JPH11216478A - Flocculation treatment of water and inorganic flocculant - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a flocculating method using silica and iron suitable for the flocculation treatment of city water in combination, which is attained in excellent flocculation, small in the injecting quantity of silica, small in the generation of sludge and short in the preparation time of a flocculant, and a silica-containing iron based flocculant. SOLUTION: The flocculation is performed by adding a polymerized silica with a ferric inorganic flocculant in an original water so as to be in a condition of the molar ratio of silica to iron of 0.5-2. And the silica-containing ferric based inorganic flocculant aq. solution containing the polymerized silica and the ferric inorganic fluocculant and having <5% silica acid concentration in the aq. solution and the molar ratio of silica to iron of 0.5-2 is used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for coagulating various kinds of wastewater and a coagulant used for coagulating various kinds of wastewater, and more particularly to a novel method of coagulating water suitable for water treatment and an inorganic coagulant. It is about.

[0002]

2. Description of the Related Art Conventionally, agglomeration of water has been widely performed to remove suspended solids, dissolved organic substances, and phosphate ions from various kinds of wastewater. In these water coagulation treatments, aluminum sulfate, polyaluminum chloride,
Aluminum-based or iron-based inorganic metal salt coagulants such as ferric sulfate, ferric sulfate and ferric chloride are used. Conventionally, iron-based coagulants have not been used in water treatment, and only aluminum-based coagulants have been used. Since these inorganic flocculants do not form a sufficiently large block when used alone, they have the disadvantage that the solid-liquid separation speed in the flocculation-sedimentation step and the sand filtration step is low. There is also a disadvantage that the sludge discharged from the coagulation separation step has poor sedimentation, concentration, and dewatering properties. Therefore, in the field of wastewater treatment, various polymer flocculants have been widely used to promote floc formation.However, in water treatment, there are concerns about the safety of organic polymer flocculants, so they are not used in Japan. Not authorized.

[0003] In addition, in the case of raw water containing a large amount of algae such as microcystis due to the progress of eutrophication of the raw water in water treatment,
In the PAC or sulfate band, only flocks having extremely poor sedimentation are formed, and the flocs may float, and there is a problem that the algae cannot be effectively removed. Furthermore, in the field of water treatment, it has been suspected that aluminum remaining in the treated water, that is, drinking water, causes Alzheimer's disease. Is desired. In the water treatment field, the use of activated silica discovered by Baylis of the United States in the 1950s as a highly safe coagulant was considered, but gelation troubles during the production of activated silica frequently occurred, and Since it is very difficult to produce activated silica, it has not been put to practical use in Japan.

[0004] In addition, the active silica produced by the Baylis method has a disadvantage that the effect of forming flocculated flocs is not so high. In addition, the Baylis method is that "water glass is diluted with water to form an aqueous solution having a silicic acid concentration of 1.5%,
Sulfuric acid is added thereto to adjust the pH to 8.5, and the mixture is stirred at room temperature for 2 hours to polymerize the silicic acid monomer to obtain polymerized silicic acid, that is, activated silica. " However, recently, there has been a movement to re-evaluate activated silica. For example, Japanese Patent Publication No. 4-75796, "Water Treatment Method and Flocculant for Water Treatment" includes "water treatment method and coagulant for water treatment," Polymeric silicic acid having an intrinsic viscosity and a soluble salt of a metal capable of forming a hydroxide in water are injected into the water to be treated at a ratio such that the molar ratio of silicon to the metal is 2 or more. A coagulation treatment method is disclosed.
One of the features of the present invention is that the silicon / metal element molar ratio is 2
It is to inject so that it becomes above.

However, when the present inventor studied this technique in detail, the following various problems were found. Even when the molar ratio of silicon / metal element is set to 2 or more, good aggregation does not occur, but rather aggregation may be deteriorated. Since the molar ratio of silicon / metal ions is as large as 2 or more, the injection rate of silica is inevitably considerably increased, and the cost of chemical injection is increased. Since the silica injection rate is high, the amount of sludge generated due to precipitated silica increases. Production of the polymerized silica flocculant requires a long time of 2 hours or more. Since the measurement of the intrinsic viscosity takes one hour or more even for an expert, the method of controlling the polymerization time while measuring the intrinsic viscosity is not practically applicable to industrial-scale production. Since the Si / Fe molar ratio is 2 or more, gelation of silica is likely to occur during storage of the aqueous flocculant solution, and the preservability of the flocculant is poor (gelation of silica is more likely to occur as the silica concentration is higher).

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a coagulation treatment method using silica and iron in combination, which solves the disadvantages of the prior art, and a silica-containing iron-based coagulant. An object of the present invention is to provide a flocculant and a flocculation treatment method suitable for the treatment.

[0007]

SUMMARY OF THE INVENTION The present inventor has proposed a method of coagulating raw water to be treated, which has not been known before.
It has been found that when polymerized silica and ferric ion are added to raw water under the condition of the Fe molar ratio, extremely effective coagulation treatment can be performed, and the present invention has been completed based on this. That is, the present invention has solved the above-mentioned problems by the following means. (1) Polymerized silica is mixed with ferric inorganic coagulant together with silicon /
A method of coagulating water, wherein iron is added to raw water and coagulated so that the molar ratio of iron is 0.5 to 2. (2) A silica-containing ferric inorganic material containing polymerized silica and a ferric inorganic coagulant, wherein the concentration of silicic acid in the aqueous solution is less than 5% and the molar ratio of silicon / iron is 0.5 to 2. Flocculant aqueous solution.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The production of an aqueous solution of an inorganic flocculant of the present invention will be specifically described with reference to the drawings. An example of the manufacturing process is shown in FIG. In FIG. 1, the silica concentration is about 30.
% Water glass stock solution, water 2 is added to a sodium silicate aqueous solution 1 diluted to a silicic acid concentration of 5 to 11%, and an acid 2 such as sulfuric acid or hydrochloric acid is added. ~
The acidic silicic acid aqueous solution 4 of 4.5) is prepared. Thereafter, the viscosity of the acidic silicic acid aqueous solution 4 is measured continuously or over time, and when the rotational viscosity increases to 10 mPa · s or more, a predetermined amount of ferric chloride, ferric sulfate, and ferric polysulfate are added. Is added in the form of a solid or aqueous solution, and the molar ratio of silicon / iron is adjusted to 0.5 to 2 in the addition, whereby the coagulant of the present invention can be produced. The increase in the rotational viscosity is such that the higher the concentration of silicic acid and the higher the pH in the stirring tank 3, the shorter the polymerization proceeds and the higher the viscosity.

Hereinafter, the "rotational viscosity" as defined above is defined as a viscosity at a water temperature of 20 to 25 ° C. measured by a Viscometer VT-03 type viscometer manufactured by Rion Corporation. This rotational viscometer measures the viscous resistance generated in a rotor by rotating a cylindrical rotor at a constant speed in a sample liquid. This is a simple viscosity measurement method that can be performed in a very short time of about 3 minutes.

As a raw material of the polymerized silica, a water glass solution is preferable. The acid mixed with sodium silicate is preferably sulfuric acid or hydrochloric acid. Nitric acid is not preferred because it adds nitric nitrogen, which is a substance causing eutrophication, to the raw water to be added. In the present invention, when the silicic acid concentration of the stirring tank 3 is less than 6%, there is no problem even if an acid is added to the aqueous solution of water glass. It is important to add soda. When an acid is added to an aqueous solution of sodium silicate having a silicic acid concentration of 6% or more, the silicic acid passes through the alkaline region, and thus a gelation trouble of the silicic acid is easily caused. Further, when sodium silicate is added to acidic water having a pH of 1 or more, the pH after the addition increases, and there is a great risk of causing gelling trouble.

It is very important to set the silica concentration at the time of mixing the ferric salt such as ferric chloride and the polymerized silica, and it is important to set the silicic acid concentration to 2 to 5%, It is important that the molar ratio of satisfies 0.5 to 2. In the above case, if the silica concentration is less than 2%, it takes a long time to produce the polymerized silica, and if the silica concentration is 5% or more, gelation tends to occur and the storage stability is poor. Further, it is important to set the pH of the solution after the addition of the ferric salt to 1 to 1.5, most preferably to pH 1 to 1.2, in order to increase the gelling time of the flocculant and to improve the storage stability. is there.

By setting the conditions as described above, a silica-iron composite flocculant having high flocculation ability and excellent long-term storage properties is produced, and when the flocculant is injected into raw water to be subjected to flocculation treatment, a flocculation stirring tank is formed. , A very large floc is quickly formed, and solid-liquid separation can be performed at a high speed in a sedimentation tank or the like. In the water coagulation treatment method of the present invention, instead of adding silica-iron composite coagulant obtained by mixing polymerized silica and iron-based coagulant to raw water, polymerized silica and iron-based coagulant are separately
It can also be performed by injecting into raw water under the condition that the / Fe molar ratio becomes 0.5 to 2.

The following important effects can be obtained by the above method. No gelation trouble of silicic acid occurs during production of activated silica. The polymerization time of the silicic acid may be zero to one hour, and the polymerized silica / iron composite flocculant can be produced in a short time. The produced flocculant has a large flocculating effect and can be stored for a long time without gelation. Since the injection rate of silica can be made smaller than before, the amount of generated sludge is small. The chemical injection cost is also low.

[0014]

The present invention will be described in detail with reference to the following examples. However, the present invention is not limited to only these examples.

Example 1 The required degree of polymerization of silica when ferric chloride and polymerized silica were used in combination was examined. That is, the relationship between the rotational viscosity and the aggregation effect was examined by changing the polymerization time of the aqueous solution of silicic acid. It has been found that a floc-forming effect is exhibited. In addition, it was found that when the rotational viscosity exceeded 300 mPa · s, the gelation was in a gelation-prone region immediately before gelation. FIG. 2 shows the results of measuring the relationship between the polymerization time and the viscosity of the aqueous solution of silicic acid at a constant pH of 4 at various silicic acid concentrations. The viscosities on the vertical axis of the graph indicate rotational viscosities measured by a viscometer rotational viscometer manufactured by Rion.

FIG. 2 shows that when the silicic acid concentration of the aqueous solution of silicic acid is less than 5%, the time required for the rotational viscosity to reach 10 mPa · s or more is as long as 2 hours or more, and a polymerization tank for polymerizing silicic acid is indispensable. At 6% or more, the rotational viscosity reaches 10 mPa · s in a few minutes to 1 hour. Therefore, a polymerized silica exhibiting an effective aggregation floc forming effect only by mixing water glass with strongly acidic water and stirring for 0 to 1 hour is used. It was found to be manufactured. If the concentration of silicic acid exceeds 11%, gelling occurs almost instantaneously when strongly acidic water and water glass are mixed, so that this condition cannot be adopted.

Example 2 Table 1 shows the relationship between the polymerization time and the time required for the rotational viscosity to reach 10 mPa · s when the pH of the aqueous solution of silicic acid was kept constant at 10% and the pH was changed from 2 to 5. In the condition of pH 4 to 4.5, 2 minutes or less is sufficient, and active silica having a high aggregation effect can be produced only by mixing the strongly acidic water and water glass for 1 to 2 minutes under the conditions of the present invention. At pH 2, it is a long time of 228 minutes, which is not reasonable because a large-capacity polymerization tank is required.
If H is 5, gelling occurs immediately after mixing the mineral acid and water glass, so this is not applicable. Therefore, the preferred pH was found to be between 3 and 4.5.

[0018]

[Table 1]

Example 3 15 cc of concentrated sulfuric acid was added to 985 cc of tap water, and pH 0.3 was added.
Made strong acid water. Then, No. 3 water glass was diluted about 3 times with water, and 1 kg of a solution adjusted to a silicic acid concentration of 10% was added thereto and mixed. The pH of this solution was 1.6. Then 40
% Caustic soda aqueous solution, silicic acid concentration 5%, pH 3.8
And stirred. When the rotational viscosity of the liquid reached 12 mPa · s at a temperature of 25 ° C., the pH was lowered to 1 to stop the polymerization, and a polymerized silica liquid having a high aggregation effect was obtained.

Example 4 Production Example of Silica / Iron Composite Coagulant 15 cc of concentrated sulfuric acid was added to 985 cc of tap water to obtain a pH of 0.3.
Made strong acid water. Next, No. 3 water glass was diluted about 3 times with water, and 1 kg of a solution adjusted to a silicic acid concentration of 10% was added and mixed. The pH of this solution was 1.6. Next, a 40% aqueous solution of caustic soda was added, the concentration of silicic acid was adjusted to 5%, and the pH was adjusted to 3.8, followed by stirring. The rotational viscosity of this liquid is 1 at a temperature of 25 ° C.
When the pressure reached 2 mPa · s, a 20% aqueous ferric chloride solution was added and mixed to produce a silica-iron composite flocculant having a Si / Fe molar ratio of 0.5 to 2.

Example 5 Coagulation treatment test Polymerized silica produced in Example 3 (SiO ₂ concentration 5
%, PH 1, and rotational viscosity of 12 mPa · s), a coagulation test was performed by a jar test. The jar test was conducted at 150 rpm for 2 minutes and 30 rpm for 5 minutes, and the supernatant water turbidity was measured 5 minutes after the stirring was stopped. The raw water to be treated was prepared by adding kaolin clay to tap water and adjusting the turbidity to 100 degrees, the M alkalinity to 55 mg / liter, and the pH to 7.3. Ferric chloride is used as the inorganic flocculant.
30 mg / liter was injected as iron. This result is
It is shown in the table.

[0022]

[Table 2]

The active silica injection rate is an injection rate as silicic acid. The floc size class is represented by the following criteria. Special A: Pellet formation, A: 5 mm or more, B: 2
３3 mm, C: 1-2 mm, D: less than 1 mm. As is apparent from Table 2, the molar ratio of Si / Fe is 0.5 to 2.7.
Under the conditions (1) and (2), a pellet-like floc having extremely large sedimentation is formed, and at a molar ratio higher than this, the floc-forming effect deteriorates. Hitherto, it has been considered that a higher flocculation rate of polymerized silica forms a better floc, and it has not been known that an appropriate range exists.
As the molar ratio of Si / Fe increases, the amount of sludge generated due to precipitated silica increases, and the preservability of the flocculant aqueous solution deteriorates. Therefore, the amount of sludge generated is reduced, pellets are formed, and the preservability is reduced. Considering three good conditions, appropriate S
It can be said that the i / Fe molar ratio is 0.5 to 2.

[0024]

According to the present invention, the following remarkable effects can be obtained. Polymerized silica having a large agglomeration effect can be continuously produced in a very short time of about several minutes, and the gelation trouble of silicic acid during the production of the polymerized silica can be surely prevented. S which ensures formation of flocks with extremely good sedimentation
Since the i / Fe molar ratio has been found, the aggregation / separation step can be greatly rationalized. Since the Si / Fe molar ratio is smaller than before, the chemical injection cost and the amount of sludge generated due to silica can be reduced, and the preservability of the flocculant is good. It is not necessary to use an aluminum-based coagulant in the water treatment, and the amount of residual aluminum in drinking water can be reduced.

[Brief description of the drawings]

FIG. 1 shows a schematic view of an example of a production process of an inorganic flocculant of the present invention.

FIG. 2 is a graph showing the relationship between the polymerization time of silicic acid and the viscosity of an aqueous solution of silicic acid at a constant pH of 4 at various silicic acid concentrations.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Sodium silicate aqueous solution 2 Acid 3 Stirring tank 4 Acidic silicic acid aqueous solution 5 pH meter 6 Rotational viscosity measuring instrument 7 Ferric salt 8 Silica iron composite flocculant

Claims (2)

[Claims] 1. Coagulation of water by adding a polymerized silica together with a ferric inorganic coagulant to raw water so that the molar ratio of silicon / iron becomes 0.5 to 2 to perform coagulation treatment. Processing method. 2. Silica-containing ferric iron containing polymerized silica and a ferric inorganic coagulant, wherein the concentration of silicic acid in the aqueous solution is less than 5% and the molar ratio of silicon / iron is 0.5 to 2. Aqueous inorganic flocculant aqueous solution.