Method for reducing nitrous oxide emissions from a fixed biomass reactor used for treating nitrogen pollution from an effluent

Abstract

The invention relates to a method for reducing the ecological footprint of a wastewater treatment plant which includes at least one fixed biomass reactor used to nitrify, in the presence of oxygen, the nitrogen pollution of the water, characterised in that it comprises a step consisting of supplying the fixed biomass of the reactor with pure oxygen or oxygen-enriched air by means of oxygen supply means, so as to reduce the nitrous oxide (N2O) emissions emitted by the reactor.

Description

[0001]Method for reducing nitrous oxide emissions from a fixed biomass reactor used for treating nitrogen pollution in effluent. Technical Field: The invention falls within the field of processes used to reduce the environmental footprint of industrial facilities by combating their greenhouse gas emissions. More specifically, the invention relates to combating greenhouse gas emissions from biological wastewater treatment plants. Greenhouse gases emitted by human activities are the primary cause of global warming and the resulting climate change. In order to reduce the environmental footprint of water treatment facilities, particularly wastewater treatment plants, numerous measures, notably those aimed at achieving energy neutrality, have already been implemented, and the search for new solutions to this objective is constantly underway.This research investigated the impact of biological processes implemented by wastewater treatment plants on their carbon footprint. It revealed that greenhouse gas emissions from these biological processes account for 18% to 83% of the carbon footprint of wastewater treatment plants (Vasilaki V. et al., 2019 - A decade of nitrous oxide (N2O) monitoring in full-scale wastewater treatment processes: A critical review - Water Research 161 (2019) 392-412). Among the gases involved in these biological processes, nitrous oxide, also known as nitrous oxide, was identified as being of particular interest for considering ways to reduce the impact of wastewater treatment plants on global warming.It has indeed been demonstrated that most wastewater treatment plants emit very significant quantities of this gas, and that nitrous oxide emissions from wastewater treatment facilities currently represent approximately 3.5% of global anthropogenic emissions of this gas. This gas has a global warming potential nearly 300 times greater than that of carbon dioxide. It was known, even before combating climate change became a major concern, that the nitrous oxide produced in biological wastewater treatment plants originates from the parts of these plants dedicated to treating nitrogen pollution in the wastewater. Nitrogen pollution removal is a crucial aspect of wastewater treatment, as it protects the ecosystems of the receiving environments from eutrophication.In practice, nitrogenous organic matter is present in water after ammonification as ammonium. Thanks to nitritizing (or nitrosating) bacteria (Ammonium Oxidizing Bacteria - AOB) and nitrifying bacteria (Nitrite Oxidizing Bacteria - NOB), it is possible, by alternating nitrification stages carried out in the presence of oxygen and denitrification stages carried out in anoxic conditions, to transform ammonium into dinitrogen. During the biological process, ammonium is NH4. +Nitrogen is oxidized during the nitrification stage by AOB and NOB into nitrates (NO3-) via the hydroxylamine (NH2OH) and nitrite (NO2-) stages. The nitrates are then reduced during the denitrification stage by denitrifying bacteria into nitrogen (N2) via nitrites (NO2-), nitric oxide (NO), and nitrous oxide (N2O). This process is shown in Figure 1. Studies have also shown that, in wastewater treatment plants, the quantities of nitrous oxide emitted depend on: - local operating conditions, such as ambient temperature, seasonal temperature variations, and the pollutant load of the effluents to be treated; - the conditions under which the biological processes for treating nitrogen pollution operate, such as pH and oxygen concentration in the environment; and - above all, the type of biological treatment implemented.To assess the impact of this last parameter, numerous wastewater treatment plants were compared with regard to their nitrous oxide emissions. Indeed, while previous literature had perfectly identified the gases produced or likely to be produced during the various biological treatment processes implemented in wastewater treatment plants, until a few years ago it had not been precisely evaluated which of these processes produced the most nitrous oxide and in what quantities.Thus, comparative studies have shown that facilities implementing free biomass biological treatment processes (activated sludge) had nitrous oxide emission factors of between 0 and 0.05% of nitrous oxide emitted relative to nitrified ammonium, while facilities implementing fixed biomass processes, in which the biomass forms a film, commonly called "biofilm", on solid supports which can be of different kinds, such as polystyrene beads or elements of plastic material with a high specific surface area (of the type manufactured by the company Anox Kaldnes), had nitrous oxide emission factors ranging from 1% to 4%, i.e. at least twenty times more.These studies have clearly shown that wastewater treatment plants using fixed-film biomass reactors to remove nitrogen pollution produce the most nitrous oxide, a gas with a particularly high greenhouse effect. More specifically, it has been established that the high nitrous oxide emissions in fixed-film biomass reactors are linked to oxygen deficiency, which occurs primarily in the deeper layers of the biofilm, i.e., those closest to the substrate. This oxygen deficiency causes competition between AOB bacteria, involved in the nitritation reaction, and NOB bacteria, involved in the nitration reaction (see Figure 1). This competition results in an accumulation of nitrites in the bioreactor.This increase in nitrite concentration leads to increased nitrous oxide formation during the nitrification stage by promoting: - hybrid oxidation of hydroxylamine (NH2OH), - autotrophic denitrification in AOBs. While hybrid oxidation of hydroxylamine has a limited impact on nitrous oxide emissions, autotrophic denitrification is the primary pathway for the production of this gas. To reduce nitrous oxide emissions from wastewater treatment plants, prior art has proposed collecting and treating these emissions. Such an approach can be described as remedial. However, it has the drawback of requiring an additional treatment process, which is neither economically nor environmentally sound.The present invention, on the contrary, aims to prevent the emission of nitrous oxide in wastewater treatment plants implementing nitrogen pollution treatment in one or more reactors using biomass fixed to a support, that is to say, to prevent its production rather than having to eliminate it once produced. Description of the invention: The invention proposes a method for reducing the ecological footprint of a wastewater treatment plant including at least one reactor containing biomass that is at least partially fixed. This reactor is used to nitrify the nitrogen pollution of said wastewater in the presence of oxygen. The method is characterized in that it consists of supplying the fixed biomass of said reactor with pure oxygen or oxygen-enriched air, using oxygen supply means, so as to reduce the emissions of nitrous oxide (N2O) emitted by said reactor.It should be noted that prior art established that the biological nitrification process of nitrogen pollution, used to treat nitrogen pollution in wastewater, could produce nitrous oxide at various stages. Many scientists therefore sought to reduce the emission factors of this compound. It should also be noted that prior art had proposed aerating the nitrifying biomass with pure oxygen or oxygen-enriched air, solely to improve oxygen availability. However, this prior art aimed only to increase the efficiency of the nitrification stage and not to reduce the environmental footprint of the facilities implementing it.To the Applicant's knowledge, however, the use of pure oxygen or oxygen-enriched air for the specific purpose of reducing the environmental footprint of wastewater treatment plants including at least one reactor used for nitrification had never been considered. When the problem linking nitrous oxide emissions from wastewater treatment plants and their environmental footprint was established, those skilled in the art turned to a curative approach to these emissions, consisting of capturing the nitrous oxygen for treatment. In contrast, the inventors opted for a preventive approach, aiming to minimize the formation of nitrous oxide within such plants. According to a preferred embodiment of the invention, pure oxygen is used to feed biomass. Using pure oxygen optimizes the biomass's performance in nitrifying wastewater.However, according to another embodiment, oxygen-enriched air may suffice, as oxygen-enriched air is less expensive than pure oxygen, provided that it contains at least 30% oxygen by volume, preferably 50%, and even more preferably 60%. According to one aspect of the invention, said oxygen supply means for said fixed biomass may be implemented intermittently or sequentially. By limiting the oxygen flow rate, it is thus possible to reduce gas emissions. Advantageously, the process according to the invention may also include a step of conveying all or part of a gaseous headspace of said fixed biomass reactor, said headspace containing oxygen not consumed by said fixed biomass, to said oxygen supply means for said fixed biomass.Thus, it will be possible to recycle oxygen, reduce the quantities of this compound required for the process, and consequently lower the associated costs. According to one embodiment of the invention, the biomass can be fixed to at least one type of mobile support on which the biomass forms a biofilm. This support forms at least one bed within the reactor, and this bed can be wholly or partially fluidized. These supports can, for example, be made of polystyrene beads and / or high specific surface area plastic elements, such as those marketed under the name AnoxKaldnes®. According to another embodiment, the biomass can also be used to denitrify nitrogen pollution in the water without the addition of oxygen. In this case, means for recirculating the water within the reactor will be provided.According to yet another variant, the biomass can be fixed onto at least one membrane, the step of supplying the biomass of the reactor with pure oxygen or oxygen-enriched air being carried out by injecting the pure oxygen or oxygen-enriched air into the membrane. Interestingly, the oxygen used in the process according to the invention can advantageously be a by-product of a hydrogen production plant using water electrolysis. Also in the context of the fight against climate change, the production of this compound is growing rapidly. Indeed, the "green hydrogen" thus produced can be used as an environmentally friendly energy source and, in the long term, cheaper than fossil fuels. By using the oxygen produced by these plants in the process, the latter becomes part of a circular economy that is even more environmentally sound.The hydrogen production facility may advantageously be located near the wastewater treatment plant to which the invention is applied in order to reduce oxygen transport between the two facilities. This will be particularly feasible in large cities, which have wastewater treatment plants and may also have hydrogen production facilities or plans to acquire them. Preferably, the wastewater is first denitrified in a denitrification compartment, with some of the water from the reactor used to nitrify the nitrogen pollution in the presence of oxygen being recirculated to the head of said denitrification compartment. This variant allows for even greater N2O removal. Finally, the wastewater treated by the facilities to which the process of the invention is applied may be municipal or industrial wastewater.According to one variant of the process, pure oxygen is supplied to the biomass by injecting it into the reactor at a rate of less than 4 m / h, preferably less than 2 m / h. According to this variant, pure oxygen is supplied to the biomass by injecting it into the reactor at a rate of more than 0.7 m / h, preferably more than 1 m / h. According to a preferred variant, pure oxygen is supplied to the biomass by injecting it into the reactor at a rate of between 1 m / h and 2 m / h. Such rates allow for both an effective reduction in nitrous oxide (N₂O) emissions into the reactor and an improvement, even without degrading, the efficiency of the nitrification treatment, compared to a process in which air, rather than pure oxygen, is injected.To avoid a loss of nitrification performance, the inventors have demonstrated that the rate of injection of pure O2 into the biomass must not be too low. Advantageously, nitrous oxide (N2O) emissions are further reduced when pure O2 is injected intermittently or in a sequential manner. The invention is exemplified by the following description of an embodiment thereof, with reference to Figures 1 to 3. Brief description of the drawings: The accompanying drawings are schematic and are intended primarily to illustrate the principles of the invention. [Fig. 1] is a schematic representation of the main biochemical reactions involved in the nitrogen pollution treatment steps (nitrification and denitrification). [Fig. 2] is a schematic representation of a pilot plant used to demonstrate the advantages of the process according to the invention; [Fig.Figure 3 illustrates a curve of the variation in N2O emission factors between "test" and "control" trials as a function of the pure O2 velocity. Description of an embodiment: Figure 1 shows the steps of nitration, nitritation, heterotrophic denitrification, and autotrophic denitrification. This last step is carried out by AOB bacteria and is characterized by being an incomplete reaction due to the absence or deactivation of an enzyme, N2O reductase, in these bacteria (this absence or deactivation is symbolized by the cross in Figure 1). In a first series of tests, six trials demonstrating the effectiveness of the process according to the invention were carried out using a pilot plant comprising two identical nitrification reactors 1,1a. This pilot plant is shown in Figure 1. It was used to nitrify an effluent to be treated in the presence of oxygen.Referring to Figure 1, the degradation of nitrogen pollution in an effluent by biomass comprises the steps of nitration, nitrification, heterotrophic denitrification, and autotrophic denitrification of the effluent, which may have been pretreated, notably to remove suspended solids. The autotrophic denitrification step is carried out by AOB bacteria and is characterized by being an incomplete reaction due to the absence or deactivation of an enzyme, N₂O reductase, in these bacteria (this absence or deactivation is symbolized by the cross in Figure 1). The pilot plant shown in Figure 2 was used to nitrify the effluent. Since nitrous oxide emissions occur only during the nitrification phase, no denitrification phase, which could have been carried out in the reactor by placing it under anoxic conditions, was included in the tests.The pilot plant was also used to reduce the COD of the effluent to determine whether the process had an impact on carbon pollution reduction. In the pilot plant shown, each of the two reactors 1,1a consists of the same equipment, known to those skilled in the art and marketed by Veolia under the Biostyr® brand. Each of these units comprises a 5-meter-high column 2,2a containing a bed 3,3a made of polystyrene beads to which a biomass film (biofilm) is attached; this biomass is identical in both reactors. Means for supplying the effluent to be treated 4,4a are provided in the lower part of each column, in a space provided below the bed, from which it is separated by a grid. Means for discharging the treated effluent 5,5a are provided from the upper part of the column, the effluent thus following an upward path within it.These discharge systems recover the treated water after it has passed through bed 3, 3a and is in the overflow 14, 14a of the reactors. Each reactor 1, 1a is also equipped at its base with means 6, 6a to distribute a gas at the base of the column into the space provided below the bed that receives the water to be treated. This gas provides the biomass present on the supports with oxygen, enabling it to degrade carbon pollution and to nitrify nitrogen pollution in the effluent. In this pilot plant, the two bioreactors are designed to operate in parallel, their feed systems each being connected to a common wastewater inlet pipe 7, which is linked to feed systems 8 for this wastewater. The pilot plant shown in Figure 2 also includes a probe 11 for measuring the pH and temperature of the effluent to be treated and a probe 12 for measuring its ammonium (NH4) concentration. +These probes are positioned upstream of reactors 1 and 1a at a tank 13 receiving the effluent to be treated. Each reactor 1 and 1a is also equipped with three probes 17, 17a; 18, 18a; and 19, 19a (marketed by Hach) to measure, respectively, the pH, oxygen concentration, and nitrous oxide concentration in the liquid phase in the overflow 14, 14a of columns 2 and 2a. The pilot plant also includes a device 20 comprising two gas analyzers (marketed under the name VA-3000 by Horiba) to continuously monitor the concentrations of nitrous oxide, oxygen, and carbon monoxide in the gaseous heads 15, 15a of column 2 of test reactor 1 and column 2a of control reactor 1a, respectively. 1) First series of tests Conditions for carrying out the first series of tests During the first series of tests, the two reactors were supplied in parallel with the same wastewater.The method according to the invention was implemented in one of these reactors, namely reactor 1 (“test reactor”), while the other reactor, namely reactor 1a (control reactor), was implemented according to the prior art. Thus, in control reactor 1a, the oxygen supply means 6a for the fixed biomass were connected to a compressed air source arriving via a pipe 9, while in test reactor 1, the oxygen supply means 6 for the fixed biomass were connected to a cylinder 10 containing pure oxygen. The compressed air flow rate and the pure oxygen flow rate were adjusted such that the biomass in test reactor 1 received the same amount of oxygen as that in control reactor 1a. The operation of the control reactor and that of the test reactor differed only in the use of air to oxygenate the biomass of the former and pure oxygen to oxygenate that of the latter.Each test was carried out over a period of 4 hours. During this period, reactors 1 and 1a were continuously supplied with wastewater to be treated at a feed rate of 100 L / h. This wastewater had a different total COD (TOD) load for each test, varying from 2.80 to 5.83 Kg O2.m. -3 .j -1 and a charge of ammonium (NH4 + ) different for each test varying from 0.50 to 0.80 Kg N-NH4 + m -3 .j -1 (See Table 1 below). The biomass in column 2a of control reactor 1a was fed with a compressed air flow rate of 500 NL / h, and column 2 of test reactor 1 with a pure oxygen flow rate of 100 NL / h. Thus, the same amount of oxygen was distributed to both columns. COD s , the DCO t And 4 NH 4+parameters were continuously measured at the outlet of each column throughout the duration of each test, and an average was calculated over 4 hours for each parameter. The average nitrogen pollution load (NH₄⁺) 4+ ) and in carbon pollution (COD) t ) relative to the incoming load could thus be calculated. The N2O content in the gas heads of each reactor was continuously measured over the duration of each test using the gas analyzer included in device 20, and an average was calculated over 4 hours. The concentration of dissolved N2O in the treated water was also measured using this analyzer and probes 19 and 19a, and an average was calculated over 4 hours. Results of the first series of tests According to the first test of the first series of tests, the CODt loads (Kg O2..m- 3 .j -1 ) and NH4 + (Kg N-NH4 + ..m -3 .j -1The values ​​applied to column 2a of the control reactor 1a and those obtained at its outlet, as well as the nitrification rate (%), are shown in Table 1 below. COD Tests t DCO t NH4 + NH4 + Rate of incoming eliminated incoming eliminated nitrification 1 4.54 3.92 0.80 0.29 36.25 2 3.57 2.98 0.74 0.28 3784 3 5.83 4.75 0.35 0.17 48.57 4 5.04 4.29 0.75 0.32 42.67 5 4.03 3.37 0.79 0.31 39.24 6 2.80 2.36 0.50 0.29 58.00 [Table 1] COD loads t (Kg O2.m -3 .j -1 ) and NH4 + (Kg N-NH4 + .m -3 .j -1 The values ​​applied to column 2 of test reactor 1 and those obtained at its outlet, as well as the nitrification rate (%), are shown in Table 2 below. CODt Tests CODt NH4 + NH4 +Rate of incoming eliminated incoming eliminated nitrification 1 4.54 3.95 0.80 0.50 62.50 2 3.57 3.00 0.74 0.47 63.51 3 5.83 4.77 0.35 0.24 68.57 4 5.04 4.48 0.75 0.43 57.33 5 4.03 3.43 0.79 0.50 63.29 6 2.80 2.36 0.50 0.38 76.00 [Table 2] The COD removed averaged 3.66 kg O2.m -3 .j -1 in the control reactor and 3.66 kg O2.m -3 .j -1 in the test reactor. These results show that the COD reductions T The concentrations in the test reactor remained essentially unchanged compared to the control reactor. Therefore, injecting pure oxygen instead of air into a nitrification reactor has no impact on the removal of carbonaceous pollution. The concentration of N-NH4 + The amount eliminated was on average 0.28 kg N-NH4 + .m -3 .j -1 in the control reactor and 0.42 Kg N-NH4 + .m -3 .j -1in the test reactor. These results therefore show that nitrification performance was significantly improved in the test reactor using pure oxygen injection compared to the control reactor using air. More specifically, the nitrification rate in the control reactor averaged 42%, while in the test reactor it averaged 65%. Thus, the nitrification yield was improved by an average of 50% in the test reactor compared to the control reactor. Regarding nitrous oxide, its production was measured throughout 4 each test in the gaseous phase (N2O Gaz ) present in the gaseous headspace of each reactor and in the liquid phase (N2O Liquide ) of the overflow from each reactor. Using these measurements, the total concentrations of nitrous acid produced by the test reactor and the control reactor were calculated during each test (N2O Total = N2O Gaz + N2OLiquide The overall N2O production factor of each reactor could be calculated by relating the total nitrous oxide production to the nitrified nitrogen pollution (N2O). Total / NH4 + (treated). The results obtained are summarized in Table 3 below, in which the factors of production are expressed as a percentage of N-N2O Total produced by quantity of NH4 +Nitrified. Tests Production Factor Production Factor Total N2OT Control Reactor Total N2OT Test Reactor 1 5.17 1.94 2 5.64 2.04 3 2.42 1.77 4 4.16 1.54 5 4.63 1.77 6 3.17 1.24 [Table 3] According to these results, the total N2OT emission factor in the control reactor averaged 4.20%, while in the test reactor it averaged 1.72%. Thus, when pure oxygen is used as an alternative to compressed air, the total N2OT emission factors decreased by an average of 59%. Since nitrous oxide is a greenhouse gas, the gaseous nitrous oxide emission factors were also calculated solely on the basis of the quantities of N2O emitted in the gaseous skies of columns 2, 2a of reactors 1, 1a. The emission factor for each reactor was calculated by relating the gaseous N2O emissions for each test to the nitrified nitrogen pollution (N2OGaz / NH4). +nitrified). The results obtained are summarized in Table 4 below, in which the gaseous N2O emission factors are expressed as a percentage of N-N2O gas produced per quantity of NH4 + Nitrified. Tests Emission Factor N2OGaz Emission Factor N2OGaz Test reactor Control reactor 1 4.15 0.71 2 4.61 0.84 3 2.08 0.83 4 3.37 0.65 5 3.77 0.77 6 3.17 0.66 [Table 4] According to these results, the N2OGaz emission factor in the control reactor averaged 3.44%, while in the test reactor it averaged 0.73%. Thus, when pure oxygen is used as an alternative to compressed air, the N2O emission factors Gazdecreased by an average of 79%. These results highlight that using pure oxygen to supply oxygen to a fixed-biomass nitrifying reactor improves both the reactor's nitrification performance and the emission of a gas with a lower N2O content. They therefore demonstrate that supplying a fixed-biomass nitrifying reactor with pure oxygen reduces the environmental footprint of a wastewater treatment plant hosting at least one such reactor. 2) Second series of tests A second series of tests was carried out to evaluate the impact of pure O2 velocities on N2O emission factors (FE-N2O) in a bioreactor using the Biostyr process. TM(described above) aerated with pure O2. Six trials, listed in the "test" column, were carried out at different pure O2 velocities (ranging from 0.71 to 3.5 m / h) and compared to six trials carried out at a constant biomass injection velocity of 7.1 m / h, listed in the "control" column. The conditions and results of these trials are presented in Tables 5a and 5b below. Load applied to the columns "Control" column "Control" and "Test" Tests Factor V in water Load [N-NH4+] applied Velocity [O2] Load [N-NH4+] of emission of treated air (kg N / m3 / d) (m / h) (mg / l) (kg N / m3 / d) N2O (% of treated N-NH4+) 1 0.71 0.42 7.1 5.6 0.37 4 2 1.42 0.94 7.1 5.5 0.43 3.2 3 0.71 0.43 7.1 6.9 0.43 4 4 0.71 0.42 7.1 7.6 0.42 3.6 5 0.71 0.42 7.1 7.4 0.42 2.9 6 1 0.62 7.1 6.1 0.4 2,54 [Table 5a] "Test" Column Comparison of "Test" and "Control" Columns Tests Velocity Load [N- Factor O2 [O2] NH4+] Emission Rate Δ Performance of Δ Factor (m / h) (mg / l) treated (kg N2O (% of N- treatment (%) N2O emission (%) N / m3 / d) NH4+ treated) 1 3.5 34.6 0.43 2 16.22 -50.00 2 1.1 11.3 0.6 0.5 39.53 -84.38 3 3.5 37.7 0.43 2.06 0.00 -48.50 4 0.71 24.3 0.42 0.3 0.00 -91.67 5 0.71 26.1 0.41 0.35 -2.38 -87.93 6 0.71 19.2 0.49 0.21 22.50 -91.73 [Table 5b] Tables 5a and 5b allow for a comparison of the N2O emission factors from the "test" trials with the N2O emission factors from the "control" trials. This yields percentage variations in emission factors. The impact of the O2pur velocity on the N2O emission factors (FE-N2O) of the Biostyr process, TMis also represented in the graph in Figure 3, illustrating the variation of N2O emission factors (FE-N2O) between the "test" and "control" trials as a function of the pure O2 velocity. The x-axis represents the pure O2 velocity, and the y-axis represents the variation of N2O emission factors (FE-N2O) between the two trials (in %). A negative value for the variation of emission factors indicates a decrease in the N2O emission factors in the "Test" column compared to the N2O emission factors in the "Control" column. The results of these trials show that the reduction of the N2O emission factor with the Biostyr bioreactor TMAeration with pure O2 is directly correlated to the pure O2 velocity, and more specifically, the N2O emission factor decreases linearly as the pure O2 velocity decreases. The N2O emission factor is lowest (approximately -89%) at a pure O2 velocity of 0.71 m / h. The N2O emission factor is highest (-50%) at a pure O2 velocity of 3.5 m / h. 3) Third series of tests A third series of tests was also carried out to evaluate the impact of pure O2 velocities on the treatment performance of the Biostyr bioreactor TMaerated with pure O2. Four trials, listed in the "test" column, were carried out at different O2 pure velocities (ranging from 0.71 to 7.1 m / h) and compared to four trials carried out at a constant air injection velocity of 7.1 m / h, listed in the "control" column. The conditions and results of these trials are presented in Tables 6a and 6b below.Load applied to the "Control" columns and "Control" "Test" columns Test Load [N-NH4+] Water velocity Air velocity (m / h) (m / h) [O2] Load [N-NH4+] applied (kg N / m (mg / l) treated (kg N / m3 / d) 1 0.58 1.56 7.1 6.3 0.38 2 0.83 1.56 7.1 5.9 0.35 3 0.63 1.43 7.1 6.8 0.31 4 0.54 1.56 7.1 6.7 0.34 [Table 6a] Comparison of the "Test" columns Test V Water velocity O2 Load [N-NH4+] [O2] (mg / l) treatment Δ Performance of (m / h) (m / h) treatment (kg N / m³ / day) (%) 1 1.56 0.71 17 0.32 -15.79 2 1.56 1.4 20 0.47 34.29 3 1.43 2.1 42.9 0.59 90.32 4 1.56 7.1 20 0.54 58.82 [Table 6b] Tables 6a and 6b allow comparison of the water treatment (nitrification) performance of the "test" trials with the water treatment performance of the "control" trials. The percentage variations in water treatment performance are thus obtained.The results of these tests show that the reduction below a certain limit of the velocity of pure O2 in the Biostyr reactor. TM Aeration with pure O2 is associated with a loss of water treatment performance. Indeed, in test #1, conducted at a pure O2 flow rate of 0.71 m / h, nitrification performance was reduced by approximately 15%. The dissolved O2 concentration was 17 mg / L. In test #3, conducted at a pure O2 flow rate of 2.1 m / h, nitrification performance was increased by approximately 90%. The dissolved O2 concentration was approximately 43 mg / L. The dissolved O2 concentration values ​​vary between 17 mg / L and 43 mg / L, indicating a decrease in the purification capacity of the Biostyr bioreactor. TMThese issues are not related to oxygen deficiencies but to the increased velocity of pure oxygen in the bioreactor. The first and second series of tests show that a compromise must be made between the need to reduce nitrous oxide (N2O) emissions and nitrification performance. Therefore, it is necessary to apply a pure oxygen velocity that is low enough to promote the reduction of nitrous oxide (N2O) emissions but also high enough to avoid degrading water treatment performance, and in particular nitrification performance.

Claims

CLAIMS

1. A method for reducing the environmental footprint of a wastewater treatment plant comprising at least one reactor containing at least partially fixed biomass, said reactor being used to nitrify nitrogen pollution in said water in the presence of oxygen, said method comprising a step of supplying, by means of oxygen supply, the fixed biomass of said reactor with pure oxygen or oxygen-enriched air, so as to reduce the emissions of nitrous oxide (N2O) emitted by said reactor.

2. A method according to claim 1, comprising using pure oxygen to supply said means of oxygen supply to said at least partially fixed biomass.

3. A method according to claim 1, comprising using oxygen-enriched air comprising at least 30%, preferably at least 50%, and preferably at least 60% by volume of oxygen to supply said oxygen supply means to said fixed biomass.

4. A method according to any one of claims 1 to 3, wherein said oxygen supply means to said fixed biomass are operated intermittently or sequentially.

5. A method according to any one of claims 1 to 4, comprising a step of conveying all or part of the air head of said fixed biomass reactor to said oxygen supply means, said air head containing oxygen not consumed by said fixed biomass.

6. A method according to any one of claims 1 to 5 wherein said biomass can be fixed on at least one type of mobile support on which the biomass forms a biofilm, said support forming at least one bed within said reactor and said at least one bed being able to be wholly or partly fluidized.

7. A method according to claim 6, characterized in that said biomass, at least partially fixed, is also used to denitrify nitrogen pollution in said waters in the absence of oxygen.

8. A method according to any one of claims 1 to 5, wherein the biomass is fixed onto at least one membrane, said step of supplying the biomass of said reactor with pure oxygen or oxygen-enriched air being carried out by injecting the pure oxygen or oxygen-enriched oxygen into said membrane.

9. A method according to any one of claims 1 to 8, wherein the oxygen used is a by-product of a hydrogen production plant by water electrolysis.

10. A method according to any one of claims 1 to 9, wherein said waters are first denitrified in a denitrification compartment, the waters from the reactor used to nitrify the nitrogen pollution of said waters in the presence of oxygen being partially recirculated to the top of said denitrification compartment.

11. A method according to any one of claims 1 to 10, wherein said waters are municipal or industrial wastewater.

12. A method according to any one of claims 2 to 11, wherein the supply of pure oxygen to the biomass is achieved by injecting pure oxygen into the reactor at a rate of less than 4 m / h, preferably less than 2 m / h.

13. A method according to claim 12, wherein the supply of pure oxygen to the biomass is achieved by injecting pure oxygen into the reactor at a rate greater than 0.7 m / h, preferably greater than 1 m / h.

14. A method according to claims 12 and 13, wherein the supply of pure oxygen to the biomass is achieved by injecting pure oxygen into the reactor at a rate between 1 m / h and 2 m / h.

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