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

Abstract

Method for reducing the ecological footprint of a wastewater treatment plant including at least one fixed biomass reactor used to nitrify nitrogen pollution of said water in the presence of oxygen, characterized in that it includes 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.

Description

Title of the invention: Method for reducing nitrous oxide emissions from a fixed biomass reactor used for treating nitrogen pollution from an effluent technical field

[0001] The invention falls within the field of methods 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.

[0002] Greenhouse gases emitted by human activities are the main cause of global warming and the resulting climate disruption.

[0003] In order to reduce the ecological footprint of water treatment facilities, in particular wastewater treatment plants, many measures, including those aimed at achieving energy neutrality, have already been put in place and the search for new solutions aimed at this objective is constantly being explored.

[0004] As part of this research, the impact of the biological processes implemented by wastewater treatment plants on their carbon footprint was studied. It was thus revealed that the greenhouse gas emissions emitted by these biological processes represented 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).

[0005] Among the gases involved in these biological processes, nitrous oxide, also known as nitrous oxide, has been identified as being of interest for considering ways to reduce the impact of wastewater treatment plants on global warming. It has been shown that most wastewater treatment plants emit very large quantities of this gas and that nitrous oxide emissions from water treatment facilities currently represent approximately 3.5% of anthropogenic emissions of this gas worldwide. This gas has a global warming potential nearly 300 times greater than that of carbon dioxide.

[0006] It was known, even before the fight against global warming became a major concern, that the nitrous oxide produced in biological wastewater treatment plants comes from the parts of these plants dedicated to treating nitrogen pollution in these waters.

[0007] Nitrogen pollution removal is an essential aspect of wastewater treatment, as it protects the ecosystems of the receiving environments from eutrophication. In practice, nitrogenous organic matter is present in the water after ammonification in the form of ammonium. Thanks to nitritizing (or nitrosating) bacteria (Ammonium Oxidizing Bacteria - AOB) and nitrating 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 NH4+ is oxidized during the nitrification step by AOB and NOB to nitrates NO3 via the hydroxylamine NH2OH and nitrite NO2 stages, and the nitrates are then reduced, during the denitrification step, by denitrifying bacteria to dinitrogen N2 via the nitrite NO2, nitric oxide NO and nitrous oxide N2O stages. This process is shown in [Fig. 1].

[0008] Studies have also shown that, in wastewater treatment plants, the quantities of nitrous oxide emitted depended on:

[0009] local operating specificities of the facilities, such as in particular ambient temperature, temperature variations according to the seasons and the pollutant load of the effluents to be treated;

[0010] the conditions for implementing biological processes for treating nitrogen pollution, such as pH or oxygen concentration in the environment; and,

[0011] especially the type of biological treatment implemented in these.

[0012] 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.

[0013] Thus, comparative studies have shown that installations implementing free-biomass biological treatment processes (activated sludge) exhibited nitrous oxide emission factors of between 0 and 0.05% of nitrous oxide emitted relative to nitrified ammonium, while installations implementing fixed-biomass processes, in which the biomass forms a film, commonly called a "biofilm," on solid supports that can be of various kinds, such as, for example, polystyrene beads or elements made of plastic material with a high specific surface area (of the type manufactured by the company Anox Kaldnes), have presented nitrous oxide emission factors ranging from 1% to 4%, i.e. at least twenty times more.

[0014] It has therefore become clear following these studies that wastewater treatment plants using fixed biomass reactors to reduce nitrogen pollution are those producing the most nitrous oxide, a gas with a particularly high greenhouse effect.

[0015] More specifically, it has been established that high nitrous oxide emissions in fixed-biomass reactors are linked to the oxygen deficit that occurs mainly in the deeper layers of the biofilm, i.e., those closest to the support. This oxygen deficit causes competition between AOB bacteria involved in the nitritation reaction and NOB bacteria involved in the nitration reaction (see [Fig. 1]). This competition results in an accumulation of nitrites in the bioreactor. This increase in nitrite concentration consequently increases nitrous oxide formation during the nitrification step by promoting: - the hybrid oxidation of hydroxylamine (NH20H), - autotrophic denitrification in AOB.

[0016] While hybrid oxidation of hydroxylamine has a limited impact on nitrous oxide emissions, autotrophic denitrification represents the main route of production of this gas.

[0017] In order to reduce nitrous oxide emissions from wastewater treatment plants, it has been proposed in the prior art to collect and treat these emissions. Such an approach can be described as remedial. It has the drawback of requiring the implementation of an additional treatment process, which is neither economically nor environmentally satisfactory.

[0018] 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 with biomass fixed on a support, that is to say, to prevent its production rather than having to eliminate it once produced. Description of the invention

[0019] The invention proposes a method for reducing the ecological 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 being characterized in that it consists of supplying the fixed biomass with oxygen by means of oxygen supplying said reactor with pure oxygen or oxygen-enriched air, so as to reduce the emissions of nitrous oxide (N2O) emitted by said reactor.

[0020] It should be noted that it was established in the prior art that the biological process of nitrification of nitrogen pollution implemented to treat nitrogen pollution in wastewater could produce nitrous oxide at various stages. Many scientists have therefore sought to reduce the emission factors of this compound.

[0021] It should also be noted that prior art had proposed aerating nitrifying biomass with pure oxygen or oxygen-enriched air, solely for the purpose of improving oxygen availability to the biomass. However, this prior art aimed only to increase the yields of the nitrification step and not to reduce the environmental footprint of the installations implementing this step. To the Applicant's knowledge, however, the use of pure oxygen or oxygen-enriched air had never been considered for the specific purpose of reducing the environmental footprint of wastewater treatment plants that include at least one reactor used for nitrification.

[0022] When the problem linking nitrous oxide emissions from water 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.

[0023] According to a preferred embodiment of the invention, pure oxygen is used to feed the biomass. Using pure oxygen optimizes its operation for nitrifying wastewater.

[0024] However, according to another variant, it may be sufficient to use oxygen-enriched air, oxygen-enriched air being less expensive than pure oxygen, provided that it comprises at least 30% by volume of oxygen, preferably 50%, and even more preferably 60%.

[0025] According to one aspect of the invention, said means for supplying oxygen to said fixed biomass can be implemented intermittently or sequentially. By limiting the oxygen flow rate, it is thus possible to reduce gas emissions.

[0026] Advantageously, the process according to the invention may also include a step of conveying all or part of a gaseous headspace from said fixed biomass reactor, said gaseous 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 the oxygen, reduce the quantities of this compound required for the implementation of the process, and consequently reduce the costs associated with it.

[0027] According to a variant of the invention, said biomass may 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.

[0028] These supports may, for example, be made of polystyrene beads and / or elements of plastic material with a high specific surface area, such as those marketed under the name AnoxKaldnes ®.

[0029] According to one variant, said biomass may also be used to denitrify nitrogen pollution in said waters in the absence of oxygen supply. In this case, means for recirculating the water in said reactor will be provided.

[0030] According to yet another variant, said biomass may be fixed on 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 pure oxygen or oxygen-enriched air into said membrane.

[0031] Interestingly, the oxygen used in the process according to the invention could advantageously be a by-product of a hydrogen production plant using water electrolysis. Also within 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 then becomes part of a circular economy that is even more environmentally beneficial.

[0032] 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.

[0033] Preferably, 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 partly redirected to the head of said denitrification compartment. Such a variant makes it possible to further reduce the N2O

[0034] Finally, the water treated by the installations to which the process of the invention is applied may be municipal water or industrial water.

[0035] The invention is exemplified by the following description of an embodiment thereof, with reference to Figures 1 and 2. Brief description of the drawings

[0036] The attached drawings are schematic and are intended primarily to illustrate the principles of the invention.

[0037] [Fig-1] is a schematic representation of the main biochemical reactions involved in the nitrogen pollution treatment stages (nitrification and denitrification).

[0038] [Fig.2] is a schematic representation of a pilot installation used to demonstrate the advantages of the process according to the invention. Description of a method of implementation

[0039] 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).

[0040] Six tests 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 [Fig. 1]. It was used to nitrify an effluent to be treated in the presence of oxygen.

[0041] With reference to [Fig. 1], the degradation of nitrogen pollution in an effluent by biomass comprises the steps of nitration, nitrification, heterotrophic denitrification, and autotrophic denitrification of said effluent, which may have been pretreated, in particular 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 [Fig. 1]).

[0042] The pilot plant shown in [Fig. 2] was used to nitrify the effluent. Since nitrous oxide emissions only occur during the nitrification phase, no denitrification phase, which could have been carried out in the reactor by placing them under anoxia, was included in the tests.

[0043] The pilot was also used to reduce the COD of the effluent in order to determine whether the process had an impact on the reduction of carbon pollution.

[0044] 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 brand name Biostyr®. Each of these units comprises a 5 m high column 2,2a containing a bed 3,3a made up of polystyrene beads onto which a biomass film (biofilm) is fixed, said biomass being identical in both Reactors. Means 4,4a for supplying the effluent to be treated 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 5,5a for discharging the treated effluent are provided from the upper part of the column, the effluent thus following an upward path within it. These discharge means recover the treated water after it has passed through the 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 for distributing a gas at the bottom 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 installation, the two bioreactors are designed to operate in parallel, their feeding means each being in communication with a common wastewater supply pipe 7 connected to means 8 for supplying it.

[0045] The pilot plant shown in [Fig. 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 the reactors 1, at the level of a tank 13 receiving the effluent to be treated.

[0046] Each reactor 1, the is also equipped with three probes 17, 17a; 18, 18a; and 19, 19a (marketed by the company Hach) allowing to measure respectively the pH, the concentration of dioxygen and the concentration of nitrous oxide in liquid phase in the overflow 14, 14a of the columns 2,2a.

[0047] The pilot also includes a device 20 including two gas analyzers (marketed under the name VA-3000 by the company Horiba) allowing continuous monitoring of the concentrations of nitrous oxide, dioxygen and carbon monoxide in the gaseous skies 15, 15a respectively of column 2 of test reactor 1 and of column 2a of control reactor 1a. Conditions for conducting the tests

[0048] During the tests, the two reactors were fed in parallel with the same wastewater. The process according to the invention was implemented in one of them, 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 means 6a for supplying oxygen to the fixed biomass were connected to a compressed air source arriving via a pipe 9, while, in test reactor 1, the means 6 for supplying oxygen to 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 the control reactor. a. The operation of the control reactor and that of the test reactor therefore differed only in the use of air to oxygenate the biomass of the former and of pure oxygen to oxygenate that of the latter.

[0049] Each test was carried out over a period of 4 hours. During this period, reactors 1 and 1 were continuously fed 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, ranging from 2.80 to 5.83 kg O2.m³.d⁻¹, and a different ammonium (NH₄⁺) load for each test, ranging from 0.50 to 0.80 kg N-NH₄⁺ m³.d⁻¹ (see Table 1 below). The biomass in column 2a of the control reactor 1 was fed with a compressed air flow rate of 500 NL / h, and column 2 of the test reactor 1 with a pure oxygen flow rate of 100 NL / h. Thus, the same quantity of oxygen was distributed in both columns.

[0050] The COD, CODt, and NH4+ 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 of these parameters. The average nitrogen (NH4+) and carbon (CODt) pollution load relative to the incoming load could thus be calculated.

[0051] The N2O content in the gaseous 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. Test results

[0052] The COD (Kg O 2. .m 3 .j ! ) and NH4+ (Kg N-NH 4 + . .m 3 .j) charges applied to column 2a of the control reactor and those obtained at the outlet of the latter as well as the nitrification rate (%) are indicated in Table 1 below. Test COD (Chemical Oxygen Demand) COD (Chemical Oxygen Demand) NH4+ (Nitritivated) NH4+ (Nitritivated) Nitriding Rate 1 4.54 3.92 0.80 0.29 36.25 2 3.57 2.98 0.74 0.28 37 84 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

[0053] [Table 1]

[0054] The CODt (Kg O2.m 3.d1 ) and NH4+ (Kg N-NH / .m 3.d ') charges 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. Test COD (Chemical Oxygen Demand) COD (Chemical Oxygen Demand) NH4+ (Nitrification Rate) 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

[0055] [Table 2]

[0056] The COD removed averaged 3.66 kg O2.m³.d' in the control reactor and 3.66 kg O2.m³.d' in the test reactor. These results show that the COD removal rates did not vary significantly in the test reactor 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.

[0057] The concentration of N-NH4+ removed averaged 0.28 kg N-NH4 / m³ / day in the control reactor and 0.42 kg O2 / m³ / day in the test reactor. These results therefore show that the 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.

[0058] Regarding nitrous oxide, its production was measured throughout each test in the gaseous phase (N2OGaz) present in the gaseous head of each reactor and in the liquid phase (N2OLiquide) of the overflow of each reactor.

[0059] Thanks to these measurements, the total concentrations of nitrous acid produced by the test reactor and by the control reactor were calculated during each test (N2OTotai = N2OGaz + N2OLiquide) ■

[0060] The overall N2O production factor of each reactor was calculated by relating the total nitrous oxide production to the nitrified nitrogen pollution (N2Oot / NH4+ treated). The results obtained are summarized in Table 3 below. where the factors of production are expressed as a percentage of N-N2OTotai produced per quantity of NH4+ nitrified. Test N2OTatai Production Factor Control Reactor N2OTatai Production Factor 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

[0061] [Table 3]

[0062] According to these results, the N2OTotai 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 N2OTotai emission factors decreased by an average of 59%.

[0063] Since nitrous oxide is a greenhouse gas, the nitrous oxide emission factors in the gas phase were also calculated solely on the basis of the quantities of N₂O emitted in the gas phase present in the gaseous heads of columns 2, 2a of reactors 1, 1a. The emission factor for each reactor was calculated by relating, for each test, the gaseous N₂O emissions to the nitrified nitrogen pollution (N₂O₆ / nitrified NH₄⁺). The results obtained are summarized in Table 4 below, in which the gaseous N₂O emission factors are expressed as a percentage of N₂O₆ produced per quantity of nitrified NH₄⁺. Test N2O Gaz emission factor control reactor N2O Gaz emission factor test 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

[0064] According to these results, the N2OGaz emission factor in the control reactor was on average 3.44%, while in the test reactor it was on average 0.73%. %. Thus, when pure oxygen is used as an alternative to compressed air, the emission factors of N2OGaz decreased on average by 79%.

[0065] These results demonstrate that using pure oxygen to supply oxygen to a fixed-biomass nitrifying reactor improves the reactor's nitrification performance and also results in 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 water treatment plant housing at least one such reactor.

Claims

Demands

1. A method for reducing the ecological footprint of a wastewater treatment plant including at least one reactor receiving at least partially fixed biomass, said reactor being used to nitrify nitrogen pollution of said water in the presence of oxygen, said method including 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 oxygen supply means to said biomass at least partially fixed.

3. A method according to claim 1, comprising using oxygen-enriched air comprising at least 30%, preferably at least 50% and preferably among all 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 means for supplying oxygen to said fixed biomass are implemented 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 gaseous head of said fixed biomass reactor to said oxygen supply means, said gaseous head containing oxygen not consumed by said fixed biomass.

6. A method according to any one of claims 1 to 5 wherein said biomass is 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 wholly or partly fluidized.

7. The process according to claim 6 characterized in that said biomass, at least partially fixed, is also used to denitrify nitrogen pollution of 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 consisting of supplying the biomass of said reactor with oxygen

9.

10.

11. pure or oxygen-enriched air being produced by injecting pure oxygen or oxygen-enriched air into said membrane. 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. 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 in the presence of oxygen the nitrogen pollution of said waters being partly rerouted to the head of said denitrification compartment. A method according to any one of claims 1 to 10 wherein said waters are municipal waters or industrial waters.

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