Method for biological treatment of nitrogen of effluents by nitritation

Abstract

The invention relates to a method for biological treatment of nitrogen in the form of ammonium in wastewater by nitritation in a biological reactor, comprising: at least one step a of aeration of the biological reactor containing the wastewater to be treated, at least one step b of removing at least a portion of the nitrites produced in step a, and a step c of extracting, from the reactor, a fraction of the sludge resulting from steps a and b. The invention also concerns a method for biological treatment of nitrogen in wastewater by nitritation / de-nitritation and / or de-ammonification, wherein the nitritation is implemented using the nitritation method according to the invention.

Description

[0001] BIOLOGICAL NITROGEN TREATMENT PROCESS

[0002] EFFLUENTS BY NITRITATION The invention relates to a biological treatment process for nitrogen in effluents by nitritation. The invention also relates to a biological treatment process for nitrogen in effluents by nitritation-denitritration and / or deammonification, also known as Partial Nitritation - Anammox, in which nitritation is carried out by the nitritation process according to the invention.

[0003] Typically, wastewater treatment plants are primarily designed to remove carbon, nitrogen, and phosphorus through biological processes using specific bacteria.

[0004] Nitrogen treatment generally involves nitrification followed by denitrification. Nitrification is an oxidation reaction, carried out by autotrophic bacteria, of ammoniacal nitrogen or ammonium, also known as NH4 or NH4+, into:

[0005] • Nitrous nitrogen, also known as nitrite, NO2 or NO2 ~ ;

[0006] • then nitric nitrogen, also known as nitrate, NO3 or NO3 " .

[0007] Denitrification consists of the reduction of nitrites or nitrates produced during nitrification reactions into nitrogen gas, known as N2, by denitrifying heterotrophic bacteria.

[0008] More precisely, nitrification consists of two sub-steps: a first nitritation step followed by a second nitration step. Nitritation involves the oxidation of ammonium to nitrite by autotrophic nitritizing bacteria, known as AOB or "Ammonia Oxidizing Bacteria," the predominant genus of which is Nitrosomonas. Nitrification involves the oxidation of nitrite to nitrate by other autotrophic bacteria, known as NOB or "Nitrite Oxidizing Bacteria," the predominant genus of which is Nitrobacter.

[0009] Denitrification can also be broken down into two sub-steps: a denitrification step which will transform nitrates into nitrite, and a denitrification step which will transform these nitrites into nitrogen gas.

[0010] To reduce the amount of energy used for nitrogen processing, other metabolic pathways can be considered: nitritation-denitritration and de-ammonification.

[0011] The nitritation-denitritation process aims to halt nitrogen oxidation at the nitrite stage, thus preventing nitrate production and bypassing the "nitrate portion" of the cycle. To implement nitritation-denitritation, it is therefore necessary to suppress NOB in favor of AOB. This process allows for a 25% reduction in oxygen requirements and a 40% reduction in carbon requirements.

[0012] Another process, called deammonification or partial nitritation / anammox, uses bacteria known as Anammox (for "ANAerobic AMMonium Oxidation"), which have the unique ability to transform nitrites and ammonium into N2 under anoxic conditions, without the need for external carbon. The Anammox deammonification process involves the partial nitritation of ammonium. Approximately 57% of the ammonium must be converted to nitrite to achieve complete deammonification.

[0013] The suppression of NOB has been extensively studied. However, to date, the implementation of this suppression in the treatment of nitrogen contained in effluents by nitritation-denitritration and de-ammonification is not optimized and remains difficult to implement.

[0014] The invention aims to optimize nitrogen treatment by nitritation-denitritration and de-ammonification by providing a nitritation process that effectively represses NOB activity and thus minimizes nitrate production compared to nitrite production, i.e., blocks nitrogen oxidation in the form of ammonium at the nitrite stage.

[0015] Consequently, the present invention relates to a biological treatment process for nitrogen in the form of ammonium in wastewater by nitritation in a biological reactor, comprising:

[0016] • at least one aeration step of the biological reactor containing the wastewater to be treated to obtain a dissolved oxygen concentration in the reactor equal to or greater than 1 mg / L, so as to oxidize at least part of the ammonium to nitrites by the ammonium-oxidizing bacteria present in said reactor,

[0017] characterized in that the process further comprises:

[0018] • at least one step b for removing at least some of the nitrites produced in step a by:

[0019] - a sample bl of water treated at step a outside the reactor, and / or by

[0020] - a biological transformation b2, under anoxia

[0021] steps a and b being implemented cyclically such that the nitrite concentration in the reactor at the start of step a is less than 2 mg N-NO2 / L or at least twice less than the ammonium concentration in the reactor, and

[0022] • a step c of extraction of a fraction of the reactor sludge resulting from steps a and b, per unit of time, calculated so that an effective aerated sludge age of the reactor is obtained which is less than or equal to a theoretical minimum aerated sludge age required for nitrification as defined by an exponential law decreasing with respect to temperature,

[0023] in such a way that the production of nitrates, i.e. nitration, is minimized relative to the production of nitrites, i.e. nitritation, in the reactor by suppressing the activity of nitrite-oxidizing bacteria, by said aeration of step a, said nitrite removal of step b and said effective aerated sludge age of step c.

[0024] In this application, the concentrations of nitrates, nitrites, and ammonium are expressed as nitrogen content. Specifically, N-NO2 for the nitrite concentration; N-NO3 for the nitrate concentration; and N-NH4 for the ammonium concentration.

[0025] Wastewater refers to any type of urban, industrial, or domestic effluent containing nitrogen, at least partially in the form of ammonium. Industrial effluent refers to effluent from any type of industry, including agriculture.

[0026] The wastewater that feeds the reactor is either raw or previously treated water, notably through a carbon treatment step.

[0027] The biological reactor is a reactor suitable for nitritation, i.e., comprising ammonium-oxidizing bacteria, such as a biological reactor conventionally used for nitrogen treatment in effluents. In this application, the biological reactor is referred to as a nitritation reactor or biological nitritation reactor. This reactor also includes nitrite-oxidizing bacteria, which are present in biological reactors for nitrogen treatment in effluents. Specifically, it is an activated sludge reactor with microbial cultures, where the hydraulic retention time is separated from the sludge retention time, the sludge being in the form of bacterial aggregates such as flocs or granules.

[0028] The reactor can be a closed tank or an open basin. The reactor can be a simple reactor, i.e. containing a single basin or tank, or a multiple reactor containing several basins or tanks, possibly connected to each other, in particular to allow the recirculation of water and / or sludge.

[0029] The reactor feed can be continuous, corresponding to a continuous process, or discontinuous, corresponding to a batch process. A continuously fed reactor is typically a fully mixed, plug flow, staged feed, or channel type. A discontinuously fed reactor is typically a sequencing batch reactor (SBR), also known as a sequencing batch reactor, particularly a constant-level sequencing batch reactor.

[0030] In the various embodiments of the process of the invention, reactor aeration, namely step a, is advantageously carried out by injecting or blowing gaseous oxygen into the reactor. The oxygen can be supplied in pure form or, advantageously, as air, which typically contains approximately 20% oxygen. The oxygen supply means can consist of a fan, blower, or compressor delivering air to the biological reactor. The dissolved oxygen concentration in the reactor during aeration step a is an average concentration resulting from continuous or discontinuous aeration of the reactor. The air flow rate injected into the reactor during the aeration step is controlled so that the dissolved oxygen (DO) concentration in the reactor is maintained at an average value equal to or greater than 1 mg / L.The reactor advantageously includes means for measuring and possibly regulating the concentration of dissolved oxygen in the reactor, such as, for example, a dissolved oxygen sensor and / or analyzer.

[0031] The reactor advantageously includes a mixer which allows the reactor to be stirred during the anoxic stage, or stage b2, and possibly also during the aeration stage, or stage a.

[0032] The concentration of dissolved oxygen, or OD, in the reactor during the aeration step a is advantageously equal to or greater than 1.5; 2; 2.5; 3 or 3.5 mg / L, preferably equal to or greater than 2 mg / L.

[0033] Step a allows for the oxidation of at least some of the ammonium, i.e., at least partial nitritation. Partial nitritation is important for nitrogen treatment by deammonification. To achieve partial nitritation, aeration, or step a, is stopped when some of the ammonium has been oxidized to nitrites. To control the percentage of ammonium oxidized during the aeration step, the reactor advantageously includes means for measuring and possibly regulating the ammonium concentration in the reactor, such as, for example, an ammonium sensor and / or analyzer.

[0034] In the various embodiments of the process of the invention, the withdrawal bl of treated water from step a, outside the reactor, implemented for the removal of at least a portion of the nitrites produced in step a, is advantageously carried out by draining b3 at least a fraction of the reactor contents resulting from step a and / or recirculating b4 at least a fraction of the reactor contents resulting from step a to a second biological reactor that feeds the first. This fraction is either a raw fraction consisting of unsettled sludge, or a fraction clarified by a prior sludge settling step between steps a and b.

[0035] The b3 draining step is generally implemented in batch processes using SBR-type reactors, while the b4 recirculation step is implemented in continuous processes using any type of reactor, such as a closed tank or an open basin, for example, a channel-type open basin. The biological removal of nitrites in step b2 consists of the biological transformation, under anoxic conditions, of nitrites into nitrogen gas. The b2 biological transformation of nitrites under anoxic conditions is carried out in the biological nitritation reactor, also known as the main reactor or first reactor, or in another biological reactor, called the biological nitrite treatment reactor, also known as the second reactor or auxiliary reactor. This b2 biological transformation of nitrites under anoxic conditions is advantageously carried out by heterotrophic denitrification in the presence of carbon and / or deammonification by Anammox.

[0036] Furthermore, bl sampling and b2 biological transformation are advantageously combined.

[0037] According to a first advantageous implementation of the process, the removal of at least some of the nitrites produced in step a is achieved by an anoxic biological transformation b2, and the sequence of steps a and b2 is repeated cyclically in the biological nitritation reactor. The nitrites produced during step a of a cycle, called cycle n, are removed during step b2 under anoxia, such that the nitrite concentration, or N-NO2, in the reactor at the beginning of step a of the following cycle, also called cycle n+1, is less than 2 mg N-NO2 / L or at least twice the ammonium concentration, or N-NH4, in the reactor. This implementation is preferred for continuous processes using any type of continuously fed reactor, such as, in particular, a reactor consisting of one or more closed tanks or one or more open basins, for example, a channel-type open basin reactor.

[0038] According to a second advantageous embodiment of the process, the removal of at least a portion of the nitrites produced in step a is achieved by recirculating at least a fraction of the reactor contents resulting from step a to a second biological reactor that feeds the first, and by the biological transformation (b2) of the nitrites in said fraction under anoxic conditions in said second reactor. The fraction of the reactor contents recirculated in step b4 is preferably an unclarified fraction. The biological nitrite treatment reactor, also known as the second reactor or auxiliary reactor, can be fed with raw water, as well as with sludge containing nitrite-laden water from the nitritation reactor, also known as the main reactor or first reactor. The second reactor removes the nitrites by biological transformation, in particular by heterotrophic denitrification in the presence of carbon.Consequently, the treated sludge exiting the second reactor, which feeds the nitritation reactor, has a nitrite concentration of less than 2 mg N-NO2 / L or at least twice the ammonium or N-NH4 concentration. This implementation method is preferred for continuous processes using any type of continuously fed reactor, such as a reactor consisting of one or more closed tanks or one or more open basins, for example, a channel-type open basin reactor.

[0039] According to a third advantageous embodiment of the process, the removal of at least some of the nitrites produced in step a is achieved by emptying b3 at least a fraction of the reactor contents resulting from step a and the biological transformation b2 under anoxic conditions of any remaining fraction in said biological reactor. Preferably, the fraction of the reactor contents that is emptied in step b3 is a fraction clarified by a prior sludge settling step between steps a and b3.

[0040] When the contents of the reactor have been completely emptied, there is no fraction remaining in the reactor and the biological transformation step b2 is omitted.

[0041] Steps a and b3 or a, b3 and b2 are repeated cyclically in the biological nitritation reactor. The nitrites produced during step a of a cycle or cycle n are removed, either entirely during the emptying step b3, or partly during the emptying step b3 and partly during the biological transformation step b2 in anoxia, so that the nitrite concentration in the reactor at the beginning of step a of the next cycle or cycle n+1 is less than 2 mg N-NO2 / L or at least 2 times less than the ammonium concentration in the reactor.

[0042] The third implementation method is preferred for discontinuous processes using a sequenced batch reactor or SBR, in particular an SBR reactor characterized by simultaneous feeding and emptying, especially simultaneous feeding and emptying at a constant level.

[0043] The fraction, preferably clarified, of the reactor contents that is drained in step b3 is advantageously discharged into a second biological reactor, and the nitrites present are removed by a biological transformation b2 under anoxia, in particular by heterotrophic denitrification in the presence of carbon and / or de-ammonification by Anammox, preferably by de-ammonification by Anammox. To control the removal of nitrites in step b and to verify the nitrite concentration or the ratio of ammonium to nitrite concentrations at the beginning of the aeration step a, the reactor advantageously includes means for measuring and possibly regulating the nitrite concentration in the reactor, possibly combined with means for measuring and possibly regulating the ammonium concentration in the reactor, such as, for example, a nitrite sensor and / or analyzer, possibly combined with an ammonium sensor and / or analyzer.

[0044] In addition, to control the relative production of nitrites and nitrates, i.e. the majority production of nitrites compared to the minority production of nitrates, during the aeration stage, the reactor advantageously includes means for measuring and possibly regulating the concentration of nitrates in the reactor, such as, for example, a nitrate sensor and / or analyzer.

[0045] In the various embodiments of the process of the invention, step c, which extracts a fraction of sludge from the reactor resulting from steps a and b per unit of time, is calculated so that, at the end of the treatment of steps a and b, the effective aerated sludge age of the reactor is less than or equal to a theoretical minimum aerated sludge age required for nitrification, as defined by an exponential law decreasing with temperature. The sludge fraction extracted in step c is, in particular, a mass fraction.

[0046] The effective aerated sludge age of the reactor, or A e ff ect if, represents the residence time of the sludge in the reactor during aeration. It corresponds to the ratio of the quantity of sludge present in kg of dry matter or DM in the biological reactor to the quantity of excess sludge or E to be removed per day, in kg DM / day.

[0047] The minimum theoretical aerated sludge age, or Atheodque, required for nitrification is defined in particular by the curve in Figure 1. It can also be defined by the equation established by the ATV or "Abwasser Technische Vereinigung eV":

[0048] Atheoretical = A = SF x 3.4 x 1.103 (15"T°C)

[0049] with T°C the temperature in degrees Celsius and SF a correction factor of 1.45 to 1.8 depending on the size of the biological treatment plant for nitrogen in effluents.

[0050] Step c is advantageously implemented by extracting, per unit of time, for example every day, a determined mass fraction of the reactor sludge, to finally obtain the said effective aerated sludge age, as illustrated in the examples.

[0051] The effective aerated sludge age of the reactor is advantageously between 50% and 90, preferably between 60% and 80% of the theoretical minimum aerated sludge age. Steps a, b, and c of the process according to the invention are advantageously repeated until the ratio of the nitrite concentration (or [N-NO2]) to the sum of the nitrite and nitrate concentrations (or [N-NO2] + [N-NO3]) at the end of aeration step a is equal to or greater than 0.8% (or 80%) when this ratio is expressed as a percentage, preferably equal to or greater than 0.9% (or 90%). The ratio of concentrations [N-NO2] / ([N-NO2]+ [N-NO3]) can be replaced by the ratio of the change in concentrations Δ[N-NO2] / (Δ[N-NO2] + Δ[N-NO3]) during the aeration step a.When the ratio of nitrite concentration to the sum of nitrite and nitrate concentrations reaches a value equal to or greater than 0.8 or 80, the repression of NOB and the associated blocking of nitrogen oxidation to ammonium at the nitrite stage—that is, the nitritation stage—are considered established in the biological reactor. After this establishment phase, nitritation can continue under the same conditions or under less repressive conditions during the maintenance phase. Less repressive conditions are achieved by decreasing the dissolved oxygen concentration in step a, by eliminating nitrite removal step b, and / or by modifying sludge removal step c, so that the effective aerated sludge age of the reactor is greater than or equal to the theoretical minimum aerated sludge age as defined previously.

[0052] The biological process for treating nitrogen in the form of ammonium by nitritation according to the invention is advantageously implemented without prior seeding of the biological nitritation reactor with nitritating bacteria.

[0053] The biological process for treating nitrogen in the form of ammonium by nitritation according to the invention, which effectively suppresses the activity of NOB and thus allows the oxidation of nitrogen to be blocked at the nitrite stage, is advantageously used in a process for treating nitrogen from wastewater by nitritation-denitritration and / or de-ammonification.

[0054] Consequently, the present invention also relates to a biological treatment process for nitrogen in the form of ammonium in wastewater by nitritation-denitritration and / or de-ammonification, characterized in that the nitritation of nitrogen in the form of ammonium is carried out by the biological treatment process for nitrogen by nitritation as defined above.

[0055] In addition to the foregoing provisions, the invention includes further provisions, which will be apparent from the following description, which refers to examples of implementation of the object of the present invention which are in no way limiting, with reference to the attached drawings in which: - figure 1 represents the decreasing exponential curve of the minimum theoretical aerated sludge age required for nitrification or A in days as a function of temperature T in °C.

[0056] - Figure 2 represents the effect of dissolved oxygen concentration during the aeration phase, nitrite removal before the next aeration phase and effective aerated sludge age on the establishment of nitritation, measured by the evolution over time in days of the [N-NO2] / [N-NOx] ratio at the end of the aeration stage, in a continuous-feed biological reactor for treating wastewater nitrogen by nitritation-denitritation.

[0057] - Figure 3 shows the evolution of concentrations of ammonium, N-NH4, nitrites,

[0058] N-NO2 and nitrates, N-NO3, during the aeration and anoxic stages, after the establishment of nitritation-denitritation in the biological reactor of Figure 2, corresponding to phase S3.3. The measurements were carried out at two different times. A. The N-NO2 concentration at the beginning of the aeration stage is less than 2 mg / LB. The N-NO2 concentration at the beginning of the aeration stage is at least twice as low as the N-NH4 concentration.

[0059] - Figure 4 represents the effect of dissolved oxygen concentration during the aeration phase, nitrite removal and effective aerated sludge age on the establishment of nitritation, measured by the evolution over time of the ratio [N-NO2] / ([N-NO2] + [N-NO3]) or [N-NOx] in the outlet water, in a biological reactor for the treatment of nitrogen from wastewater by nitritation-denitritation, with discontinuous feeding (SBR reactor).

[0060] Figure 5 shows the evolution of ammonium (NH4), nitrite (NO2), and nitrate (NO3) concentrations during the aeration and anoxic stages, after the establishment of nitritation-denitritation in the biological reactor shown in Figure 4, corresponding to the second half of phase P3 (time > 70 days). The NO2 concentration at the beginning of the aeration stage is at least half the NH4 concentration.

[0061] Figure 6 illustrates the effect of dissolved oxygen concentration during the aeration phase, nitrite removal, and the effective age of aerated sludge on the evolution of nitrifying populations (AOB and NOB) in activated sludge. The results are derived from molecular biology analysis using qPCR (quantitative Polymerase Chain Reaction), which quantifies the DNA of each microbial population considered. The results are expressed as a percentage enrichment (or depletion if less than 0) of the population relative to a starting date, namely day 16 (Period 1). EXAMPLE 1: Establishment and maintenance of nitritation by NOB repression in a continuous-feed nitritation-denitritation reactor

[0062] The biological nitrogen treatment system for the effluent consists of two activated sludge biological reactors of 4 and 8.9 m³ respectively and a 10.6 m³ clarifier. 3continuously powered at 2 m 3 / h, using urban wastewater pre-treated to remove particulate and colloidal carbon. The biological reactors are equipped with a mixer, an aeration system, and means for measuring and regulating dissolved oxygen concentration, as well as means for measuring nitrite, nitrate, and ammonium concentrations. Nitrogen treatment in the form of ammonium was started without prior seeding of the biological reactor. The effect of dissolved oxygen concentration during the aeration phase, nitrite removal before the aeration phase, and the effective age of the aerated sludge on NOB suppression and nitritation establishment in the reactor were tested over time, according to the following experimental design:

[0063] SI phase:

[0064] Start of the installation without seeding to avoid having NOBs and to form only AOBs in phase S 1.1 then draining of the installation in phase S 1.2 and restart without seeding.

[0065] - phase S2:

[0066] The standard nitritation parameters are applied with a low dissolved oxygen concentration, on the order of 0.35 mg / L, and without aerated sludge age control or sequenced aeration to remove the nitrites produced. Control refers to maintaining the system at a fixed aerated sludge age by modulating the sludge extraction volume.

[0067] - phase S3:

[0068] The three parameters are implemented with a sequenced aeration process consisting of successive cycles of a 30-minute aeration phase and a 30-minute anoxic phase. The dissolved oxygen concentration in the reactor during the aeration phase is initially 1 mg / L in phase S3.1, then decreased to 0.6 mg / L in phase S3.2, and subsequently increased to 2.5 mg / L in phase S3.3. Sludge extraction is carried out daily to obtain an effective aerated sludge age equal to 70% of the minimum theoretical aerated sludge age required for nitrification.

[0069] At the start-up in phase SI, which was carried out without seeding, a nitritation phase was observed with a [N-NO2] / [N-NOx] ratio of approximately 80, linked to the expression of AOB. Without bacterial population management through controlled sludge extraction and nitrite removal before the aeration phase, nitritation alone was quickly lost. Draining the system in phase S1.2 and restarting it quickly restored nitritation, but then nitration became dominant. NOB suppression was ineffective. In phase S3, sludge extraction management, coupled with a dissolved oxygen concentration of 1 mg / L in phase S3.1 and sequenced aeration to remove nitrites formed by heterotrophic bacteria, allowed nitritation to be established after 2 to 3 effective aerated sludge ages. In phase S3.2. A test with a dissolved oxygen concentration of 0.6 mg / L results in the immediate nitration of nitrites and therefore the expression of NOB. Once the dissolved oxygen concentration is increased to 2.5 mg / L from phase S3.3, nitritation is reinstated after the implementation of NOB suppression and is maintained permanently over several months with a [N-NO2] / [N-NOx] ratio equal to or greater than 85%.

[0070] The establishment of effective NOB suppression in the biological reactor is accompanied by a blocking of ammonium oxidation at the nitrite stages, resulting in minimal nitrate production relative to nitrite production during the aeration stage. Consequently, measuring the ratio of nitrite concentration to the sum of nitrite and nitrate concentrations at the end of the aeration stage allows for the investigation of effective NOB suppression in the biological reactor. This ratio was measured in the biological reactor over time under the different nitritation conditions mentioned above, and the results are presented in Figure 2.

[0071] The evolution of ammonium, nitrite, and nitrate concentrations in the reactor was monitored at two different times during phase S3.3, the establishment and maintenance of NOB suppression. The results presented in Figures 3A and 3B show that during the aeration phase, ammonium is converted into nitrites; the ammonium concentration in the reactor decreases, and the nitrite concentration, initially low (less than 2 mg N-NO2 / L, Figure 3A) or at least twice the ammonium concentration (N-NH4, Figure 3B), increases. In contrast, the nitrate concentration does not change during the aeration phase and remains low (less than 1 mg / L, Figures 3A and 3B). The [N-NO2] / [N-NOx] ratio at the end of the aeration step is greater than 0.8 (Figures 3A and 3B). In the anoxic phase, a decrease in nitrites is observed, which are eliminated by heterotrophs in the presence of carbon supplied by raw water.At the end of the anoxic phase, the nitrite concentration is low, less than 2 mg N-NO2 / L (Figure 3A) or at least half the ammonium concentration N-NH4 (Figure 3B). Since the aeration and anoxic phases are repeated cyclically, the nitrite concentration at the beginning of the aeration phase of the next cycle is also low, less than 2 mg N-NO2 / L or at least half the ammonium concentration N-NH4. Nitrite removal limits the growth of NOBs because they lack a suitable substrate when oxygen is present. By combining nitrite removal with sludge extraction, the NOBs are ultimately washed out of the reactor.

[0072] EXAMPLE 2: Establishment and maintenance of nitritation by NOB suppression in a discontinuous-fed SBR nitritation-denitritation reactor. The biological reactor is a constant-level SBR activated sludge reactor fed discontinuously at 13.7 m 3 / j, using urban wastewater that has been pre-treated to remove particulate and colloidal carbon as well as suspended solids. The reactor dimensions are as follows: diameter: 1.2 m; surface area: 1.13 m²; total height: 4 m; water depth: 3.18 m; volume: 3.6 m³ 3The reactor is equipped with a mixer, an aeration system, and means for measuring and regulating dissolved oxygen concentration, as well as means for measuring nitrite, nitrate, and ammonium concentrations. Nitrogen treatment is carried out through successive cycles of the following four steps: a simultaneous feeding and emptying step lasting 60 minutes, an anoxic step of 30 minutes, an aeration step of 60 minutes, and a settling step of 60 minutes.

[0073] The effect of dissolved oxygen concentration during the aeration phase, nitrite removal before the aeration phase, and effective aerated sludge age on NOB bacteria repression and nitritation establishment in the reactor were tested over time, according to the following experimental scheme:

[0074] - Period 1 or PI

[0075] During this period, the dissolved oxygen concentration during the aeration phase is 0.85 mg / L. 57% of the total reactor volume is renewed during the emptying / feeding cycle. Consequently, the nitrites produced during the aeration stage are removed, partly by reactor emptying (57%) and partly (43%) by denitrification by heterotrophic bacteria. No sludge removal is performed during this period.

[0076] Period 2 or P2

[0077] During this period, the dissolved oxygen concentration during the aeration phase is 0.85 mg / L. The removal of nitrites produced during the aeration step is carried out as in period 1. Sludge extraction is carried out so that the effective aerated sludge age of the reactor is equal to 70% of the minimum theoretical aerated sludge age required for nitrification at this temperature.

[0078] Period 3 or P3

[0079] During this period, the dissolved oxygen concentration during the aeration phase is 3.5 mg / L. The removal of nitrites produced during the aeration step is carried out as in period 1. Sludge extraction is carried out so that the effective aerated sludge age of the reactor is equal to 70% of the minimum theoretical aerated sludge age required for nitrification at this temperature.

[0080] The paragraph below provides an example of sludge age management based on Period 3 data. During this period, the temperature in the reactor was 19°C. Applying the ATV formula to calculate the minimum theoretical aerated sludge age required for nitrification yields:

[0081] 1.8 x 3.4 x 1.103 (15"19) = 4.1 days with SF=1.8

[0082] Considering a reactor aeration rate of 30% (fraction of time in aeration out of the total time), a minimum theoretical or Atheoretical aerated sludge age of 13.8 days is obtained. The following table gives the actual sludge ages (A e ff ect if) over four one-week periods. The effective sludge ages are calculated according to the definition given on page 7, line

[0083] 20.

[0084] The extracted volumes are calculated by considering the mass flow rate of sludge removed (in kgMS / day) divided by the sludge concentration in the reactor (ratio of the weight of sludge to the volume of said reactor).

[0085] The [N-NO2] / [N-NOx] ratio in the biological reactor at the end of the aeration stage was measured over time under the different nitritation conditions mentioned above and the results are shown in Figure 4.

[0086] In the absence of sufficient dissolved oxygen concentration during the aeration stage or sufficient sludge removal, nitrite production in the reactor is negligible. Conversely, when nitrite removal prior to the aeration stage is combined with sufficient dissolved oxygen concentration during the aeration stage and sufficient sludge removal, nitrite production predominates, with nitrate production being the minority, resulting in a [N-NO2] / [N-NOx] ratio exceeding 80%.

[0087] The evolution of ammonium, nitrite, and nitrate concentrations in the reactor was monitored during the establishment and maintenance phase of NOB suppression (second part of period P3, duration > 70 days). The results are presented in Figure 5. During the anoxic phase, a decrease in nitrites is observed, as they are eliminated by heterotrophs in the presence of carbon supplied by the raw water. At the end of the anoxic phase, the nitrite concentration is low, at least half the ammonium concentration (N-NH4) (Figure 5). Since the anoxic phase is followed by an aeration phase, the nitrite concentration at the beginning of the aeration phase is also low, at least half the ammonium concentration (N-NH4) (Figure 5).During the aeration phase, ammonium is converted to nitrites; the ammonium concentration in the reactor decreases, and the nitrite concentration, initially low and at least twice as low as the ammonium concentration (N-NH4), increases (Figure 5). In contrast, the nitrate concentration does not change during the aeration phase and remains low, below 1 mg / L (Figure 5). The [N-NO2] / [N-NOx] ratio at the end of the aeration stage is greater than 0.8 (Figure 5). The removal of nitrites limits the growth of NOBs since they lack a suitable substrate when oxygen is present.

[0088] By combining nitrite removal with sludge extraction, the NOB is ultimately washed away from the reactor, as shown in Figure 6. It appears that during the first two periods, the two microbial populations, AOB and NOB, while showing similar upward trends, exhibit different rates of increase. Indeed, at the end of P2, the NOB population had increased by 60% compared to the starting date (Day 16 (Period 1)), while AOB had increased by 550%. Furthermore, during the third period, the combination of three parameters—dissolved oxygen concentration during the aeration phase, nitrite removal, and management of the effective aerated sludge age—allows for a drastic washing away of the NOB population. By day 115, 85% of the nitrate population had been washed away. At the same time, the quantity of AOB increased sixteenfold.The operating conditions therefore generated a modification of the microbial community in favour of the AOB population, allowing the establishment of nitritation in the biological reactor.

Claims

DEMANDS 1. A process for the biological treatment of nitrogen in the form of ammonium in wastewater by nitritation in a biological reactor, comprising: • at least one aeration step of the biological reactor containing the wastewater to be treated to obtain a dissolved oxygen concentration in the reactor equal to or greater than 1 mg / L, so as to oxidize at least part of the ammonium to nitrites by the ammonium-oxidizing bacteria present in said reactor, characterized in that the process further comprises: • at least one step b for removing at least some of the nitrites produced in step a by: - a sample bl of water treated at step a outside the reactor, and / or by - a biological transformation b2, under anoxia, steps a and b being implemented cyclically such that the nitrite concentration in the reactor at the start of step a is less than 2 mg N-NO2 / L or at least twice less than the ammonium concentration in the reactor, and • a step c of extraction of a fraction of the reactor sludge resulting from steps a and b, per unit of time, calculated so that an effective aerated sludge age of the reactor is obtained which is less than or equal to a theoretical minimum aerated sludge age required for nitrification as defined by an exponential law decreasing with respect to temperature, such that nitrate production is minimized relative to nitrite production in the reactor by suppressing the activity of nitrite-oxidizing bacteria by said aeration of step a, said nitrite removal of step b and said effective aerated sludge age of step c.

2. A process according to claim 1, characterized in that the biological reactor is an activated sludge reactor.

3. A process according to any one of the preceding claims, characterized in that the dissolved oxygen concentration at step a is equal to or greater than 2 mg / L.

4. A method according to any one of the preceding claims, characterized in that the withdrawal bl of treated water in step a, outside the reactor, is carried out by emptying b3 of at least a fraction of the reactor contents resulting from step a and / or recirculating b4 of at least a fraction of the reactor contents resulting from step a to a second biological reactor which feeds the first.

5. A process according to any one of the preceding claims, characterized in that the biological transformation b2, in anoxia, is carried out by heterotrophic denitrification in the presence of carbon and / or de-ammonification by Anammox.

6. A process according to any one of claims 1 to 5, characterized in that the removal of at least part of the nitrites produced in step a is achieved by a biological transformation b2 in anoxia and the sequence of steps a and b2 being repeated cyclically in said biological reactor.

7. A process according to any one of claims 1 to 5, characterized in that the removal of at least a part of the nitrites produced in step a is achieved by recirculating b4 at least a fraction of the reactor contents resulting from step a to a second biological reactor which feeds the first and by the biological transformation b2 into anoxia of said fraction in said second reactor.

8. A process according to any one of claims 1 to 5, characterized in that the removal of at least a part of the nitrites produced in step a is carried out by emptying b3 of at least a fraction of the contents of the reactor resulting from step a and the biological transformation b2 into anoxia of the fraction possibly remaining in said biological reactor.

9. A process according to any one of the preceding claims, characterized in that the effective aerated sludge age of said at least one reactor is between 50% and 90, preferably between 60% and 80% of the theoretical minimum aerated sludge age.

10. A process according to any one of the preceding claims, characterized in that steps a, b and c are repeated until the ratio of the nitrite concentration to the sum of the nitrite and nitrate concentrations at the end of the aeration step a is greater than 0.8, preferably greater than 0.

9.

11. A biological treatment process for nitrogen in wastewater by nitritation-denitritration and / or de-ammonification, characterized in that nitritation is carried out by the process according to one of the preceding claims.

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