Method for treating sludge water and method for producing treated sludge water

Abstract

To provide a method of treating sludge water, capable of decreasing the concentration of hexavalent chromium contained in sludge water, and a method of producing sludge treatment water.SOLUTION: The method of treating sludge water according to the present invention, or a method of treating sludge water containing the solid component of sludge, comprising a blowing step of blowing gas including carbon dioxide into the sludge water to let the carbon dioxide and a hydration product constituting the solid component of sludge react with each other to form calcium carbonate, and an adding step of adding a reducing agent to the sludge water subsequent to the blowing step, is characterized in that the amount of the reducing agent added in the adding step is not less than 5 mg / kg.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for treating sludge water and a method for producing treated sludge water. [Background technology]

[0002] The water from the washing wastewater used to wash vehicles, mixers, etc. in ready-mixed concrete plants, excluding aggregate, is called recovered water. The recovered water is divided into supernatant water containing hydration products such as calcium hydroxide and calcium silicate hydrate eluted from cement, and sludge water containing sludge solids mostly composed of the hydration products.

[0003] When using recovered water as mixing water in ready-mixed concrete plants, the Reclaimed Water Utilization Committee of the Japan Concrete Institute states that the supernatant water can be used as mixing water in the same way as tap water. On the other hand, for sludge water, the sludge solid content must not exceed 3% based on the quality standards stipulated in JIS A 5308;2019, so various methods have been proposed for treating sludge water and recovered water and reusing it as mixing water (for example, Patent Documents 1 to 3). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2011-072947 A [Patent Document 2] JP 2003-190998 A [Patent Document 3] JP 2003-071466 A Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, most of the raw materials and fuels for cement are natural resources present in the earth's crust, so cement contains trace elements in addition to the main constituent elements. Among the trace elements, chromium is mostly present as trivalent chromium, but some of the trivalent chromium is oxidized and converted to hexavalent chromium during the production of cement clinker. This hexavalent chromium is contained in the sludge water in a fixed state in the hydration products that constitute the sludge solids, but there is a risk that it will dissolve into the liquid phase of the sludge water and have a negative impact on the environment, so it is desirable to reduce the concentration.

[0006] The present invention has been made to solve such problems, and an object of the present invention is to provide a sludge water treatment method capable of reducing the concentration of hexavalent chromium contained in the sludge water, and a method for producing treated sludge water. [Means for solving the problem]

[0007] The sludge water treatment method of the present invention is a method for treating sludge water containing sludge solids, and includes an injection step of injecting a gas containing carbon dioxide into the sludge water to react the carbon dioxide with a hydration product constituting the sludge solids to produce calcium carbonate, and an addition step of adding a reducing agent to the sludge water after the injection step, wherein the amount of the reducing agent added in the addition step is 5 mg / kg or more.

[0008] In the sludge water treatment method, the blowing step causes carbon dioxide to react with the hydration product constituting the sludge solid content, thereby efficiently dissolving hexavalent chromium fixed to the hydration product constituting the sludge solid content into the liquid phase. In the adding step, a reducing agent is added to the sludge water, thereby reducing the hexavalent chromium in the liquid phase to trivalent chromium, thereby reducing the concentration of hexavalent chromium in the sludge water.

[0009] The method for treating sludge water according to the present invention may include a separation and recovery step of separating and recovering the calcium carbonate after the blowing step and before the adding step.

[0010] The sludge water treatment method according to the present invention can efficiently fix carbon dioxide as calcium carbonate, thereby suppressing the amount of carbon dioxide released into the atmosphere and contributing to the reduction of greenhouse gases.

[0011] In the sludge water treatment method according to the present invention, in the blowing step, a gas containing carbon dioxide may be blown into the sludge water to adjust the pH of the sludge water to 12 or less.

[0012] With this configuration, the sludge water treatment method can more efficiently dissolve the hexavalent chromium fixed to the hydration product constituting the sludge solid content into the liquid phase, thereby further reducing the concentration of hexavalent chromium contained in the sludge water.

[0013] In the sludge water treatment method according to the present invention, in the blowing step, the gas containing carbon dioxide may be a gas containing carbon dioxide generated during burning of cement clinker.

[0014] With such a configuration, the sludge water treatment method can effectively utilize carbon dioxide generated during the burning of cement clinker and suppress the amount of carbon dioxide released into the atmosphere, thereby contributing to the reduction of greenhouse gases.

[0015] In the sludge water treatment method according to the present invention, in the blowing step, the hydration product may contain at least one selected from calcium hydroxide and calcium silicate hydrate.

[0016] With this configuration, the method for treating sludge water makes it easier to adjust the pH of the sludge water to 12 or less.

[0017] In the sludge water treatment method according to the present invention, the amount of the reducing agent added in the adding step may be 5 mg / kg or more and 1000 mg / kg or less.

[0018] With such a configuration, the sludge water treatment method can sufficiently and efficiently reduce hexavalent chromium in the liquid phase to trivalent chromium, and as a result, the concentration of hexavalent chromium contained in the sludge water can be further reduced.

[0019] In the sludge water treatment method according to the present invention, in the adding step, the reducing agent may be at least one selected from the group consisting of ferrous sulfate monohydrate and powdered blast furnace slag.

[0020] With such a configuration, the sludge water treatment method can efficiently reduce hexavalent chromium in the liquid phase to trivalent chromium, and as a result, the concentration of hexavalent chromium contained in the sludge water can be further reduced.

[0021] The method for producing treated sludge water of the present invention is a method for producing treated sludge water by treating sludge water containing sludge solids, and includes an injection process in which a gas containing carbon dioxide is injected into the sludge water to react with the carbon dioxide and hydration products that constitute the sludge solids to produce calcium carbonate, and an addition process in which a reducing agent is added to the sludge water after the injection process, wherein the amount of the reducing agent added in the addition process is 5 mg / kg or more.

[0022] With this configuration, the method for producing treated sludge water can produce treated sludge water with a reduced concentration of hexavalent chromium. Effect of the Invention

[0023] According to the present invention, it is possible to provide a method for treating sludge water and a method for producing treated sludge water that are capable of reducing the concentration of hexavalent chromium contained in the sludge water. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Hereinafter, the method for treating sludge water and the method for producing treated sludge water according to this embodiment will be described.

[0025] <Sludge water treatment method> The sludge water treatment method according to this embodiment is a method for treating sludge water containing sludge solids.

[0026] Sludge water is water that contains sludge solids and is obtained by removing aggregate from the washing wastewater used to wash vehicles, mixers, etc. in ready-mixed concrete plants (i.e., recovered water). The sludge water contains, for example, 1% by mass or more and 50% by mass or less of sludge solids.

[0027] The sludge solids are mostly composed of hydration products, and further contain aggregate fine particles, etc. The hydration products include, for example, calcium hydroxide, calcium silicate hydrate, monosulfate, ettringite, etc., which are eluted from cement. Among these, the hydration products preferably contain at least one selected from calcium hydroxide and calcium silicate hydrate, from the viewpoint of facilitating adjustment of the pH of the sludge water to 12 or less.

[0028] The sludge water treatment method according to this embodiment includes an injection process in which a gas containing carbon dioxide is injected into the sludge water to cause a reaction between the carbon dioxide and a hydration product constituting the sludge solids to produce calcium carbonate, and an addition process in which a reducing agent is added to the sludge water after the injection process.

[0029] In the blowing step, examples of the carbon dioxide-containing gas to be blown into the sludge water include carbon dioxide as an industrial product, air, exhaust gas containing carbon dioxide and nitrogen oxides generated during the burning of cement clinker, etc. Among these, the carbon dioxide-containing gas is preferably a gas containing carbon dioxide generated during the burning of cement clinker, from the viewpoints of effectively utilizing the carbon dioxide generated during the burning of cement clinker and suppressing the amount of carbon dioxide released into the atmosphere.

[0030] Examples of the method of blowing in the gas containing carbon dioxide include a simple bubbling method, a bubbling method while stirring, and a bubbling method while irradiating ultrasonic waves. Among these, the method of blowing in the gas containing carbon dioxide is preferably a bubbling method while stirring from the viewpoint of improving the reaction efficiency and improving the economic efficiency. The concentration of the gas containing carbon dioxide is preferably 10% or more and 100% or less. The blowing conditions of the gas containing carbon dioxide can be, for example, a blowing rate of 1 mL / min or more and 1000 mL / min or less per 1 g of sludge solid content, and a total blowing amount of 50 L or more and 1000 L or less.

[0031] The washing wastewater used for washing vehicles, mixers, etc. in ready-mixed concrete plants is sent to a sludge tank after aggregates are removed. The gas containing carbon dioxide may be blown into the sludge water in the sludge tank, or may be blown into the sludge water before it is sent to the sludge tank or into the sludge water after it is collected from the sludge tank.

[0032] In the blowing step, from the viewpoint of more efficiently dissolving the hexavalent chromium fixed to the hydration product constituting the sludge solid content into the liquid phase, it is preferable to adjust the pH of the sludge water to 12 or less, and more preferably to adjust the pH to 10 or less, by blowing a gas containing carbon dioxide into the sludge water.

[0033] In the adding step, the amount of the reducing agent added is 5 mg / kg or more, and preferably 5 mg / kg or more and 1000 mg / kg or less, from the viewpoint of reducing the concentration of hexavalent chromium contained in the sludge water. Note that the amount of the reducing agent added is the amount added per 1 kg of the sludge water.

[0034] Examples of the reducing agent include sulfites such as ammonium sulfite, potassium sulfite, calcium sulfite, and sodium sulfite; hydrogen sulfites such as potassium hydrogen sulfite and sodium hydrogen sulfite; thiosulfates such as sodium thiosulfate and potassium thiosulfate; sulfides such as sodium sulfide, potassium sulfide, and calcium sulfide; hydrosulfides such as sodium hydrosulfide, potassium hydrosulfide, and calcium hydrosulfide; polysulfides such as sodium polysulfide, potassium polysulfide, and calcium polysulfide; sulfur compounds such as thioacid salts, sulfur dioxide, and sulfur; metal salts and hydrates thereof such as ferrous salts and trivalent titanium salts such as ferrous sulfate, ferrous nitrate, and ferrous chloride; organic compounds such as aldehydes, sugars, formic acid, oxalic acid, and ascorbic acid; blast furnace slag powder, peat, lignite, iodine, and iron powder. Among these, the reducing agent is preferably at least one selected from ferrous sulfate monohydrate and blast furnace slag powder from the viewpoint of efficiently reducing hexavalent chromium in the liquid phase to trivalent chromium. The reducing agent may be a solid or a liquid.

[0035] In the adding step, from the viewpoint of efficiently reducing hexavalent chromium in the liquid phase to trivalent chromium, it is preferable to add the reducing agent to the sludge water and then stir the mixture for 0.5 to 6 hours.

[0036] In the sludge water treatment method according to the present embodiment, it is preferable to include a separation and recovery step of separating and recovering the calcium carbonate after the blowing step and before the adding step.

[0037] In the sludge water treatment method according to the present embodiment, in the blowing step, a gas containing carbon dioxide is blown into the sludge water, and the carbon dioxide is reacted with the hydration product constituting the sludge solid content, thereby efficiently dissolving the hexavalent chromium fixed in the hydration product constituting the sludge solid content into the liquid phase. In the adding step, 5 mg / kg or more of a reducing agent is added to the sludge water, thereby reducing the hexavalent chromium in the liquid phase to trivalent chromium, and as a result, the concentration of hexavalent chromium contained in the sludge water can be reduced.

[0038] The sludge water treatment method according to the present embodiment may include a separation and recovery step of separating and recovering the calcium carbonate after the blowing step and before the adding step. With such a configuration, carbon dioxide can be efficiently fixed as calcium carbonate. As a result, the amount of carbon dioxide released into the atmosphere can be suppressed, which contributes to the reduction of greenhouse gases.

[0039] In the sludge water treatment method according to the present embodiment, in the blowing step, a gas containing carbon dioxide may be blown into the sludge water to adjust the pH of the sludge water to 12 or less. With this configuration, the hexavalent chromium fixed to the hydration product constituting the sludge solid content can be more efficiently eluted into the liquid phase. As a result, the concentration of hexavalent chromium contained in the sludge water can be further reduced.

[0040] In the sludge water treatment method according to the present embodiment, the gas containing carbon dioxide in the blowing step may be a gas containing carbon dioxide generated during the burning of cement clinker. With this configuration, the carbon dioxide generated during the burning of cement clinker can be effectively utilized, and the amount of carbon dioxide released into the atmosphere can be suppressed, thereby contributing to the reduction of greenhouse gases.

[0041] In the sludge water treatment method according to the present embodiment, the hydration product may contain at least one selected from calcium hydroxide and calcium silicate hydrate in the blowing step, which makes it easier to adjust the pH of the sludge water to 12 or less.

[0042] In the sludge water treatment method according to the present embodiment, the amount of the reducing agent added in the adding step may be 5 mg / kg or more and 1000 mg / kg or less. With this configuration, the hexavalent chromium in the liquid phase can be sufficiently and efficiently reduced to trivalent chromium, and as a result, the concentration of hexavalent chromium contained in the sludge water can be further reduced.

[0043] In the sludge water treatment method according to the present embodiment, the reducing agent may be at least one selected from the group consisting of ferrous sulfate monohydrate and blast furnace slag powder. With this configuration, hexavalent chromium in the liquid phase can be efficiently reduced to trivalent chromium, and as a result, the concentration of hexavalent chromium contained in the sludge water can be further reduced.

[0044] <Method of manufacturing treated sludge water> The method for producing treated sludge water according to this embodiment is a method for producing treated sludge water by treating sludge water containing sludge solids. The sludge solids and sludge water can be the same as those used in the above-mentioned method for treating sludge water. In particular, the sludge solids are mostly composed of hydration products, and from the viewpoint of easily adjusting the pH of the sludge water to 12 or less, it is preferable that the hydration products contain at least one selected from calcium hydroxide and calcium silicate hydrate.

[0045] The method for producing sludge treated water in this embodiment includes an injection process in which a gas containing carbon dioxide is injected into the sludge water to react with the carbon dioxide and the hydration products that constitute the sludge solids to produce calcium carbonate, and an addition process in which a reducing agent is added to the sludge water after the injection process.

[0046] The blowing step and the adding step can be carried out in the same manner as in the above-mentioned sludge water treatment method.

[0047] In particular, in the blowing process, it is preferable that the gas containing carbon dioxide blown into the sludge water is a gas containing carbon dioxide generated during the burning of cement clinker, from the viewpoint of effectively utilizing the carbon dioxide generated during the burning of cement clinker and suppressing the amount of carbon dioxide released into the atmosphere.

[0048] In addition, in the blowing step, from the viewpoint of more efficiently dissolving the hexavalent chromium fixed to the hydration product constituting the sludge solid content into the liquid phase, it is preferable to adjust the pH of the sludge water to 12 or less, and more preferably to adjust the pH to 10 or less, by blowing a gas containing carbon dioxide into the sludge water.

[0049] Furthermore, in the adding step, the amount of the reducing agent added is 5 mg / kg or more, and preferably 5 mg / kg or more and 1000 mg / kg or less, from the viewpoint of reducing the concentration of hexavalent chromium contained in the sludge treatment water. From the viewpoint of efficiently reducing hexavalent chromium in the liquid phase to trivalent chromium, the reducing agent is preferably at least one selected from ferrous sulfate monohydrate and blast furnace slag powder.

[0050] In addition, the method for producing treated sludge water according to this embodiment preferably includes a separation and recovery step of separating and recovering the calcium carbonate after the blowing step and before the adding step.

[0051] In the method for producing treated sludge water according to the present embodiment, in the blowing step, a gas containing carbon dioxide is blown into the sludge water, and the carbon dioxide is reacted with the hydration product constituting the sludge solid content, thereby efficiently dissolving the hexavalent chromium fixed in the hydration product constituting the sludge solid content into the liquid phase. In the adding step, 5 mg / kg or more of a reducing agent is added to the sludge water, thereby reducing the hexavalent chromium in the liquid phase to trivalent chromium, and as a result, treated sludge water having a reduced concentration of hexavalent chromium can be obtained. EXAMPLES

[0052] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0053] (Sludge water) The sludge water used contained coarse aggregate, fine aggregate, and hydration products as sludge solids, and also contained calcium hydroxide, calcium silicate hydrate, monosulfate, ettringite, etc. as hydration products. The content of the sludge solids was 10 mass% relative to the sludge water. The chromium content of the sludge solids was 100 mg / g. The chromium content was measured using a fluorescent X-ray analyzer (PANalytical Epsilon 3).

[0054] (Sludge water treatment) After blowing 20% ​​carbon dioxide into the sludge water in the sludge tank at 100mL / min for 24 hours, ferrous sulfate monohydrate or blast furnace slag powder was added as a reducing agent in the amounts shown in Table 1, and the sludge water was treated by stirring for 1 hour to obtain treated sludge water. Note that in the "blank" in Table 1, no reducing agent was added.

[0055] The concentration of hexavalent chromium in the treated sludge water was measured in accordance with JIS K 0102 65.2.1 "Diphenylcarbazide spectrophotometry method." The measurement results of the hexavalent chromium concentration are shown in Table 1.

[0056] The pH and oxidation-reduction potential (ORP) of the sludge treated water were measured using a tabletop pH meter F-73 (pH electrode: 9615S-10D, ORP electrode: 9300-10D) manufactured by Horiba Ltd. The measurement results of pH and oxidation-reduction potential (ORP) are shown in Table 1.

[0057] [Table 1]

[0058] As can be seen from the results in Table 1, the sludge treated water of each Example that satisfies all the constituent requirements of the present invention had a hexavalent chromium concentration less than the detection limit of 0.005 mg / L, and therefore it can be said that the hexavalent chromium concentration was reduced. From the measurement results of pH and redox potential of each Example, and referring to the pH-redox potential diagram of chromium described in M. POURBAIX, ATLAS OF ELECTROCHEMICAL EQUILIBRIA in AQUEOUS SOLUTIONS, NACE, HOUSTON (1966), it is considered that in the sludge treated water of each Example, hexavalent chromium was reduced to trivalent chromium, and as a result, the hexavalent chromium concentration was reduced. In other words, it can be seen that the sludge water treatment method of the present invention was able to reduce the hexavalent chromium concentration.

[0059] On the other hand, in the sludge treatment water of Comparative Example 1, in which the amount of ferrous sulfate monohydrate added was 1 mg / kg, and Comparative Example 2, in which the amount of blast furnace slag powder added was 2 mg / kg, the concentration of hexavalent chromium could not be reduced because the amount of reducing agent added was small.

Claims

1. 1. A method for treating sludge water containing sludge solids, comprising: an injection step of injecting a gas containing carbon dioxide into the sludge water to react the carbon dioxide with a hydration product constituting the sludge solid content to produce calcium carbonate; and an adding step of adding a reducing agent to the sludge water after the blowing step, In the adding step, the amount of the reducing agent added is 5 mg / kg or more; In the blowing step, a gas containing carbon dioxide is blown into the sludge water to adjust the pH of the sludge water to 8.8 or more and 12 or less; A method for treating sludge water, wherein in the adding step, the reducing agent is at least one selected from the group consisting of ferrous sulfate monohydrate and blast furnace slag powder.

2. 2. The method for treating sludge water according to claim 1, further comprising a separation and recovery step of separating and recovering the calcium carbonate after the blowing step and before the adding step.

3. 3. The method for treating sludge water according to claim 1, wherein in the blowing step, the gas containing carbon dioxide is a gas containing carbon dioxide generated during burning of cement clinker.

4. 4. The method for treating sludge water according to claim 1, wherein in the blowing step, the hydration product contains at least one selected from calcium hydroxide and calcium silicate hydrate.

5. 5. The method for treating sludge water according to claim 1, wherein the amount of the reducing agent added in the adding step is 5 mg / kg or more and 1000 mg / kg or less.

6. A method for producing treated sludge water by treating sludge water containing sludge solids, comprising the steps of: an injection step of injecting a gas containing carbon dioxide into the sludge water to react the carbon dioxide with a hydration product constituting the sludge solid content to produce calcium carbonate; and an adding step of adding a reducing agent to the sludge water after the blowing step, In the adding step, the amount of the reducing agent added is 5 mg / kg or more; In the blowing step, a gas containing carbon dioxide is blown into the sludge water to adjust the pH of the sludge water to 8.8 or more and 12 or less; A method for producing treated sludge water, wherein in the adding step, the reducing agent is at least one selected from the group consisting of ferrous sulfate monohydrate and blast furnace slag powder.

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