JPH0416239B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Application of the Invention) The present invention relates to a sewage treatment method for removing phosphorus by anaerobic-aerobic activated sludge treatment from organic wastewater containing phosphorus such as sewage and industrial wastewater.
Specifically, the present invention relates to a method for treating surplus sludge generated by the sewage treatment. (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. (primary treatment step), then sent to an aeration tank to decompose BOD, and this aeration tank outlet mixed liquid is sent to a final settling tank (hereinafter abbreviated as final settling) to settle and separate the sludge (as described above). The aeration tank and final settling tank are called the secondary treatment process), and the supernatant liquid after separation is sent to the drainage system as treated water, while a part of the sedimented and separated sludge is sent to the BOD decomposition in the secondary treatment process. This is a well-known method in which the sludge is sent back to the inlet of the aeration tank as necessary sludge, and the remaining sludge is pulled out from the final sedimentation stage as surplus sludge and sent to the dewatering and solidification process. In particular, in this method, when the primary treated water and retransferred sludge flowing in from the initial settling side are passed through the anaerobic tank and the aerobic tank in the order of the secondary treatment process, the BOD in the wastewater is A method that can perform denitrification at the same time as removal (in addition, if a denitrification tank is installed between the anaerobic tank and the aerobic tank and the recycled water from the aerobic tank is supplied, denitrification is performed in addition to dephosphorization). However, in the following explanation, the biological treatment method that has dephosphorization ability, including both of these, will be used as an A/O method.
It 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. There is. By the way, phosphorus removal in the A/O method refers to accumulating phosphorus in the liquid into sludge, and if the 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, in the sewage treatment using the A/O method, it is necessary to devise measures to prevent phosphorus regression from occurring in the treatment system for excess sludge, and several proposals have been made in the past. An example of this is shown in Japanese Patent Application Laid-Open No. 150487/1987, in which excess sludge is aerated for 2 to 18 hours and then sent to a dewatering step. However, this method requires a large amount of air for a long time, for example when maintaining a large amount of surplus sludge generated from a sewage treatment plant in an aerobic state, so it is difficult to use in terms of equipment and running costs. There are many difficulties, especially in terms of applicability to large-scale processing equipment, which poses many practical problems. As another example, according to Japanese Patent Application Laid-open No. 56-150500, in the desorbed water generated in the treatment system for excess sludge,
A method has been proposed in which phosphorus is separated and recovered as an insoluble salt by adding Ca ²⁺ , Al ³⁺ , Fe ^{3+ ,} etc. However, this method has the disadvantage that additional equipment is required to remove phosphorus from the desorbed water. (Object of the invention) In view of the above-mentioned current situation, the present invention provides an A/O
This provides a suitable treatment method for phosphorus-containing surplus sludge generated in a wastewater treatment system that has a secondary treatment process, and one of its purposes is to obtain surplus sludge through rapid mechanical thickening treatment. This prevents the occurrence of anaerobic conditions during treatment, and enables dewatering treatment while phosphorus is accumulated in excess sludge without requiring aeration treatment such as aeration. Another object of the present invention is to minimize the release of phosphorus into the liquid during the process of concentrating and dewatering surplus sludge, and return it to the sewage treatment system via the desorbed water generated in the process. The aim is to significantly reduce the amount of phosphorus. Still another object of the present invention is to reduce the amount of chemicals added for conditioning such as flocculation, which are generally used in the dewatering process, by increasing the concentration of excess sludge, and to reduce the capacity of dewatering equipment. It is about realization. Furthermore, another object of the present invention is to achieve the above-mentioned various objects and to make it possible to perform the two processes of mechanical concentration and dehydration without temporal succession, thereby improving the efficiency of the dehydration treatment equipment. The objective is to be able to stop operation at night or on holidays. (Summary of the Invention) In order to achieve the above-mentioned object, the present invention provides A/
In a sewage treatment system that has a secondary treatment process according to the O method, the conventional way of treating excess sludge is to separate it by sedimentation through final settling, and remove the excess sludge (generally sludge concentration is about 1.0 to 2.0% by weight) from below. By completely changing the concept of drawing, a part of the aerobic tank outlet mixture in an aerobic state is directly sent to a centrifugal or cross-type mechanical concentrator, which is newly provided in the present invention, for rapid concentration. However, the first aspect of the present invention is characterized in that this concentrated surplus sludge is sent to the dewatering process. In the sewage treatment system, the activated sludge, that is, the retransferred sludge, which needs to flow into the aeration tank inlet together with the sewage, may be obtained by final settling as in the conventional method. Therefore, in other words, the amount of retransferred sludge necessary and sufficient to biodegrade the substrate in the water to be treated that flows into the secondary treatment process from the initial sedimentation will settle the solid content of the mixed liquid at the outlet of the aerobic tank. In order to avoid the drawbacks of the method of securing excess sludge through final settling, on the other hand, the aerobic tank outlet mixed solution containing the remaining sludge should be kept under aerobic conditions as much as possible. It can be said that the present invention is characterized in that it is rapidly concentrated to a high concentration using a mechanical concentrator while maintaining the concentration, and then sent to the dehydration process. The degree of rapid concentration by the mechanical concentrator in the present invention is generally desirably as high as possible; The sludge concentration is is preferably 3 to 8
The amount is 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 is done by controlling the amount of liquid. In this case, the pumping amount control of the pump is related to the fact that the amount of sludge obtained by sedimentation and separation in the final sedimentation changes depending on the discharge amount of the mixed liquid at the outlet of the aerobic tank, the sludge concentration in the treated water, etc. Since it is necessary to always secure a portion of the redirected sludge in the final settling, for example, each of the above-mentioned feedings must be adjusted so that the interface level of the sludge settled and separated in the final settling is within a certain range. Amount controlled. in particular,
For example, a final settling sludge interface level detector may be installed, and the detector may be linked to the drive mechanism of the pump. It goes without saying that the present invention is not limited to the type and type of the aerobic tank outlet mixed liquid feeding means and distribution control means. In the present invention, the mechanical thickener is specified as a centrifugal type or a centrifugal type because it shortens the residence time of excess sludge in the device to prevent the occurrence of anaerobic conditions and achieve high concentration concentration. Furthermore, this is to ensure that the effects of the present invention are fully exhibited, taking into consideration the 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 dehydrators include belt press dehydrators, screw press dehydrators, centrifugal dehydrators, vacuum dehydrators, etc. that can perform rapid dehydration (in about 1 hour or less). Next, to describe the features of the surplus sludge treatment of the second invention of the present application, in the surplus sludge treatment using the mechanical thickener, in order to prevent the release of phosphorus from the sludge and complete the dewatering treatment, It is necessary to operate the concentration and dewatering treatment equipment day and night in the same way as the sewage treatment system. This is because if secondary treated water or thickened sludge is stored somewhere for a long period of time, it will become anaerobic and it will be difficult to prevent phosphorus release. However, as mentioned above, operating the equipment at night or on holidays is difficult from a human standpoint, such as securing personnel. Therefore, the second invention of the present application is an advanced aspect of the first invention that is suitably applied to a sewage treatment system having a secondary treatment step of the A/O method. The rapidly concentrated surplus sludge is stored in a storage tank for a predetermined period of time, and then sent to the dewatering process, and furthermore, in the storage tank,
Slaked lime [Ca (OH ₂ )], quicklime [CaO] in excess sludge
It is designed to add Ca salt such as Here, surplus sludge before mechanical thickening is not stored because the sludge concentration is generally 2000~2000~
This is because it is dilute at 2500 mg/ml, so storing it would have the disadvantage of requiring a huge amount, but storing it after concentration would be extremely advantageous in terms of the amount of storage. The reason why Ca salt is added to the sludge is that the Ca salt has the effect of capturing and insolubilizing phosphorus released from the sludge.
It is advantageous in terms of mixing to add Ca salt when or before excess sludge flows into the storage tank, and the amount of addition depends on factors such as storage time, sludge concentration of excess sludge to be stored, and temperature. Although it is determined depending on the relationship with the solid content of surplus sludge, it is generally preferable that the solid content ratio of excess sludge is 5 to 10% by weight. Note that maintaining the surplus sludge in an aerobic state by aeration in the storage tank is not practical because the surplus sludge has a relatively high viscosity. (Embodiments of the invention) Example 1 Figure 1 shows the flow of an embodiment of the present invention, in which raw sewage is fed into the equipment 2 of the secondary treatment process in accordance with the A/O method. After the first sinking 1,
The aerobic tank outlet has a tank for anaerobic-aerobic treatment consisting of an anaerobic tank 3, an aerobic tank 4 downstream of the anaerobic tank 3, and a final sedimentation tank 6, in which phosphorus is accumulated in the sludge due to the treatment. The mixed liquid is sent from the outlet of the aerobic tank 4 through an outflow conduit 5 and mainly to a final sedimentation tank 6. Further, a part of the mixed liquid in the outflow conduit 5 is pumped up by a pump (not shown) and sent to the mechanical concentrator 7. The mechanical thickener 7 is configured as a centrifugal type (or pressurized type) thickener, and preferably reduces the sludge concentration of the mixed liquid to 3 within a residence time of about 1 hour.
Concentrate to ~8% by weight, optimally 4-6% by weight. Since this concentration process is carried out quickly as described above, the concentration is completed before the dissolved oxygen (DO) of the mixed liquid begins to decrease. It only contains phosphorus at the same level as the subsequent treated water. Note that this desorbed water S ₁ ^* is returned to the inlet of the initial settling 1. The mixed liquid sent to the final settling 6 stays in the final settling for a predetermined 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 anaerobic tank as re-sludge. delivered to the entrance. The amount of secondary treated water flowing into the final settling 6 that performs such treatment is determined by using a sludge interface level detection means (not shown) to the mechanical thickener so that the level is within a certain range. This is done by controlling the amount of pumping. Further, the thickened sludge obtained from the mechanical thickener 7 is sent to a dewatering step by a pump 9 via a cushion tank 8 in this example. The dewatering process of this example is configured in the same manner as before, with a conditioning tank 1 in which a polymer flocculant is added to condition the thickened sludge.
0 and a belt press type dehydrator 11 at the subsequent stage, and the cake after dewatering is processed according to a conventional method. In addition, the desorbed water S ₂ ^* from the dehydrator 11 is
returned to the entrance. The treatment system shown in the flow above was tested in comparison with a conventional case in which surplus sludge is extracted from final settling. The results are shown in Table 1 below. The test conditions are as follows. Anaerobic tank retention time 1.5hr Aerobic tank retention time 3.5hr MLSS 2000~2500mg/Sludge return amount 20% of raw water inflow Type of raw water Public sewage Raw water quality (initial settling and overflow water) PH 6.7~7.2 BOD 100-130mg/ SS 80-100mg/ T-P 4.5-5.5mg/

[Table] From the above results, the thickened sludge before dewatering obtained in Example 1 of the present invention has a higher sludge concentration than the sludge drawn from final settling in the conventional method, and also has a higher sludge concentration than that produced during mechanical thickening. It was confirmed that the total phosphorus concentration in the desorbed water was kept at a low value of 0.4 to 0.8 mg/, and as a result, the absolute amount of phosphorus returned to the wastewater treatment system was kept at an extremely low value. Example 2 In this example shown in FIG. 2, a cushion tank located next to the mechanical thickener 7 is provided as a storage tank 8', and Ca salt is added to the stored thickened sludge and stirred. However, the other basic configuration is the same as that shown in FIG. 1, so the same components are denoted by the same reference numerals and their explanation will be omitted. According to such a configuration, the advantages of the sewage treatment system shown in FIG. 1 can be realized in a more practically developed device.
In other words, the addition of Ca salt makes it possible to store the thickened sludge for a predetermined period of time while preventing the disadvantage of releasing phosphorus into the liquid, making it possible to stop the dewatering process at night or on holidays. It is. Example 1 except that Ca salt was added and stored in the storage tank 8' using the flow treatment system shown in Figure 2.
The test was conducted according to the following conditions. The results are shown in the table below, and it was confirmed that the present invention exhibited superior effects compared to conventional examples.

[Table] (Effects of the invention) As explained above, according to the method of the present invention, A/
In the sludge treatment system according to the O method, it is possible to concentrate and dewater the excess sludge without releasing phosphorus, which greatly reduces phosphorus return to the sewage treatment system, making it possible to improve the secondary treatment process of the A/O method. It is possible to fully demonstrate the function of sludge, and there is no need for aeration to prevent the release of phosphorus from sludge, which has advantages in terms of equipment and lining costs.Furthermore, by adopting mechanical concentration, Since the sludge concentration in the dewatering process is increased, the effect of reducing the flocculant addition rate and the cake water content can be obtained. In addition, an additional effect of the method of the present invention is that the dewatered surplus sludge contains high phosphorus,
Composting, which is a type of sludge reuse, also has the advantage of providing a highly effective fertilizer. Furthermore, according to the second invention method of the present application, the effects and advantages of the various inventions described above are satisfied, in accordance with the current situation where dehydration process equipment is generally operated for about 6 to 8 hours during the day. This means that you will be able to do so. (Modified Examples and Application Examples of the Invention) It is obvious that the present invention is not limited to the embodiments described above, and that various modified embodiments can be considered. As shown by the broken line frame, iron salts such as ferric salts and
The thickened sludge may be refined by adding Ca salt or hydrogen peroxide. Further, the concentrated sludge may be further dehydrated, and in the case of adding Ca salt, it may be dehydrated using a filter press 11' which takes a relatively long time to dehydrate. In addition, in the case of Example 2 shown in FIG. 2, the addition of Ca salt in the storage tank 8' prevents the problems associated with the release of phosphorus from the sludge into the liquid. After concentrating the sludge in the concentrating tank 12, the sludge may be fed to the storage tank 8' and mixed, thereby being treated at the same time. This is because the problems associated with phosphorus release can be effectively prevented even by mixing the initially settled and thickened sludge.

[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 sedimentation, 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...Pump, 10, 1
0'... Tempering tank, 11... Belt press machine, 1
1'... Filter press machine, 12... Concentrator.

Claims (1)

[Scope of Claims] 1. The method includes a step of passing treated water, which is a mixture of sewage and retransfer sludge, through an anaerobic tank and an aerobic tank in order to accumulate and retain phosphorus in the sludge, and the mixed liquid at the outlet of the aerobic tank A biological sewage treatment method in which a part of the sludge is sedimented and separated in a sedimentation tank installed in the next stage to become the retransfer sludge, and the remaining mixed liquid is used as surplus sludge to be sent to the sludge treatment process. The sludge treatment process includes a process of directly feeding the mixed liquid at the outlet of the aerobic tank to a centrifugal or permeable mechanical thickener to quickly concentrate it, and a process of dewatering excess sludge after mechanical thickening. A sewage treatment method characterized by: 2. Includes a step in which sewage and retransferred sludge are passed through an anaerobic tank and then an aerobic tank to accumulate and retain phosphorus in the sludge, and a part of the mixed liquid at the outlet of the aerobic tank is transferred to a settling tank installed at the next stage. The biological sewage treatment method is a biological sewage treatment method in which the mixed liquid at the outlet of the aerobic tank is converted into the retransferred sludge by sedimentation and separation, and the remaining mixed liquid is made into sludge to be sent to the sludge treatment process. A process in which the thickened sludge is directly fed to a centrifugal or over-type mechanical thickener and rapidly concentrated, a process in which 5 to 10% by weight of Ca salt (solid content) is added to the thickened sludge and stored, and a subsequent stage. A sewage treatment method characterized by having a dehydration process.