Anaerobic digestion and struvite recovery process and installation

Abstract

The invention relates to a method and installation for anaerobic digestion and struvite formation. The present invention comprises an anaerobic digestion step in a digester (110), a struvite formation step, and implements a step for extracting the ammoniacal nitrogen formed during the anaerobic digestion, in which: - a fraction of the digester contents is withdrawn, - the withdrawn fraction is subjected to evaporation by lowering the pressure, during which gaseous CO2, ammonia-enriched water vapor, and a liquid residue are formed, - an aqueous solution enriched in ammoniacal nitrogen is formed by condensation of the ammonia-enriched water vapor, - the liquid residue is returned to the digester. The aqueous solution enriched in ammoniacal nitrogen is sent to a struvite precipitation reactor (120) to provide at least part of the ammoniacal nitrogen necessary for struvite precipitation.Figure from the summary: Figure 1.

Description

Title of the invention: Method and installation for anaerobic digestion and recovery of struvite. Field 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 particularly, the invention relates to an anaerobic digestion process in a digester allowing the accumulation of struvite to be reduced in the digester and downstream of it and to produce usable struvite. Description of the state of the art

[0003] During the biological treatment of wastewater, based on the transformation of 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 comprises several treatment stages, generally primary treatment, secondary treatment, and tertiary treatment.

[0004] As part of the primary treatment, settling may be carried out, and the sludge produced, called primary sludge, is composed mainly of suspended solids carried by the wastewater. The primary wastewater leaving the primary treatment 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 designated 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 treatments, anaerobic digestion (or methanization) is commonly used. It allows the treatment of primary sludge, secondary sludge, or mixtures of primary and secondary sludge called 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] Mg2+ + HPO4 2 + NH4 + + 6 H2O ->MgNH4PO4.6H2O + H+(l)

[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 devices downstream of the digester. The accumulation of struvite in this equipment can then lead to its clogging and / or impair its operation, necessitating the shutdown of the installation 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] Moreover, since struvite can be used as a 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. Summary of the invention

[0013] To this end, the invention relates to a process for anaerobic digestion and struvite formation, said process comprising: - an anaerobic digestion stage 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 implemented in a struvite precipitation reactor, during which a second phosphorus-containing effluent is brought into the presence of ammoniacal nitrogen, and optionally a magnesium source, 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: - a fraction of the digester contents is taken, - the extracted fraction is subjected to evaporation by lowering the pressure, in particular by lowering it to a pressure less than or equal to the prevailing pressure in the digester, during which gaseous CO2, ammonia-enriched water vapor and a liquid residue are formed, - an aqueous solution enriched in ammoniacal nitrogen is formed by condensation of the ammonia-enriched water vapor, - the liquid residue is returned to the digester.

[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 form usable struvite using the extracted ammonia nitrogen. Furthermore, since only a fraction of the digester contents is extracted, the volume of the aqueous solution enriched with ammonia nitrogen is moderate, so that 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 sent to the struvite precipitation reactor has a basic pH, typically above 8, which can limit, or even eliminate, the addition of basic compounds to the struvite precipitation reactor that is 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 addition of chemical compounds to the struvite precipitation reactor and the associated costs. In addition, the aqueous solution enriched with ammonia nitrogen is relatively pure and contains 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 inside the digester, in particular the temperature of the liquid phase of this medium.

[0019] In other words, 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 carried out at a pressure low enough to induce the boiling of at least part of the water at this 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, will be chosen.

[0022] Advantageously, the condensation of ammonia-enriched water vapor can be carried out in a heat exchanger. This heat exchanger can be direct or indirect, preferably indirect. The use of an indirect heat exchanger avoids dilution or contamination of the aqueous solution with other chemical compounds: the aqueous solution then contains mainly 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 in ammonia can be carried out at the same pressure as evaporation.

[0024] Advantageously, a predetermined quantity of aqueous solution enriched in ammoniacal nitrogen can be fed into the struvite precipitation reactor, for example, just the amount necessary for struvite precipitation corresponding to an amount of ammoniacal nitrogen equimolar to the amount of phosphate ions, or in slight excess (ammoniacal nitrogen / phosphate ion molar ratio greater than 1, for example, from 2 to 8). This optimizes the operation of the struvite precipitation reactor and, in particular, avoids having to return the excess ammoniacal nitrogen exiting the struvite precipitation reactor to a nitrification / denitrification treatment. Furthermore, the remaining aqueous solution enriched in ammoniacal nitrogen (which is not used for struvite precipitation) can then be used to manufacture fertilizers or for other purposes.

[0025] In a first embodiment, the method may further comprise:

[0026] - the supply of a wastewater effluent comprising accumulating organisms of phosphorus-loaded polyphosphates,

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

[0028] In this embodiment:

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

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

[0031] The wastewater effluent may comprise, 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 stay 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 from digestate dewatering, which can then be combined with the aqueous solution enriched in ammoniacal nitrogen in the struvite precipitation reactor to form struvite.

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

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

[0036] - optionally a step of hydrolysis 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 method may further comprise:

[0039] - the supply of a wastewater effluent comprising accumulating organisms of phosphorus-loaded polyphosphates,

[0040] - a step of phosphorus release by accumulating organisms phosphorus-loaded polyphosphates contained in wastewater effluent, this step producing an effluent enriched in dissolved phosphorus and containing phosphorus-discharged polyphosphate-accumulating organisms,

[0041] - a step of separating the effluent enriched in dissolved phosphorus into a first a 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 also reduces the ammonia nitrogen concentration within the digester. Thus, this embodiment reduces the concentrations of two struvite constituents (phosphate ions 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 step, the process may include a thickening step of at least a portion of the wastewater effluent.

[0045] In a variant of the second embodiment, the method may further comprise:

[0046] - a second step of struvite precipitation in a second reactor, during of which a third effluent containing phosphorus is placed in the presence of a source of ammoniacal nitrogen, and optionally a source of magnesium, to form struvite,

[0047] - a step of dehydrating 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 is made up of, the liquid effluent from the digestate dewatering stage, and

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

[0051] This variant makes it possible to maximize the amount of struvite formed and the utilization of phosphorus.

[0052] The invention also relates to an anaerobic digestion and struvite recovery installation, in particular adapted to implement the process according to the invention, comprising: - an anaerobic digester comprising an inlet pipe receiving a first effluent and an outlet pipe for the digestate produced, - a struvite precipitation reactor comprising an inlet pipe receiving a second effluent, a first struvite discharge pipe and a second discharge pipe of a liquid effluent depleted in phosphorus and nitrogen.

[0053] According to the invention, the installation further comprises an ammonia nitrogen extraction system comprising: - an evaporation device connected to the anaerobic digester by a pipe for collecting the contents of the digester, including a liquid fraction of these contents, and by a return pipe for a liquid residue, - a condenser equipped with an inlet pipe connected to the evaporation device and receiving from it water vapor enriched with ammonia, and a condensate drain pipe, - at least one pump connected at least to the evaporation device, the condensate drain line of the extraction system being connected to an inlet of the struvite precipitation reactor.

[0054] The installation may further include at least one of the following features:

[0055] - the condenser is a heat exchanger, preferably a heat exchanger indirect,

[0056] - a pressure management system within the configured evaporation device to apply inside the evaporation device a pressure less than or equal to, preferably strictly less than, the saturated vapor pressure of water at a temperature lower than the temperature of the medium contained inside the digester, in particular of a liquid fraction of that medium

[0057] - 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 to precipitate struvite or in slight excess as previously described.

[0058] In a first embodiment, particularly adapted to implement the process according to the first embodiment, the installation further comprises:

[0059] - a dehydration device equipped with an inlet connected to the pipe digestate evacuation, a liquid fraction evacuation outlet and a dry matter concentrated fraction evacuation line, the outlet being connected to the inlet line of the struvite precipitation reactor.

[0060] In a variant of the first embodiment, the installation may include:

[0061] - 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.

[0062] In a second embodiment, particularly adapted to implement the process according to the second embodiment, the installation further comprises:

[0063] - a phosphorus release reactor equipped with an effluent inlet pipe wastewater containing polyphosphate-accumulating organisms loaded with phosphorus, and an outlet pipe of said effluent enriched with dissolved phosphorus and containing the polyphosphate-accumulating organisms discharged from phosphorus,

[0064] - optionally a thickening device mounted on the inlet pipe of the phosphorus release reactor,

[0065] - a separation device comprising an inlet connected to the outlet pipe from 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.

[0066] 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 comprise:

[0067] - 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,

[0068] - a dehydration device equipped with an inlet connected to the pipe digestate evacuation from the digester, a liquid fraction discharge outlet and a dry matter concentrated fraction discharge line, the outlet being connected to the inlet line of the second struvite reactor,

[0069] and wherein the condensate drain line of the extraction system is also connected to an inlet of the second struvite precipitation reactor. Detailed description of the invention

[0070] 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:

[0071] [Fig-1] is a schematic representation of an installation according to a mode of realization of the invention,

[0072] [Fig.2] is a schematic representation of an installation according to a mode of a specific implementation of the invention,

[0073] [Fig.3] is a schematic representation of an installation according to another mode of a specific implementation of the invention,

[0074] [Fig.4] is a schematic representation of an installation according to a variant of the embodiment shown [Fig.3].

[0075] In the figures, the same references designate the same elements.

[0076] Definitions / abbreviations

[0077] Volatile matter (VM), or volatile suspended matter, refers to the portion of suspended matter capable of volatilizing at 550 °C. The volatile matter content of a sample is determined by calcining the suspended matter obtained after centrifugation and drying at 105 °C at 550 °C. The volatile matter content, expressed in kg / m³ of sample, can be determined gravimetrically according to standard NF T90-105-2: 1997.

[0078] Dry matter (denoted DM) includes both suspended solids and dissolved salts. The dry matter content is expressed in g / L of sample and can be determined according to standard NF EN 12880- Nov 2000.

[0079] 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.

[0080] Anaerobic digestion stage

[0081] The process according to the invention comprises an anaerobic digestion or methanation step, 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 essentially 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.

[0082] This step is advantageously an anaerobic digestion step carried out in liquid form.

[0083] 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 carrying out this step, in particular the temperature, pH, and residence time, can advantageously be chosen to maximize biogas production.

[0084] The first effluent entering the anaerobic digestion stage is typically formed of secondary sludge or mixed sludge (mixtures of secondary and primary sludge).

[0085] This first effluent is thus generally a wastewater effluent comprising polyphosphate accumulating organisms loaded with phosphorus or rich in phosphorus resulting from the release of phosphorus by these PAOs.

[0086] The process according to the invention may thus include a step of supplying such an effluent.

[0087] This supply step may include a biological wastewater treatment step to obtain biological or secondary sludge.

[0088] This supply step may alternatively include a primary wastewater treatment step to obtain primary sludge, a biological wastewater treatment step to obtain biological or secondary sludge, and a primary and secondary sludge mixing step to form a first effluent.

[0089] The primary treatment stage generally reduces the solids and / or organic matter content of the wastewater to be treated. It is typically 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 contained in the wastewater thus settle to the bottom of the tank where they are collected. This stage produces so-called primary sludge and an effluent with a reduced solids content, which is then sent to the biological treatment stage.

[0090] Biological wastewater treatment typically includes a biological nutrient removal step, known in English as "Biological Nutrient Removal" (BNR) or "Enhanced Biological Nutrient Removal" (EBNR). Biological nutrient removal typically comprises three sub-steps, the order of which may vary, but which generally follow this order: an anaerobic step, an anoxic step, and an aerobic step. Examples of such a process 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 skilled professional knows how to implement such a biological nutrient removal stage, particularly depending on the region and the characteristics of the wastewater to be treated by the treatment plant. Biological treatment generally includes a settling and / or clarification stage.

[0091] The wastewater effluent from the biological treatment stage typically comprises a mixture of solids, liquids, and microorganisms. It may, in particular, include or consist of biological sludge or activated sludge, including excess biological sludge (EBS).

[0092] 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.

[0093] 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.

[0094] In a first embodiment, the first effluent may comprise 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.

[0095] Typically, a thickening step allows the dry matter content to increase from 1 to 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).

[0096] This thickening step can be carried out using one of the following techniques:

[0097] - Decantation / clarification: gravity is used to separate the solid particles water.

[0098] - Dissolved air flotation (DAF): air bubbles are injected into the sludge, this which allows solid particles to float to the surface and be skimmed off.

[0099] - Centrifugation: a centrifugal force is used to separate the solid particles water.

[0100] This thickening step may optionally be followed by a pretreatment step by hydrolysis, which increases the yield of the anaerobic digestion step by promoting the degradation of organic matter by microorganisms. This may be acid or basic hydrolysis, generally carried out at a temperature below 100°C, thermal hydrolysis, or biological hydrolysis.

[0101] Biological hydrolysis is for example a step of fermentation / hydrolysis under mesophilic (30 - 42°C) or thermophilic (45 - 70°C) conditions with a residence time of the order of 1 to 3 days.

[0102] Thermal hydrolysis can be a thermal hydrolysis (THP) process, which is a process consisting of maintaining sludge, typically having 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 generally cooled in a flash tank before being introduced into the digester.

[0103] 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, carried out in a dedicated reactor.

[0104] After hydrolysis, the liquid effluent is generally diluted to about 10% dry matter before being injected into the digester.

[0105] In a second embodiment, the first effluent may have a high phosphorus content due to phosphorus release by the PAOs.

[0106] The process then includes a step of supplying a wastewater effluent comprising phosphorus-loaded polyphosphate-accumulating organisms. This is secondary sludge which is then subjected to a phosphorus release step by the phosphorus-loaded PAOs.

[0107] This phosphorus release step produces an effluent enriched in dissolved phosphorus and containing the polyphosphate accumulating organisms discharged from phosphorus.

[0108] It is typically implemented in a tank, or more specifically in a release reactor. The phosphorus release step 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 step 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 in particular between 2 and 12 hours.

[0109] According to an advantageous embodiment, the phosphorus release step may include the addition of biodegradable carbon to the wastewater effluent. The addition of biodegradable carbon promotes the release of phosphorus 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 not, or of supernatant or settled water from primary sludge.

[0110] Typically, when biodegradable carbon is added during the phosphorus release step, the required residence time is reduced, particularly in the case of RBC. The required residence time is then generally between 1 h and 8 h.

[0111] 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.

[0112] This separation step is typically a thickening step, for example as described above.

[0113] The first fraction, alone or mixed with primary sludge, then forms a first effluent within the meaning of the present invention.

[0114] The second effluent defined in the present invention then comprises, or is made up of, the second liquid fraction.

[0115] Upstream of the release step, an optional pre-thickening step of at least part of the input of this release step may be provided, similar to the thickening step mentioned for the first embodiment.

[0116] Advantageously, the working concentration of sludge selected 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, the proportion of pre-thickened sludge being chosen to achieve this objective. 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.

[0117] 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 towards the second liquid fraction.

[0118] The greater the working concentration of sludge chosen in the anaerobic release reactor, the greater the concentration of soluble phosphorus in the second fraction of separated liquid will be, which has an advantage for the struvite precipitation yield.

[0119] 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 the amount of water recovered in the second phosphorus-enriched liquid fraction is.

[0120] 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).

[0121] A person skilled in the art will be able to advantageously determine a working concentration in the sludge 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.

[0122] 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.

[0123] Struvite precipitation stage

[0124] The process according to the invention also includes one or two struvite precipitation steps in which a phosphorus-rich effluent is brought into the presence of ammoniacal nitrogen, and optionally a source of magnesium, to form struvite.

[0125] Struvite is a neutral complex consisting of magnesium (Mg2+), ammonium (NH4+) or phosphate (PO43+) which precipitates under conditions of equimolarity between its constituents under conditions of pH typically from 7.5 to 8.5.

[0126] Struvite precipitation is obtained when a phosphorus-rich effluent is mixed with ammoniacal nitrogen (NH4+) and magnesium (Mg2+) according to reaction (1).

[0127] To improve the yield of the struvite precipitation step, at least one counterion source may be added. Typically, the counterion source is magnesium and / or ammonium.

[0128] An external addition of magnesium, in particular in the form of magnesium chloride and / or magnesium oxide, can be carried out during the struvite precipitation step.

[0129] 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.

[0130] Struvite precipitation 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 a fertilizer.

[0131] In one embodiment, the phosphate is supplied by a liquid fraction rich in dissolved phosphorus from the digestate, anaerobic digestion having been carried out on a wastewater effluent containing phosphorus-laden PAO. The digestion step is then followed by a dewatering step of the digestate allowing the separation of a liquid effluent and a concentrated dry matter fraction (or "cake"). This liquid effluent is rich in phosphates which have been released by the PAO under the anaerobic conditions of digestion and which are found dissolved in the liquid fraction.

[0132] The digestate dewatering step also aims to maximize the reduction of the water content of the sludge to obtain a more stable and easier-to-manage final product.

[0133] Typically, during the dehydration stage, the dry matter content of the digestate changes from 30-60 g / L to a content of 150-300 g / L for the cake.

[0134] By way of example, the dehydration step may implement one or more of the following mechanical processes:

[0135] - Screw press / Belt filter / Filter press: the sludge is pressed through a filter to extract the water.

[0136] - Centrifugation.

[0137] In another embodiment, the phosphate is supplied by the effluent exiting the phosphorus release stage, more specifically by the second liquid fraction from the separation of the effluent exiting the phosphorus release stage.

[0138] Two struvite precipitation stages can be provided, 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.

[0139] 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.

[0140] 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.

[0141] Furthermore, regardless of the embodiment, depending on the magnesium ion content of the effluents supplying the phosphate, magnesium may be added, for example in the form of Mg2+ ions in solution. Preferably, magnesium will be added in such a way that the total quantity of magnesium present corresponds to a ratio of 1.0 to 1.5 moles to the number of moles of phosphate.

[0142] Alternatively or in combination, a basic compound may be added to reach the pH range favouring struvite formation.

[0143] Step of extraction of ammonia nitrogen formed during anaerobic digestion

[0144] During this step:

[0145] - a fraction of the digester contents is taken, in particular a liquid fraction of this content,

[0146] - the extracted fraction is subjected to evaporation by lowering the pressure to during which gaseous CO2, water vapor enriched with ammonia, and a liquid residue are formed,

[0147] - an aqueous solution enriched in ammoniacal nitrogen is formed by condensation of the water vapor enriched with ammonia,

[0148] - the liquid residue is returned to the digester.

[0149] The aqueous solution enriched in ammoniacal nitrogen can then be used in the struvite precipitation reactor to provide the ammoniacal nitrogen necessary for the precipitation of struvite.

[0150] The fraction of the digester contents taken 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.

[0151] During evaporation by lowering the pressure, the microorganisms (and possibly the enzymes) remain in solution and form the liquid residue which is then returned to the digester. The volatile compounds, namely CO2, ammonia, and also dihydrogen and methane, are evaporated.

[0152] When the pressure is lowered, particularly to a pressure lower than that prevailing inside the digester, CO2 degassing occurs, which increases the pH of the remaining extracted fraction and thus shifts the solubility equilibria. Furthermore, water vapor is also formed. This water vapor acts as a carrier gas and carries with it the volatile products, namely ammonia, and possibly methane and / or dihydrogen. This water vapor is thus enriched in ammonia (NH3).

[0153] 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.

[0154] Typically, the temperature of the water (in particular 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 contents of the digester, in particular the liquid phase of these contents.

[0155] In general, the evaporation step is preferably carried out under temperature and pressure conditions chosen so as to avoid degradation Microorganisms (and any enzymes) are produced during evaporation. A skilled professional knows how to choose a temperature and / or pressure that does not damage and / or degrade the microorganisms and / or enzymes according to their nature.

[0156] The applied pressure is typically lower than the pressure inside the digester.

[0157] 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.

[0158] Typically, the pressure of the evaporation stage is 33 to 350 millibars absolute pressure.

[0159] The evaporation step 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.

[0160] The ammonia-enriched water vapor is then condensed, allowing the separation of an aqueous solution enriched in ammonia nitrogen and the non-condensable gases (CO2, CH4, H2). The non-condensable gases can be returned to the digestion step.

[0161] This condensation is typically carried out at the same pressure as the evaporation step, the condensation resulting from a lowering of the temperature.

[0162] Condensation can be implemented in a direct heat exchanger, by contact with a cold liquid, for example an acid, or in an indirect heat exchanger through which a cold fluid, generally water, circulates. It is preferable to use an indirect heat exchanger in order to limit chemical consumption and avoid contamination of the aqueous solution enriched with ammonia nitrogen.

[0163] 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.

[0164] The aqueous solution enriched with ammoniacal 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.

[0165] 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 the precipitation of the struvite or, preferably, to an excess amount, more preferably to a slight excess amount (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.

[0166] Installation

[0167] Figure 1 schematically represents an installation 100 implementing the process according to the invention. This installation 100 comprises 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.

[0168] 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.

[0169] 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.

[0170] The installation 100 also includes a struvite precipitation reactor 120 suitable for carrying out 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®.

[0171] The reactor 120 is equipped with an inlet line 122 receiving a second 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 struvite formed and a discharge line 127 for a liquid effluent depleted in phosphorus and nitrogen. It may also be equipped, as required, with one or more additional magnesium feed lines 128 (if the second effluent does not contain enough magnesium) and / or basic compound (to adjust the pH if necessary).

[0172] Installation 100 also includes an ammonia nitrogen extraction system 130 suitable for implementing the step of extracting the ammonia nitrogen formed during anaerobic digestion.

[0173] 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 by a pipe 132b for returning the liquid residue to the anaerobic digester. Pumps 133a and 133b provide the extraction of the digester contents and the return of 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.

[0174] 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.

[0175] 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.

[0176] 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.

[0177] According to the invention, the condensate drain line 134b of the extraction system is connected to the inlet 124 of the struvite precipitation reactor.

[0178] The extraction system 130 operates as follows:

[0179] - a fraction of the contents is first drawn via pump 133a and line 132a from digester 110, including a liquid fraction of its contents,

[0180] - the extracted fraction 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.

[0181] - an aqueous solution enriched in ammoniacal nitrogen is formed by condensation of the water vapor enriched with ammonia in condenser 134, this condensed fraction being discharged via pipe 134b,

[0182] - the liquid residue is returned to the digester by means of pipe 132b and pump 133b.

[0183] At least a portion of the aqueous solution enriched with ammoniacal nitrogen is sent to the struvite precipitation reactor 120 via line 134b and inlet 124 to supply the ammoniacal nitrogen required for struvite precipitation. The remainder of the aqueous solution enriched with ammoniacal nitrogen can be discharged via line 135 for further use.

[0184] 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.

[0185] This management system may include a valve or other pressure-reducing system, advantageously computer-controlled, and optionally a pressure sensor for the evaporation device and / or a pressure sensor for the digester.

[0186] Advantageously, the management system 150 can be configured to apply inside 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 inside the digester.

[0187] 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 the amount necessary (equimolar ratio of ammoniacal nitrogen to phosphate ions) for the precipitation of struvite or preferably an excess amount, particularly a slight excess (ammoniacal nitrogen / phosphate ion molar ratio of 2 to 8). This control system 160 may include a valve, a pump, or other device for regulating the flow rate of the quantity of condensate sent to the struvite reactor, advantageously computer-controlled.

[0188] Fig. 2 represents an embodiment of the installation 100 in which the second effluent feeding the struvite precipitation reactor is derived from the digestate.

[0189] In this embodiment, the installation thus comprises a dewatering device 140 including 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 dehydration device 140 may include one or more of the following mechanical devices: a screw press, a belt filter, a filter press, a centrifuge.

[0190] This embodiment is particularly well suited to the treatment of secondary sludge and mixed sludge.

[0191] 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 or grid or drum or cloth, a screw or piston press, a belt or plate filter.

[0192] It may be possible to provide, downstream of the thickening device and upstream of the digester 110, an optional hydrolysis step implemented in a hydrolysis reactor 20 to facilitate the degradation of organic matter within the digester 110.

[0193] Fig. 3 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.

[0194] The release reactor 30 is equipped with an inlet pipe 32 for wastewater effluent comprising 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.

[0195] 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.

[0196] Upstream of the release reactor 30, a pre-thickening device 14 may be provided, comprising 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 circulate only a portion of the effluent through the pre-thickening device 14. This allows the working sludge concentration in the release reactor 30 to be adjusted.

[0197] The first effluent entering the anaerobic digester can also be supplied partly with primary sludge by an inlet pipe 112'.

[0198] 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.

[0199] Figure 4 represents a variant of the embodiment of Figure 3 comprising A second struvite reactor 120'. This second struvite reactor 120' is equipped with an inlet pipe 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 pipe 126' for the formed struvite and a discharge pipe 127' for a liquid effluent depleted in phosphorus and nitrogen. It can also be equipped, as needed, 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). The third effluent in this case comes from the digestate. The installation thus includes a dehydration device 140 as described with reference to [Fig.2], but whose outlet 144 is here connected to the inlet pipe 122' of the second struvite precipitation reactor 120'.

Claims

Demands

1. A process for anaerobic digestion and struvite formation, 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 source of magnesium, to form struvite, characterized in that it comprises a step for extracting the ammoniacal nitrogen formed during the anaerobic digestion during which: - a fraction of the digester contents is taken, - the taken fraction is subjected to evaporation by lowering the pressure during which gaseous CO2 is formed,of water vapor enriched with ammonia, and a liquid residue, - an aqueous solution enriched with ammoniacal nitrogen is formed by condensation of the water vapor enriched with ammonia, - the liquid residue is returned to the digester, and in that at least part of the aqueous solution enriched with ammoniacal nitrogen is sent to the struvite precipitation reactor (120) to supply at least part of the ammoniacal nitrogen necessary for the precipitation of the struvite.

2. 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).

3. Anaerobic digestion and struvite recovery process according to claim 1 or 2, characterized in that the condensation of ammonia-enriched water vapor is carried out in a heat exchanger (134).

4. 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.

5. 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).

6. 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.

7. 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, and optionally hydrolyzed.

8. A process for anaerobic digestion and struvite recovery according to any one of claims 1 to 5, characterized in that it further comprises: - supplying 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-discharged polyphosphate-accumulating organisms, - a step of separating the effluent enriched in dissolved phosphorus into a first fraction concentrated in struvite-accumulating organisms polyphosphate discharged of phosphorus and in a second liquid fraction rich in dissolved phosphorus, the first effluent comprising the first fraction and the second effluent comprising the second liquid fraction.

9. 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 portion of the wastewater effluent.

10. Anaerobic digestion and struvite recovery process according to claim 8 or 9, characterized in that it comprises: - a second struvite precipitation step in a second reactor, during which a third phosphorus-containing effluent is brought into the presence of an ammoniacal nitrogen source, and optionally a magnesium source, to form struvite, - a digestate dewatering step into a liquid effluent and a dry matter concentrated fraction, and in that - the third effluent comprises the liquid effluent from the digestate dewatering 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.

11. 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) including: - an evaporation device (132) connected to the anaerobic digester (110) by a pipe (132a) for sampling the contents of the digester and by a pipe (132b) for returning a liquid residue, - a condenser (134) equipped with an inlet pipe (134a) connected to the evaporation device (132) and receiving from it steam 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 pipe (134b) of the extraction system is connected to an inlet (124) of the struvite precipitation reactor.

12. An anaerobic digestion and struvite recovery installation according to claim 11 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.

13. An anaerobic digestion and struvite recovery installation (100) according to claim 11 or 12, further comprising: - a dewatering device (140) equipped with an inlet (142) connected to the digestate discharge line (114), an outlet (144) for 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).

14. An anaerobic digestion and struvite recovery installation (100) according to claim 13, further comprising: - a thickening device (10), and optionally a hydrolysis reactor (20), the thickening device (10) being equipped with a wastewater effluent inlet (10a) and a 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),

15. An anaerobic digestion and struvite recovery plant according to claim 11 or 12, further comprising: - a phosphorus release reactor (30) equipped with an inlet pipe (32) for a wastewater effluent comprising phosphorus-loaded polyphosphate accumulating organisms, and an outlet pipe (34) for said effluent enriched with dissolved phosphorus and containing the 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 polyphosphate storage organisms discharged from phosphorus 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.

16. An anaerobic digestion and struvite recovery plant according to claim 15, further comprising: - a second struvite precipitation reactor (122') comprising an inlet pipe (122') receiving a third effluent, a first struvite discharge pipe (126') and a second discharge pipe (127') of a liquid effluent depleted in phosphorus and nitrogen, - a dewatering device (140) equipped with an inlet (142) connected to the digestate discharge line (114), an outlet (144) for discharging a liquid fraction and a discharge line (146) for discharging a dry matter concentrated fraction, the outlet (144) being connected to the inlet line (122') of the second struvite reactor, and in which the condensate discharge line (134b) of the extraction system is also connected to an inlet (124') of the second struvite precipitation reactor.

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