Sludge treatment method and apparatus

Abstract

The present invention provides a sludge treatment method and apparatus that rapidly grasps the changes in the properties of sludge introduced into a digester and adds an appropriate amount of iron and / or aluminum-containing chemical agent to suppress scale formation based on the phosphate ion concentration in the sludge. [Solution] A cationic agent that insolubilizes phosphate ions is added to the sludge collected from the digester tank 1, and the concentration of specific ions is measured using an ion-selective electrode 31 to quantitatively analyze the phosphate ion concentration in the sludge in the digester tank in situ. Based on the quantitatively analyzed phosphate ion concentration in the sludge in the digester tank, Fe 3+ Source or / and Al 3+ A sludge treatment method characterized by controlling the amount of iron / aluminum chemicals containing the source added to the sludge in the digester, thereby suppressing MAP formation in the digester.

Description

[Technical Field]

[0001] The present invention relates to a sludge treatment method and apparatus, and more particularly to Fe based on the phosphate ion concentration contained in the sludge in the digester. 3+ Source or / and Al 3+ This invention relates to a sludge treatment method and apparatus that controls the amount of chemicals added, including the source, to prevent the crystallization of magnesium ammonium phosphate and the formation of scale caused by the added chemicals in sludge treatment facilities other than magnesium ammonium phosphate crystallization tanks (especially digester tanks), and to suppress the generation of hydrogen sulfide. [Background technology]

[0002] In recent years, with the 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. Biological phosphorus removal utilizes the action of microorganisms called polyphosphate-accumulating bacteria (PAOs) to absorb phosphorus from sewage into the PAOs' bodies in a reaction tank, and then removes it from the water treatment system as excess sludge. However, PAOs release the phosphorus they absorbed in subsequent digesters. In addition, a large amount of ammonia is generated as proteins in the sludge are broken down, so digested sludge is rich in phosphorus and ammonia. Therefore, many treatment plants employ methods to crystallize magnesium ammonium phosphate (MAP), which is produced from phosphorus, ammonia, and magnesium.

[0003] A technology has been developed to remove and recover phosphorus contained in sludge as MAP crystals by adding a magnesium-containing chemical to sludge containing phosphorus and ammonia to adjust the pH to an appropriate level. For example, Japanese Patent Publication No. 2004-941 (Patent Document 1) describes a method for treating organic wastewater or sludge that recovers phosphorus as MAP crystals from wastewater containing high concentrations of organic matter, phosphorus, and nitrogen, such as human waste, dewatered sludge from septic tanks, digested sludge, and wastewater from chemical industries. The method comprises an anaerobic treatment step and a step of removing phosphorus and nitrogen from the organic wastewater or sludge in the form of magnesium ammonium phosphate from the system, wherein the method includes a step of adding a magnesium source in or before the anaerobic treatment step according to the target 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, leading a portion of the sludge to a sludge dewatering step after the recovery of magnesium ammonium phosphate, and returning the remainder of the sludge to the anaerobic treatment step.

[0004] The method described in Patent Document 1 is useful because it can recover not only phosphorus present as dissolved ions in the sludge, but also naturally occurring MAPs in digesters, etc. However, Patent Document 1 describes a method for several thousand m³ of sludge with a typical residence time of about 30 days. 3 In a large-capacity digester, the stirring power required to rapidly disperse the input magnesium source and uniformly flow the MAP seed crystals becomes significant.

[0005] Furthermore, in the MAP crystallization reaction, if the amount of magnesium added is too little, phosphate ions will remain, and conversely, if the amount of magnesium added is too much, the degree of supersaturation will rise above the appropriate range, resulting in the generation of a large amount of fine crystals that cannot be recovered. Therefore, insufficient or excessive magnesium addition for the MAP crystallization reaction will result in phosphate ions remaining in the sludge, causing scale to form in treatment equipment other than the MAP crystallization tank.

[0006] MAP is an inhibitory factor in methane fermentation. Therefore, it is desirable to suppress MAP crystallization in the digester. Japanese Patent Publication No. 2003-275726 (Patent Document 2) describes adding an iron-based flocculant to suppress MAP crystallization and hydrogen sulfide generation when performing methane fermentation under anaerobic conditions.

[0007] To suppress MAP crystallization in digesters, it is desirable to periodically collect sludge from the digester, measure the phosphorus concentration in the sludge, and add an appropriate amount of iron-based coagulant. The official method for phosphate ions is based on the molybdenum blue (ascorbic acid reduction) spectrophotometric method (JIS K 0102-2016) specified in the Industrial Wastewater Testing Methods. However, because the quantitative range of the molybdenum blue spectrophotometric method is low, sludge separation, ultrafiltration, and dilution are required as pretreatment for the water sample, making the procedure complicated and difficult to perform frequently. Furthermore, organic wastewater or sludge containing phosphorus is often relatively viscous and muddy, requiring time and effort to measure the concentration. Moreover, by the time the concentration measurement results are obtained, further changes in the properties of the organic wastewater or sludge may have occurred. Furthermore, while phosphorus recovery from digested sludge and subsequent sludge dewatering are generally carried out as 24-hour continuous processes, the manual analysis of sludge components, especially at night, and the ability to track changes in sludge properties are particularly difficult. Therefore, with such methods, it is difficult to quickly adjust the operating conditions of the crystallization reaction tank to changes in the properties of the input sludge. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2004-941 [Patent Document 2] Japanese Patent Publication No. 2003-275726 [Overview of the project] [Problems that the invention aims to solve]

[0009] Conventionally, since the measurement of phosphate ion concentration in sludge has been performed by official methods, the pretreatment for quantitative analysis is complicated and frequent measurement cannot be performed, making it difficult to add an amount of iron-based flocculant that conforms to fluctuating sludge properties. If the addition amount of the iron-based flocculant is too small, phosphate ions remain in the treated sludge. Phosphate ions remaining in sludge precipitate in pipes and tanks of treatment facilities other than MAP crystallization reaction tanks, adhere to wall surfaces to form scale, and cause problems such as blockage. On the other hand, if an excessive amount of a chemical for crystallizing phosphate ions is added to prevent residual phosphate ions in sludge, scale derived from the chemical is also generated.

[0010] An object of the present invention is to provide a sludge treatment method and apparatus that prevent phosphorus contained in digested sludge from crystallizing in treatment facilities other than MAP crystallization tanks, particularly in digestion tanks. Specifically, even when the sludge in the digestion tank contains sulfide, after accurately grasping the influence of the sulfide, the change in properties of the sludge in the digestion tank is quickly grasped, and an amount of chemical suitable for the phosphate ion concentration in the sludge in the digestion tank is added, thereby suppressing MAP formation in the digestion tank and scale generation in other treatment facilities, and it is an object of the present invention to provide a sludge treatment method and apparatus that suppress the generation of hydrogen sulfide when the sludge in the digestion tank contains sulfide. [Means for Solving the Problem]

[0011] The present invention provides the following sludge treatment apparatus. [1] An anaerobic digestion treatment unit that performs anaerobic digestion treatment on sludge, a phosphate ion quantitative analysis unit that quantitatively analyzes the phosphate ion concentration in sludge, and Fe based on the phosphate ion quantitative analysis result 3+ source or / and Al 3+ and an iron / aluminum chemical addition control mechanism that controls the addition amount of an iron / aluminum chemical containing a source, the anaerobic digestion treatment unit includes a digestion tank that performs anaerobic digestion treatment on sludge, an iron / aluminum chemical supply line that supplies the iron / aluminum chemical to the digestion tank, and an iron / aluminum chemical addition amount adjusting means, Said phosphate ion quantitative analysis unit comprises: a reaction tank provided with an ion-selective electrode; an analytical sludge introduction line for introducing sludge from the digestion tank into the reaction tank; a cationic chemical addition line for adding a cationic chemical that insolubilizes phosphate ions to the reaction tank; and a phosphate ion concentration calculation unit electrically connected to the ion-selective electrode and calculating a phosphate ion concentration based on a measurement result of the ion-selective electrode; The iron / aluminum chemical addition control mechanism comprises: a calculation unit that determines an addition amount of an iron / aluminum chemical to the digestion tank based on a quantitative analysis result from the phosphate ion quantitative analysis unit; and a control unit that outputs a control signal for the iron / aluminum chemical addition amount adjusting means based on the quantitative analysis result from the phosphate ion quantitative analysis unit. [2] The sludge treatment apparatus according to [1] above, wherein a solid-liquid separation device that performs solid-liquid separation on sludge before being introduced into the reaction tank is provided in the analytical sludge introduction line of the phosphate ion quantitative analysis unit. [3] The sludge treatment apparatus according to [1] or [2] above, wherein the ion-selective electrode is a magnesium ion-selective electrode, a calcium ion-selective electrode, or an ammonium ion-selective electrode.

[0012] The present invention also provides the following sludge treatment method. [4] A cationic chemical that insolubilizes phosphate ions is added to sludge collected from a digestion tank, and the phosphate ion concentration in the sludge in the digestion tank is quantitatively analyzed in situ by detection monitoring or concentration measurement of specific ions using an ion-selective electrode, based on the quantitatively analyzed phosphate ion concentration in the sludge in the digestion tank, Fe 3+ source or / and Al 3+ A sludge treatment method characterized in that the addition amount of an iron / aluminum chemical containing a source to sludge in a digestion tank is controlled to suppress MAP formation in the digestion tank. [5] The quantitative analysis of the phosphate ion concentration is performed by: introducing a predetermined amount of sludge collected from the digestion tank into a reaction tank having an ion-selective electrode, The cation agent is added to the sludge in the reaction tank, and the detection of specific ions by the ion-selective electrode is monitored. The sludge treatment method according to [4] above, characterized in that the phosphate ion concentration in the sludge in the digester is calculated using formula 1 based on the amount of specific ions that reacted with phosphate ions.

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[10] The sludge treatment method according to [4] above, wherein the control of the addition amount of the iron / aluminum chemical to the sludge in the digestion tank based on the quantitatively analyzed phosphate ion concentration in the sludge in the digestion tank is performed by controlling the addition amount of said iron / aluminum chemical such that the phosphate ion concentration in the treated sludge drawn out from the digestion tank is not less than 10 mg / L and not more than 40 mg / L.

Effects of the Invention

[0013] According to the present invention, even when the sludge in the digestion tank contains sulfide, the influence of sulfide can be accurately grasped, the change in properties of the sludge in the digestion tank can be quickly grasped, and an amount of Fe suitable for the phosphate ion concentration in the sludge in the digestion tank 3+ source or / and Al 3+ There are provided a sludge treatment method and apparatus that add an iron / aluminum chemical containing a source to suppress MAP formation in a digestion tank and scale generation in other treatment facilities.

[0014] Furthermore, according to the present invention, if the sludge in the digester contains sulfides, the generation of hydrogen sulfide can also be suppressed.

[0015] In the sludge treatment method of the present invention, an appropriate amount of iron / aluminum chemical agent is added in accordance with fluctuations in the phosphate ion concentration in the sludge in the digester, thereby suppressing the amount of unnecessary chemicals added and reducing chemical costs.

[0016] According to the sludge treatment apparatus of the present invention, it is possible to automatically measure the phosphate ion concentration in the sludge in the digester and to automatically control the addition of an appropriate amount of iron / aluminum chemical agent. [Brief explanation of the drawing]

[0017] [Figure 1] Schematic diagram illustrating the sludge treatment apparatus of the present invention. [Figure 2] Flowchart for quantitative analysis of phosphate ions [Figure 3] Flowchart for quantitative analysis of phosphate ions in a titration method where the amount of cationic drug added is determined from the endpoint of the insolubilization reaction. [Figure 4] Flowchart for quantitative analysis of phosphate ions when determining the amount of cationic drug consumed in reaction with phosphate ions. [Figure 5] A schematic diagram showing one embodiment (with pretreatment) of the phosphate ion quantitative analyzer used in Example 1. [Figure 6] A schematic diagram showing one embodiment (without pretreatment) of the phosphate ion quantitative analyzer used in Example 1. [Figure 7] A graph showing the phosphate ion concentration using the official method with pretreatment as in Example 1, and the analytical value obtained by the phosphate ion quantitative analysis method used in the present invention. [Figure 8] A graph showing the phosphate ion concentration using the official method without pretreatment in Example 1, and the analytical value obtained by the phosphate ion quantitative analysis method used in the present invention. [Modes for carrying out the invention]

[0018] The present invention will be described below with reference to the attached drawings. Figure 1 shows a schematic of the sludge treatment apparatus of the present invention. The sludge treatment apparatus of the present invention comprises an anaerobic digestion section for anaerobic digestion of sludge, a phosphate ion quantitative analysis section for quantitative analysis of phosphate ion concentration in sludge, and based on the phosphate ion quantitative analysis results, Fe 3+ Source or / and Al 3+ It includes an iron / aluminum agent addition control mechanism that controls the amount of iron / aluminum agent added, including the source.

[0019] The anaerobic digestion treatment unit includes a digester 1 for anaerobic digestion of sludge, an iron / aluminum chemical supply line 21 for supplying the iron / aluminum chemical to the digester 1, and an iron / aluminum chemical addition amount adjustment means P1. Figure 1 also shows an iron / aluminum chemical storage tank 22 for storing the iron / aluminum chemical supplied to the digester 1.

[0020] The phosphate ion quantitative analysis unit includes a reaction tank 30 equipped with an ion-selective electrode 31, an analytical sludge introduction line 10 for introducing sludge from the digester 1 into the reaction tank 30 and a pump P2 for controlling the amount of analytical sludge supplied, a cationic agent addition line 23 for adding a cationic agent to the reaction tank 30 to insolubilize phosphate ions and a pump P3 for controlling the amount of cationic agent added, and a phosphate ion concentration calculation unit 32 which is electrically connected to the ion-selective electrode 31 and calculates the phosphate ion concentration based on the measurement results of the ion-selective electrode 31. Figure 1 also shows a cationic agent storage tank 24 for storing the cationic agent supplied to the reaction tank 30. In the illustrated embodiment, a pump P3 for controlling the amount of cationic agent added is provided in the cationic agent addition line 23, but it is not limited to a pump as long as it is a mechanism that can supply a predetermined amount of cationic agent to the reaction tank 30.

[0021] The ion-selective electrode 31 is capable of selectively detecting soluble cations derived from the cationic agent or ammonium ions in the sample water as specific ions. For example, when a calcium salt is used as the cationic agent, a calcium ion-selective electrode can be used; when a magnesium salt is used as the cationic agent, a magnesium ion-selective electrode or an ammonium ion-selective electrode can be used. Various commercially available ion meters may be used as the ion-selective electrode 31.

[0022] It is preferable that the reaction tank 30 is equipped with a stirrer (not shown). By providing a stirrer, the sludge and the cationic agent can be brought into uniform and good contact, promoting the insolubilization reaction uniformly and improving the accuracy of detecting the completion of the insolubilization reaction.

[0023] The reaction tank 30 may be equipped with means (not shown) for introducing dilution water such as tap water, as well as pH adjusters and chelating agents. When the reaction tank 30 is in standby mode and not performing quantitative analysis, introducing tap water can clean the inside of the tank and prevent the accumulation of dirt. Also, by keeping tap water in the tank, the adhesion of dirt inside the reaction tank 30 can be prevented. In addition, although not shown, a solid-liquid separation membrane device may be provided in the analytical sludge introduction line 10, and the separated water may be introduced into the reaction tank 30. If the sludge contains insoluble matter or substances that inhibit ion-selective electrodes, removing these by solid-liquid separation can increase the detection sensitivity of specific ions.

[0024] When quantitatively analyzing phosphate ions in the sludge from the digester 1, the sludge collected from the digester 1 is introduced into the reaction tank 30 of the phosphate ion quantitative analyzer via the analytical sludge introduction line 10, and the cationic agent is added to the reaction tank 30 of the phosphate ion quantitative analysis unit via the cationic agent addition line 23. The supply amount of analytical sludge is controlled by pump P2. The amount of cationic agent added is controlled by pump P3.

[0025] The iron / aluminum drug addition control mechanism includes a calculation unit 41 that determines the amount of iron / aluminum drug to be added to the digester 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 iron / aluminum drug based on the quantitative analysis results from the phosphate ion quantitative analysis unit, and an iron / aluminum drug addition amount adjustment means P1 controlled by the control signal from the control unit 42. In Figure 1, the iron / aluminum drug addition amount adjustment means P1 is provided in the iron / aluminum drug supply line 21.

[0026] The calculation unit 41 monitors the specific ion measurement results of the ion-selective electrode 31 of the phosphate ion quantitative analysis unit and calculates the phosphate ion concentration in the sludge and the appropriate amount of iron / aluminum agent to be added, based on the specific ion concentration when predetermined conditions described later are reached. The calculation unit 41 only needs to be electrically connected to the ion-selective electrode 31 and the iron / aluminum agent addition amount adjustment means P1, and may be a calculation device provided in the phosphate ion quantitative analysis unit or a calculation device provided on a server in the cloud.

[0027] The control unit 42 issues a control signal to control the iron / aluminum drug addition amount adjustment means P1 provided in the iron / aluminum drug supply line 21, so that an appropriate amount of iron / aluminum drug is supplied to the digester 1 via the iron / aluminum drug supply line 21, based on the phosphate ion concentration calculated by the calculation unit 31 of the phosphate ion quantitative analysis unit. The control unit 42 only needs to be electrically connected to the calculation unit 41 and the iron / aluminum drug addition amount adjustment means P1 so as to issue a signal to control the operation of the iron / aluminum drug addition amount adjustment means P1 in response to instructions from the calculation unit 41. This control signal may be a control signal generator provided in the phosphate ion quantitative analysis unit or a control signal generator provided on a server in the cloud. In the illustrated embodiment, the iron / aluminum drug addition amount adjustment means P1 is a pump, but is not limited to this, and may be any means known to those skilled in the art that can adjust the addition amount.

[0028] Next, the sludge treatment method of the present invention will be described. The sludge treatment method of the present invention is characterized by quantitatively analyzing the phosphate ion concentration in the sludge in the digester tank at the site and controlling the amount of iron / aluminum agent added based on the quantitatively analyzed phosphate ion concentration. According to the sludge treatment method of the present invention, by supplying a suitable amount of iron / aluminum agent for removing phosphate ions from the sludge in the digester tank based on the results of quantitative analysis of the phosphate ion concentration in the sludge at a high frequency and in a simple manner, it is possible to remove phosphate ions in accordance with fluctuations in the sludge properties, thereby preventing MAP crystallization in the digester tank and suppressing scale formation.

[0029] As for iron / aluminum drugs, trivalent iron ions (Fe 3+ ) or aluminum ions (Al 3+ Drugs containing ) such as ferric polysulfate, iron chloride, polyaluminum chloride, and aluminum sulfate can be suitably used. The reaction between iron ions or aluminum ions and phosphate ions is shown below.

[0030] [ka]

[0031] Sulfate ions (SO4) in the drug 2- ) can be reduced in the digester to produce sulfides and sulfide ions, which may inhibit phosphorus removal or promote hydrogen sulfide generation. Therefore, as iron / aluminum agents, ferric chloride and polyaluminum chloride, etc., are used. 2- Drugs that do not contain, or SO4 for iron 2- A relatively low content of polyferric sulfate is preferred. Among iron-based additives, polyferric sulfate is less corrosive and is therefore preferred from the standpoint of equipment maintenance.

[0032] If the sludge in the digester contains sulfides, Fe 3+It preferentially reacts with sulfide ions rather than phosphate ions to form iron sulfide, suppressing the generation of hydrogen sulfide. When sulfides in the sludge of the digester are consumed, it reacts with phosphate ions to form iron phosphate, thus removing phosphate ions.

[0033] If the sludge in the digester contains sulfides, Al 3+ It preferentially reacts with phosphate ions to form aluminum phosphate, and can remove phosphate ions. 3+ Because it has low reactivity with sulfide ions, it is less affected by sulfides in the sludge in the digester. Therefore, a small amount of Al 3+ This can remove phosphate ions. Also, Al 3+ This method does not change the ion charge and can stably remove phosphate ions.

[0034] If the sludge in the digester contains sulfides, adding a chemical containing both iron and aluminum will result in Fe 3+ It reacts with sulfide ions to suppress the generation of hydrogen sulfide, Al 3+ This is more preferable because it can react with phosphate ions to stably remove them.

[0035] The present invention is characterized by determining the amount of iron / aluminum agent to be added to the sludge in the digester based on the results of in-situ quantitative analysis of the phosphate ion concentration in the sludge in the digester. The quantitative analysis of the phosphate ion concentration in the sludge in the digester can be performed by taking a sample of the sludge from the digester, introducing it into a reaction vessel equipped with an ion-selective electrode, adding a cationic agent that can react with phosphate ions to make them insoluble, and measuring the concentration of a specific ion using the ion-selective electrode. When an amount of cationic agent exceeding the stoichiometric ratio of a specific ion to phosphate ions is added, the specific ion remains and is detected by an ion-selective electrode. The titration termination point is when the detection of the specific ion by the ion-selective electrode increases sharply or when the concentration of the specific ion stops changing. The phosphate ion concentration in the sludge is determined by the amount of phosphate ions at the stoichiometric ratio of the specific ion amount immediately before the end of the titration.

[0036] The reaction between phosphate ions and cationic agents can be represented by the following reaction equation.

[0037] [ka]

[0038] As shown in the reaction equations (1) or (2) above, when an amount of cationic agent (calcium salt in reaction equation (1), magnesium salt in reaction equation (2)) exceeding the stoichiometric ratio of each cation to the phosphate ion is added, the cations that were not consumed in the insolubilization reaction (Ca in reaction equation (1)) 2+ In reaction equation (2), Mg 2+ The remaining ions are detected by ion-selective electrodes (calcium ion-selective electrode in reaction equation (1), magnesium ion-selective electrode in reaction equation (2)) and the soluble cations are detected. The point at which the detection of soluble cations by the ion-selective electrode increases sharply is the endpoint of the insolubilization reaction (titration endpoint), and the amount of phosphate ions in stoichiometric ratios of the amount of cations (calcium ions in reaction equation (1), magnesium ions in reaction equation (2)) just before the end of the titration becomes the phosphate ion concentration in the sample water.

[0039] Furthermore, in the above reaction equation (2), when a magnesium salt is added as a cationic agent, NH4 is added to the insolubilization reaction. + The ions are consumed and the soluble cation (NH4) is detected by an ion-selective electrode (ammonium ion-selective electrode). + The concentration of phosphate ions decreases. The point at which the fluctuation in soluble cation detection by the ion-selective electrode ceases is the endpoint of the insolubilization reaction (titration termination point), and the amount of phosphate ions added immediately before the end of the titration in stoichiometric ratio to the amount of cations (magnesium ions) becomes the phosphate ion concentration in the sample water.

[0040]

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[0041] For example, in reaction equation (1), 3 moles of phosphate ions react with 5 moles of calcium ions, and in reaction equation (2), 1 mole of phosphate ions reacts with 1 mole of magnesium or ammonium ions. Therefore, in reaction equation (1), α in equation 1 and equations 2-3 described later is 3 / 5, and in reaction equation (2), α in equation 1 and equations 2-3 described later is 1.

[0042] Figure 2 shows a flowchart of the phosphate ion quantitative analysis method. Step S1 involves introducing sludge from the digester into a reaction vessel equipped with an ion-selective electrode, adding a cationic agent to the sludge in the reaction vessel (Step S2), monitoring the detection of specific ions by the ion-selective electrode (Step S3), and calculating the phosphate ion concentration in the sludge according to Equation 1 based on the amount of specific ions that reacted with phosphate ions, which is determined by the detection trends of the specific ions (Step S4).

[0043] In the phosphate ion quantitative analysis method used in the present invention, the phosphate ion concentration can be calculated by (A) adding a fixed amount of cationic agent and determining the amount of specific ions added from the endpoint of the insolubilization reaction where the concentration of specific ions rapidly increases or the fluctuation of the concentration of specific ions stops (titration method), or (B) adding an excess of cationic agent and determining the amount of specific ions that reacted with phosphate ions up to the point where the concentration of specific ions stops fluctuating.

[0044] Figure 3 shows a flowchart of the phosphate ion quantitative analysis method when (A) a titration method is used to determine the amount of a specific ion added from the endpoint of the insolubilization reaction. First, a predetermined amount A0 (L) of sludge from the digester is introduced into a reaction vessel equipped with an ion-selective electrode (Step S1). Next, a cationic agent is added to the sludge in the reaction vessel at a predetermined concentration M (mg / L) and a constant flow rate L (L / hr) (Step S2-1). The detection of specific ions by the ion-selective electrode is monitored (Step S3). Steps S2-1 to S3 are repeated until the concentration of the specific ions rises rapidly (Step S4-1-1). The titration endpoint is defined as the point just before the rapid increase in the concentration of the specific ions (Step S4-1-2). The phosphate ion concentration (mg-P / L) is calculated according to the following formula 2 based on the amount of cationic agent added at the titration endpoint A (L) (= L (L / hr) × [elapsed time to the titration endpoint] (hr)) and the concentration M (mg / L) (S4-1-3).

[0045]

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[0046] Figure 4 shows a flowchart of the phosphate ion quantitative analysis method when (B) the method is used to determine the amount of specific ions that reacted with phosphate ions up to the point where the concentration of the specific ion stops fluctuating. First, a predetermined amount A0 (L) of sludge from the digester is introduced into a reaction vessel equipped with an ion-selective electrode (Step S1). Next, a predetermined concentration M (mg / L) of cationic agent and an excess amount A1 (L) are introduced into the sludge in the reaction vessel (Step S2-2). The detection of specific ions by the ion-selective electrode is monitored (Step S3). Step S3 is repeated until the concentration of the specific ions stops fluctuating (Step S4-2-1). The specific ion concentration M1 (mg / L) at the point where the specific ion concentration stops fluctuating is determined, and the phosphate ion concentration in the sample water is calculated according to the following formula 3 based on the amount of specific ions that reacted with phosphate ions, which is determined from the difference between the amount of specific ions added [M (mg / L) × A1 (L)] and the amount of specific ions at the point where the specific ion concentration stops fluctuating [M1 (mg / L) × A0 (L)] (Step S4-2-2).

[0047]

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[0048] In quantitative phosphate analysis, calcium chloride, calcium hydroxide, magnesium chloride, and magnesium hydroxide are particularly preferred as cationic agents, with neutral salts that exhibit little pH fluctuation, such as calcium chloride and magnesium chloride, being even more preferred. If the concentration ratio of ammonium nitrogen (NH4-N) to phosphate phosphorus (PO4-P) in the water sample is 1.6 or less, preferably 0.45 or less, and more preferably no ammonium ions are present, it is preferable to use the above cationic agents other than magnesium salts. If the concentration ratio of ammonium nitrogen to phosphate phosphorus in the water sample exceeds 1.6, it is preferable to use a magnesium salt as the cationic agent.

[0049] If the ammonium ion concentration in the water sample exceeds 1.6 in terms of the concentration ratio of ammonium nitrogen to phosphate phosphorus, a magnesium salt can be used as a cationic agent, and ammonium ions can be detected as a soluble cation. The phosphate ion concentration can be calculated by (A) adding a fixed amount of magnesium ions at a time, consuming ammonium ions and phosphate ions according to the above reaction equation (2), and determining the amount of magnesium ions added from the endpoint of the insolubilization reaction where no further decrease in ammonium ion concentration is observed (titration method); or (B) adding an equal or greater amount of magnesium ions to determine the amount of ammonium ions consumed by the reaction with phosphate ions up to the point where the ammonium ion concentration no longer fluctuates.

[0050] An ion-selective electrode is an electrode that generates an electric potential in response to the activity of a specific ion (JIS K0122-1997 3.(1)). In the quantitative analysis of phosphate ions of the present invention, an ion-selective electrode is used that generates an electric potential in response to the activity of a cation (specific ion) derived from a cationic agent that insolubilizes phosphate ions. That is, when a calcium salt is used as the cationic agent, the specific ion is Ca 2+When using magnesium salts as cationic agents, the specific ion is Mg 2+ or NH4 + An example of a combination of a cationic agent and an ion-selective electrode is a calcium salt and calcium ion (Ca 2+ ) Selective electrode, magnesium salt and magnesium ion (Mg 2+ ) Selective electrode, magnesium salt and ammonium ion (NH4 + Selective electrodes can be used as ion-selective electrodes. Commercially available calcium ion analyzers, magnesium ion analyzers, and ammonium ion analyzers can also be used. By using an ammonium ion analyzer, which is becoming increasingly practical, inexpensive and highly accurate measurements can be performed.

[0051] If the sample water contains coexisting ions that inhibit the reaction of reaction equation (1) or (2) above, or inhibitory substances that reduce the detection sensitivity of the ion-selective electrode, the measurement accuracy can be improved by adding a pH adjuster to control the pH to an appropriate range to preferentially allow the reaction of reaction equation (1) or (2) above to proceed, or by using a chelating agent to insolubilize or dilute the coexisting ions, thereby reducing side reactions of the coexisting ions or inhibitory substances.

[0052] Fluctuations in pH can shift the equilibrium of the insolubilization reaction or alter the concentration of ammonium ions, potentially compromising the accuracy of quantitative analysis of phosphate ion concentration. Furthermore, the equilibrium of the insolubilization reaction may also shift depending on the concentrations of calcium, magnesium, and ammonium in reaction equation (1) or (2). It is desirable to maintain the pH of the sample water between 7.5 and 8.5, preferably between 7.5 and 8.0, and this can be adjusted by adding a pH adjusting agent as appropriate. Additionally, the calcium ion, magnesium ion, and ammonium ion concentrations in the sample water before the addition of the cationic agent can be measured using corresponding ion-selective electrodes, and the measured values ​​of the quantitative analysis of phosphate ions can be corrected.

[0053] Furthermore, since ion-selective electrodes are used under conditions that induce insolubilization reactions, scale tends to adhere to the electrodes. To suppress or remove such deposits, it is preferable to periodically clean the electrodes with water containing low concentrations of acids, chelating agents, or scale-causing substances, such as tap water, industrial water, or process water from treatment facilities, or to immerse the electrodes in such water to remove deposits that cause scale formation, either when abnormalities are observed in the measurement values ​​or when abnormalities are detected. It is also preferable to periodically perform physical cleaning, such as ultrasonic cleaning.

[0054] Even when the water sample contains a large amount of solids, solid-liquid separation is not essential, and measurement can be performed simply without pretreatment such as solid-liquid separation. However, it is also possible to perform solid-liquid separation beforehand and use the separated water as the sample. By performing solid-liquid separation beforehand, coexisting ions and inhibiting substances can be eliminated, allowing the insolubilization reaction to proceed more stably.

[0055] If an iron / aluminum agent is added in a stoichiometric ratio close to the phosphate ion concentration in the sludge of the digester, phosphate ions will be removed without excess or deficiency. However, the properties of the sludge in the digester vary due to various factors. In this invention, the amount of iron / aluminum agent to be added, based on the phosphate ion concentration in the sludge of the digester, which is quantitatively analyzed in situ, can be controlled in the following manner. (1) A method of controlling the amount of iron / aluminum chemical added so that the ratio of specific ions / P in the sludge inside the digester tank remains constant. (2) A method of controlling the amount of iron / aluminum chemical added so that the phosphate ion concentration in the sludge drawn from the digester remains constant.

[0056] Each aspect will be described below. (1) A method of controlling the amount of iron / aluminum chemical added so that the ratio of specific ions / P in the sludge inside the digester tank remains constant. In this embodiment, an amount of iron / aluminum agent is added to maintain a preset specific ion / P ratio based on the measured phosphate ion concentration in the sludge of the digester, so that a suitable amount of specific ions is present in the digester that follows the fluctuations in the phosphate ion concentration in the sludge of the digester. By setting the specific ion / P ratio to more than 1.0, almost all of the phosphate ions in the sludge of the digester can be removed.

[0057] The ratio of specific ion to P should preferably be 1.00 or more and 1.25 or less, more preferably 1.05 or more and 1.20 or less, and more preferably 1.10 or more and 1.15 or less. For example, if the measured concentration of phosphate ions (PO4-P) in the sludge of the digester is 350 mg / L, the molar concentration of P will be 350 / 31 = 11.3 mmol. In terms of stoichiometric ratio (specific ion / P = 1), if the specific ion is trivalent iron ions, then 55.8 × 11.3 = 630.5 mg / L of iron ions will be required in the sludge of the digester, and if the specific ion is trivalent aluminum ions, then 27 × 11.3 = 305 mg / L of aluminum ions will be required in the sludge of the digester. When setting the specific ion / P ratio to 1.1, adjust the amount of iron / aluminum agent added so that approximately 693.5 mg / L (= 630.5 × 1.1) of iron ions and approximately 336 mg / L (= 305 × 1.1) of aluminum ions are supplied.

[0058] (2) A method of controlling the amount of iron / aluminum chemical added so that the phosphate ion concentration in the sludge drawn from the digester remains constant. In this embodiment, to prevent an excess of specific ions by ensuring that specific ions are always present in slightly less than the stoichiometric ratio in the digester, an amount of iron / aluminum agent is added to the sludge drawn from the digester, based on the measured concentration of phosphate ions in the sludge within the digester, so that a certain amount of phosphate ions remain in the sludge. Depending on how the treated sludge is used, it may be preferable that a predetermined amount of phosphate ions remain without being completely removed. This embodiment is a suitable control method when it is desirable for a predetermined amount of phosphate ions to remain in the sludge drawn from the digester.

[0059] The concentration of phosphate ions remaining in the treated sludge withdrawn from the digester should preferably be 10 mg / L or higher, more preferably 20 mg / L or higher. If a large amount of phosphate ions remain in the treated sludge, there is a risk that MAP may crystallize in the piping downstream of the MAP crystallization reaction tank, so it is desirable to keep it at 40 mg / L or lower, more preferably 30 mg / L or lower. For example, if the measured concentration of phosphate ions (PO4-P) in the sludge of the digester is 350 mg / L, and the phosphate ion concentration in the treated sludge withdrawn from the digester is to be maintained at 20 mg / L, the molar concentration of P to be removed will be (350-20) / 31 = 10.6 mmol. If the specific ion is trivalent iron ions, the amount of iron / aluminum agent added will be adjusted so that there are 55.8 × 10.6 = 591.5 mg / L of iron ions in the sludge of the digester, and if the specific ion is trivalent aluminum ions, the amount of aluminum ions added will be adjusted so that there are 27 × 10.6 = 286.2 mg / L of aluminum ions in the sludge of the digester.

[0060] The present invention provides a sludge treatment method that allows for the suppression of MAP formation, prevention of scale formation, or control of residual phosphate ion concentration in the digester, by measuring the phosphate ion concentration in the sludge in the digester, whose properties fluctuate due to various factors, in a timely manner and controlling the amount of iron / aluminum agent added. This enables the sludge digestion reaction to follow the fluctuations in sludge properties, thereby enabling desired behaviors such as suppression of MAP formation, prevention of scale formation, or control of residual phosphate ion concentration in the digester. Furthermore, if the sludge in the digester contains sulfides, the generation of hydrogen sulfide can be suppressed. [Examples]

[0061] [Example 1] Analysis of phosphate ion concentration in digested sludge As a sample, digested sludge from a sewage treatment plant having the properties shown in Table 1 was used. Magnesium chloride (10 mmol) was used as a cationic agent to insolubilize phosphate ions, and a magnesium ion-selective electrode was used as the ion-selective electrode. Quantitative analysis of the phosphate ion concentration shown in Figure 3 was performed using the quantitative analyzer shown in Figure 5 or Figure 6. As a pretreatment for quantitative analysis, the digested sludge was passed through a separation membrane for solid-liquid separation (Figure 6: with pretreatment), the digested sludge was analyzed directly without solid-liquid separation (Figure 5: without pretreatment), and quantitative analysis was performed using the official method (molybdenum blue spectrophotometric method). The results were compared. The results are shown in Table 2 and Figures 7 (without pretreatment) to 8 (with pretreatment).

[0062] [Table 1]

[0063] [Table 2]

[0064] The quantitative analysis method used in this invention, in the high concentration range of 178 mg-P / L to 315 mg-P / L, falls within ±10 mg of the analytical values ​​obtained by the official method, regardless of whether or not the digested sludge is pretreated. As shown below, it was confirmed to be a highly accurate and simple analytical method with a high correlation. Pre-processing applied: R 2 =0.986 [Official method phosphate ion concentration] = 0.9424 × [Analytical value of phosphate ion concentration] + 20.274 No pretreatment: R 2 =0.978 [Official phosphate ion concentration] = 0.8892 × [Analytical value of phosphate ion concentration] + 30.622

[0065] [Example 2] At sewage treatment plant A, which employs anaerobic sludge digestion, a pilot facility in the digester tank was used to process mixed raw sludge at a flow rate of 1.0 m³. 3The sample was added to the digester at a rate of / h, and ferric polysulfate (11% w / w, specific gravity 1.45) was used as the iron / aluminum agent. The system was operated for approximately 3 months, and the risk of scale formation was evaluated by observing the increase in the torque of the digester's agitator.

[0066] The phosphate ion concentration in the sludge of the digester was measured twice a day, and the flow rate of ferric polysulfate was repeatedly adjusted to achieve a Fe / P ratio of 1.1 relative to the measured phosphate ion concentration. This process was repeated for approximately three months. Table 3 shows the results of measuring the phosphate ion (PO4-P) concentration in the sludge of the digester, the flow rate of ferric polysulfate, and the phosphate ion (PO4-P) concentration in the withdrawn sludge from the start of operation up to 30 days later, and calculating the phosphate ion (PO4-P) removal rate. Table 6 shows the results of checking the amount of ferric polysulfate used and the increase in agitator torque after approximately three months of operation.

[0067] [Table 3]

[0068] The phosphate ion removal rate was high, ranging from 95.7% to 98.3%. The residual phosphate ions in the extracted sludge were low, ranging from 3.2 mg / L to 8.9 mg / L, with an average of 5.5 mg / L. It is thought that the reaction between phosphate ions and iron ions proceeded at an appropriate rate because the flow rate of ferric polysulfate was changed in response to fluctuations in the phosphate ion concentration in the sludge within the digester.

[0069] [Control System 1] At sewage treatment plant A, which employs anaerobic sludge digestion, a pilot facility in the digester tank was used to process mixed raw sludge at a flow rate of 1.0 m³. 3The system was introduced into the digester at a rate of / h, and ferric polysulfate (11% w / w, specific gravity 1.45) was used as the iron / aluminum agent. Control system 1 was the conventional operating method, and the initial phosphate ion concentration in the sludge of the digester was measured using the molybdenum method. The phosphate ion concentration in the sludge of the digester was set to 250 mg / L, which was measured at the time of design. Since the molar concentration of P is 250 / 31 = 8.1 mmol, the amount of Fe required to remove all of this P is 55.8 × 8.1 = 452 mg / L. Ferric polysulfate was continuously added at a flow rate of 590 L / d to add 452 × 1.1 = 497.2 mg / L of Fe, which is an excess Fe / P ratio of 1.1 compared to the stoichiometric ratio of Fe to P, and the system was operated for approximately 3 months. Table 4 shows the results of measuring the phosphate ion (PO4-P) concentration in the digester sludge, the ferric polysulfate flow rate, and the phosphate ion (PO4-P) concentration in the extracted sludge from the start of operation up to 30 days later, and calculating the phosphate ion (PO4-P) removal rate. Table 6 shows the results of checking the amount of ferric polysulfate used and the increase in agitator torque after approximately 3 months of operation.

[0070] [Table 4]

[0071] The measured phosphate ion concentration in the sludge within the digester fluctuated significantly during the one-month operation period, ranging from 197 mg / L to 241 mg / L, and was lower than the set value of 250 mg / L. The phosphate ion removal rate was in the range of 95.1% to 96.4%, which was lower than that of Example 2.

[0072] [Example 3] The phosphate ion concentration in the sludge inside the digester and the phosphate ion concentration in the treated sludge withdrawn from the digester were measured twice a day. Ferric polysulfate was added repeatedly until the phosphate ion concentration in the treated sludge reached 20 mg / L, and the system was operated for approximately 3 months. The phosphate ion (PO4-P) concentration in the sludge inside the digester, the flow rate of ferric polysulfate, and the phosphate ion (PO4-P) concentration in the withdrawn sludge were measured from the start of operation up to 30 days later, and the phosphate ion (PO4-P) removal rate was calculated, as shown in Table 5. After approximately 3 months of operation, the amount of ferric polysulfate used and the increase in agitator torque were confirmed, as shown in Table 6.

[0073] [Table 5]

[0074] The flow rate of ferric polysulfate was adjusted so that 20 mg / L of phosphate ions remained in the treated sludge (extracted sludge). As a result, the phosphate ion removal rate was lower than that of control system 1, at 88.3% to 91.7%, but the amount of ferric polysulfate used was 10.1 m³. 3 This was lower than in Example 2.

[0075] [Table 6]

[0076] In control system 1, the phosphate ion concentration in the sludge within the digester was fixed to the initial setting, and the amount of ferric polysulfate added was also fixed, resulting in a usage of 53.1 m³ of ferric polysulfate. 3 This resulted in the highest reading. Furthermore, a torque increase of over 20% in the agitator was observed, suggesting that excess ferric polysulfate was remaining in the sludge in the digester. This means that excess ferric polysulfate is present not only in the sludge in the digester but also in the treated sludge, indicating a high risk of scale formation in the treatment facility.

[0077] In Example 2, the phosphate ion concentration in the sludge of the digester was measured twice a day, and the amount of ferric polysulfate added was frequently adjusted based on the measured phosphate ion concentration. As a result, the amount of ferric polysulfate used was less than in Control System 1 (36.5 m³). 3 The excessive addition of iron ions was suppressed. Both phosphate ions and iron ions were consumed in the digester, and the residual amounts in the sludge withdrawn from the digester were significantly reduced. No increase in agitator torque was observed, and it was determined that the risk of MAP formation in the digester and scale generation in the treatment equipment was low.

[0078] In Example 3, the amount of ferric polysulfate added was adjusted to keep the phosphate ion concentration in the sludge drawn from the digester constant (20 mg / L), so the amount of ferric polysulfate used was 30.4 m 3 The torque was lowest in this case. The increase in agitator torque was 10-20%, and the risk of scale formation was judged to be lower than in control system 1 but higher than in Example 2. Although the increase in agitator torque was slightly higher compared to Example 2 because the phosphate ion concentration in the sludge drawn from the digester was maintained at 20 mg / L, it is thought that setting the phosphate ion concentration in the drawn sludge lower than 20 mg / L would improve the risk of MAP formation in the digester and scale formation in the treatment equipment. [Explanation of Symbols]

[0079] 1: Digestion tank 10: Sludge introduction line for analysis P2: Mechanism for adjusting the amount of sludge collected for analysis (pump) 21: Iron / aluminum chemical additive line 22: Iron / aluminum chemical storage tank P1: Means for adjusting the amount of iron / aluminum chemical added (pump) 23::Cational drug supply line 24: Cationic drug storage tank P3: Means for adjusting the amount of cationic agent added (pump) 31: Reaction vessel 31: Ion-selective electrodes 32: Phosphate ion concentration calculation unit 41: Arithmetic section 42: Control Unit

Claims

1. An anaerobic digestion section for anaerobic digestion treatment of sludge, a phosphate ion quantitative analysis section for quantitative analysis of phosphate ion concentration in the sludge, and Fe based on the phosphate ion quantitative analysis results ３＋ Source or / and Al ３＋ It includes an iron / aluminum agent addition control mechanism that controls the amount of iron / aluminum agent added, including the source, The anaerobic digestion treatment unit includes a digester for anaerobic digestion of sludge, an iron / aluminum chemical supply line for supplying the iron / aluminum chemical to the digester, and means for adjusting the amount of iron / aluminum chemical added. The phosphate ion quantitative analysis unit includes a reaction tank equipped with an ion-selective electrode, an analytical sludge introduction line for introducing sludge from the digester into the reaction tank, a cationic agent addition line for adding a cationic agent to the reaction tank to insolubilize phosphate ions, and a phosphate ion concentration calculation unit that is electrically connected to the ion-selective electrode and calculates the phosphate ion concentration based on the measurement results of the ion-selective electrode. The sludge treatment apparatus is characterized in that the iron / aluminum chemical additive control mechanism includes a calculation unit that determines the amount of iron / aluminum chemical additive to be added to the digester based on the quantitative analysis results from the phosphate ion quantitative analysis unit, and a control unit that issues a control signal for the iron / aluminum chemical additive amount adjustment means based on the quantitative analysis results from the phosphate ion quantitative analysis unit.

2. The sludge treatment apparatus according to claim 1, characterized in that the analytical sludge introduction line of the phosphate ion quantitative analysis unit is provided with a solid-liquid separation device for separating the sludge into solid and liquid components before it is introduced into the reaction tank.

3. The sludge treatment apparatus according to claim 1 or 2, characterized in that the ion-selective electrode is a magnesium ion-selective electrode, a calcium ion-selective electrode, or an ammonium ion-selective electrode.

4. A cationic agent that insolubilizes phosphate ions is added to the sludge collected from the digester, and the concentration of phosphate ions in the sludge in the digester is quantitatively analyzed in situ by measuring the concentration of specific ions using an ion-selective electrode. Based on the phosphate ion concentration in the sludge inside the digester tank, which was quantitatively analyzed, Fe ３＋ Source or / and Al ３＋ A sludge treatment method characterized by controlling the amount of iron / aluminum chemicals containing the source added to the sludge in the digester, thereby suppressing MAP formation in the digester.

5. The quantitative analysis of the phosphate ion concentration is performed as follows: A predetermined amount of sludge collected from the digester is introduced into a reaction vessel having an ion-selective electrode. The cation agent is added to the sludge in the reaction tank, and the detection of specific ions by the ion-selective electrode is monitored. The sludge treatment method according to claim 4, characterized in that the concentration of phosphate ions in the sludge in the digester is calculated using formula 1 based on the amount of specific ions that reacted with phosphate ions. [Math 1]

6. The quantitative analysis of the phosphate ion concentration is performed as follows: A predetermined amount A0 (L) of sludge collected from the digester is introduced into a reaction vessel equipped with an ion-selective electrode. A predetermined amount of the cationic agent at a predetermined concentration M (mg / L) is added to the sludge in the reaction tank, and the detection of specific ions using an ion-selective electrode is monitored to determine the endpoint of the insolubilization reaction. The sludge treatment method according to claim 4, characterized in that the phosphate ion concentration (mg-P / L) in the sludge in the digester is calculated by formula 2 below, based on the amount of specific ions that reacted with phosphate ions [M (mg / L) × A (L)] determined from the amount of cationic agent added A (L) up to the endpoint of the insolubilization reaction and the concentration M (mg / L) of the cationic agent. [Math 2]

7. The quantitative analysis of the phosphate ion concentration is performed as follows: A predetermined amount A0 (L) of sludge collected from the digester is introduced into a reaction vessel equipped with an ion-selective electrode. A predetermined concentration M (mg / L) and a predetermined amount A1 (L) of the cationic agent are added to the sludge in the reaction tank, the concentration of the specific ion is measured using an ion-selective electrode, and the specific ion concentration M1 (mg / L) at which the specific ion concentration stops fluctuating is determined. The sludge treatment method according to claim 4, characterized in that the phosphate ion concentration in the sludge in the digester is calculated using the following formula 3, based on the amount of specific ions that reacted with phosphate ions, which is determined from the difference between the amount of specific ions added [M (mg / L) × A1 (L)] and the amount of specific ions at the point where the concentration of specific ions stops fluctuating [M1 (mg / L) × A0 (L)]. [Math 3]

8. The control of the amount of iron / aluminum agent added to the sludge in the digester tank based on the phosphate ion concentration in the sludge in the digester tank, as quantitatively analyzed, (1) A method of controlling the amount of the iron / aluminum agent added so that the ratio of specific ions / P in the sludge in the digester tank remains constant, or (2) A mode in which the amount of iron / aluminum agent added is controlled so that the phosphate ion concentration in the treated sludge withdrawn from the digester remains constant. The sludge treatment method according to claim 4, characterized in that it is carried out in any of the following ways.

9. The sludge treatment method according to claim 4, characterized in that the control of the amount of iron / aluminum agent added to the sludge in the digester based on the phosphate ion concentration in the sludge in the digester that has been quantitatively analyzed is performed by controlling the amount of iron / aluminum agent added so that the ratio of specific ions / P in the sludge in the digester is 1.00 or more and 1.25 or less.

10. The sludge treatment method according to claim 4, characterized in that the control of the amount of iron / aluminum agent added to the sludge in the digester tank based on the phosphate ion concentration in the sludge in the digester tank that has been quantitatively analyzed is performed by controlling the amount of iron / aluminum agent added so that the phosphate ion concentration in the treated sludge withdrawn from the digester tank is 10 mg / L or more and 40 mg / L or less.

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