Phosphorus recovery method and phosphorus recovery device

Abstract

To provide a method and an apparatus for increasing phosphorus recovery from biologically treated excess sludge by adding an inorganic coagulant containing iron or aluminum.SOLUTION: Excess sludge from biological treatment tank 20, which has been biologically treated by adding inorganic coagulant containing iron or aluminum, is immobilized with iron or aluminum ions by adding sulfide or chelating agents in phosphate ion release tank 40 under pH less than 9 and under anaerobic conditions to form sludge containing phosphate ions released from the phosphorus accumulating microbes in the excess sludge. The phosphate ion-containing sludge from the phosphate ion release tank 40 is concentrated and separated in the concentrator 60, and the phosphate ions are recovered as MAP or HAP from the concentrated and separated liquid in the phosphorus recovery tank 80.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method and an apparatus for recovering phosphorus from excess sludge after biological treatment. [Background technology]

[0002] While phosphorus is the main component of fertilizer, it is a depletable resource, and recycling is required from the viewpoint of effective resource utilization. Therefore, focusing on the fact that various waste liquids and sludges such as drinking water, sewage, human wastewater, industrial water, and factory wastewater contain phosphorus, various devices and methods for removing and recovering phosphorus from them have been studied. As a method for recovering phosphorus from waste liquids and sludge as fertilizer, for example, a method of crystallizing and recovering phosphoric acid as magnesium ammonium phosphate (hereinafter sometimes abbreviated as "MAP") and hydroxyapatite (hereinafter sometimes abbreviated as "HAP") is known. Hereinafter, the recovery method that forms MAP may be referred to as the MAP method, and the recovery method that forms HAP may be referred to as the HAP method.

[0003] In addition, in order to recover phosphorus, it is necessary that phosphorus is solubilized in the form of phosphoric acid, and the higher the phosphoric acid concentration, the greater the amount of phosphorus that can be recovered. The following methods are known for increasing the phosphoric acid concentration in sludge:

[0004] Japanese Patent Application Laid-Open No. 2013-119081 discloses a technical idea for the purpose of efficiently recovering phosphorus from sludge after phosphorus release treatment, in which excess sludge obtained by biological treatment of wastewater is supplied to a phosphorus release tank while maintained in an aerobic state for biological dephosphorization, and then phosphorus is recovered from the filtrate obtained by solid-liquid separation, thereby improving the phosphorus recovery rate. Specifically, the method for treating phosphorus-containing wastewater includes a biological treatment step of aerobically treating phosphorus-containing wastewater, a re-aerobic treatment step of further aerobically treating the first excess sludge obtained by the biological treatment step, a concentration step of adding a flocculant to the second excess sludge obtained by the re-aerobic treatment step and then concentrating it by solid-liquid separation, a phosphorus release step of retaining the concentrated sludge obtained by the concentration treatment step under anaerobic conditions at 15°C to 35°C for 3 hours to 48 hours to release phosphorus from the concentrated sludge, a solid-liquid separation step of adding the primary settling sludge obtained from the wastewater before the first aerobic treatment to the concentrated sludge after the phosphorus release step, adding a flocculant and performing solid-liquid separation, and a phosphorus recovery step of adding a phosphorus recovery agent to the filtrate obtained by the solid-liquid separation to crystallize and recover phosphorus contained in the filtrate as a salt. The phosphorus release step uses a biological phosphorus removal method based on the metabolism of microorganisms, and is a step of releasing phosphorus accumulated in microorganisms by maintaining the temperature at 15°C to 35°C, at which the metabolism of phosphorus-accumulating bacteria is active. The flocculants used in the solid-liquid separation process and the concentration process are organic flocculants or polymer flocculants, rather than aluminum-based or iron-based flocculants, which tend to capture phosphorus, to prevent the phosphorus in the sludge from being fixed as an inorganic salt.

[0005] Japanese Patent Application Laid-Open No. 2017-124351 discloses a technical idea for dissolving nitrogen and phosphorus components in sludge in a balanced manner and efficiently recovering magnesium ammonium phosphate, in which an alkaline agent is added to the sludge to maintain the pH of the sludge at 11.0 or higher and the sludge is retained for a specified period of time. Specifically, the phosphorus recovery method includes a step of adding an alkaline agent to sludge after biological treatment, a step of retaining the sludge to which the alkaline agent has been added, and a step of adding a magnesium salt to the retained sludge, in which the alkaline agent is added so that the pH of the sludge is 11.0 to 13.0 in the step of adding the alkaline agent to the sludge, and in which the sludge is retained at a temperature of 20°C to 50°C for 30 to 144 hours while maintaining the pH of the sludge at 8.4 to 10.5, thereby eluting ammoniacal nitrogen and phosphate-type phosphorus, and then adding magnesium to the sludge containing the ammoniacal nitrogen and phosphate-type phosphorus to form MAP. Here, phosphorus elution is performed by adding an alkaline agent to make the pH strongly alkaline at 11.0 or more, thereby dissolving the cell membranes of phosphorus-accumulating microorganisms and eluting the phosphorus accumulated in the microorganisms. In the alkaline range, if magnesium is present, nitrogen, phosphorus, and magnesium will immediately react to form MAP, so an alkaline treatment tank and a phosphorus reaction tank are provided separately to suppress the progress of the MAP formation reaction. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2013-119081 A [Patent Document 2] JP 2017-124351 A Summary of the Invention [Problem to be solved by the invention]

[0007] In recent years, in order to improve the solid-liquid separation in biological treatment and to remove refractory organic matter in wastewater, treatments in which inorganic coagulants containing iron or aluminum are directly added to biological treatment tanks have become more common. The excess sludge after biological treatment with the addition of inorganic coagulants containing iron or aluminum contains metal ions derived from the inorganic coagulants. When an alkaline agent is added in a conventional phosphorus recovery method to create a strong alkaline atmosphere and leach phosphate ions from phosphorus-accumulating microorganisms, the leach phosphate ions react with the metal ions and are re-immobilized as metal phosphates, so that the amount of phosphorus in the form of phosphorus does not increase and the amount of phosphorus recovered does not increase. In addition, the method of adjusting the excess sludge to an alkaline region to elute phosphate phosphorus requires the addition of a large amount of chemicals, which poses safety issues on-site.

[0008] An object of the present invention is to provide a method and apparatus for increasing the amount of phosphorus recovered from excess sludge that has been biologically treated by adding an inorganic flocculant containing iron or aluminum. [Means for solving the problem]

[0009] In conventional phosphorus recovery methods, since phosphorus in excess sludge after biological treatment is accumulated in the biofilm of phosphorus-accumulating microorganisms, an alkaline agent is added to the excess sludge to create a strongly alkaline atmosphere of about pH 11, destroying the biofilm of the phosphorus-accumulating microorganisms and eluting the phosphorus. However, metal ions derived from the inorganic coagulant are present in the excess sludge that has been biologically treated with the addition of an inorganic coagulant, and the eluted phosphorus binds to the metal ions to become phosphates and is immobilized again, so the number of phosphate ions in the excess sludge does not increase, making it difficult to improve phosphorus recovery efficiency.

[0010] The present inventors discovered that, rather than using the conventional method of destroying the biofilm of phosphorus-accumulating microorganisms in a strongly alkaline atmosphere by adding an alkaline agent, a method of releasing the phosphorus that the phosphorus-accumulating microorganisms have accumulated as polyphosphate through their metabolism under weakly alkaline to weakly acidic and anaerobic conditions as phosphate-phosphorus, and adding a sulfide or a chelating agent to immobilize metal ions derived from inorganic coagulants and suppress their reaction with phosphate-phosphorus, can suppress the re-immobilization of phosphorus even in excess sludge after biological treatment that contains a large amount of metal ions derived from inorganic coagulants, and can increase the amount of phosphorus recovered by the MAP method or HAP method, thereby completing the present invention.

[0011] According to the present invention, there is provided a method for recovering phosphorus, which comprises adding a sulfide or a chelating agent to excess sludge that has been subjected to biological treatment by adding an iron-containing inorganic coagulant under anaerobic conditions at a pH of less than 9 to immobilize iron ions, thereby forming phosphate ion-containing sludge that contains phosphate ions released from phosphorus-accumulating microorganisms in the excess sludge, and adding a magnesium salt or a calcium salt to a concentrated separation liquid obtained by concentrating and separating the phosphate ion-containing sludge, thereby recovering phosphate ions as magnesium ammonium phosphate or hydroxyapatite.

[0012] The present invention also provides a method for recovering phosphorus, comprising the steps of: adding a chelating agent to excess sludge that has been subjected to biological treatment by adding an aluminum-containing inorganic coagulant under anaerobic conditions at a pH of less than 9 to immobilize aluminum ions, thereby forming phosphate ion-containing sludge that contains phosphate ions released from phosphorus-accumulating microorganisms in the excess sludge; and adding a magnesium salt or a calcium salt to a concentrated separation liquid obtained by concentrating and separating the phosphate ion-containing sludge, thereby recovering phosphate ions as magnesium ammonium phosphate or hydroxyapatite.

[0013] The excess sludge to be treated in the present invention is excess sludge after biological treatment with the addition of an inorganic flocculant containing iron or aluminum, and contains iron or aluminum and phosphorus. For example, preferred examples include excess sludge that has been biologically treated by adding an inorganic flocculant containing iron or aluminum to septic tank sludge generated in a sewage treatment plant or a private factory, or excess sludge that has been biologically treated by adding an inorganic flocculant containing iron or aluminum to supernatant water from a settling tank of a sewage treatment plant. The properties of the excess sludge are that the total phosphorus (TP) is 10 to 2000 mg-P / L, and the total amount of iron or aluminum is about 0.1 to 10 times, preferably about 0.5 to 5 times, the substance amount ratio relative to the total phosphorus, that is, the total amount of iron or aluminum is desirably 0.1 mol to 10 mol, preferably 0.5 mol to 5 mol, relative to 1 mol of total phosphorus.

[0014] The inorganic flocculant added during biological treatment of sludge or water may be any that contains iron or aluminum, and may be any inorganic flocculant commonly used in the relevant technical field, without limitation, such as polyferric sulfate (polyiron), aluminum sulfate (aluminum sulfate), polyaluminum chloride (PAC), ferric chloride, ferrous chloride, and ferrous sulfate. In particular, when a sulfide, as described below, is added, it is preferable to add an iron-based flocculant such as polyiron or ferric chloride.

[0015] The sulfide to be added to the excess sludge is preferably one or more sulfides selected from hydrogen sulfide, sodium sulfide, sodium hydrogen sulfide, and any mixture thereof. As the hydrogen sulfide, a gas containing hydrogen sulfide, for example, a high-concentration odorous gas in a treatment plant, can be used. The amount of sulfide to be added is an amount in which the amount of sulfur is 0.2 to 3 times, preferably 0.5 to 1.5 times, the amount of iron in the excess sludge, that is, 0.2 mol to 3.0 mol, preferably 0.5 mol to 1.5 mol of sulfur per mol of iron in the excess sludge. In the present invention, most of the iron or aluminum in the excess sludge to be treated is considered to be derived from the inorganic flocculant added during biological treatment, so the amount of sulfide to be added to the excess sludge can be roughly determined based on the iron content in the inorganic flocculant added during biological treatment.

[0016] The chelating agent to be added to the excess sludge may be any agent capable of forming a complex with iron or aluminum, and may be any chelating agent commonly used in the art, without limitation. For example, one or more chelating agents selected from EDTA, NTA, citric acid, oxalic acid, salicylic acid, tartaric acid, tannin, and any mixture thereof may be preferably used. The amount of the chelating agent to be added varies depending on the physical properties of the chelating agent, but can be determined based on the reaction equivalent with iron or aluminum specific to each chelating agent. For example, EDTA reacts with iron at a substance ratio of 1:1, so the theoretical amount of EDTA to be added is 1 times the substance ratio of iron in the excess sludge. In this way, the amount of each chelating agent to be added can be determined to be 0.2 to 3 times, preferably 0.5 to 1.5 times the theoretical amount of addition determined depending on the metal element.

[0017] In the present invention, the addition of sulfide or a chelating agent to excess sludge is carried out under anaerobic conditions with a pH of less than 9. The anaerobic conditions are S-COD Cr / MLSS is 0.0 2 by upper 0.1 5 by below, preferably 0.0 4 by Top 0. 1 byThe anaerobic conditions can be adjusted to below pH 6.0. The anaerobic conditions can be adjusted by adding primary settling sludge, sewage sludge or septic tank sludge that has not been subjected to biological treatment, or organic matter (methanol, ethanol, acetic acid, soluble organic matter, etc.) to the excess sludge after biological treatment. Here, if the pH is high, the biofilm of the phosphorus-accumulating microorganisms will be destroyed before the metal ions derived from the inorganic flocculant bind to the sulfide or chelating agent, and the phosphorus accumulated by the phosphorus-accumulating microorganisms will be eluted and bound to the metal ions derived from the inorganic flocculant, resulting in the phosphate phosphorus being immobilized again. Therefore, it is preferable that the pH of the excess sludge is weakly acidic or weakly alkaline to such an extent that the release of phosphate ions due to the metabolism of the microorganisms proceeds, but the destruction of the biofilm of the microorganisms does not occur or occurs slowly. In addition, in the case of sludge treatment, the supernatant water after phosphorus recovery is subjected to biological treatment, so that the excess sludge after biological treatment may contain magnesium ions or calcium ions, which react with the phosphate ions eluted from the excess sludge at a high pH to form MAP or HAP, which precipitates, and the phosphate ion concentration in the phosphate ion-containing sludge decreases. In addition, because sulfides are added, hydrogen sulfide gas may be generated in a strongly acidic range. Therefore, the excess sludge when releasing phosphate ions from phosphorus-accumulating microorganisms in the excess sludge should have a pH of less than 9.0, preferably a pH of 8.5 or less, more preferably a pH of 8.0 or less, a pH of 5.5 or more, preferably a pH of 6.0 or more, and more preferably a pH of 6.5 or more.

[0018] Under anaerobic conditions, polyphosphates accumulated in phosphorus-accumulating microorganisms in excess sludge are released as phosphate ions. Iron phosphate in excess sludge is [ka] When sulfide is added to excess sludge in a weakly acidic or weakly alkaline atmosphere with a pH of less than 9, the equilibrium of the above formula (1) is lost, and iron phosphate is converted into phosphate ion and Fe 3+ and Fe 2+The sludge is shifted in the direction of dissociation to phosphate ion. When a certain amount of sulfide is added, the iron ions derived from the iron phosphate and the iron ions derived from the inorganic flocculant are preferentially combined with sulfur to form iron sulfide, and the combination of the phosphate ions released from the phosphorus-accumulating microorganisms with the iron ions is inhibited, so that sludge containing phosphate ions is formed. Alternatively, when a chelating agent is added, the metal components of the metal phosphates in the excess sludge and the metal ions derived from the inorganic flocculant are adsorbed, and the combination with the released phosphate ions is inhibited, so that sludge containing phosphate ions is formed. The sludge containing phosphate ions is concentrated and separated, and the concentrated separation liquid containing phosphate ions is subjected to the MAP method or the HAP method to recover the phosphate ions as magnesium phosphate (MAP) or hydroxyapatite (HAP).

[0019] The MAP method is suitable when the amount of ammonium ions in the concentrated separated liquid is greater than that of phosphate ions. A magnesium salt (magnesium hydroxide, magnesium chloride, etc.) and an alkaline agent are added to the concentrated separated liquid containing phosphate ions converted from metal phosphate salts, and the pH is adjusted to 7.5 to 9.5, preferably 8.0 to 9.0, to precipitate the phosphate ions as magnesium ammonium phosphate (MAP), and phosphorus can be recovered. The amount of magnesium salt added is preferably 1 to 1.2 times the amount of phosphate ions in the concentrated separated liquid, that is, 1 mol to 1.2 mol of magnesium ions per 1 mol of phosphate ions.

[0020] The HAP method is suitable when the amount of ammonium ions in the concentrated separated liquid is less than that of phosphate ions. A calcium salt (calcium hydroxide, calcium chloride, etc.) and an alkaline agent are added to the concentrated separated liquid to adjust the pH to 7.0 to 8.5, preferably 7.5 to 8.0, to precipitate phosphate ions as hydroxyapatite, and phosphorus can be recovered. The amount of calcium salt added is preferably 1.7 to 1.9 times the amount of phosphate ions in the concentrated separated liquid, that is, 1.7 to 1.9 mol of calcium ions per 1 mol of phosphate ions.

[0021] The concentrated separated liquid can be formed by adding a polymer flocculant to phosphate ion-containing sludge to flocculate it, and then concentrating and separating it into concentrated sludge and concentrated separated liquid using a concentrator. As the polymer flocculant, any commonly used polymer flocculant can be used without any particular limitation.

[0022] In a general sludge treatment method, an inorganic flocculant is added to improve the dewaterability of the sludge. In the present invention, when an inorganic flocculant is added to sludge containing phosphate ions, the phosphate ions recombine with the metal ions in the inorganic flocculant, so the inorganic flocculant is added to the concentrated sludge after concentration and separation. The concentrated sludge to which the inorganic flocculant has been added is subjected to a dehydration treatment to form a dehydrated cake and a dehydrated separated liquid. The dehydrated separated liquid obtained by the dehydration treatment can be subjected to biological treatment by adding an inorganic flocculant. The treated liquid after phosphorus is recovered from the concentrated separated liquid containing phosphate ions can be subjected to biological treatment together with the dehydrated separated liquid. The excess sludge after biological treatment can be subjected to phosphate ion release again.

[0023] In a typical sewage treatment method, when concentrated sludge is dehydrated without adding an inorganic flocculant, the dehydrated separated liquid can be subjected to phosphorus recovery together with the concentrated separated liquid.

[0024] Furthermore, according to the present invention, there is provided a phosphorus recovery device comprising: a biological treatment tank for adding an inorganic coagulant containing iron; a phosphate ion release tank for adding a sulfide or a chelating agent to excess sludge from the biological treatment tank under anaerobic conditions at a pH of less than 9 to immobilize iron ions and to release polyphosphoric acid and metal phosphates in the excess sludge as phosphate ions; and a phosphorus recovery tank for recovering phosphate ions as magnesium ammonium phosphate or hydroxyapatite from a concentrated separation liquid obtained by concentrating and separating the phosphate ion-containing sludge from the phosphate ion release tank.

[0025] Furthermore, according to the present invention, there is provided a phosphorus recovery device comprising: a biological treatment tank for adding an inorganic coagulant containing aluminum; a phosphate ion release tank for adding a chelating agent to excess sludge from the biological treatment tank under anaerobic conditions at a pH of less than 9 to immobilize aluminum ions and to release polyphosphoric acid and metal phosphates in the excess sludge as phosphate ions; and a phosphorus recovery tank for recovering phosphate ions as magnesium ammonium phosphate or hydroxyapatite from a concentrated separation liquid obtained by concentrating and separating the phosphate ion-containing sludge from the phosphate ion release tank.

[0026] The phosphorus recovery apparatus of the present invention preferably further comprises a coagulation reaction tank for adding a polymer coagulant to the phosphate ion-containing sludge from the phosphate ion release tank, a concentrator for concentrating and separating the phosphate ion-containing sludge from the coagulation reaction tank to which the polymer coagulant has been added into a concentrated separation liquid and sludge, and a dehydrator for dehydrating the sludge from the concentrator into a dehydrated cake and a dehydrated separation liquid. The concentrated separation liquid and the dehydrated separation liquid are supplied to the phosphorus recovery tank, and phosphate ions can be recovered as magnesium ammonium phosphate or hydroxyapatite.

[0027] The phosphorus recovery apparatus of the present invention further includes a COD filter for the excess sludge between the biological treatment tank and the phosphate ion release tank. Cr / MLSS to 0.0 2 by upper 0.1 5 by By installing an anaerobic tank before the phosphate ion release tank, the excess sludge can be put into anaerobic conditions in advance, and the sulfate ions (SO4 2- ) is reduced to sulfide ions (S 2- ) and hydrogen sulfide ion (HS - ), the amount of sulfide added to the phosphate ion release tank can be reduced. Effect of the Invention

[0028] According to the phosphorus recovery method of the present invention, even if the excess sludge after biological treatment contains a large amount of metal ions derived from an inorganic flocculant, the amount of phosphorus recovered by the MAP method or the HAP method can be increased.

[0029] Furthermore, unlike conventional methods in which a large amount of alkaline agents are added to create a strongly alkaline atmosphere, the phosphorus recovery method of the present invention promotes the release of phosphate ions through the metabolism of phosphorus-accumulating microorganisms and creates a weak acidity or alkalinity that does not destroy biofilms or only gently destroys them, thereby contributing to a reduction in the amount of alkaline agents added and a reduction in the risk of danger at the site. [Brief description of the drawings]

[0030]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

[0031] The present invention will now be described in more detail with reference to the accompanying drawings, but the present invention is not limited thereto.

[0032] Fig. 1 is a schematic diagram showing a treatment flow using the phosphorus recovery apparatus of the present invention when applied to a general activated sludge treatment. The phosphorus recovery apparatus in Fig. 1 includes a settling tank 10, an activated sludge tank 20, a final settling tank 30, a phosphate ion release tank 40, a flocculation reaction tank 50, a thickener 60, a dehydrator 70, and a phosphorus recovery tank 80.

[0033] In FIG. 1, raw water to be treated, such as sewage or industrial wastewater, is sent to a sedimentation tank 10 to remove large garbage, then sent to an activated sludge tank 20, where an inorganic flocculant is added and the water is subjected to activated sludge treatment (biological treatment). The activated sludge treated water is sent to a final sedimentation tank 30 and separated into treated water and excess sludge. The excess sludge is sent to a phosphate ion release tank 40, where a sulfide or a chelating agent is added under anaerobic conditions at a pH of less than 9, whereby metal ions derived from the inorganic flocculant are immobilized, and polyphosphoric acid accumulated by phosphorus-accumulating microorganisms in the excess sludge and metal phosphates in the excess sludge are released as phosphate ions, forming phosphate ion-containing sludge. The phosphate ion-containing sludge is sent to a flocculation reaction tank 50, where a polymer flocculant is added to form flocculated sludge. The flocculated sludge is sent to a thickener 60 and separated into thickened sludge and thickened separated liquid. The thickened sludge is sent to a dehydrator 70 and separated into a dehydrated cake and a dehydrated separated liquid. The concentrated separated liquid from the concentrator 60 and the dehydrated separated liquid from the dehydrator 70 are sent to a phosphorus recovery tank 80, where an alkaline agent and a magnesium salt or a calcium salt are added to form magnesium ammonium phosphate (MAP) or hydroxyapatite (HAP). The formed MAP or HAP is recovered from the phosphorus recovery tank 80.

[0034] If the pH of the phosphate ion releasing tank 40 is high, phosphate ions from the phosphorus-accumulating microorganisms are rapidly eluted and may bind with metal ions in the excess sludge to form metal phosphates and be re-immobilized, so the pH is set to less than 9.0, preferably 8.5 or less, more preferably 8.0 or less, 5.5 or more, preferably 6.0 or more, more preferably 6.5 or more. The pH of the phosphate ion releasing tank 40 can be adjusted by adding an appropriate amount of a pH adjuster. As the pH adjuster, a pH adjuster generally used in the technical field can be used. For example, sulfuric acid, hydrochloric acid, etc. can be used as the acid, and caustic soda, sodium carbonate, sodium bicarbonate, etc. can be used as the alkali.

[0035] The anaerobic conditions in the phosphate ion release tank 40 are Cr / MLSS is 0.02 mg or more and 0.15 mg or less, preferably 0.0 4 by upper 0.1 0 by The excess sludge introduced into the phosphate ion release tank 40 is preferably 0.00 2 processes Low COD Cr / MLSS, the COD can be reduced by adding an appropriate amount of primary sludge from the sedimentation tank 10 that is not biologically treated, or methanol, ethanol, acetic acid, dissolved organic matter, etc. Cr The phosphate ion releasing tank 40 is preferably connected to a primary sludge supply line 42 extending from the settling tank 10.

[0036] In the phosphate ion release tank 40, the excess sludge is stored in the phosphate ion release tank 40 for a predetermined time or more, whereby the phosphate ions from the phosphorus accumulating microorganisms are released, and the excess sludge is sufficiently reacted with the sulfide or the chelating agent, so that the iron ions derived from the inorganic flocculant are immobilized by the sulfide or the chelating agent, and the aluminum ions are immobilized by the chelating agent. Therefore, the phosphate ions released from the phosphorus accumulating microorganisms in the excess sludge are less likely to bind to the metal ions derived from the inorganic flocculant, and are maintained as phosphate ions. The retention time of the excess sludge in the phosphate ion release tank 40 is desirably 2 hours or more, preferably 4 hours or more. On the other hand, if the excess sludge is retained for too long, the release of phosphate ions is saturated, and the effect of inhibiting the binding of phosphate ions and metal ions is also saturated, so that the retention time is desirably 24 hours or less, preferably 12 hours or less.

[0037] In addition, by adding a predetermined amount of sulfide or chelating agent to the excess sludge in the phosphate ion release tank 40, the iron ions in the excess sludge are preferentially reacted with the sulfide or chelating agent to immobilize the iron ions, or the aluminum ions in the excess sludge are preferentially reacted with the chelating agent to immobilize the aluminum ions, thereby inhibiting the binding of the phosphate ions released from the excess sludge with the iron ions or aluminum ions. The sulfide can be appropriately selected from hydrogen sulfide, sodium sulfide, sodium hydrogen sulfide, and any mixture thereof. The amount of sulfide added is preferably an amount such that the molar ratio of the total iron (T-Fe) in the excess sludge to the sulfur (S) in the sulfide is 0.2 or more, preferably 0.5 or more, and more preferably 1 or more. As the chelating agent, a chelating agent capable of immobilizing iron or aluminum can be used appropriately and in an appropriate amount.

[0038] Regarding the retention time of excess sludge and the amount of sulfide added in the phosphate ion release tank 40, as shown in the examples described later, it has been confirmed that when the molar ratio S / T-Fe of the total iron amount in the excess sludge to the sulfur amount in the sulfide is 0.5 or more, the phosphate ion concentration is saturated at a retention time of about 2 hours, and when S / T-Fe is 0.2 or more and less than 0.5, the phosphate ion concentration is saturated at a retention time of about 6 hours.

[0039] Figure 2 shows a modification of the phosphorus recovery apparatus of Figure 1, in which an anaerobic tank 90 for putting excess sludge into anaerobic conditions is provided between the activated sludge tank (biological treatment tank) 20 and the phosphate ion release tank 40. Since the other configuration is the same as that of the phosphorus recovery apparatus shown in Figure 1, only the differences will be described and overlapping descriptions will be omitted.

[0040] In Fig. 2, the anaerobic tank 90 is provided between the final settling tank 30 and the phosphate ion release tank 40 in order to make the excess sludge from the final settling tank 30 under anaerobic conditions. In order to maintain a constant sludge concentration in the activated sludge tank 20, a sludge return line 34 is provided to return a portion of the excess sludge from the final settling tank 30 to the activated sludge tank 20. The anaerobic tank 90 is preferably provided immediately before the phosphate ion release tank 40, downstream of the branch point where the sludge return line 34 to the activated sludge tank 20 is connected, in the excess sludge discharge line 32 that extracts the excess sludge from the final settling tank 30. A primary sludge supply line 92 that receives the primary sludge from the settling tank 10 is preferably connected to the anaerobic tank 90 in order to adjust the anaerobic conditions.

[0041] Fig. 3 is a schematic diagram showing the treatment flow in the phosphorus recovery apparatus of the present invention when applied to general biological treatment of sewage and septic tank sludge. The phosphorus recovery apparatus shown in Fig. 3 includes a sludge storage tank 110 for receiving the sewage and septic tank sludge to be treated, a flocculation reaction tank 120, a thickener 130, a dehydrator 140, a biological treatment tank 150, a phosphate ion release tank 160, and a phosphorus recovery tank 170.

[0042] In FIG. 3, sewage and septic tank sludge to be treated is received in a sludge storage tank 110, and after large impurities are removed, it is sent to a coagulation reaction tank 120 where a polymer coagulant is added to form coagulated flocs. The coagulated flocs are sent to a thickener 130 where they are thickened and separated into thickened sludge and thickened separated liquid. The thickened sludge is sent to a dehydrator 140 where they are separated into a dehydrated cake and a dehydrated separated liquid. The dehydrated separated liquid is sent to a biological treatment tank 150 where an inorganic coagulant is added to perform biological treatment to form treated water and excess sludge. The excess sludge after biological treatment is sent to a phosphate ion release tank 160 where a sulfide or a chelating agent is added under anaerobic conditions of less than pH 9, and metal ions derived from the inorganic coagulant are fixed and phosphate ions are released from phosphorus-accumulating microorganisms in the excess sludge, forming phosphate ion-containing sludge. The phosphate ion-containing sludge is sent to a sludge storage tank 110 where it is mixed with untreated sludge. The mixed sludge containing phosphate ion-containing sludge is sent to the coagulation reaction tank 120, where a polymer coagulant is added, and coagulated flocs containing phosphate ion-containing sludge are formed. The coagulated flocs containing phosphate ion-containing sludge are sent to the concentrator 130, where they are concentrated and separated into concentrated sludge containing phosphate ion-containing sludge and concentrated separated liquid containing phosphate ion. The concentrated separated liquid containing phosphate ion is sent to the phosphorus recovery tank 170, where a magnesium salt or a calcium salt and an alkaline agent are added to form magnesium ammonium phosphate (MAP) or hydroxyapatite (HAP). The formed MAP or HAP is recovered from the phosphorus recovery tank 170. The concentrated sludge is sent to the dehydrator 140, where it is separated into a dehydrated cake and a dehydrated separated liquid. The supernatant water from the phosphorus recovery tank 170 and the dehydrated separated liquid from the dehydrator 140 are sent to the biological treatment tank 150, where an inorganic coagulant containing iron or aluminum is added to perform biological treatment.

[0043] If the pH of the phosphate ion release tank 160 is high, phosphate ions from the phosphorus-accumulating microorganisms are rapidly eluted and may bind with metal ions in the excess sludge to form metal phosphates and be re-immobilized, so the pH is set to less than 9.0, preferably 8.5 or less, more preferably 8.0 or less, 5.5 or more, preferably 6.0 or more, more preferably 6.5 or more. The pH of the phosphate ion release tank 170 can be adjusted by adding an appropriate amount of a pH adjuster. As the pH adjuster, a pH adjuster generally used in the technical field can be used. For example, sulfuric acid, hydrochloric acid, etc. can be used as the acid, and caustic soda, sodium carbonate, sodium bicarbonate, etc. can be used as the alkali.

[0044] The anaerobic conditions in the phosphate ion release tank 160 are Cr / MLSS is 0.0 2 by upper 0.1 5 by Below, preferably 0.0 4 by upper 0.1 0 by The excess sludge introduced into the phosphate ion release tank 160 is preferably 0.00 2 processes Low COD Cr Since it has MLSS, COD can be reduced by adding an appropriate amount of sewage / septic tank sludge before it is stored in the sludge storage tank 110 that has not been biologically treated, or methanol, ethanol, acetic acid, dissolved organic matter, etc. Cr In order to adjust the anaerobic conditions in the phosphate ion releasing tank 160, it is preferable that a line 162 for supplying untreated sewage / septic tank sludge is connected to the phosphate ion releasing tank 160.

[0045] In the phosphate ion releasing tank 160, the excess sludge is stored in the phosphate ion releasing tank 160 for a predetermined time or more to release phosphate ions from the phosphorus accumulating microorganisms, and the excess sludge is sufficiently reacted with a sulfide or a chelating agent to immobilize metal ions derived from the inorganic coagulant, thereby inhibiting binding between the phosphate ions released from the phosphorus accumulating microorganisms in the excess sludge and the metal ions. The retention time of the excess sludge in the phosphate ion releasing tank 160 is desirably 2 hours or more, preferably 4 hours or more. On the other hand, if the excess sludge is retained for too long, the effect of releasing phosphate ions becomes saturated, so the retention time is desirably 24 hours or less, preferably 12 hours or less.

[0046] In addition, in the phosphate ion releasing tank 160, a predetermined amount of sulfide or chelating agent is added to the excess sludge, so that the iron ions in the excess sludge are preferentially reacted with the sulfide or chelating agent to immobilize the iron ions, or the aluminum ions in the excess sludge are immobilized in the chelating agent, thereby inhibiting the binding of the phosphate ions released from the excess sludge with the metal ions. The sulfide can be appropriately selected from hydrogen sulfide, sodium sulfide, sodium hydrogen sulfide, and any mixture thereof. The amount of sulfide added is preferably an amount such that the molar ratio of the total iron (T-Fe) in the excess sludge to the sulfur (S) in the sulfide, S / T-Fe or S / T-Al, is 0.2 or more, preferably 0.5 or more, and more preferably 1 or more. As the chelating agent, a chelating agent capable of immobilizing iron or aluminum can be appropriately used in an appropriate amount.

[0047] Regarding the retention time of excess sludge and the amount of sulfide added in the phosphate ion release tank 160, as shown in the examples described later, it has been confirmed that when the molar ratio S / T-Fe of the total iron amount in the excess sludge to the sulfur amount in the sulfide is 0.5 or more, the phosphate ion concentration is saturated with a retention time of about 2 hours, and when S / T-Fe is 0.2 or more and less than 0.5, the phosphate ion concentration is saturated with a retention time of about 6 hours.

[0048] Figure 4 shows a modification of the phosphorus recovery apparatus shown in Figure 3, in which an anaerobic tank 180 for putting excess sludge into anaerobic conditions is provided between the biological treatment tank 150 and the phosphate ion release tank 160. Since the other configuration is the same as that of the phosphorus recovery apparatus shown in Figure 3, only the differences will be described and overlapping descriptions will be omitted.

[0049] The anaerobic tank 180 receives excess sludge extracted from the biological treatment tank 150, adjusts the conditions to anaerobic conditions, and then sends the phosphate ion releasing tank 160, so it is preferably provided on the excess sludge extraction line 152 from the biological treatment tank 150 just before the phosphate ion releasing tank 160. The anaerobic tank 180 is preferably connected to an untreated sludge receiving line 182 that can receive untreated sewage / septic tank sludge for adjustment of anaerobic conditions. EXAMPLES

[0050] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. The sludge from sewage and septic tanks and excess sludge after biological treatment in an actual sewage treatment facility were treated according to the treatment flow shown in Figure 3. In this sewage treatment facility, the sludge from sewage and septic tanks is stored in a sludge storage tank, then a polymer flocculant is added to form flocs, which are then concentrated in a thickener to separate the sludge from concentrated sludge and concentrated separated liquid. The concentrated sludge is directly dehydrated in a dehydrator, and the dehydrated separated liquid is subjected to biological treatment. In the biological treatment tank, an inorganic flocculant containing iron (polyferric sulfate) is added to remove persistent COD and color. In this example, in order to put the excess sludge under anaerobic conditions, untreated sewage and septic tank sludge is added to the excess sludge, and mixed sludge in which the excess sludge:untreated sewage and septic tank sludge are mixed in a 1:1 ratio is used to release phosphate ions. The properties of the excess sludge, untreated sewage and septic tank sludge, and mixed sludge are shown in Table 1.

[0051] [Table 1]

[0052] Sodium sulfide was added to mixed sludge under anaerobic conditions at pH 7.0 so that the molar ratios of S / T-Fe (sulfur / total iron) were 0.01, 0.1, 0.2, 0.5, 1.0, 1.5, and 3.0, and the change in phosphate ion concentration over time was examined. The results are shown in Figure 5.

[0053] From FIG. 5, it can be seen that by setting the molar ratio of S / T-Fe to 0.2 or more, the amount of phosphate ions increases to about 5 mg-P / L in a residence time of 2 hours, sulfur quickly binds to iron ions, and the released phosphate ions can be maintained as ions. It can also be seen that by setting the molar ratio of S / T-Fe to 0.5 or more, the amount of phosphate ions increases to about 5 mg-P / L in a residence time of about 1 hour, and the binding between sulfur and iron ions progresses in a shorter time, and the released phosphate ions can be maintained. It can also be seen that by setting the molar ratio of S / T-Fe to 1.0 or more, the amount of phosphate ions increases rapidly, increasing to about 35 mg-P / L in a residence time of about 2 hours, and the binding between sulfur and iron ions progresses in a very short time, and the released phosphate ions can be maintained. On the other hand, if the molar ratio of S / T-Fe is 1.0 or more, even if it is 3.0, the phosphate ion concentration is about 40 mg-P / L, and the concentration of phosphate ions does not increase as much as the molar ratio of S / T-Fe increases, and it can be confirmed that the binding of sulfur and iron ions and the release of phosphate ions are almost saturated. In addition, if the molar ratio of S / T-Fe is 0.2 or more, the phosphate ion concentration increases with a residence time of about 1 hour in any case of the molar ratio, and is almost saturated with about 2 hours. From the above, it can be said that the released phosphate ions can be well maintained by setting the S / T-Fe molar ratio to 0.2 or more, preferably 0.5 or more, more preferably 1.0 or more, and setting the residence time to 1 hour or more, preferably 2 hours or more.

Claims

1. A method for recovering phosphorus, comprising the steps of: adding a sulfide or a chelating agent to excess sludge that has been subjected to biological treatment by adding an iron-containing inorganic coagulant under anaerobic conditions at a pH of less than 9 to immobilize iron ions and releasing polyphosphoric acid and metal phosphates in the excess sludge as phosphate ions to form phosphate ion-containing sludge; adding a magnesium salt or a calcium salt to a concentrated separation liquid obtained by concentrating and separating the phosphate ion-containing sludge, and recovering the phosphate ions as magnesium ammonium phosphate or hydroxyapatite.

2. the sulfide is selected from hydrogen sulfide, sodium sulfide, sodium hydrosulfide, and any mixture thereof; 2. The method for recovering phosphorus according to claim 1, wherein the chelating agent is selected from the group consisting of EDTA, NTA, citric acid, oxalic acid, salicylic acid, tartaric acid, tannin, and any mixture thereof.

3. 3. The method for recovering phosphorus according to claim 1, wherein the sulfide is added in an amount such that the amount of sulfur per 1 mol of iron in the excess sludge is 0.2 mol to 3.0 mol.

4. A method for recovering phosphorus, comprising the steps of: adding a chelating agent to excess sludge that has been subjected to biological treatment by adding an aluminum-containing inorganic coagulant under anaerobic conditions at a pH of less than 9 to immobilize aluminum ions and releasing polyphosphoric acid and metal phosphates in the excess sludge as phosphate ions to form phosphate ion-containing sludge; concentrating and separating the phosphate ion-containing sludge to obtain a concentrated separation liquid; and adding a magnesium salt or a calcium salt to the concentrated separation liquid to recover the phosphate ions as magnesium ammonium phosphate or hydroxyapatite.

5. The anaerobic conditions are the S-COD of the excess sludge. Ｃｒ The method for recovering phosphorus according to any one of claims 1 to 4, characterized in that / MLSS is 0.02 or more and 0.15 or less.

6. 6. The phosphorus recovery method according to claim 5, wherein the anaerobic conditions are adjusted by adding sludge that has not been subjected to biological treatment, septic tank sludge, or organic matter to the excess sludge after the biological treatment.

7. a biological treatment tank in which an inorganic flocculant containing iron is added to the material to be treated for biological treatment; a phosphate ion release tank in which a sulfide or a chelating agent is added to the excess sludge from the biological treatment tank under anaerobic conditions at a pH of less than 9 to immobilize iron ions and release polyphosphoric acid and metal phosphates in the excess sludge as phosphate ions to form sludge containing phosphate ions; a phosphorus recovery tank in which a magnesium salt or a calcium salt is added to a concentrated and separated liquid obtained by concentrating and separating the phosphate ion-containing sludge from the phosphate ion release tank, and phosphate ions are recovered as magnesium ammonium phosphate or hydroxyapatite; A phosphorus recovery apparatus comprising:

8. a biological treatment tank in which an inorganic flocculant containing aluminum is added to the material to be treated for biological treatment; a phosphate ion release tank in which a chelating agent is added to the excess sludge from the biological treatment tank under anaerobic conditions at a pH of less than 9 to immobilize aluminum ions and release polyphosphoric acid and metal phosphates in the excess sludge as phosphate ions to form phosphate ion-containing sludge; a phosphorus recovery tank in which a magnesium salt or a calcium salt is added to a concentrated and separated liquid obtained by concentrating and separating the phosphate ion-containing sludge from the phosphate ion release tank, and phosphate ions are recovered as magnesium ammonium phosphate or hydroxyapatite; A phosphorus recovery apparatus comprising:

9. a flocculation reaction tank in which a polymer flocculant is added to the phosphate ion-containing sludge from the phosphate ion release tank; a thickener for concentrating and separating the phosphate ion-containing sludge to which the polymer flocculant has been added from the flocculation reaction tank into a concentrated separation liquid and a concentrated sludge; a dehydrator that dehydrates the sludge from the thickener and separates it into a dehydrated cake and a dehydrated separated liquid; Further comprising:

9. The phosphorus recovery apparatus according to claim 7, wherein the concentrated separated liquid and the dehydrated separated liquid are supplied to the phosphorus recovery tank, and phosphate ions are recovered as magnesium ammonium phosphate or hydroxyapatite.

10. Between the biological treatment tank and the phosphate ion release tank, S-COD of excess sludge is Ｃｒ 10. The phosphorus recovery apparatus according to claim 7, further comprising an anaerobic tank for adjusting the MLSS to 0.02 or more and 0.15 or less.

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