JP2007283223A - Method for recovering phosphorus from sludge - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for certainly recovering phosphorus with high efficiency by efficiently discharging phosphorus contained in the excess sludge formed from a biological dephosphorization process. <P>SOLUTION: In a biological dephosphorization process having an anaerobic tank and an aerobic tank, produced excess sludge is recovered as a slurry and, after the recovered phosphate type phosphorus (PO<SB>4</SB>-P) is discharged into slurry water, the slurry after discharge is concentrated to be separated into active sludge and phosphorus concentrated water and the phosphorus concentrated water after separation is charged in a fluidized bed filled with steel sludge to crystallize phosphorus component as calcium appatite to be recovered. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for recovering phosphorus in excess sludge generated from biological water treatment processes such as waste water and sewage.

Phosphorus contained in urban sewage and industrial wastewater is one of the eutrophication-causing substances in lakes and marine areas, and effective phosphorus removal technology from sewage and wastewater is required from the viewpoint of environmental conservation. In addition to removal from water, phosphorus has been strongly required to recover from sludge generated from the biological phosphorus removal process of sewage and wastewater due to the problem of depletion of phosphorus resources in the future.
The background art will be described below.

The total phosphorus concentration in municipal sewage is usually about 3 to 5 mg / L, and biological phosphorus removal methods have been actively implemented as a method for removing phosphorus in sewage. That is, some bacterial groups (hereinafter referred to as polyphosphate-accumulating bacteria) in activated sludge (aggregates of microorganisms) are PO under anaerobic conditions (a state in which there is neither dissolved oxygen nor bound oxygen such as nitrate ions). _When phosphorus is released as _4- P (indicated as phosphorus contained in phosphate ions, commonly referred to as phosphate phosphorus; the same shall apply hereinafter), PO ₄ -P will normally be released under aerobic conditions. In comparison with activated sludge, the excess intake PO ₄ -P is retained in the cells as polyphosphate granules. If such a system is adopted, it is said that the phosphorus concentration in the activated sludge of sewage increases from about 2% by mass to about 6% by mass at maximum.

Although this process is intended to remove phosphorus from water, it is a technique that concentrates PO ₄ -P dissolved in water as granules of polyphosphoric acid in activated sludge rather than completely removing it. This method has been put into practical use in the field of municipal sewage treatment.
Below, the conventional method of biological phosphorus removal and excess sludge treatment according to the present invention will be described.

The biological phosphorus removal process generally comprises a first sedimentation tank, a reaction tank (anaerobic tank and aerobic tank), and a final sedimentation tank (hereinafter referred to as AO process). Moreover, when removing phosphorus and nitrogen simultaneously, it comprises a first sedimentation tank, a reaction tank (anaerobic tank, anaerobic tank and aerobic tank), and a final sedimentation tank (hereinafter referred to as A ₂ O process).

Here, a conventional A ₂ O process will be described as an example (see FIG. 1).
The first sedimentation basin (2) settles and removes coarse suspended solids in the sewage (1) and reduces the organic acid load in the reaction tank. The reaction tank comprises an anaerobic tank (4), an oxygen-free tank (5), and an aerobic tank (6). In the anaerobic tank (4), as mentioned above, polyphosphate-accumulating bacteria in activated sludge discharge phosphorus. In the anaerobic tank (5), sludge circulation in which the nitrification reaction (nitric nitrogen (NO ₃ -N) accumulates as a result of oxidation of ammonia nitrogen (NH ₄ -N)) progressed from the aerobic tank (6) Water (14) is returned. In the anaerobic tank (5), nitrite nitrogen (NO ₂ -N) and NO ₃ -N in this nitrification solution are denitrified with organic acids in sewage by denitrifying bacteria in activated sludge. Is removed as nitrogen gas. In the aerobic tank (6), oxidation of NH ₄ -N (nitrification reaction) is performed by nitrifying bacteria in activated sludge. A part of this is circulated to the anoxic tank (5) as described above. Polyphosphate accumulating bacteria overdose phosphorus in the anoxic tank (5) and aerobic tank (6). In the final sedimentation basin (7), activated sludge excessively ingested with phosphorus is settled and the supernatant is discharged as treated water (8). The concentrated activated sludge (19) that has been separated by settling is partially removed as excess sludge, and the remainder is returned to the anaerobic tank (4) by the return sludge pump (9) as return sludge (10). Excess sludge is concentrated by a concentrator (20) (generally, a mechanical concentrator such as a centrifugal concentrator is used), then dehydrated by a dehydrator (21) and disposed as dehydrated sludge (24). .

However, in such a process, phosphorus contained in the excess sludge is in the form of PO ₄ -P in the concentrated separated water (20) and dehydrated filtrate (22) generated during the above-described concentration and dehydration operations. May be released again. Since the concentrated separated water (20) and the dehydrated filtrate (22) are returned to the sewage (1) as return water generated from the sludge treatment, phosphorus is circulated in the water treatment system. In order to suppress such re-release of phosphorus, a method of adding aluminum or an iron-based flocculant in the concentration or dehydration process is effective, but the amount of excess sludge and its treatment cost increase. . Moreover, in the case of phosphorus combined with aluminum or iron, it becomes difficult to reuse phosphorus.

As a method for removing phosphorus without using a flocculant, a method for removing phosphorus using a crystallization reaction is known (for example, Non-Patent Document 1 and Non-Patent Document 2). In the crystallization reaction, phosphate ore is used as a nucleus, phosphate ions (PO ₄ ³⁻ ) dissolved in water are reacted with calcium ions, and phosphorus is grown as insoluble calcium hydroxyapatite on the phosphate ore. To do. This reaction is represented by the following reaction formula (1).
5Ca ²⁺ + 3PO ₄ ^3- + OH ⁻ → Ca ₅ (OH) (PO ₄ ) ₃ (1)
From this reaction formula (1), it can be seen that 1.7 mol of Ca ²⁺ is required to remove 1 mol of PO ₄ -P. In terms of mass ratio, it is 2.2 times.

However, such crystallization reactors using phosphorus ore as the core were originally considered for removal of low-concentration phosphorus contained in sewage and wastewater, and many fixed-bed systems are obstructed. It has not been widely used because there are issues such as easy handling and high cost.
On the other hand, in recent years, the importance of effective utilization of phosphorus has been recognized again, and various methods for recovering phosphorus from surplus sludge (sludge with accumulated phosphorus) generated from biological dephosphorization processes will be proposed. It has become.

For example, Patent Document 1 is a proposal for releasing PO ₄ -P from excess sludge and recovering PO ₄ -P. In order to release phosphorus, about 2 to 20% of the sludge that first settled in the settling basin is added to the excess sludge, and after anaerobic conditions are allowed to stand for an appropriate period of time, the sludge and the concentrated phosphorus solution are separated. It is said that it is acceptable to first ferment the sedimentation sludge in the settling basin and use the organic acid in the fermentation product. As a method for recovering phosphorus, a method of adding an aluminum or iron-based flocculant or a MAP method (a method of recovering as magnesium ammonium phosphate) has been proposed.

However, in fact, the sludge and fermentation products that settled in the first sedimentation basin have severe changes in properties, and even if this is added to the excess sludge at a certain rate, it is difficult to stably release PO ₄ -P. Conceivable. In addition, the sedimentation sludge in the first sedimentation basin is a mass of miscellaneous suspended matter contained in the sewage, and since it contains heavy metals, it is easily estimated that the properties of the recovered phosphorus are not stable.

Patent Document 2 is also a proposal for releasing PO ₄ -P from excess sludge and recovering PO ₄ -P. In order to release phosphorus, a part of the sewage is added to the excess sludge to make anaerobic conditions. Thereafter, the sludge and the phosphorus concentrate are separated by membrane separation or sedimentation separation. Phosphorus is obtained by using calcium compounds such as calcium chloride and slaked lime, maintaining pH = 7.5 to 9.0, forming calcium phosphate, separating and recovering. Specifically, although a crystallization tank is recommended, a filler is not particularly required, and if calcium phosphate seed crystals are used at 50 g / L to 200 g / L at the start, the calcium phosphate crystals It is said that it is easy to obtain.

However, the actual sewage has a severe property change, and even if sewage is added to the excess sludge at a certain rate, it is considered difficult to stably release PO ₄ -P. In addition, since sewage contains some heavy metals, it is easily estimated that the properties of the recovered phosphorus are not stable. The operation method of the crystallization tank is not clear, and in reality, a large number of fine crystals are generated and are expected to flow out as SS (suspended solid). The particle size of the calcium phosphate seed crystal is not specified, and application is difficult as it is.
Water recycling (applied), Yoroku Wada, p15-p23, 1992 Advanced treatment facility design manual, Japan Sewerage Association, 1994 Japanese Patent Application No. 7-44686 JP 11-133931

The methods for recovering phosphorus from excess sludge generated from biological dephosphorization processes that have been proposed so far have the following problems.
First, the conventional methods are very unclear and abstract in terms of the release conditions of PO ₄ -P from excess sludge.

(1) It is unknown how much PO ₄ -P contained in activated sludge is released. In activated sludge, there are phosphorus originally contained in activated sludge cells and phosphorus originally present as PO ₄ -P in the sewage, and excessively taken up by the sludge and retained as polyphosphoric acid granules. It is not clear which is the target.
(2) Although it is clearly stated that anaerobic conditions are necessary to release PO ₄ -P, it is not clear how much PO ₄ -P can be released under such anaerobic conditions.

(3) Furthermore, as a means to achieve anaerobic conditions, methods using sewage itself or fermentation products such as sedimentation sludge or sludge in the first sedimentation basin have been proposed, but the properties of these substances are extremely unstable. As such, it is not stable to PO ₄ -P release and is actually very difficult to control. Even if only the amount ratio of the additive is set, the anaerobic condition and the amount of released PO ₄ -P cannot be accurately controlled.
(4) In the means proposed so far, heavy metals and other trace organic acids in sewage are contained in the recovered material, and the purity of the recovered phosphorus is lowered.

Therefore, it is extremely difficult to control the stable release of PO ₄ -P from the excess sludge generated from the biological dephosphorization process with the conventional proposed method.

In addition, it should be noted that the release conditions of PO ₄ -P from excess sludge are greatly affected by the operating state of the biological phosphorus removal process in the previous stage. That is, the reaction tank of the biological phosphorus removal process consists of an anaerobic tank and an aerobic tank (so-called AO method) or an anaerobic tank and an anaerobic tank and an aerobic tank (so-called A ₂ O method). In any method, activated sludge is placed under anaerobic conditions in an anaerobic tank to release PO ₄ -P, and then PO ₄ -P is excessively consumed in the aerobic tank. Also in the phosphorus removal process, the operating conditions of the anaerobic tank and the aerobic tank are extremely important for phosphorus recovery from the excess sludge, and the AO method and the A ₂ O method have different aspects.

That is,
(1) If the anaerobic conditions in the anaerobic tank are not sufficiently achieved in both the A ₂ O method and the AO method, the amount of PO ₄ -P released from the anaerobic tank decreases. If the release of PO ₄ -P is small, excessive intake of PO ₄ -P in the aerobic tank will not occur, and as a result, the phosphorus content held in the excess sludge will decrease. Therefore, it is necessary to sufficiently achieve the anaerobic conditions in the anaerobic tank in both the A ₂ O method and the AO method.

(2) In the A ₂ O method, NH ₄ —N contained in sewage is oxidized to NO ₃ —N (nitrification reaction) in an aerobic tank. However, if excessive aeration is performed and a large amount of dissolved oxygen (DO) remains, it will affect the release of PO ₄ -P from excess sludge in the phosphorus release tank. In the final sedimentation basin, DO and NO ₃ -N in the settled sludge decrease with time, but if the DO is high, the residual amount of NO ₃ -N in the sludge increases. The more NO ₃ -N concentration remains in the excess sludge generated from the final sedimentation basin, the more difficult it becomes to achieve the subsequent anaerobic conditions, and the release of PO ₄ -P from the excess sludge in the phosphorus release tank is suppressed. .

(3) On the other hand, in the case of the AO method, the nitrification reaction is generally suppressed in an aerobic tank. That is, in such a case, since there is no DO or NO ₃ -N, the anaerobic degree of sedimentation sludge in the final sedimentation basin is likely to increase. As a result, PO ₄ -P is easily released into the supernatant of the final sedimentation basin. The quality of treated water is likely to deteriorate. At the same time, the phosphorus content in the excess sludge generated from the final sedimentation basin also decreases.

Therefore, in the AO method and the A ₂ O method, it is necessary to fully consider the operating conditions of the AO method and the A ₂ O method when considering the recovery of phosphorus from excess sludge. More specifically, in both aerobic tanks, it is necessary for both methods to promote the nitrification reaction, but on the other hand, it is not preferable that excessive DO exists in the outflow water from the aerobic tank.
The crystallization method for recovering phosphorus also has the following problems.

That is, in the conventional way of thinking, the main purpose is to control the addition amount and pH of calcium ions so as to keep the region where the crystallization reaction is likely to occur (metastable region) (Non-patent Document 2). This is presumably because the chemical reaction between PO ₄ -P and calcium ions represented by the reaction formula (1) is considered to be the rate-determining reaction rate. However, in the case of a conventional fixed bed or the like, in actuality, clogging is likely to occur and countermeasures are difficult (being easily affected by fine SS), and the filler is not efficient (the specific surface area is small and the reaction efficiency is poor). In the case where the filler is phosphate ore, it is inconsistent with phosphorus depletion, but in the case of other fillers, it is considered that the spread is hindered due to the high cost.

  An object of the present invention is to solve the conventional problems in recovering phosphorus from surplus sludge generated from a biological phosphorus removal process, and to provide a method for recovering phosphorus reliably and efficiently.

  As a result of repeated studies to solve the above problems, the present inventors have succeeded in stably recovering phosphorus from excess sludge generated from the biological phosphorus removal process by the following method.

The gist of the present invention is the following (1) to (14).
(1) In the method of recovering phosphorus, in the biological dephosphorization process having an anaerobic tank and an aerobic tank, the generated excess sludge is recovered as a slurry, and phosphate phosphorus (PO ₄ -P) is recovered from the recovered excess sludge. ) In the slurry water, the slurry after the release is concentrated and separated into activated sludge and phosphorus concentrated water. The separated phosphorus concentrated water is put into a fluidized bed filled with steel slag, and phosphorus is added. The component is crystallized and recovered as calcium apatite.

(2) In the phosphorus recovery method according to (1), the biological dephosphorization process having an anaerobic tank and an aerobic tank is an AO process comprising an initial sedimentation tank, an anaerobic tank, an aerobic tank, and a final sedimentation tank. Or an A ₂ O process comprising a first settling tank, an anaerobic tank, an oxygen-free tank, an aerobic tank, and a final settling tank.

(3) In the phosphorus recovery method according to (1) or (2), the excess sludge recovered as the slurry is activated sludge in an aerobic tank, or activated sludge that has settled and separated in a final sedimentation tank. It is characterized by that.

(4) In the phosphorus recovery method according to any one of (1) to (3), PO ₄ -P is released from the excess sludge in a phosphorus release tank, and the phosphorus in the phosphorus release tank Stir the excess sludge so that the ORP (redox potential / silver / silver chloride electrode standard) of the slurry water is −350 mV (silver / silver chloride electrode standard) or less and −400 mV (silver / silver chloride electrode standard) or more. It is characterized by promoting the release of PO ₄ -P.

(5) In the phosphorus recovery method according to any one of (1) to (4), PO ₄ -P is released from the activated sludge in a phosphorus release tank, and the phosphorus in the phosphorus release tank The ORP (oxidation-reduction potential / silver / silver chloride electrode standard) of slurry water is −350 mV (silver / silver chloride electrode standard) or less, −400 mV (silver / silver chloride electrode standard) or more (that is, a range of −400 to −350 mV). The above is characterized in that at least one of organic acid, first sedimentation tank sedimentation sludge and anaerobic tank sludge is added to the surplus sludge.

(6) In the method for recovering phosphorus according to (5), acetic acid or sodium acetate is used as the organic acid.
(7) In the phosphorus recovery method according to (5) or (6), the amount of the organic acid added is 4 times or more the amount of PO ₄ -P released from the excess sludge. Features.

(8) In the method for recovering phosphorus according to any one of (1) to (7), NH ₄ —N in sewage is biologically oxidized to NO ₃ —N in an aerobic tank, The NH ₄ —N concentration is 0.1 mg / L or more and 0.5 mg / L or less.
(9) In the phosphorus recovery method according to any one of (1) to (8), PO ₄ -P is released from the excess sludge in a phosphorus release tank, and the phosphorus in the phosphorus release tank The pH of the slurry water is maintained at 7.5 or more and 9 or less.

(10) In the phosphorus recovery method according to any one of (1) to (9), as the steel slag filled in the fluidized bed, at least of blast furnace slag and converter slag that are by-products of the steel manufacturing process. Any one of them is used.
(11) The phosphorus recovery method according to (10), wherein the steel slag has a particle size range of 100 μm to 300 μm.

(12) The method for recovering phosphorus according to any one of (1) to (11), wherein, in addition to the phosphorus concentrated water, a calcium ion source is also charged into a fluidized bed filled with the steel slag. To do.
(13) In the phosphorus recovery method of (12), the calcium ion source is calcium chloride.

(14) In the phosphorus recovery method according to the above (12) or (13), the calcium ion amount is 2.2 by the mass ratio of Ca / P with respect to the PO ₄ -P amount contained in the phosphorus concentrated water. The calcium ion source is charged into the fluidized bed so as to be 2.5 or less.

According to the present invention, phosphate phosphorus (PO ₄ -P) can be stably and reliably released from excess sludge generated from a biological dephosphorization process, and this phosphorus can be stably recovered as calcium hydroxyapatite. .

In the method for recovering phosphorus according to the present invention, surplus sludge generated in a biological dephosphorization process having an anaerobic tank and an aerobic tank is recovered as a slurry, and phosphate phosphorus (PO ₄ -P) is recovered from the recovered excess sludge. ) Is discharged into the slurry water, and the slurry after the discharge is concentrated and separated into activated sludge and phosphorus concentrated water, and the separated phosphorus concentrated water is put into a fluidized bed filled with steel slag, The phosphorus component is crystallized and recovered as calcium apatite.

Hereinafter, specific embodiments will be described.
A biological dephosphorization process having an anaerobic tank and an aerobic tank includes an AO process comprising an initial settling tank, an anaerobic tank, an aerobic tank, and a final settling tank, or an initial settling tank, an anaerobic tank, an anaerobic tank, and an aerobic tank The A ₂ O process consisting of a final sedimentation tank is desirable.

In the present invention, when releasing PO ₄ -P from excess sludge, the release of components other than PO ₄ -P, such as heavy metal ions, ammonium ions, and COD components, is suppressed to the maximum. Then, PO ₄ -P be released from the excess sludge, hackers originally, PO ₄ -P contained in sewage, in other words, to overdose than the activated sludge is normally held, polyphosphoric acid The phosphorus compound held as a granule is PO ₄ -P which is decomposed and generated.

The following means are known as a method for forcibly releasing PO ₄ -P from excess sludge.
1) Addition of sewage or sedimentation sludge from the first sedimentation basin to release phosphorus under anaerobic conditions 2) Dissolution of activated sludge with strong acid such as hydrochloric acid or sulfuric acid or strong alkali such as NaOH 3) Ozone Method to dissolve activated sludge using oxidizing agent 4) Method to dissolve activated sludge using special enzyme or microorganism 5) Method to dissolve activated sludge under high heat and / or high pressure

However, except for 1), in the case of such a method, not only PO ₄ -P, but also the heavy metal ions and ammonium ions adsorbed on the activated sludge from the activated sludge are dissolved in the water. Resulting in. As a result, even if PO ₄ -P is selectively recovered by a crystallization method or the like, many components other than PO ₄ -P are adsorbed, and the purity of the recovered phosphorus is lowered.

  Therefore, in the present invention, as the phosphorus to be recovered, activated sludge is excessively consumed in the process of sewage treatment, and the phosphorus compound retained as polyphosphoric acid granules is targeted, and the activated sludge cells themselves originally have. The basic principle is not to release the phosphorus component.

Hereinafter, the present invention will be described in detail with reference to FIG.
The biological dephosphorization process in FIG. 2 is a so-called A ₂ O method comprising an anaerobic tank (4), an oxygen-free tank (5), and an aerobic tank (6). First, in the anaerobic tank (4), the ORP value of the ORP meter (15) is controlled to be -270 mV or less, and the PO ₄ -P concentration in the anaerobic tank (4) from the activated sludge is 10 mg / L or more. To release. In order to achieve the anaerobic condition in the anaerobic tank (4), an organic acid (33) such as acetic acid can be used. After the release of PO ₄ -P, the activated sludge is excessively ingested with PO ₄ -P in the anaerobic tank (5) and the aerobic tank (6). In this case, the operating conditions of the aerobic tank (6) are extremely important for phosphorus recovery from excess sludge. That is, in the A ₂ O method, aeration is usually performed in an aerobic tank (6), and NH ₄ —N contained in sewage is almost completely biooxidized (nitrification reaction) to NO ₃ —N. However, if aeration is excessively performed in the aerobic tank (6) and a large amount of DO remains, NO ₃ -N and / or NO ₂ -N (NOx in the settled sludge (9) in the final sedimentation basin (7). -N) Residual amount increases. Therefore, it is desirable to lower DO as much as possible in the aerobic tank (6).

Therefore, established the aerobic tank NH ₄ -N concentration meter (6) (39), the NH ₄ -N concentration in the effluent to measure continuously, NH ₄ -N concentration to fit this range It is desirable to adjust the amount of aeration by the blower (12) by the NH ₄ -N concentration meter (39). NH ₄ -N concentration by NH ₄ -N concentration meter (39), 0.1 mg / L or more 0.5 mg / L or less. If the NH ₄ -N concentration is set to less than 0.1 mg / L, DO in the aerobic tank (6) rises, and excessive NOx-N is contained in the excess sludge (19) generated from the final sedimentation basin (7). Easy to remain. Moreover, if the NH ₄ —N concentration is oxidized to about 0.5 g / L, the anaerobic degree of the settled sludge in the final sedimentation basin (7) can be increased, and the release of phosphorus can be suppressed.

In the case of the AO method, it is a process in which the anaerobic tank (5) is eliminated from the A ₂ O method of FIG. 2, and the nitrification reaction is suppressed in the aerobic tank (6) (remains as NH ₄ -N). It was common. However, when the nitrification reaction is suppressed in this way, there is almost no DO or NO ₃ -N in the effluent from the aerobic tank (6), so the anaerobic degree of the settled sludge in the final sedimentation basin (7) is extremely likely to increase. As a result, PO ₄ -P is easily released into the supernatant of the final sedimentation basin (7), and the quality of the treated water is lowered. At the same time, the phosphorus content in the excess sludge (19) generated from the final sedimentation tank (7) also decreases.

Therefore, in order to recover phosphorus, NH ₄ -N contained in sewage is biologically oxidized (nitrification reaction) to NO ₃ -N by aeration with a blower (12) in an aerobic tank (6) even in the AO method. . Also in this case, it is desirable that the NH ₄ —N concentration remaining in the aerobic tank (6) is 0.1 mg / L or more and 0.5 mg / L or less. Specifically, NH ₄ -N concentration meter (39) placed in the aerobic tank (6), continuously measuring the NH ₄ -N concentration in the effluent, NH ₄ -N concentration to fall within this range The aeration amount of the aerobic tank (6) by the blower (12) may be adjusted. If the NH ₄ —N concentration is oxidized to about 0.5 mg / L, an increase in the anaerobic degree of the settled sludge in the final sedimentation basin (7) can be suppressed. If the NH ₄ -N concentration is set to less than 0.1 mg / L, DO in the aerobic tank (6) rises, and excessive NOx-N is contained in the excess sludge (19) generated from the final sedimentation basin (7). Easy to remain.

Next, the recovery of phosphorus from excess sludge (19) is carried out according to the following procedure.
First, surplus sludge (19) is collected and put into a phosphorus release tank (25). As the excess sludge (19), the settling sludge (9) of the final settling basin (7) is usually used. In some cases, activated sludge (31) in the aerobic tank (6) may be used as excess sludge. This is because these activated sludges are in a state of excessive intake of PO ₄ -P to the maximum extent. Since these excess sludge contains a large amount of moisture, it is recovered as a slurry. This slurry is composed of an SS (Suspended Solid) component and water (also referred to as slurry water), but excess sludge is contained in the SS component. In addition, since the collect | recovered slurry controls the density | concentration of SS component as needed, water addition and further concentration can be performed suitably.

In the present invention, in order to control the anaerobic degree of the slurry in the phosphorus release tank (25) by introducing the slurry containing excess sludge into the phosphorus release tank (25), an ORP meter (15 ) To measure the ORP (silver / silver chloride electrode reference) value. Slowly agitate the excess sludge (19) (slurry containing excess sludge) in the phosphorus release tank (25) until the ORP value in the phosphorus release tank (25) falls below -350 mV (silver / silver chloride electrode standard), Promotes the release of PO ₄ -P.

In this case, as a means for quickly reducing the ORP value of the ORP meter (15) of the phosphorus release tank (25), an organic acid may be added from the organic acid addition device (33) to the phosphorus release tank (25). Specifically, first, the target amount of phosphorus recovered is the difference between the phosphorus content in the activated sludge in the aerobic tank (6) and the activated sludge in the anaerobic tank (4) and the sludge in the surplus sludge that is the target. Obtained from concentration integration. When recovering this phosphorus as PO ₄ -P, the amount of organic acid added to surplus sludge was examined to determine that almost the entire amount could be recovered. As a result, the amount of organic acid added was the recovery target PO ₄ -P amount. On the other hand, it was experimentally confirmed that it should be added 4 times by mass or more.

FIG. 3 shows an example in which the PO ₄ -P concentration of the recovery target in the phosphorus release tank (25) is 40 mg / L, and the target value is almost achieved at 200 mg / L for acetic acid. The upper limit of the amount of the organic acid added is not particularly defined, but it causes an increase in cost, so 5 mass times or less is sufficient for the recovery target PO ₄ -P amount.

  As the organic acid, it is desirable to use acetic acid or sodium acetate. Acetic acid or sodium acetate is most effective in releasing phosphorus from sludge and has the advantage that it can be easily obtained in high purity.

Instead of the organic acid, a part of the activated sludge (32) in the anaerobic tank (4) of the biological dephosphorization process may be collected and added to the phosphorus release tank (25). However, when sludge from the anaerobic tank (4) of the biological dephosphorization process is added to the phosphorus release tank (25), the ORP value of the ORP meter (15) of the anaerobic tank (4) of the biological dephosphorization process is It is necessary to maintain −270 mV or less, and −300 mV or less if possible. In such a case, using the sludge in the anaerobic tank (4), it was confirmed that the ORP value of the ORP meter (15) in the phosphorus release tank (25) was -350 mV or less. The amount of sludge added to the anaerobic tank (4) is preferably 20 V / V% (volume / volume%) or more and 50 V / V% or less in the excess sludge. If it is less than 20 V / V%, the effect of lowering the ORP value of the ORP meter (15) of the phosphorus release tank (25) is small, and if it exceeds 50 V / V%, the amount of PO ₄ -P recovered is halved.

Moreover, the sedimentation sludge (17) of the first sedimentation basin (2) may be added to the phosphorus release tank (25). In this case, the amount of sedimentation sludge (17) in the first sedimentation basin (2) is measured by measuring the amount of acetic acid in the sedimentation sludge (17) in the first sedimentation basin (2), and the amount of organic acid added is the target of recovery. It is desirable to set it so that it can be kept at least 4 times the mass. Regardless of which method is used, if the ORP value of the ORP meter (15) in the phosphorus release tank (25) decreases to -350 mV or less, the target release of PO ₄ -P from the excess sludge (19) is judged to have been completed. it can. The lower limit of the ORP value of the ORP meter (15) of the phosphorus release tank (25) is preferably set to -400 mV or more. This is because when the ORP value of the ORP meter (15) of the phosphorus release tank (25) falls below -400 mV, the sludge decays and the generation of hydrogen sulfide and ammonia proceeds.

  In addition, the pH value of the pH meter (30) of the phosphorus release tank (25) is set so that the excess sludge (19) is not completely killed and the operating conditions of the subsequent crystallizer (27) are taken into account. 5 to 9 is maintained. That is, the production of calcium apatite can be promoted at pH = 7.5 or more, and the production of fine calcium apatite can be suppressed at pH = 9 or less. Furthermore, if pH is 7.5 or more and 9 or less, the fall of the function of activated sludge is not seen.

  After confirming that the ORP value of the ORP meter (15) in the phosphorus release tank (25) is -350 mV or less, the activated sludge is transported to the sludge concentrator (18) by the pump, and the sludge concentrator (18) is used. Solid-liquid separation into dephosphorized sludge (28) and phosphorus concentrated water (20).

The sludge concentrating device (18) may be either a sedimentation type concentrating device or a mechanical concentrating device such as a centrifugal concentrating device. Further, subsequently, this phosphorus concentrated water (20) is conveyed to the crystallizer (27) by a pump, and high concentration PO ₄ -P is recovered as calcium apatite. If the SS is high in the phosphorous concentrate (20), the slag surface of the crystallizer (27) is covered with SS and the crystallization reaction stops, so the crystallizer (27) A sand filter device or a membrane separation device may be provided in the previous stage.

The details of the crystallizer (27) comprising a fluidized bed are shown in FIG.
In the case of the present invention, the crystallizer (27) includes steel slag (38) generated from the steel production process at the steelworks, that is, blast furnace slow-cooled slag, blast furnace water granulated slag, blast furnace water granulated slag, steelmaking One type or two or more types of slag are used. Blast furnace slow-cooled slag, blast furnace water granulated slag, and blast furnace water granulated slag are all slag generated from the blast furnace of the ironworks. Blast furnace slow-cooled slag is obtained by pouring molten slag discharged from the blast furnace into a yard or the like and slowly cooling and solidifying it, and is crystalline. Blast furnace granulated slag is obtained by spraying water directly on the molten slag from the blast furnace and rapidly solidifying it, and is amorphous. The ground granulated slag from the blast furnace is slag obtained by carrying the molten slag to a place away from the blast furnace with a receiving pan. Therefore, it is a blast furnace granulated slag including a part that has been partially solidified to become crystalline. Steelmaking slag is crystalline, such as converter slag and pretreatment slag.

  These are generated in the steel manufacturing process, and a large amount of easily stable products can be obtained. Moreover, since such blast furnace slag and steelmaking slag contain 30-50 mass% of calcium oxide (CaO), they are slightly alkaline while releasing calcium ions when immersed in water. Therefore, immediately after the start of operation, it is possible to reduce the amount of calcium ions and pH control alkaline agent as described later. Such an effect cannot be expected from phosphate ore that has been used in the past. These slags can be used alone or in any desired mixture.

The principle of phosphorus removal of the present invention is a crystallization reaction. Calcium ions released from slag filled in a crystallizer (27) composed of a fluidized bed, or calcium ions (34) from a calcium ion source added from the outside and PO ₄ -P in water to be treated are converted into steel slag (38 ) And is recovered as calcium apatite.

  During recovery, the steel slag (38) itself, which is packed in a fluidized bed and crystallized with calcium apatite on the surface (referred to as phosphorus-attached slag), can be recovered and used as fertilizer. Therefore, it is possible to effectively utilize the collected phosphorus-attached slag (36) as it is as a fertilizer.

  The pH value of the pH meter (30) of the crystallizer (27) gradually decreases because the hydroxide ions are removed as shown in the reaction formula (1). For this reason, it is desirable that the pH value is continuously measured and controlled with an alkaline agent (29) such as NaOH so that the pH value of the pH meter (30) is 8 or more and less than 9. However, in the present invention, since the pH value is already maintained at 7.5 or more and 9 or less in the phosphorus release tank (25), the pH control of the crystallizer (27) cannot be performed. In the conventional method, an alkaline agent (29) such as NaOH is added to increase the pH of the crystallizer (27), but a large amount of fine calcium apatite is produced due to a local rapid increase in pH at the injection site. This is likely to cause clogging in the crystallizer (27). In the present invention, such troubles can be avoided.

Further, in the crystallizer (27), in order to generate _{Ca 5 (OH) (PO 4} ) 3 , it is necessary to supply continuously calcium ions (34). The required amount of calcium ions (34) is such that the calcium ions (34) with respect to PO ₄ -P have a Ca / P mass ratio of 2.2 or more, as shown in reaction formula (1). It is desirable to add.

The presence of excess CO ₃ ²⁻ is a factor that inhibits the formation of calcium apatite. This is because CO ₃ ²⁻ binds to calcium ions to easily form CaCO ₃ and inhibits the formation of calcium apatite.

In the A ₂ O method, since the nitrification reaction is proceeding in the aerobic tank (6), H ⁺ is likely to be generated, and this reacts with CO ₃ ²⁻ to form H ₂ CO _3, which is then removed as CO _2. It is to be done. Therefore, in the case of the A ₂ O method, the maximum Ca / P mass ratio is 2.5 or less.

On the other hand, when the biological dephosphorization process in the previous stage is the AO method, the nitrification reaction is generally suppressed in the aerobic tank (6). However, in such an operation, CO ₃ ²⁻ is not reduced in the aerobic tank (6), and the production of calcium apatite is easily inhibited. Furthermore, the anaerobic degree of the sedimentation sludge (9) in the final sedimentation basin (7) is likely to increase. As a result, PO ₄ -P is easily released into the supernatant of the final sedimentation basin (7) and the quality of treated water is reduced. It becomes easy to do. At the same time, the phosphorus content in the sedimentation sludge (9) generated from the final sedimentation basin (7) also decreases. For these reasons, even in the case of the AO method, it is an essential condition from the viewpoint of phosphorus recovery to promote the nitrification reaction in the aerobic tank (6). Specifically, the aerobic tank (6) is adjusted so that the NH ₄ -N concentration of the NH ₄ -N meter (39) at the end of the aerobic tank (6) is 0.1 mg / L or more and 0.5 mg / L or less. It can be aerated with a blower (12). Therefore, even in the case of the AO method, a Ca / P mass ratio of 2.5 or less is sufficient under the condition that nitrification is sufficiently advanced.

As the calcium ion source (34) added to the crystallizer (27), highly soluble CaCl ₂ may be used. In addition, Ca (OH) ₂ , CaSO ₄ , and CaO may be used, but the solubility is small and it is likely to cause clogging.

  As a method of contacting the phosphorus-enriched water (20) and the steel slag (38), after pouring the phosphorus-enriched water (20) into the crystallizer (27) filled with the steel slag (38), the treated water (26) is used. There are a batch processing method for drawing out and a continuous processing method in which phosphorus concentrated water (20) is brought into contact with the crystallizer (27) filled with steel slag (38) from the lower end to the upper end while passing through a pump. In this, phosphorus concentrated water (20) was passed upward from the lower end of the filling tank filled with steel slag (38) with a small particle size in the crystallizer (27), and the steel slag (38) was expanded and expanded. Most preferred is a fluidized bed system that is fluidized and efficiently contacted with phosphorus concentrate (20) to remove phosphorus.

  The particle size of the steel slag (38) is preferably as fine as possible from the standpoint that the contact area with the phosphorus-enriched water (20) can be increased and fluidization is easy. In particular, in the case of a fluidized bed, energy for fluidization is generally a problem. In the case of a fluidized bed reactor, the treated water is often circulated again and used for the flow of the carrier.

  However, in the case of steel slag (38), fine particles can be obtained at low cost as a by-product of the steel mill, and the fluidization energy can be easily reduced. Of course, if it becomes too fine, the steel slag (38) itself tends to flow out with the treated water. As a result of intensive studies, the inventors have found that the range of the particle size of the steel slag (38) to be used is preferably 0.1 mm or more and 0.3 mm or less. That is, the steel slag (38) to be used is preferably classified into a range of 0.1 mm to 0.3 mm by means such as a sieve.

  The linear flow rate in the crystallizer (27) of the phosphorus concentrate (20) is 5 m / day or more and 15 m / day when the range of the particle size of the steel slag (38) used is 0.1 mm or more and 0.3 mm or less. The following should be set. When the linear flow velocity is less than 5 m / day, the steel slag (38) does not expand and flow sufficiently, and the mixing with the added calcium ions tends to be insufficient, and the calcium hydroxyapatite is formed below the crystallizer (27). Becomes easier to accumulate. On the other hand, when the linear flow rate is 15 m / day or more, the produced calcium hydroxyapatite is less likely to adhere to and grow on the surface of the steel slag (38), and easily flows out of the crystallizer (27). Also, the steel slag (38) itself tends to flow out of the crystallizer (27). Therefore, the linear flow velocity in the crystallizer (27) of the phosphorus-enriched water (20) may be set to 5 m / day or more and 15 m / day or less.

[Reference Example 1: Promotion of PO ₄ -P release from surplus sludge by addition of organic acid and pH control]
(Pretreatment of Example)
Efficient PO from excess sludge generated from A ₂ O process consisting of first sedimentation tank (2), anaerobic tank (4), anoxic tank (5), aerobic tank (6) and final sedimentation tank (7) _{The 4-} P release method was examined (see FIG. 2).

The surplus sludge was activated sludge (31) extracted from the aerobic tank (6) in which the NH ₄ -N concentration in the aerobic tank 6 was 0.2 mg / L and nitrification was sufficiently advanced. The DO concentration in the aerobic tank (6) was around 1 mg / L, and was kept lower than the normal DO management value (1.5 mg / L or more).

The activated sludge (31) in the aerobic tank (6) has a phosphorus content of 3.1 to 3.3% by mass. When the phosphorus content held by normal activated sludge is 2% by mass, the recovered PO ₄ -The amount of P was set to 1.2% of the average phosphorus contained in the sludge. For example, the activated sludge in the aerobic tank (6) is estimated to have a PO ₄ -P release concentration of about 42 to 45 mg / L when the MLSS (Mixed liquor Suspended Solid) concentration is 3500 to 3700 mg / L.

In the phosphorus release tank (25), the activated sludge (31) extracted from the aerobic tank (6) was left with stirring. As a result, as shown in Table 1, no release of PO ₄ -P was observed after standing for about 4 hours, and the release of PO ₄ -P was 1 mg / L or less even after 24 hours.

At the same time, the activated sludge (31) extracted from the aerobic tank (6) was filtered, and the NOx-N concentration was measured. NOx-N was initially present at 4.36 mg / L, but remained at 2.45 mg / L after 4 hours and 0.34 mg / L after 24 hours.
From these results, it is considered that the presence of NOx-N in the activated sludge (31) suppressed the release of PO ₄ -P in the phosphorus release tank (25).

Therefore, in order to remove NOx-N from the activated sludge (31), acetic acid is selected as the organic acid, and acetic acid (33) is added to the phosphorus release tank (25) so as to be 200 mg / surplus sludge-L. The promotion of the release of PO ₄ -P by selenium was examined.

Table 2 shows the results of adding acetic acid without pH control. As a result, it dropped to 5 from 7.6 pH value after 1 hour in pH meter (30) of phosphorus release tank (25), decreased function of activated sludge was observed, PO ₄ in phosphate release tank (25) - The ability to release P into water decreased, and the target PO ₄ -P release concentration of 42 to 45 mg / L was not reached even after 24 hours. Thus, the PO ₄ -P release promoting effect is not sufficient only by adding the organic acid to the phosphorus release tank (25).

Therefore, the NaOH solution was added from the NaOH addition device (34) so that the pH value of the pH meter (30) was maintained at 7.5 or more even when acetic acid was added to the phosphorus release tank (25). The results are shown in Table 3. As a result, when 200 mg / L of acetic acid was added while the pH value of the phosphorus release tank (25) was 7.5, PO ₄ -P having a maximum concentration of about 49 mg / L was obtained after 24 hours. This is almost consistent with the previously calculated phosphorus release concentration. It was also confirmed that the phosphorus concentration in the dephosphorization activated sludge (28) after the phosphorus release operation was reduced to 2.0 mass% in terms of mass% per MLSS.
In addition, if sodium acetate is used instead of acetic acid, it is possible to prevent a rapid drop in pH, so sodium acetate may be dissolved and added.

Furthermore, as shown in FIG. 3, the amount of organic acid added was varied from 40 to 400 mg / L under the control of pH = 7.5 in the phosphorus release tank (25), and the phosphorus release concentration was examined. As a result, it became clear that the target phosphorus release concentration can be obtained when the amount of organic acid added is about 200 mg / L. However, this sludge contains NOx-N, and acetic acid is consumed by NOx-N (4.0 mg / L), so it is necessary to subtract this. That is, when NOx-N is present, the organic acid necessary for the release of phosphorus as shown in the reaction formula (2) is immediately decomposed, and this amount is subtracted.
^{_{8NO 3 - + 5CH 3 COOH →}} 4N 2 + 10CO 2 + 6H 2 O + 8OH - (2)
As a result of subtraction, the net amount of acetic acid required for phosphorus release is estimated as follows.
(200mg-acetic acid / L)-(4mg / L x 2.7) = 190mg-acetic acid / L
From these results, it is considered that the net amount of acetic acid required for phosphorus release is about 4 times the amount of PO ₄ -P.

In this way, activated sludge (31) in the aerobic tank with a high phosphorus content or settled sludge (9) from the final sedimentation basin (7) is selected as the excess sludge, and phosphorus release in the phosphorus release tank (25) is selected. When considering, the presence of NOx-N in the sludge greatly suppresses the release of PO ₄ -P. In such a case, in the phosphorus release tank (25), the method of adding the organic acid (33) to the activated sludge (31) or the final sedimentation basin settling sludge (9) under pH control is extremely effective for phosphorus release. It is considered effective.

[Reference Example 2: Promotion of PO ₄ -P release by addition of anaerobic tank sludge]
(Pretreatment of Example)
Efficient PO from excess sludge generated from A ₂ O process consisting of first sedimentation tank (2), anaerobic tank (4), anoxic tank (5), aerobic tank (6) and final sedimentation tank (7) _{The 4-} P release method was examined (see FIG. 2).

In the anaerobic tank (4) of the A ₂ O process, the ORP value of the ORP meter (15) was controlled to −270 mV or less. The surplus sludge was activated sludge (31) extracted from the aerobic tank (6) in which the NH ₄ -N concentration in the aerobic tank (6) was 0.2 mg / L and was sufficiently nitrified. The DO concentration in the aerobic tank (6) was around 1 mg / L, and was kept lower than the normal DO management value (1.5 mg / L or more).

The activated sludge (31) extracted from the aerobic tank (6) contained about 4 mg / L of NOx-N. For this reason, it has been extremely difficult to release PO ₄ -P in the phosphorus release tank (25) in a short time. Therefore, the activated sludge (32) was removed from the anaerobic tank (4), and 50% by volume was added to the activated sludge (31) extracted from the aerobic tank (6), and this was used as excess sludge.

The activated sludge in the aerobic tank (6) has a phosphorus content of 3.2% by mass and the activated sludge (32) in the anaerobic tank (4) is 2% by mass, so the excess sludge has an MLSS concentration of the excess sludge. Of 2400 mg / L and an average phosphorus content of 2.6% by mass. If the phosphorus content held by normal activated sludge is 2% by mass, the amount of PO ₄ -P to be recovered is 0.6% by mass of phosphorus contained in the excess sludge. From these results, it was estimated that the PO ₄ -P release concentration in the phosphorus release tank (25) was about 24 mg / L.

While slowly stirring in the phosphorus release tank (25), the ORP value and PO ₄ -P concentration of the ORP meter (15) in this phosphorus release tank (25) were measured. The result is shown in FIG. The PO ₄ -P concentration reached 27 mg / L, which was almost close to the estimated value after 24 hours. Moreover, the ORP value in the ORP meter (15) of the phosphorus release tank (25) decreased from -30 mV to -350 mV.

From these results, it is possible to reduce the ORP value in the ORP meter (15) of the phosphorus release tank (25) and promote the release of PO ₄ -P even if the sludge of the anaerobic tank (4) is used. In addition, the state of PO ₄ -P release in the phosphorus release tank (25) can be estimated from the ORP value of the ORP meter (15).

[Example 1: Recovery of PO ₄ -P by a fluidized bed crystallizer filled with steel slag]
(Pretreatment + crystallization operation)
Efficient PO from excess sludge generated from A ₂ O process consisting of first sedimentation tank (2), anaerobic tank (4), anoxic tank (5), aerobic tank (6) and final sedimentation tank (7) _4- P release and recovery methods were examined (see FIGS. 2 and 5). The surplus sludge was activated sludge (31) (excess sludge) extracted from the aerobic tank (6) with sufficient NH ₄ -N concentration of 0.1 mg / L in the aerobic tank (6). . The DO concentration in the aerobic tank (6) was around 1 mg / L, and was kept lower than the normal DO management value (1.5 mg / L or more).

After the activated sludge (31) from the aerobic tank (6) is put into the phosphorus release tank (25) as a slurry, acetic acid (33) is 200 mg / L under the control of the pH value of the pH meter (30) of 8. After adding and confirming that the ORP value in the ORP meter (15) of the phosphorus release tank (25) is -350 mV or less, the slurry containing activated sludge is conveyed to the concentrator (18) by a pump, Solid-liquid separation was carried out into dephosphorized sludge (28) and phosphorus-enriched water (20) as activated sludge by a concentrator (18).
The results are shown in Table 4.

The phosphorus-concentrated water (20) had TP and PO ₄ -P concentrations of 41 mg / L and 38 mg / L, respectively, and a pH of about 8.3. Subsequently, phosphorus-concentrated water (20) was passed through the crystallizer (27) by a pump.

A detailed view of the crystallizer (27) is shown in FIG.
The crystallizer (27) is a cylindrical fluidized bed reactor, and filled with granulated blast furnace slag (particle size: 0.10 to 0.3 mm), which is a kind of steel slag (38). The phosphorus-enriched water (20) was passed upward from the lower end of the crystallizer (27) at a flow rate of 10 m / hr. As calcium ions, an aqueous solution of calcium chloride (34) was added from the lower end of the crystallizer (27) so that the Ca / P mass ratio was 2.2. Further, the pH value of the pH meter (30) at the upper end of the crystallizer (27) was measured, and NaOH was added from the lower end of the crystallizer (27) so that the pH value was maintained at 8.5.

As a result, PO ₄ -P in the treated water (26) of the fluidized bed crystallizer (27) filled with the steel slag (38) decreased to 3 mg / L. Moreover, TP was about 7 mg / L and the outflow of calcium apatite to the treated water (26) could be considerably suppressed.
As a result, it was revealed that about 85% of PO ₄ -P in the concentrated phosphorus water (20) can be recovered as calcium apatite.

  Furthermore, Table 5 shows the component analysis results of the phosphorus-attached slag (36) recovered. Thus, according to the method of the present invention, a phosphorus compound having a small amount of heavy metal components can be obtained, and application to fertilizers and the like becomes easy.

It is a schematic diagram of a conventional A ₂ 0 method and sludge treatment. It is a schematic diagram of a phosphorus incorporating recovery process A ₂ 0 method (present invention). Is a diagram illustrating a PO ₄ -P concentration relationship was released acetic acid concentrations added to excess sludge. It is a diagram phosphorus showing the relationship between PO ₄ -P concentrations released ORP release tank when adding the anaerobic tank sludge. It is a figure which shows the fluidized bed type crystallizer.

Explanation of symbols

1: Sewage
2: First sedimentation basin
3: First settling basin effluent
4: Anaerobic tank
5: Anoxic tank (denitrification tank)
6: Aerobic tank
7: Final sedimentation basin
8: treated water
9: Final sedimentation basin sedimentation sludge
10: Return sludge
11: Underwater stirrer
12: Blower
13: Sludge circulation pump
14: Sludge circulation water
15: ORP meter
16: DO meter
17: First settling tank sludge
18: Sludge concentrator
19: Surplus sludge
20: Phosphorous concentrate
21: Dehydrator
22: Dehydrated filtrate
23: Concentrated sludge
24: Dehydrated sludge
25: Phosphorus release tank
26: Crystallizer treated water
27: Crystallizer
28: Dephosphorized sludge
29: NaOH addition device
30: pH meter
31: Aerobic tank activated sludge
32: Anaerobic tank activated sludge
33: Organic acid addition equipment
34: Calcium preparation equipment
35: Phosphorus removal water
36: Phosphorus adhesion slag
37: Return sludge pump
38: Steel slag
39: NH ₄ -N meter

Claims (14)

In a biological dephosphorization process having an anaerobic tank and an aerobic tank, the generated excess sludge was recovered as a slurry, and phosphate phosphorus (PO ₄ -P) was released from the recovered excess sludge into the slurry water. Then, the slurry after the release is concentrated to separate activated sludge and phosphorus concentrated water, and the separated phosphorus concentrated water is put into a fluidized bed filled with steel slag to crystallize the phosphorus component as calcium apatite. A method for recovering phosphorus, comprising collecting the phosphorus. A biological dephosphorization process having an anaerobic tank and an aerobic tank is an AO process comprising an initial settling tank, an anaerobic tank, an aerobic tank, and a final settling tank, or an initial settling tank, an anaerobic tank, an anaerobic tank, and an aerobic tank The method for recovering phosphorus according to claim 1, which is an A ₂ O process comprising a final sedimentation tank.   The method for recovering phosphorus according to claim 1 or 2, wherein the excess sludge recovered as the slurry is activated sludge in an aerobic tank or activated sludge settled and separated in a final sedimentation tank. Phosphate phosphorus (PO ₄ -P) is released from the excess sludge in a phosphorus release tank, and the ORP (oxidation reduction potential / silver / silver chloride electrode standard) of the slurry water in the phosphorus release tank is −350 mV. (Silver / silver chloride electrode standard) Less than -400 mV (silver / silver chloride electrode standard) The excess sludge is agitated to promote the release of phosphorous phosphorus (PO ₄ -P). The method for recovering phosphorus according to any one of claims 1 to 3. Phosphate phosphate (PO ₄ -P) is released from the activated sludge in a phosphorus release tank, and the ORP (oxidation reduction potential / silver / silver chloride electrode standard) of the slurry water in the phosphorus release tank is −350 mV. (Silver / silver chloride electrode standard) Less than -400 mV (silver / silver chloride electrode standard) or higher, at least one of organic acid, first sedimentation basin sediment sludge, anaerobic tank sludge with respect to surplus sludge The method for recovering phosphorus according to any one of claims 1 to 4, which is added.   The method for recovering phosphorus according to claim 5, wherein acetic acid or sodium acetate is used as the organic acid. The amount of the organic acid added is 4 mass times or more with respect to the mass of phosphoric acid phosphorus (PO ₄ -P) released from the excess sludge. Collection method. It is characterized in that NH ₄ -N in sewage is biologically oxidized to NO ₃ -N in an aerobic tank, and the remaining NH ₄ -N concentration is 0.1 mg / L or more and 0.5 mg / L or less. The method for recovering phosphorus according to any one of claims 1 to 7. Phosphorus phosphorus (PO ₄ -P) is released from the excess sludge in a phosphorus release tank, and the pH of the slurry water in the phosphorus release tank is maintained at 7.5 or more and 9 or less. The method for recovering phosphorus according to any one of claims 1 to 8.   The recovery of phosphorus according to any one of claims 1 to 9, wherein at least one of a blast furnace slag and a converter slag, which are byproducts of a steel manufacturing process, is used as the steel slag to be filled in the fluidized bed. Method.   The method for recovering phosphorus according to claim 10, wherein a particle size range of the steel slag is 100 μm or more and 300 μm or less.   The method for recovering phosphorus according to any one of claims 1 to 11, wherein a calcium ion source is also added to the fluidized bed filled with the steel slag in addition to the phosphorus concentrate.   The method for recovering phosphorus according to claim 12, wherein the calcium ion source is calcium chloride. The calcium ions so that the amount of calcium ions is 2.2 to 2.5 in terms of the mass ratio of Ca / P with respect to the amount of phosphate phosphorus (PO ₄ -P) contained in the phosphorus-enriched water. The method for recovering phosphorus according to claim 12 or 13, wherein a source is put into the fluidized bed.

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