Method and facility for anaerobic digestion of an effluent and degradation of ammonium

Abstract

The invention relates to a method for anaerobic digestion of an effluent and reduction of its nitrogen load, the method comprising: - a step (102) of anaerobic digestion of the effluent in a digester (4), producing a digestate and ammonia nitrogen; - a step (106) of degrading ammonium ions originating from the digester in a bioreactor (24) for degrading ammonium ions, characterised in that the method comprises a step (104) of extracting the ammonia nitrogen formed during the anaerobic digestion step (102), during which a stream exiting the anaerobic digester is subjected to evaporation and condensation in order to produce an aqueous solution enriched in ammonium ions, and in that at least part of the aqueous solution enriched in ammonium ions is sent to the bioreactor (24) in order to supply at least some of the ammonium ions to be degraded.

Description

Process and installation for anaerobic digestion and degradation of ammonium

[0001] The invention relates to the field of effluent treatment by anaerobic digestion, and in particular the coupling of a digester with an assembly enabling the reduction of the quantity of ammonium ions present in a digester.

[0002] For the purposes of this invention, effluent refers to urban wastewater, industrial wastewater, agricultural wastewater, leachate or hydrolyzed sludge, and more generally any effluent containing organic matter. We will subsequently describe the scenario of wastewater treatment.

[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] Anaerobic digestion involves a cascade of biochemical reactions that convert the organic matter in the digester into biogas, primarily a mixture of carbon dioxide and methane. The remaining material is called digestate.

[0008] The anaerobic digestion of organic matter releases nitrogen from the substrates in the form of ammonia. This nitrogen is present in the digestate exiting the digester and is also found in the digestate / filtrates (corresponding to a liquid fraction of the digestate) following the dewatering stage downstream of the anaerobic digestion stage. This creates a flow laden with ammoniacal nitrogen (in other words, ammonium ions), which is destined to return to the main water treatment system (also known as "return to the head"), for example, upstream of the primary treatment. These returns introduce an ammonia overload and can be problematic for the water treatment system (insufficient volume for treatment, insufficient aeration for ammonium oxidation, treatment compliance). In fact, these returns can represent up to 15% of the nitrogen flow to be treated.This figure is even more important when external sludge is brought back to the level of a treatment plant (a case called "SludgeTreatmentCenter").

[0009] With increasingly restrictive European legislation and international standards on the quality of discharges into natural environments, particularly nitrogen, this issue is hindering the adoption of advanced digestion technologies that are necessary and relevant to meet the challenges of maximizing energy production via methane produced by anaerobic digestion and minimizing the volume of sludge produced to be disposed of.

[0010] In order to reduce the nitrogen load at the digester outlet, a dedicated treatment (conventionally called "sidestream") can be implemented to treat the nitrogen at the outlet of the anaerobic digester.

[0011] This treatment can be biological, involving conventional nitrification and denitrification. In the case of nitrogen, a distinction is essentially made between nitrification and denitrification treatments. During nitrification, the ammonium exiting the anaerobic digester is oxidized under aerated conditions by autotrophic bacteria in two stages: first, into nitrites by ammonium-oxidizing bacteria (AOB), then into nitrates by nitrite-oxidizing bacteria (NOB). During denitrification, the nitrates produced are ultimately reduced to nitrogen gas under anoxic conditions by a consortium of heterotrophic bacteria, requiring readily biodegradable carbon.

[0012] This treatment can also be carried out by partial nitrification and deammonification reactions (or Anammox reaction). In this case, it is necessary to first oxidize a portion of the ammonium exiting the digester to nitrites (by nitritation) using the following reaction:

[0013] NH4 + + 1.5 O2 → NO2 + H2O + 2 H +

[0014] During this aeration phase, the objective is to regulate aeration so that only this first oxidation reaction can take place. As a result, approximately half of the ammonia is converted into nitrite with a very low concentration of dissolved oxygen by ammonium oxidizing bacteria (AOB).

[0015] The Annamox reaction is then carried out. This reaction is triggered by the remaining ammonium and the nitrites produced to form nitrogen gas (N2) according to the following reaction:

[0016] NH4 + NO2 → N2 + 2 H2O

[0017] This reaction occurs in the absence of oxygen during the anoxic phase, carried out by bacteria performing anaerobic ammonium oxidation (Anammox). It produces up to 89% nitrogen gas (N2). The critical aspect of this reaction is maintaining a stable nitrite / ammonium ratio during the first treatment stage. The advantages over conventional biological nitrogen treatment are associated with savings on aeration energy requirements, which are reduced by 60%, and the elimination of the need for readily biodegradable carbon for denitrification.

[0018] Even though the biological process of partial nitrification and deammonification allows for a significant reduction in energy requirements (less aeration needed) and does not require carbon input, unlike conventional denitrification, several inherent limitations of this process exist, such as: Slow growth of Anammox bacteria: The biomass yield of the Anammox process is very low, which saves on sludge treatment costs. However, this implies a very long start-up time with a gradual ramp-up; typically between 6 and 9 months. This long start-up therefore impacts schedules and associated costs. Sensitivity to environmental conditions: Anammox bacteria are sensitive to variations in pH, temperature, dissolved oxygen, and ammonium and nitrite concentrations. Significant fluctuations can disrupt the process and reduce treatment efficiency.This therefore implies significant monitoring and precise temperature control, typically above 25°C, resulting in substantial operating costs. Inhibition by certain compounds: Some compounds present in digestate centrates / filtrates, such as sulfides and heavy metals, and polymer residues used in the dewatering of digested sludge, can inhibit Anammox bacteria. This is especially true when thermal treatments are implemented, such as thermal hydrolysis, which generates compounds toxic to Anammox bacteria. Therefore, additional pretreatments or prior dilution are systematically implemented to eliminate / reduce the impact of these compounds before applying the Anammox process. This, in turn, creates issues regarding the treatment system's footprint, heating requirements, and consequently, installation costs.Nitrous oxide (N₂O) release: N₂O is a potent greenhouse gas, with a global warming potential (GWP) 298 times greater than that of CO₂ over 100 years. When ammonium is not completely converted to nitrogen gas, some can be transformed into N₂O through undesirable denitrification reactions. Typically, variations in pH, temperature, dissolved oxygen, and ammonium concentration can promote N₂O production.

[0019] The invention aims in particular to provide a process for treating ammoniacal nitrogen in order to eliminate the aforementioned problems.

[0020] To this end, the invention relates to a process for the anaerobic digestion and reduction of the nitrogen load of an effluent, said process comprising: an anaerobic digestion step of the effluent in an anaerobic digester producing a digestate, the effluent containing organic matter including nitrogen, and the anaerobic digestion producing ammoniacal nitrogen and dissolved CO2; a degradation step of ammonium ions from the digester in an ammonium ion degradation bioreactor, during which ammonium ions are partially oxidized to form nitrites, then brought into contact with nitrites to form nitrogen gas; the process comprising a step of extracting the ammoniacal nitrogen formed during the anaerobic digestion during which: an output stream from the anaerobic digester is subjected to evaporation by lowering the pressure during which gaseous CO2, water vapor enriched in ammonia, and a liquid residue are formed.An aqueous solution enriched in ammonium ions is formed by condensation of water vapor enriched in ammonia; at least a portion of this aqueous solution is sent to the bioreactor to provide at least some of the ammonium ions for degradation.

[0021] Thus, ammonia nitrogen is extracted by an evaporation / condensation process to produce a solution enriched in ammonium ions, this solution being sent to an ammonium ion degradation bioreactor, which reduces the amount of ammonia nitrogen returning to the main water treatment system, for example upstream of primary treatment, thus avoiding an ammonia overload.

[0022] Furthermore, because the ammonium-enriched solution is small compared to the digestate, the volume of the ammonium ion degradation bioreactor can be drastically reduced compared to a prior art bioreactor receiving all the filtrates / centrates from anaerobic digestion and dehydration, thus controlling manufacturing and operating costs. The ammonium-enriched solution represents approximately 0.1 to 15% of the total hydraulic flow undergoing digestion. The size of the bioreactor can theoretically be reduced by up to 99% compared to a prior art bioreactor.

[0023] Furthermore, and because the solution enriched in ammonium ions is of a small volume compared to the digestate, at a pH between 8 and 9 and at a temperature above 30°C, it is possible to maximize the kinetics of partial nitrification by facilitating the establishment of favorable operating conditions both on the quantity of ammonium ions supplied to the bioreactor and on the parameters inside the bioreactor (temperature, pH), and thus avoid / minimize the production of nitrous oxide.

[0024] A faster and more controlled installation of the first phase of partial nitrification (i.e., nitrate bypass) than with prior art also allows for rapid bioreactor startup. Furthermore, given the significantly reduced volumes of the ammonium-rich solution, inoculation becomes feasible for startup within a few days or weeks (compared to 6 to 9 months with prior art).

[0025] Finally, the evaporation / concentration method allows for precise control of the ammonium ion-rich solution. It yields a solution free of polymer residues, suspended solids, and COD (limiting the risk of competition with heterotrophic bacteria), with a controlled and stable ammonia concentration. This maximizes ammonium ion degradation performance and reduces the risk of biomass leaching. In other words, the aforementioned prior dilution to eliminate / reduce the impact of these compounds before applying the Anammox process is unnecessary. This also allows for maintaining the parameters of the aqueous ammonium ion-rich solution (temperature, pH, volume), thereby reducing the system's footprint, heating requirements, and consequently, installation costs.

[0026] The process according to the invention may further comprise at least one of the following features: the evaporation of the ammonia nitrogen extraction step is carried out at a pressure less than or equal to the saturated vapor pressure of water at a temperature lower than the temperature of the medium contained within the anaerobic digester; the condensation of the ammonia-enriched water vapor is carried out in a heat exchanger; the condensation of the ammonia-enriched water vapor is carried out at the same pressure as the evaporation; a predetermined quantity of aqueous solution enriched in ammonium ions is sent into the bioreactor; the output stream from the anaerobic digester consists of a first liquid fraction taken from the anaerobic digester; a volume of the first fraction is determined as a function of a predetermined concentration of ammonia nitrogen in the digestate.The process further comprises a digestate dewatering step, with a liquid fraction from the dewatering being sent to the bioreactor; the ammonia nitrogen extraction step, which produces a liquid residue returned to the anaerobic digester; the anaerobic digester outlet stream consists of the digestate from anaerobic digestion; the evaporation and condensation steps separate the digestate to form an aqueous solution enriched in ammonium ions and an ammonium ion-depleted fraction; the ammonium ion-depleted digestate fraction is subjected to a digestate dewatering step, with a liquid fraction from the dewatering being sent to the bioreactor; and the effluent is selected from urban wastewater, industrial wastewater, agricultural wastewater, leachate, or hydrolyzed sludge.

[0027] The invention also relates to an installation for the anaerobic digestion and reduction of the nitrogen load of an effluent, comprising: an anaerobic digester including an inlet pipe receiving an effluent and a discharge pipe for the digestate produced, a bioreactor for the degradation of ammonium ions from the anaerobic digester including an inlet receiving an aqueous solution enriched in ammonium ions and a discharge pipe for a liquid effluent depleted in ammonium ions, further comprising 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 water vapor enriched in ammonia, and a discharge pipe for an aqueous solution rich in ammonium ions,and in that the discharge pipe for an aqueous solution rich in ammonium ions from the extraction system is connected to the inlet of the ammonium ion degradation bioreactor.

[0028] The installation according to the invention may further comprise at least one of the following features: the installation comprises at least one of the following features: the condenser is a heat exchanger, a pressure management system within the evaporation device configured to apply a pressure inside the evaporation device less than or equal to the saturated vapor pressure of water at a temperature lower than the temperature of the medium contained inside the anaerobic digester, a system for controlling the quantity of aqueous solution enriched with ammonium ions sent to the bioreactor configured to send a predetermined quantity of aqueous solution enriched with ammonium ions to the bioreactor, the bioreactor is closed so as to form a gas-tight internal volume, the bioreactor comprises an enclosure comprising a heat-insulating material, for example, rock wool or glass wool.The evaporation device is connected to the anaerobic digester by a pipe for collecting the contents of the anaerobic digester. The installation further includes a dewatering unit equipped with an inlet connected to the digestate discharge pipe, an outlet pipe for a liquid fraction, and a discharge pipe for a dry matter concentrated fraction. The liquid fraction outlet pipe includes a sub-pipe connected to the inlet of the bioreactor. The ammonia nitrogen extraction system further includes a return pipe for a liquid residue to the anaerobic digester. The evaporation device is connected to the anaerobic digester by the discharge pipe for the produced digestate. The ammonia extraction system is configured to separate the digestate to form an aqueous solution enriched in ammonium ions and a fraction depleted in ammonium ions.The installation further comprises a dehydration unit equipped with an inlet connected to the evaporation device to receive the ammonium ion-depleted fraction, an outlet line for a liquid fraction, and a discharge for a dry matter-concentrated fraction; the liquid fraction outlet line includes a sub-pipe connected to the bioreactor inlet. Brief description of the figures

[0029] The invention will be better understood upon reading the following description, given solely by way of example and made with reference to the accompanying drawings in which:

[0030] is a schematic representation of an installation according to a first embodiment of the invention,

[0031] is a schematic representation of an installation according to a second embodiment of the invention,

[0032] is a schematic representation of an installation according to a third embodiment of the invention,

[0033] is a schematic representation of an installation according to a fourth embodiment of the invention, and

[0034] is a flowchart of an anaerobic digestion and nitrogen load reduction process according to the invention. Detailed description of the invention

[0035] Reference is now made to Figure 2, illustrating an anaerobic digestion and nitrogen reduction installation for an effluent, and to Figure 100, illustrating a flowchart of an anaerobic digestion and nitrogen reduction process for an effluent according to the invention. In what follows, certain aspects of the invention will be described using wastewater treatment as an example. Generally, the effluent can be chosen from urban wastewater, industrial wastewater, agricultural wastewater, leachate, or hydrolyzed sludge. Therefore, all the elements described below relating to wastewater treatment are applicable to the treatment of these types of effluents.

[0036] Anaerobic digestion stage

[0037] 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 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 step can be the same for all embodiments of the invention.

[0038] This step is advantageously an anaerobic digestion step implemented in liquid form.

[0039] 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.

[0040] The first effluent entering the anaerobic digestion stage is typically made up of primary sludge, secondary sludge or mixed sludge (mixtures of secondary and primary sludge).

[0041] The process according to the invention may thus include a step of supplying such an effluent. An anaerobic digester 4 is continuously fed with effluent.

[0042] This supply step may include a biological wastewater treatment step to obtain biological or secondary sludge.

[0043] 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 the effluent.

[0044] 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.

[0045] The biological treatment of wastewater typically includes a biological nutrient removal stage, known in English 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.

[0046] 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).

[0047] 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.

[0048] Step of extraction of ammonia nitrogen formed during anaerobic digestion

[0049] In what follows, the terms "ammonium ions" and "ammoniacal nitrogen" are used interchangeably, as these two terms are equivalent.

[0050] The anaerobic digestion step 102 of the effluent in an anaerobic digester 4 therefore produces a digestate, the effluent containing organic matter and nitrogen, and the anaerobic digestion producing ammoniacal nitrogen and dissolved carbon dioxide.

[0051] As explained above, this ammonia nitrogen must undergo treatment, for example degradation, to meet regulatory standards imposing maximum quantities of nitrogen discharges in effluent treatment systems such as wastewater.

[0052] In the first and second embodiments of the invention, this ammoniacal nitrogen is initially recovered and concentrated in a solution rich in ammonium ions in an ammoniacal nitrogen extraction system as follows: a first fraction of the contents of the anaerobic digester 4 is taken (the output from the anaerobic digester subjected to the evaporation and condensation steps is therefore the first fraction taken from the anaerobic digester 4), in particular a liquid fraction of this content; the fraction taken is subjected to evaporation by lowering the pressure during which gaseous CO2, water vapor enriched in ammonia, and a liquid residue are formed; an aqueous solution enriched in ammonium ions is formed by condensation of the water vapor enriched in ammonia.at least a portion of the aqueous solution enriched in ammonium ions is sent to bioreactor 24 to provide at least some ammonium ions for degradation.

[0053] The fraction of the contents of anaerobic digester 4 that is sampled 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.

[0054] During evaporation by lowering the pressure, microorganisms (and possibly enzymes) remain in solution and form a liquid residue that can then be returned to the digester. Volatile compounds, namely CO2, ammonia, and also dihydrogen and methane, are evaporated.

[0055] 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).

[0056] 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.

[0057] 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.

[0058] 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.

[0059] The applied pressure is typically lower than the pressure inside the digester.

[0060] 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.

[0061] Typically, the pressure in the evaporation stage is 33 to 350 millibars absolute pressure.

[0062] 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.

[0063] The ammonia-enriched steam is then condensed, separating an aqueous solution enriched in ammonia nitrogen from the non-condensable gases (CO2, CH4, H2). These non-condensable gases can then be returned to the digestion stage or fed into the biogas network downstream of digestion.

[0064] This condensation typically occurs at the same pressure as the evaporation stage, with condensation resulting from a drop in temperature.

[0065] 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 ammonium ions.

[0066] This aqueous solution enriched in ammonium ions has the advantage of having a basic pH, typically greater than 8. This configuration is a favorable factor for the rapid implementation of the next step of ammonium ion degradation and makes it possible not to add an additional basic compound.

[0067] Advantageously, the quantity of aqueous solution enriched with ammonium ions that is sent to bioreactor 24 is controlled and corresponds to an amount that allows for optimal degradation of the ammonium ions. This quantity can be determined beforehand by tests and / or modeling based on the ammonium ion concentration of the aqueous solution enriched with ammonium ions.

[0068] Advantageously, the volume of the first fraction taken from the anaerobic digester 4 is determined based on a predetermined concentration of ammonium ions in the digestate. In other words, a maximum acceptable quantity of ammonium ions in the digestate is determined, the latter being determined based on the maximum acceptable quantity of ammonium ions returning to the main water treatment system. Thus, to reduce the quantity of ammonium ions returning to the main water treatment system (primarily through the return of a centrate / filtrate representing the liquid fraction resulting from digestate dewatering), the quantity of ammonium ions in the digestate is controlled by adjusting the volume of the fraction taken to obtain the ammonium-enriched solution.

[0069] Figures 3 and 4 illustrate two different embodiments from those of Figures 1 and 2. For these two embodiments, we will only describe the differences with the first two embodiments.

[0070] In both of these embodiments, 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.

[0071] In other words, and as can be seen in figures 3 and 4, a sampling pump assembly 16 / ammonia extraction system / vacuum pump 22 can be directly connected to the digester 8 discharge line in order to receive the digestate directly instead of a fraction taken as is the case for the first two embodiments.

[0072] 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 4. Indeed, ammonium ions play a buffering role, maintaining an optimal pH in the anaerobic digester 4, 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 4. In the embodiments of Figures 3 and 4, 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 (arrow 29), for example, between 50 and 100 mg / L.This eliminates the need for a return line to digester 14 and its associated return pump 18. A sampling pump 16 (to supply the extraction system) and a vacuum pump 22 are retained, but their flow rate is equal to the organic matter inlet flow rate of the anaerobic digester 4, unlike the first two embodiments where the flow rates of the sampling and return pumps 16 are between 3 and 10 times greater than the organic matter inlet flow rate of the anaerobic digester 4. This results in energy savings. It also allows for the capture of dissolved gases by passing the digestate through the evaporation device 9 and the condenser 10. 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.

[0073] Stage of degradation of ammonium ions from the digester

[0074] This step is carried out in an ammonium ion degradation bioreactor and is identical for all embodiments. Typically, the bioreactor can be a sequential bioreactor (SBR) implementing a treatment cycle in which the aerated and non-aerated reaction phases have a limited duration and can be regulated based at least on the conductivity measured in the biological reactor, and preferably also on the dissolved oxygen concentration measured in the reactor. This type of reactor typically includes nitrifying bacteria, with operating conditions designed to favor the action of nitritating bacteria and minimize the action of nitrate-producing bacteria.A volume to be treated, here at least a fraction of the ammonium-enriched solution, is introduced into the bioreactor and undergoes an aeration phase to induce partial nitrification, transforming some of the ammonium ions into nitrites. This is followed by an anoxic phase during which aeration is stopped and a carbon source is introduced into the reactor to transform the nitrites and ammonium ions into nitrogen. The two reaction equations were mentioned above. In other words, the ammonium ions are first partially oxidized to form nitrites, and then reacted with nitrites to form nitrogen gas.

[0075] The degradation of ammonium ions can be carried out in a mixed culture reactor, with bacteria on a support or with bacteria forming compact granules.

[0076] During the first stage of partial nitrification, an oxygen supply (for example via an aerator) is implemented in order to allow the oxidation reaction of ammonium ions into nitrite by the action of ammonium oxidizing bacteria (AOB).

[0077] Following this oxidation, nitrites accumulate by maintaining operating conditions such as low dissolved oxygen concentrations, a predetermined pH value (recalling here that the pH of the solution enriched with ammonium ions is between 8 and 9, which is ideal), a high temperature (recalling here that the temperature of the solution enriched with ammonium ions is above 30°C, which is ideal) and high ammonium concentrations (given that the solution entering the bioreactor is enriched with ammonium ions and that partial nitrification oxidizes between 55 and 60% of the ammonium ions, the quantity of ammonium ions remains high after the partial nitrification step).

[0078] This is one of the advantages of the invention. The treatment process according to the invention allows a pollutant-free solution (including inhibitors of partial nitrification), with a small volume, concentrated in ammonium ions, a temperature typically above 30°C, and a pH between 8 and 9, to be sent to the bioreactor 24. Compared to the prior art, any need for heating or dilution (which could also lead to cooling) is avoided by injecting into the bioreactor 24 a solution whose parameters are conducive to the rapid oxidation of ammonium ions to nitrite (thus bypassing nitrates). Furthermore, nitrous oxide production is reduced because this production competes with partial nitrification. Promoting partial nitrification leads to a reduction in nitrous oxide production.Finally, these conditions allow for a faster ramp-up of the bioreactor 24 than according to the prior art.

[0079] During the second stage leading to the production of nitrogen gas, aeration is stopped and Anammox bacteria degrade ammonium ions, reacting with nitrites to form nitrogen gas, nitrates and water.

[0080] Advantageously, a predetermined quantity of aqueous solution enriched with ammonium ions is introduced into bioreactor 24. This quantity allows for the treatment of a satisfactory amount of ammonium ions while ensuring, within the bioreactor, operating conditions that allow for good degradation of the ammonium ions, for example, by avoiding inhibition of Anammox bacteria that would be caused by a high nitrite concentration, while also allowing for the establishment of stable conditions for maintaining the (N-NO2:N-NH4) ratio. It is possible to introduce the ammonium-enriched solution in successive volume fractions into bioreactor 24, each volume fraction being treated during a subcycle. The successive introduction of volume fractions allows for bacterial selection pressure favorable to the development and activity of nitritogenic bacteria through surges in ammonia load.

[0081] It is important to supply the bioreactor 24 with readily available carbonaceous material to enable the non-aerated, anoxic stage during which the nitrites produced and ammonium are converted into nitrogen gas. In the context of this invention, the ammonium-enriched solution includes dissolved CO2 trapped during the condensation phase, which serves as a carbon source.

[0082] Advantageously, it is possible to send a liquid fraction resulting from the dewatering of the digestate (i.e., the centrates / filtrates) to the bioreactor 24. These centrates / filtrates are rich in carbonaceous matter, which will be available to the bacteria. In other words, and for the first and second embodiments of the invention, the digestate exiting the anaerobic digester 4 can enter a dewatering unit 30 to separate the digestate into a solid fraction and a liquid fraction, the liquid fraction being returned to the bioreactor 24. Generally, the installation 2 can include one or more optional digestate treatment systems configured to separate the liquid fraction from the digestate. The optional treatment system can thus be a phase separation system typically comprising one or more settling tanks, clarifiers, or filters.The optional treatment system can also be a system for removing impurities, for example by sieving.

[0083] For the embodiments of figures 3 and 4, the extraction of ammonium ions in the digestate precedes dewatering. A fraction depleted in ammonium ions (and potentially degassed) is sent to the dewatering unit 30, for example via a sending pump 31, so as to separate it into a solid fraction and a liquid fraction, the liquid fraction being able to be returned to the bioreactor 24 at the level of the main treatment line () or to the bioreactor 24 ().

[0084] Alternatively, it is possible to inject CO2 or sodium bicarbonate and trace elements into bioreactor 24 in order to supply bioreactor 24 with carbonaceous matter.

[0085] Facility

[0086] Figure 1 schematically represents an installation 2 implementing process 100 according to the invention. This installation 2 comprises an anaerobic digester 4 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.

[0087] The anaerobic digester 4 is equipped with an inlet pipe 6 receiving a first effluent, an outlet pipe 8 for the digestate produced during anaerobic digestion and an outlet pipe for the biogas produced during digestion.

[0088] 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+).

[0089] Anaerobic digester 4 can comprise one or more bioreactors connected in parallel or in series, adapted for carrying out anaerobic digestion in the presence of suitable microorganisms. These can be continuous, semi-continuous, or batch bioreactors. For example, a sequential batch reactor (SBR), a mixed-culture reactor with continuous feeding, a fixed-culture or free-culture reactor, etc., can be used.

[0090] Installation 2 further includes a digestate dewatering unit 30 comprising an inlet 32 ​​connected to the digestate discharge line 8 from digester 4, an outlet line 34 for the discharge of a liquid fraction, and a discharge 36 for the discharge of a concentrated dry matter fraction (cake). This dewatering unit 30 may include one or more of the following mechanical devices: a screw press, a belt filter, a filter press, or a centrifuge.

[0091] Installation 2 further includes the bioreactor 24, which degrades ammonium ions as described above. This bioreactor comprises an inlet 26 for receiving at least a portion of the ammonium-enriched solution and an outlet 28 for a solution depleted of ammonium ions. The ammonium-depleted solution is conventionally returned to the main treatment line, as illustrated by arrow 29.

[0092] As stated above, the bioreactor 24 can be formed from one or more reactors fed by at least part of the solution enriched in ammonium ions, and potentially a liquid fraction from a dehydration of the digestate as a source of carbonaceous material.

[0093] Advantageously, bioreactor 24 is closed to form a gas-tight volume. This is made possible by the reduced size of bioreactor 24, which only needs to process a small volume of solution, namely the ammonium ion-enriched solution. This volume reduction allows for completely closed bioreactors, enabling total control of the gas flows emitted by the process. This prevents the release of nitrous oxide, leading to an improved environmental footprint.

[0094] Furthermore, with such a closure, it is possible to optimally and simply control the operating conditions within bioreactor 24, including, for example, the temperature, knowing that the ammonium ion-rich solution is already at a temperature above 30°C when it arrives in bioreactor 24. This allows for a reduction in the heating costs of bioreactor 24.

[0095] Advantageously, and in order to control heating costs, the bioreactor 24 can include an enclosure containing a heat-insulating material, for example, rock wool or glass wool. This prevents heat loss and therefore excessively rapid cooling of the ammonium ion-enriched solution entering the bioreactor 24.

[0096] Installation 2 also includes an ammonia nitrogen extraction system 10 suitable for implementing the step of extracting the ammonia nitrogen formed during anaerobic digestion.

[0097] This extraction system includes an evaporation device 9 connected to the anaerobic digester 4 by a pipe 12 for sampling the contents of the digester and by a pipe 14 for returning the liquid residue to the anaerobic digester 4. A sampling pump 16 and a return pump 18 respectively ensure the sampling of the fraction of the contents of the anaerobic digester 4 to the extraction system and the return of the liquid residue to the digester 4. The evaporation device 9 is external to the anaerobic digester 4. It is typically an evaporator, such as 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.

[0098] The extraction system also includes a condenser 10 receiving from the evaporation device 9 via an inlet pipe water vapor enriched with ammonia (NH3) and producing a condensate discharged by a discharge pipe of the solution enriched with ammonium ions 20. This condenser 10 can be a direct or indirect heat exchanger, preferably indirect.

[0099] The extraction system includes a pump connected to the evaporation device 9, here via the condenser 10. This arrangement allows the non-condensables from the condenser 10 to be returned to the inlet of the digester 4 via a pipe or directly into the gaseous head of the digester.

[0100] Finally, the extraction system includes a vacuum pump 22 to maintain the extraction system under vacuum in order to allow evaporation and condensation to take place.

[0101] 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 9, and preferably also to the condenser 10.

[0102] According to the invention, the 20 evacuation line for the ammonium ion-enriched solution from the extraction system is connected to the inlet 26 of the bioreactor 24.

[0103] The extraction system operates as follows: First, a fraction of the contents of the anaerobic digester 4, specifically a liquid fraction, is extracted via the sampling pump 16 and sampling line 12. The extracted fraction is subjected to evaporation by lowering the pressure in the evaporation device 9 using the vacuum pump 22, which allows the formation of CO2 and a liquid residue discharged via the return line 14. An aqueous solution enriched with ammonium ions is formed by condensing the ammonia-enriched water vapor in the condenser 10. This condensed fraction is discharged via the ammonium-enriched solution discharge line 20. The liquid residue is returned to the digester 4 via the return line 14 and the return pump 18.

[0104] At least a portion of the ammonium-enriched aqueous solution is sent to the ammonium ion degradation bioreactor 24 via the ammonium-enriched solution discharge line 20 and the inlet 26 to supply the ammonia nitrogen for degradation. The remainder of the ammonium-enriched aqueous solution, if only a fraction of it is sent to the bioreactor 24, can be sent there at a later date or can be discharged for another use.

[0105] Installation 2 may include a set of systems enabling the control of the installation based on the quantity of ammonium ions to be degraded, so as not to exceed a certain amount of ammonium ions returned to a main treatment line. Control is achieved by monitoring pressures and recirculations at the digester 4 and the ammonia extraction system. This system notably allows for the control of the operation of the sampling pump 16 and the vacuum pump 22, particularly as a function of the ammonium ion concentration in the ammonium-enriched solution and in the digestate.

[0106] Advantageously, the installation 2 may include a pressure management system within the evaporation device 9, configured to lower its pressure to a pressure lower than the pressure prevailing inside the anaerobic digester 4.

[0107] This management system may include a valve or other pressure-reducing system, advantageously computer-controlled, and possibly a pressure sensor for the evaporation device 9 and / or a pressure sensor for the digester.

[0108] Advantageously, the management system can be configured to apply within the evaporation device 9 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.

[0109] Advantageously, the installation 2 may include a control system for the quantity of ammonium ion-enriched solution sent to the bioreactor 24 to send a predetermined quantity of this solution to the latter, for example an optimal quantity for proper operation of the bioreactor 24. This control system may include a valve, a pump, or other device to regulate the flow of the quantity of ammonium ion-enriched solution sent, advantageously computer-controlled.

[0110] Lare represents an embodiment of the installation 2 in which, as explained above, the liquid fraction from the dehydration unit 30 is sent to the bioreactor 24. The outlet pipe 34 of the dehydration unit 30 can, for example, be divided into two sub-pipes 34' and 34'', the first allowing the liquid fraction to be sent to the bioreactor 24 and the second allowing direct return of this liquid fraction to the inlet. Alternatively, it would be possible to provide only one return option for the liquid fraction from the dehydration unit 30 to the bioreactor 24.

[0111] This 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 4 and creating a parallel flow to the digestate. All the components enabling ammonium ion extraction are therefore located downstream of the discharge pipe 8 of the anaerobic digester 4. The aqueous solution enriched in ammonium ions is sent to the bioreactor 24. The fraction depleted in ammonium ions is sent to the dewatering unit 30 to form a liquid fraction and a fraction enriched in dry matter; the liquid fraction is returned to the main treatment line, as is the liquid exiting the bioreactor.

[0112] Lare represents an embodiment close to that of ladans in which, as explained above and as is the case for la, the liquid fraction from the dehydration unit 30 is used which is sent in whole or in part to the bioreactor 24. List of references

[0113] 2: Anaerobic digestion and nitrogen reduction system for effluent; 4: Anaerobic digester; 6: Digester inlet pipe

[0114] 8: Digester discharge pipe

[0115] 9: Evaporation device

[0116] 10: Condenser

[0117] 12: Sampling procedure

[0118] 14: Return line to the digester

[0119] 16: Sampling pump

[0120] 18: Return pump

[0121] 20: Conducting an aqueous solution rich in ammonium ions

[0122] 22: vacuum pump

[0123] 24: bioreactor

[0124] 26: bioreactor inlet

[0125] 28: Bioreactor discharge pipe

[0126] 29: return of a fraction depleted in ammonium ions

[0127] 30: Dehydration unit

[0128] 31: Sending pump

[0129] 32: entrance to the dehydration unit

[0130] 34: Dehydration unit outlet pipe

[0131] 34', 34'': outlet sub-pipes of the dehydration unit

[0132] 36: Evacuation of the dehydration unit

[0133] 100: Anaerobic digestion process and reduction of the nitrogen load of an effluent

[0134] 102: Anaerobic digestion stage

[0135] 104: Ammonia nitrogen extraction step

[0136] 106: ammonium ion degradation stage

[0137] Stage 108: Back in the lead

[0138] 110: Dehydration stage

[0139] 112: Step of sending a liquid fraction of the digestate to the bioreactor

Claims

A process (100) for the anaerobic digestion and reduction of the nitrogen load of an effluent, said process comprising: an anaerobic digestion step (102) of the effluent in an anaerobic digester (4) producing a digestate, the effluent containing organic matter including nitrogen, and the anaerobic digestion producing ammoniacal nitrogen and dissolved CO2; an ammonium ion degradation step (106) from the digester in an ammonium ion degradation bioreactor (24), during which ammonium ions are partially oxidized to form nitrites, and then reacted with nitrites to form nitrogen gas, characterized in that it comprises an extraction step of the ammoniacal nitrogen (104) formed during the anaerobic digestion, during which: an outgoing stream from the anaerobic digester is subjected to pressure-reducing evaporation, during which CO2 gas is formed. water vapor enriched with ammonia, and a liquid residue,An aqueous solution enriched in ammonium ions is formed by condensation of water vapor enriched in ammonia, and in that at least a part of the aqueous solution enriched in ammonium ions is sent to the bioreactor (24) to provide at least a part of the ammonium ions to be degraded. Method (100) according to claim 1, wherein the evaporation of the ammonia nitrogen extraction step (104) 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 anaerobic digester (4). A method (100) according to any one of the preceding claims, wherein the condensation of the ammonia-enriched water vapor is carried out at the same pressure as the evaporation. Method (100) according to any one of the preceding claims, wherein a predetermined quantity of aqueous solution enriched in ammonium ions is sent into the bioreactor (24). Method (100) according to any one of the preceding claims, wherein the output stream from the anaerobic digester is formed by a first liquid fraction taken from the anaerobic digester (4). Method (100) according to the preceding claim, wherein a volume of the first fraction is determined as a function of a predetermined concentration of ammoniacal nitrogen in the digestate. A process (100) according to any one of claims 5 and 6, further comprising a step of dehydrating the digestate, a liquid fraction from the dehydration being sent to the bioreactor (24). A process (100) according to any one of claims 1 to 4, 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. Method (100) according to the preceding claim, 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 (24). A process (100) according to any one of the preceding claims, wherein the effluent is selected from urban wastewater, industrial wastewater, agricultural wastewater, leachate, or hydrolyzed sludge. An installation (2) for the anaerobic digestion and nitrogen reduction of an effluent, comprising: an anaerobic digester (4) including an inlet pipe (6) receiving an effluent and a discharge pipe (8) for the digestate produced; a bioreactor (24) for the degradation of ammonium ions from the anaerobic digester (4) including an inlet (26) receiving an aqueous solution enriched in ammonium ions and a discharge pipe (28) for a liquid effluent depleted in ammonium ions, characterized in that it further comprises an ammonia nitrogen extraction system including: an evaporation device (9) configured to receive an output stream from the anaerobic digester (4); a condenser (10) equipped with an inlet pipe connected to the evaporation device (9) and receiving from it steam enriched in ammonia, and a discharge pipe (20) for a solution aqueous solution rich in ammonium ions,and in that the discharge pipe (28) of an aqueous solution rich in ammonium ions from the extraction system is connected to the inlet (26) of the ammonium ion degradation bioreactor (24). Installation (2) according to claim 11, comprising at least one of the following features: the condenser (10) is a heat exchanger, a pressure management system within the evaporation device (9) configured to apply inside the evaporation device (9) a pressure less than or equal to the value of the saturated vapor pressure of water at a temperature lower than the temperature of the medium contained inside the anaerobic digester (4), a system for controlling the quantity of aqueous solution enriched with ammonium ions sent to the bioreactor (24) configured to send to the bioreactor (24) a predetermined quantity of aqueous solution enriched with ammonium ions. Installation (2) according to any one of claims 11 and 12, wherein the bioreactor (24) is closed so as to form a gas-tight internal volume. Installation (2) according to any one of claims 11 to 13, wherein the bioreactor (24) comprises an enclosure comprising a heat-insulating material, for example, comprising rock wool or glass wool. Installation (2) according to any one of claims 11 to 14, wherein the evaporation device (9) is connected to the anaerobic digester (4) by a sampling line (12) for the contents of the anaerobic digester (4). Installation (2) according to the preceding claim, further comprising a dehydration unit (30) equipped with an inlet (32) connected to the digestate discharge line (8), an outlet line (34) of a liquid fraction and a discharge (36) of a dry matter concentrated fraction, the outlet line (34) of a liquid fraction comprising a sub-pipe (34') connected to the inlet (26) of the bioreactor (24). Installation (2) according to any one of claims 11 to 14, wherein the evaporation device (9) is connected to the anaerobic digester (4) by the discharge line (8) of the produced digestate, 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 (2) according to the preceding claim, further comprising a dehydration unit (30) equipped with an inlet (32) connected to the evaporation device (9) so as to receive the ammonium ion depleted fraction, an outlet line (34) of a liquid fraction and an outlet (36) of a dry matter concentrated fraction, the outlet line (34) of a liquid fraction comprising a sub-pipe (34') connected to the inlet (26) of the bioreactor (24).

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