JPH11226302A - Wastewater treatment method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To significantly lessen the amount of waste and to efficiently treat wastewater containing salts by a simple apparatus and simple operation. SOLUTION: After raw water 11 containing salts is concentrated by an evaporation apparatus 1 for concentration, the resultant raw water is further concentrated by an evaporation apparatus 2 for crystallization to precipitate salt crystals. The obtained suspended liquid is solid-liquid separated by a solid-liquid separation apparatus. The separated liquid 21 is turned back to the evaporation apparatus 1 for concentration and a part of the liquid is discharged out of a discharging route 22 and treated for purification. On the other hand, the salt crystals 17 are dissolved in a dissolution tank 4 and then released.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating effluents containing salts, and more particularly to a method for treating effluents containing salts at a high concentration such as flue gas desulfurization effluent. About.

[0002]

2. Description of the Related Art In a thermal power plant or the like, when flue gas generated by the combustion of fuel such as coal or petroleum is subjected to wet desulfurization in a desulfurization unit, the flue gas desulfurization effluent discharged from the desulfurization unit includes sodium chloride and chloride. It contains a large amount of salts such as calcium and sodium sulfate, and CO such as dithionic acid.
It also contains incompatible substances such as D component, heavy metals, fluorine, and boron compounds.

Conventional methods for treating such flue gas desulfurization effluent include JP-A-3-8411, JP-A-5-4080,
Japanese Patent Application Laid-Open No. 5-123532 describes a method in which a wastewater is concentrated, and then a solidifying agent such as coal ash, cement, kaolinite or the like is added to the concentrated liquid to solidify salts and other components. However, in the conventional method, since the amount of salts contained in the effluent is large, the amount of the solidifying agent to be added is also large, and the solidification also requires the presence of water. There is a problem that the amount becomes large.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for treating wastewater which can reduce the amount of waste extremely and can efficiently treat wastewater containing salts with a simple apparatus and operation. It is to propose.

[0005]

SUMMARY OF THE INVENTION The present invention is directed to a concentration and crystallization step of evaporating and concentrating an effluent containing salts to precipitate crystals of salts, and a step of forming crystals in the concentration and crystallization steps. A solid-liquid separation step of solid-liquid separation of the suspension into crystals and a separation liquid, a dissolution release step of dissolving the separated solids in water and discharging it out of the system, and a return of the separated liquid to the concentration and crystallization step And a wastewater treatment method comprising the steps of:

The effluent to be treated in the present invention is not particularly limited as long as it is an effluent in which salts are dissolved, but is preferably an effluent in which salts are dissolved at a high concentration. It may be. Specific examples of the salts include sodium chloride, calcium chloride, sodium sulfate and the like. Examples other than salts include COD components such as dithionic acid, heavy metals, fluorine and boron compounds. Specific examples of the effluent include flue gas desulfurization effluent discharged from a device for wet desulfurization of flue gas generated by combustion of fuel such as coal and petroleum.

[0007] The effluent (raw water) containing salts is subjected to a coagulation treatment (hereinafter referred to as a first coagulation agent) by adding an inorganic coagulant such as a calcium salt or an aluminum salt or a coagulant such as a chelating agent before being subjected to a concentration and crystallization step. ), And heavy metals and fluorine compounds in raw water by solid-liquid separation such as sedimentation.
It is preferable to remove boron compounds and the like. Thereby, corrosion of the evaporator and generation of scale can be prevented.

[0008] In the concentration and crystallization step of the present invention, the raw water is evaporated and concentrated to precipitate salt crystals. In the concentration and crystallization steps, concentration and crystallization can be performed in one step using one evaporator. However, a concentration step in which raw water is evaporated using two evaporators to obtain a concentrate, It is preferable to perform the two steps separately from a crystallization step of further evaporating the liquid to precipitate salt crystals.

In either the case where the concentration and crystallization steps are performed in one step or in the case of two steps, any evaporating apparatus such as heating evaporation or vacuum evaporation can be adopted as the evaporating apparatus, such as flash type, film type, and forced circulation type. , Any type of evaporator can be used according to its characteristics.

[0010] The degree of concentration in the concentration step is not particularly limited, but it is preferable to concentrate until just before the precipitation of crystals, whereby the formation of scale can be suppressed. Even when crystals are precipitated, they can be directly transferred to the subsequent crystallization step as a concentrated liquid, and can be directly processed in the crystallization step.

Although the concentrated solution obtained in the concentration step can be directly treated in the crystallization step, it is preferable that the solid content be separated by solid-liquid separation before that. As a result, S, such as crystals precipitated in the concentration step and gypsum precipitated by the concentration,
When the S component and the first aggregation treatment step are provided, the residual SS component and the like are removed. Solid-liquid separation can be performed by a known method such as precipitation, filtration, membrane separation, and centrifugation. Further, instead of the solid-liquid separation, the same coagulation treatment as in the first coagulation treatment step can be performed. In either case, the separated liquid is processed as a concentrated liquid in the subsequent crystallization step.

In the crystallization step, the concentrated liquid obtained in the above-mentioned concentration step is further evaporated and concentrated to precipitate salt crystals to obtain a suspension in which crystals are suspended. In the concentration in the crystallization step, the concentration is set in consideration of the components in the effluent. For example, flue gas desulfurization effluent contains unremovable salts and organic substances (COD) together with releasable salts,
Usually, the former has a high concentration and the latter has a low concentration. Therefore, it is preferable that the salts which can be discharged crystallize, but the substances which are unfavorable for the discharge are hardly crystallized and are concentrated to a state of being dissolved in the concentrate.

Needless to say, it is not necessary to control the degree of concentration so as to strictly separate the dischargeable substance and the discharge inconvenient substance, and it is only necessary to roughly separate them. If some of the substances that are inconvenient for release are crystallized, dissolve the crystals and release them.
It can be removed by providing a separation step such as an aggregation treatment.
If the concentration is progressed until crystals are precipitated, a scale is also formed. Therefore, it is preferable to wash the evaporator every 1 to 4 weeks.

The steam generated in the concentration and crystallization steps is cooled and condensed, and discharged as condensed water. The discharged condensed water contains almost no impurities and can be collected and reused.

In the solid-liquid separation step, the suspension in which the crystals have been precipitated in the concentration and crystallization steps is subjected to solid-liquid separation into crystals and a separated liquid. As the solid-liquid separator, an ordinary solid-liquid separator can be used, and for example, a centrifugal separator, a cyclone, a filter using a filter cloth, a sedimentation separator, and the like can be used. There is also an evaporator equipped with a separator as a crystallization evaporator. By using such a crystallization evaporator, the concentration and crystallization step and the solid-liquid separation step can be performed in one stage.

Since the crystals separated in the solid-liquid separation step are usually salts that can be discharged, they are dissolved in water and discharged in the dissolution and discharge step. The water used here is not particularly limited, but it is preferable to use the condensed water recovered in the concentration and crystallization steps, and it is particularly preferable to use the condensed water recovered in the crystallization step. When the entire amount of condensed water recovered in the crystallization step is used, the concentration does not become unnecessarily low and the salts can be dissolved almost certainly. Since the separation liquid adheres to the crystal surface, if a substance that is inconvenient to be released is mixed into the aqueous solution if dissolved as it is, the crystal is washed, dissolved and discharged. The washing solution is preferably returned to the concentration and crystallization step as a separation solution.

If the crystal contains a substance which is inconvenient to be discharged, the crystal is dissolved in the same water as described above, and this aqueous solution is subjected to the same coagulation treatment as in the first coagulation treatment step (hereinafter, referred to as “the first coagulation treatment”). The supernatant water can be discharged after removing the inconvenient substance to be discharged in the second coagulation treatment step).

The separated liquid separated in the solid-liquid separation step is returned to the concentration and crystallization step, and the concentration and crystallization treatment is repeated with the raw water. When the concentration and crystallization steps are performed in two steps, the mixture is returned to the concentration step. In the case where the first coagulation treatment step is provided, it is preferable to return to the first coagulation treatment step.

The separated liquid generally contains substances which are inconvenient to be discharged. For example, the exhaust gas from flue gas desulfurization contains CO such as dithionic acid.
Since the D component, heavy metal, fluorine, boron compound and the like are contained, if the separation liquid is returned and the processing is continued, the concentration of the insoluble substances in the system increases. For this reason, the separated liquid is discharged partly out of the system. In this case, in the purification process, the inconvenient substances to be discharged can be removed. The purification treatment can be performed at any position in the system, but is preferably performed on the separated liquid separated in the solid-liquid separation step. In this case, the amount of water to be treated can be reduced, and the apparatus can be downsized.

The purification treatment is preferably carried out by extracting a part of the separated liquid. Specifically, the same coagulation treatment as in the first coagulation treatment step, and the solidification using a solidifying agent such as coal ash, cement or kaolin. Treatment, an acid heat decomposition treatment, an ion exchange treatment, and a combination thereof.
Although COD components such as dithionic acid cannot be removed by the coagulation treatment, they can be removed by ion exchange treatment, preferably by ion exchange treatment with a weakly basic anion exchange resin, or by acid thermal decomposition treatment.

When the first coagulation treatment step is provided and the separated liquid is returned to the first coagulation treatment step, the first coagulation treatment step can be regarded as a purification treatment step. When a COD component that cannot be removed by the coagulation treatment is contained, it is preferable to perform an acid heat decomposition treatment or an ion exchange treatment as a purification treatment, and then return to the first coagulation treatment step.

As described above, according to the present invention, since the salts in the waste liquid are separated and discharged, the amount of generated waste can be reduced. Moreover, it is not necessary to completely dispose of the separated liquid from which the salts have been removed, so that the amount of waste can be further reduced.

[0023]

According to the wastewater treatment method of the present invention, the wastewater containing salts is evaporated to precipitate crystals, and the crystals are dissolved in water and discharged out of the system. Since the wastewater is returned to the conversion step, the amount of waste can be extremely reduced, and the wastewater containing salts can be efficiently treated by a simple apparatus and operation.

[0024]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a system diagram showing a drainage treatment apparatus according to an embodiment of the present invention. In FIG. 1, 1 is an evaporator for concentration, 2 is an evaporator for crystallization, 3 is a solid-liquid separator,
4 is a dissolution tank.

In the treatment method using the apparatus shown in FIG. 1, raw water is first introduced into the evaporator 1 for concentration from the raw water channel 11 and concentrated by heating to evaporate. The evaporated water vapor is cooled and condensed, and is recovered from the water recovery path 12 as condensed water. Here, it is preferable to concentrate until just before the crystals precipitate. The concentrated raw water is introduced into the crystallization evaporator 2 from the communication path 13 as a concentrated liquid.

Next, the concentrated liquid is further heated and evaporated in the crystallization evaporator 2 and concentrated to precipitate salt crystals. Here, the salts which can be discharged are crystallized, but the substances which are inconvenient to be discharged hardly crystallize, and it is preferable to concentrate them to a state in which they are still dissolved in the concentrated liquid.

The suspension in which the crystals are precipitated is introduced into the solid-liquid separation device 3 through the communication line 14. The vapor which evaporates is cooled, condensed and recovered, and the condensed water is passed through the connecting line 15 to the dissolving tank 4.
To be introduced.

Next, the suspension transferred from the crystallization evaporator 2 is introduced into the solid-liquid separator 3 to be separated into a solid and a separated liquid containing salts crystals. Separation solid content is connection 1
7, the mixture is introduced into the dissolution tank 4 and mixed with the condensed water transferred from the crystallization evaporator 2, and is stirred and dissolved by the stirrer 18. The obtained aqueous solution of salts is discharged from the drainage channel 19 to the outside of the system as treated water.

The separated liquid separated by the solid-liquid separation device 3 is mixed with the raw water in the raw water channel 11 via the return water channel 21 so as to be returned to the evaporating apparatus 1 for concentration. At this time, return water channel 2
A part of the separated liquid to be returned is drawn out from the drawing path 22 branched from 1 and discharged out of the system. In this case, it can be rendered harmless by performing a purification process. As a purification process,
Solidification treatment for mixing and solidifying a solidifying agent such as cement, coal ash, and kaolin; solidification treatment for adding inorganic coagulant such as calcium salt and aluminum salt and coagulant such as chelating agent to form floc; ion exchange treatment Acid thermal decomposition treatments; treatments combining these; and the like.

When a solidification treatment is performed as a purification treatment, the solidified matter is discharged as waste. When performing the coagulation treatment, the separated solids are discharged as waste. The separated liquid can be returned to the evaporator 1 for concentration. In addition, when the acid thermal decomposition treatment is performed, almost no waste is generated, and the treated water can be returned to the evaporator 1 for concentration.

According to such a treatment method, since the solid content obtained in the solid-liquid separation device 3 is a solid content of salts that can be discharged, the aqueous solution can be discharged from the drainage channel 19 as it is. When a substance which is inconvenient to be discharged adheres to the solid content in the form of adhering water, it can be dissolved and discharged after washing. In this case, the washing liquid can be returned to the evaporating apparatus 1 for concentration. If the solid content contains a substance that is inconvenient to be discharged, the aqueous solution can be discharged after being removed by a method such as coagulation treatment.

In the treatment method shown in FIG. 1, the salts contained in the raw water are separated and discharged, and furthermore, it is not necessary to purify the whole separated liquid, so that the amount of waste can be extremely reduced. The wastewater containing salts can be efficiently treated by a simple apparatus and operation.

FIG. 2 is a system diagram showing a wastewater treatment apparatus suitable for treating flue gas desulfurization wastewater according to an embodiment of the present invention. 1 denote the same or corresponding parts.
In FIG. 2, reference numeral 31 denotes a first coagulation-sedimentation device,
, 33 is a second solid-liquid separator, 34 is an acid thermal decomposition device, and 35 is a second coagulation-sedimentation device. In some cases, the coagulation and sedimentation apparatuses 31 and 35 are provided with a reaction tank and a solid-liquid separation tank (precipitation tank) separately, but these are collectively illustrated as a coagulation and sedimentation apparatus.

In the treatment method using the apparatus shown in FIG. 2, first, raw water (flue gas desulfurization effluent) is supplied from a raw water channel 41 to a first coagulating sedimentation device 3.
The mixture is added to the mixture, and an inorganic flocculant such as a calcium salt or an aluminum salt or a flocculant such as a chelating agent is added to form a floc and then subjected to solid-liquid separation. The obtained separated solids are discharged from the discharge path 42 to the outside of the system. Thereby, heavy metals, fluorine compounds, boron compounds, and other SS components in the raw water are removed. The separated liquid from which these are separated is introduced into the evaporating apparatus 1 for concentration through the communication path 43.

In the evaporator 1 for concentration, the same treatment as in FIG. 1 is performed to obtain a concentrated liquid. This concentrated liquid is introduced into the first solid-liquid separation device 32 through the communication line 44, and the condensed water is
Collect from

In the first solid-liquid separator 32, the concentrated liquid concentrated by the evaporator for concentration 1 is subjected to solid-liquid separation. The obtained separated solids are discharged from the discharge path 45 to the outside of the system. This allows
The residual SS component in the coagulation sedimentation process in the former stage, a small amount of crystals generated by the evaporation and concentration process, and other SS components are removed from the concentrate. The separated liquid from which these are separated is introduced into the crystallization evaporator 2 through the communication line 46 as a concentrated liquid.

In the crystallization evaporator 2, the same treatment as in FIG. 1 is performed to obtain a suspension in which salt crystals are suspended. This suspension is introduced into the second solid-liquid separation device 33 through the communication line 47, and the condensed water is introduced into the dissolution tank 4 through the communication line 48.

The second solid-liquid separator 33 performs the same processing as the solid-liquid separator 3 shown in FIG. The solid content obtained here is introduced into the dissolution tank 4 through the communication path 54, mixed with the condensed water transferred from the communication path 48, and dissolved by stirring with the stirrer 55.

The aqueous solution obtained in the dissolving tank 4 is introduced into the second coagulation-sedimentation apparatus 35 through the communication path 56 and subjected to coagulation-sedimentation treatment in the same manner as in the first coagulation-sedimentation apparatus 31. The separated solids obtained by the coagulation and sedimentation process are discharged from the discharge path 57 to the outside of the system. This makes it possible to remove insoluble substances that are attached to or contained in the solids separated by the second solid-liquid separator 33. The separated liquid from which the undesired substances have been removed is discharged from the drainage channel 58 to the outside as treated water.

The separated liquid separated by the second solid-liquid separation device 33 is introduced into the acid pyrolysis device 34 from the communication line 51. In the acid thermal decomposition device 34, an acid such as sulfuric acid is added from the chemical injection path 52 and heated to 90 ° C. or higher to decompose COD components such as dithionic acid. The treatment liquid is mixed with the raw water in the raw water channel 41 via the return water channel 53 so as to be returned to the first flocculation settling device 31.

In the treatment method using the apparatus shown in FIG. 2, the salts contained in the raw water at a high concentration are discharged, and the discharge paths 42, 4
Since the separated solids discharged from 5, 57 become waste, the generation amount is extremely small, and the wastewater containing salts can be efficiently treated by a simple apparatus and operation.

[0042]

Next, an embodiment of the present invention will be described. Example 1 A flue gas desulfurization effluent was evaporated to obtain a concentrated solution approximately 10 times concentrated. The concentration of each component in the separated liquid obtained by solid-liquid separation of this concentrated liquid was TDS = 1114 mg / l, Na = 13000
mg / l, Ca = 330 mg / l, Mg = 11000 m
g / l, Cl = 21100 mg / l, SO ₄ = 3800
0 mg / l, F = 170 mg / l, COD _Mn = 75 mg
/ L.

6 liters of the above separated solution was further evaporated at 100 ° C. and concentrated 6 times to precipitate crystals, and condensed water was recovered. The suspension was subjected to solid-liquid separation to obtain a separated solution having a COD _Mn concentration of 450 mg / l and 260 g of crystals. The crystals were dissolved in 5 liters of the above condensed water.

The obtained aqueous solution was F = 100 mg / l, C
It had a concentration of OD _Mn = 5.1 mg / l, and did not contain any substances that would cause inconvenience in releasing heavy metals. 12,000 mg / l of a sulfuric acid band and 4500 of NaOH were added to this aqueous solution.
The solution was added at a rate of mg / l (pH = 7.0), and allowed to react for 30 minutes to perform an aggregation treatment. Thereafter, the F concentration in the separated liquid subjected to solid-liquid separation was 4.0 mg / l. The generated SS was a 2.7 kg / m ³ -10-fold concentrated liquid.

Example 2 The separation liquid (COD _Mn concentration 450 mg / l) obtained by solid-liquid separation of the suspension in Example 1 was subjected to COD component purification treatment. That is, 8 H ₂ SO ₄ was added to the separated liquid.
0 g / l was added and reacted at 95 ° C. for 1 hour. As a result, the COD _Mn concentration was reduced to 37 mg / l. When this purified water is returned to the flue gas desulfurization effluent before the evaporation and concentration treatment, the increased concentration of COD _Mn is about 0.6 mg / l.

Comparative Example 1 The concentrated solution of about 10 times concentration obtained in Example 1 was further concentrated 3 times. To 1 part by weight of the 30-fold concentrated liquid, 0.7 part by weight of cement and 1.1 parts by weight of coal ash were added for solidification treatment. As a result, a large amount of solidified waste of about 100 kg / m ³ -10 times concentrated liquid was generated. When the amount of addition of cement and coal ash was less than the above, the solidification state was poor. When the concentration was more than 30 times, addition of water was required to solidify.

[Brief description of the drawings]

FIG. 1 is a system diagram showing a wastewater treatment apparatus according to an embodiment of the present invention.

FIG. 2 is a system diagram showing a wastewater treatment apparatus suitable for treating flue gas desulfurization wastewater according to an embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Concentration evaporator 2 Crystallization evaporator 3 Solid-liquid separator 4 Dissolution tank 11, 41 Raw water channel 12 Water recovery channel 13, 14, 15, 17, 43, 44, 46, 47, 4
8, 51, 54, 56 communication path 18, 55 stirrer 19, 58 drainage path 21, 53 return water path 22 extraction path 31 first coagulation / sedimentation device 32 first solid-liquid separation device 33 second solid-liquid separation device 34 Acid pyrolysis device 35 Second coagulation sedimentation device 42, 45, 57 Discharge route 52 Chemical injection route

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI C02F 1/58 C02F 1/58 M J 1/62 1/62 Z

Claims (1)

[Claims] 1. A concentration and crystallization step of evaporating and concentrating an effluent containing salts to precipitate crystals of salts, and converting a suspension in which crystals are precipitated in the concentration and crystallization steps into crystals and a separated liquid. Wastewater treatment comprising: a solid-liquid separation step of solid-liquid separation into water; a dissolution release step of dissolving the separated solid content in water and discharging it outside the system; and a return step of returning the separated liquid to the concentration and crystallization step. Method.