Method and plant for anaerobic digestion and struvite recovery

Abstract

The invention relates to a method and a plant for anaerobic digestion and struvite formation. The present invention comprises a step of anaerobic digestion in a digester (110), a step of forming struvite, and implements a step of extracting the ammoniacal nitrogen formed during the anaerobic digestion, during which step: - a stream exiting the anaerobic digester is subjected to evaporation by lowering the pressure, during which gaseous CO2, ammonia-enriched water vapour and a liquid residue are formed, - an aqueous solution enriched with ammoniacal nitrogen is formed by condensing the ammonia-enriched water vapour. The aqueous solution enriched with ammoniacal nitrogen is sent to a struvite precipitation reactor (120) in order to at least partially supply the ammoniacal nitrogen required for struvite precipitation.

Description

Anaerobic digestion and struvite recovery process and installation Scope of the invention

[0001] The invention relates to the field of wastewater treatment and more specifically to the treatment of sludge from wastewater treatment, such as activated sludge or mixed sludge.

[0002] More specifically, the invention relates to an anaerobic digestion process in a digester that reduces the accumulation of struvite in the digester and downstream of it and produces usable struvite. Description of the state of the art

[0003] During the biological treatment of wastewater, which is based on transforming pollution into biomass (called sludge), wastewater treatment plants produce different types of sludge depending on their treatment stages. This sludge is composed of water and matter containing mineral and organic substances. A biological wastewater treatment system typically includes several treatment stages, generally primary, secondary, and tertiary treatment.

[0004] As part of primary treatment, settling may be carried out, and the resulting sludge, called primary sludge, consists mainly of suspended solids carried by the wastewater. Primary wastewater leaving the primary treatment process has a reduced suspended solids content.

[0005] In secondary wastewater treatment, primary wastewater is treated biologically in the presence of air or pure oxygen. A commonly used process, called the "activated sludge" process, involves using oxygen-dependent microorganisms to metabolize the incoming wastewater and form a mixture of microorganisms and wastewater known as a "mixed liquor." This mixture is transferred to a settling tank or clarifier to separate the treated water and form concentrated activated sludge, also called biological sludge or secondary sludge. Most of this concentrated activated sludge is returned to the activated sludge treatment tank. This recirculated biological sludge is referred to in English by the acronym RAS (Recirculated Activated Sludge).The excess portion of the biological sludge (designated in English by the acronym WAS: Waste Activated Sludge) is extracted and sent to a sludge treatment system.

[0006] Among existing sludge treatment methods, anaerobic digestion (or methanization) is commonly used. It allows for the treatment of primary sludge, secondary sludge, or mixtures of primary and secondary sludge known as mixed sludge.

[0007] Secondary sludge usually contains phosphorus and magnesium, which, when brought into contact with ammonium ions, precipitate as struvite (MgNH4PO4.6H2O), according to the reaction:

[0008] Mg 2+ + HPO4 2- + NH4 + + 6 H2O -> MgNH4PO4.6H2O + H + (1)

[0009] Such precipitation can be observed in anaerobic digesters due to the formation of ammoniacal nitrogen during the degradation of nitrogen compounds contained in the sludge. Depending on the conditions, struvite precipitation can occur spontaneously in the digester and / or in the pipes, pumps, and other equipment downstream of the digester. The accumulation of struvite in this equipment can then lead to blockages and / or impair its operation, necessitating the shutdown of the plant for cleaning.

[0010] It is therefore preferable to avoid the precipitation of struvite in an anaerobic sludge treatment digester, and downstream of the latter.

[0011] Furthermore, since struvite can be used as fertilizer, it is advantageous to be able to recover it.

[0012] The invention aims to overcome all or part of the aforementioned disadvantages and to enable the valorization of struvite.

[0013] To this end, the invention relates to a process for anaerobic digestion and struvite formation, said process comprising: - an anaerobic digestion step of a first effluent in a digester producing a digestate, the first effluent containing organic matter including nitrogen and phosphorus, and the anaerobic digestion producing ammoniacal nitrogen and dissolved CO2, - a struvite precipitation step carried out in a struvite precipitation reactor, during which a second effluent containing phosphorus is brought into the presence of ammoniacal nitrogen, and optionally a source of magnesium, to form struvite.

[0014] According to the invention, the process further comprises a step of extracting the ammoniacal nitrogen formed during anaerobic digestion in which: - an outgoing stream from the anaerobic digester is subjected to evaporation by lowering the pressure, in particular by lowering it to a pressure less than or equal to the pressure prevailing in the digester, during which gaseous CO2, water vapor enriched in ammonia and a liquid residue are formed, - an aqueous solution enriched in ammoniacal nitrogen is formed by condensation of the water vapor enriched in ammonia.

[0015] According to the invention, at least part of the aqueous solution enriched in ammoniacal nitrogen is sent into the struvite precipitation reactor to provide at least part of the ammoniacal nitrogen required for the precipitation of struvite.

[0016] The sequence of steps in the process according to the invention makes it possible, on the one hand, to reduce the ammonia nitrogen content of the digester contents and the digestate, thereby reducing the risk of spontaneous struvite precipitation within and downstream of the digester, and on the other hand, to produce usable struvite using the extracted ammonia nitrogen. Furthermore, because the ammonium ion-enriched solution has a small volume compared to the digestate, the volume of the aqueous ammonia nitrogen-enriched solution is moderate, so the volume of the struvite precipitation reactor used does not need to be increased, thus controlling manufacturing and operating costs.

[0017] Furthermore, the aqueous solution enriched with ammonia nitrogen, which is fed into the struvite precipitation reactor, has a basic pH, typically above 8. This can limit, or even eliminate, the need for additional basic compounds in the reactor, which are usually required to reach the optimal pH range (typically 7 to 9) for struvite precipitation. This makes it possible to reduce, or even eliminate, the need for chemical compound additions in the struvite precipitation reactor, and the associated costs. In addition, the aqueous solution enriched with ammonia nitrogen is relatively pure, containing few or no other elements, particularly no suspended solids, so the struvite obtained after mixing it with the second effluent is also relatively pure.

[0018] Advantageously, in order to facilitate the recovery of ammonia, the evaporation of the ammonia nitrogen extraction step can be implemented at a pressure less than or equal to, preferably strictly less than, the value of the saturated vapor pressure of water at the temperature of the medium contained within the digester, in particular the temperature of the liquid phase of this medium.

[0019] Put another way, for a given temperature, corresponding in particular to the temperature of the liquid phase of the medium contained inside the digester, the evaporation step is advantageously carried out at a pressure which is less than or equal to, preferably strictly less than, the saturated vapor pressure of water, in particular pure water, at that given temperature.

[0020] Put another way, for a given temperature, corresponding in particular to the temperature of the liquid phase of the medium contained inside the digester, the evaporation step is advantageously implemented at a pressure low enough to induce the boiling of at least part of the water at that given temperature.

[0021] Typically, for example, an operating pressure equal to the value of the saturated vapor pressure of water at a temperature lower (-0.01 to -10°C) than that of the medium contained in the digester, in particular its liquid phase.

[0022] Advantageously, the condensation of ammonia-enriched steam can be carried out in a heat exchanger. This heat exchanger can be direct or indirect, preferably indirect. Using an indirect heat exchanger avoids dilution or contamination of the aqueous solution with other chemical compounds: the aqueous solution then contains mostly water and ammonium ions, or even consists solely of water and ammonium ions. This improves the purity of the struvite formed when this aqueous solution is added to the second effluent.

[0023] Advantageously, the condensation of water vapor enriched with ammonia can be carried out at the same pressure as evaporation.

[0024] Advantageously, a predetermined quantity of aqueous solution enriched with ammonia nitrogen can be fed into the struvite precipitation reactor. This could be just the amount necessary for struvite precipitation, corresponding to an ammonia nitrogen concentration equal to the phosphate ion concentration, or a slight excess (ammonia nitrogen / phosphate ion molar ratio greater than 1, for example, 2 to 8). This optimizes the operation of the struvite precipitation reactor and, in particular, avoids the need to return excess ammonia nitrogen from the reactor to a nitrification / denitrification treatment. Furthermore, the remaining aqueous solution enriched with ammonia nitrogen (not used for struvite precipitation) can then be used to produce fertilizers or for other applications.

[0025] In a first embodiment, the process may further include:

[0026] - the supply of wastewater effluent containing phosphorus-laden polyphosphate-accumulating organisms,

[0027] - a dehydration stage of the digestate into a liquid effluent and a fraction concentrated in dry matter.

[0028] In this embodiment:

[0029] - the first effluent comprises, or is composed of, said wastewater effluent, and

[0030] - the second effluent comprises, or is made up of, the liquid effluent from the dewatering stage of the digestate.

[0031] Wastewater effluent may include, or be made up of, secondary sludge, alone or mixed with primary sludge.

[0032] Secondary sludge contains microorganisms, including polyphosphate-accumulating organisms (PAOs). These PAOs are bacteria that exhibit the characteristic of overaccumulating phosphorus when subjected to alternating anaerobic and aerobic conditions. PAOs release phosphates during their time in anaerobic conditions, and upon transitioning to aerobic conditions, they accumulate a greater quantity of phosphates than that released under anaerobic conditions.

[0033] Thus, in this first embodiment, during the anaerobic digestion of wastewater effluent containing phosphorus-laden PAO (optionally thickened and optionally hydrolyzed effluent), the PAO releases phosphorus into the digester contents. Despite the presence of phosphorus in the digester, the extraction of ammoniacal nitrogen limits, or even eliminates, the risk of struvite precipitation within the digester or in downstream equipment. Furthermore, the phosphorus thus released is found in the digestate and in the liquid effluent resulting from digestate dewatering, which can then be combined with the ammoniacal nitrogen-enriched aqueous solution in the struvite precipitation reactor to form struvite.

[0034] Advantageously, in this first embodiment, the process may further include:

[0035] - a thickening step of said wastewater effluent,

[0036] - optionally a hydrolysis step of the thickened wastewater effluent,

[0037] the first effluent then comprises, or is made up of, said thickened wastewater effluent, and optionally hydrolyzed.

[0038] In a second embodiment, the process may further include:

[0039] - the supply of wastewater effluent containing phosphorus-laden polyphosphate-accumulating organisms,

[0040] - a phosphorus release stage by phosphorus-laden polyphosphate-accumulating organisms contained in the wastewater effluent, this stage producing an effluent enriched in dissolved phosphorus and containing the phosphorus-laden polyphosphate-accumulating organisms,

[0041] - a step of separating the effluent enriched in dissolved phosphorus into a first fraction concentrated in polyphosphate-accumulating organisms discharged from phosphorus and a second liquid fraction rich in dissolved phosphorus,

[0042] the first effluent comprising, or consisting of, the first fraction and the second effluent comprising, or consisting of, the second liquid fraction.

[0043] In this embodiment, the PAOs release phosphorus before entering the digester, which also helps to limit, or even eliminate, struvite formation within the digester or in downstream equipment. This risk is further reduced because the ammonia nitrogen extraction step can also reduce the ammonia nitrogen concentration within the digester. Thus, this embodiment reduces the concentrations of two struvite constituents (phosphate and ammonium ions) within the digester, further limiting the risk of struvite precipitation compared to the first embodiment.

[0044] Advantageously, in order to reduce the volume of effluent to be treated, prior to the phosphorus release stage, the process may include a thickening stage of at least part of the wastewater effluent.

[0045] In a variant of the second embodiment, the process may further include:

[0046] - a second struvite precipitation stage in a second reactor, during which a third effluent containing phosphorus is brought into contact with a source of ammoniacal nitrogen, and optionally a source of magnesium, to form struvite,

[0047] - a dehydration stage of the digestate into a liquid effluent and a fraction concentrated in dry matter.

[0048] In this variant of the second embodiment:

[0049] The third effluent comprises, or consists of, the liquid effluent from the digestate dewatering stage, and

[0050] a portion of the aqueous solution enriched in ammoniacal nitrogen is sent to the second struvite precipitation reactor to provide the ammoniacal nitrogen needed for struvite precipitation.

[0051] This variant maximizes the amount of struvite formed and the utilization of phosphorus.

[0052] According to one embodiment of the invention, the output stream from the anaerobic digester is formed by a liquid fraction taken from the anaerobic digester. This allows for the aforementioned advantages related to the extraction

[0053] According to one embodiment of the invention, the output stream from the anaerobic digester is formed by the digestate from the anaerobic digestion, the evaporation and condensation steps separating the digestate so as to form the aqueous solution enriched in ammonium ions and a fraction depleted in ammonium ions.

[0054] According to an advantageous variant of this embodiment, the fraction of the digestate depleted in ammonium ions is subjected to a digestate dehydration step, a liquid fraction resulting from the dehydration being sent to the bioreactor.

[0055] The invention also relates to an anaerobic digestion and struvite recovery installation, particularly adapted to implement the process according to the invention, comprising: - an anaerobic digester including an inlet pipe receiving a first effluent and an outlet pipe for the digestate produced, - a struvite precipitation reactor including an inlet pipe receiving a second effluent, a first outlet pipe for the struvite and a second outlet pipe for a liquid effluent depleted in phosphorus and nitrogen.

[0056] According to the invention, the installation further comprises an ammonia nitrogen extraction system including: - an evaporation device configured to receive an output flow from the anaerobic digester, - a condenser equipped with an inlet pipe connected to the evaporation device and receiving from it steam enriched in ammonia, and a condensate discharge pipe, - at least one pump connected at least to the evaporation device, the condensate discharge pipe of the extraction system being connected to an inlet of the struvite precipitation reactor.

[0057] The installation may also include at least one of the following features:

[0058] - the condenser is a heat exchanger, preferably an indirect heat exchanger,

[0059] - a pressure management system within the evaporation device configured to apply a pressure inside the evaporation device that is less than or equal to, and preferably strictly less than, the saturated vapor pressure of water at a temperature lower than the temperature of the medium contained within the digester, in particular a liquid fraction of that medium

[0060] - a system for controlling the amount of condensate sent to the struvite reactor configured to send to the struvite reactor a predetermined amount of aqueous solution enriched in ammoniacal nitrogen, for example just enough for the precipitation of struvite or in slight excess as previously described.

[0061] In a first embodiment, particularly adapted to implement the process according to the first embodiment, the installation further comprises:

[0062] - a dewatering device equipped with an inlet connected to the digestate discharge line, an outlet for the discharge of a liquid fraction and an outlet for the discharge of a fraction concentrated in dry matter, the outlet being connected to the inlet line of the struvite precipitation reactor.

[0063] In a variant of the first embodiment, the installation may include:

[0064] - a thickening device, and optionally a hydrolysis reactor, the thickening device being equipped with an inlet pipe for a wastewater effluent and an outlet pipe for said thickened wastewater effluent connected to the inlet pipe of the anaerobic digester, either directly or via the hydrolysis reactor.

[0065] In a second embodiment, particularly adapted to implement the process according to the second embodiment, the installation further comprises:

[0066] - a phosphorus release reactor equipped with an inlet pipe for a wastewater effluent comprising phosphorus-loaded polyphosphate accumulating organisms, and an outlet pipe for said effluent enriched with dissolved phosphorus and containing the phosphorus-discharged polyphosphate accumulating organisms,

[0067] - optionally a thickening device mounted on the inlet pipe of the phosphorus release reactor,

[0068] - a separation device comprising an inlet connected to the outlet pipe of the phosphorus release reactor, a first outlet of a first fraction concentrated in polyphosphate accumulating organisms discharged from phosphorus and a second outlet of a second liquid fraction rich in dissolved phosphorus, the first outlet being connected to the inlet pipe of the anaerobic digester and the second outlet being connected to the inlet pipe of the struvite precipitation reactor.

[0069] In a variant of the second embodiment, particularly adapted to implement the process according to the variant of the second embodiment, the installation may further include:

[0070] - a second struvite precipitation reactor comprising an inlet pipe receiving a third effluent, a first struvite discharge pipe and a second discharge pipe for a liquid effluent depleted in phosphorus and nitrogen,

[0071] - a dewatering device equipped with an inlet connected to the digestate discharge line of the digester, an outlet for a liquid fraction and a discharge line for a dry matter concentrated fraction, the outlet being connected to the inlet line of the second struvite reactor,

[0072] and in which the condensate drain line of the extraction system is also connected to an inlet of the second struvite precipitation reactor.

[0073] According to one embodiment of the invention, the evaporation device is connected to the anaerobic digester by a pipe for collecting the contents of the anaerobic digester and by a return pipe for a liquid residue.

[0074] According to another embodiment of the invention, the evaporation device is connected to the anaerobic digester by the digestate discharge line, the ammonia extraction system being configured to separate the digestate so as to form the aqueous solution enriched in ammonium ions and a fraction depleted in ammonium ions.

[0075] According to a variant of this embodiment of the invention, the installation further comprises a dehydration device equipped with an inlet connected to the evaporation device so as to receive the fraction depleted in ammonium ions, an outlet line for a fraction concentrated in dry matter and an outlet for a liquid fraction, the outlet being connected to the inlet line of the struvite precipitation reactor. Detailed description of the invention

[0076] Other features and advantages of the invention will become apparent from the following description of a particular embodiment of the invention, given by way of example, but not limitation, with reference to the accompanying drawings in which:

[0077] is a schematic representation of an installation according to an embodiment of the invention,

[0078] is a schematic representation of an installation according to a particular embodiment of the invention,

[0079] is a schematic representation of an installation according to another particular embodiment of the invention,

[0080] is a schematic representation of an installation according to a variant of the embodiment shown,

[0081] is a schematic representation of an installation according to an embodiment of the invention,

[0082] is a schematic representation of an installation according to an embodiment of the invention.

[0083] In the figures, the same references designate the same elements.

[0084] Definitions / abbreviations

[0085] Volatile solids (VS), or volatile suspended solids, refers to the portion of suspended solids that can be volatilized at 550 °C. The volatile solids content of a sample is determined by calcining the suspended solids obtained after centrifugation and drying at 105 °C at 550 °C. The volatile solids content, expressed in kg / m³ of sample, can be determined gravimetrically according to standard NF T90-105-2: 1997.

[0086] Dry matter (DM) includes both suspended solids and dissolved salts. Dry matter content is expressed in g / L of sample and can be determined according to standard NF EN 12880- Nov 2000.

[0087] In what follows, the dry matter content is expressed as a percentage. The dry matter content corresponds to the ratio DM / MB of the mass of dry matter (DM) obtained after 24 hours of drying at 105°C to the gross mass (GM), which corresponds to the mass of raw material before drying at 105°C, and is expressed as a percentage.

[0088] Anaerobic digestion stage

[0089] The process according to the invention comprises an anaerobic digestion or methanation stage, which corresponds to a cascade of well-known biochemical reactions enabling methanogenic microorganisms to convert the organic matter present in a digester into biogas, an energy-rich gas consisting primarily of methane (60 to 65% v / v) and carbon dioxide (35 to 40% v / v). The biogas may also contain other compounds in very small proportions: CO, N2, hydrocarbons, H2S, mercaptants, and volatile organic compounds. The biogas can be utilized, possibly after purification. The remaining material is called digestate. This stage is the same for all embodiments of the invention.

[0090] This step is advantageously an anaerobic digestion step implemented in liquid form.

[0091] In general, anaerobic digestion can be carried out at a temperature of 5 to 60°C, under psychrophilic, mesophilic, or thermophilic conditions. The conditions for implementing this step, particularly temperature, pH, and residence time, can be advantageously chosen to maximize biogas production.

[0092] The first effluent entering the anaerobic digestion stage is typically formed from secondary sludge or mixed sludge (mixtures of secondary and primary sludge).

[0093] This first effluent is therefore generally a wastewater effluent comprising polyphosphate accumulating organisms loaded with phosphorus or rich in phosphorus resulting from the release of phosphorus by these PAOs.

[0094] The process according to the invention may therefore include a step of supplying such an effluent.

[0095] This supply step may include a biological wastewater treatment step to obtain biological or secondary sludge.

[0096] This supply stage can alternatively include a primary wastewater treatment stage to obtain primary sludge, a biological wastewater treatment stage to obtain biological or secondary sludge, and a stage of mixing primary and secondary sludge to form a first effluent.

[0097] The primary treatment stage generally reduces the solids and / or organic matter content of the wastewater to be treated. It typically involves a gravity separation stage (sedimentation and / or clarification), possibly assisted by the prior addition of a coagulant and flocculant, during which the wastewater is placed in a holding tank or settling basin. The solids in the wastewater settle to the bottom of the tank where they are collected. This stage produces primary sludge and an effluent with a reduced solids content, which is then sent to the biological treatment stage.

[0098] The biological treatment of wastewater typically includes a biological nutrient removal stage, known as Biological Nutrient Removal (BNR) or Enhanced Biological Nutrient Removal (EBNR). Biological nutrient removal typically comprises three substages, the order of which may vary, but which generally follow this sequence: an anaerobic stage, an anoxic stage, and an aerobic stage. Examples of such processes include the Phostrip, Bardenpho, and UCT (University of Cape Town) processes. Such processes are described, for example, by Brett et al. ("Phosphorus Removal and Recovery Technologies," European Centre for Polyphosphate Studies EV, 1997, published by Selper Publications (ISBN: 094841 1 10 0), Chapter 4).A professional skilled in the art knows how to implement such a biological nutrient removal stage, taking into account the specific characteristics of the region and the wastewater to be treated by the treatment plant. Biological treatment generally includes a settling and / or clarification stage.

[0099] The wastewater effluent from the biological treatment stage typically comprises a mixture of solids, liquids, and microorganisms. It may include or consist of biological sludge or activated sludge, including excess biological sludge (EBS).

[0100] Among the microorganisms present in wastewater effluents from a biological treatment stage, there are polyphosphate accumulating organisms which are loaded with phosphorus due to the succession of aerobic / anaerobic treatments undergone by the wastewater.

[0101] The first effluent used in the present invention typically has a dry matter content of 1 to 25 g / L, preferably 4 to 15 g / L.

[0102] In a first embodiment, the first effluent may include secondary or mixed sludge, which is subjected to a thickening step aimed at increasing the dry matter concentration of the sludge by separating some of the free water. This results in a reduction in the volume of the sludge, which facilitates its transport and subsequent use.

[0103] Typically, a thickening step allows the dry matter content to increase from 1-25 g / L to 40-100 g / L. This minimizes the volume of anaerobic digestion, for example, while remaining within a sludge concentration range that is pumpable and mixable with conventional pump and agitation technologies (liquid anaerobic digestion).

[0104] This thickening step can be implemented using one of the following techniques:

[0105] - Decantation / clarification: gravity is used to separate solid particles from water.

[0106] - Dissolved air flotation (DAF): air bubbles are injected into the sludge, allowing solid particles to float to the surface and be skimmed off.

[0107] - Centrifugation: a centrifugal force is used to separate solid particles from water.

[0108] This thickening step may optionally be followed by a pretreatment step using hydrolysis to increase the yield of the anaerobic digestion step by promoting the degradation of organic matter by microorganisms. This may involve acid or basic hydrolysis, generally carried out at a temperature below 100°C, thermal hydrolysis, or biological hydrolysis.

[0109] Biological hydrolysis is, for example, a fermentation / hydrolysis step under mesophilic (30-42°C) or thermophilic (45-70°C) conditions with a residence time of approximately 1 to 3 days.

[0110] Thermal hydrolysis (THP) can be a process that involves holding sludge, typically with a dry matter content of 12% to 25%, at a temperature between 140°C and 170°C for a treatment time of 30 to 60 minutes. The hydrolyzed sludge is then usually cooled in a flash tank before being introduced into the digester.

[0111] Hydrolysis aims to improve digestion performance and sludge dewatering by breaking down the cell walls of microorganisms, thus making the cell contents easily degradable anaerobically. It is, for example, implemented in a dedicated reactor.

[0112] After hydrolysis, the liquid effluent is usually diluted to about 10% dry matter before being injected into the digester.

[0113] In a second embodiment, the first effluent may have a high phosphorus content due to phosphorus release by the PAO.

[0114] The process then includes a step of supplying wastewater effluent containing phosphorus-laden polyphosphate-accumulating organisms (PAOs). This secondary sludge is subsequently subjected to a phosphorus release step by the phosphorus-laden PAOs.

[0115] This phosphorus release stage produces an effluent enriched in dissolved phosphorus and containing polyphosphate-accumulating organisms that have been discharged of phosphorus.

[0116] It is typically implemented in a tank, or more specifically in a release reactor. The phosphorus release stage takes place under anaerobic conditions: under these conditions, it is known to those skilled in the art that PAOs release phosphorus. Those skilled in the art will know how to choose, in particular, the temperature, pH, and residence time conditions for this stage to obtain the desired result. For example, the residence time in the tank is between 1 and 48 hours, preferably between 1 and 24 hours, and especially between 2 and 12 hours.

[0117] In an advantageous embodiment, the phosphorus release step may include the addition of biodegradable carbon to the wastewater effluent. The addition of biodegradable carbon promotes phosphorus release by microorganisms. The biodegradable carbon may be readily biodegradable carbon (RBC), such as volatile fatty acids, advantageously acetic acid or propionic acid. Alternatively, it may be slowly biodegradable carbon (SBC), and / or precursors of biodegradable carbon. In particular, it may consist of primary sludge, fermented or unfermented, or the supernatant or settled water from primary sludge.

[0118] Typically, when biodegradable carbon is added during the phosphorus release step, the required residence time is reduced, particularly in the case of RBCs. The required residence time is then generally between 1 and 8 hours.

[0119] The step of releasing phosphorus by phosphorus-loaded PAOs is advantageously followed by a step of separating the effluent enriched in dissolved phosphorus into a first fraction concentrated in phosphorus-discharged polyphosphate accumulating organisms and into a second liquid fraction rich in dissolved phosphorus.

[0120] This separation step is typically a thickening step, for example as described above.

[0121] The first fraction, alone or mixed with primary sludge, then forms a first effluent within the meaning of the present invention.

[0122] The second effluent defined in the present invention then comprises, or is made up of, the second liquid fraction.

[0123] An optional pre-thickening step of at least part of the input of this release step may be provided prior to the release step, similar to the thickening step mentioned for the first embodiment.

[0124] Advantageously, the working concentration of sludge chosen in the anaerobic release reactor (expressed as mass of suspended solids (SS) per liter of the medium present in the release reactor) is 10 to 35 g / L of suspended solids, preferably 15 to 25 g / L, with the proportion of pre-thickened sludge upstream being chosen to achieve this target. In other words, by adjusting the proportion of pre-thickened sludge in the sludge entering the release reactor, it is possible to adjust this working concentration.

[0125] It is preferable that the working concentration of sludge chosen in the anaerobic release reactor remains lower than that of the first fraction targeted in feeding the digester so that the second separated liquid fraction has a non-negligible volume, allowing to effectively redirect a large part of the phosphorus released in soluble phase to the second liquid fraction.

[0126] The higher the working sludge concentration chosen in the anaerobic release reactor, the higher the concentration of soluble phosphorus in the second fraction of separated liquid will be, which is advantageous for struvite precipitation yield.

[0127] However, the closer the working concentration of sludge chosen in the anaerobic release reactor is to that of the first fraction (the thickened sludge), the less water is recovered in the second phosphorus-enriched liquid fraction.

[0128] A high working concentration can therefore lead to reduced protection of the digester against uncontrolled struvite precipitation because the majority of the phosphorus released by the PAO is likely to reach the digester with the first fraction (the thickened sludge downstream of the release reactor).

[0129] A person skilled in the art will be able to advantageously determine a working concentration in the anaerobic release reactor resulting from a compromise between the phosphorus flow directed to the digester and the recovery potential in the struvite precipitation reactor, itself maximized by the highest concentrations of dissolved phosphorus.

[0130] Advantageously, the working sludge concentration chosen in the anaerobic release reactor is less than or equal to half that targeted in the first fraction concentrated in phosphorus-discharged polyphosphate-accumulating organisms (intended for digester feed, optionally preceded by hydrolysis) so that the second separated liquid fraction has a greater volume than the first fraction, effectively helping to redirect a large part of the phosphorus released in soluble phase to the second liquid fraction (towards struvite precipitation) rather than to the digester.

[0131] Struvite precipitation stage

[0132] The process according to the invention also includes one or two struvite precipitation steps in which a phosphorus-rich effluent is treated with ammonia nitrogen, and optionally a magnesium source, to form struvite. This step is the same for all embodiments of the invention.

[0133] Struvite is a neutral complex made up of magnesium (Mg 2+ ), ammonium (NH4 + ) or phosphate (PO4 3+ ) which precipitates under conditions of equimolarity between its constituents under conditions of pH typically from 7.5 to 8.5.

[0134] Struvite precipitation is obtained when a phosphorus-rich effluent is mixed with ammoniacal nitrogen (NH4 + ) and magnesium (Mg 2+ ) according to reaction (1).

[0135] To improve the yield of the struvite precipitation step, at least one counterion source can be added. Typically, the counterion source is magnesium and / or ammonium.

[0136] An external addition of magnesium, particularly in the form of magnesium chloride and / or magnesium oxide, can be carried out during the struvite precipitation step.

[0137] During this step, a base such as sodium hydroxide (NaOH) can also be added to adjust the pH within a range that optimizes struvite precipitation. Advantageously, the pH can be adjusted to a value of 7.5 to 8.5.

[0138] The precipitation of struvite allows for the recovery of phosphorus. Once collected, the struvite can be washed, dried, and preferably packaged. The struvite can then be used, for example, as fertilizer.

[0139] In one embodiment, phosphate is supplied by a liquid fraction rich in dissolved phosphorus from the digestate, following anaerobic digestion of a wastewater effluent containing phosphorus-laden PAO (polycyclic aromatic hydrocarbons). The digestion step is then followed by a digestate dewatering step, separating a liquid effluent from a concentrated dry matter fraction (or "cake"). This liquid effluent is rich in phosphates released by the PAO under the anaerobic conditions of digestion and dissolved in the liquid fraction.

[0140] The digestate dewatering stage also aims to maximize the reduction of the water content of the sludge to obtain a more stable and easier-to-manage final product.

[0141] Typically, during the dehydration stage, the dry matter content of the digestate increases from 30-60 g / L to 150-300 g / L for the cake.

[0142] As an example, the dehydration stage may employ one or more of the following mechanical processes:

[0143] - Screw press / Belt filter / Filter press: sludge is pressed through a filter to extract water.

[0144] - Centrifugation.

[0145] In another embodiment, the phosphate is supplied by the effluent exiting the phosphorus release stage, more precisely by the second liquid fraction from the separation of the effluent exiting the phosphorus release stage.

[0146] We can foresee two struvite precipitation stages implemented in two struvite precipitation reactors, one of which receives phosphates supplied by an effluent exiting the phosphorus release stage, and the other receives phosphates supplied by a liquid fraction rich in dissolved phosphorus from the digestate.

[0147] Regardless of the embodiment, the ammonium ions required for struvite precipitation are supplied, in whole or in part, preferably in whole, by the ammonia nitrogen extraction step described below. These ions may be supplied in large excess. However, advantageously, they are supplied in quantities with a molar ratio of 2 to 8 times that of the phosphate ions entering the precipitation reactor.

[0148] In general, the quantity of ammonium ions supplied by the aqueous solution enriched in ammoniacal nitrogen from the ammoniacal nitrogen extraction step of the present invention is much greater than the quantity required for struvite precipitation and an external supply of ammonium ions is not necessary.

[0149] Furthermore, regardless of the implementation method, depending on the magnesium ion content of the effluents supplying the phosphate, it may be necessary to add magnesium, for example in the form of Mg ions. 2+ in solution. Preferably, magnesium will then be added so that the total quantity of magnesium present corresponds to a number of moles of 1.0 to 1.5 in ratio to the number of moles of phosphates.

[0150] Alternatively or in combination, a basic compound may be added to reach the pH range that favors struvite formation.

[0151] Step of extraction of ammonia nitrogen formed during anaerobic digestion

[0152] During this step, and according to the embodiments and variants of figures 1 to 4:

[0153] - a fraction of the digester's contents is taken, specifically a liquid fraction of these contents,

[0154] - the extracted fraction is subjected to evaporation by lowering the pressure, during which gaseous CO2, water vapor enriched in ammonia, and a liquid residue are formed.

[0155] - an aqueous solution enriched in ammoniacal nitrogen is formed by condensation of water vapor enriched in ammonia,

[0156] - the liquid residue is returned to the digester.

[0157] The aqueous solution enriched with ammoniacal nitrogen can then be used in the struvite precipitation reactor to provide the ammoniacal nitrogen needed for struvite precipitation.

[0158] The fraction of the digester contents that is collected is a liquid medium containing microorganisms, products of anaerobic digestion, and water. It may also contain enzymes. The products of anaerobic digestion typically include ammonium ions, dissolved CO2, but also volatile fatty acids, dihydrogen, methane, alcohols, aldehydes, and / or ketones.

[0159] During evaporation by lowering the pressure, microorganisms (and possibly enzymes) remain in solution and form the liquid residue, which is then returned to the digester. Volatile compounds, namely CO2, ammonia, and also dihydrogen and methane, are evaporated.

[0160] When the pressure is lowered, particularly to a pressure below that inside the digester, CO2 degassing occurs, which increases the pH of the remaining extracted fraction and thus shifts the solubility equilibrium. Furthermore, water vapor is also produced. This water vapor acts as a carrier gas, carrying with it volatile products, namely ammonia, and possibly methane and / or hydrogen. This water vapor is therefore enriched in ammonia (NH3).

[0161] This formation of water vapor can be promoted by implementing evaporation at a pressure less than or equal to, preferably strictly less than, the value of the saturated vapor pressure of water at a temperature lower than the temperature of the medium contained inside the digester.

[0162] Typically, the water temperature (especially pure water) used to determine the saturated vapor pressure is 0.01 to 10 °C lower, preferably 1 to 10 °C lower, more preferably 2 to 10 °C lower, than the temperature of the digester contents, especially the liquid phase of those contents.

[0163] Generally, the evaporation stage is preferably carried out under temperature and pressure conditions chosen to avoid degradation of microorganisms (and any enzymes) during evaporation. Those skilled in the art know how to select a temperature and / or pressure that does not damage and / or degrade the microorganisms and / or enzymes according to their nature.

[0164] The applied pressure is typically lower than the pressure inside the digester.

[0165] Typically, the evaporation stage temperature is as high as possible, but similar to or lower than the digester temperature, ideally within the thermophilic range, although the invention also applies to mesophilic bioreactors. The temperature can be between 45 and 75 °C when the digester operates under thermophilic conditions and between 20 and 45 °C when the digester operates under mesophilic conditions.

[0166] Typically, the pressure in the evaporation stage is 33 to 350 millibars absolute pressure.

[0167] The evaporation stage is advantageously carried out in an evaporator external to the digester, such as, but not limited to, a forced circulation evaporator, or an upward or downward film evaporator, or a stirred thin film evaporator, or a multi-effect evaporator or a self-cleaning evaporator, or even a flash evaporator, also known by some as the flash cooling process.

[0168] The ammonia-enriched water vapor is then condensed, separating an aqueous solution enriched in ammonia nitrogen from the non-condensable gases (CO2, CH4, H2). The non-condensable gases can then be returned to the digestion stage.

[0169] This condensation typically occurs at the same pressure as the evaporation stage, with condensation resulting from a drop in temperature.

[0170] Condensation can be achieved in a direct heat exchanger, by contact with a cold liquid, such as an acid, or in an indirect heat exchanger through which a cold fluid, usually water, circulates. An indirect heat exchanger is preferable to limit chemical consumption and avoid contamination of the aqueous solution enriched with ammonia nitrogen.

[0171] This aqueous solution enriched in ammoniacal nitrogen has the advantage of having a basic pH, typically greater than 8. The struvite precipitation step can then be carried out with little or no addition of a base and / or an additional basic compound.

[0172] The aqueous solution enriched with ammonia nitrogen contains few or no other elements, and in particular no suspended solids. Its suspended solids content is typically less than 500 mg / L, most often less than 100 mg / L.

[0173] Advantageously, the quantity of aqueous solution enriched with ammonia nitrogen that is sent to each struvite precipitation stage is determined, and in particular controlled, and corresponds to just the amount necessary for struvite precipitation or, preferably, to an excess amount, more preferably to a slight excess (ammonia nitrogen / phosphate ion molar ratio greater than 1, for example, from 2 to 8). This quantity can be determined beforehand by tests and / or modeling based on the phosphate content of the effluent entering the struvite reactor and the ammonium ion concentration of the aqueous solution enriched with ammonia nitrogen.

[0174] Figures 5 and 6 illustrate two other embodiments of the invention. For these two embodiments, we will only describe the differences with the first embodiments described.

[0175] In both of these embodiments, the anaerobic digestion output stream is formed by the digestate from the anaerobic digestion, the evaporation and condensation steps separating the digestate so as to form the aqueous solution enriched in ammonium ions and a fraction depleted in ammonium ions.

[0176] In other words, and as can be seen in figures 5 and 6, a pump assembly 133a / extraction system 130 / pump 136 can be directly connected to the discharge line of the digester 114 in order to receive the digestate directly instead of a fraction taken as is the case for the first embodiments.

[0177] The advantages of direct digestate treatment are as follows: It eliminates the need to retain a minimum quantity of ammonium ions in the anaerobic digester 110. Indeed, ammonium ions play a buffering role, maintaining an optimal pH in the anaerobic digester 110, typically above 7. In the first two embodiments, it is necessary to avoid removing too many ammonium ions to maintain an ammonium ion concentration preferably greater than or equal to 500 mg / L in the anaerobic digester 110. In the embodiments of Figures 5 and 6, it is possible to overcome this limitation and treat significantly more ammonium ions via the digestate ammonia extraction system in order to achieve a low ammonium ion concentration in the liquid returned to a main treatment line, for example, between 50 and 100 mg / L.This eliminates the need for a return line 137 to the digester and its associated pump. A sampling pump 133a (to supply the extraction system) and a pump 136 are retained, but their flow rate is equal to the organic matter inlet flow rate of the anaerobic digester 110, unlike the earlier embodiments in which the flow rates of pumps 133a and 137 are between 3 and 10 times greater than the organic matter inlet flow rate of the anaerobic digester 110. This results in energy savings. It also allows for the capture of dissolved gases by passing the digestate through the evaporation device 132 and the condenser 134. The biogas can then be recovered and fed into a biogas network to prevent its escape, thus enabling its utilization rather than the release of greenhouse gases.

[0178] In embodiments 5 and 6, ammonium ion extraction from the digestate precedes dewatering. The ammonium-depleted (and potentially degassed) fraction is sent to the dewatering unit 140, for example via a pump 141, to separate it into a solid fraction and a liquid fraction. In embodiment 1, the liquid fraction can be returned to the struvite precipitation reactor 120. In embodiment 2, the liquid fraction resulting from dewatering can be returned to the main treatment line.

[0179] Facility

[0180] Figure 100 schematically represents an installation implementing the process according to the invention. This installation includes an anaerobic digester 110 capable of carrying out an anaerobic digestion step on a first effluent. This can be any enclosure capable of carrying out anaerobic digestion with biogas formation.

[0181] The anaerobic digester 110 is equipped with an inlet pipe 112 receiving a first effluent, an outlet pipe 114 for the digestate produced during anaerobic digestion and an outlet pipe 113 for the biogas produced during digestion.

[0182] The first effluent typically contains organic matter, including nitrogen and phosphorus. During anaerobic digestion, the nitrogen it contains will be transformed into ammoniacal nitrogen (ammonium ions NH4). +Depending on any prior treatments present, the first effluent may also contain phosphorus, particularly present in PAOs, or in the form of phosphates previously released by PAOs.

[0183] Installation 100 also includes a struvite precipitation reactor 120 suitable for implementing the struvite precipitation step of the process, such as a continuously stirred tank reactor, or a fluidized bed reactor, as described in particular in Chapter 5 of Brett et al. An example of a commercial precipitation reactor, useful in particular for precipitating phosphorus as struvite, is the Crystallactor®.

[0184] Reactor 120 is equipped with an inlet pipe 122 receiving a second effluent containing phosphorus and an inlet 124 for an ammonia nitrogen source. Reactor 120 is also equipped with a discharge pipe 126 for the struvite formed and a discharge pipe 127 for a liquid effluent depleted in phosphorus and nitrogen. It can also be equipped, as required, with one or more additional feed pipes 128 for magnesium (if the second effluent does not contain sufficient magnesium) and / or a basic compound (to adjust the pH if necessary).

[0185] Installation 100 also includes an ammonia nitrogen extraction system 130 suitable for implementing the step of extracting ammonia nitrogen formed during anaerobic digestion.

[0186] This extraction system 130 includes an evaporation device 132 connected to the anaerobic digester 110 by a pipe 132a for extracting the digester contents and a pipe 132b for returning the liquid residue to the anaerobic digester. Pumps 133a and 133b extract the digester contents and return the liquid residue to the digester. The evaporation device is external to the digester. It is typically an evaporator, such as a forced circulation evaporator, an upward or downward film evaporator, a stirred thin-film evaporator, a multi-effect evaporator, a self-cleaning evaporator, or a flash evaporator.

[0187] The extraction system 130 also includes a condenser 134 receiving water vapor enriched in ammonia (NH3) from the evaporation device 132 via an inlet line 134a and producing a condensate discharged by a condensate discharge line 134b. This condenser 134 can be a direct or indirect heat exchanger, preferably indirect.

[0188] The extraction system 130 finally includes a pump 136 connected to the evaporation device, here via the condenser 134. This arrangement allows the non-condensables from the condenser 134 to be returned to the inlet of the digester 110 via a pipe 137 or directly into the gaseous head of the digester.

[0189] The invention is not limited, however, by the number of pumps used, nor their position, provided that a vacuum can be applied to the evaporation device 132, and preferably also to the condenser 134.

[0190] According to the invention, the condensate drain line 134b of the extraction system is connected to the inlet 124 of the struvite precipitation reactor.

[0191] The 130 extraction system works as follows:

[0192] - First, a fraction of the contents of digester 110, specifically a liquid fraction of these contents, is extracted via pump 133a and pipe 132a.

[0193] - the fraction taken is subjected to evaporation by lowering the pressure in the evaporation device 132 by means of the pump 136 which allows the formation of gaseous CO2 discharged via the pipe 132c, water vapor enriched in ammonia discharged by the pipe 134a, and a liquid residue discharged by the pipe 132b.

[0194] - an aqueous solution enriched in ammoniacal nitrogen is formed by condensation of water vapor enriched in ammonia in condenser 134, this condensed fraction being discharged via pipe 134b,

[0195] - the liquid residue is returned to the digester by means of pipe 132b and pump 133b.

[0196] At least a portion of the ammonia nitrogen-enriched aqueous solution is sent to the struvite precipitation reactor 120 via line 134b and inlet 124 to supply the ammonia nitrogen required for struvite precipitation. The remaining ammonia nitrogen-enriched aqueous solution can be discharged via line 135 for further use.

[0197] Advantageously, the installation 100 may include a pressure management system 150 within the evaporation device 132, configured to lower its pressure to a pressure lower than the pressure prevailing inside the digester.

[0198] This management system may include a valve or other pressure-reducing system, advantageously computer-controlled, and possibly a pressure sensor for the evaporation device and / or a pressure sensor for the digester.

[0199] Advantageously, the 150 management system can be configured to apply within the evaporation device an operating pressure less than or equal to, preferably strictly less than, the value of the saturated vapor pressure of water at a temperature lower than a temperature of the medium contained within the digester.

[0200] Advantageously, the installation 100 may include a control system 160 for the quantity of condensate sent to the struvite reactor, configured to send to the struvite reactor a predetermined quantity of aqueous solution enriched in ammoniacal nitrogen, for example, just enough (equimolar ratio of ammoniacal nitrogen to phosphate ions) for struvite precipitation or, preferably, an excess quantity, particularly a slight excess (ammoniacal nitrogen / phosphate ion molar ratio of 2 to 8). This control system 160 may include a valve, pump, or other device for regulating the flow rate of the quantity of condensate sent to the struvite reactor, advantageously computer-controlled.

[0201] Lare represents an embodiment of installation 100 in which the second effluent feeding the struvite precipitation reactor is derived from the digestate.

[0202] In this embodiment, the installation thus includes a dewatering device 140 comprising an inlet 142 connected to the digestate discharge line 114 of the digester, an outlet 144 for the discharge of a liquid fraction and a discharge line 146 for the discharge of a dry matter concentrated fraction (cake), the outlet 144 being connected to the inlet line 122 of the struvite precipitation reactor 120. This dewatering device 140 may include one or more of the following mechanical devices: a screw press, a belt filter, a filter press, a centrifuge.

[0203] This embodiment is particularly well suited to the treatment of secondary sludge and mixed sludge.

[0204] Preferably, as shown, before entering the anaerobic digestion reactor 110, the sludge to be treated is thickened in a thickening device 10, thereby reducing the volume of the treated sludge. This thickening device 10, suitable for carrying out the thickening step of the process, may include one or more of the following devices: a settling tank, a clarifier, a dissolved air flotation device, a centrifuge, a dewatering table, grid, drum, or cloth, a screw or piston press, or a belt or plate filter.

[0205] It may be possible to provide, downstream of the thickening device and upstream of digester 110, an optional hydrolysis step implemented in a hydrolysis reactor 20 to facilitate the degradation of organic matter within digester 110.

[0206] Lare represents an embodiment of the installation 100 in which the second effluent feeding the struvite precipitation reactor comes from a phosphorus release reactor 30, capable of implementing the phosphorus release step of the first effluent before its entry into the digester.

[0207] The release reactor 30 is equipped with an inlet pipe 32 for wastewater effluent containing phosphorus-loaded polyphosphate-accumulating organisms, and an outlet pipe 34 for said effluent enriched with dissolved phosphorus and containing the phosphorus-depleted polyphosphate-accumulating organisms. This effluent, enriched with dissolved phosphorus, is then subjected to a separation step in a separation device 12.

[0208] This separation device 12 comprises an inlet 12a connected to the outlet line 34 of the phosphorus release reactor 30, a first outlet 12b of a first fraction concentrated in phosphorus-discharged polyphosphate-accumulating organisms, and a second outlet 12c of a second liquid fraction rich in dissolved phosphorus. The first outlet 12b is connected to the inlet line 112 of the anaerobic digester 110, and the second outlet 12c is connected to the inlet line 122 of the struvite precipitation reactor 120. The separation device 12, suitable for carrying out the separation step of the process, may include one or more of the following devices: a decanter, a clarifier, a dissolved air flotation device, a centrifuge, a dewatering table, grid, drum, or cloth, a screw or piston press, or a belt or plate filter.

[0209] Furthermore, upstream of the release reactor 30, a pre-thickening device 14 may be provided, which could include one or more of the following devices: a settling tank, a clarifier, a dissolved air flotation device, or a centrifuge. A bypass line 15 may be provided to allow only a portion of the effluent to pass through the pre-thickening device 14. This makes it possible to adjust the working sludge concentration in the release reactor 30.

[0210] The first effluent entering the anaerobic digester can also be partly supplied with primary sludge via an inlet pipe 112'.

[0211] The release reactor 30 can also be fed by an inlet pipe 31, to provide a carbon source, either by an input containing RBC, or by primary sludge, or a primary thickener overflow.

[0212] This represents a variant of the embodiment of the reactor comprising a second struvite reactor 120'. This second struvite reactor 120' is equipped with an inlet line 122' receiving a third effluent containing phosphorus and an inlet 124' for an ammonia nitrogen source. The reactor 120' is also equipped with a discharge line 126' for the formed struvite and a discharge line 127' for a liquid effluent depleted in phosphorus and nitrogen. It can also be equipped, as required, with one or more additional lines 128' for supplying magnesium (if the second effluent does not contain sufficient magnesium) and / or a basic compound (to adjust the pH if necessary). The third effluent in this case comes from the digestate. The installation thus includes a dehydration device 140 as described in reference to the, but whose outlet 144 is here connected to the inlet pipe 122' of the second struvite precipitation reactor 120'.

[0213] Lare represents an embodiment in which, as explained above, ammonium ion extraction is carried out directly on the digestate and not on a fraction taken from the anaerobic digester 110 and creating a parallel flow to the digestate. All the components enabling ammonium ion extraction are therefore located downstream of the discharge line 114 of the anaerobic digester 110. The aqueous solution enriched in ammonium ions is sent to the struvite precipitation reactor 120. The fraction depleted in ammonium ions is sent to the dewatering unit 140 to form a liquid fraction and a fraction concentrated in dry matter; the liquid fraction can be returned to the struvite precipitation reactor 120, for example, via line 122'.

[0214] Lare represents an embodiment close to that of ladans in which the liquid fraction is evacuated, for example by being sent back to a main treatment line just like the liquid at the outlet of struvite precipitation reactor 120.

Claims

Anaerobic digestion and struvite formation process, said process comprising: - an anaerobic digestion step of a first effluent in a digester (110) producing a digestate, the first effluent containing organic matter comprising nitrogen and phosphorus, and the anaerobic digestion producing ammoniacal nitrogen and dissolved CO2, - a struvite precipitation step carried out in a struvite precipitation reactor (120), during which a second effluent containing phosphorus is brought into contact with ammoniacal nitrogen, and optionally a magnesium source, to form struvite, characterized in that it comprises a step of extracting the ammoniacal nitrogen formed during the anaerobic digestion during which: - an output stream from the anaerobic digester is subjected to pressure-reducing evaporation during which gaseous CO2, ammonia-enriched water vapor, and a liquid residue,- an aqueous solution enriched in ammoniacal nitrogen is formed by condensation of water vapor enriched in ammonia, and in that at least a part of the aqueous solution enriched in ammoniacal nitrogen is sent to the struvite precipitation reactor (120) to supply at least a part of the ammoniacal nitrogen necessary for the precipitation of the struvite. Anaerobic digestion and struvite recovery process according to claim 1, characterized in that the evaporation of the ammonia nitrogen extraction step is carried out at a pressure less than or equal to the value of the saturated vapor pressure of water at a temperature lower than a temperature of the medium contained inside the digester (110). Anaerobic digestion and struvite recovery process according to claim 1 or 2, characterized in that the condensation of water vapor enriched in ammonia is carried out in a heat exchanger (134). Anaerobic digestion and struvite recovery process according to any one of claims 1 to 3, characterized in that the condensation of the ammonia-enriched water vapor is carried out at the same pressure as the evaporation. Anaerobic digestion and struvite recovery process according to any one of claims 1 to 4, characterized in that a predetermined quantity of aqueous solution enriched in ammoniacal nitrogen is sent into the struvite precipitation reactor (120). Anaerobic digestion and struvite recovery process according to any one of claims 1 to 5, characterized in that it further comprises: - the supply of a wastewater effluent comprising phosphorus-loaded polyphosphate accumulating organisms, - a step of dewatering the digestate into a liquid effluent and a dry matter concentrated fraction, and in that - the first effluent comprises said wastewater effluent, and - the second effluent comprises the liquid effluent from the digestate dewatering step. Anaerobic digestion and struvite recovery process according to claim 6, characterized in that it further comprises: - a thickening step of said wastewater effluent, - optionally a hydrolysis step of the thickened wastewater effluent, and in that the first effluent comprises said thickened wastewater effluent, optionally hydrolyzed. Anaerobic digestion and struvite recovery process according to any one of claims 1 to 5, characterized in that it further comprises: - the supply of a wastewater effluent comprising phosphorus-loaded polyphosphate accumulating organisms, - a step of releasing phosphorus from the phosphorus-loaded polyphosphate accumulating organisms contained in the wastewater effluent, this step producing an effluent enriched in dissolved phosphorus and containing the phosphorus-depleted polyphosphate accumulating organisms, - a step of separating the dissolved phosphorus-enriched effluent into a first fraction concentrated in phosphorus-depleted polyphosphate accumulating organisms and into a second liquid fraction rich in dissolved phosphorus, the first effluent comprising the first fraction and the second effluent comprising the second liquid fraction. Anaerobic digestion and struvite recovery process according to claim 8, characterized in that it comprises, prior to the phosphorus release step, a thickening step of at least a part of the wastewater effluent. Anaerobic digestion and struvite recovery process according to claim 8 or 9, characterized in that it comprises: - a second step of struvite precipitation in a second reactor, during which a third effluent containing phosphorus is brought into the presence of an ammoniacal nitrogen source, and optionally a magnesium source, to form struvite, - a step of dehydrating the digestate into a liquid effluent and a dry matter concentrated fraction, and in that - the third effluent comprises the liquid effluent from the digestate dehydration step, and - a portion of the aqueous solution enriched in ammoniacal nitrogen is sent into the second struvite precipitation reactor to supply the ammoniacal nitrogen necessary for struvite precipitation. Anaerobic digestion and struvite recovery process according to any one of claims 1 to 10, wherein the output stream from the anaerobic digester is formed by a fraction taken from the anaerobic digester (110). A process according to any one of claims 1 to 5, 8 or 9, wherein the output stream from the anaerobic digester is formed by the digestate from anaerobic digestion, the evaporation and condensation steps separating the digestate so as to form the aqueous solution enriched in ammonium ions and a fraction depleted in ammonium ions. Anaerobic digestion and struvite recovery process according to claim 12, wherein the ammonium ion-depleted digestate fraction is subjected to a digestate dehydration step, a liquid fraction from the dehydration being sent to the bioreactor. An anaerobic digestion and struvite recovery installation (100) comprising: - an anaerobic digester (110) including an inlet pipe (112) receiving a first effluent and a discharge pipe (114) for the produced digestate, - a struvite precipitation reactor (120) including an inlet pipe (122) receiving a second effluent, a first discharge pipe (126) for the struvite, and a second discharge pipe (127) for a liquid effluent depleted in phosphorus and nitrogen, characterized in that it further comprises: an ammonia nitrogen extraction system (130) comprising: - an evaporation device (132) configured to receive an output stream from the anaerobic digester (110), - a condenser (134) equipped with an inlet pipe (134a) connected to the evaporation device (132) and receiving from this, water vapor enriched in ammonia, and a condensate discharge pipe (134b),- at least one pump (136) connected to the evaporation device (132), and in that the condensate discharge line (134b) of the extraction system is connected to an inlet (124) of the struvite precipitation reactor. Anaerobic digestion and struvite recovery installation according to claim 14 comprising at least one of the following features: - the condenser is a heat exchanger, - a pressure management system within the evaporation device configured to apply inside the evaporation device a pressure less than or equal to the value of the saturated vapor pressure of water at a temperature lower than a temperature of the medium contained inside the digester, - a condensate quantity control system sent to the struvite reactor configured to send to the struvite reactor a predetermined quantity of aqueous solution enriched in ammoniacal nitrogen. Installation (100) for anaerobic digestion and struvite recovery according to claim 14 or 15, further comprising: - a dehydration device (140) equipped with an inlet (142) connected to the digestate discharge line (114), an outlet (144) for the discharge of a liquid fraction and a discharge line (146) for a dry matter concentrated fraction, the outlet (144) being connected to the inlet line (122) of the struvite precipitation reactor (120). Anaerobic digestion and struvite recovery installation (100) according to claim 16, further comprising: - a thickening device (10), and optionally a hydrolysis reactor (20), the thickening device (10) being equipped with an inlet pipe (10a) of a wastewater effluent and an outlet pipe (10b) of said thickened wastewater effluent connected to the inlet pipe (112) of the anaerobic digester, either directly or via the hydrolysis reactor (20), Anaerobic digestion and struvite recovery installation according to claim 14 or 15, further comprising: - a phosphorus release reactor (30) equipped with an inlet pipe (32) for wastewater effluent comprising phosphorus-loaded polyphosphate-accumulating organisms, and an outlet pipe (34) for said effluent enriched in dissolved phosphorus and containing phosphorus-discharged polyphosphate-accumulating organisms, - optionally a thickening device (14) mounted on the inlet pipe (32) of the phosphorus release reactor, - a separation device (12) comprising an inlet (12a) connected to the outlet pipe (34) of the phosphorus release reactor, a first outlet (12b) of a first fraction concentrated in phosphorus-discharged polyphosphate-accumulating organisms and a second outlet (12c) of a second liquid fraction rich in dissolved phosphorus,the first outlet (12b) being connected to the inlet pipe (112) of the anaerobic digester and the second outlet (12c) being connected to the inlet pipe (122) of the struvite precipitation reactor. An anaerobic digestion and struvite recovery plant according to claim 18, further comprising: - a second struvite precipitation reactor (122') including an inlet pipe (122') receiving a third effluent, a first struvite discharge pipe (126') and a second discharge pipe (127') of a phosphorus- and nitrogen-depleted liquid effluent, - a dewatering device (140) equipped with an inlet (142) connected to the digestate discharge pipe (114), an outlet (144) for discharging a liquid fraction and a discharge pipe (146) for discharging a dry matter-concentrated fraction, the outlet (144) being connected to the inlet pipe (122') of the second struvite reactor, and wherein the condensate discharge pipe (134b) of the extraction system is also connected to an inlet (124') second struvite precipitation reactor. Installation (100) according to any one of claims 14 to 19, wherein the evaporation device (132) is connected to the anaerobic digester (110) by a sampling line (132a) of the contents of the anaerobic digester (110) and by a return line (132b) of a liquid residue. Installation (100) according to any one of claims 14, 15 or 18, wherein the evaporation device (132) is connected to the anaerobic digester (110) by the digestate discharge line (114), the ammonia extraction system being configured to separate the digestate so as to form the aqueous solution enriched in ammonium ions and a fraction depleted in ammonium ions. Installation (100) according to the preceding claim, further comprising a dehydration device (30) equipped with an inlet (142) connected to the evaporation device (132) so as to receive the ammonium ion depleted fraction, an outlet line (146) of a dry matter concentrated fraction and an outlet (144) of a liquid fraction, the outlet (144) being connected to the inlet line (122) of the struvite precipitation reactor (120).

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