Method for recovering phosphorus present in water to be treated, and associated plant

Abstract

The present invention relates to a method for recovering phosphorus present in water to be treated (12), comprising: producing a biological sludge (14A); salting out the biological sludge to produce a phosphorus-rich and magnesium-rich liquid phase (28) and a thickened phase (30); anaerobic digestion of the thickened phase (30) to produce a liquid centrate (38) enriched in phosphorus and in aqueous ammonia; treating the phosphorus-rich and magnesium-rich liquid phase (28) in a main struvite precipitation reactor (66) to produce struvite; characterized by a step of intermediate treatment of the liquid centrate (38) to produce at least one liquid effluent (44), the liquid effluent (44) being at least partially introduced into the main struvite precipitation reactor (66) in order to react with the phosphorus-rich and magnesium-rich liquid phase (28).

Description

[0001] TITLE: Process for recovering phosphorus from water to be treated and associated installation

[0002] The present invention relates to a process for recovering phosphorus from water to be treated, comprising the following steps: production of a biological sludge containing microorganisms concentrated in phosphorus and magnesium; release, in a release reactor, of a portion of the phosphorus and magnesium contained in the microorganisms of the biological sludge, to produce a treated solid / liquid mixture having an aqueous phase comprising magnesium and phosphorus; separation of the treated solid / liquid mixture into a liquid phase rich in phosphorus and magnesium and into a thickened phase; anaerobic digestion of the thickened phase in at least one anaerobic digester to release residual phosphorus and magnesium, to produce ammonia and to obtain a solid / liquid mixture after digestion, the liquid / solid mixture after digestion having a liquid phase containing phosphorus, magnesium and ammonia;dehydration of the solid / liquid mixture after digestion to produce a liquid centrate enriched in phosphorus and ammonia and a dehydrated solid phase; treatment of the phosphorus and magnesium-rich liquid phase in a main struvite precipitation reactor to produce struvite.

[0003] Environmental protection requires the implementation of water treatment, particularly for municipal and industrial wastewater and rainwater.

[0004] Wastewater and stormwater treatment aims to eliminate potential pollutants, particularly particulate, carbonaceous, nitrogenous, and phosphorus pollution. This treatment also has the secondary objective of recovering valuable mineral or energy resources.

[0005] In this context, it is known to recover phosphorus present in the water being treated for reuse. This recovery is generally carried out by precipitation of struvite, a mineral formed by crystallization from ammonia, magnesium, and phosphate.

[0006] Wastewater treatment at a treatment plant produces biological sludge from the bacteria that break down pollutants (also known as "waste-activated sludge"). This sludge is regularly removed from the biological treatment process to maintain a constant concentration. Any excess sludge is then treated in the sludge treatment lines.

[0007] For example, EP 2 238 081 B1 describes a process for treating biological sludge from wastewater. This process includes successive steps of extracting phosphorus and magnesium from the biological sludge to create a first stream rich in magnesium and phosphorus in a dedicated reactor, and the creation of a second stream rich in ammonia, for example, from an anaerobic digester located downstream of the dedicated reactor. The combination of these two streams in a struvite precipitation reactor induces struvite precipitation.

[0008] However, such a process is not entirely satisfactory. Indeed, the ammonia concentration of the ammonia-rich stream added to the precipitation reactor in this process is not always optimized, which can, in some unfavorable cases, lead to incomplete phosphorus recovery, or costly recovery and treatment of excess ammonia, due to the diluted ammonia concentration in the precipitation reactor.

[0009] In addition, uncontrolled struvite precipitation can occur in unwanted locations, for example within the digester, and lead to clogging of piping in the biological sludge treatment stages.

[0010] One aim of the invention is therefore to propose a process optimizing the recovery of phosphorus present in water to be treated, in order to valorize it, while limiting uncontrolled struvite precipitation.

[0011] To this end, the invention relates to a precipitate-type recovery process, characterized by an intermediate treatment step of the liquid centrate to produce at least one liquid effluent, the liquid effluent being at least partially introduced into the main struvite precipitation reactor to react with the phosphorus and magnesium-rich liquid phase.

[0012] Such a process involves an intermediate treatment step of the liquid centrate from dehydration to produce at least one liquid effluent. This liquid effluent is at least partially introduced into the precipitation reactor to react with the phosphorus- and magnesium-rich liquid phase and to produce struvite by precipitation.

[0013] Thanks to this intermediate treatment stage, the ammonia concentration is more easily adjusted, allowing for better control of the relative amounts of ammonia, phosphorus, and magnesium entering the main struvite precipitation reactor. Struvite precipitation is thus better controlled, optimizing phosphorus recovery. Furthermore, the remaining ammonia-rich liquid effluent can be treated directly without passing through the precipitation reactor, via a process optimized for ammonia recovery and valorization, while avoiding dilution by mixing with the phosphorus- and magnesium-rich liquid phase.

[0014] This intermediate treatment step of the liquid centrate generally removes excess ammonia and limits or avoids uncontrolled clogging problems by struvite precipitation which increase maintenance and equipment renewal costs.

[0015] According to particular embodiments of the invention, the fractionation process according to the invention may comprise one or more of the following features, taken individually or in any technically feasible combination(s):

[0016] - the intermediate treatment step includes the introduction of the liquid centrate into an intermediate precipitation reactor, the precipitation of struvite in the intermediate precipitation reactor, and the production of at least one liquid effluent from the intermediate precipitation reactor;

[0017] - the intermediate treatment step includes the addition of a stream containing magnesium to the intermediate precipitation reactor;

[0018] - the intermediate treatment stage includes ammonia stripping of the liquid centrate in a stripping system to produce an ammonia-concentrated stream and an ammonia-depleted liquid stream, the liquid effluent being produced from one of the ammonia-rich liquid stream and the ammonia-depleted liquid stream;

[0019] - the intermediate treatment stage includes the addition of an acid stream, in particular a sulfuric acid stream, into the stripping system; biological sludge is sludge resulting from biological treatment of wastewater;

[0020] - the release stage is carried out by maintaining the biological sludge in a release reactor for at least 3 hours, in particular between 4 hours and 24 hours;

[0021] - the release stage involves adding easily biodegradable carbons, including volatile fatty acids, into the release reactor, advantageously maintaining the biological sludge in the release reactor for between 30 minutes and 4 hours;

[0022] - the separation of the treated solid / liquid mixture having an aqueous phase is carried out in a static or dynamic thickener; - the concentration of phosphorus and magnesium in the liquid phase rich in phosphorus and magnesium is higher than that of the liquid phase of the biological sludge;

[0023] - the concentration of phosphorus in the liquid phase rich in phosphorus and magnesium is lower than that of the thickened phase;

[0024] - the thickened phase treatment step in the anaerobic digester is carried out by maintaining the thickened phase in the anaerobic digester for a period of between 8 and 40 days;

[0025] - the concentration of phosphorus and ammonia in the concentrated liquid centrate is higher than that of the thickened phase entering the anaerobic digester;

[0026] - the intermediate treatment step lacks an intermediate struvite precipitation step;

[0027] - the intermediate treatment stage includes ammonia stripping of the liquid centrate in a stripping system to produce an ammonia-concentrated stream and an ammonia-depleted liquid stream, the liquid effluent being produced from the ammonia-depleted liquid stream.

[0028] The invention further relates to a production installation for recovering phosphorus present in water to be treated, comprising:

[0029] - a station configured to produce biological sludge containing microorganisms concentrated in phosphorus and magnesium;

[0030] - a reactor for releasing some of the phosphorus and magnesium contained in the microorganisms of the biological sludge to produce a treated solid / liquid mixture having an aqueous phase comprising magnesium and phosphorus;

[0031] - a system for separating the treated solid / liquid mixture into a liquid phase rich in phosphorus and magnesium and into a thickened phase;

[0032] - an anaerobic digester configured to process the thickened phase in order to release residual phosphorus and magnesium and to produce ammonia to obtain a solid / liquid mixture after digestion, the liquid / solid mixture after digestion having a liquid phase containing phosphorus, magnesium and ammonia;

[0033] - a dehydration reactor for the solid / liquid mixture after digestion to produce a liquid centrate enriched in phosphorus and ammonia and a dehydrated solid phase;

[0034] - a phosphorus and magnesium rich liquid phase treatment unit comprising a main struvite precipitation reactor for producing struvite; characterized by an intermediate liquid centrate treatment unit for producing at least one liquid effluent, the intermediate liquid centrate treatment unit being connected to the main struvite precipitation reactor to introduce at least part of the liquid effluent into the main struvite precipitation reactor in order to react with the phosphorus and magnesium rich liquid phase.

[0035] The installation according to the invention may include one or more of the following features, taken individually or in any technically feasible combination:

[0036] - the intermediate treatment unit includes an intermediate precipitation reactor or an ammonia stripping system;

[0037] - the intermediate treatment unit lacks an intermediate struvite precipitation reactor;

[0038] - the intermediate treatment unit is configured to introduce a liquid effluent depleted in ammonia into the main struvite precipitation reactor.

[0039] The invention will be better understood upon reading the following description, given solely by way of example, and made with reference to the attached drawings, in which:

[0040] - [Fig 1] Figure 1 is a synoptic diagram illustrating a phosphorus recovery unit in sludge from water to be treated, for the implementation of a first process according to the invention, in which the intermediate treatment includes the precipitation of a liquid centrate from dewatering, conducted in an intermediate struvite precipitation reactor and the introduction of a liquid effluent from the intermediate reactor into the main struvite precipitation reactor;

[0041] - [Fig 2] Figure 2 is a synoptic diagram similar to Figure 1, for the implementation of a second process according to the invention, in which the intermediate treatment includes stripping ammonia from a liquid centrate obtained from dehydration and introducing a liquid effluent with reduced ammonia content to the struvite precipitation reactor;

[0042] - [Fig 3] Figure 3 is a synoptic diagram similar to Figure 1, in which the intermediate treatment corresponds to the stripping of ammonia from a liquid centrate obtained from dehydration and the introduction of a liquid effluent rich in ammonia towards the struvite precipitation reactor, for the implementation of a third process according to the invention.

[0043] The terms "including" and "comprises" as used herein are synonymous with "including", "includes" or "contains", "containing", and are inclusive or boundless and do not exclude additional features, elements or unspecified method steps.

[0044] Unless otherwise indicated, percentages used are mass percentages, and pressures are absolute pressures. The terms % by mass and % mass have equivalent meanings and refer to the proportion of the mass of a product relative to 100 g of a composition containing it.

[0045] The expressions "rich in..." / "depleted in..." mean that the compounds mentioned are respectively more / less concentrated after a processing step than before the processing.

[0046] A first installation 10 for recovering phosphorus present in water to be treated 12 is schematically illustrated in figure 1.

[0047] Installation 10 is intended to recover phosphorus present in a biological sludge 14A containing microorganisms concentrated in phosphorus and magnesium, from the water to be treated 12, by precipitation of struvite 16.

[0048] Struvite 16 is a mineral belonging to the hydrated phosphate family. It is advantageously formed from a double phosphate of ammonium and magnesium hexahydrate with the chemical formula NH4MgPO4• 6 H2O.

[0049] Installation 10 also produces a dehydrated solid phase 18, after dehydration. The dehydrated solid phase has a water mass concentration of less than 45%, and generally comprises between 20% and 35% dry matter by mass, relative to the total mass.

[0050] The mass concentrations of water, ammonia and phosphorus of the dehydrated phase 18 are each lower than the mass concentrations of water, ammonia and phosphorus of the solid / liquid mixture obtained after digestion 36.

[0051] As mentioned above, the biological sludge 14A to be treated by the installation 10 comes from the water to be treated 12, which is, for example, industrial or municipal wastewater, or water from agriculture or the agri-food industry.

[0052] Preferably, the water to be treated 12 is purified in a wastewater treatment plant by several successive treatment units.

[0053] A first treatment unit 20 is intended to collect and pass water to be treated through sequential stages advantageously including lifting, screening, grit removal, grease removal and primary settling, to produce by settling a primary sludge 22A and a raw liquid phase 22B.

[0054] Then, the primary sludge 22A is discharged from the treatment unit, followed by further treatment to potentially produce biogas. The raw liquid phase 22B is introduced into the second treatment unit 20A, which includes at least one anaerobic treatment zone and one aerobic treatment zone with a solid / liquid mixture recirculation system between these two zones, and a solid / liquid separation system. The treatment unit 20A is therefore configured to produce a biological sludge 14A (waste-activated sludge, WAS) and a treated water stream 14B.

[0055] Biological sludge 14A comprises a solid / liquid mixture containing water and dissolved materials such as nitrogen and / or phosphorus compounds originating, for example, from human waste.

[0056] Generally, biological sludge 14A has a higher mass concentration of phosphorus and magnesium than the water to be treated 12 before treatment.

[0057] During the treatment of the raw liquid phase 22B, under the influence of the treatment conditions, microorganisms present in the raw liquid phase accumulate phosphorus- and magnesium-rich compounds in the aerobic phase and release them in the anaerobic phase. They induce the eventual formation of biological sludge 14A. These microorganisms include, for example, bacteria capable of extracting magnesium and phosphorus by releasing them from the primary or biological sludge. These microorganisms are generally referred to as phosphorus-accumulating organisms (PAOs).

[0058] These microorganisms include, for example, Candidatus Phosphoribacter, Candidatus Accumlibacter phosphatis, Candidatus dechloromonas, Candidatus accumulimonas, Microlunatis phosphovorus, Pseudomonas spp., Paracoccus denitrificans, Quatrionicoccus aussiasis, Malikia granosa, Lampropedia spp., Candidatus Microthrix, and Gemmatimonas aurantiaca.

[0059] Phosphorus-accumulating microorganisms are generally able to concentrate phosphorus, particularly in the form of polyphosphate granules, when subjected to alternating anaerobic and aerobic conditions.

[0060] Phosphorus and magnesium are generally released by microorganisms that accumulate phosphorus under anaerobic conditions.

[0061] The term "anaerobic" as used here indicates a reaction condition in which the concentration of oxygen present in the reaction environment is less than 0.5 ppm.

[0062] The term "aerobic" as used here indicates a reaction condition where the concentration of oxygen present in the reaction environment is greater than 2 ppm.

[0063] With reference to Figure 1, the installation 10 includes, upstream, a unit 24 for extracting phosphorus and magnesium from the biological sludge 14A, producing a treated solid / liquid mixture 26 and separating it into a liquid phase 28 rich in phosphorus and magnesium and into a thickened phase 30. The installation 10 further includes a unit 32 for digesting and dewatering the thickened phase 30 to produce a solid / liquid mixture after digestion 36 which is dewatered and separated into a liquid centrate 38 concentrated in phosphorus and ammonia and into the dewatered solid phase 18.

[0064] Installation 10 further includes a unit 40 for the treatment of the phosphorus and magnesium rich liquid phase 28 from the treated liquid / solid mixture 26 to precipitate struvite 16.

[0065] According to the invention, it comprises an intermediate treatment unit 42 for the liquid centrate 38 from the digestion and dehydration unit 32 to produce at least one liquid effluent 44, the liquid effluent 44 being at least partially introduced into the treatment unit 40 to react with the phosphorus- and magnesium-rich liquid phase 28

[0066] The phosphorus and magnesium extraction unit 24 receives the biological sludge 14A containing microorganisms concentrated in phosphorus and magnesium. It includes at least one phosphorus and magnesium release reactor 46 and at least one solid / liquid separation unit, such as a thickener 48.

[0067] The phosphorus and magnesium release reactor 46 is configured to extract phosphorus and magnesium contained in biological sludge 14A from microorganisms.

[0068] The release reactor 46 has at least one feed inlet for introducing the biological sludge 14A, at least one anaerobic treatment zone for producing the treated solid / liquid mixture 26. The release reactor 46 has at least one discharge outlet connected to the thickener 48 to bring the treated solid / liquid mixture 26 to the thickener 48. In a variant, the release reactor 46 and the thickener 48 are formed within the same reactor.

[0069] Preferably, a volatile fatty acid addition tap 50 is disposed upstream of the feed inlet of the release reactor 46 or in the release reactor 46 to allow phosphorus-accumulating microorganisms to consume volatile fatty acids in order to form, for example, polyhydroxyalkanoates, promoting the release of phosphate and magnesium in this anaerobic reactor.

[0070] The term "volatile fatty acids (VFAs)" as used here refers to short-chain fatty acids, for example, those with fewer than six carbon atoms. Examples of volatile fatty acids include acetic acid, propionic acid, and butyric acid.

[0071] Advantageously, the addition of volatile fatty acids is replaced by the addition of at least one readily biodegradable carbon (RBC) source. This accelerates and optimizes the release of phosphorus and magnesium by phosphorus-accumulating microorganisms. An example of a readily biodegradable carbon source is advantageously fermented primary sludge. The thickener 48 is configured to separate the treated solid / liquid mixture 26 from the release reactor 46 into the phosphorus- and magnesium-rich liquid phase 28 and the thickened phase 30. The phosphorus- and magnesium-rich liquid phase is relatively more concentrated in phosphorus and magnesium than the biological sludge 14A introduced into the release reactor 46.

[0072] It has a first discharge outlet 52 to bring the thickened phase 30 to the digestion and dehydration unit 32, and at least a second discharge outlet 54 to bring the phosphorus and magnesium rich liquid phase 28 to the treatment unit 40 to carry out struvite precipitation.

[0073] Preferably, the thickener 48 is selected from a static thickener or a dynamic thickener.

[0074] With reference to Figure 1, the digestion and dehydration unit 32, which receives upstream the thickened phase 30 from the thickener 48, is configured to produce downstream a dehydrated solid phase 18 depleted in water and depleted in phosphorus, magnesium and ammonia, and the liquid centrate 38 containing phosphorus, magnesium and possibly ammonia.

[0075] The digestion and dehydration unit 32 comprises at least one anaerobic digester 56, into which the thickened phase 30 is introduced to release at least some of the phosphorus and magnesium remaining in the phosphorus-accumulating microorganisms and to produce ammonia by release. In some cases, the digestion and dehydration unit 32 comprises at least two anaerobic digesters 56, connected in series.

[0076] Digestion (sometimes referred to as "fermentation" in the technical field of the invention) in the digester 56 generates the production of the liquid / solid mixture after digestion 36.

[0077] The digestion and dehydration unit 32 further includes a dehydration reactor 58 to separate the solid / liquid mixture after digestion 36 into the liquid centrate 38, which is more concentrated in phosphorus and ammonia than the thickened phase 30 entering the digester 56, and into the dehydrated solid phase 18 intended to be removed from the installation 10.

[0078] In the example shown in Figure 1, the intermediate treatment unit 42 includes an intermediate struvite precipitation reactor 60 located downstream of the dewatering reactor 58 to receive the liquid centrate 38 and perform intermediate struvite precipitation. It optionally includes a feed inlet (not shown) for a magnesium source. It includes a struvite recovery outlet 62 and a liquid effluent recovery outlet 64, which is less concentrated in ammonia and phosphorus than the liquid centrate 38. The liquid phase treatment unit 28 includes a main struvite precipitation reactor 66, located downstream of the intermediate struvite precipitation reactor 60.

[0079] The main struvite precipitation reactor 66 receives the phosphorus and magnesium rich liquid phase 28 from the thickener 48 and at least a part 67 of the effluent 44 from the intermediate struvite precipitation reactor 60, to produce precipitated struvite.

[0080] Optionally, the intermediate reactor 60 is configured to be supplied with an additional source of magnesium. It is configured to receive sodium hydroxide and / or to allow carbon dioxide (CO2) degassing, in order to adjust the pH of the precipitation reaction.

[0081] An example of a process for recovering phosphorus present in water to be treated 12, implemented in the installation 10 shown in Figure 1, will now be described.

[0082] As mentioned above, the biological sludge 14A containing microorganisms concentrated in phosphorus and magnesium to be treated by the installation is initially produced from the water to be treated 12.

[0083] As described above, the water to be treated 12 is, for example, purified in the wastewater treatment plant by a first treatment unit 20 to produce the primary sludge 22A and the raw liquid phase 22B. The raw liquid phase 22B is then treated in the second treatment unit 20A in the presence of microorganisms, passing successively through the anaerobic treatment zone and then the aerobic treatment zone and possibly through recirculation, to produce the biological sludge 14A, which is introduced into the installation 10.

[0084] As mentioned above, these microorganisms are in particular phosphorus-accumulating microorganisms (in English, "Phosphorous Accumulating Organisms" or PAO).

[0085] Biological sludge 14A generally contains at least 3% by mass of solids, for example between 5% by mass of solids and 8% by mass of solids.

[0086] Biological sludge 14A is generally rich in phosphorus and magnesium. Its phosphorus mass content is advantageously greater than 1.5% by mass and is generally between 2% by mass and 3% by mass.

[0087] The biological sludge 14A containing phosphorus-accumulating microorganisms is then introduced into the release reactor 46.

[0088] It remains in the release reactor 46 for a residence time exceeding 3 hours and advantageously between 4 and 24 hours. In the release reactor 46, the biological sludge 14A passes through at least one anaerobic treatment zone.

[0089] In the anaerobic treatment zone, the mass concentration of dioxygen is less than 0.5 ppm and is specifically between 0 ppm and 0.5 ppm. The temperature is advantageously between 8 °C and 25 °C.

[0090] A treated solid / liquid mixture 26 is then produced at the outlet of the release reactor. This mixture has an aqueous phase containing magnesium and phosphorus dissolved in water at higher mass concentrations than in the biological sludge 14A. The mass content of phosphorus and magnesium still present in the phosphorus-accumulating microorganisms in the treated solid / liquid mixture 26 is reduced compared to that in the biological sludge 14A.

[0091] Preferably, an easily biodegradable carbon source, in particular volatile fatty acids, is added to the release reactor 46 through the tap 50. The biological sludge 14A is introduced into the release reactor 46 before the addition of the volatile fatty acids through the tap 50.

[0092] The release reaction rate is then accelerated. The residence time of the biological sludge 14A in the release reactor can advantageously be less than 6 hours.

[0093] Next, the treated solid / liquid mixture 26 is separated in the thickener 48 into the phosphorus and magnesium rich liquid phase 28 and the thickened phase 30.

[0094] The phosphorus- and magnesium-rich liquid phase 28 generally contains more than 1.5% by mass of phosphorus and in the treated solid / liquid mixture 26. It preferably contains less than 1% by mass of solids.

[0095] After thickening, the thickened phase 30 is introduced into the anaerobic digester 56.

[0096] It remains in the anaerobic digester 56 with a residence time greater than 8 days and including between 8 days and 40 days.

[0097] Anaerobic digestion can be carried out in one or more stages.

[0098] The temperature in the anaerobic digester 56 is generally between 30 °C and 75 °C, and the pressure in the anaerobic digester 56 is generally between 1 bar and 2 bar. The oxygen concentration is preferably less than 500 ppm, and between 10 ppm and 100 ppm.

[0099] The pH in the anaerobic digester 56 is maintained below 7 and in particular between 6.5 and 6.8.

[0100] The liquid / solid mixture obtained after digestion 36 has a liquid phase containing phosphorus, magnesium, and possibly ammonia. This liquid / solid mixture is then introduced into the dehydration reactor 58.

[0101] Liquid centrate 38 is recovered at the outlet of the dehydration reactor. It is concentrated in phosphorus and ammonia.

[0102] Liquid centrate 38 generally contains more than 70 mg / L of phosphorus and more than 500 mg / L of ammonia. It preferably contains less than 1% by mass of solids.

[0103] The dehydrated solid phase 18 has a water concentration of less than 82% by mass and a solids content of more than 18% by mass

[0104] In the embodiment of Figure 1, the liquid centrate 38 is brought into the intermediate struvite precipitation reactor 60.

[0105] Advantageously, a magnesium source is added in the struvite precipitation intermediate reactor 60.

[0106] Thus, the struvite precipitation intermediate reactor 60 is fed with a liquid containing more than 70 mg / L of phosphorus and 500 mg / l of ammonia in the struvite precipitation intermediate reactor 60.

[0107] The treatment implemented converts phosphorus, magnesium and ammonia to precipitate struvite which is recovered at the recovery outlet 62.

[0108] The residence time in the struvite 60 precipitation intermediate reactor is advantageously between 0.5 hours and 2 hours. The temperature in the struvite 60 precipitation intermediate reactor is generally between 20 °C and 30 °C and the pressure in the struvite 60 precipitation intermediate reactor is generally between 1 bar and 1.5 bar.

[0109] The pH value is adjusted by adding a base, such as sodium hydroxide, or by degassing carbon dioxide, to between 8 and 9.5.

[0110] Liquid effluent 44 is recovered at the recovery outlet 64. It is depleted in ammonia. Its ammonia content is lower than that of the reactor inlet.

[0111] At least a part 67, for example between 20% by mass and 100% by mass, of the liquid effluent 44 is then injected into the main struvite precipitation reactor 66 to mix with the phosphorus and magnesium rich liquid phase 28.

[0112] The treatment implemented converts phosphorus, magnesium and ammonia to precipitate struvite 16 which is recovered.

[0113] The residence time in the main struvite 66 precipitation reactor is advantageously between 0.5 and 4 hours. The temperature in the main struvite 66 precipitation reactor is generally between 15 °C and 30 °C, and the pressure in the main struvite 66 precipitation reactor is generally between 1 bar and 15 bar. The pH value is adjusted by adding sodium hydroxide or degassing carbon dioxide, to between 8 and 9.5.

[0114] The volume of liquid effluent 44 introduced into the main struvite precipitation reactor 66 is controlled according to the ratio of ammonia, phosphorus and magnesium present in the main struvite precipitation reactor 66, in order to control struvite precipitation.

[0115] Optionally, a magnesium stream is added to the main struvite precipitation reactor 66.

[0116] Thanks to the presence of the intermediate treatment unit 42, which includes an intermediate struvite precipitation reactor 60, it is easier to control struvite precipitation in the plant 10 by having two reaction sites with adjustable conditions. This maximizes phosphorus recovery while preventing unwanted precipitation in the plant 10. Furthermore, the intermediate treatment unit 42 avoids mixing the phosphorus- and magnesium-rich liquid phase 28 with the liquid centrate 38. It allows only a portion 67 of the liquid effluent 44 to be injected into the main struvite precipitation reactor 66, thus retaining the remaining ammonia stream 68. The ammonia stream 68 can be recovered or treated more easily and economically due to its high ammonia concentration, without dilution by the phosphorus- and magnesium-rich liquid phase 28.

[0117] In the variant illustrated by Figure 2, the intermediate treatment unit 42 does not include an intermediate struvite precipitation reactor 60. It includes an ammonia stripping system 70 to recover the ammonia present in the liquid centrate 38 and to produce an ammonia-concentrated stream 72 and the liquid effluent 44 which is depleted in ammonia.

[0118] The stripping system 70 comprises at least two stripping columns. For example, it includes a stripping column where ammonia is released from the liquid centrate 38 as a gas. The addition of sodium hydroxide (NaOH) may be necessary to increase the pH of the solution and promote ammonia release. The gaseous ammonia is then introduced into a sulfuric acid (H2SO4) scrubbing tower. In this tower, the ammonia is contacted with an acid solution, for example, sulfuric acid, nitric acid, or phosphoric acid, where it dissolves to form a complex, such as ammonium sulfate ((NH4)2SO4).

[0119] For example, the concentrated ammonia 72 stream contains more than 5% by mass of ammonium sulfate.

[0120] The liquid effluent 44 introduced into the main struvite precipitation reactor 68 contains less than 50% by mass of the ammonia present in the liquid centrate 38. In the variant illustrated in Figure 3, the ammonia-concentrated stream from the ammonia stripping system 70 forms the liquid effluent 44, at least a portion 67 of which is introduced into the main struvite precipitation reactor 66. An ammonia-depleted stream 74 is extracted separately from the ammonia stripping system 70.

[0121] In one variant (shown in dotted lines in Figures 1 to 3), at least part of the primary sludge 22A is introduced into the release reactor 46 of the phosphorus and magnesium extraction unit 24 and / or directly into the anaerobic digester 56 without passing through the phosphorus and magnesium extraction unit 24.

Claims

DEMANDS 1. Process for recovering phosphorus present in water to be treated (12), comprising the following steps: production of a biological sludge (14A) containing microorganisms concentrated in phosphorus and magnesium; release, in a release reactor (46), of part of the phosphorus and magnesium contained in the microorganisms of the biological sludge (14A), to produce a treated solid / liquid mixture (26) having an aqueous phase comprising magnesium and phosphorus; separation of the treated solid / liquid mixture (26) into a liquid phase rich in phosphorus and magnesium (28) and into a thickened phase (30);anaerobic digestion of the thickened phase (30) in at least one anaerobic digester (56) to release residual phosphorus and magnesium, to produce ammonia and obtain a solid / liquid mixture after digestion (36), the liquid / solid mixture after digestion (36) having a liquid phase containing phosphorus, magnesium and ammonia; dehydration of the solid / liquid mixture after digestion (36) to produce a liquid centrate (38) enriched in phosphorus and ammonia and a dehydrated solid phase (18); treatment of the phosphorus- and magnesium-enriched liquid phase (28) in a main struvite precipitation reactor (66) to produce struvite;characterized by an intermediate treatment step of the liquid centrate (38) to produce at least one liquid effluent (44), the liquid effluent (44) being at least partially introduced into the main struvite precipitation reactor (66) to react with the phosphorus and magnesium rich liquid phase (28).

2. A phosphorus recovery process according to claim 1, wherein the intermediate treatment step comprises introducing the liquid centrate (38) into an intermediate precipitation reactor (60), precipitating struvite in the intermediate precipitation reactor (60), and producing at least one liquid effluent (44) from the intermediate precipitation reactor (60).

3. Phosphorus recovery process according to claim 2, comprising adding a stream containing magnesium to the intermediate precipitation reactor (60).

4. A phosphorus recovery process according to claim 1, wherein the intermediate treatment step comprises ammonia stripping (70) from the liquid centrate (38) in a stripping system (70) to produce an ammonia-concentrated stream (72) and an ammonia-depleted liquid stream, the liquid effluent (44) being produced from one of the ammonia-rich liquid stream (72) and the ammonia-depleted liquid stream.

5. Phosphorus recovery process according to claim 4, wherein the intermediate treatment step includes the addition of an acid stream, in particular a sulfuric acid stream, into the stripping system (70).

6. A process for recovering phosphorus according to any one of the preceding claims, wherein the biological sludge (14A) is sludge from a biological treatment of wastewater.

7. A method for recovering phosphorus according to any one of the preceding claims, wherein the release step is carried out by maintaining the biological sludge (14A) in a release reactor (46) for at least 3 hours, in particular between 4 hours and 24 hours.

8. A method for recovering phosphorus according to any one of the preceding claims, wherein the release step includes the addition of readily biodegradable carbons (50), in particular volatile fatty acids, into the release reactor (46), advantageously, by maintaining the biological sludge (14A) in the release reactor (46) for between 30 minutes and 4 hours.

9. A method for recovering phosphorus according to any one of the preceding claims, wherein the separation of the treated solid / liquid mixture (26) having an aqueous phase is carried out in a static or dynamic thickener (48).

10. A method for recovering phosphorus according to any one of the preceding claims, wherein the concentration of phosphorus and magnesium in the liquid phase rich in phosphorus and magnesium (28) is greater than that of the liquid phase of the biological sludge (14A).

11. A process for recovering phosphorus according to any one of the preceding claims, wherein the concentration of phosphorus in the phosphorus-magnesium-rich liquid phase (28) is lower than that of the thickened phase (30).

12. A method for recovering phosphorus according to any one of the preceding claims, wherein the step of treating the thickened phase (30) in the anaerobic digester (56) is carried out by maintaining the thickened phase (30) in the anaerobic digester (56) for a period of between 8 and 40 days.

13. A process for recovering phosphorus according to any one of the preceding claims, wherein the concentration of phosphorus and ammonia in the liquid centrate (38) concentrated in phosphorus and ammonia is greater than that of the thickened phase (30) entering the anaerobic digester (56).

14. Installation (10) for recovering phosphorus present in water to be treated (12), comprising: - a station (20) configured to produce a biological sludge (14A) containing microorganisms concentrated in phosphorus and magnesium; - a release reactor (46) of part of the phosphorus and magnesium contained in the microorganisms of the biological sludge (14A) to produce a treated solid / liquid mixture (26) having an aqueous phase comprising magnesium and phosphorus; - a system for separating the treated solid / liquid mixture (26) into a phosphorus and magnesium rich liquid phase (28) and a thickened phase (30); - an anaerobic digester (56) configured to treat the thickened phase (30) in order to release residual phosphorus and magnesium and to produce ammonia to obtain a solid / liquid mixture after digestion (36), the liquid / solid mixture after digestion (36) having a liquid phase containing phosphorus, magnesium and ammonia; - a dehydration reactor (58) of the solid / liquid mixture after digestion (36) to produce a liquid centrate (38) enriched in phosphorus and ammonia and a dehydrated solid phase (18); - a unit (40) for treating the phosphorus and magnesium-rich liquid phase (28) comprising a main struvite precipitation reactor (66) for producing struvite; characterized by an intermediate liquid centrate treatment unit (42) (38) for producing at least one liquid effluent (44), the intermediate liquid centrate treatment unit (42) (38) being connected to the main struvite precipitation reactor (66) to introduce at least part of the liquid effluent (44) into the main struvite precipitation reactor (66) in order to react with the phosphorus and magnesium-rich liquid phase (28).

15. Installation of claim 14, wherein the intermediate treatment unit (42) comprises an intermediate precipitation reactor (60) or an ammonia stripping system (70).

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