Sewage treatment method using highly basic polyaluminum chloride solution

Abstract

To provide an improvement beyond an injection of normal-basicity PAC which is effective in promoting sludge floc formation and phosphorus removal.SOLUTION: Provided is a sewage treatment method using a high basicity polyaluminum chloride solution (high basicity PAC). The high basicity PAC has a basicity of 67 to 75%, an Al2O3 concentration of 9.0 to 11.0 mass%, and an SO4 concentration of 1.0 to 3.5 mass%. When a mean value and a standard deviation of an injection rate in terms of aluminum oxide in the case of injection of a predetermined normal-basicity polyaluminum chloride solution as inorganic coagulant are each 100 and a mean value of SS concentration in the supernatant in the final sedimentation tank is 100, an injection rate in terms of aluminum oxide of the high basicity PAC to allow the mean value of the SS concentration in the supernatant in the final sedimentation tank to be within a range of 100±15 is 75 or less in the mean value and 75 or less in the standard deviation.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a sewage treatment method using a high basicity polyaluminum chloride solution.

Background Art

[0002] A sewage treatment plant (referring to a sewage treatment plant actually operated by a sewage authority such as a municipality; the same applies hereinafter) is also called a final treatment plant. In the sewage treatment, the main processes generally include a process of treating in the order of an initial sedimentation tank, a biological reaction tank, and a final sedimentation tank.

[0003] In the initial sedimentation tank, a part of the suspended organic pollutants is sedimented and separated. Next, in the biological reaction tank, aeration is performed to remove the organic pollutants by activated sludge. The biological reaction tank is also called an aeration tank or an aeration tank. In addition to the standard activated sludge method in which the whole tank is aerated, an anaerobic-aerobic activated sludge method or the like is used for the treatment in the biological reaction tank. The anaerobic-aerobic activated sludge method is particularly used for phosphorus removal, and there are cases where the tank is divided into anaerobic and aerobic parts. In the present invention, including these, it is called a biological reaction tank. In the final sedimentation tank, the clarified water and the sludge flocs containing activated sludge are separated by gravitational sedimentation. A part of the sludge flocs is returned to the biological reaction tank as return sludge.

[0004] In the outlet region of the biological reaction tank or after leaving the outlet, in order to promote the formation of sludge flocs for the purpose of SS removal, and also for the purpose of generating poorly water-soluble aluminum phosphate salts for phosphorus removal, a normal basicity polyaluminum chloride solution (basicity: approximately 50%, Al 2 O 3 concentration: 10 to 11% by mass, SO 4 concentration: 2.0 to 3.0% by mass) (hereinafter referred to as "normal basicity PAC") may be injected.

[0005] Non-Patent Document 1 describes adding inorganic flocculants such as aluminum sulfate (alum) and polyaluminum chloride for the purpose of phosphorus removal in sewage treatment. Patent Document 1 describes adding an inorganic flocculant between the outlet of the biological reaction tank and the final sedimentation tank. Patent Documents 2 and 3 describe adding an inorganic flocculant to the outlet region of the biological reaction tank. Further, Patent Document 4 describes adding an inorganic flocculant near the outlet (outlet region) of the biological reaction tank in the treatment of organic wastewater such as electronic industry wastewater. Patent Documents 1 to 4 each exemplify polyaluminum chloride as the inorganic flocculant.

[0006] Patent Document 5 describes that in a method for producing purified water, when using PAC with a basicity of 70 to 75% as a flocculant, organic substances are likely to aggregate, organic substances can be sufficiently removed from the water to be treated, and the chromaticity is also likely to be reduced.

[0007] Patent Document 6 describes adding PAC with a basicity of 60 to 70% as an inorganic flocculant in the membrane filtration treatment of water to be treated containing organic substances. Among them, PAC with a basicity of 70% is described as being excellent in low residual Al, organic substance removal property, and flocculability.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Non-Patent Documents

[0009] [Non-Patent Document 1] Chemistry and Education, Vol. 45, No. 4 (1997), pp. 208-211, Koichi Fujie, "Current Status and Future Outlook of Sewage Treatment Technology" [Summary of the Invention] [Problems to be Solved by the Invention]

[0010] In Japanese Industrial Standards (JIS), the standard for polyaluminum chloride for water supply was established as K1475 in 1978, and since then the basicity standard in JIS has been [45 - 65%]. This basicity standard [45 - 65%] was also carried over when the Japan Water Works Association (JWWA) established the standard for polyaluminum chloride for water supply in 2000. The basicity standard in JWWA was revised to [45 - 75%] on August 9, 2016. From the history of the basicity standard for polyaluminum chloride for water supply above, and the fact that the most commonly used one up to now has been normal basicity PAC with a basicity of around 50%, it is reasonable to consider the "polyaluminum chloride" described in Non-Patent Document 1 and Patent Documents 1 to 4 as normal basicity PAC with a basicity of around 50%.

[0011] Patent Document 5 does not disclose at all the degree of ease of coagulation of organic substances when using PAC with a basicity of 70 - 75%, nor does it disclose anything about the treatment capacity when applying PAC with a basicity of 70 - 75% to sewage.

[0012] Similar to Patent Document 5, Patent Document 6 does not disclose at all the degree of ease of coagulation of organic substances when using PAC with a basicity of 60 to 70%, nor does it disclose anything about the treatment capacity when applying PAC with a basicity of 60 to 70% to sewage. By the way, in paragraph

[0027] of Patent Document 6, it is described that PAC with a basicity of 60 to 70% can form flocs with good peelability for the immersion membrane even with a smaller addition amount than PAC with a basicity of 40 to 50%. However, this description is about the formation of flocs with good peelability, which is an issue in membrane filtration, and not about coagulability.

[0013] The injection of normal basicity PAC is effective in promoting the formation of sludge flocs and phosphorus removal. However, the inventors of the present invention believe that there is room for further improvement beyond these, and have made this an issue.

Means for Solving the Problems

[0014] As a result of intensive studies on the above problems, the inventors of the present invention have found that by using a predetermined high basicity polyaluminum chloride solution as a coagulant, not only can the injection rate of the coagulant be significantly reduced, but also surprisingly, the variation in the injection rate can be significantly suppressed. Based on this finding, the present invention has been completed.

[0015] The present invention is as follows. [1] It includes a process of treating sewage in the order of the primary sedimentation tank, biological reaction tank, and final sedimentation tank. In a sewage treatment method of injecting an inorganic coagulant at an arbitrary location from the outlet region of the biological reaction tank to before reaching the final sedimentation tank, The inorganic coagulant is a high basicity polyaluminum chloride solution with a basicity of 67 to 75%, an Al 2 O 3 concentration of 9.0 to 11.0 mass% and an SO 4 concentration of 1.0 to 3.5 mass%, As the inorganic coagulant, a basicity of 51 ± 2%, Al 2 O 3 concentration of 10.3 ± 0.2 mass% and SO 4When injecting the ordinary basicity polyaluminum chloride solution "PAC250A" manufactured by Takaki Chemical Co., Ltd. with a concentration of 2.6 ± 0.3 mass%, the average value and standard deviation of the injection rate in terms of aluminum oxide conversion are set to 100 respectively, and when the average value of the SS concentration in the supernatant in the final sedimentation tank is set to 100, The injection rate of the above high basicity polyaluminum chloride solution in terms of aluminum oxide conversion for the average value of the SS concentration in the supernatant in the final sedimentation tank to be within the range of 100 ± 15 is 75 or less in the average value and 75 or less in the standard deviation. A sewage treatment method using a high basicity polyaluminum chloride solution.

Embodiments for Carrying Out the Invention

[0016] Hereinafter, the present invention will be described in detail based on preferred embodiments. However, the present invention is not limited to the following embodiments, and various modifications can be made within the scope shown in the claims. In the present invention, the notation "numerical value 1 to numerical value 2" regarding a numerical range means a numerical range including both numerical values 1 and 2 with numerical value 1 as the lower limit value and numerical value 2 as the upper limit value, and is synonymous with "numerical value 1 or more and numerical value 2 or less".

[0017] The present invention includes a process of treating sewage in the order of the primary sedimentation tank, biological reaction tank, and final sedimentation tank. In a sewage treatment method of injecting an inorganic flocculant at any location from the outlet area of the biological reaction tank until reaching the final sedimentation tank, the inorganic flocculant has a basicity of 67 to 75%, Al 2 O 3 a concentration of 9.0 to 11.0 mass% and SO 4 a concentration of 1.0 to 3.5 mass%, which is a high basicity polyaluminum chloride solution, and as the inorganic flocculant, a basicity of 51 ± 2%, Al 2 O 3 a concentration of 10.3 ± 0.2 mass% and SO 4When injecting the normal basicity polyaluminum chloride solution "PAC250A" manufactured by Taki Chemical Co., Ltd. with a concentration of 2.6 ± 0.3 mass%, the average value and standard deviation of the injection rate in terms of aluminum oxide conversion are set to 100 respectively, and when the average value of the SS concentration in the supernatant in the final sedimentation tank is set to 100, the injection rate of the high basicity polyaluminum chloride solution in terms of aluminum oxide conversion for the average value of the SS concentration in the supernatant in the final sedimentation tank to be within the range of 100 ± 15 is 75 or less for the average value and 75 or less for the standard deviation. The present invention relates to a sewage treatment method using a high basicity polyaluminum chloride solution (hereinafter referred to as "the sewage treatment method of the present invention"). Hereinafter, the high basicity polyaluminum chloride solution is referred to as "high basicity PAC", and the normal basicity polyaluminum chloride solution "PAC250A" manufactured by Taki Chemical Co., Ltd. is referred to as "normal basicity PAC-1". The basicity is determined by the procedure described in the "Method for Measuring Basicity" in the Japan Water Works Association Standard "Polyaluminum Chloride for Waterworks JWWA K 154:2016" of JWWA.

[0018] Among the sewage treatment methods of the present invention, the part of "a sewage treatment method including a process of treating sewage in the order of the primary sedimentation tank, biological reaction tank, and final sedimentation tank, and injecting an inorganic flocculant at any location from the outlet area of the biological reaction tank to before reaching the final sedimentation tank" is a known method as described in, for example, Patent Documents 1 to 3, and is also a method widely implemented in sewage treatment plants. As an example of rephrasing this part in another expression, there is "a sewage treatment method including a process of treating sewage in the order of the primary sedimentation tank, biological reaction tank, and final sedimentation tank, and the influent water to the final sedimentation tank is the one injected with an inorganic flocculant". Before the primary sedimentation tank, processes such as a screen and a grit chamber may be provided, and after the final sedimentation tank, processes such as sand filtration and disinfection may be provided. Also, from the viewpoint of the mixing property with sludge, the injection position of the inorganic flocculant is preferably the outlet area of the biological reaction tank.

[0019] In the sewage treatment method of the present invention, high basicity PAC is injected as the inorganic flocculant. The high basicity PAC has a basicity within the range of 67 to 75%, and Al 2 O 3In the range of 9.0 to 11.0 mass%, and SO 4 In the range of 1.0 to 3.5 mass%. The lower limit of the basicity is preferably 68%. The upper limit of the basicity is preferably 73%. Therefore, within the preferable range of basicity, 67 - 73%, 68 - 75%, 68 - 73% can be mentioned. Al 2 O 3 Concentration is preferably within the range of 10.0 to 11.0 mass% from the perspective of general PAC standards. SO 4 Concentration is preferably within the range of 1.5 to 2.5 mass%.

[0020] As ordinary basicity PAC - 1, basicity: 51 ± 2%, Al 2 O 3 Concentration: 10.3 ± 0.2 mass%, SO 4 Concentration: 2.6 ± 0.3 mass% is used. Each physical property value has a certain range, but such a range does not pose a problem in comparison with high basicity PAC. Most preferably, basicity: 50%, Al 2 O 3 Concentration: 10.2 mass%, SO 4 Concentration: 2.6 mass%, but in the tests at sewage treatment plants, a large amount of PAC is used over a long period, so it is necessary to use industrially produced PAC, and it is acceptable that such PAC has a certain range in physical property values.

[0021] As described above, ordinary basicity PAC has been used in sewage treatment, and its injection amount has been set so that treated water with a predetermined water quality, that is, treated water having the water quality targeted in the actual operation of the sewage treatment plant, can be obtained. As the water quality, for example, phosphorus concentration, SS concentration, etc. are used as indicators. As an example of treated water with a predetermined water quality, in the supernatant in the final sedimentation tank, the average value of the phosphorus concentration is preferably, for example, 0.4 mg / L or less, and the average value of the SS concentration is preferably, for example, 10 mg / L or less. Ordinary basicity PAC - 1 or high basicity PAC is injected so that such treated water with a predetermined water quality can be obtained.

[0022] The injection rate in terms of aluminum oxide of the inorganic coagulant of normal basicity PAC-1 or high basicity PAC is determined from the data on a daily basis. Let the daily injection volume of the inorganic coagulant be a (L), the specific gravity of the inorganic coagulant be b, the concentration of aluminum oxide (Al 2 O 3 ) be c (mass %), and the daily inflow volume into the biological reaction tank be d (m 3 ). In this case, The injection rate in terms of aluminum oxide of the inorganic coagulant (g / m 3 ) = (a × b × c / 100 × 1000) / d can be obtained by the formula.

[0023] For both normal basicity PAC-1 and high basicity PAC, when determining the average value and standard deviation of the injection rate, it is preferable to use data for at least a consecutive one-month period. The data is preferably obtained at a level of a pilot plant or higher, and more preferably at a sewage treatment plant.

[0024] When comparing the average value and standard deviation of the injection rate in terms of aluminum oxide of normal basicity PAC-1 and high basicity PAC at a sewage treatment plant, it is preferable to use the following procedure. In the following, a case where the injection period of high basicity PAC is set after the injection period of normal basicity PAC-1 will be described, but the same applies when this order is reversed. (i) When a PAC different from normal basicity PAC-1 is being used, switch to normal basicity PAC-1. After switching and after a certain period of time, obtain data on the injection rate of normal basicity PAC-1 for at least a consecutive one-month period. The certain period after switching is preferably at least one month. This is because the SS in the biological reaction tank including the returned sludge may be affected by the aluminum species in the PAC, and a period is required for complete replacement from the aluminum species derived from the PAC before switching to the aluminum species derived from the PAC after switching. (ii) After the injection period of normal basicity PAC-1 ends, switch to high basicity PAC. After the switch, after a certain period of time, obtain data for at least one continuous month regarding the injection rate of high basicity PAC. Set the certain period in the same manner as in (i) above.

[0025] Here, when injecting normal basicity PAC-1 to obtain treated water of a predetermined water quality, set the average value of the injection rate in terms of aluminum oxide conversion of normal basicity PAC-1 to 100 and its standard deviation to 100, and set the average value of the SS concentration in the supernatant in the final sedimentation tank to 100. Also, set the range of water quality similar to the average value of the SS concentration in the supernatant in this final sedimentation tank within the range of 100 ± 15. When injecting high basicity PAC, when the average value of the SS concentration in the supernatant in the final sedimentation tank falls within the range of 100 ± 15, it can be determined that the treatment can be carried out to the same extent as when injecting normal basicity PAC-1. When more strictly defining that it is the same extent, the above range may be set to 100 ± 10.

[0026] The sewage treatment method of the present invention is such that the injection rate in terms of aluminum oxide conversion of high basicity PAC for the average value of the SS concentration in the supernatant in the final sedimentation tank to be within the range of 100 ± 15 is 75 or less in terms of the average value and 75 or less in terms of the standard deviation.

[0027] Regarding the above average value, the fact that the average value of high basicity PAC is 75 or less when the average value of normal basicity PAC-1 is 100 cannot be predicted from known techniques and is significantly excellent. The average value of the above high basicity PAC is preferably 70 or less. There is no particular limitation on the lower limit value, and it is at least more than 0. If the lower limit value is deliberately set, for example, it is 10, 15, 20, 25, 30, etc. When using these lower limit values, examples of the range of the average value of the above high basicity PAC are more than 0 and 75 or less, 10 - 75, 15 - 75, 20 - 75, 25 - 75, 30 - 75, more than 0 and 70 or less, 10 - 70, 15 - 70, 20 - 70, 25 - 70, 30 - 70.

[0028] Regarding the above standard deviation, when the standard deviation of normal basicity PAC-1 is set to 100, the standard deviation of high basicity PAC being 75 or less is not disclosed in the prior art regarding the standard deviation. Therefore, it is even less predictable from the prior art than the above average value. And the standard deviation not disclosed in the prior art can be said to be heterogeneous from the prior art. The standard deviation of the above high basicity PAC is preferably 70 or less. There is no particular limitation on the lower limit value, and it is at least more than 0. If the lower limit value is deliberately set, for example, it can be 10, 15, 20, 25, 30, etc. When these are used, the range of the standard deviation of the above high basicity PAC is exemplified as more than 0 and 75 or less, 10 - 75, 15 - 75, 20 - 75, 25 - 75, 30 - 75, more than 0 and 70 or less, 10 - 70, 15 - 70, 20 - 70, 25 - 70, 30 - 70.

[0029] The fact that the average value of the above high basicity PAC is 75 or less means that the injection amount can be reduced to a considerable extent, and since the delivery frequency of PAC can be reduced to a considerable extent, it can contribute to the logistics aspect. Also, the fact that the standard deviation of the above high basicity PAC is 75 or less means that the variation in the injection rate is suppressed to a considerable extent, leading to ease of operation management that is highly welcomed by sewage treatment plants.

Examples

[0030] Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited thereto.

[0031] As normal basicity PAC-1, among "PAC250A" manufactured by Taki Chemical Co., Ltd., those corresponding to basicity: 51 ± 2%, Al 2 O 3 concentration: 10.3 ± 0.2 mass%, SO 4 concentration: 2.6 ± 0.3 mass% were used. The specific gravity was approximately 1.2.

[0032] As high basicity PAC, "PAC700A" manufactured by Taki Chemical Co., Ltd. (basicity: 68 - 72%, Al 2 O 3Concentration: 10.1 - 10.5 mass%, SO 4 Concentration: 1.7 - 2.3 mass%) was used. The specific gravity was approximately 1.2.

[0033] Tests were conducted at a sewage treatment plant in a certain city. There were two treatment lines, and the daily influent volume for each treatment line was approximately 3000 - 5000 m 3 .

[0034] Ordinary basicity PAC - 1 was applied to the two treatment lines, and then high - basicity PAC was applied to the two treatment lines.

[0035] Regarding ordinary basicity PAC - 1, it had been applied previously, and data for approximately two years and three months before the switch to high - basicity PAC were used.

[0036] After switching from ordinary basicity PAC - 1 to high - basicity PAC, while confirming that the predetermined treated water quality was obtained, the injection rate of high - basicity PAC was gradually decreased. The data for high - basicity PAC were those for approximately one year and one month after about two months had passed since the switch and the injection rate had stabilized.

[0037] The results regarding the injection rate are shown in Table 1. The index is the relative value when the ordinary basicity PAC - 1 in each line is set to 100. Also, as the treated water quality, Table 2 shows the average values of the phosphorus (P) concentration and the SS concentration in the supernatant of the final sedimentation tank. The indexes for the phosphorus (P) concentration and the SS concentration are the relative values when the ordinary basicity PAC - 1 in each line is set to 100.

[0038]

Table 1

[0039]

Table 2

[0040] From Table 2, it can be seen that there was no difference in the treated water quality between normal basicity PAC-1 and high basicity PAC. This can be confirmed by the fact that when the average value of normal basicity PAC-1 was set to 100 each in the indexes of phosphorus (P) concentration and SS concentration in the supernatant in the final sedimentation tank, the average values of phosphorus (P) concentration and SS concentration of high basicity PAC were both within the range of 100 ± 15. On the other hand, from Table 1, it can be seen that high basicity PAC fully satisfied 75 or less in both the average value and the standard deviation of the injection rate when normal basicity PAC-1 was set to 100, realizing the reduction of the injection amount and the suppression of the variation of the injection rate, and contributing to the efficiency improvement of the operation management of the sewage treatment plant.

[0041] From the above, the application of high basicity PAC to sewage treatment can significantly reduce the injection rate compared to normal basicity PAC-1 to obtain treated water of a predetermined water quality, and can further significantly suppress the variation of the injection rate. Therefore, its industrial applicability is great.

Claims

[Claim 1] This involves a process of treating sewage in the order of a primary sedimentation tank, a biological reactor, and a final sedimentation tank. A sewage treatment method in which an inorganic coagulant is injected at any point from the outlet region of a biological reactor to before the final sedimentation tank, Inorganic flocculant, basicity: 67-75%, Al 2 O 3 Concentration: 9.0 to 11.0 mass% and SO 4 A high-basicity polyaluminum chloride solution having a concentration of 1.0 to 3.5 mass%. Inorganic flocculant Basicity: 51±2%, Al 2 O 3 Concentration: 10.3±0.2 mass% and SO 4 When the normal basicity polyaluminum chloride solution "PAC250A" manufactured by Taki Chemical Industry Co., Ltd., with a concentration of 2.6±0.3% by mass is injected, the average injection rate and standard deviation in terms of aluminum oxide are set to 100, and the average SS concentration in the supernatant in the final settling tank is set to 100. The injection rate of the high basicity polyaluminum chloride solution, calculated as aluminum oxide, for keeping the average SS concentration in the supernatant in the final settling tank within the range of 100±15 is set to 75 or less on average and 75 or less on standard deviation. A method for treating sewage using a highly basic polyaluminum chloride solution.