JP2004243288A - Sewage treatment method and system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To effectively and stably eliminate phosphorus with a little amount of coagulant while keeping high activity of biological treatment. <P>SOLUTION: This sewage treatment system is provided with: an aerobic tank 24 aerating organic sewage 20 containing phosphorus in the presence of activated sludge; a sedimentation basin 26 separating effluent of the aerobic tank 24 into a primary treated water 32 and the sludge; a sludge drawing pump 40 drawing out a prescribed amount of sludge-mixed water in the aerobic tank 24 so as to obtain a prescribed concentration of the sludge in the aerobic tank 24; a mixing tank 48 for mixing at least a part of the primary treated water 32 discharged from the basin 26 with the sludge-mixed water drawn by the pump 40 and added with the coagulant thereto; and a settler 56 for separating aggregate formed in the mixing tank 48. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for treating wastewater, and more particularly to a method and an apparatus for treating organic wastewater containing phosphorus.
[0002]
[Prior art]
As a method for biologically treating organic wastewater containing phosphorus such as sewage, there is known an anaerobic-aerobic method (commonly called AO method) in which the wastewater is treated in the order of an anaerobic tank-aerobic tank-sedimentation basin. I have. As shown in FIG. 4, in the anaerobic tank 2 into which the water to be treated 1 flows, the activated sludge returned from the sedimentation tank 4 releases phosphorus under anaerobic conditions. In the aerobic tank 3, the activated sludge placed under aerobic conditions by aeration proliferates using the organic matter in the wastewater as a nutrient source, and takes in the phosphorus in the wastewater and the phosphorus released in the anaerobic tank into the sludge body. As a result, organic matter and phosphorus in the wastewater are removed at the same time. Activated sludge is settled and separated in the sedimentation basin 4, and the supernatant water is discharged out of the system as treated water 5. Most of the settled sludge is returned to the anaerobic tank 2 as returned sludge 6 and is repeatedly circulated. The remaining sludge substantially corresponds to the amount of activated sludge multiplied in the aerobic tank, and is sent to the post-processing step 8 as surplus sludge 7.
[0003]
However, in the above-described anaerobic-aerobic method, when activated sludge that has incorporated phosphorus into the body is placed under anaerobic conditions, phosphorus is released again in an unstable state. In the sedimentation basin 4, an anaerobic condition may be caused due to the residence time. Phosphorus is released again in the sedimentation basin 4 and moves to the supernatant water side, and the situation where the phosphorus concentration of the treated water 5 rises often occurs. Was. Further, the excess sludge 7 drawn from the sedimentation basin 4 re-releases phosphorus in the post-treatment step 8 such as concentration and dehydration in the same manner as when placed under anaerobic conditions, and the re-released phosphorus is used in the post-treatment step. The process moves to the desorbing liquid 9. The desorbed liquid 9 is usually combined with the water to be treated 1 on the upstream side and undergoes reprocessing. As a result, there is a problem in that the phosphorus load of the processing equipment increases, which adversely affects the phosphorus removal performance of the entire processing equipment.
[0004]
In order to improve such a problem, for example, Patent Literature 1 and Patent Literature 2 disclose the excess sludge in an anaerobic state and forcibly release the phosphorus from the excess sludge to the liquid side. A method for chemical removal and recovery is described. However, these methods have complicated processes and cannot solve the above-mentioned problem that phosphorus is re-released in the sedimentation basin 4 and the phosphorus concentration of the treated water 5 increases.
[0005]
On the other hand, for example, Patent Literature 3 and Patent Literature 4 describe a method in which a coagulant is added to a biological treatment tank such as the aerobic tank 3 to coagulate phosphorus in the water to be treated as a stable solid. According to these methods, the problem of re-release of phosphorus from sludge does not occur. However, there is a problem that the metal component caused by the flocculant accumulates at a high concentration in the circulating sludge 6, and this metal component lowers the biological treatment activity of the activated sludge.
[0006]
It is also conceivable to add a coagulant to the treated water 5 to coagulate and separate phosphorus in the treated water as a stable solid. However, this method is not practical because it requires a large amount of flocculant.
[0007]
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2000-317492 [Patent Document 2]
Japanese Patent Application Laid-Open No. 2002-192185 [Patent Document 3]
JP-A-11-169866 [Patent Document 4]
JP-A-2002-307094
[Problems to be solved by the invention]
The present invention has been made in view of the above background of the prior art, and an object of the present invention is to effectively and stably remove phosphorus by a small amount of a flocculant while maintaining a high biological treatment activity. It is to provide a possible wastewater treatment method and apparatus.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, a wastewater treatment method according to the present invention comprises: a primary treatment step of introducing an organic wastewater containing phosphorus into an aerobic tank holding activated sludge for biological treatment; And a secondary treatment step of mixing the excess sludge generated in the primary treatment step with the primary treatment water that has passed through, and adding a coagulant to coagulate and separate the phosphorus remaining in the primary treatment water. .
[0010]
Further, the present invention is characterized in that, in the above configuration, only a part of the primary treatment water is treated in the secondary treatment step according to the phosphorus concentration in the primary treatment water. Further, according to the present invention, in the above-described configuration, in the primary treatment step, a fixed amount of the sludge mixed water in the aerobic tank is withdrawn so that the sludge concentration in the aerobic tank becomes a predetermined value. The sludge is mixed with the primary treatment water.
[0011]
Further, the wastewater treatment apparatus according to the present invention includes an aerobic tank for aerating organic wastewater containing phosphorus in the presence of activated sludge, and a sedimentation tank for separating the effluent of the aerobic tank into primary treated water and sludge. Sludge return means for returning the sludge separated in the sedimentation basin to the aerobic tank, excess sludge withdrawal means for withdrawing the sludge mixed water from the aerobic tank, and primary treatment water discharged from the sedimentation basin A mixing tank in which a coagulant is added while mixing at least a part of the sludge mixed water extracted by the excess sludge extraction means, and a solid-liquid separation means for separating aggregates generated in the mixing tank. It is characterized by.
[0012]
[Action]
According to the present invention, the excess sludge generated in the primary treatment step is mixed with the primary treatment water that has passed through the primary treatment step, and a coagulant is added, so that the phosphorus remaining in the primary treatment water is subjected to the coagulation separation treatment. The mixed excess sludge becomes a nucleus for the formation of flocculated flocs as a suspended substance, and in the case of sedimentation and separation, the sedimentation of fine flocs is promoted by coprecipitation. For this reason, efficient coagulation separation processing is performed only by adding a small amount of coagulant, and phosphorus is removed from the primary treatment water. The separated excess sludge contains flocculent floc and unreacted flocculant. For this reason, even if phosphorus is re-released from the excess sludge put in the anaerobic state in the post-treatment step, these phosphorus are taken into the flocculent floc and the unreacted flocculant, and do not migrate to the desorbing liquid side. . Further, in the primary treatment step, since the metal component caused by the coagulant is not mixed, the activity of the biological treatment can be kept high.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a treatment system diagram showing a first embodiment of a wastewater treatment method and apparatus according to the present invention. The primary treatment step is performed by passing the water 20 to be treated through the anaerobic tank 22, the aerobic tank 24, and the sedimentation tank 26 in this order. The treated water 20 and the activated sludge 28 returned from the sedimentation basin 26 flow into the anaerobic tank 22, and are mixed. The anaerobic tank 22 is maintained in an anaerobic state, and the activated sludge flowing in releases the phosphorus. The aerobic tank 24 is maintained in an aerobic state by aeration from the blower 30. In the aerobic tank 24, the activated sludge grows using the organic matter in the water to be treated as a nutrient source, and takes in the phosphorus in the water to be treated and the phosphorus released in the anaerobic tank 22 into the sludge. As a result, organic matter and phosphorus in the water to be treated are simultaneously removed. The activated sludge is settled and separated in the sedimentation basin 26, and the supernatant water is sent to the secondary treatment step as the primary treatment water 32. The entire amount of the settled sludge is returned to the anaerobic tank 22 as returned sludge 28 and is repeatedly circulated.
[0014]
As a result of the activated sludge multiplying in the aerobic tank 24, the concentration of the activated sludge circulating in the primary treatment step gradually increases. Therefore, the sludge concentration meter 34 is installed in the aerobic tank 24 and detects the sludge concentration in the tank. The aerobic tank 24 is connected to a sludge extraction pipe 38 for extracting excess sludge, and the sludge extraction pipe 38 is provided with a sludge extraction pump 40. The detected value of the sludge concentration meter 34 is sent to the first controller 36. The first controller 36 calculates the flow rate of the mixed sludge water withdrawn from the sludge withdrawal pipe 38 and controls the flow rate of the sludge withdrawal pump 40 so that the detected value of the sludge concentration meter 34 becomes a predetermined preferable value. As a result, the sludge in the mixed sludge water extracted from the sludge extraction pipe 38 is discharged from the primary treatment step as surplus sludge, and the concentration of the activated activated sludge circulating in the primary treatment step is maintained at a predetermined preferable value.
[0015]
As described above, in the aerobic tank 24, phosphorus in the water to be treated 20 is removed by the action of taking in phosphorus by activated sludge. For this reason, the phosphorus concentration in the primary treated water 32, which is the supernatant water of the sedimentation basin 26, is usually lower than the discharge reference value in many cases. However, the phosphorus concentration in the primary treated water 32 may exceed the target value due to a temporary increase in the phosphorus load of the incoming treated water 20 or other various factors. In addition, in the sedimentation basin 26, the inflow sludge mixed water may stay for a considerably long time due to the sedimentation treatment, and the inside of the pond may be in an anaerobic state. Then, the phosphorus taken in the activated sludge is re-released, and the case where the phosphorus concentration of the primary treatment water 32 rises often occurs.
[0016]
In such a case, a secondary processing step is performed. This secondary treatment step is performed by subjecting at least a part of the primary treatment water 32 to a coagulation separation treatment. That is, a pipe 46 provided with an extraction pump 44 is branched from the discharge pipe 42 of the primary treatment water 32, and a part of the primary treatment water 32 is supplied to the mixing tank 48. The remaining portion of the primary treatment water 32 is discharged directly from the discharge pipe 42 to the outside of the system. The mixed water from the sludge extraction pipe 38 is also supplied to the mixing tank 48, and the primary treated water and the mixed sludge water are mixed. Further, the coagulant in the coagulant tank 50 is added to the mixing tank 48 through a pipe 54 provided with an injection pump 52. The coagulant to be used may be an aluminum coagulant such as polyaluminum chloride and aluminum sulfate, an iron coagulant such as ferric chloride, an organic polymer coagulant, or a combination thereof. As a result, the flocculation floc is formed in the mixing tank 48 by the flocculating action of the flocculant. Phosphorus remaining in the primary treatment water at the time of formation of the aggregated flocs is taken in. The excess sludge in the sludge mixed water mixed with the primary treatment water becomes a nucleus for forming flocculated flocs as a suspended substance and promotes the flocculating action. The primary treated water subjected to the coagulation treatment is sent to the next settling tank 56. In the sedimentation tank 56, the flocculated floc is settled and separated together with the excess sludge. The sludge suspended during the sedimentation and separation promotes the sedimentation of fine flocs by co-precipitation. The supernatant water from which phosphorus has been removed is discharged from the pipe 58 as secondary treated water. The secondary treated water from the pipe 58 merges with the remaining portion of the primary treated water 32 flowing through the discharge pipe 42, and then is discharged from the pipe 60 to the outside of the system.
[0017]
A phosphorus concentration meter 62 for detecting the phosphorus concentration in the treated water discharged from the pipe 60 is provided, and the detected value of the phosphorus concentration meter 62 is transmitted to the second controller 64. The second controller 64 calculates the flow rate of the primary treatment water requiring the secondary treatment step so that the detection value of the phosphorus concentration meter 62 is equal to or less than the target value. Based on the calculation result, the second controller 64 controls the extraction pump 44 and also controls the injection pump 54 so as to add the coagulant in an amount proportional to the flow rate to the mixing tank 48.
[0018]
The sludge containing flocculated floc separated and settled in the settling tank 56 is extracted from the pipe 66 and undergoes a necessary post-treatment step. This excess sludge contains flocculent floc and unreacted flocculant. For this reason, even if phosphorus is re-released from the excess sludge put in the anaerobic state in the post-treatment step, these phosphorus are taken into the flocculent floc and the unreacted flocculant, and do not migrate to the desorbing liquid side. . In addition, it is preferable that a part of the extracted sludge is circulated to the mixing tank 48 via a pipe 68 as necessary. That is, the extracted sludge may contain a large amount of unreacted coagulant having a cohesive force. In such a case, if the sludge is circulated through the mixing tank 48, the coagulant can be used without waste. Further, by increasing the sludge concentration in the mixing tank 48 or the sedimentation tank 56, it is possible to enhance the coagulation promoting action and the coprecipitation action of the sludge. Although the amount of circulation of this sludge is empirically obtained, the total amount of sludge supplied to the mixing tank 48 is preferably kept constant. Therefore, when the amount of mixed sludge supplied from the sludge extraction pipe 38 to the mixing tank 48 is small, it is preferable to control the circulation pump 70 with the first controller 36 so as to increase the amount of sludge circulated.
[0019]
FIG. 2 is a material balance diagram of phosphorus in the secondary processing step according to the present embodiment. In the figure, the total flow rate Q ₁ of the primary treated water and the phosphorus concentration are C ₁ . The flow rate of the sludge mixed water supplied to the mixing tank 48 is withdrawn from the sludge extraction pipe 38 and Q _2. Let R be the ratio of the primary treated water, which is supplied from the conduit 46 to the mixing tank 48 and undergoes the secondary treatment step. The phosphorus concentration of secondary treatment water discharged from the precipitation tank 56 via a secondary treatment step and C _2. Secondary treatment primary treatment water not subject to process and secondary treatment water is a target concentration of phosphorus after the confluence to C _t. Then, Equation (1) for determining the ratio R is determined from the phosphorus mass balance at the junction X as shown in FIG. Using this equation (1), for example, total flow rate _{Q 1} 100 of the primary treated water, the phosphorus concentration _{C 1} 2.0 mg / L, the flow rate _{Q 2} of the sludge mixed water 10 weight phosphorus secondary treated water When determining the ratio R of the concentration _{C 2} 0.1mg / L, the target phosphorus concentration _{C t} after the confluence as a 1.0 mg / L, the ratio R is about 0.48. That is, it is possible to clear the target phosphorus concentration C _t of treated water after the confluence only handling about half of the total amount of primary treated water in the secondary treatment process. For this reason, compared with the case where the whole amount of the primary treatment water is treated in the secondary treatment step, the equipment cost and the consumption amount of the coagulant can be significantly reduced.
[0020]
As described above, according to the wastewater treatment method and apparatus of the present embodiment, a part of the primary treatment water that has passed through the primary treatment step is mixed with the sludge mixed water equivalent to the excess sludge generated in the primary treatment step, and is coagulated. An agent was added so as to coagulate and separate phosphorus remaining in the primary treatment water. For this reason, efficient coagulation separation processing is performed only by adding a small amount of coagulant, and phosphorus is removed from the primary treatment water. The separated excess sludge contains flocculent floc and unreacted flocculant. For this reason, even if phosphorus is re-released from the excess sludge put in the anaerobic state in the post-treatment step, these phosphorus are taken into the flocculent floc and the unreacted flocculant, and do not migrate to the desorbing liquid side. . Further, in the primary treatment step, since the metal component caused by the coagulant is not mixed, the activity of the biological treatment can be kept high.
[0021]
In the present embodiment, the first controller 36 and the second controller 64 have been described to automatically control the flow rate of the sludge mixed water, the flow rate of the primary treatment water requiring the secondary treatment step, and the like. However, the invention is not limited to such a method. For example, the inspector manually detects the sludge concentration in the aerobic tank 24 and the phosphorus concentration of the treated water discharged from the pipeline 60 once a day. Based on the detection result, the flow rate of the sludge mixed water, the flow rate of the primary treatment water requiring the secondary treatment step, and the like may be manually adjusted.
[0022]
FIG. 3 is a treatment system diagram showing a second embodiment of the wastewater treatment method and apparatus according to the present invention. In FIG. 3, elements denoted by the same reference numerals as those in FIG. 1 have the same functions as the elements described in the first embodiment, and a description thereof will be omitted. The second embodiment has a configuration in which the first treatment step is commonly referred to by a person skilled in the art as a modified activated sludge circulation method (or A2O method). That is, the anoxic tank 23 is provided between the anaerobic tank 22 and the aerobic tank 24, and the mixed sludge water (nitrified liquid) in the aerobic tank 24 is circulated from the pipe 25 to the anoxic tank 23. ing. In this configuration, nitrifying bacteria and denitrifying bacteria proliferate in the activated sludge using nitrogen components contained in the water to be treated as nutrients, and nitrogen components are efficiently removed from the water to be treated in addition to organic substances and phosphorus.
[0023]
In the second embodiment, the excess sludge settled and separated in the sedimentation basin 26 is supplied to the mixing tank 48 from the pipeline 27 instead of the sludge mixed water. Further, a phosphorus concentration meter 62 is disposed in the discharge pipe 42 of the primary treatment water 32, and a detection value of the phosphorus concentration meter 62 is transmitted to the third controller 63. The third controller 63 calculates the flow rate of the primary treatment water requiring the secondary treatment step based on the detection result of the phosphorus concentration meter 62, controls the extraction pump 44, and mixes the coagulant in an amount proportional to the flow rate. The injection pump 52 is controlled to be added to the tank 48. Further, a membrane separation tank 57 is provided at the next stage of the mixing tank 48 as solid-liquid separation means instead of the precipitation tank 56 of the first embodiment. The sludge containing flocculated flocs separated by the membrane separator 57 is withdrawn from the pipe 67, a part of the sludge is circulated to the mixing tank 48 via the pipe 69, and the remaining part is subjected to a post-treatment step required as excess sludge. receive. The secondary treatment water that has passed through the membrane 57A of the membrane separation tank 57 is discharged from the pipe 58 as in the first embodiment.
[0024]
In the second embodiment, the same operation and effect as those in the first embodiment can be obtained. The present invention is not limited to the above embodiments. For example, the first processing step may include a single aerobic tank in which the anaerobic tank and the anoxic tank are omitted. In addition, the coagulation / separation processing means according to the second processing step is not limited to the combination of the mixing tank and the solid-liquid separation means of the next stage, and the functions of mixing and solid-liquid separation are achieved in parallel in the same tank. Means may be used.
[0025]
【The invention's effect】
According to the present invention, the excess sludge generated in the primary treatment step is mixed with the primary treatment water that has passed through the primary treatment step, and a coagulant is added, so that the phosphorus remaining in the primary treatment water is subjected to the coagulation separation treatment. For this reason, the mixed excess sludge becomes a nucleus in the formation of flocculated flocs as a suspended substance, and an efficient flocculation and separation treatment is performed simply by adding a small amount of flocculant, thereby removing phosphorus from the primary treated water. The separated excess sludge contains flocculent floc and unreacted flocculant. For this reason, even if phosphorus is re-released from excess sludge put in an anaerobic state in the post-treatment step, these phosphorus are taken into the flocculent floc and the unreacted flocculant, and do not migrate to the desorbed liquid side. . Further, in the primary treatment step, the metal component caused by the coagulant is not mixed, so that the activity of the biological treatment can be kept high.
[Brief description of the drawings]
FIG. 1 is a treatment system diagram showing a first embodiment of a wastewater treatment method and apparatus according to the present invention.
FIG. 2 is a mass balance diagram of phosphorus in a secondary processing step according to the first embodiment.
FIG. 3 is a treatment system diagram showing a second embodiment of the wastewater treatment method and apparatus according to the present invention.
FIG. 4 is a processing system diagram of an anaerobic-aerobic method according to the related art.
[Explanation of symbols]
20 ... water to be treated, 22 ... anaerobic tank, 24 ... aerobic tank, 26 ... sedimentation basin, 32 ... primary treatment water, 34 ... sludge concentration meter, 36 ... 1st controller, 38 ... sludge extraction pipe, 40 ... sludge extraction pump, 42 ... discharge pipe, 44 ... extraction pump, 48 ... mixing tank, 50 ... flocculant tank, 52 ..., Injection pump, 56, sedimentation tank, 62, phosphorus concentration meter, 64, second controller.

Claims (4)

A primary treatment step of introducing an organic wastewater containing phosphorus into an aerobic tank holding activated sludge for biological treatment, and mixing excess sludge generated in the primary treatment step with the primary treatment water passing through the primary treatment step. And a secondary treatment step of adding a coagulant and coagulating and separating phosphorus remaining in the primary treatment water. 2. The wastewater treatment method according to claim 1, wherein only a part of the primary treatment water is treated in the secondary treatment step according to the phosphorus concentration in the primary treatment water. 3. In the primary treatment step, a certain amount of the sludge mixed water in the aerobic tank is drawn out so that the sludge concentration in the aerobic tank becomes a predetermined value, and the sludge in the sludge mixed water is mixed with the excess sludge mixed with the primary treatment water. The wastewater treatment method according to claim 1 or 2, wherein An aerobic tank that aerates the organic wastewater containing phosphorus in the presence of activated sludge, a sedimentation tank that separates the effluent of this aerobic tank into primary treated water and sludge, and a sludge separated by this sedimentation tank Sludge return means for returning to the aerobic tank, excess sludge withdrawal means for extracting sludge mixed water from the aerobic tank, and at least a part of the primary treatment water discharged from the sedimentation basin and withdrawal by the excess sludge withdrawal means A wastewater treatment apparatus, comprising: a mixing tank for mixing the mixed sludge mixed water and adding a flocculant; and a solid-liquid separating means for separating aggregates generated in the mixing tank.

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