JP2018199101A - Sludge treatment system and sludge treatment method - Google Patents

Abstract

To provide a sludge treatment system and a sludge treatment method capable of efficiently recovering and removing phosphorus in digested sludge without using medicine.SOLUTION: A sludge treatment system according to an embodiment includes: a digestion tank, a MAP production tank, a carbon dioxide diffuser, and a magnesium source supply device. The digestion tank serves as digesting organic waste to obtain digested sludge and biogas. The MAP production tank serves as stirring and mixing the digested sludge or a solubilized product of the digested sludge and the magnesium source to obtain magnesium ammonium phosphate (MAP) and dephosphorylated product. The carbon dioxide diffuser serves as diffusing carbon dioxide into the digested sludge or the solubilized product of the digested sludge in the MAP production tank. The magnesium source supply device serves as supplying a magnesium source to the MAP production tank.SELECTED DRAWING: Figure 1

Description

  Embodiments described herein relate generally to a sludge treatment system and a sludge treatment method.

  An example of a method for recovering and removing phosphorus in a liquid to be treated in a wastewater treatment facility is an anaerobic aerobic method (AO method). In this AO method, first, microorganisms release phosphoric acid into the liquid to be treated in an anaerobic tank. Thereafter, the liquid to be treated is sent to the aerobic tank together with the microorganisms for aerobic treatment. At this time, the microorganisms sent to the aerobic tank ingest phosphorus more than the amount of phosphoric acid released in the anaerobic tank as polyphosphoric acid. Therefore, a large amount of phosphorus is contained in excess sludge, which is a microorganism that proliferated excessively in the aerobic tank. This surplus sludge is sent to a sludge treatment facility as sewage sludge and processed.

  In the sludge treatment facility, organic waste including sewage sludge sent from the wastewater treatment facility and wastewater sent from the food industry is processed. The organic waste sent to the sludge treatment facility is subjected to anaerobic treatment (digestion treatment) in a digestion tank to produce digested sludge and biogas. At this time, the above-described microorganism containing a large amount of phosphorus releases phosphoric acid into the digested sludge. Therefore, the detachment liquid obtained by dewatering digested sludge contains a large amount of phosphorus. This desorbed liquid is returned to the wastewater treatment facility for wastewater treatment. However, if a desorbed liquid containing a large amount of phosphorus is sent to a wastewater treatment facility, the phosphorus load on the wastewater treatment facility increases. Therefore, removal of phosphorus in digested sludge or detachment liquid is performed.

  A MAP (Magnesium Ammonium Phosphate) method is known as a method for collecting phosphorus in sludge treatment. In the MAP method, in order to improve the recovery efficiency of phosphorus, the pH of the liquid to be treated is adjusted to be alkaline, so that crystals containing phosphorus are crystallized and grown, and the crystals are collected to recover the liquid in the liquid to be treated. Collect and remove phosphorus.

Japanese Patent Laid-Open No. 2015-20160

  The problem to be solved by the present invention is to provide a sludge treatment system and a sludge treatment method that can efficiently recover and remove phosphorus in digested sludge without using chemicals.

The sludge treatment system of the embodiment includes a digestion tank, a MAP generation tank, a carbon dioxide aeration device, and a magnesium source supply device.
The digester digests organic waste to obtain digested sludge and biogas.
In the MAP production tank, the digested sludge or the solubilized digested sludge and a magnesium source are stirred and mixed to obtain magnesium ammonium phosphate (MAP) and a dephosphorized product.
The carbon dioxide diffuser diffuses carbon dioxide into the digested sludge or the solubilized digested sludge in the MAP production tank.
The magnesium source supply device supplies a magnesium source to the MAP production tank.

The schematic diagram of the sludge processing system which is a 1st embodiment. The schematic diagram of the sludge processing system which is 2nd Embodiment. The schematic diagram of the sludge processing system which is 3rd Embodiment. The figure which shows the relationship between pH of digested sludge and tap water, and the aeration time of a carbon dioxide. The figure which shows the relationship between the phosphate phosphorus density | concentration in digested sludge, and the stirring time after a magnesium source addition. The figure which shows the result of Example 1-2.

  Hereinafter, a sludge treatment system and a sludge treatment method of an embodiment will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a diagram showing a sludge treatment system of the present embodiment.
The sludge treatment system 100 of the present embodiment includes a digestion tank 1, a MAP generation tank 2, a carbon dioxide aeration device 4, and a magnesium source supply device 5.
Moreover, as shown in FIG. 1, the sludge treatment system 100 may include a dehydration device 3, a pH meter 6, and a flocculant supply device 7.

The digestion tank 1 digests the organic waste sent from the wastewater treatment facility (not shown) through the organic waste inflow line with anaerobic microorganisms, and decomposes it into carbon dioxide, methane, ammonia, and digested sludge. It is a tank which performs a digestion process. The organic waste sent to the digester 1 contains sewage sludge, and this sewage sludge contains excess sludge.
The digestion tank 1 is connected to the MAP generation tank 2 through a digested sludge feed mud line. In addition, the digestion tank 1 can discharge biogas to the outside of the digestion tank 1 through a biogas air supply line (not shown).

  The MAP production tank 2 is a tank in which the digested sludge sent from the digestion tank 1 and a magnesium source are stirred and mixed to produce magnesium ammonium phosphate (MAP) to obtain MAP and a dephosphorized product. In addition, in this embodiment, a dephosphorization processed material means the digested sludge after phosphorus is removed by the MAP production tank 2. FIG. The MAP generation tank 2 includes a stirring means (not shown).

  The MAP production tank 2 is connected to the dehydration device 3 via a dephosphorized material feed mud line. Further, the MAP generation tank 2 can discharge MAP to the outside of the MAP generation tank 2 through the MAP discharge line.

The carbon dioxide diffuser 4 is a device that diffuses carbon dioxide into the digested sludge in the MAP production tank 2. The carbon dioxide diffused by the carbon dioxide diffuser 4 is, for example, carbon dioxide in a commercially available carbon dioxide cylinder, carbon dioxide in the atmosphere, carbon dioxide in biogas generated in the digestion tank 1, Examples include carbon dioxide obtained by baking the produced biogas.
The carbon dioxide diffuser 4 is connected to the MAP generation tank 2 via a carbon dioxide supply line.

The magnesium source supply device 5 is a device that supplies a magnesium source serving as a seed crystal into the MAP generation tank 2. Examples of the magnesium source include magnesium chloride and magnesium hydroxide.
The magnesium source supply device 5 is connected to the MAP generation tank 2 via a magnesium source supply line.

  The pH meter 6 is a measuring instrument that measures the pH of the digested sludge in the MAP production tank 2.

  The dehydrating device 3 is a device that dehydrates the dephosphorization treatment sent from the MAP generation tank 2 to obtain dehydrated sludge and a desorbed liquid. Examples of the dehydrator 3 include a belt press dehydrator, a filter press dehydrator, a rotary press dehydrator, a screw press dehydrator, a centrifugal dehydrator, a multiple disk dehydrator, and the like.

  The dewatering device 3 is connected to a drying device (not shown) via a dewatered sludge feed mud line. Further, the dehydrator 3 is connected to a wastewater treatment facility (not shown) via a desorbed liquid feed line. Further, the dehydrating device 3 is connected to the flocculant supply device 7 via the flocculant supply line.

  The flocculant supply device 7 is a device that supplies the flocculant to the dehydrator 3. Examples of the flocculant supplied by the flocculant supply device 7 include inorganic flocculants and organic polymers such as polyaluminum chloride, sulfate band, ferric chloride, ferrous sulfate, sodium silicate solution, and polysilica iron. Organic flocculants such as flocculants are listed.

  Next, the operation of each component of the sludge treatment system 100 of the present embodiment will be described, and a sludge treatment method using the sludge treatment system 100 will be described.

  In the sludge treatment method of the present embodiment, a digestion treatment step of digesting organic waste to obtain digested sludge, and after agitating carbon dioxide into the digested sludge, the digested sludge and a magnesium source are stirred and mixed. And MAP production step for obtaining a magnesium ammonium phosphate (MAP) and a dephosphorylated product.

Further, in the sludge treatment method of the present embodiment, the flocculant is added from the flocculant supply device 7 to the dephosphorization-treated product sent from the MAP generation tank 2 to the dehydrator 3, and then dehydrated to remove the desorbed liquid and dehydrated liquid. You may have the dehydration process which obtains sludge.
Hereinafter, each step will be described.

  First, the digestion process will be described. In the digestion treatment step, carbon dioxide, methane, ammonia and digested sludge are obtained by digesting organic waste sent from the wastewater treatment facility by the digestion tank 1. In the digestion process, excess sludge in the organic waste releases phosphorus in the form of phosphoric acid. The phosphoric acid released from the excess sludge dissolves in the water contained in the digested sludge.

In addition, a part of the ammonia generated in the digestion process becomes biogas together with some carbon dioxide and methane. The remaining ammonia and carbon dioxide are dissolved in the moisture contained in the digested sludge and become a state contained in the digested sludge. That is, the moisture in the digested sludge generated in the digestion process includes phosphoric acid, ammonia, and carbon dioxide.
Components other than moisture in the digested sludge are organic waste (organic matter) residues and anaerobic microorganisms that the anaerobic microorganisms could not be completely decomposed.

  The biogas obtained in the digestion tank 1 is discharged out of the digestion tank 1 via a biogas supply line (not shown). On the other hand, the digested sludge generated in the digestion tank 1 is sent to the MAP generation tank 2 through the digested sludge feed mud line.

  Next, the MAP generation process will be described. In the MAP generation step, carbon dioxide is diffused to the digested sludge sent from the digestion tank 1 to the MAP generation tank 2, and then the magnesium source is supplied to the digested sludge and mixed by stirring, whereby the dephosphorized product and MAP Get.

In the MAP generation step, phosphorus is collected and removed by the MAP method. In the MAP method, a magnesium magnesium phosphate hexahydrate (MgNH ₄ PO ₄ .6H _{2) is} prepared by adding a liquid magnesium source or a solid magnesium source as a seed crystal to digested sludge containing a large amount of phosphoric acid and ammonia. This is a method for crystallizing and growing O) crystals. By recovering crystals (solid matter) containing a large amount of phosphorus, phosphorus and ammonia can be recovered and removed from the digested sludge.

  In the MAP method, the present inventors have found that the pH of the digested sludge that is the saturated solubility of MAP is 6.23. That is, it was found that MAP does not crystallize in the region where the pH of digested sludge is less than 6.23. Further, in the region where the pH of the liquid to be treated is 6.23 or more, as the pH increases, MAP does not crystallize, and a large amount of fine MAP is generated in a supersaturated region where MAP is generated in the form of growing seed crystals. It was found that there is a region to crystallize. If the digested sludge has a high pH and is a region where a large amount of fine MAP is crystallized, the finely crystallized MAP does not settle and is sent to the subsequent dewatering device together with the digested sludge. Phosphorus may not be recovered / removed. The pH of the digested sludge that has undergone the digestion process is 7.5 to 8.5, and if phosphorus is recovered by the MAP method as it is, fine MAP is crystallized in a large amount. Therefore, in order to efficiently recover phosphorus in the digested sludge, the pH of the digested sludge is lowered, and the pH of the digested sludge is 6.23 or higher, which is a pH at which MAP does not crystallize, and seed crystals are grown. It is important to be in the above supersaturated region, which is the pH at which MAP is produced.

In the sludge treatment method of the present embodiment, the pH of the digested sludge is reduced by aeration of carbon dioxide into the digested sludge in the MAP generation tank 2 by the carbon dioxide aeration device 4. Carbon dioxide (CO ₂ ) diffused in the digested sludge combines with water (H ₂ O) to form carbonic acid (H ₂ CO ₃ ). When this carbonic acid (H ₂ CO ₃ ) is separated into hydrogen ions (H ⁺ ) and carbonate ions (HCO ₃ ⁻ ), the hydrogen ion concentration in the digested sludge increases, and the pH of the digested sludge decreases.

Further, the present inventors have found that the pH of the digested sludge decreases only to about 7 even if carbon dioxide is continuously diffused into the digested sludge. Furthermore, since this pH is in the range of the supersaturated region, it was found that digested sludge having a reduced pH by aeration of carbon dioxide can be suitably used for the MAP method.
Even if carbon dioxide is continuously diffused into the digested sludge, the pH of the digested sludge does not drop to about 7. The reason is that the carbon dioxide generated by the digestion process is already dissolved in the digested sludge. This is probably because only a certain amount of carbon dioxide dissolves even if it is noticed.

  The lower limit of the time during which carbon dioxide is diffused into the digested sludge is preferably 15 minutes or more, and more preferably 30 minutes or more in order to lower the pH of the digested sludge to the pH of the supersaturated region. From the viewpoint of shortening the processing time in the MAP generation tank 2, the upper limit of the time for aeration of carbon dioxide in the digested sludge is preferably set to 1 hour or less, for example.

Next, the digested sludge whose pH has been lowered by aeration of carbon dioxide and the magnesium source supplied from the magnesium source supply device 5 are stirred and mixed. The supply amount of the magnesium source may be appropriately adjusted according to the phosphorus concentration in the digested sludge.
As described above, the pH of the digested sludge diffused with carbon dioxide is the pH in the supersaturated region where MAP is generated in a form in which the MAP is not crystallized and a magnesium source as a seed crystal is grown. Therefore, when the magnesium source that becomes the seed crystal and the digested sludge are mixed with stirring, phosphoric acid and ammonia in the digested sludge do not crystallize in large quantities as fine MAP, but grow the magnesium source that is the seed crystal. In the form of MAP crystals.

  The grown MAP crystals settle in the lower layer of the MAP production tank 2. The settled MAP is discharged out of the MAP generation tank 2 through the MAP discharge line. On the other hand, the dephosphorized product after the phosphorus is removed is sent to the dehydrator 3 via the dephosphorized product feed mud line. The pH of the dephosphorized product is the same as that of the digested sludge, and the pH is approximately 7.

Next, after adding a flocculant to the dephosphorization sludge sent from the MAP production tank 2 to the dehydrator 3, the dehydrator dehydrates to obtain dehydrated sludge and a desorbed liquid (dehydration step).
In the dehydration step, the flocculant is added from the flocculant supply device 7 to the dephosphorized product in the dehydrator 3. When the flocculant is added to the dephosphorized sludge, the solid matter in the dephosphorized sludge is polymerized.

  After adding the flocculant, the dephosphorization product is dehydrated by the dehydrator 3. The dewatered sludge obtained by dehydrating the dephosphorized material is sent to a drying device (not shown) via the dewatered sludge feed mud line. The dewatered sludge sent to the drying device is discarded after being dried. On the other hand, the detachment liquid is sent to a wastewater treatment facility (not shown) via the detachment liquid supply line for processing.

Note that the sludge treatment system 100 of the present embodiment operates in the MAP generation tank 2 so as to supply a magnesium source after aeration of carbon dioxide in the digested sludge. That is, in the MAP production tank 2, the carbon dioxide diffuser 4 diffuses carbon dioxide into the digested sludge and the magnesium source supply device 5 operates to supply the magnesium source after the pH of the digested sludge is lowered. To do.
The timing of adding the magnesium source to the digested sludge in the MAP generation tank 2 is measured by a timer and may be added manually after a certain period of time, and the pH of the digested sludge in the MAP generation tank 2 is measured. Then, after confirming that the pH of the digested sludge has decreased, it may be added manually, or any of these may be performed automatically by the control device.

  According to the above configuration, by aeration of carbon dioxide in the digested sludge, the pH of the digested sludge can be lowered without using a chemical, and the pH of the digested sludge can be set to the pH of the supersaturated region. . Therefore, in the MAP production tank 2, coarse MAP can be grown without crystallizing fine MAP. Since the coarsely grown MAP easily settles, it can easily settle in the MAP production tank 2 and be recovered. Thereby, phosphorus in digested sludge can be efficiently recovered and removed. Further, the phosphorus concentration in the desorbed liquid obtained by dehydrating the dephosphorized product can be reduced, and when this desorbed liquid is sent to the wastewater treatment facility, the phosphorus load in the wastewater treatment facility can be reduced. Thereby, the operating costs such as aeration required for phosphorus removal in the wastewater treatment facility can be reduced.

  In addition, by aeration of carbon dioxide in the digested sludge, it is possible to lower the pH of the digested sludge without using chemicals. Therefore, in the dehydrated sludge and desorbed liquid obtained by dehydrating the dephosphorized material. The drug never remains. Therefore, when the dried sludge obtained by drying the dehydrated sludge is discarded by a method such as landfill, it is not necessary to consider the environmental load. Furthermore, when the desorbed liquid is sent to the wastewater treatment facility for processing, the biological treatment at the wastewater treatment facility is not adversely affected.

  Further, since the pH of the digested sludge is lowered by aeration of carbon dioxide, the pH of the digested sludge can be easily set to the pH of the supersaturated region as compared with the case where the pH is adjusted with a chemical. The pH of the supersaturated region is in the neutral region, and it is very difficult to adjust the pH to the neutral region using chemicals such as organic acids and inorganic acids. However, in the above-described embodiment, even if carbon dioxide is continuously diffused into the digested sludge, the pH is reduced only to about 7, and thus it is easy to set the pH to the supersaturated region.

  Moreover, since the dephosphorization process thing sent to the dehydration apparatus 3 from the MAP production | generation tank 2 has low pH, it can improve the agglomeration effect by a flocculant. Thereby, the dewatering efficiency in the dehydrating apparatus 3 can be improved. Furthermore, the amount of the flocculant added to the dephosphorized product can be reduced.

(Second Embodiment)
As in the sludge treatment system 100 of the first embodiment, the sludge treatment system 200 of the present embodiment shown in FIG. 2 is a digester tank 1, a MAP generation tank 2, a carbon dioxide diffuser 4, and a magnesium source supply. And a device 5.
Moreover, the sludge treatment system 200 of this embodiment includes a solubilization treatment tank 8 between the digestion tank 1 and the MAP generation tank 2.
Moreover, the sludge treatment system 200 of this embodiment may be equipped with the dehydration apparatus 3, the pH meter 6, and the flocculant supply apparatus 7, as shown in FIG. Among the components of the sludge treatment system 200 shown in FIG. 2, the same components as those of the sludge treatment system 100 of the first embodiment are denoted by the same reference numerals as those in FIG. .

  The solubilization treatment tank 8 is a tank for solubilizing the digested sludge sent from the digestion tank 1 through the digested sludge feed mud line. The solubilization treatment tank 8 is connected to the digestion tank 1 via a digested sludge feed mud line, and receives digested sludge. Moreover, the solubilization processing tank 8 is connected with the MAP production tank 2 through the solubilized processed material feeding line.

  Next, the operation of each component of the sludge treatment system 200 of the present embodiment will be described, and a sludge treatment method using the sludge treatment system will be described.

  The sludge treatment method of the present embodiment includes a digestion treatment step for digesting organic waste to obtain digestion sludge, a solubilization treatment step for solubilizing digested sludge, and carbon dioxide in the digested sludge solubilized. After diffusing, the solubilized digested sludge and the magnesium source are stirred and mixed to obtain a dephosphorized product and a magnesium ammonium phosphate (MAP) production step.

Further, in the sludge treatment method of the present embodiment, the flocculant is added from the flocculant supply device 7 to the dephosphorization-treated product sent from the MAP generation tank 2 to the dehydrator 3, and then dehydrated to remove the desorbed liquid and dehydrated liquid. You may have the dehydration process which obtains sludge.
Hereinafter, the description of the same steps as those in the first embodiment is omitted or simplified.

First, carbon dioxide, methane, ammonia, and digested sludge are obtained by digesting the organic waste sent from the wastewater treatment facility by the digestion tank 1 (digestion treatment step). The moisture in the digested sludge obtained in the digestion process includes phosphoric acid released by excess sludge, carbon dioxide and ammonia generated by the digestion process.
Digested sludge generated in the digestion tank 1 is sent to the solubilization treatment tank 8 through the digested sludge feed mud line.

  Next, the digested sludge sent to the solubilization processing tank 8 from the digestion tank 1 is solubilized (solubilization processing step). The solubilization method includes, for example, a chemical method using ozone or a chemical, a physicochemical method using heat treatment or microwave heating, a mechanical method of crushing using ultrasonic waves or a bead mill, An electrochemical method by decomposition, an enzyme reaction method using an enzyme, or the like can be used.

When the digested sludge is solubilized, the hardly decomposable organic matter in the digested sludge is reduced to a readily decomposable organic matter. This easily decomposable organic substance is easily dissolved in moisture. Therefore, the easily decomposable organic substance produced by reducing the molecular weight of the hardly decomposable organic substance is dissolved in the moisture in the digested sludge. Then, the amount of solid matter in the digested sludge decreases. Moreover, the phosphorus contained in the hardly decomposable organic substance is reduced in molecular weight by the solubilization treatment, and is dissolved in the moisture in the digested sludge as phosphoric acid. Then, the phosphorus concentration in the digested sludge increases.
The digested sludge subjected to the solubilization treatment is sent to the MAP generation tank 2 through the solubilization treatment product feeding mud line.

  Next, phosphorus in the digested sludge after the solubilization process sent to the MAP generation tank 2 is recovered and removed (MAP generation step). Thereafter, the dephosphorized product from which phosphorus has been removed is sent to the dehydrator 3 and separated into a desorbed liquid and dehydrated sludge (dehydration step). The dewatered sludge obtained by dehydrating the dephosphorized material is sent to a drying device (not shown) via a dewatered sludge feed mud line. On the other hand, the detachment liquid is sent to a wastewater treatment facility (not shown) via a detachment liquid liquid supply line and is subjected to wastewater treatment.

  According to the above configuration, by aeration of carbon dioxide in the digested sludge, the pH of the digested sludge can be lowered without using a chemical, and the pH of the digested sludge can be set to the pH of the supersaturated region. . Therefore, in the MAP production tank 2, coarse MAP can be grown without crystallizing fine MAP. Thereby, phosphorus in digested sludge can be efficiently recovered and removed. Further, when the phosphorus concentration in the desorbed liquid obtained by dehydrating the dephosphorized product is reduced and this desorbed liquid is sent to the wastewater treatment facility, the phosphorus load in the wastewater treatment facility can be reduced. Thereby, the operating costs such as aeration required for phosphorus removal in the wastewater treatment facility can be reduced.

In addition, by aeration of carbon dioxide in the digested sludge, the pH of the digested sludge can be lowered without using chemicals. The drug never remains.
In addition, since the pH of the digested sludge is lowered by aeration of carbon dioxide, the pH in the supersaturated region can be easily set as compared with the case where the pH is adjusted with a chemical.

  Moreover, since the dephosphorization process thing sent to the dehydration apparatus 3 from the MAP production | generation tank 2 has low pH, it can improve the agglomeration effect by a flocculant. Thereby, the dehydrating efficiency in the dehydrating apparatus 3 can be improved, and the amount of the flocculant added to the dephosphorized product can be reduced.

  In addition, since the digested sludge generated in the digestion tank 1 is solubilized, the organic matter in the digested sludge can be reduced in molecular weight, and the amount of solid matter in the digested sludge can be reduced. Therefore, the amount of dewatered sludge obtained by dehydrating the dephosphorized product can be reduced, and the amount of dry sludge to be finally disposed can be reduced. Thereby, the cost required for disposal when landfilling and incinerating dry sludge can be reduced. Further, by solubilizing the digested sludge, a large amount of phosphorus can be dissolved in the water in the digested sludge solubilized and the recovery efficiency of phosphorus in the MAP production tank 2 can be improved.

  Moreover, since the digested sludge is solubilized in the solubilization tank 8, solubilization efficiency can be improved as compared with the case where the organic waste is solubilized. When the organic waste is solubilized, the organic matter that can be decomposed by the anaerobic microorganisms in the digestion tank 1 is reduced in molecular weight. Then, the chemical | medical agent and energy which are required for a solubilization process may increase. However, as described above, in this embodiment, digested sludge is solubilized. For this reason, organic substances that cannot be completely decomposed by anaerobic microorganisms are solubilized, and the chemicals and energy required for the solubilization process can be reduced.

(Third embodiment)
The sludge treatment system 300 of the present embodiment shown in FIG. 3 is similar to the sludge treatment system 100 of the first embodiment, in the digestion tank 1, the MAP generation tank 2, the carbon dioxide diffuser 4, and the magnesium source supply. And a device 5.
Further, the sludge treatment system 300 of this embodiment includes a solubilization treatment tank 8, a mud sending means 10, and a returning means 11.
Moreover, as shown in FIG. 3, the sludge treatment system 300 of the present embodiment may include a dehydrator 3, a pH meter 6, and a flocculant supply device 7. Among the components of the sludge treatment system 300 shown in FIG. 3, the same components as those of the sludge treatment system 100 of the first embodiment are denoted by the same reference numerals as those in FIG. .

  The mud sending means 10 is means for sending a part of the digested sludge generated in the digestion tank 1 to the solubilization treatment tank 8. An example of the mud sending means 10 is a pipe having a pump.

The solubilization treatment tank 8 is a tank for solubilizing the digested sludge sent from the digestion tank 1 by the mud feeding means 10.
The solubilization treatment tank 8 is connected to the digestion tank 1 through the mud feeding means 10. Further, the solubilization treatment tank 8 is connected to the digestion tank 1 through the return means 11.

  The return means 11 is means for sending the digested sludge solubilized in the solubilization tank 8 to the digester 1. An example of the return means 11 is a pipe having a pump.

  Next, the operation of each component of the sludge treatment system 300 of the present embodiment will be described, and a sludge treatment method using the sludge treatment system will be described.

  The sludge treatment method of the present embodiment comprises a digestion treatment step for digesting organic waste to obtain a digested sludge, a solubilization treatment step for solubilizing a part of the digested sludge, and a solubilized digested sludge. After returning the carbon dioxide to the digested sludge sent from the digester 1 and the return process sent to the digester 1, the digested sludge and the magnesium source are stirred and mixed to remove the dephosphorized product and magnesium ammonium phosphate (MAP) MAP generation step to obtain

Further, in the sludge treatment method of the present embodiment, the flocculant is added from the flocculant supply device 7 to the dephosphorization-treated product sent from the MAP generation tank 2 to the dehydrator 3, and then dehydrated to remove the desorbed liquid and dehydrated liquid. You may have the dehydration process which obtains sludge.
Hereinafter, the description of the same steps as those in the first embodiment is omitted or simplified. In this embodiment, the sludge sent from the digestion tank 1 to the MAP generation tank 2 is described as digested sludge. That is, sludge generated by digesting organic waste in the digestion tank 1 and sludge generated by digesting solubilized digested sludge are referred to as digested sludge.

First, carbon dioxide, methane, ammonia, and digested sludge are obtained by digesting the organic waste sent from the wastewater treatment facility by the digestion tank 1 (digestion treatment step). The moisture in the digested sludge obtained in the digestion process includes phosphoric acid released by excess sludge, carbon dioxide and ammonia generated by the digestion process.
Part of the digested sludge generated in the digestion tank 1 is sent to the MAP generation tank 2 through the digested sludge feed mud line. The remainder of the digested sludge is sent to the solubilization tank 8 by the mud sending means 10.

  Next, the digested sludge sent from the digestion tank 1 by the mud sending means 10 is solubilized (solubilization process step). Examples of the solubilization method include chemical methods using ozone and chemicals, physicochemical methods using heat treatment and microwave heating, mechanical methods of crushing using ultrasonic waves and bead mills, and electrolysis. An electrochemical method, an enzyme reaction method using an enzyme, or the like can be used. When the digested sludge is solubilized, the hardly decomposable organic matter in the digested sludge is reduced to a readily decomposable organic matter. This easily decomposable organic substance is easily dissolved in moisture. Therefore, the easily decomposable organic substance produced by reducing the molecular weight of the hardly decomposable organic substance is dissolved in the moisture in the digested sludge. Then, the amount of solid matter in the digested sludge decreases. Moreover, the phosphorus contained in the hardly decomposable organic substance is reduced in molecular weight by the solubilization treatment, and is dissolved in the moisture in the digested sludge as phosphoric acid. Then, the phosphorus concentration in the digested sludge increases.

  Next, the digested sludge solubilized in the solubilization tank 8 is sent to the digester 1 by the return means 11 (returning step). The digested sludge after the solubilization process sent to the digester 1 is digested with the organic waste by the digester 1. Since the organic matter in the digested sludge after the solubilization treatment is in a form in which the anaerobic microorganisms in the digestion tank 1 are easily decomposed, the digestion treatment with the anaerobic microorganisms is performed efficiently. Digested sludge produced by digesting organic waste and solubilized digested sludge is partly sent to the MAP generation tank 2 via the digested sludge feed mud line, and the remainder is sent by the mud feed means 10. It is sent to the solubilization treatment tank 8.

  Next, phosphorus in the digested sludge sent from the digestion tank 1 to the MAP generation tank 2 is recovered and removed (MAP generation step). The digested sludge sent from the digestion tank 1 also includes digested sludge solubilized in the solubilization treatment tank 8, and the digested sludge has a high phosphorus concentration while the amount of solid-liquid is small.

  Next, the dephosphorized product from which phosphorus has been removed after the MAP generation step is sent to the dehydration apparatus 3 and is subjected to the dehydration step. In the dehydration step, the dephosphorized product is dehydrated by the dehydrator 3 and separated into a desorbed liquid and dehydrated sludge. The dewatered sludge obtained by dehydrating the dephosphorized material is sent to a drying device (not shown) through the dewatered sludge feed mud line. On the other hand, the desorbed liquid is sent to a wastewater treatment facility (not shown) via a desorbed liquid feed line and is subjected to wastewater treatment.

  According to the above configuration, by aeration of carbon dioxide in the digested sludge, the pH of the digested sludge can be lowered without using a chemical, and the pH of the digested sludge can be set to the pH of the supersaturated region. . Therefore, in the MAP production tank 2, coarse MAP can be grown without crystallizing fine MAP. Thereby, phosphorus in digested sludge can be efficiently recovered and removed. Further, when the phosphorus concentration in the desorbed liquid obtained by dehydrating the dephosphorized product is reduced and the desorbed liquid is sent to the wastewater treatment facility, the phosphorus load in the wastewater treatment facility can be reduced. Thereby, the operating costs such as aeration required for phosphorus removal in the wastewater treatment facility can be reduced.

In addition, by aeration of carbon dioxide in the digested sludge, it is possible to lower the pH of the digested sludge without using chemicals. Therefore, in the dehydrated sludge and desorbed liquid obtained by dehydrating the dephosphorized material. The drug never remains.
In addition, since the pH of the digested sludge is lowered by aeration of carbon dioxide, the pH in the supersaturated region can be easily set as compared with the case where the pH is adjusted with a chemical.

  Moreover, since the dephosphorization process thing sent to the dehydration apparatus 3 from the MAP production | generation tank 2 has low pH, it can improve the agglomeration effect by a flocculant. Thereby, the dehydrating efficiency in the dehydrating apparatus 3 can be improved, and the amount of the flocculant added to the dephosphorized product can be reduced.

  Moreover, since the digested sludge produced in the digestion tank 1 is solubilized by the solubilization treatment tank 8, the organic matter in the digested sludge can be reduced in molecular weight. Moreover, the digesting sludge in the digestion tank 1 can be improved by returning the solubilized digested sludge in a form in which the organic matter in the digested sludge is easily decomposed by the anaerobic microorganisms to the digestion tank 1 by the return means 11. . Thereby, while being able to increase the biogas generation amount in the digestion tank 1, the amount of solid substances in digested sludge can be reduced. Since the amount of solid matter in the digested sludge can be reduced, the amount of dehydrated sludge obtained by dehydrating the dephosphorized material can be reduced, and the amount of dry sludge to be finally disposed can be reduced. Thereby, the cost required for disposal when landfilling and incinerating dry sludge can be reduced. Further, by solubilizing the digested sludge, a large amount of phosphorus can be dissolved in the water in the digested sludge solubilized and the recovery efficiency of phosphorus in the MAP production tank 2 can be improved.

  According to at least one embodiment described above, the pH of the digested sludge can be lowered without using a chemical agent by aeration of carbon dioxide in the digested sludge. The pH can be set. Thereby, phosphorus in digested sludge can be efficiently recovered and removed. Further, when the phosphorus concentration in the desorbed liquid obtained by dehydrating the dephosphorized product is reduced and the desorbed liquid is sent to the wastewater treatment facility, the phosphorus load in the wastewater treatment facility can be reduced. Thereby, the operating costs such as aeration required for phosphorus removal in the wastewater treatment facility can be reduced.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

(Example 1-1)
Carbon dioxide was diffused into the digested sludge, and the relationship between the carbon dioxide aeration time and the pH of the digested sludge was examined by the following method.
Carbon dioxide was diffused into three kinds of digested sludges having different pHs, and changes in pH of each digested sludge were observed over time. The pH of the digested sludge before aeration of carbon dioxide was 7.5, 7.6, and 8.1, respectively. For comparison, carbon dioxide was diffused into tap water, and changes in the pH of tap water over time were observed. The pH of tap water before aeration of carbon dioxide was 7.2.
The results are shown in FIG. In addition, FIG. 4 is a figure which shows the relationship between digested sludge and tap water pH, and a carbon dioxide aeration time.

  From FIG. 4, the pH of the digested sludge whose pH was 7.5 to 8.1 was 6.9 to 7.2 in 15 minutes after the carbon dioxide was diffused, and the carbon dioxide was diffused. 30 minutes from 6.8 to 7.0. Further, after 15 minutes from the aeration of carbon dioxide, the pH of the digested sludge has remained substantially flat. On the other hand, the tap water in which carbon dioxide was diffused had a pH of around 7 and the pH dropped to about 5 in 8 minutes after the carbon dioxide was diffused.

  From the above results, by setting the carbon dioxide aeration time to 15 minutes, the pH of the digested sludge, which was 7.5 to 8.1, can be lowered to 6.9 to 7.2, and the aeration time is set to 30. It turns out that the pH of digested sludge can be reduced to 6.8-7.0 by setting it as minutes. In addition, after 15 minutes have passed since the carbon dioxide was diffused, unlike tap water, it was found that even when the carbon dioxide was diffused, the pH hardly decreased and remained flat. Therefore, it turns out that the aeration of carbon dioxide becomes excessive, and the pH of the digested sludge does not decrease to a pH at which MAP does not crystallize (less than 6.23). Therefore, it turns out that it is not necessary to control the amount of aeration of carbon dioxide and the time for aeration while measuring the pH of digested sludge.

(Example 1-2)
Using the digested sludge whose pH was lowered by aeration of carbon dioxide, the recovery rate / removal rate of phosphorus when the phosphorus recovery rate / removal rate was performed by the MAP method was examined by the following method.
FIG. 5 is a diagram showing the relationship between the phosphate phosphorus concentration (PO ₄ ^3- P) in the digested sludge and the stirring time. FIG. 5 will be described in detail below.
In this example, in Example 1-1, the digested sludge whose pH was lowered to 7.0 by aeration of carbon dioxide for 30 minutes was used. This digested sludge had a phosphate phosphorus concentration (PO ₄ ^3-- P) of 200 mg / L and an ammonia nitrogen concentration (NH ₄ ⁺ -N) of 700 mg / L.

  A magnesium chloride solution was added to the digested sludge so that the Mg / P ratio was 1.5 wt / wt, and the mixture was stirred and mixed. And the phosphate-type phosphorus density | concentration in digested sludge was measured for every fixed time. As shown in FIG. 5, at the time when 1 hour had passed since the magnesium chloride solution was added, the phosphate phosphorus concentration hardly changed.

  This shows that the pH of the digested sludge is a pH at which MAP does not crystallize. That is, it can be seen that phosphoric acid and ammonia in digested sludge having a pH of 7.0 do not crystallize as MAP even when a liquid magnesium source is added.

  Next, a commercially available MAP crystal serving as a seed crystal was added to the digested sludge and mixed with stirring. As shown in FIG. 5, after adding MAP crystals, the phosphate phosphorus concentration in the digested sludge gradually decreased. Five hours after adding the MAP crystals, the phosphate phosphorus concentration in the digested sludge was 30 mg / L. That is, it can be seen that 85% of the total amount of phosphate phosphorus in the digested sludge has been removed. In other words, it can be seen that 85% of the total amount of phosphate phosphorus in the digested sludge became MAP crystals.

  From this, it can be seen that phosphoric acid and ammonia in the digested sludge became MAP by growing MAP crystals as seed crystals by adding MAP crystals. That is, it can be seen that phosphoric acid and ammonia in digested sludge having a pH of 7.0 do not crystallize as fine MAP, but become MAP in the form of growing a magnesium source as a seed crystal. Therefore, it can be seen that digested sludge having a pH reduced to about 7 by aeration of carbon dioxide can be suitably used for phosphorus removal by the MAP method.

Next, stirring was stopped and the mixture was allowed to stand for 30 minutes to precipitate MAP. Thereafter, the treatment liquid (dephosphorization product) was discharged, and the solid matter settled at the bottom was collected. The collected solid was rinsed with water and then dried at 45 ° C. until a constant weight was reached. XRD analysis was performed on the dried product after drying. As a result of XRD analysis, this dried product was identified as MAP (MgNH ₄ PO ₄ .6H ₂ O).

  As a result of subtracting the weight of the MAP crystal charged from the weight of the dried product and calculating the amount of phosphorus from the difference, the amount of phosphorus contained in the dried product is equivalent to 150 mg / L of phosphate phosphorus in the digested sludge. It was an amount. That is, it can be seen that 75% of the total amount of phosphate phosphorus in the digested sludge was recovered as MAP crystals. As described above, the amount of phosphate phosphorus in the digested sludge that has become MAP crystals is 85% of the total amount, so there is a difference of 10% from the amount of phosphate phosphorus recovered. This difference of 10% was presumed that the MAP crystals were small and were discharged together with the treatment liquid (dephosphorization product).

  The results regarding the removal rate / recovery rate of phosphate phosphorus described above are shown in FIG. In addition, FIG. 6 is a figure which shows the result of the phosphorus removal rate in digested sludge, and a MAP collection | recovery rate in Example 1-2.

  From the above results, phosphoric acid and ammonia in the digested sludge having a pH of 6.23-7.0 do not crystallize as fine MAP, but grow MAP as a seed crystal magnesium source (MAP). It turns out that it becomes. Therefore, it can be seen that the digested sludge having a pH reduced to about 7 by aeration of carbon dioxide can be suitably used for phosphorus recovery and removal by the MAP method. Moreover, when MAP used as a seed crystal is added to digested sludge having a pH of 7.0, 75% of the total amount of phosphate phosphorus in the digested sludge can be recovered as MAP.

DESCRIPTION OF SYMBOLS 100,200,300 ... Sludge processing system, 1 ... Digestion tank, 2 ... MAP production tank, 3 ... Dehydration apparatus, 4 ... Carbon dioxide aeration apparatus, 5 ... Magnesium source supply apparatus, 7 ... Coagulant supply apparatus, 8 ... Solubilization treatment tank, 10 ... mud feeding means, 11 ... return means.

Claims (8)

A digester that digests organic waste to obtain digested sludge;
A MAP production tank in which the digested sludge or the solubilized digested sludge and a magnesium source are mixed to obtain a MAP and a dephosphorized product;
A magnesium source supply device for supplying a magnesium source to the MAP generation tank;
A carbon dioxide diffuser that diffuses carbon dioxide into the digested sludge in the MAP generation tank or the digested sludge solubilized;
In the MAP generation tank, a sludge treatment system that operates to supply a magnesium source after aeration of carbon dioxide in the digested sludge or the solubilized digested sludge. A dehydrating apparatus for dehydrating the dephosphorized material to obtain dehydrated sludge and desorbed liquid;
A flocculant supply device for adding a flocculant to the dephosphorized product in the dehydrator;
A sludge treatment system according to claim 1.   The sludge treatment system according to claim 1 or 2, comprising a solubilization treatment tank for solubilizing the digested sludge between the digestion tank and the MAP generation tank. A solubilization tank for solubilizing the digested sludge;
Mud feeding means for sending the digested sludge from the digestion tank to the solubilization treatment tank;
Return means for sending the digested sludge solubilized from the solubilization tank to the digester,
The sludge treatment system of Claim 1 or Claim 2 provided with these. A digestion process that digests organic waste to obtain digested sludge;
After the carbon dioxide is diffused into the digested sludge or the solubilized digested sludge, the digested sludge or the solubilized digested sludge and the magnesium source are stirred and mixed to obtain MAP and a dephosphorized product. Process,
Provided sludge treatment method.   The sludge treatment method according to claim 5, further comprising a dehydration step of obtaining a dehydrated sludge and a desorbed liquid after adding a flocculant to the dephosphorized product.   The sludge treatment method according to claim 5 or 6, further comprising a solubilization treatment step of solubilizing the digestion sludge generated in the digestion treatment step after the digestion treatment step. A solubilization process for solubilizing a part of the digested sludge produced in the digestion process; and
A return step of returning the digested sludge solubilized to the digestion step;
A sludge treatment method according to claim 5 or claim 6 comprising:

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