Wastewater treatment method using a high-basicity 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 polyaluminum chloride solution with a high basicity.

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 shall apply hereinafter) is also called a final treatment plant. In the sewage treatment process, the main processes generally include a primary sedimentation tank, a biological reaction tank, and a final sedimentation tank in this order.

[0003] In the primary sedimentation tank, a part of the suspended organic pollutants is separated by sedimentation. Next, in the biological reaction tank, aeration is carried out to remove 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 of aerating the entire tank, anaerobic-aerobic activated sludge methods are also used for the treatment in the biological reaction tank. The anaerobic-aerobic activated sludge method is particularly used for phosphorus removal, and the tank may be divided into anaerobic and aerobic parts in some cases. 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 gravity sedimentation. A part of the sludge flocs is returned to the biological reaction tank as return sludge.

[0004] In the outlet area 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 to generate poorly water-soluble aluminum phosphate salts for the purpose of phosphorus removal, an ordinary basicity polyaluminum chloride solution (basicity: approximately 50%, Al2O3 concentration: 10 - 11% by mass, SO4 concentration: 2.0 - 3.0% by mass) (hereinafter referred to as "ordinary basicity PAC") may be injected as a flocculant.

[0005] Non-patent document 1 describes the addition of inorganic coagulants such as aluminum sulfate (alum) and polyaluminum chloride for the purpose of removing phosphorus in sewage treatment. Patent document 1 describes the addition of inorganic coagulants between the outlet of the biological reaction tank and the final sedimentation tank. Patent documents 2 and 3 describe the addition of inorganic coagulants in the outlet region of the biological reaction tank. Furthermore, patent document 4 describes the addition of inorganic coagulants near the outlet (outlet region) of the biological reaction tank in the treatment of organic wastewater such as wastewater from the electronics industry. Patent documents 1 to 4 each exemplify polyaluminum chloride as an inorganic coagulant.

[0006] Patent Document 5 describes that in a water purification method, using PAC with a basicity of 70-75% as a coagulant makes it easier for organic matter to coagulate, allowing for sufficient removal of organic matter from the treated water and also making it easier to reduce color.

[0007] Patent document 6 describes the addition of PAC with a basicity of 60-70% as an inorganic coagulant in membrane filtration treatment of water to be treated that contains organic matter, and states that PAC with a basicity of 70% has low residual Al, excellent organic matter removal ability and coagulation properties. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2008-238015 [Patent Document 2] Japanese Patent Application Publication No. 11-169887 [Patent Document 3] Japanese Patent Application Publication No. 10-99887 [Patent Document 4] Patent No. 4997724 [Patent Document 5] Japanese Patent Publication No. 2022-152649 [Patent Document 6] Japanese Patent Publication No. 2020-108874 [Non-patent literature]

[0009] [Non-Patent Document 1] Chemistry and Education, Vol. 45, No. 4 (1997), pp. 208-211, Koichi Fujie, "Current Status and Future Prospects of Wastewater Treatment Technology." [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] In the 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 adopted when the Japan Water Works Association (JWWA) established its standard for polyaluminum chloride for water supply (2000). The JWWA revised its basicity standard to [45-75%] on August 9, 2016. Given the history of basicity standards for polyaluminum chloride for water supply, and the fact that ordinary basicity PAC with a basicity of around 50% has been the most commonly used up to the present day, it is reasonable to consider the "polyaluminum chloride" described in Non-Patent Document 1 and Patent Documents 1-4 as ordinary basicity PAC with a basicity of around 50%.

[0011] Patent Document 5 does not disclose anything about the degree to which organic matter is likely to flocculate when using PAC with a basicity of 70-75%, nor does it disclose anything about the treatment capacity when PAC with a basicity of 70-75% is applied to sewage.

[0012] Similar to Patent Document 5, Patent Document 6 does not disclose anything about the degree of organic matter aggregation when using PAC with a basicity of 60-70%, nor does it disclose anything about the treatment capacity when PAC with a basicity of 60-70% is applied to sewage. Incidentally, paragraph

[0027] of Patent Document 6 states that PAC with a basicity of 60-70% can form flocs with good detachability on the immersion membrane even with a smaller amount added than PAC with a basicity of 40-50%, but this statement concerns the formation of flocs with good detachability, which is a problem in membrane filtration, and does not concern aggregation.

[0013] Injecting ordinary basicity PAC is effective in promoting sludge floc formation and removing phosphorus. However, the inventors of this invention believe there is room for further improvement beyond these points, and have made this their research objective. [Means for solving the problem]

[0014] As a result of diligent research into the above-mentioned problems, the inventors of this invention discovered that by using a predetermined high-basicity polyaluminum chloride solution as a flocculant, not only can the injection rate of the flocculant be reduced to a considerable extent, but surprisingly, fluctuations in the injection rate can also be suppressed to a considerable extent. Based on this finding, the inventors completed the present invention.

[0015] The present invention is as follows: [1] The process includes treating wastewater in the order of primary sedimentation tank, biological reaction tank, and final sedimentation tank, In a wastewater treatment method in which an inorganic coagulant is injected at any point from the outlet region of the biological reaction tank to before it reaches the final sedimentation tank, The inorganic flocculant is a highly basic polyaluminum chloride solution with a basicity of 67-75%, an Al2O3 concentration of 9.0-11.0% by mass, and an SO4 concentration of 1.0-3.5% by mass. When injecting the normal basicity polyaluminum chloride solution "PAC250A" manufactured by Takagi Chemical Co., Ltd. with a basicity of 51 ± 2%, an Al2O3 concentration of 10.3 ± 0.2 mass%, and a SO4 concentration of 2.6 ± 0.3 mass% as an inorganic flocculant, taking the average value and standard deviation of the injection rate in terms of aluminum oxide conversion as 100 respectively, and taking the average value of the SS concentration in the supernatant in the final sedimentation tank as 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 terms of the average value and 75 or less in terms of the standard deviation. A sewage treatment method using a high basicity polyaluminum chloride solution.

Mode for Carrying Out the Invention

[0016] Hereinafter, the present invention will be described in detail based on preferred embodiments, but the present invention is not limited to the following embodiments, and various modifications are possible 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 end numerical values 1 and numerical value 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 relates to a wastewater treatment method comprising a process of treating wastewater in the order of a primary sedimentation tank, a biological reaction tank, and a final sedimentation tank, wherein an inorganic coagulant is injected at any point from the outlet region of the biological reaction tank to before reaching the final sedimentation tank, wherein the inorganic coagulant is a high-basicity polyaluminum chloride solution having a basicity of 67-75%, an Al2O3 concentration of 9.0-11.0% by mass, and an SO4 concentration of 1.0-3.5% by mass, and the inorganic coagulant is a normal basicity polyaluminum chloride solution manufactured by Taki Chemical Co., Ltd., having a basicity of 51±2%, an Al2O3 concentration of 10.3±0.2% by mass, and an SO4 concentration of 2.6±0.3% by mass. This invention relates to a wastewater treatment method using a high-basicity polyaluminum chloride solution (hereinafter referred to as "the wastewater treatment method of the present invention"). When the average value and standard deviation of the injection rate in terms of aluminum oxide when the luminium solution "PAC250A" is injected are set to 100, and 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 is set to 75 or less in average value and 75 or less in standard deviation, so that the average value of the SS concentration in the supernatant in the final sedimentation tank is within the range of 100±15. Hereinafter, the high-basicity polyaluminum chloride solution will be referred to as "high-basicity PAC," and the ordinary-basicity polyaluminum chloride solution "PAC250A" manufactured by Taki Chemical Co., Ltd. will be referred to as "ordinary-basicity PAC-1." The basicity is determined by the procedure described in "Method for measuring basicity" in the JWWA Japan Water Works Association standard "Polyaluminum Chloride for Water Supply JWWA K 154:2016."

[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 a primary sedimentation tank, a biological reaction tank, and a final sedimentation tank, and injecting an inorganic flocculant at any location from the outlet region of the biological reaction tank until reaching the final sedimentation tank" is a well-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 a primary sedimentation tank, a biological reaction tank, and a final sedimentation tank, and the influent water to the final sedimentation tank is one injected with an inorganic flocculant". Note that 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 perspective of the mixing property with sludge, the injection position of the inorganic flocculant is preferably the outlet region of the biological reaction tank.

[0019] In the sewage treatment method of the present invention, highly basic PAC is injected as the inorganic flocculant. The highly basic PAC has a basicity within the range of 67 to 75%, an Al2O3 concentration within the range of 9.0 to 11.0% by mass, and a SO4 concentration within the range of 1.0 to 3.5% by mass. The lower limit value of the basicity is preferably 68%. The upper limit value of the basicity is preferably 73%. Therefore, the preferable ranges of the basicity include 67 to 73%, 68 to 75%, and 68 to 73%. From the perspective of the specifications of general PAC, the Al2O3 concentration is preferably within the range of 10.0 to 11.0% by mass. The SO4 concentration is preferably within the range of 1.5 to 2.5% by mass.

[0020] As the ordinary basicity PAC-1, those with a basicity of 51 ± 2%, an Al2O3 concentration of 10.3 ± 0.2% by mass, and a SO4 concentration of 2.6 ± 0.3% by mass are used. Each physical property value has a certain range, but such a range does not pose a problem in comparison with highly basic PAC. Most preferably, the basicity is 50%, the Al2O3 concentration is 10.2% by mass, and the SO4 concentration is 2.6% by mass. However, in 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 mentioned above, ordinary basicity PAC has been used in sewage treatment, and the injection amount has been set so that treated water of a predetermined quality is obtained, that is, treated water with the target water quality for actual operation of the sewage treatment plant. For example, phosphorus concentration and SS concentration are used as indicators of this water quality. As an example of treated water of a predetermined quality, in the supernatant of the final sedimentation tank, the average value of the phosphorus concentration is preferably 0.4 mg / L or less, and the average value of the SS concentration is preferably 10 mg / L or less. Ordinary basicity PAC-1 or high basicity PAC is injected to obtain treated water of such predetermined quality.

[0022] The injection rate of inorganic coagulants in terms of aluminum oxide for normal basicity PAC-1 or high basicity PAC is determined from daily data, where a (L) is the daily injection volume of inorganic coagulant, b is the specific gravity of the inorganic coagulant, c (mass%) is the aluminum oxide (Al2O3) concentration of the inorganic coagulant, and d (m³) is the daily inflow volume of water into the biological reaction tank. 3 ) If so, Injection rate of inorganic flocculant in terms of aluminum oxide (g / m³) 3 ) = (a × b × c / 100 × 1000) / d It can be calculated using the following formula.

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

[0024] When comparing the average and standard deviation of the injection rates of ordinary basicity PAC-1 and high basicity PAC in terms of aluminum oxide equivalent at a sewage treatment plant, it is preferable to use the following procedure. Note that the following explanation assumes that the injection period for high basicity PAC follows the injection period for ordinary basicity PAC-1, but the same procedure applies if the order is reversed. (i) If a PAC different from the standard basicity PAC-1 is being used, switch from that PAC to the standard basicity PAC-1. After the switch, after a certain period of time, obtain data on the injection rate of the standard basicity PAC-1 for at least one consecutive month. Preferably, the certain period after the switch should be at least one month. This is because, during the certain period after the switch, the SS in the biological reaction tank, including the returned sludge, may be affected by the aluminum species in the PAC, and a period of time is needed for the aluminum species derived from the PAC before the switch to be completely replaced by the aluminum species derived from the PAC after the switch. (ii) After the period of injection with normal basicity PAC-1 ends, switch to high basicity PAC. After the switch, after a certain period of time has passed, acquire data on the injection rate of high basicity PAC for at least one consecutive month. The certain period will be set in the same manner as in (i) above.

[0025] Here, when ordinary basicity PAC-1 is injected to obtain treated water of a predetermined quality, the average value of the injection rate of ordinary basicity PAC-1 in terms of aluminum oxide is set to 100 and its standard deviation to 100, and the average value of the SS concentration in the supernatant in the final sedimentation tank is set to 100. Furthermore, the range of water quality equivalent to this average value of the SS concentration in the supernatant in the final sedimentation tank is set to within 100 ± 15. When high basicity PAC is injected, if 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 has been performed to the same extent as when ordinary basicity PAC-1 is injected. If the equivalent extent is to be defined more strictly, the above range may be set to 100 ± 10.

[0026] The wastewater treatment method of the present invention involves setting the injection rate of high-basicity PAC, in terms of aluminum oxide, to 75 or less in average value and 75 or less in standard deviation, in order to keep the average value of the SS concentration in the supernatant of the final sedimentation tank within the range of 100 ± 15.

[0027] Regarding the above average values, the fact that the average value of high basicity PAC is 75 or less when the average value of normal basicity PAC-1 is set to 100 is unpredictable from known technology and is remarkably excellent. The above average value of high basicity PAC is preferably 70 or less. The lower limit is not particularly limited, but is at least greater than 0. If a lower limit were to be set, it would be, for example, 10, 15, 20, 25, 30, etc. When these lower limits are used, the range of the above average value of high basicity PAC can be exemplified as greater than 0 and 75 or less, 10 to 75, 15 to 75, 20 to 75, 25 to 75, 30 to 75, greater than 0 and 70 or less, 10 to 70, 15 to 70, 20 to 70, 25 to 70, 30 to 70.

[0028] Regarding the above standard deviation, the fact that the standard deviation of high basicity PAC is 75 or less when the standard deviation of ordinary basicity PAC-1 is set to 100 is even more unpredictable from the publicly known technology than the above average value, because the publicly known technology does not disclose anything about the standard deviation. Furthermore, a standard deviation that is not disclosed in the publicly known technology can be said to be alien from the publicly known technology. The above standard deviation of high basicity PAC is preferably 70 or less. The lower limit is not particularly limited, but is at least greater than 0. If a lower limit is to be set, for example, it would be 10, 15, 20, 25, 30, etc. If these are used, the range of the above standard deviation of high basicity PAC can be exemplified as greater than 0 and less than or equal to 75, 10 to 75, 15 to 75, 20 to 75, 25 to 75, 30 to 75, greater than 0 and less than or equal to 70, 10 to 70, 15 to 70, 20 to 70, 25 to 70, 30 to 70.

[0029] The fact that the average value of the high-basicity PAC is 75 or less means that the injection volume can be reduced to a considerable extent, and the frequency of PAC deliveries can be reduced to a considerable extent, thus contributing to logistics. Furthermore, the fact that the standard deviation of the high-basicity PAC is 75 or less means that fluctuations in the injection rate are suppressed to a considerable extent, which leads to easier operation and management, which is highly welcomed by sewage treatment plants. [Examples]

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

[0031] As a standard PAC-1, we used "PAC250A" manufactured by Taki Chemical Co., Ltd., which has a basicity of 51±2%, an Al2O3 concentration of 10.3±0.2% by mass, and an SO4 concentration of 2.6±0.3% by mass. The specific gravity was approximately 1.2.

[0032] As a high-basicity PAC, we used "PAC700A" manufactured by Taki Chemical Co., Ltd. (basicity: 68-72%, Al2O3 concentration: 10.1-10.5% by mass, SO4 concentration: 1.7-2.3% by mass). The specific gravity was approximately 1.2.

[0033] The test was conducted at a sewage treatment plant in a certain city. There were two treatment lines, and the daily inflow volume of each treatment line was approximately 3000-5000 m³. 3 That is the case.

[0034] After applying normal-grade PAC-1 to two processing lines, high-grade PAC was applied to two processing lines.

[0035] For standard basicity PAC-1, which had been applied previously, data from approximately 2 years and 3 months prior to the switch to high basicity PAC was used.

[0036] After switching from standard basicity PAC-1 to high basicity PAC, the injection rate of high basicity PAC was gradually reduced while confirming that the desired treated water quality was obtained. The data for high basicity PAC used was collected over a period of approximately 1 year and 1 month, starting about 2 months after the switch and once the injection rate had stabilized.

[0037] The results regarding the injection rate are shown in Table 1. The index is a relative value for each system, with the normal basicity PAC-1 set to 100. Furthermore, as a measure of treated water quality, Table 2 shows the average values ​​of phosphorus (P) concentration and suspended solids (SS) concentration in the supernatant of the final sedimentation tank. The indices for phosphorus (P) concentration and SS concentration are relative values ​​for each system, with the normal basicity PAC-1 set to 100.

[0038] [Table 1]

[0039] [Table 2]

[0040] Table 2 shows that there was no difference in treated water quality between normal basicity PAC-1 and high basicity PAC. This can be confirmed by the fact that, when the average values ​​of phosphorus (P) concentration and suspended solids (SS) concentration in the supernatant of the final sedimentation tank are set to 100 for normal basicity PAC-1, the average values ​​of phosphorus (P) concentration and SS concentration for high basicity PAC were both within the range of 100 ± 15. On the other hand, as shown in Table 1, the high-basicity PAC sufficiently met the requirement that the average value and standard deviation of the injection rate, when normal-basicity PAC-1 is set to 100, be 75 or less in both cases. This resulted in a reduction in the amount of injection and suppression of fluctuations in the injection rate, contributing to the improved efficiency of sewage treatment plant operation and management.

[0041] Based on the above, the application of high-basicity PAC to wastewater treatment has great industrial potential because it can significantly reduce the injection rate compared to ordinary-basicity PAC-1 in order to obtain treated water of a specified quality, and can also significantly suppress fluctuations in the injection rate.

Claims

[Claim 1] The process includes treating wastewater in the following order: primary sedimentation tank, biological reaction tank, and final sedimentation tank. In a wastewater treatment method in which an inorganic coagulant is injected at any point from the outlet region of the biological reaction tank to before it reaches the final sedimentation tank, Inorganic flocculants have a basicity of 67-75% and Al 2 O 3 Concentration: 9.0–11.0% by mass and SO 4 A highly basic polyaluminum chloride solution with a concentration of 1.0 to 3.5% by mass. As an inorganic flocculant, basicity: 51±2%, Al 2 O 3 Concentration: 10.3 ± 0.2% by mass and SO 4 When a polyaluminum chloride solution with a normal basicity of 2.6 ± 0.3 mass% is injected, the mean and standard deviation of the injection rate in terms of aluminum oxide are set to 100, and the mean values ​​of the phosphorus (P) concentration and SS concentration in the supernatant of the final sedimentation tank are also set to 100. To ensure that the average values ​​of phosphorus (P) and SS concentrations in the supernatant of the final sedimentation tank are within the range of 100 ± 15, the injection rate of the above-mentioned high-basicity polyaluminum chloride solution in terms of aluminum oxide shall be 75 or less in average value and 75 or less in standard deviation. A wastewater treatment method using a highly basic polyaluminum chloride solution.

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