration, which is a major feature of the present invention, and the filtration loss becomes unstable due to a large increase in pressure loss over time of the filtration layer. In some cases, the quality of the filtered water may not be sufficiently improved due to a change in the charge of the suspended substance or re-stabilization. Conversely, if the concentration of the coagulant is too low, sufficient coagulation ability cannot be exhibited, resulting in deterioration of water quality. Therefore, the concentration of added chitosan is important. The raw material water may contain various kinds of salts, such as seawater, or may be fresh water. If the raw material water contains a suspended substance, it can be treated. The processing can be performed in a wide temperature range from 0 ° C. to 100 ° C. for the raw water. Chitosan is used by dissolving it uniformly in water to which an acid such as hydrochloric acid or acetic acid has been previously added by a usual dissolving method such as a stirrer. Stirring at the time of addition
Any method capable of promptly and uniformly dispersing the coagulant solution in the raw material water can be used without any particular problem from the stirring by a usual stirrer to the stirring by a pump.

Filter media used for filtration include granular filter media having a single layer or multilayer of sand, garnet, pumice, anthracite, manganese sand, granular activated carbon, etc. Woven and non-woven filter media,
A filter medium exhibiting a filtration phenomenon called deep-layer filtration, such as a membrane filter medium, can be used without limitation. Here, deep-filtration means, as described on pages 105-127 of "Filtration Mechanism and Filter Media / Filter Aid" at Jinjinshokan,
This filtration is mainly for clarifying filtration, and is different from the strain filtration in which solids are removed through the pores of a filter medium by removing the solids, and the cake filtration in which a cake is formed on the surface of the filter layer and filtered. This is a filtration utilizing a filtration phenomenon such that particles having a particle size smaller than the gap can be removed while flowing through the gap. For this reason, the filter layer needs to form a thickness several hundred times the size of the particles of the suspended substance in the raw water,
The present invention only needs to be a filter medium capable of forming such a thickness. In the volume filtration of the present invention, the use of such a filter medium does not mean that there is no phenomenon of strain filtration or cake filtration, but rather the volume of the filter layer is increased by mainly using depth filtration. This is a filtration method used effectively for In the case of a granular filter medium which is a typical example of this volume filtration, for example, in the case of sand, those having an effective diameter of 200 μm to 1000 μm can be used, and particularly 300 μm to 800 μm.
Filter media having an effective diameter of m are preferred. Under these conditions, the rise in pressure loss over time of the filter layer is small, and the quality of the filtered water is good. The coagulant and the filter layer are very well matched, and the filter can be operated stably. If the effective diameter of the filter medium is smaller than 200 μm, the increase in pressure loss over time of the filter layer becomes extremely large. Conversely, if the effective diameter of the filter medium is larger than 1000 μm, the increase in pressure loss is small, but the quality of the filtered water is low. , The stable operation of the filter becomes difficult under both conditions. Here, the effective diameter refers to the size of the opening of the sieve that passes by 10% on a weight basis. In addition, the uniformity coefficient means an opening size / effective diameter of a sieve that passes through 60% on a weight basis, and a sieving operation uses a dry sieving test method of JISK0069. The concentration of a suspended substance is a value measured using the method for measuring a suspended substance in JIS K0101 Industrial Water Test Method.

[0012]

The chitosan is expressed in the unit of mg / l in the water for raw material whose pH is adjusted to 3.0 to 8.5 with an acid, and the pH is adjusted to 0.02 to 0.
By adding and filtering in the range of 5, the filtration layer can be effectively used in volume, and the obtained filtered water has a higher degree of clarification in terms of turbidity, number of particles, etc. as compared with the conventional filtration method. Things are obtained. In the case of filtering using a particulate filter medium, if a filter medium having an effective diameter of 200 μm to 1000 μm is selected, the filtration becomes more stable.

[0013]

EXAMPLES The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to these examples. Example 1 An effective diameter of 380 μm was applied to a 500 mmφ vinyl chloride pipe.
m, a seawater containing 6 mg / l as a suspended substance was adjusted to pH 7 with hydrochloric acid in a primary filtration tower filled with sand having a uniformity coefficient of 1.4 at a thickness of 600 mm, and the pH was adjusted to 7.5 m / hr at 7.5 m / hr. Run down and filter. Next, press aid 1 made by Kurita Kogyo
0.1 was added to the primary filtered water at 0.1 mg / l, and the mixture was stirred by a pump. After that, the solution was passed downward through a secondary filtration tower having the same specifications as the primary at 15 m / hr, and filtered. The primary filtration tower flows clear seawater at 40 m / hr for 4 minutes every 6 hours for 4 minutes, and the secondary filtration tower flows upward at the same seawater, flow rate and time every 8 hours for backwashing. The pressure loss over time of the secondary filtration tower and the turbidity of the secondary filtered water using a turbidity meter 2100N manufactured by Hach,
The number of particles was measured using a Coulter Counter Multisizer. Table 1 shows the results. The measurement conditions for the Coulter Counter Multisizer were as follows:
The test was carried out in a siphon mode at 00 μm and a sample volume of 2 μl. A turbidimeter manufactured by Hach Co. uses formazine as a turbidity standard solution for calibration, and is expressed in NTU, that is, turbidity unit. FI value was 2.1 kg / cm @ 2 using an 8 μm polycarbonate membrane filter.
The raw water is filtered while applying pressure. First, the time T ↓ 0 for collecting 500 ml from the start of filtration was determined, and 50 minutes after 15 minutes.
Assuming that the time for collecting 0 ml is T ↓ 1, FI = (1-T ↓
0 / T ↓ 1) Calculate from × 100/15. Example 2 Filtration by adding 0.7 mg / l of sodium hypochlorite before secondary filtration under the same seawater, apparatus, method, type of coagulant and concentration of coagulant as in Example 1 Then, the measurement was performed using the same items and methods as in Example 1. Table 1 shows the results.
Even if an oxidizing substance such as sodium hypochlorite is added together with the flocculant, the quality of the filtered water is good as in this example. Comparative Example 1 The same seawater, apparatus, and method as in Example 1 were used for filtration without addition of a coagulant, and the pressure loss over time in secondary filtration and the quality of the filtered water were measured using the same measurement method as in Example 1. It was measured. Table 1 shows the results. Comparative Example 2 In the same seawater, apparatus and method as in Example 1, 0.01 mg / l of the same type of coagulant was added, followed by filtration. It measured using the same measuring method as 1. Table 1 shows the results. Comparative Example 3 In the same seawater, apparatus and method as in Example 1, the same type of coagulant was added at 1 mg / l, followed by filtration. The time-dependent pressure loss of the secondary filtration and the quality of the filtered water were the same as in Example 1. The measurement was performed using the same measurement method. It is shown in Table 1. Comparing Example 1 with Comparative Example 1, the water quality of the filtered water is improved by about １ ／ in both the turbidity and the number of particles. The quality of the water can be improved. Further, even if the concentration of the coagulant is lower than the range of the present invention as in Comparative Examples 2 and 3, or conversely, the water quality is lowered.

Example 3 After adjusting the pH of seawater containing 6 mg / l as a suspended substance to 7 with hydrochloric acid, 0.1 mg / l of press aid 101 manufactured by Kurita Kogyo was added and stirred with a stirrer for 7 minutes. hand,
Effective diameter 380μm for 40mmφ vinyl chloride pipe
The mixture was filtered at 7.5 m / hr downward through a filtration tower filled with sand having a uniformity coefficient of 1.4 and a thickness of 600 mm. Changes in pressure loss over time of the filtration tower and Hach
Turbidity was measured by a turbidity meter manufactured by the company. The change in pressure loss over time is the change in pressure at the supply port to the filtration tower when the raw material solution is supplied to the filtration tower at a constant flow rate. Means the change in the water head. The unit is expressed in cm / hr. (Less than,
It is called pressure loss gradient. If the flocculant is not added as in Comparative Example 4, the quality of the filtered water will be several times worse. When the effective diameter of the filter is 200 μm or less as in Comparative Example 6, the water quality is not so good despite the high pressure loss gradient. Example 4 In the same seawater, apparatus, and method as in Example 3, the filtration material was filtered using pumice having an effective diameter of 600 μm and a uniformity coefficient of 1.5, and the pressure loss gradient of the filtration layer and the quality of the filtered water were measured. It measured using the same measuring method as 3. Table 2 shows the results. Compared with Example 3, the pressure loss gradient can be reduced by increasing the effective diameter of the filter medium. If the coagulant is not added as in Comparative Example 5, the quality of the filtered water deteriorates. Comparative Example 4 The same seawater, apparatus, filter material and filtration method as in Example 3 were used for filtration without the addition of a coagulant, and the pressure loss gradient of the filtration layer and the quality of the filtered water were measured using the same measurement method as in Example 3. It was measured. Table 2 shows the results. Comparative Example 5 The same seawater, apparatus, filter material, and filtration method as in Example 4 were used for filtration without the addition of a coagulant, and the pressure loss gradient of the filtration layer and the quality of the filtered water were measured using the same measurement method as in Example 3. It was measured. Table 2 shows the results. Comparative Example 6 With the same seawater, apparatus, method, type of flocculant and flocculant concentration as in Example 3, an effective diameter of 150 μm as a filter medium and a uniformity coefficient of 1.
The mixture was filtered using the sand of No. 4, and the pressure drop gradient of the filtration layer and the quality of the filtered water were measured using the same measuring method as in Example 3. Table 2 shows the results.

Example 5 Hydrochloric acid was added to seawater containing 6 mg / l as a suspended substance to adjust the pH to 7, and then press aid 1 manufactured by Kurita Kogyo was used.
01 was added in an amount of 0.1 mg / l, and the mixture was stirred by a pump circulation.
A primary filter having a 1000 mm diameter inner rubber-lined iron filter container filled with Unitika made of 10 denier polyester thread filter material with a thickness of 1000 mm was placed in a primary filter.
The mixture was filtered by flowing downward at 8 m / hr. Further, as a secondary filtration, a filtration tower filled with sand of Example 1 to a thickness of 600 mm in a pipe made of vinyl chloride of 500 mmφ and having a thickness of 600 mm, was subjected to 15 m / h.
The mixture was filtered by flowing downward at r. The pressure loss gradient of the primary filter and the quality of the secondary filtered water were measured by the same measurement method as in Example 1. Table 3 shows the results. Even if a filter medium other than the granular filter medium is used, the effect of the present invention can be similarly expected. Comparative Example 7 The addition of the flocculant was stopped using the same seawater, apparatus, and method as in Example 5, and the pressure loss gradient of the filtration layer and the quality of the filtered water were measured using the same measurement method as in Example 3. Table 3 shows the results.

Example 6 Filtered water obtained by filtering seawater by the same filtration apparatus, type of coagulant, concentration of coagulant, pH, and filtration method as in Example 1 was used to obtain a filter having a length of 28 cm.
cm, a small ion exchange membrane electrodialyzer of 24 cm in width (hereinafter referred to as a dialysis machine) without flowing an electric current.
The solution was supplied at a constant flow rate of cm / sec, and the pressure at the inlet of the dialyzer was measured to evaluate the obstruction. The results are shown in Table 4. The initial inlet pressure at the start of supplying the filtered water gradually increases over time due to the progress of clogging of the dialyzer, contamination of the membrane, and the like. The relationship between the change in the inlet pressure and the time was determined by a regression line, and the slope was defined as the pressure rise per unit time.
(Hereinafter referred to as a pressure gradient.) Since the operable pressure of the dialysis device is limited, this pressure gradient determines the operable time of the dialysis device. It is necessary to make the pressure gradient as small as possible. Comparative Example 8 Filtration water obtained by filtering seawater by the same filtration device and filtration method as in Comparative Example 1 was supplied to the same dialysis device as in Example 6 under the same conditions as in Example 6, and the inlet of the dialysis device was supplied. The pressure was measured. The results are shown in Table 4. Comparing Example 6 with Comparative Example 8, since the pressure gradient of Example 6 is reduced to about 1/4, the number of days in which the dialysis device can be operated is several times as long as the pretreatment of the present invention is performed. You can see that it can be extended.

According to the present invention, the pH of the raw material water is adjusted, and then chitosan is added at an appropriate concentration as a coagulant and the mixture is filtered to improve the quality of filtered water and stabilize filtration. Efficient filtration can be achieved. In the case of performing filtration using a particulate filter medium, this effect can be further exhibited by properly selecting the effective diameter. ADVANTAGE OF THE INVENTION According to this invention, the efficient pretreatment method of raw material water suitable for severe requirements, such as an electrodialysis method, a reverse osmosis method, and water pretreatment, can be provided.

Claims (2)

[Claims] 1. An acid is added to raw water containing a suspended substance to adjust the pH to 3 to 8.5, and then chitosan is added in mg / mg.
This is a pretreatment method characterized by adding and filtering in units of 1 so as to fall within a range of 0.02 to 0.5. 2. After the raw material water of claim 1 is adjusted to the pH and chitosan concentration of claim 1, the effective diameter is 200 μm to 1000 μm.
It is a pretreatment method characterized by filtering using a particulate filter medium of μm.