JP4596897B2 - Anaerobic digestion of organic waste - Google Patents

Description

  The present invention relates to an anaerobic digestion method for organic waste, and in particular, it is treated in an anaerobic digestion tank in order to improve the efficiency of anaerobic digestion when anaerobic digestion of organic waste and recover methane gas. The present invention relates to a method for anaerobic digestion of organic waste in which organic waste is solubilized in advance by crushing or the like.

  Conventionally, in anaerobic digesters that treat organic sludge generated by sewage treatment and human waste treatment, or anaerobic digesters that treat biomass such as livestock waste and food waste, Biodegradable organic substances have been reduced in molecular weight, liquefied, and gasified and converted to digested gas and digested sludge to reduce sludge generation and stabilize sludge properties.

  However, the rate of digestion, which is the rate at which organic components in organic waste introduced into an anaerobic digestion tank are gasified and reduced, is about 50% in an intermediate temperature digestion anaerobic digestion tank that is widely used in sewage treatment plants. The solids reduction rate is only about 40%.

  In order to improve such points, various methods have been proposed in which sludge solubilization is combined with an anaerobic digester. Examples of such methods include a method of treating a portion of digested sludge discharged from an anaerobic digestion tank with an alkali and then returning it to the anaerobic digestion tank (see Patent Document 1), an ultrahigh pressure jet pump prior to anaerobic digestion. (See Patent Document 2), a method in which sludge is ejected from a nozzle by crushing (see Patent Document 2), a method in which a part of digested sludge is solubilized by an ozone reaction and then returned to an anaerobic digester (see Patent Document 3), digestion There is a method of milling sludge and performing anaerobic digestion again (see Patent Document 4). According to these methods, the reduction in the molecular weight, liquefaction, and gasification of organic waste in the digestion tank is further promoted, and the solids reduction rate is higher than in the case where the treatment is performed in an anaerobic digestion tank alone as in the prior art. It is said that the amount of sludge to be finally disposed of can be reduced.

  On the other hand, as a problem in the maintenance management of anaerobic digesters, as organic waste proceeds in anaerobic digesters, nitrogen and phosphorus components elute and react with magnesium ions in anaerobic digested sludge. There was a case where scales occurred. The scale problem occurs not only in a sewage treatment plant, but also in a human waste treatment facility (see Non-Patent Document 1) and a piggery waste treatment facility (see Non-Patent Document 2), which is a serious problem. If scale occurs in an anaerobic digestion tank or anaerobic digestion sludge transfer pipe, etc., it will lead to troubles such as pipe clogging, in which case work such as washing the pipe with strong acid or laying the pipe again is necessary. It was a big maintenance problem. Thus, the main component of the scale generated in and around the anaerobic digester is said to be magnesium ammonium phosphate, which is a crystal in which phosphate ions, magnesium ions, and ammonium ions are combined (non-patented). Reference 3).

As described above, when sludge solubilization is combined with an anaerobic digestion method for the purpose of improving the efficiency of an anaerobic digester, solubilization of organic substances is promoted in the anaerobic digester. As a result, since elution of nitrogen and phosphorus components is promoted, there is a problem that the risk of scale formation in anaerobic digested sludge is further increased. In order to solve this problem, a magnesium source is added to an anaerobic treatment step to promote a magnesium ammonium phosphate production reaction, and a magnesium ammonium phosphate crystal produced in anaerobic digested sludge is recovered (Patent Document 5). Have been proposed). However, it is difficult to control the production of magnesium ammonium phosphate crystals, and it has not been put into practical use. In addition, an ammonia removal agent such as magnesium phosphate or magnesium ammonium phosphate is added to anaerobic sludge to immobilize ammonia nitrogen, which is one of the main elements constituting magnesium ammonium phosphate crystals, in the form of sparingly soluble salts. A method has also been proposed (see Patent Document 6). However, while it is necessary to continue adding an ammonia removing agent containing phosphorus, there is a problem that there is no way to recover magnesium ammonium phosphate which is a phosphorus resource.
Japanese Patent Application Laid-Open No. 64-080499 Japanese Examined Patent Publication No. 03-065999 JP 2002-361291 A JP 2000-167597 A PPM, Vol.19, No.4, p.18-28, (1988) Livestock Research, Vol.57, No.1, p.83-90, (2003) Water and wastewater, Vol.29, No.7, p.636-640, (1987) JP 2004-000941 A JP 2004-000910 A

  The basics of organic waste treatment are to reduce the amount that needs final disposal, to stabilize the properties, and from the aspect of organic waste as biomass, there are processing and adjustments there. It is to reuse the contained resources. When these basic conditions are applied to an anaerobic digester, the anaerobic digester alone cannot sufficiently achieve the first weight reduction. Therefore, it is necessary to combine the above-described sludge solubilization treatment in order to improve the efficiency of the anaerobic digester. In order to carry out the reuse of resources, which is the third condition, it is necessary to eliminate scale troubles in the anaerobic digester and its surrounding facilities. In other words, it can be said that a system that realizes the efficiency improvement of the anaerobic digester and the resolution of the scale trouble at the same time is a truly appropriate disposal method for organic waste.

  The present invention reduces the volume of organic waste, that is, anaerobic digesters that perform stabilization and stabilization of organic sludge generated during sewage treatment and human waste treatment, or livestock waste, food waste, etc. In an anaerobic digester that performs stabilization, it is possible to simultaneously achieve low molecular weight, liquefaction, and gasification by anaerobic digestion of biodegradable organic substances, and elimination of scale troubles in the anaerobic digester and its surrounding equipment. An object of the present invention is to provide a method for anaerobic digestion of organic waste.

  As a result of intensive studies to solve such problems, the present inventors have focused on magnesium ions as a causative substance of scale trouble, and this organic waste is preceded by anaerobic digestion of organic waste. By solubilizing the product, eluting the magnesium in the organic waste as magnesium ions in advance, and then anaerobically digesting the organic waste from which the magnesium ions have been removed from the solubilized organic waste, The present inventors have found the fact that scale trouble can be avoided and have reached the present invention.

That is, the present invention is a method for anaerobic digestion treatment of organic waste, and includes the following steps (a), (b), and (c).
(B) Step of solubilizing organic waste and eluting magnesium in the organic waste (b) By solid-liquid separation of the organic waste solubilized in the step (b) The step of removing magnesium as a sparingly soluble magnesium salt by separating the liquid phase containing magnesium and adding an alkaline agent to the separated liquid phase (c) The magnesium is removed in the step (b), And anaerobic digestion of the liquid phase after removal of the sparingly soluble magnesium salt together with organic waste that has not been subjected to the process of eluting new magnesium

Also, in the above, step (b) be summarized in that a wet bead mill stirring and grinding processes.

Moreover, this invention makes it a summary to include the following processes further in the above.
(D) A step of dehydrating the digested sludge treated in the step (c) to obtain a dehydrated filtrate (e) Nitrogen and phosphorus contained in the dehydrated filtrate obtained in the step (d) are magnesium phosphate. Process to recover by ammonium crystallization method

  The gist of the present invention is that the magnesium removed in the step (b) is used as a magnesium source in the step (e).

  According to the present invention, the organic waste is solubilized in the first stage before anaerobic digestion of the organic waste, so that magnesium which is one of the scale causative substances is solubilized, and then the magnesium is removed. Moreover, since the anaerobic digestion process is performed, the amount of magnesium flowing into the anaerobic digester can be significantly reduced. As a result, it is possible to suppress the generation of scale in the anaerobic digestion tank or the transfer pipe of the anaerobic digestion sludge. In addition, since the organic waste is solubilized in the previous stage of anaerobic digestion, gasification of organic matter in the anaerobic digestion tank is promoted, and the amount of methane generated per unit organic waste can be increased. As a result, the solids reduction rate can be increased, and the amount of sludge to be finally disposed can be reduced.

  According to the present invention, since the liquid phase containing magnesium is separated from the solubilized organic waste, the liquid phase after separation can be used as a magnesium source. In addition, when recovering magnesium from the liquid phase, the recovery operation becomes easy. Separation by liquid cyclone, separation by gravity concentration, separation by pressure flotation device, separation by sand filtration, separation by fiber filtration, membrane filtration Various separation methods such as separation can be applied.

  Further, according to the present invention, the hardly soluble magnesium salt obtained by solid-liquid separation of the solubilized organic waste, separating the liquid phase containing magnesium, and adding an alkaline agent to the separated liquid phase to cause precipitation. Can be reused as a magnesium resource.

  In addition, according to the present invention, the degree of solubilization of organic waste can be manipulated by the residence time in the wet bead mill apparatus, so that there are few management items in the field, and process maintenance is easy.

  According to the present invention, the treated digested sludge is dehydrated to obtain a dehydrated filtrate, and nitrogen and phosphorus contained in the dehydrated filtrate obtained in this step are recovered by a magnesium ammonium phosphate crystallization method. Process, the phosphate that has been scaled in the conventional method, that is, the phosphate that has been insolubilized in the anaerobic digester, will be contained in the dehydrated filtrate while being dissolved. The phosphate concentration can be increased more than the dehydrated filtrate of anaerobic digested sludge. Therefore, the amount of magnesium ammonium phosphate recovered from the dehydrated filtrate can be increased compared to the conventional method.

  According to the present invention, the magnesium removed in the previous stage of anaerobic digestion of organic waste is used as a magnesium source in the step of recovering nitrogen and phosphorus contained in the dehydrated filtrate by magnesium ammonium phosphate crystallization. Since it is used, the amount of magnesium supplied from outside the system can be reduced.

  The organic waste to be treated in the present invention is organic waste that has been treated in an anaerobic digestion tank, and organic sludge generated by sewage treatment or human waste treatment, domestic wastewater treatment, and livestock waste disposal. And food waste. In particular, in the anaerobic digester for treating sewage treatment sludge, human waste treatment sludge, and piggery waste, many of the above-mentioned scale problems related to the apparatus and peripheral devices have been reported, and the present invention can greatly benefit.

  In the present invention, it is necessary to first perform a step of solubilizing organic waste and eluting magnesium in the organic waste. Organic waste solubilization methods in this step include mechanical crushing methods such as wet bead mill stirring, ultrasonic irradiation methods, heat solubilization methods at high temperatures, chemical addition treatment methods such as ozone treatment, etc. A known solubilization method can be used as appropriate. Of these, wet bead mill stirring is preferred in view of the elution of magnesium and process maintenance. The solubilization rate by the wet bead mill stirring treatment can be controlled by the residence time in the bead mill apparatus, but is also affected by the bead material and the stirring peripheral speed.

  In order to suppress the generation of scale in an anaerobic digester, the ratio of soluble magnesium to total magnesium after the solubilization step needs to be at least 50%, preferably 70% or more. As an example, when solubilizing concentrated excess sludge of MLSS = 3 to 4% generated in a sewage treatment plant with a bead mill device, it is better to use beads of fine ceramics with a higher specific gravity than glass beads There is an effect to increase. At this time, the residence time in the bead mill apparatus is at least 5 minutes or more, preferably about 10 minutes, and a desired solubilization rate can be obtained. Further, prior to this wet bead mill stirring treatment, a solubilizing effect can be further enhanced by providing a step of adding an acid such as sulfuric acid or hydrochloric acid to the organic waste to make it weakly acidic.

  By this step, organic waste, particularly magnesium present in the microorganisms constituting the sludge is eluted out of the microorganisms when the microorganisms are crushed. When concentrated excess sludge of MLSS = 3 to 4% generated in a sewage treatment plant is solubilized by a bead mill device, the dissolved magnesium concentration can be about 100 to 200 mg / L.

  In addition to magnesium, organic substances contained in organic waste, metals such as potassium, organic nitrogen and phosphorus are also eluted by solubilization. In this state, the pH of the solubilized organic waste is from the neutral range to the acidic range, and the eluted magnesium exists in a state dissolved in the solubilized organic waste.

  In the present invention, next, a step of removing magnesium from the organic waste solubilized in the above step is necessary. As a simple method for that purpose, there is a method of solid-liquid separation of solubilized organic waste. The optimal solid-liquid separation method depends on the concentration of organic waste to be solubilized. Specifically, a gravity concentration separation device, a pressure levitation device, a membrane filtration device, or the like can be used. In the case of organic waste having a solid concentration exceeding 1%, solid-liquid separation can also be performed by adding a polymer flocculant and concentrating.

In addition, in order to remove magnesium from the solubilized organic waste, the solubilized organic waste is separated into solid and liquid phases, a liquid phase containing magnesium is separated, and an alkaline agent is added to the separated liquid phase. Need to be removed as a sparingly soluble magnesium salt. If it does in this way, magnesium content rate can be raised compared with the state in which magnesium is solubilized in the liquid phase. As an example of the magnesium salt, magnesium hydroxide has a solubility product Ks = 10 ^−10.74 , and has a property of ^{forming a} white precipitate in the alkaline region. That is, the magnesium ion can be removed by separating the magnesium ion into hardly soluble magnesium hydroxide by adding an alkali agent. As the alkaline agent, caustic soda, slaked lime, and the like can be used. As a result of solubilizing the target organic waste, if the range of magnesium concentration to be solubilized is known in advance, the amount of alkali agent added can be practically determined by the pH after adding the alkali agent. It is. Furthermore, since the supernatant water After removing the addition of A alkali agent as the magnesium salt of poorly soluble in the separated liquid phase it contains soluble organic substances, in order to recover biogas, anaerobic put into the sex digester.

In the present invention, next, an organic anaerobic digestion process is performed on the organic waste from which magnesium has been removed and from which new magnesium has not been eluted . This anaerobic digestion treatment may be either an intermediate temperature treatment or a high temperature treatment. When performed under medium temperature treatment conditions, a digestion temperature of 30 to 37 ° C. and a treatment period of 10 to 20 days are appropriate. When performed under high temperature treatment conditions, a digestion temperature of 50 to 55 ° C. and a treatment period of 7 to 10 days are appropriate. Is appropriate. Further, the anaerobic digestion method may be either one-stage digestion or two-stage digestion.

In the present invention, after the step of anaerobic digestion of organic waste from which magnesium has been removed and from which new magnesium has not been eluted, the digested sludge treated in this anaerobic digestion step is dehydrated and dehydrated. It is preferable to execute a step of obtaining a liquid and a step of crystallizing and recovering nitrogen and phosphorus contained in the dehydrated filtrate obtained in this dehydration step as magnesium ammonium phosphate.

  In the step of obtaining a dehydrated filtrate, a dehydrator such as a belt press filter, a centrifugal dewaterer, or a screw press dewaterer may be used for dewatering the digested sludge. At this time, if an inorganic flocculant such as an iron salt, aluminum salt, or calcium salt is added to perform dehydration, the phosphate concentration in the dehydrated filtrate is significantly reduced. It is not suitable for recovering magnesium ammonium as a phosphorus resource.

  To recover nitrogen and phosphorus contained in the dehydrated filtrate as magnesium ammonium phosphate crystals in the process of crystallizing and collecting nitrogen and phosphorus contained in the dehydrated filtrate obtained in the dehydration process as magnesium ammonium phosphate Requires at least an equimolar concentration of magnesium with phosphate. For example, a magnesium ammonium phosphate crystallizer is supplied with any magnesium compound such as magnesium hydroxide, magnesium chloride, magnesium oxide or seawater as a magnesium source, and the pH in the crystallizer is adjusted to 8 to 8 by adding an alkali agent. A method of crystallizing magnesium ammonium phosphate by controlling the pressure within the range of 9 and agitating the inside of the crystallizer with aeration can be employed.

  In the present invention, the magnesium obtained in the step of removing magnesium from the solubilized organic waste described above can be reused as part of the magnesium source in the magnesium ammonium phosphate crystallizer. In this case, the organic waste solubilized as described above is subjected to solid-liquid separation to separate a liquid phase containing magnesium, and an alkali agent is added to the separated liquid phase to obtain a hardly soluble magnesium salt. When utilizing what was excluded, this alkaline agent can be reused as a part of the alkaline agent used for controlling the pH in the crystallizer to the above-mentioned range of 8-9.

FIG. 1 shows a schematic configuration of an example of an apparatus for carrying out a conventional method for anaerobic digestion of organic waste. The organic waste 1 is solubilized by the solubilizer 2, whereby a part of magnesium contained in the organic waste 1 is solubilized. The solubilized sludge 3 is sent to the magnesium removing device 4 and separated into the magnesium-removed sludge 6 and the magnesium recovered product 7. The magnesium-removed sludge 6 is sent to the anaerobic digestion tank 5 to generate methane gas 17 and digested sludge 8.

FIG. 2 shows a process in which the digested sludge 8 treated in the anaerobic digester 5 is dehydrated by a dehydrator 14 to obtain a dehydrated filtrate 15, and the nitrogen and phosphorus contained in the dehydrated filtrate 15 are phosphorus as a crystallizer. The schematic structure of an example of the conventional apparatus including the process of crystallizing and recovering as magnesium ammonium phosphate 13 in the recovery apparatus 12 is shown.

  Here, the organic waste 1 is solubilized by the solubilizer 2, and a part of magnesium contained in the organic waste is solubilized. The solubilized sludge 3 is separated into a solubilized concentrated sludge 10 and a separation liquid 11 by a solid-liquid separator 9. Here, the solubilized magnesium moves to the separation liquid 11 side, and the amount of magnesium flowing into the anaerobic digester 5 is significantly reduced. The solubilized concentrated sludge 10 is digested in the anaerobic digestion tank 5, and the digested sludge 8 is separated into a dehydrated filtrate 15 and a dehydrated sludge 16 by a dehydrator 14. The dehydrated filtrate 15 is supplied to the phosphorus recovery device 12, and phosphate and ammonium salt are recovered as magnesium ammonium phosphate crystals. In the phosphorus recovery apparatus 12, it is necessary to add a magnesium source in order to advance the magnesium ammonium phosphate crystallization reaction, but it is also possible to reuse the separation liquid 11 as a part of the magnesium source.

FIG. 3 shows a schematic configuration of an example of an apparatus for reusing the separation liquid 11 as a part of the magnesium source and carrying out the organic waste anaerobic digestion method of the present invention . The difference from the apparatus of FIG. 2 will be described in detail. A chemical addition tank 18 is formed by adding a chemical such as caustic soda to the separation liquid 11 from the solid-liquid separation apparatus 9 to form a magnesium salt floc. It is what makes it settle and separate. The precipitate 19 containing the magnesium salt is supplied to the phosphorus recovery apparatus 12 as a crystallization apparatus, and is reused as a part of the magnesium source in the phosphorus recovery apparatus 12 as described above. The supernatant 20 after the magnesium salt is settled and separated in the chemical addition tank 18 is mixed with the solubilized concentrated sludge 10 and supplied to the anaerobic digestion tank 5.

[Example]
Processing was performed using the apparatus shown in FIG. That is, the organic waste 1 to be treated was concentrated excess sludge 0.01 m ³ by the standard activated sludge method. Sulfuric acid was added to this concentrated excess sludge, and after 30 minutes of stirring with a stirrer (60 rpm), solubilization was performed by the solubilizer 2. The addition amount of sulfuric acid, was per sludge 1m ³ 1.2kg. As the solubilizer 2, a wet bead mill was used, and the treatment time here was 10 minutes.

Subsequently, the solubilized sludge 6 was subjected to solid-liquid separation by a solid-liquid separation device 9 using a centrifuge to obtain a solubilized concentrated sludge 10 (4 L) and a separated liquid 11 (6 L). Then, caustic soda was added to the separation liquid 11 containing magnesium in the chemical addition tank 18 to a pH of 11, and the flocs of magnesium salt were formed. The required addition amount of caustic soda was 2.5 kg per 1 m ^{3 of the} separation liquid 11. A white precipitate 19 (0.6 L) obtained by precipitating this magnesium salt floc by centrifugation in the chemical addition tank 18 was recovered. The supernatant water 20 after the precipitation separation was mixed with the solubilized concentrated sludge 10.

  This solubilized concentrated sludge 10 and supernatant water 20 are digested in a batch system for 20 days using a small anaerobic digester 5 (effective capacity 1 L) filled with 0.5 L of digested sludge as seed sludge, Digested sludge 8 and methane gas 17 as biogas were obtained. The biogas generation amount for 20 days was 300 NL / kg-VS with respect to the input sludge. Digested sludge 8 was filtered with a dehydrator 14 and separated into filtrate 15 (0.4 L) and sludge 16.

  Table 1 shows the water quality at each stage of the above process.

  The ammonium ion concentration of the dehydrated filtrate 15 was sufficient to produce magnesium ammonium phosphate, but the magnesium concentration was insufficient. Therefore, a magnesium salt precipitate 19 recovered from the separation liquid 11 was added in a phosphorus recovery device 12 as a crystallizer. At this time, the precipitate concentration of the magnesium salt was 5.4 g / L with respect to magnesium hydroxide, and 30 mL was sufficient for the amount of the precipitate 19 added to 0.4 L of the dehydrated filtrate 15. As a result, since the pH inside the phosphorus recovery apparatus 12 for crystallization increased to 8.2, it was not necessary to add an alkali agent. Crystallization product 13 was obtained by performing crystallization of magnesium ammonium phosphate in a batch system with a stirring time of 30 minutes and a precipitation time of 30 minutes.

[Comparative example]
The organic waste 1 to be treated was 0.01 m ³ of concentrated excess sludge of the standard activated sludge method similar to that of the example. Sulfuric acid was added to this concentrated excess sludge, and after 30 minutes of stirring with a stirrer (60 rpm), solubilization was performed by the solubilizer 2. The addition amount of sulfuric acid, was per sludge 1m ³ 1.2kg. As the solubilizer 2, a wet bead mill was used, and the treatment time here was 10 minutes.

  Next, the solubilized sludge 3 is used for 20 days using a small anaerobic digester 5 (effective capacity 1 L) filled with 0.5 L of digested sludge as seed sludge without solid-liquid separation, that is, without removing magnesium. Digestion was carried out batchwise to obtain digested sludge 8 and biogas 17. The biogas generation amount for 20 days was 250 NL / kg-VS with respect to the input sludge. And the digested sludge 8 was filtered with the dehydrator 14, and it isolate | separated into the filtrate 15 (0.4L) and the sludge 16. FIG. Properties of the filtrate 15 are shown in Table 2.

  The ammonium ion concentration was sufficient to produce magnesium ammonium phosphate relative to the phosphate ion concentration of the dehydrated filtrate 15, but the magnesium concentration was insufficient. Therefore, in the phosphorus recovery apparatus 12 as the crystallization apparatus, an Mg salt was added so as to be equimolar to the phosphate ion concentration using a magnesium chloride aqueous solution (1 g-Mg / L) as the magnesium agent. Since the pH inside the phosphorus recovery apparatus 12 for crystallization was as low as 7.6, it was necessary to add caustic soda as an alkaline agent in order to advance the crystallization reaction. Magnesium ammonium phosphate crystallization was carried out batchwise, with stirring time of 30 minutes and precipitation time of 30 minutes to obtain magnesium ammonium phosphate 13 as a crystallized product. The recovered crystallized product was weighed after drying at room temperature. Then, the recovered amount was 300 mg, which was only about 40% with respect to the recovered amount in the example of the present invention.

  From the above results, the organic waste is solubilized in the first stage before anaerobic digestion of the organic waste, and the magnesium in the organic waste is eluted in advance as magnesium ions. By anaerobic digestion of organic waste from which magnesium ions have been removed from waste, it has been found that most of the phosphorus in the sludge can be recovered by crystallizing magnesium ammonium phosphate while avoiding scale troubles.

It is a figure which shows schematic structure of an example of the apparatus for enforcing the anaerobic digestion method of the organic waste of this invention. It is a figure which shows schematic structure of the other example of the apparatus for enforcing the anaerobic digestion method of the organic waste of this invention. It is a figure which shows schematic structure of the further another example of the apparatus for implementing the anaerobic digestion method of the organic waste of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Organic waste 2 Solubilization device 6 Magnesium removal sludge 5 Anaerobic digestion tank 9 Solid-liquid separation device 12 Phosphorus collection device 14 Dehydrator 18 Chemical addition tank

Claims (4)

A method for anaerobic digestion of organic waste, comprising the following steps (a), (b) and (c):
(B) Step of solubilizing organic waste and eluting magnesium in the organic waste (b) By solid-liquid separation of the organic waste solubilized in the step (b) The step of removing magnesium as a sparingly soluble magnesium salt by separating the liquid phase containing magnesium and adding an alkaline agent to the separated liquid phase (c) The magnesium is removed in the step (b), And anaerobic digestion of the liquid phase after removal of the sparingly soluble magnesium salt together with organic waste that has not been subjected to the process of eluting new magnesium   2. The method for anaerobic digestion of organic waste according to claim 1, wherein the step (a) is a wet bead mill stirring and crushing step. Furthermore, the following process is included, The anaerobic digestion method of the organic waste of Claim 1 or 2 characterized by the above-mentioned.
(D) A step of dehydrating the digested sludge treated in the step (c) to obtain a dehydrated filtrate (e) Nitrogen and phosphorus contained in the dehydrated filtrate obtained in the step (d) are magnesium phosphate. Process to recover by ammonium crystallization method   The method of anaerobic digestion of organic waste according to claim 3, wherein the magnesium removed in the step (b) is used as a magnesium source in the step (e).

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