JPH0420676B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Application of the Invention) The present invention targets organic wastewater containing phosphorus (hereinafter referred to as wastewater) such as sewage and industrial wastewater.
The present invention relates to a method for treating excess sludge generated in a sewage treatment method for removing phosphorus from sewage using anaerobic-aerobic activated sludge. (Background of the invention) In general, biological wastewater treatment methods using activated sludge pass wastewater (raw water) through an initial settling tank (hereinafter referred to as initial settling tank) to remove solids and a portion of BOD. The sludge is then sent to the secondary treatment process to decompose the BOD, and the mixed liquid at the outlet of the aeration tank is sent to the final settling tank (hereinafter referred to as final settling) to settle and separate the sludge. The clear liquid is sent to the drainage system as treated water, while a portion of the sedimented and separated sludge is returned to the aeration tank inlet as return sludge necessary for BOD decomposition in the aeration tank, and the remaining sludge is used as surplus sludge. This is a well-known method in which sludge is extracted from the final sedimentation stage and sent to a dehydration and solidification treatment process. In particular, in this method, during the secondary treatment process, the primary treatment water and return sludge flowing in from the initial settling are
If the water is passed in the order of anaerobic tank and aerobic tank, it is possible to remove BOD from the wastewater and also dephosphorize it at the same time (in addition, a denitrification tank is provided between the anaerobic tank and the aerobic tank, When circulating water from the aerobic tank is supplied, it is a method that can perform denitrification in addition to dephosphorization, but in the following explanation, A biological treatment method that has dephosphorization ability, including both of these, will be used. /O method), which has recently attracted attention as a treatment method that can remove phosphorus, which is one of the sources of eutrophication in treated water discharge areas, without using chemicals at the secondary treatment stage. It's coming. By the way, phosphorus removal in the A/O method refers to accumulating phosphorus in the liquid into sludge, and if this mixed liquid is then placed under anaerobic conditions, the sludge will release phosphorus back into the liquid. It has a nature. Therefore, when the surplus sludge extracted from final settling is treated under anaerobic conditions, most of the phosphorus in the surplus sludge is released into the liquid and is absorbed through the desorbed water generated during the treatment of the surplus sludge. The phosphorus then returns to the sewage treatment system, resulting in a vicious cycle in which the phosphorus returns. For these reasons, it is desirable to prevent and reduce regressive phosphorus in wastewater treatment systems using the A/O method, and proposals such as those in JP-A-56-150487 and JP-A-56-150500 have been proposed. However, all of them have drawbacks in terms of practicality, especially operating costs and sludge treatment and disposal. Therefore, the inventors of the present invention have investigated how to treat a mixed liquid with generally high dissolved oxygen, which is generally discharged from an aerobic tank in a secondary treatment process according to the A/O method, to the extent that the sludge contained therein should be treated as surplus sludge. The novel method is to directly feed the liquid to a centrifugal or permeable mechanical concentrator, where it is quickly concentrated before the occurrence of anaerobic conditions due to a decrease in dissolved oxygen over time, and then sent to a dehydration process. A method for treating surplus sludge was proposed. According to this method, the phosphorus concentration released from sludge into the liquid can be reduced to 0.4 to 0.4 with rapid concentration within about one hour.
The phosphorus concentration can be suppressed to 0.8mg/, which is lower than the normal phosphorus concentration when using the conventional method of drawing out excess sludge from final settling.
It is extremely effective in reducing regressive phosphorus, and the degree of concentration of sludge sent to the dewatering process is several times that of conventional methods. As a result, the amount of flocculant added as a pretreatment for dehydration was significantly reduced, and various effects were obtained. By the way, in addition to the above-mentioned improvements, various improvements have been made in recent years in the treatment method of the dewatering process.In particular, in the treatment of sludge by adding a flocculant in the front stage of the dehydrator, polymer flocculants have been used. There has been a remarkable development, and the opportunities for this to be used are increasing, and along with this, the types of dehydrators used have also changed from vacuum dehydrators to belt press dehydrators, centrifugal dehydrators, etc. There is a current situation. However, such treatment using a polymer flocculant is different from the surplus sludge treatment method based on the rapid concentration method of the aerobic tank outlet mixture using a mechanical thickener, developed by the present inventors. Since the organic relationship between the two has not been improved and developed, when the above-mentioned steps are combined, i.e., rapid concentration using a mechanical concentrator and dehydration using a polymer flocculant, It is recognized that there are some problems that need to be further improved in order to bring out the excellent effects of these two treatments. For example, the polymer flocculant does not show any particular effect on fixing (insolubilizing) phosphorus when it is released from sludge into the liquid, so it is sufficient to sufficiently reduce phosphorus return via desorbed water. In order to achieve this, it is desirable to perform dehydration treatment quickly or to devise a separate method for phosphorus fixation. On the other hand, the thickened sludge obtained by rapid concentration using the mechanical thickener cannot be dehydrated as it is unless a polymer flocculant or the like is added to the sludge for tempering treatment. (Object of the Invention) In view of these various points, the present invention has been developed by obtaining thick sludge by rapidly concentrating the mixed liquid at the outlet of an aerobic tank from surplus sludge in the A/O method, and then preparing this with a polymer flocculant. The present invention aims to provide a new sludge treatment method in which the effects of each of these series of steps can be effectively exhibited in a sludge treatment method in which sludge is hydrated and dehydrated. Another object of the present invention is to achieve the above-mentioned object and to perform two processes, mechanical concentration treatment and dehydration treatment.
The purpose is to make it possible to perform the dehydration treatment without continuous time, thereby making it possible to stop the operation of the dehydration treatment equipment at night or on holidays. (Summary of the Invention) The basic gist of the present invention for achieving the above-mentioned object is that the concentrated sludge A obtained by concentrating the mixed liquid at the outlet of the aerobic tank of the A/O method is subjected to initial sedimentation with a high fiber content. The first settled sludge B from the sludge B is combined with the first sludge B, thereby increasing the overspeed during dewatering treatment and reducing the moisture content of the cake. Specifically, the gist of the first invention of the present application is In a wastewater treatment method that has a secondary treatment step in which the water to be treated is passed through an anaerobic tank and an aerobic tank in order to accumulate phosphorus in the sludge after the primary treatment by sedimentation, the mixed liquid at the outlet of the aerobic tank is Thickened sludge (hereinafter referred to as sludge A), a portion of which was quickly concentrated directly using a centrifugal or permeable mechanical thickener;
After the initial settling sludge (hereinafter referred to as sludge B) from the initial settling tank is combined, it is immediately sent to the dewatering process, and is subjected to tempering treatment using a polymer flocculant as a pretreatment for dewatering after mechanical concentration. A sludge treatment method characterized by: In addition, the second invention of the present application provides the above-mentioned method in order to achieve the operating conditions of the equipment found in actual sewage treatment facilities, that is, in particular, to stop the dewatering treatment equipment at night and on holidays, while ensuring the effects of the method. Thickened sludge A after mechanical thickening in the first invention
The sludge is characterized by being able to be stored in a storage tank (however, the sludge is tempered after storage), provided that Ca salt, which is effective in fixing phosphorus eluted from the sludge, is added. In addition, the storage of sludge A with the addition of Ca salt is as follows:
It may be carried out at the time of merging with sludge B or before merging with sludge B. In the latter case in particular, a preferable embodiment of merging sludge A and sludge B immediately before dewatering treatment can be realized. There are advantages. In the present invention, it is generally desirable for the level of rapid concentration by the mechanical concentrator to be as high as possible, but the flow rate of the mixed liquid fed to the concentrator, the capacity and capacity of the concentrator, In order to prevent the negative effects of sludge concentration, the retention time should be kept as short as possible (normally, it is desirable to keep the retention time in the device within 1 hour), etc., and the sludge concentration is preferably 3. The content is preferably about 8% by weight, most preferably about 4% to 6% by weight. In the present invention, the feeding amount distribution control of the aerobic tank outlet mixed liquid in two directions is performed, for example, by pumping the aerobic tank outlet mixed liquid from the outflow conduit from the aerobic tank to the final sedimentation to the mechanical concentrator. This can be done by controlling the amount of liquid. In this case, the amount of pumping by the pump is controlled because the amount of sludge obtained by sedimentation and separation in the final sedimentation changes depending on the amount of secondary treated water discharged from the aerobic tank, the sludge concentration in the treated water, etc. Correspondingly, since it is necessary to always secure the amount of returned sludge in the final settling, for example, each of the above-mentioned feeding rates is controlled so that the interface level of the sludge separated by sedimentation in the final settling is within a predetermined range. It will be done. Specifically, for example, a sludge interface level detector for final settling may be installed, and the detector and the drive mechanism of the pump may be linked together. In the present invention, the mechanical concentrator is specified as a centrifugal type or a centrifugal type because it shortens the residence time of treated water in the device to prevent the appearance of anaerobic conditions.
This is to achieve high concentration concentration and to fully exhibit the effects of the present invention, taking into account equipment size, operability, etc. Examples of mechanical concentrators include horizontal centrifugal concentrators,
Typical examples include a vertical centrifugal concentrator and a cloth tube concentrator proposed in Japanese Unexamined Patent Publication No. 138596/1983. Typical examples of the dehydrator include those that can perform rapid treatment (dehydration treatment in about 1 hour or less), such as belt press dehydrators, screw press dehydrators, and centrifugal dehydrators. In the present invention, initial settling sludge B is combined with thickened sludge A rapidly thickened by a mechanical thickener.
is obtained by sedimentation and separation in a sedimentation tank,
It is preferable to use sludge that has been further sedimented and separated in a thickening tank. The degree of concentration of this sludge B varies depending on the capacity of the thickening tank, residence time, type of raw water, etc., but in general it is often set to a high concentration, preferably 3 to 8% by weight as a solid content. Optimally 4-6% by weight
It is desirable to keep it at a certain level. It is desirable to use the same kind of polymer flocculant used in the present invention even when sludge A and sludge B are subjected to separate tempering treatments.As the polymer flocculant, known cationic polymers or cationic Anionic polymers can be used in combination. Specifically, examples of cationic organic polymer flocculants include acid salts of N,N'-dimethylaminoalkyl acrylate or methacrylate with an alkyl group having 2 or 3 carbon atoms, and vinylbenzylmonoditrimethylammonium acid salts. Homopolymers and copolymers of cationic monomers such as , acid salts of cationically modified acrylamide, vinylpyridine and substituted derivatives thereof, acrylamine and substituted derivatives thereof, and the like can be used. In addition, copolymers of the above-mentioned cationic monomers and monomers such as acrylamide, acrylonitrile, and acrylic acid alkyl esters, as well as polyvinylimidazoline acid salts, chitosan acid salts,
Cationized starch can also be used. In addition, when using a cationic organic polymer flocculant and an anionic organic polymer flocculant together, in addition to the above cationic organic polymer flocculant, acrylic acid, methacrylic acid, and their alkali Metal salts, sulfomethylated acrylamide and its alkali metal salts, vinylbenzenesulfonic acid, styrenesulfonic acid and its alkali metal salts, vinylsulfonic acid and its alkali metal salts, maleic anhydride, and other homopolymers and copolymers etc. are used.
Further, copolymers of the above-mentioned anionic monomers with monomers such as acrylamide, acrylonitrile, and acrylic acid alkyl esters, as well as anion-modified products of sodium alginate and chitin, can also be used. In the first invention of the present application, when sludge A and sludge B are combined, the refining treatment by adding a polymer flocculant may be performed on each sludge before the combination, or it may be performed after the combination. However, since the anaerobization of sludge A is generally promoted by the merging, in order to reduce as much as possible the regressed phosphorus contained in the desorbed water during dewatering, it is necessary to Sludge A and B immediately before being led to the dewatering treatment equipment
It is desirable to merge the two. It is practically desirable to stir the combined sludges A and B to eliminate partial uneven distribution of the sludge. In the second invention of the present application, it is generally preferable to store the sludge A in a storage tank in a state that has been quickly concentrated using a mechanical thickener, but the sludge B is combined with the first settled sludge B in the storage tank. You may let them. When storing sludge A, sludge A is different from initially settled sludge B in that it has accumulated phosphorus, so as the storage time becomes longer and dissolved oxygen (DO) decreases, anaerobic conditions will appear. This causes the accumulated phosphorus to be released into the liquid. Therefore, by fixing such released phosphorus, Ca salt is added in order to minimize the return of phosphorus to the sewage treatment system via the water released during dehydration treatment. The amount of Ca salt added is selected depending on the relationship with the storage time of sludge A, sludge concentration, temperature, amount of DO in the secondary treatment water, etc., but it is generally determined by the solid content ratio of sludge A. It is preferably 5 to 10% by weight. Specific examples of the Ca salt include slaked lime [Ca(OH) ₂ ] and quicklime [CaO]. (Embodiments of the Invention) Example 1 Figure 1 shows a general flow of an embodiment of the present invention, in which primary treatment is performed in the initial settling 1 where raw water is introduced, and the primary treated water is sent to the next stage. It is fed to a secondary treatment facility 2 configured according to the A/O method. The secondary treatment equipment 2 first puts inflow water into an anaerobic tank 3, then transfers it to an aerobic tank 4 on the downstream side, and
In addition to biodegrading BOD through aerobic treatment,
Phosphorus is removed from the liquid by accumulating it in the sludge. After this, the mixed liquid flows out from the outlet of the aerobic tank 4 into the conduit 5.
The mixed liquid is mainly sent to the final settling 6, and a part of the mixed liquid is pumped up from the outflow conduit 5 by a pump (not shown) and sent to the mechanical concentrator 7. This mechanical concentrator 7 is configured as a centrifugal type (or pressurized type) concentrator, and the residence time is usually within about 1 hour, preferably about 30 minutes, and the secondary treated water is preferably
Concentrate to a sludge concentration of 4-6% by weight, optimally 4-6% by weight. In this concentration process, the process is extremely rapid, so the concentration progresses before the dissolved oxygen DO decreases, and the phosphorus contained in the liquid returned to the inlet of initial precipitation 1 as desorbed water is removed from the treated water. It is approximately the same level as the phosphorus concentration. The concentrated sludge A is sent via the cushion tank 8 to the conditioning tank 10 by the pump 9, and after being subjected to a predetermined conditioning by adding polymer, it is combined with the initial settling sludge B, which will be described later, and mixed. Immediately after being stirred in tank 11, it is led to belt press dehydrator 12. The desorbed water S ^* during dehydration is returned to the initial settling port. On the other hand, the combined initial settling sludge B is obtained by sending the sludge that has been settled and separated in the initial settling 1 to the thickening tank 13, where it is further settled and separated by gravity settling, and is obtained as sludge that is separated by the pump 14. It is fed to another tempering tank 15 and merged with sludge A after a predetermined tempering treatment. In the above, the polymers (polymer flocculants) used in the two tempering tanks 10 and 15 are preferably of the same type in order to equalize the degree of aggregation. In addition, the mixed liquid sent to the final settling 6 is retained in the final settling for a predetermined period of time to settle and separate the sludge,
The supernatant is sent to the drainage system as treated water, and the sludge is sent to the inlet of the anaerobic tank as return sludge. The amount of secondary treated water flowing into the final sedimentation unit 6 that performs such treatment is determined using, for example, a sludge interface level detection means (not shown) in the mechanical thickener so that the level is within a certain range. This is done by controlling the amount of pumping into the tank. The test results of sludge treatment carried out in the sewage treatment system shown in the above flow are shown in Table 1 below, in comparison with the results when sludge A is not combined with sludge B.

[Table] As is clear from the above table, compared to when sludge A is dehydrated alone, when sludge A and B are combined and treated, the time required for dewatering treatment, the moisture content of the cake, etc. Recognized as excellent. In addition, in Example 1 shown in FIG. 1, the sludge obtained from the mechanical thickener is immediately fed to the belt press machine without being stored on the way, thereby reducing the amount of phosphorus in the liquid due to the emergence of an anaerobic state. It is desirable to prevent the release of sludge into the sludge as much as possible, and therefore it is better to combine the initial settling sludge B as much as possible immediately before the dewatering process, but in order to simplify the refining operation by adding a polymer flocculant In this case, the combined sludge A and B may be stirred in one tempering tank, and in this case as well, sludge treatment can be carried out with the amount of regressed phosphorus kept within a range that does not cause any practical problems. Example 2 In the present Example 2 shown in FIG. 2, the thickened sludge A that was rapidly concentrated using the mechanical thickener 7 compared to that in FIG. 1 was stored in a storage tank 8', and a predetermined amount of Ca salt was added. A further feature is that the initial settling sludge B is merged into this storage tank 8'. Therefore, only one refining tank 10 is provided for the combined sludges A and B. According to such a configuration, it is possible to secure the advantages of sludge treatment shown in FIG. 1 and realize a more practically excellent sludge treatment facility. In other words, even if an anaerobic condition develops in the storage tank 8' due to sludge retention for a long time, and phosphorus is released from the sludge into the liquid, this phosphorus is fixed by the added Ca salt and dewatering takes place. This will effectively prevent large amounts of phosphorus from being contained in the desorbed water S ^* during treatment, and on the other hand, the operation of the belt press dewatering machine will be stopped at night and on holidays due to the accumulation of sludge A. This is because it becomes possible.
The table below shows the results when sludge A and B were stored for 36 hours with addition of Ca salt.

[Table] In addition, in the case of adding Ca salt, it is also possible to use a filter press dehydrator, which takes a relatively long time to dehydrate. (Effect of the invention) As explained above, according to the method of the present invention, A/
In sewage treatment systems that have a secondary treatment process in accordance with the O method, it is possible to concentrate and dehydrate excess sludge without releasing phosphorus, which greatly reduces phosphorus return to the sewage treatment system and improves A/O This makes it possible to fully utilize the functions of the secondary treatment process of the method, and also to fully demonstrate the effects of rapid concentration using a mechanical concentrator. Its usefulness is extremely great, as the problems that arise when it is used in combination with dewatering treatment can be effectively resolved by combining it with the initial settling sludge. Additionally, the method of the present invention has the additional advantage that, since the surplus sludge that has been dehydrated contains high phosphorus, it can provide a highly effective fertilizer in composting, which is one of the ways to reuse sludge. ing.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the flow of the first embodiment of the present invention, and FIG. 2 is a diagram showing the flow of the second embodiment. 1... Initial settling, 2 ... Secondary treatment process equipment, 3...
Anaerobic tank, 4...Aerobic tank, 5...Outlet drain, 6...Final sedimentation, 7...Mechanical thickener, 8...Cushion tank, 8'...Storage tank, 9, 14...Pump, 10,
15... Tempering tank, 11... Mixing tank, 12... Belt press dehydrator, 13... Concentrating tank.

Claims (1)

[Scope of Claims] 1. A sewage treatment method having a secondary treatment step in which the water to be treated is passed through an anaerobic tank and an aerobic tank in order to accumulate phosphorus in the sludge after the primary treatment in a sedimentation tank, Thickened sludge A, which is obtained by directly concentrating a part of the mixed liquid at the outlet of the aerobic tank using a centrifugal or cross-type mechanical thickener, and sludge B drawn from the first settling tank are combined and then quickly dehydrated. A method for treating sludge, characterized by feeding the sludge to a treatment process and subjecting it to tempering treatment using a polymer flocculant as a pretreatment for dehydration after mechanical concentration. 2. The sludge treatment method as set forth in claim 1, wherein the sludge B initially drawn from the sedimentation tank has passed through a thickening tank. 3 In a sewage treatment method that has a secondary treatment process in which the water to be treated is passed through an anaerobic tank and an aerobic tank in order to accumulate phosphorus in the sludge after the primary treatment in the initial settling tank, the aerobic tank outlet Thickened sludge A, which is a part of the mixed liquid, is quickly concentrated directly with a centrifugal or cross-type mechanical thickener, and sludge B drawn from the first settling tank,
This is a method in which the concentrated sludge A is fed to a dewatering process after being combined, and is subjected to tempering treatment using a polymer flocculant as a pre-dehydration treatment, wherein the thickened sludge A is freed from Ca at the time of or before joining with the sludge B. A sludge treatment method characterized by storing sludge with the addition of salt. 4. The sludge treatment method as set forth in claim 3, wherein the sludge initially drawn from the sedimentation tank is passed through a thickening tank.