JPH1110196A - Thermal power plant wastewater treatment method - Google Patents

Abstract

(57) [Summary] [Problem] To coagulate wastewater from a thermal power plant and perform membrane separation treatment of the coagulation treatment liquid to prevent a decrease in membrane flux when switching from steady-state wastewater to non-steady-state wastewater treatment. Reduce the frequency of chemical cleaning. SOLUTION: At the time of switching from the treatment of the steady wastewater to the treatment of the unsteady wastewater, the pH of the coagulation treatment is adjusted to 8 to 9 and the pH of the water supplied to the membrane separation device 3 becomes 8 to 9 The ferrous ion concentration of this water is 100 mg / L
Adjust so that it becomes the above. [Effect] When switching from regular drainage to unsteady drainage,
When the pH of the coagulation treatment is increased to 8 to 9, sludge derived from steady drainage becomes poorly coagulated and becomes finer, but ferrous ions are reduced to 10
When the concentration is adjusted to be 0 mg / L or more, the cohesiveness is increased, and an aggregated floc having good filterability is formed together with the ferrous ions, so that a decrease in membrane flux is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of treating wastewater from a thermal power plant, and more particularly, to a treatment system in which a stationary wastewater from a thermal power plant is subjected to a membrane separation treatment after a coagulation treatment, and the liquid to be treated is removed from the steady wastewater. The present invention relates to a method for performing a stable and efficient treatment by preventing a decrease in flux of a membrane when performing a treatment by switching to an unsteady drainage.

[0002]

2. Description of the Related Art Thermal power plant wastewater is generally treated by PA.
Aluminum salts such as C (polyaluminum chloride) and Fe
The treatment is performed by adding a ferric salt such as Cl ₃ (ferric chloride) as a flocculant, adjusting the pH and performing a flocculation treatment, and subjecting the resulting flocculation treatment liquid to solid-liquid separation. As the solid-liquid separation means, a precipitation method has been adopted. However, a membrane separation method has recently been adopted because it has an advantage that a precipitation tank can be omitted.

[0003] In such treatment of thermal power plant wastewater, most of the period is steady wastewater (for example, flue gas desulfurization wastewater, power plant cleaning wastewater, domestic wastewater, etc., which contains fluorine, metal, SS, etc.). (1) Fe-ion content is low, and unsteady wastewater is generated about 1 to 3 times a year (for example, electric dust collector washing wastewater, air heater washing wastewater, etc. In addition to SS, a relatively large number of first wastewater is used. Each time the iron ions are generated, the supply of the steady-state wastewater to the water treatment device is temporarily stopped, and the unsteady wastewater is supplied instead, thereby being treated in the water treatment device.

When supplying the unsteady wastewater to the water treatment apparatus, the unsteady wastewater is usually introduced into the water treatment system for treating the steady wastewater instead of the steady wastewater. In the treatment system, almost all of the wastewater is replaced with unsteady wastewater. It takes several hours from switching.

[0005] In the treatment of steady wastewater, the adjusted pH value for stable membrane separation of coagulated sludge is as follows:
It is 6-7.5, preferably 6-7. However, when the wastewater to be treated is replaced from the steady wastewater to the non-steady wastewater, the ferrous ion is increased (usually several hundreds to several hundreds hundreds) with the coagulation treatment conditions of the steady wastewater, that is, pH 6 to 7.5. mg / L
), The unsteady drainage including
Since the pH is slightly lower for the coagulation of iron ions, a part of ferrous ions leaks into the treated water, and the generated flocs become flocs smaller than the floc diameter suitable for membrane separation, thereby reducing the membrane flux. Problem arises.

In order to avoid such problems, it is necessary to increase the pH of the coagulation treatment to about 8 to 9 when treating unsteady wastewater.

[0007]

However, simply increasing the pH of the coagulation treatment at the time of switching from the steady drainage to the non-steady drainage causes clogging of the membrane in a short time. This is because the sludge of the steady drainage remains in the water treatment system at the time of switching, so if only the pH is adjusted, the sludge becomes finer due to the increased pH,
This sludge with reduced filtration properties contaminates the membrane surface,
By causing a decrease in membrane flux.

In general, in a membrane separation device, clogging of a membrane is prevented by alternately repeating a water-passing process of collecting permeated water and a backwashing of backflow of permeated water or the like from a permeated water side. However, when the recovery of the film flux due to the backwashing is poor, the operation of the apparatus is stopped and the cleaning with the chemical is performed. When the frequency of such chemical cleaning increases, not only does the cost of chemicals rise, but also the operation rate of the apparatus decreases. Therefore, it is important to prevent film clogging and reduce the frequency of chemical cleaning.

SUMMARY OF THE INVENTION The present invention solves the above-mentioned conventional problems and reduces the frequency of chemical cleaning by preventing a decrease in membrane flux in the treatment of thermal power plant wastewater by switching from steady wastewater to unsteady wastewater. The aim is to provide a method.

[0010]

SUMMARY OF THE INVENTION A thermal power plant wastewater treatment method of the present invention is a thermal power plant wastewater which is subjected to a coagulation treatment, and a coagulation treatment liquid is passed through a membrane separation device to make permeated water into treated water. It is a method of treating power plant wastewater, and as thermal power plant wastewater,
In a method for treating wastewater from a thermal power plant, the method includes treating a stationary wastewater having a relatively low ferrous ion content and treating an unsteady wastewater having a relatively high ferrous ion content. At the time of switching from the treatment of the stationary wastewater to the treatment of the unsteady wastewater, the pH of the coagulation treatment is adjusted to 8 to 9 and when the pH of the water supplied to the membrane separation device becomes 8 to 9, Is characterized in that the ferrous ion concentration is adjusted to be 100 mg / L or more.

As described above, when the pH of the coagulation treatment is increased to 8 to 9 when switching from the regular drainage to the unsteady drainage, the sludge derived from the regular drainage becomes poorly coagulated and becomes finer.
When the iron ion is adjusted to be 100 mg / L or more, the cohesiveness increases, and an aggregated floc having good filterability is formed together with the ferrous ion, so that a decrease in membrane flux is suppressed.

It should be noted that some time is required until the pH in the system becomes 8 to 9 due to the pH adjustment at the time of this switching, during which the membrane flux decreases slightly. Therefore, in order to prevent a decrease in the membrane flux during the pH adjustment, it is necessary to stop collecting the treated water from the membrane separator until the pH of the water supplied to the membrane separator is stabilized at 8 to 9. preferable.

[0013]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a system diagram showing an embodiment of a method for treating wastewater from a thermal power plant according to the present invention.

During the treatment of the stationary wastewater from the thermal power plant, the stationary wastewater is introduced into the coagulation reaction tank 1 and, if necessary, a pH adjuster of an alkali such as NaOH or an acid such as H ₂ SO ₄ is added. The pH is adjusted to 6 to 7.5, preferably to 6 to 7, and an aluminum salt such as PAC or a ferric salt such as FeCl ₃ is added as a coagulant to perform coagulation treatment. And supplied to the membrane separation device 3 by the pump P. The permeated water of the membrane separation device 3 is taken out of the system as treated water, a part of the concentrated water is returned to the coagulation reaction tank 1, and the remainder is circulated to the circulation tank 2. By returning the concentrated water to the flocculation reaction tank 1, flocs in the concentrated water become seed crystals of the flocculation reaction and exhibit an effect of accelerating the flocculation reaction. However, the return of the concentrated water is not necessarily required. May be circulated to the circulation tank 2.

The coagulant added to the wastewater in the coagulation reaction tank 1 is
Although it depends on the type of coagulant and the quality of the waste water, in general, it is preferably about several tens to several hundreds (eg, 40 to 600) mg / L.

As the membrane of the membrane separation device 3, an MF (microfiltration) membrane or a UF (ultrafiltration) membrane is used, and the membrane is preferably used at a permeate flow rate of 0.1 to 10 m ³ / m ² · day. Separation processing is performed.

The circulation tank 2 is usually provided with the same pH adjusting means as the coagulation reaction tank 1. Circulation tank 2
If no pH adjusting means is provided, the liquid in the circulation tank 2 is returned to the coagulation reaction tank 1 so that the pH of the circulation tank 2 is substantially equal to the pH of the coagulation reaction tank 1.

From the circulation tank 2 or the circulation system of the membrane separation device 3, the SS concentration in the tank is usually 1000 to 30.
Sludge is continuously or intermittently pulled out to about 000 mg / L.

When unsteady wastewater is generated and the wastewater to be treated is switched from the steady wastewater to the unsteady wastewater, the supply of the steady wastewater is stopped, the unsteady wastewater is introduced into the coagulation reaction tank 1, and the coagulation reaction is performed. Raise the set pH of tank 1 to 8-9. If the pH of the coagulation treatment is less than 8, insolubles cannot be sufficiently generated from the unsteady wastewater containing a large amount of ferrous ions. However, if the pH of the coagulation treatment exceeds 9, aluminum and ferric salt in the sludge are eluted, which is not preferable. Therefore, the set pH for the coagulation reaction of the unsteady wastewater is 8-9.
And

According to the present invention, the pH of the coagulation treatment is raised to 8 to 9 at the time of switching from the steady drainage to the unsteady drainage, and the pH of the water supplied to the membrane separation device 3 becomes 8 to 9 The ferrous ion concentration of the water is 100 mg / L
Adjust so that it becomes the above.

More specifically, after confirming that the pH in the circulating tank 2 has reached 8 to 9, ferrous ions,
For example, ferrous sulfate, ferrous chloride, etc. are added, and the circulation tank 2
Is adjusted to 100 mg / L or more, preferably 300 to 1000 mg / L. When the ferrous ion concentration is less than 100 mg / L, the cohesiveness of the sludge derived from the steady drainage remaining in the system is low because the ferrous ion concentration is low despite the alkaline pH of 8 to 9. Is reduced, and this becomes finer, which causes a reduction in the membrane flux of the membrane separation device 3. Ferrous ion concentration is especially 300
mg / L or more is preferable, but even if the ferrous ion concentration is excessively increased, there is no difference in the cohesiveness improving effect.
The amount of sludge generated is undesirably increased. Therefore, the first
The iron ion concentration is 1000 mg / L or less.

As described above, according to the present invention, by increasing the pH and the ferrous ion concentration in accordance with the present invention, the sludge derived from the stationary wastewater remaining in the system when switching from the stationary wastewater to the unsteady wastewater can be filtered. Although it can be sent to the membrane separation device 3 as a good flocculent floc, it is possible to suppress the decrease of the membrane flux, but it takes a little time to switch the set pH value when switching from the steady drainage to the unsteady drainage. In the meantime, the coagulation treatment liquid that does not satisfy the preferable coagulation conditions flows into the membrane separation device 3, thereby reducing the membrane flux.

Therefore, in the present invention, when switching from the steady drainage to the unsteady drainage, as described above, the coagulation treatment pH is increased to 8 to 9, the ferrous ion concentration is adjusted, and the valve V is closed. It is preferable to suspend the use of the permeated water from the membrane separation device 3 until the pH of the water introduced into the membrane separation device 3 becomes stable at 8 to 9. In this case, the liquid in the circulation tank 2 simply returns to the circulation tank 2 through the raw water chamber of the membrane separation device 3.

By stopping the water sampling in this way, the flocculation treatment liquid that does not satisfy the preferable flocculation conditions and contains flocculants having poor filterability simply circulates in the circulation system, Since it does not permeate the membrane, the membrane flux does not decrease, and the membrane separation performance can be maintained high.

In the illustrated method, the ferrous salt of the reagent is added to the circulation tank 2 in order to adjust the ferrous ion concentration, but the addition of the ferrous salt is not limited to the circulation tank 2. Alternatively, it may be added to a pipe for supplying water from the circulation tank 2 to the membrane separation device 3 or a pipe for circulating concentrated water from the membrane separation device 3 to the circulation tank 2. As described above, by adding the ferrous salt into the circulation system of the membrane separation device, the contact of the fine sludge which is inconvenient for the membrane separation with the membrane is reduced, which is effective in preventing the reduction of the membrane flux. However, the ferrous salt may be added to a pipe for returning concentrated water from the coagulation reaction tank 1 or the membrane separation device 3 to the coagulation reaction tank 1. In particular, it is preferable to add a ferrous salt to both the coagulation reaction tank 1 and the circulation tank 2 in order to improve cohesion and filterability.

The ferrous ion concentration is adjusted by separately adding ferrous salt from outside the system and discharging the water from the circulation tank 2 to the concentrated water tank, etc. To an appropriate level, and the unsteady wastewater can be introduced into the circulation tank 2 via the coagulation reaction tank 1 to increase the ferrous ion concentration in the circulation tank 2 to 100 mg / L or more.

The adjustment of the ferrous ion concentration by the unsteady drainage may be performed in combination with the adjustment of the ferrous ion concentration by the addition of the ferrous salt of the reagent.

In the present invention, the regular wastewater is wastewater constantly discharged from a thermal power plant such as flue gas desulfurization wastewater, power plant cleaning wastewater, domestic wastewater, etc., as described above. The ion concentration is 10 mg / L or less, especially 0.1 to
1 mg / L.

On the other hand, the unsteady wastewater is wastewater discharged from a thermal power plant such as an electric dust collector washing wastewater and an air heater washing wastewater, and usually has a ferrous iron concentration of 100 mg / L or more. Especially 200-2000mg / L
It is.

[0031]

The present invention will be described more specifically with reference to examples and comparative examples, but the present invention is not limited to the following examples unless it exceeds the gist.

First, a comparative example will be described for convenience of explanation.

Comparative Example 1 Steady wastewater (desulfurized wastewater) from a thermal power plant was treated by the apparatus shown in FIG. 1 and then switched to unsteady wastewater for treatment. The water quality of steady drainage and unsteady drainage is as follows.

Steady-state wastewater quality pH: 6.8 SS: 30 mg / L Fe: 1.3 mg / L (Fe ²⁺ : 0.1 mg / L or less) F: 19 mg / L Unsteady wastewater quality pH: 2.6 SS : 720 mg / L Fe: 1340 mg / L (Fe ²⁺ : 1280 mg / L) F: 3.8 mg / L First, steady waste water was introduced into the coagulation reaction tank (capacity 500 L) 1 at a flow rate of 1200 L / day, and the coagulant was added. PAC was added at 200 mg / L, and NaOH or H ₂ SO ₄ was added as necessary to adjust the pH to 6 to 7.
After adjustment and sufficient coagulation reaction, a circulation tank (capacity 100
0L) 2, introduced by a pump P into a membrane separator 3 equipped with a tubular MF membrane (inner diameter 5.5 mm, pore diameter 0.2 μm) and subjected to membrane separation at a constant permeate flow rate of 5 m ³ / m ² · day. .

The permeated water of the membrane separation device 3 was taken out as treated water, a part (1200 L / day) of the concentrated water was returned to the coagulation reaction tank 1, and the remainder was circulated to the circulation tank 2. Sludge was extracted from the circulation tank 2 such that the SS concentration in the circulation tank became 20000 mg / L.

In the apparatus of this embodiment, p
Since the H adjusting means is not provided, the liquid in the circulation tank 2 is returned to the coagulation reaction tank 1 at a rate of 1200 L / day, so that the pH of the liquid in the circulation tank 2 is adjusted to the liquid p in the coagulation reaction tank 1.
H.

The membrane separator 3 was continuously treated by performing backwashing with treated water for 5 seconds once every 15 minutes.

In chemical cleaning, the amount of permeated water is 5 m ³ / m
^It is performed when the value becomes ² · day or less.

As the filtration performance of the membrane at this time, the difference between the average of the circulating water side pressure and the permeate water side pressure was taken as the effective pressure, and the membrane flux was calculated at 0.5 kgf / cm ² at 25 ° C. When the amount of reduction of the membrane flux per day was calculated as the flux reduction rate, it was 0.19 m ³ /
m ² · day / day.

At a certain point in time, the unsteady wastewater was generated. Therefore, the introduction of the steady wastewater was stopped to switch to the unsteady wastewater, and the unsteady wastewater was introduced into the coagulation reaction tank 1 at a flow rate of 1200 L / day. Then, in order to sufficiently coagulate Fe ²⁺ in the unsteady wastewater, the same coagulant addition amount was set in the same manner as in the treatment of the steady wastewater except that the set pH of the agglutination reaction tank 1 was raised to 8. Continuous treatment was performed at the same permeate flow rate and backwash frequency. The Fe ²⁺ concentration in the circulation tank 2 was 0.2 mg / L immediately after switching from the steady drainage to the unsteady drainage, and gradually increased to about 100 mg / L about 0.5 days later. .

The rate of flux reduction and the frequency of chemical cleaning when the unsteady drainage treatment was continued for 7 days were as shown in Table 1.

Example 1 In Comparative Example 1, when the steady drainage was switched to the non-steady drainage, the pH of the coagulation treatment was raised to 8, and the
When the pH of the inside becomes 8, the Fe ²⁺ concentration becomes 100 mg.
/ L, the same treatment was carried out except that ferrous sulfate as a reagent was added to the circulation tank 2, and the flux reduction rate and chemical cleaning frequency during the unsteady drainage treatment were as shown in Table 1. became.

Example 2 In Example 1, the Fe ²⁺ concentration in the circulation tank 2 was 300 m
The same treatment was carried out except that ferrous sulfate was added to give g / L. The rate of flux reduction and the frequency of chemical cleaning during the unsteady drainage treatment were as shown in Table 1.

Embodiments 3 and 4 In Embodiments 1 and 2, the valve V is closed and the sampling of permeated water is temporarily stopped until the pH in the circulation tank 2 starts to rise to pH 8 and the circulation tank is stopped. When the same treatment was performed except that the sampling of permeated water was restarted when the pH in 2 became 8, the flux reduction rate and chemical cleaning frequency during unsteady drainage treatment were as shown in Table 1. became.

[0045]

[Table 1]

From Table 1, it can be seen that by adjusting the Fe ²⁺ concentration according to the method of the present invention, a decrease in the film flux can be prevented and the frequency of chemical cleaning can be significantly reduced. It is apparent that by stopping the sampling of the permeated water during the period, the decrease in the membrane flux can be further prevented.

In Comparative Example 1 and Examples 1 to 3, the Fe and Al ³⁺ concentrations in the treated water during the unsteady drainage treatment were as good as 1.0 mg / L or less in each case.

[0048]

As described in detail above, according to the method for treating thermal power plant wastewater of the present invention, it is possible to prevent a decrease in membrane flux when switching from steady wastewater to unsteady wastewater. For this reason, the frequency of chemical cleaning of the film is greatly reduced, so that the chemical cost can be reduced and the operation rate of the apparatus can be improved.

[Brief description of the drawings]

FIG. 1 is a system diagram showing an embodiment of a method for treating wastewater of a thermal power plant according to the present invention.

[Explanation of symbols]

 1 Coagulation reaction tank 2 Circulation tank 3 Membrane separation device

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI C02F 1/44 C02F 1/44 E 1/52 1/52 K (72) Inventor Hiroyuki Fujita Ichibancho, Aoba-ku, Sendai-shi, Miyagi 3-7-1, Tohoku Electric Power Co., Inc. (72) Inventor Kiyohito Chikazawa 3-4-2, Nishi-Shinjuku, Shinjuku-ku, Tokyo Kurita Kogyo Co., Ltd. Kurita Kogyo Co., Ltd. (4-7-7) Kurita Industries Co., Ltd. (72) Inventor Takeshi Sato 3-4-7 Nishi Shinjuku, Shinjuku-ku, Tokyo

Claims (2)

[Claims] Claims: 1. A method for treating thermal power plant wastewater by coagulating wastewater from a thermal power plant, passing the flocculated liquid through a membrane separation device and treating permeated water as treated water, A method for treating wastewater from a thermal power plant, comprising: treating a stationary wastewater having a relatively low ferrous ion content; and treating an unsteady wastewater having a relatively large ferrous ion content. At the time of switching from the treatment of the stationary wastewater to the treatment of the unsteady wastewater, the pH of the coagulation treatment is adjusted to 8 to 9 and when the pH of the water supplied to the membrane separation device becomes 8 to 9, Adjusting the ferrous ion concentration to be 100 mg / L or more. 2. The method according to claim 1, wherein at the time of the switching, the collection of the treated water from the membrane separation device is stopped until the pH of the water supplied to the membrane separation device becomes 8 to 9. Thermal power plant wastewater treatment method.