Crystallization method and device for magnesium ammonium phosphate

Abstract

To provide a crystallization method and device for magnesium ammonium phosphate (MAP), whereby, when recovering phosphorus contained in organic wastewater or sludge as MAP, it is possible to rapidly grasp changes in properties of the sludge supplied to a crystallization reaction tank, supply an amount of magnesium salt suitable for the phosphate ion concentration in the sludge, reduce Mg concentration in the treated sludge, and improve the MAP recovery rate.SOLUTION: The present invention provides a crystallization method for MAP, wherein the phosphate ion concentration in sludge introduced into an MAP crystallization reaction tank is quantitatively analyzed on-site at predetermined time intervals using a magnesium ion-selective electrode, and the addition amount of a magnesium salt is controlled based on the quantitatively analyzed phosphate ion concentration. The present invention also provides a crystallization device for MAP, including: an MAP crystallization reaction section that mixes sludge and a magnesium salt to generate MAP; a phosphate ion quantitative analysis section that quantitatively analyzes phosphate ion concentration in sludge; and a magnesium salt addition control mechanism that controls the amount of magnesium salt to be added based on the phosphate ion quantitative analysis result.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method and apparatus for crystallizing magnesium ammonium phosphate, and more particularly to a method and apparatus for crystallizing magnesium ammonium phosphate in which the amount of chemicals added is controlled based on the concentration of phosphate ions contained in inorganic wastewater, organic wastewater, inorganic sludge, organic sludge, or the like. [Background technology]

[0002] With the recent global depletion of phosphorus resources, there has been renewed interest in recovering phosphorus from sewage, wastewater, and human waste. For example, biological phosphorus removal is widely used in sewage treatment. This method utilizes the activity of microorganisms called polyphosphate-accumulating bacteria (PAOs). The PAOs absorb phosphorus from sewage in a reaction tank and then remove it as excess sludge from the water treatment system. However, PAOs release the absorbed phosphorus in subsequent processes such as digesters. Furthermore, digested sludge contains abundant phosphorus and ammonia because ammonia is generated in large amounts during the decomposition of proteins and other substances in the sludge. Therefore, many treatment plants use a method that crystallizes magnesium ammonium phosphate (MAP), which is produced from phosphorus, ammonia, and magnesium.

[0003] A technology has been developed to remove and recover the phosphorus contained in sludge as MAP crystals by adding chemicals containing magnesium to sludge containing phosphorus and ammonia and adjusting the pH to an appropriate level. For example, Japanese Patent Laid-Open Publication No. 2004-941 (Patent Document 1) describes a method for recovering phosphorus as MAP crystals from wastewater containing high concentrations of organic matter, phosphorus, and nitrogen, such as human waste or septic tank sludge dewatering supernatant, sludge digestion liquid, and chemical industry wastewater. The method comprises an anaerobic treatment step and a step of removing the phosphorus and nitrogen in the organic wastewater or sludge from the system in the form of magnesium ammonium phosphate, the method comprising a step of adding a magnesium source within the anaerobic treatment step or a step preceding the anaerobic treatment step in accordance with a target recovery amount of magnesium ammonium phosphate to be recovered by the process, a step of separating the magnesium ammonium phosphate crystals generated in the anaerobic treatment step, and a step of introducing a portion of the sludge after recovery of the magnesium ammonium phosphate into a sludge dewatering step and returning the remainder of the sludge after recovery of the magnesium ammonium phosphate to the anaerobic treatment step.

[0004] The method disclosed in Patent Document 1 is useful in that it can recover not only phosphorus present as dissolved ions in sludge, but also MAP and the like naturally occurring in the digester, etc. However, Patent Document 1 is a method in which the Mg supply amount and the amount of MAP seed crystals are controlled by treating the digester itself as a MAP crystallization reaction tank. Therefore, it is difficult to recover the amount of Mg supplied and the amount of MAP seed crystals in a sludge containing several thousand m3, which usually has a residence time of about 30 days. 3 In a digestion tank with a large capacity, the stirring power required to quickly disperse the input magnesium source and uniformly flow the MAP seed crystals is large. A method for efficiently recovering MAP from organic wastewater or sludge is considered to be effective by providing a separate small crystallization reaction tank for recovering MAP. However, because the properties of the organic wastewater or sludge input into the crystallization reaction tank fluctuate due to various factors, it is difficult to adjust the reaction conditions for the crystallization reaction in response to changes in the properties of the input sludge.

[0005] Furthermore, if the amount of magnesium added in the MAP crystallization reaction is too small, phosphate ions remain, whereas if the amount of magnesium added is too large, the degree of supersaturation rises above the appropriate range, resulting in the formation of a large amount of unrecoverable fine crystals. Therefore, if too little or too much magnesium is added to the MAP crystallization reaction, there is a concern that the amount of MAP recovered will ultimately decrease.

[0006] To optimize the MAP crystallization reaction, it is recommended to periodically sample the crystallization reaction solution in the crystallization reactor, measure the phosphorus concentration in the crystallization reaction solution, and add an appropriate amount of magnesium. The official method for phosphate ion determination is based on the molybdenum blue (ascorbic acid reduction) spectrophotometric method (JIS K 0102-2016), specified in the Industrial Wastewater Testing Method. Because the quantification range for molybdenum blue spectrophotometric method is low, pretreatment of the sample water is required, including sludge separation, ultrafiltration, and dilution. This complicated procedure makes frequent implementation difficult. Furthermore, organic wastewater or sludge containing phosphorus is often relatively viscous and muddy, making concentration measurement time-consuming and laborious. Furthermore, by the time concentration measurement results are obtained, further changes in the organic wastewater or sludge may have occurred. Furthermore, phosphorus recovery from digested sludge and subsequent sludge dewatering are generally performed continuously for 24 hours, but manual analysis of sludge components, especially at night, is time-consuming and it is particularly difficult to keep up with changes in sludge properties. Therefore, with this method, it is difficult to quickly adjust the operating conditions of the crystallization reactor to match changes in the properties of the input sludge, and there are cases where the production efficiency of MAP cannot be sufficiently improved. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-941 Summary of the Invention [Problem to be solved by the invention]

[0008] Conventionally, the measurement of phosphate ion concentration in sludge has been performed using official methods, but the pretreatment required for quantitative analysis is cumbersome, making frequent measurements impossible, and it has been difficult to add an appropriate amount of magnesium to match the fluctuating sludge properties. If too little magnesium is added, phosphate ions remain in the treated sludge, reducing the MAP recovery rate. If too much magnesium is added, the degree of supersaturation increases, resulting in the formation of large amounts of small, unrecoverable precipitates, reducing the MAP recovery rate. Furthermore, excess magnesium remains in the treated sludge and precipitates in pipes and tanks in the treatment facility other than the MAP crystallization reaction tank, adhering to the walls and forming scale, which can cause problems such as blockages.

[0009] The present invention aims to provide a method and apparatus for recovering magnesium ammonium phosphate (MAP) from organic wastewater or sludge, in which phosphorus contained in the sludge is recovered as MAP, by quickly detecting changes in the properties of the sludge to be fed into a crystallization reaction tank and supplying an amount of magnesium salt appropriate for the phosphate ion concentration in the sludge, thereby suppressing scale formation and improving the MAP recovery rate.

[0010] More specifically, an object of the present invention is to provide a method and apparatus for crystallizing magnesium ammonium phosphate, which allows for easy and frequent measurement of the phosphate ion concentration in sludge in a crystallization reaction tank, and controls the amount of magnesium to be added to the crystallization reaction tank based on the phosphate ion concentration. [Means for solving the problem]

[0011] The present invention provides the following MAP crystallization reaction apparatus. [1] The system comprises a MAP crystallization reaction unit that mixes sludge and magnesium salt to produce MAP, a phosphate ion quantitative analysis unit that quantitatively analyzes the phosphate ion concentration in the sludge, and a magnesium salt addition control mechanism that controls the amount of magnesium salt added based on the results of the phosphate ion quantitative analysis; the MAP crystallization reaction section includes a MAP crystallization reaction tank, a sludge supply line for supplying sludge to the MAP crystallization reaction tank, a magnesium salt supply line for supplying magnesium salt to the MAP crystallization reaction tank, and a magnesium salt storage tank; the phosphate ion quantitative analysis unit includes a reaction tank provided with a magnesium ion selective electrode, an analytical sludge introduction line for introducing sludge into the reaction tank, and an analytical magnesium salt addition line for adding a magnesium salt to the reaction tank; the sludge introduction line for analysis of the phosphate ion quantitative analysis unit is connected to the sludge supply line to the MAP crystallization reaction tank, and a route switching means is provided in the sludge supply line to switch the introduction destination of the sludge to either the MAP crystallization reaction tank or the phosphate ion quantitative analysis unit; the magnesium salt addition line for analysis of the phosphate ion quantitative analysis unit is connected to a magnesium salt supply line to the MAP crystallization reaction tank, and a path switching means is provided in the magnesium salt supply line for switching the introduction destination of the magnesium salt to either the MAP crystallization reaction tank or the phosphate ion quantitative analysis unit; The magnesium salt addition control mechanism of the MAP crystallization reaction apparatus is characterized in that it includes: a calculation unit that determines the amount of magnesium salt to be added to the MAP crystallization reaction tank based on the quantitative analysis results from the phosphate ion quantitative analysis unit; a control unit that issues a control signal for magnesium salt addition from the magnesium salt storage tank; and magnesium salt supply amount adjustment means that is provided in the magnesium salt supply line. [2] The MAP crystallization reaction apparatus according to [1] above, wherein a solid-liquid separator for separating solid and liquid of sludge before introduction into the reaction tank is provided in an analytical sludge introduction line of the phosphate ion quantitative analysis unit.

[0012] The present invention also provides the following MAP crystallization method. [3] A MAP crystallization method characterized by quantitatively analyzing the phosphate ion concentration in sludge introduced into a MAP crystallization reaction tank on-site at predetermined time intervals using a magnesium ion-selective electrode, and controlling the amount of magnesium salt added based on the quantitatively analyzed phosphate ion concentration. [4] The quantitative analysis of the phosphate ion concentration in sludge using the magnesium ion selective electrode is A predetermined amount of sludge is introduced into a reaction tank having a magnesium ion selective electrode; Magnesium salt is added to the sludge in the reaction tank, and the detection of magnesium ions is monitored using a magnesium ion selective electrode. The MAP crystallization method according to [3] above, characterized in that the phosphate ion concentration in the sludge is calculated using Equation 1 based on the amount of magnesium ions that have reacted with phosphate ions.

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[10] The MAP crystallization method according to the above item [3], characterized in that the control of the amount of magnesium salt to be added based on the quantitatively analyzed phosphate ion concentration in the sludge is carried out by controlling the amount of magnesium salt to be added so that the phosphate ion concentration in the effluent from the MAP crystallization reaction tank is 10 mg / L or more and 40 mg / L or less. [Effects of the Invention]

[0013] According to the present invention, in a method for crystallizing magnesium ammonium phosphate (MAP) to convert phosphorus contained in organic wastewater or sludge into MAP, a method and apparatus are provided that improve the MAP recovery rate by adding an appropriate amount of magnesium salt in rapid response to changes in the properties of the sludge added to a crystallization reaction tank.

[0014] According to the present invention, the phosphate ion concentration in sludge in a crystallization reaction tank or a digester can be easily measured, enabling frequent quantitative analysis of the phosphate ion concentration, and the amount of magnesium added can be controlled in accordance with fluctuations in sludge properties, thereby improving the MAP recovery rate. Furthermore, scale formation in locations other than the MAP crystallization reaction tank due to excessive addition of magnesium can also be suppressed.

[0015] In the MAP crystallization method of the present invention, an appropriate amount of magnesium salt is added in accordance with fluctuations in the phosphate ion concentration in the sludge in the MAP crystallization reaction tank, thereby reducing the amount of unnecessary chemical addition and reducing chemical costs.

[0016] The MAP crystallizer of the present invention makes it possible to automatically measure the phosphate ion concentration in the sludge in the MAP crystallization reaction tank and automatically control the addition of an appropriate amount of magnesium salt. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic explanatory diagram showing an outline of a MAP crystallization reaction apparatus according to the present invention. [Figure 2] Flowchart of the phosphate ion quantitative analysis method [Figure 3] Flowchart of the quantitative analysis method for phosphate ions when using the titration method to determine the amount of magnesium ions added from the end point of the insolubilization reaction [Figure 4] Flowchart of a method for quantitatively analyzing phosphate ions when the method is for determining the amount of magnesium ions consumed that have reacted with phosphate ions. [Figure 5]Schematic diagram of a 24-hour continuous inflow and continuous outflow phosphorus recovery pilot system used in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will now be described with reference to the accompanying drawings. An outline of the MAP crystallization reaction apparatus of the present invention is shown in Figure 1. The MAP crystallization reaction apparatus of the present invention includes a MAP crystallization reaction section that mixes sludge and magnesium salt to produce MAP, a phosphate ion quantitative analysis section that quantitatively analyzes the phosphate ion concentration in the sludge, and a magnesium salt addition control mechanism that controls the amount of magnesium salt to be added based on the results of the phosphate ion quantitative analysis.

[0019] The MAP crystallization reaction section includes a MAP crystallization reaction tank 1, sludge supply lines 10a and 10b that supply sludge to the MAP crystallization reaction tank 1 and a sludge supply amount adjustment means P2 that controls the sludge supply amount, magnesium salt supply lines 21a and 21b that supply magnesium salt to the MAP crystallization reaction tank 1 and a magnesium salt addition amount adjustment means P1 that controls the magnesium salt addition amount, and a magnesium salt storage tank 22 that stores the magnesium salt to be added to the MAP crystallization reaction tank 1.

[0020] The phosphate ion quantitative analysis unit includes a reaction tank 30 equipped with a magnesium ion selective electrode 31, an analytical sludge introduction line 10 for introducing sludge into the reaction tank 30 and a pump P2a for controlling the analytical sludge supply rate, an analytical magnesium salt addition line 21 for adding magnesium salt to the reaction tank 30 and a pump P1a for controlling the analytical magnesium salt addition rate, and a phosphate ion concentration calculation unit 32 electrically connected to the magnesium ion selective electrode 31 and calculating the phosphate ion concentration based on the measurement results of the magnesium ion selective electrode 31. In the illustrated embodiment, the analytical sludge introduction line 10 is equipped with a pump P2a for controlling the analytical sludge supply rate, and the analytical magnesium salt addition line 21 is equipped with a pump P1a for controlling the analytical magnesium salt addition rate. However, these pumps are not limited to pumps as long as they can supply predetermined amounts of analytical sludge and analytical magnesium salt to the reaction tank 30. Alternatively, the pumps P1a and P2a may be omitted and may be configured to be shared by the magnesium salt addition rate adjustment means P1 and the sludge supply rate adjustment means P2 of the MAP crystallization reaction unit, respectively.

[0021] The reaction tank 30 is preferably equipped with an agitator (not shown). The agitator allows the sludge and magnesium salt to come into uniform and good contact with each other, uniformly promoting the insolubilization reaction and improving the accuracy of detecting the end point of the insolubilization reaction. The reaction tank 30 may also be equipped with a means (not shown) for introducing dilution water such as tap water, a pH adjuster, or a chelating agent. During standby mode, when quantitative analysis is not being performed, introducing tap water into the reaction tank 30 can clean the inside of the tank and prevent the accumulation of contaminants. Furthermore, keeping tap water in the reaction tank 30 can prevent the adhesion of contaminants. Furthermore, although not shown, a solid-liquid separation membrane device may be installed on the analytical sludge introduction line 10, and separated water may be introduced into the reaction tank 30. When the sludge contains insolubilized matter or inhibitors of the magnesium ion-selective electrode, removing these by solid-liquid separation can improve the detection sensitivity of magnesium ions.

[0022] The analytical sludge inlet line 10 of the phosphate ion quantitative analysis section is connected to a sludge supply line 10a to the MAP crystallization reaction tank 1, and the sludge supply line 10a is provided with a path switching means for switching the sludge introduction destination between the sludge inlet line 10b and the analytical sludge inlet line 10. In the illustrated embodiment, the path switching means is a switching valve, but is not limited to this and may be any means known to those skilled in the art that can switch between any paths.

[0023] The analytical magnesium salt addition line 21 of the phosphate ion quantitative analysis section is connected to a magnesium salt supply line 21a to the MAP crystallization reaction tank 1, and the magnesium salt supply line 21a is provided with a path switching means for switching the introduction destination of the magnesium salt between the magnesium salt supply line 21b and the analytical magnesium salt addition line 21. In the illustrated embodiment, the path switching means is a switching valve, but is not limited to this and may be any means capable of switching paths known to those skilled in the art, such as switching by a pump.

[0024] During normal operation, sludge is supplied to the MAP crystallization reaction tank 1 via sludge supply lines 10a and 10b, and magnesium salt is supplied to the MAP crystallization reaction tank 1 via magnesium salt supply lines 21a and 21b.

[0025] When quantitatively analyzing phosphate ions in sludge, the sludge is introduced into the reaction tank 30 of the phosphate ion quantitative analyzer via a sludge supply line 10a and an analytical sludge introduction line 10, and magnesium salt is added to the reaction tank 30 of the phosphate ion quantitative analysis unit via a magnesium salt supply line 21a and an analytical magnesium salt addition line 21. The supply amount of analytical sludge is controlled by a pump P2a. The addition amount of analytical magnesium salt is controlled by a pump P1a.

[0026] The magnesium salt addition control mechanism includes a calculation unit 41 that determines the amount of magnesium salt to be added to the MAP crystallization reaction tank 1 based on the quantitative analysis results from the phosphate ion quantitative analysis unit, a control unit 42 that issues a control signal for the addition of magnesium salt from the magnesium salt storage tank 22 based on the quantitative analysis results from the phosphate ion quantitative analysis unit, and a magnesium salt addition amount adjustment means P1 provided in the magnesium salt supply line 21a.

[0027] The calculation unit 41 monitors the magnesium ion measurement results of the magnesium ion selective electrode 31 in the phosphate ion quantitative analysis unit, and calculates the phosphate ion concentration in the sludge and the optimum amount of magnesium salt to be added corresponding to the phosphate ion concentration based on the magnesium ion concentration when a predetermined condition described below is reached. The calculation unit 41 only needs to be electrically connected to the magnesium ion selective electrode 31 and the magnesium salt addition amount adjustment means P1, and may be a calculation device provided in the phosphate ion quantitative analysis unit or a calculation device provided in a server on the cloud.

[0028] Based on the phosphate ion concentration calculated by the calculation unit 31 of the phosphate ion quantitative analysis unit, the control unit 42 issues a control signal to control the magnesium salt addition amount adjustment means P1 provided on the magnesium salt supply line 21a so that an appropriate amount of magnesium salt is supplied to the MAP crystallization reaction tank 1 via the magnesium salt supply line 21a. The control unit 42 is electrically connected to the calculation unit 41 and the magnesium salt addition amount adjustment means P1 so as to receive instructions from the calculation unit 41 and issue a signal to control the operation of the magnesium salt addition amount adjustment means P1. The control unit 42 may be a control signal issuing device provided in the phosphate ion quantitative analysis unit or a control signal issuing device provided on a cloud server. In the illustrated embodiment, the magnesium salt addition amount adjustment means P1 is a pump, but is not limited thereto and may be any means known to those skilled in the art for adjusting the amount of addition, such as gravity flow and adjusting the flow rate by the opening of a valve.

[0029] Next, the MAP crystallization method of the present invention will be described. The MAP crystallization method of the present invention is characterized by quantitatively analyzing the phosphate ion concentration in sludge introduced into a MAP crystallization reaction tank on-site at predetermined time intervals, and controlling the amount of magnesium salt to be added based on the quantitatively analyzed phosphate ion concentration. According to the MAP crystallization method of the present invention, an appropriate amount of magnesium salt for MAP production is supplied based on the results of frequent and simple quantitative analysis of the phosphate ion concentration in the sludge, thereby promoting the MAP crystallization reaction in response to fluctuations in sludge properties, thereby improving the MAP recovery rate and reducing the amount of residual magnesium ions in the sludge, thereby preventing MAP crystallization in locations other than the MAP crystallization reaction tank and suppressing scale formation.

[0030] The reaction between phosphate ions and magnesium salts can be represented by the following formula:

[0031] [ka]

[0032] When an amount of magnesium salt added exceeds the stoichiometric ratio of magnesium ions to phosphate ions, residual magnesium ions are detected by a magnesium ion-selective electrode. The point at which the detected magnesium ions by the magnesium ion-selective electrode rises sharply is the end point of the titration, and the amount of magnesium ions just before the end of the titration and the amount of phosphate ions in the stoichiometric ratio represent the phosphate ion concentration in the sludge.

[0033]

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[0034] FIG. 2 shows a flowchart of the phosphate ion quantitative analysis method. A predetermined amount A0 (L) of sludge is introduced into a reaction tank having a magnesium ion selective electrode (step S1), magnesium salt is added to the sludge (step S2), the detection of magnesium ions by the magnesium ion selective electrode is monitored (step S3), and the phosphate ion concentration in the sludge is calculated according to the above formula 1 based on the amount of magnesium ions that have reacted with phosphate ions, which is determined by the detection trend of magnesium ions (step S4).

[0035] In the phosphate ion quantitative analysis method used in the present invention, the phosphate ion concentration can be calculated by detecting magnesium ions in either of two ways: (A) adding a constant amount of magnesium salt and determining the amount of magnesium ions added from the end point of the insolubilization reaction at which the magnesium ion concentration rises sharply (titration method); or (B) adding an excess amount of magnesium salt and determining the amount of magnesium ions consumed by reaction with phosphate ions up to the point at which the magnesium ion concentration no longer fluctuates.

[0036] FIG. 3 shows a flowchart of (A) a method for quantitatively analyzing phosphate ions in the case of a titration method for determining the amount of magnesium ions added from the end point of the insolubilization reaction. First, a predetermined amount of sludge, A0 (L), is introduced into a reaction tank equipped with a magnesium ion-selective electrode (step S1). Next, magnesium salt is added to the sludge at a predetermined concentration, M (mg / L), and a constant flow rate, L (L / hr) (step S2-1). The detection of magnesium ions by the magnesium ion-selective electrode is monitored (step S3). Steps S2-1 to S3 are repeated until the magnesium ion concentration increases rapidly (step S4-1-1). When the magnesium ion concentration increases rapidly, the point immediately before the increase is designated as the titration endpoint (step S4-1-2). Based on the amount of magnesium salt added at the endpoint, A (L) (= L (L / hr) × [time elapsed until the titration endpoint] (hr)) and the concentration, M (mg / L), the phosphate ion concentration (mg-P / L) is calculated according to the following formula 2 (S4-1-3).

[0037]

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[0038] FIG. 4 shows a flowchart of (B) a method for quantitatively analyzing phosphate ions in the case of a method for determining the amount of magnesium ions consumed that have reacted with phosphate ions. First, a predetermined amount A0 (L) of sludge is introduced into a reaction tank equipped with a magnesium ion-selective electrode (step S1). Next, a predetermined concentration M (mg / L) of magnesium salt is introduced into the sludge in an excess amount A1 (L) (step S2-2). The detection of magnesium ions by the magnesium ion-selective electrode is monitored (step S3). Step S3 is repeated until the magnesium ion concentration stops fluctuating (step S4-2-1). The phosphate ion concentration in the sludge is calculated according to the following formula 3 based on the consumption of magnesium salt, which is calculated from the difference between the amount of magnesium ions added: M (mg / L) × A1 (L) and the amount of magnesium ions at the point where the magnesium ion concentration stops fluctuating: M1 (mg / L) × A0 (L) (step S-4-2-2).

[0039]

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[0040] If Mg is added in a ratio close to the stoichiometric ratio relative to the phosphate ion concentration in the sludge, the MAP crystallization reaction will proceed without excess or deficiency. However, the properties of the treated sludge required after the MAP crystallization reaction vary depending on the manner in which the treated sludge is used, and there are cases in which it is desirable to leave a predetermined amount of magnesium in the treated sludge, and there are also cases in which it is desirable to leave a predetermined amount of phosphate ions in the treated sludge. In the present invention, the amount of magnesium salt to be added based on the quantitatively analyzed phosphate ion concentration in the sludge can be controlled in the following manner. (1) The amount of magnesium salt added is controlled so that the Mg / P ratio in the sludge in the MAP crystallization reaction tank is constant. (2) The amount of magnesium salt added is controlled so that the magnesium ion concentration in the effluent (treated sludge) from the MAP crystallization reaction tank is constant. (3) The amount of magnesium salt added is controlled so that the phosphate ion concentration in the effluent (treated sludge) from the MAP crystallization reaction tank is constant.

[0041] Each aspect will be described. (1) The amount of magnesium salt added is controlled so that the Mg / P ratio is constant. In this embodiment, a magnesium salt is added in an amount that maintains a preset Mg / P ratio based on the measured value of the phosphate ion concentration in the sludge, so that an appropriate amount of magnesium ions that follows fluctuations in the phosphate ion concentration in the sludge is present in the MAP crystallization reaction tank. By setting the Mg / P ratio to the stoichiometric ratio of about 1, it is possible to consume almost all of the phosphate ions in the sludge in the MAP crystallization reaction tank and produce MAP.

[0042] The Mg / P ratio is preferably 0.80 to 1.20, more preferably 0.85 to 1.15, even more preferably 0.90 to 1.10, and even more preferably 0.95 to 1.05. For example, if the measured phosphate ion (PO4-P) concentration in the sludge introduced into the MAP crystallization reaction tank is 350 mg / L, the molar concentration of P is 350 / 31 = 11.3 mmol, and a stoichiometric ratio (Mg / P = 1) requires 24.3 × 11.3 = 275 mg / L of Mg. When the Mg / P ratio is set to 1.15, the amount of magnesium salt added is controlled to supply approximately 316 mg / L (= 275 × 1.15) of magnesium ions. When the Mg / P ratio is set to 1.05, the amount of magnesium salt added is controlled to supply approximately 288 mg / L (= 275 × 1.05) of magnesium ions.

[0043] (2) The amount of magnesium salt added is controlled so that the magnesium ion concentration in the effluent (treated sludge) from the MAP crystallization reaction tank is constant. In this embodiment, the amount of magnesium salt added is such that a certain amount of magnesium ions remains in the treated sludge after the MAP crystallization reaction, based on the measured value of the phosphate ion concentration in the sludge, so that the magnesium ions in the MAP crystallization reaction tank are always slightly more than the stoichiometric ratio and there is no shortage of magnesium ions to recover the entire amount of phosphorus contained in the sludge in the MAP crystallization reaction tank as MAP.

[0044] The magnesium ions remaining in the treated sludge after the MAP crystallization reaction should be 60 mg / L or less, preferably 50 mg / L or less, more preferably 30 mg / L or less, and even more preferably 20 mg / L or less. For example, if the measured concentration of phosphate ions (PO4-P) in the sludge introduced into the MAP crystallization reaction tank is 350 mg / L, the molar concentration of P is 350 / 31 = 11.3 mmol. To convert all of this P to MAP, 24.3 × 11.3 = 275 mg / L of Mg is required. If the magnesium ion concentration remaining in the treated sludge is set to 20 mg / L, the amount of magnesium salt added is controlled so that 275 + 20 = 295 mg / L of magnesium ions are supplied.

[0045] (3) The amount of magnesium salt added is controlled so that the phosphate ion concentration in the effluent (treated sludge) from the MAP crystallization reaction tank is constant. In this embodiment, magnesium salt is added in an amount that will leave a certain amount of phosphate ions in the treated sludge after the MAP crystallization reaction, based on the measured value of the phosphate ion concentration in the sludge, so that the magnesium ions in the MAP crystallization reaction tank are always slightly less than the stoichiometric ratio and do not become excessive. This embodiment is suitable for cases where, for example, phosphorus is desirably contained in discharge water to fishing grounds, and all of the phosphorus in the sludge in the MAP crystallization reaction is left in the treated sludge without being crystallized as MAP.

[0046] The phosphate ion concentration remaining in the treated sludge after the MAP crystallization reaction is desirably 10 mg / L or higher, preferably 20 mg / L or higher. Because a large amount of phosphate ions remaining in the treated sludge may cause MAP crystallization in the piping downstream of the MAP crystallization reaction tank, the concentration is desirably 40 mg / L or lower, preferably 30 mg / L or lower. For example, if the measured phosphate ion (PO4-P) concentration in the sludge introduced into the MAP crystallization reaction tank is 350 mg / L, to ensure that 20 mg / L of phosphate ions remain in the treated sludge discharged from the MAP crystallization reaction tank, the amount of P that becomes MAP is 350 - 20 = 330 mg / L, or 10.6 mmol. The required magnesium ions are 24.3 × 10.6 = 257.6 mg / L, and the amount of magnesium salt added is controlled to supply this amount.

[0047] As the magnesium salt necessary for the MAP crystallization reaction of the present invention, any magnesium salt used in a normal MAP crystallization reaction can be used without limitation. However, since the magnesium salt is also used to measure the phosphate ion concentration in sludge, it is preferable to use a salt that does not affect the detection sensitivity of the magnesium-selective electrode, and suitable examples of the salt include magnesium hydroxide, magnesium chloride, and magnesium sulfate.

[0048] The MAP crystallization method of the present invention controls the amount of magnesium salt added by measuring the phosphate ion concentration in sludge, whose properties fluctuate due to various factors, in a timely manner, thereby enabling MAP crystallization in a desired manner, such as improving the MAP recovery rate, controlling the residual magnesium ion concentration, or controlling the residual phosphate ion concentration, in response to fluctuations in sludge properties. [Example]

[0049] At a sewage treatment plant A that employs anaerobic digestion of sludge, a 24-hour continuous inflow and continuous outflow phosphorus recovery pilot system as shown in Figure 5 was used, and magnesium hydroxide (40 wt%, specific gravity 1.32) was added to the sludge in the MAP crystallization reactor, and operation was carried out for approximately two weeks under the operating conditions shown in Table 1. The amount of sludge fed into the crystallization reactor was 10 m 3The phosphate ion concentration in the sludge, the phosphate ion removal rate, the TP removal rate, the amount of MAP recovered, and the amount of Mg remaining in the sludge were determined by the following methods.

[0050] The phosphorus recovery pilot system shown in Figure 5 comprises a MAP crystallization reaction tank, a first MAP separator, a second MAP separator, and a MAP washer / dryer. Sludge and magnesium hydroxide are supplied to the MAP crystallization reaction tank, and the resulting treated sludge containing MAP is extracted from the bottom of the MAP crystallization reaction tank. The extracted treated sludge containing MAP is separated into MAP and effluent sludge in the first MAP separator, and the MAP is returned to the MAP crystallization reaction tank as seed crystals. When recovering MAP, the extracted treated sludge containing MAP is separated into MAP and effluent sludge in the second MAP separator, and the MAP is sent to the MAP washer / dryer, where it is recovered after being washed and dried.

[0051] [Measurement of phosphate ion concentration in sludge in a crystallization reactor] A predetermined amount of collected sludge was introduced into a reaction tank equipped with a magnesium ion selective electrode, magnesium hydroxide was added, and the Mg 2+ The detection of is monitored to identify the titration point, and the phosphate ion concentration in the sludge is calculated using the following formula 1 based on the amount of magnesium ions that have reacted with phosphate ions at the titration point.

[0052]

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[0053] [Phosphate ion removal rate] The phosphate removal rate was calculated by dividing the difference between the phosphate ion concentration in the sludge in the MAP crystallization reaction tank (PO4-P concentration in raw sludge) and the phosphate ion concentration in the treated sludge (PO4-P concentration in treated sludge) by the phosphate ion concentration in the MAP crystallization reaction tank. The phosphate ion concentration in the treated sludge was measured using the molybdenum method.

[0054]

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[0055] [TP removal rate] The TP concentration in the sludge was measured based on JIS K 0170-4:2019.

[0056]

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[0057] [MAP recovery amount] The amount of MAP recovered was measured by the weight of dried MAP. The amount of MAP recovered for one day was recorded.

[0058] [Remaining magnesium in sludge] The amount of magnesium remaining in the sludge was measured using a magnesium ion meter. Measurements were taken twice a day and the average value was calculated.

[0059] [Control system 1] Under the previous operating method, the phosphate ion concentration in the sludge was measured using the molybdenum method. The phosphate ion concentration in the sludge in the MAP crystallization reactor was set to 300 mg / L, the value measured during design. Since the molar concentration of P is 300 / 31 = 9.7 mmol, the amount of magnesium required to convert all of this P to MAP is 24.3 × 9.7 = 235.7 mg / L. Magnesium hydroxide was continuously added at a flow rate of 11.8 L / h (283 L / d) to add 235.7 × 1.15 = 271.1 mg / L of magnesium, which is an excess of the stoichiometric ratio of Mg to P, resulting in a Mg / P ratio of 1.15. The measurement results are shown in Table 1. During approximately two weeks of operation, the phosphate ion (PO4-P) removal rate was 89.8-92.3% (average 91.1%), the TP removal rate was 23.2-30.3% (average 27.0%), the residual magnesium in the sludge was 47.8-129.0mg / L (average 82.6mg / L), and the MAP recovery rate was 250-270kg / d (average 259kg / d). There was a large proportion of small MAP that could not be recovered.

[0060] [Table 1]

[0061] [Control system 2] Using the previous operating method, the phosphate ion concentration in the sludge was measured using the molybdenum method, and the phosphate ion concentration in the sludge in the MAP crystallization reaction tank was set to 300 mg / L, the same value measured at the time of design. Magnesium hydroxide was added at a flow rate of 11.8 L / h (283 L / d) to achieve a Mg / P ratio of 1.15, and the system was operated for one week. After one week, the phosphate ion concentration in the sludge in the MAP crystallization reaction tank was remeasured using the molybdenum method, and the magnesium hydroxide flow rate was changed so that the Mg / P ratio relative to the measured phosphate ion concentration was 1.15, and the system was operated for another week. The measurement results are shown in Table 2. During approximately two weeks of operation, the phosphate ion (PO4-P) removal rate was 90.2-95.1% (average 93.5%), the TP removal rate was 30.5-40.2% (average 36.4%), the residual magnesium content in the sludge was 58.3-86.7 mg / L (average 75.2 mg / L), and the MAP recovery rate was 280-310 kg / d (average 298 kg / d).

[0062] [Table 2]

[0063] [Example 1] The phosphate ion concentration in the sludge in the MAP crystallization reactor was measured twice a day, and the magnesium hydroxide flow rate was varied and repeatedly added so that the Mg / P ratio relative to the measured phosphate ion concentration was 1.15. Operation was continued for approximately two weeks. The measurement results are shown in Table 3. During the approximately two-week operation, the phosphate ion (PO4-P) removal rate was 94.5-98.2% (average 96.0%), the TP removal rate was 59.4-64.4% (average 61.3%), the residual magnesium content in the sludge was 40.4-49.6 mg / L (average 45.0 mg / L), and the MAP recovery rate was 343-396 kg / d (average 360 ​​kg / d).

[0064] [Table 3]

[0065] [Example 2] The phosphate ion and magnesium concentrations in the sludge in the MAP crystallization reactor were measured twice a day, and the magnesium hydroxide flow rate was repeatedly changed and added so that the residual magnesium content in the treated sludge remained at 20 mg / L. The operation was carried out for approximately two weeks. The measurement results are shown in Table 4. During the approximately two-week operation, the phosphate ion (PO4-P) removal rate was 85.1 to 95.7% (average 89.4%), the TP removal rate was 45.5 to 54.8% (average 51.0%), and the MAP recovery rate was 295 to 320 kg / d (average 309 kg / d).

[0066] [Table 4]

[0067] [Example 3] The phosphate ion concentrations in the sludge in the MAP crystallization reactor and in the treated sludge flowing out of the MAP crystallization reactor were measured twice a day, and magnesium hydroxide was added repeatedly to maintain the phosphate ion concentration in the treated sludge at 20 mg / L. This process was carried out for approximately two weeks. The measurement results are shown in Table 5. After approximately two weeks of operation, the phosphate ion (PO4-P) removal rate was 89-90% (average 90%), the TP removal rate was 40.8-47.9% (average 44.4%), the residual magnesium content in the treated sludge was 8.5-13.2 mg / L (average 10.6 mg / L), and the MAP recovery rate was 290-300 kg / d (average 295 kg / d).

[0068] [Table 5]

[0069] Table 6 shows the operating conditions and operating results (average values) for Control Systems 1 and 2 and Examples 1 to 3.

[0070] In control system 1, the phosphate concentration in the sludge in the MAP crystallization reaction tank was fixed at the initial setting, and the amount of magnesium hydroxide added was also fixed at 283 L / d, so the remaining amount of magnesium in the sludge was the highest, indicating that excessive magnesium had been added. The excessive addition of magnesium increased the degree of supersaturation, producing many small, unrecoverable MAP particles, and resulting in the lowest TP removal rate and MAP recovery amount.

[0071] In control system 2, the phosphate ion concentration in the sludge in the MAP crystallization reaction tank was measured midway through the two-week operation period, and the amount of magnesium hydroxide added was adjusted based on the measured phosphate ion concentration, resulting in a value of 260 to 311 L / d (average 273 L / d). With the same amount of magnesium hydroxide used as in control system 1, the TP removal rate and MAP recovery rate improved slightly, but the residual magnesium in the sludge was still high, so it can be said that the amount of magnesium added was still excessive.

[0072] In Example 1, the phosphate ion concentration in the sludge in the MAP crystallization reaction tank was measured twice a day, and the amount of magnesium hydroxide added was frequently adjusted based on the measured phosphate ion concentration.As a result, excessive addition of magnesium was suppressed with a lower amount of magnesium hydroxide used (250 to 260 L / d) than in Control Systems 1 and 2, and the phosphate ion removal rate (94.5 to 98.2%), TP removal rate (59.4 to 64.4%), and MAP recovery amount (343 to 360 kg / d) were all significantly improved.

[0073] In Example 2, the amount of magnesium hydroxide added was adjusted to maintain a constant magnesium concentration (20 mg / L) in the treated sludge discharged from the MAP crystallization reaction tank, so the amount of magnesium hydroxide used was 240 to 250 L / d. Excessive addition of magnesium was suppressed by using a smaller amount of magnesium hydroxide than in Control Systems 1 and 2, and a phosphate ion removal rate (85.1 to 95.7%) equivalent to Control Systems 1 and 2, a TP removal rate (45.5 to 54.8%) improved over Control Systems 1 and 2, and a MAP recovery rate (295 to 320 kg / d) were achieved.

[0074] In Example 3, the magnesium hydroxide input rate was adjusted to maintain a constant phosphate ion concentration (20 mg / L) in the treated sludge effluent from the MAP crystallization reaction, resulting in a magnesium hydroxide usage rate of 210–230 L / d. The use of a lower amount of magnesium hydroxide than in Controls 1 and 2 and Examples 1 and 2 prevented excessive magnesium addition, reducing the residual Mg content in the treated sludge to 8.5–13.2 mg / L. This resulted in a phosphate ion removal rate (89–90%) comparable to Controls 1 and 2, and an improved TP removal rate (40.8–47.9%) and MAP recovery rate (290–300 kg / d) compared with Controls 1 and 2. While the phosphate ion removal rate, TP removal rate, and MAP recovery rate were inferior to those in Examples 1 and 2, these results are believed to be improved by setting the residual phosphate ion concentration in the treated sludge below 20 mg / L.

[0075] [Table 6] [Explanation of symbols]

[0076] 1: MAP crystallization reactor 10a, 10b: Sludge supply line P2: Sludge supply amount adjustment means 21a, 21b: Magnesium salt supply line 22: Magnesium salt storage tank P1: Magnesium salt addition amount adjustment means 10: Sludge introduction line for analysis 21: Analytical magnesium salt supply line 30: Reactor 31: Magnesium ion selective electrode 32: Phosphate ion concentration calculation unit 41: Arithmetic section 42: Control unit

Claims

1. The system comprises a MAP crystallization reaction section for producing MAP by mixing sludge and magnesium salt, a phosphate ion quantitative analysis section for quantitatively analyzing the phosphate ion concentration in the sludge, and a magnesium salt addition control mechanism for controlling the amount of magnesium salt to be added based on the results of the phosphate ion quantitative analysis, the MAP crystallization reaction section includes a MAP crystallization reaction tank, a sludge supply line for supplying sludge to the MAP crystallization reaction tank, a magnesium salt supply line for supplying magnesium salt to the MAP crystallization reaction tank, and a magnesium salt storage tank; the phosphate ion quantitative analysis unit includes a reaction tank provided with a magnesium ion selective electrode, an analytical sludge introduction line for introducing sludge into the reaction tank, and an analytical magnesium salt addition line for adding a magnesium salt to the reaction tank; the sludge introduction line for analysis of the phosphate ion quantitative analysis unit is connected to the sludge supply line to the MAP crystallization reaction tank, and a path switching means is provided in the sludge supply line for switching the introduction destination of the sludge to either the MAP crystallization reaction tank or the phosphate ion quantitative analysis unit; the magnesium salt addition line for analysis of the phosphate ion quantitative analysis unit is connected to a magnesium salt supply line to the MAP crystallization reaction tank, and a path switching means is provided in the magnesium salt supply line for switching the introduction destination of the magnesium salt to either the MAP crystallization reaction tank or the phosphate ion quantitative analysis unit; The magnesium salt addition control mechanism comprises: a calculation unit that determines the amount of magnesium salt to be added to the MAP crystallization reaction tank based on the quantitative analysis results from the phosphate ion quantitative analysis unit; a control unit that issues a control signal for magnesium salt addition from the magnesium salt storage tank; and a magnesium salt supply amount adjustment means that is provided in the magnesium salt supply line.

2. 2. The MAP crystallization reaction apparatus according to claim 1, wherein a solid-liquid separator for separating solid and liquid of sludge before introduction into the reaction tank is provided in an analytical sludge introduction line of the phosphate ion quantitative analysis section.

3. A MAP crystallization method characterized by quantitatively analyzing the phosphate ion concentration in sludge introduced into a MAP crystallization reaction tank on site at predetermined time intervals using a magnesium ion selective electrode, and controlling the amount of magnesium salt to be added based on the quantitatively analyzed phosphate ion concentration.

4. The quantitative analysis of the phosphate ion concentration in sludge using the magnesium ion selective electrode is as follows: A predetermined amount of sludge is introduced into a reaction tank having a magnesium ion selective electrode; Magnesium salt is added to the sludge in the reaction tank, and the detection of magnesium ions is monitored using a magnesium ion selective electrode.

4. The MAP crystallization method according to claim 3, wherein the phosphate ion concentration in the sludge is calculated using Equation 1 based on the amount of magnesium ions reacted with phosphate ions. [Equation 1]

5. The quantitative analysis of the phosphate ion concentration in sludge using the magnesium ion selective electrode is as follows: A predetermined amount of sludge (A0) (L) is introduced into a reaction tank having a magnesium ion selective electrode. A magnesium salt having a predetermined concentration M (mg / L) is added in predetermined amounts to the sludge in the reaction tank, and the detection of magnesium ions by a magnesium ion selective electrode is monitored to determine the end point of the insolubilization reaction.

4. The MAP crystallization method according to claim 3, wherein the phosphate ion concentration (mg-P / L) in the sludge is calculated by the following formula 2 based on the amount of magnesium ions reacted with phosphate ions [M (mg / L) × A (L)], which is calculated from the amount A (L) of magnesium salt added up to the end point of the insolubilization reaction and the concentration M (mg / L) of the magnesium salt: [Equation 2]

6. The quantitative analysis of the phosphate ion concentration in sludge using the magnesium ion selective electrode is as follows: A predetermined amount of sludge (A0) (L) is introduced into a reaction tank having a magnesium ion selective electrode. A predetermined amount A1 (L) of magnesium salt is added to the sludge in the reaction tank, the concentration of magnesium ions is measured using a magnesium ion selective electrode, and the soluble cation concentration M1 (mg / L) at the point where the magnesium ion concentration no longer fluctuates is determined; 4. The MAP crystallization method according to claim 3, wherein the phosphate ion concentration in the sludge is calculated by the following formula 3 based on the consumption amount of magnesium salt, which is determined from the difference between the amount of magnesium ions added, M (mg / L) × A1 (L), and the amount of magnesium ions at the point where the magnesium ion concentration no longer fluctuates, M1 (mg / L) × A0 (L). [Equation 3]

7. The amount of magnesium salt to be added is controlled based on the quantitatively analyzed concentration of phosphate ions in the sludge, (1) An embodiment in which the amount of magnesium salt added is controlled so that the Mg / P ratio in the sludge in the MAP crystallization reaction tank is constant; (2) An embodiment in which the amount of magnesium salt added is controlled so that the magnesium ion concentration in the effluent from the MAP crystallization reaction tank is constant, or (3) An embodiment in which the amount of magnesium salt added is controlled so that the phosphate ion concentration in the effluent from the MAP crystallization reaction tank is constant.

4. The MAP crystallization method according to claim 3, wherein the crystallization is carried out in any one of the following ways:

8. 4. The MAP crystallization method according to claim 3, wherein the amount of magnesium salt to be added based on the quantitatively analyzed concentration of phosphate ions in the sludge is controlled by controlling the amount of magnesium salt to be added so that the Mg / P ratio in the sludge in the MAP crystallization reaction tank is 0.8 or more and 1.2 or less.

9. 4. The MAP crystallization method according to claim 3, wherein the amount of magnesium salt to be added based on the quantitatively analyzed phosphate ion concentration in the sludge is controlled by controlling the amount of magnesium salt to be added so that the magnesium ion concentration in the effluent from the MAP crystallization reaction tank is 60 mg / L or less.

10. 4. The MAP crystallization method according to claim 3, wherein the amount of magnesium salt to be added based on the quantitatively analyzed phosphate ion concentration in the sludge is controlled by controlling the amount of magnesium salt to be added so that the phosphate ion concentration in the effluent from the MAP crystallization reaction tank is 10 mg / L or more and 40 mg / L or less.

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