JP4981726B2 - Aggregation-membrane filtration method - Google Patents

Description

  The present invention relates to a flocculation-membrane filtration method used for sewage reclaimed water treatment, seawater desalination pretreatment, etc., in addition to water purification treatment.

  Sand filtration has been used for many years in the field of water purification, but recently it has more stable removal performance than sand filtration and membrane filtration that can reliably remove protozoa and other microorganisms that could not be removed by sand filtration. Is spreading. However, in membrane filtration, membrane fouling that clogs the pores of the membrane due to SS in the raw water occurs, and it is inevitable that the membrane filtration flux gradually decreases. Therefore, measures such as increasing the frequency of backwashing of the separation membrane and cleaning the separation membrane with chemicals are being implemented, but these cause the operating rate of the equipment to decrease, so the frequency should be reduced as much as possible. It is desirable.

  Along with this, membrane fouling is also suppressed by examining the agglomeration treatment conditions, which are pretreatments for membrane separation. The method is roughly divided into a proper injection method and a sweep flocculation method.

  The appropriate injection method controls the injection amount of the flocculant according to the raw water turbidity so that the AL / T ratio (the ratio of the AL amount in the flocculant and the turbidity of the raw water) is an appropriate ratio of about 0.05 to 0.2. Is the method. This method can reduce the injection amount of the flocculant, but on the other hand, since the pollutants in the raw water cannot be completely agglomerated, the effect of removing pollutants is low and the effect of suppressing membrane fouling is also low. There is a disadvantage that it is low.

  On the other hand, the sweep flocculation method is a method in which a larger amount of the flocculant than the proper injection method (AL / T ratio is 0.2 or more) is injected, and fine particles are taken in and easily flocculated. Since micro particles in the raw water can be converted into micro flocs, not only is the contaminant removal effect high, but it is also effective in suppressing membrane fouling. On the other hand, however, there is a disadvantage that the amount of the flocculant injected increases and the running cost increases.

  As described above, the proper injection method and the sweep flocculation method have their merits and demerits, but usually the sweep flocculation method that is excellent in the suppression effect of membrane fouling is adopted, which causes an increase in running cost. .

  For this reason, it is required to effectively combine agglomeration treatment and membrane separation, and to suppress membrane fouling and improve the quality of treated water while suppressing the injection amount of the aggregating agent. Therefore, as shown in Patent Document 1, a proposal has been made to suppress the injection amount of the flocculant by storing the flocculant inside the reaction tank. In particular, in the example shown in FIG. 2, concentrated water (membrane return water) that has been subjected to cross flow filtration with a separation membrane installed outside the tank is returned to the reaction tank to effectively use the flocculant.

However, since the flocculant and raw water are always rapidly stirred in the flow of Patent Document 1, strong stirring with a large G value (a numerical value indicating stirring strength) is added to flocs in the return water from the separation membrane. For this reason, the floc in return water will be destroyed. Moreover, according to recent studies, it has been found that if flocs are repeatedly agglomerated and broken, the reagglomeration property gradually decreases, and it becomes difficult to form flocs by adding any flocculant. Therefore, even if the flocculant is stored in the reaction tank as in Patent Document 1 and cross-flow filtration of the water in the tank is performed, if the amount of flocculant injected is not increased, the effect of suppressing membrane fouling gradually decreases. However, the quality of treated water will also deteriorate.
JP 2001-70758 A

  Therefore, the object of the present invention is to solve the above-mentioned problems of the prior art, taking into consideration the stirring strength related to the growth and destruction of flocs, and suppressing membrane fouling and controlling the quality of treated water while suppressing the amount of flocculant injected. It is to provide a novel coagulation-membrane filtration method capable of stably achieving the improvement.

  The coagulation-membrane filtration method of the present invention made to solve the above problems is to add a flocculant to raw water to form a micro floc in a rapid stirring tank, and aeration stirring in a reaction tank to coarsen the floc. In the sedimentation tank, coarse flocs are settled and separated, and the supernatant water is sent to the separation membrane installed outside the tank and the bottom water is returned to the reaction tank. The membrane return water is returned to the reaction tank. As long as the reaction tank has a gentle stirring condition, a stirring system using a paddle may be used, and the precipitation tank does not need to be a separate tank, and a mechanism for precipitation may be provided in a part of the reaction tank.

  If the water source is a complex polluted water source containing ammonia, soluble manganese and odorous substances at the same time as in claim 2, chlorine and powdered activated carbon can be added to the raw water in the preceding stage of the rapid stirring tank. Further, as in claim 3, a flocculant can be further added to the supernatant water of the precipitation tank, and water can be sent to the separation membrane.

  In any of the inventions, as in claim 4, a part of the membrane return water can be returned to the reaction tank, and can be returned not only to the reaction tank but also to the rapid stirring tank.

  Moreover, it is preferable that the addition rate of the flocculant to the raw water is 0.5 to 20 mg-PAC / L as in claim 5, and when performing two-stage injection as in claim 2, the supernatant water of the precipitation tank It is preferable to set the flocculant addition ratio to 0.5 to 10 mg-PAC / L.

  According to the coagulation-membrane filtration method of the present invention, micro flocs are formed in a rapid stirring tank, and then flocs are grown and coarsened by gentle stirring by aeration or paddle in a reaction tank. Further, coarse flocs are settled and separated in a sedimentation tank, and only the supernatant water containing micro flocs that are not easily destroyed is sent to a separation membrane installed outside the tank, and crossflow filtration is performed. For this reason, since the particle number load of the separation membrane is reduced and the floc that is larger than the membrane pore size and is not easily destroyed is filtered, membrane fouling is suppressed and the quality of the treated water can be improved.

  Further, by returning the bottom water of the precipitation tank to the reaction tank, the concentrated unreacted flocculant and the remaining flocculent floc are brought into contact with the raw water under slow stirring conditions. As a result, the coagulant effect of the coagulant can be effectively utilized while suppressing breakage of the floc and deterioration of the reaggregability. For this reason, sweep flocculation is possible at a low injection rate, and the amount of flocculant added to the raw water can be reduced to 0.5 to 20 mg-PAC / L, so that the running cost can be reduced. In this way, it is possible to stably achieve suppression of membrane fouling and improvement of treated water quality while suppressing the injection amount of the flocculant.

  Further, as in claim 3, if a flocculant is further added to the supernatant water of the precipitation tank and then sent to the separation membrane, fine particles that could not be aggregated in the previous steps are aggregated to suppress membrane fouling. And the effect of improving treated water quality can be further enhanced. Since the second-stage flocculant injection is aimed at agglomeration of fine particles, even if the G value of the agitation is somewhat high, floc growth and reaggregation are not adversely affected. When performing such two-stage injection, the flocculant addition rate to the supernatant water of the precipitation tank may be 0.5 to 10 mg-PAC / L.

  In addition, if chlorine and powdered activated carbon are added to the raw water at the front stage of the rapid stirring tank as in claim 2, ammonia, manganese, odorous substances, dissolved organic substances, etc. in the raw water can be removed without changing the tank equipment. It is also possible to deal with complex polluted water sources. In this case, micro flocs incorporating powdered activated carbon are formed, and the activated carbon can be retained in the reaction tank for a long time, so that the removal efficiency of odorous substances, dissolved organic substances and the like can be increased.

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a view showing an embodiment of the invention of claim 1, wherein 1 is a rapid stirring tank into which raw water flows, 2 is a reaction tank disposed in the subsequent stage, 3 is a precipitation tank disposed in the subsequent stage, 4 Is a separation membrane installed outside the tank.

The raw water first enters the rapid agitation tank 1, and the aggregating agent is injected from the aggregating agent feeder 5, and rapidly agitated at a high G value by the agitation device 6. The flocculant can also be injected immediately before the rapid stirring tank 1. As a kind of the flocculant, an appropriate kind such as PAC, iron salt, PSI, or vanous sulfate may be selected according to the properties of the raw water, and this point is the same as the conventional one. However, the injection rate of the flocculant is lower than before, and it may be 0.5 to 20 mg-PAC / L. This number is expressed using PAC, but it is converted for other flocculants. If the amount is less than 0.5 mg-PAC / L, the cohesive force is insufficient, and if it exceeds 20 mg-PAC / L, the object of the present invention, that is, suppressing the injection amount of the flocculant cannot be achieved.
However, when the turbidity of the raw water rises, the injection rate of the flocculant is adjusted in consideration of the ALT ratio (0.05 to 0.2) that is a standard for proper injection.

  The raw water in which micro flocs are formed in the rapid stirring tank 1 then flows into the reaction tank 2. The reaction tank 2 is an aeration tank provided with an aeration device 7. The aeration method may be a full-surface aeration method or a swirl flow method by partial aeration, and slow agitation by the aeration apparatus 7 is performed. The G value is lower than that of the rapid stirring tank 1, and the flocs are increased in size. As will be described later, since the flocculant-containing water is returned from the latter stage, sweep flocculation is possible even if the injection rate of the flocculant in the rapid stirring tank 1 is low. In addition, by using the aeration apparatus 7, it can prevent reliably that a tank bottom part will be in an anaerobic state.

  The overflow water from the reaction tank 2 flows into the settling tank 3. Here, coarse flocs are separated by settling. Since the stirring is not performed, the G value becomes very low. In this way, in the present invention, the G value gradually decreases as it goes to the subsequent stage, so that the destruction of the floc does not proceed. The supernatant water of the sedimentation tank 3 is sent to the separation membrane 4 installed outside the tank via the pump 8, but the coarse flocs are settled and separated, and the particle number load on the separation membrane 4 is reduced. In order to filter flocs that are larger than the membrane pore size and are not easily destroyed, membrane fouling is suppressed. In this embodiment, an inclined plate 9 is installed inside the sedimentation tank 3 to improve sedimentation separation.

  The bottom water of the settling tank 3 contains unreacted flocculant in addition to the coarse flocs. Therefore, the bottom water of the sedimentation tank 3 is returned to the reaction tank 2 by the return pump 12 via the tank bottom water return line 10. As a result, the concentration of the flocculant in the reaction tank 2 is increased, and the above-described sweep flocculation becomes possible. Since the G value in the reaction tank 2 is low, the returned floc is not easily destroyed.

  The type of the separation membrane 4 is not particularly limited, but a monolith type ceramic filtration membrane used in this embodiment is more preferable. The filtration method is cross flow filtration, and the membrane permeated water is taken out as treated water. Membrane filtration is performed in a state where floc destruction does not proceed and the coarse floc is removed and the particle number load is reduced, so that membrane fouling is suppressed and stable treated water quality is achieved. can do.

  The concentrated water that has passed through the separation membrane 4 is returned to the reaction tank 2 through the membrane return water return line 11 as membrane return water. Since this membrane return water contains the concentrated flocculant, the flocculant action of the flocculant can be effectively utilized by returning it to the reaction tank 2. For these reasons, sweep flocculation is possible with a small amount of flocculant in the present invention.

FIGS. 3-1 and 2 are views showing an embodiment of claim 3. A combination with claim 1 is shown in FIG. 3-1, and a combination with claim 2 is shown in FIG. 3-2.
In this embodiment, the flocculant is further injected from the second flocculant supply device 16 into the supernatant water of the precipitation tank, stirred by the static mixer 17, and then fed to the separation membrane 4. The addition of the second-stage aggregating agent is performed to agglomerate fine particles that could not be agglomerated so far, and to reduce the number of particles affecting fouling. The addition rate of the flocculant is suitably 0.5 to 10 mg-PAC / L. If it is more than this range, it is contrary to the object of the present invention of reducing the flocculant.

  Moreover, in embodiment of FIG. 3-2, the mixing tank 14 is installed in the front | former stage of the rapid stirring tank 1, powdered activated carbon is supplied from the activated carbon supply apparatus 15, and it mixes with raw | natural water. Bound chlorine reacts with powdered activated carbon and is denitrified. In addition, odorous substances and dissolved organic substances in the raw water are also adsorbed on the powdered activated carbon.

  In this way, the raw water to which chlorine and powdered activated carbon are added is sent to the rapid stirring tank 1 and the flocculant is added to form micro flocs incorporating powdered activated carbon, as in the other embodiments. Furthermore, although aeration stirring is performed in the reaction tank 2, since the activated carbon powder is also concentrated in the reaction tank 2 in the same manner as the flocculant, the SRT can be secured for a long time of about 5 to 20 days, during which the odor substance Adsorption of dissolved organic matter is sufficiently performed. Further, in the sedimentation tank 3, the coarse floc that has taken in the powdered activated carbon is settled and separated, and the load on the separation membrane is reduced as in the other embodiments.

  Thus, if the flow of FIGS. 3A and 3B is used, it is possible to cope with a complex contaminated water source without changing the tank equipment, but the effects described in the other embodiments are obtained as they are. It goes without saying that it can be done.

  The agitation by the static mixer 17 is agitation with a high G value, but the purpose is to agglomerate fine particles that have not been agglomerated so far. Therefore, the problem of floc growth and re-aggregation does not occur. Since the other configuration is the same as that of the first embodiment, the same reference numerals are assigned to the corresponding portions, and the description is not repeated.

FIG. 2 is a view showing an embodiment of claim 2.
This is an effective flow when the water source is a combined polluted water source containing ammonia, manganese, odorous substances and the like at the same time. The tank equipment remains the same as that of the other embodiment, and only chlorine and powdered activated carbon need be added. In this embodiment, after adjusting the pH of the raw water, sodium hypochlorite is added from the chlorine supply device 13 to convert ammonia into bound chlorine (dichloromine) and to oxidize and insolubilize manganese ions. Oxidized manganese can be separated and removed in the separation membrane 4.

  In any embodiment, the sediment in the sedimentation tank 3 is returned to the reaction tank 2 and periodically discharged from the reaction tank 2 to the outside of the system. Further, depending on the operating conditions, it is also discharged out of the system from the sedimentation tank 3, and the turbid mass in the system is controlled.

  As described above, according to the present invention, the stirring strength related to the growth and breakage of flocs is taken into consideration, and the suppression of membrane fouling and the improvement of the quality of treated water are stably performed while suppressing the injection amount of the flocculant. Can be achieved.

It is a flowchart which shows embodiment of invention of Claim 1. It is a flowchart which shows embodiment of invention of Claim 2. It is a flowchart which shows embodiment of invention of Claim 3.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Rapid stirring tank 2 Reaction tank 3 Precipitation tank 4 Separation membrane 5 Coagulant feeder 6 Stirrer 7 Aeration apparatus 8 Pump 9 Inclined plate 10 Tank bottom water return line 11 Membrane return water return line 12 Return pump 16 Second flocculant Feeder 17 Static mixer 13 Chlorine feeder 14 Mixing tank 15 Activated carbon feeder

Claims (6)

  A flocculant is added to the raw water to form micro flocs in the rapid stirring tank, aeration stirring is performed in the reaction tank to coarsen the flocs, and the coarse flocs are settled and separated in the settling tank, and the supernatant water is separated outside the tank. An agglomeration-membrane filtration method characterized in that water is sent to the membrane and the bottom water is returned to the reaction vessel, and the separation membrane performs cross-flow filtration to extract treated water, while returning the membrane return water to the reaction vessel.   The aggregation-membrane filtration method according to claim 1, wherein chlorine and powdered activated carbon are added to the raw water in a stage preceding the rapid stirring tank.   The coagulation-membrane filtration method according to claim 1 or 2, wherein a flocculant is further added to the supernatant water of the precipitation tank, and water is fed to the separation membrane.   The aggregation-membrane filtration method according to any one of claims 1 to 3, wherein a part of the membrane return water is returned to the reaction tank.   The coagulation-membrane filtration method according to any one of claims 1 to 4, wherein an addition rate of the coagulant to the raw water is 0.5 to 20 mg-PAC / L.   The coagulation-membrane filtration method according to claim 3, wherein the addition rate of the flocculant to the supernatant water of the precipitation tank is 0.5 to 10 mg-PAC / L.

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