Wastewater treatment method with maximization of biogas production comprising an electro-oxidation step

Abstract

The invention relates to a method and a plant for treating wastewater and associated sludge that makes it possible to eliminate the carbon and nitrogen with maximization of biogas production. The process comprises: (a) a step of treating wastewater producing a first effluent (2) having a reduced content of carbonaceous material and a second effluent (3) having an increased content of carbonaceous material, (b) a step of treating at least one portion of the first effluent producing a third effluent (4) having a reduced nitrogen content, carried out without use of a biological nitrification under aerobic conditions and comprising at least one step of electro-oxidation during which at least one portion of the ammonium ions contained in the first effluent are oxidized to nitrites and / or nitrates, and / or to dinitrogen, (c) a step of anaerobic digestion of the second effluent to produce biogas (7) and a digestate (8).

Description

[0001] Description

[0002] Wastewater treatment process with maximization of biogas production including an electro-oxidation step

[0003] Field of invention

[0004] The invention relates to a method for treating wastewater and associated sludge, in particular a treatment for removing carbon and nitrogen with maximization of biogas production.

[0005] State of the art

[0006] Generally, wastewater treatment is a three-step process: a primary treatment step, a secondary treatment step, and a tertiary treatment step.

[0007] The first stage of primary treatment generally reduces the solids and / or organic matter content of the wastewater to be treated. This typically involves a settling stage, possibly assisted by the prior addition of coagulant and flocculant, during which the wastewater is placed in a holding tank or settling basin. The solids contained in the wastewater settle to the bottom of the tank where they are collected, and lighter substances such as grease and oils are recovered from the top of the wastewater present in the tank. This stage thus produces so-called primary sludge and an effluent with a reduced solids content. This primary sludge is generally treated by anaerobic digestion to produce biogas, an energy gas composed mainly of methane and carbon dioxide.

[0008] The secondary treatment stage reduces the carbon and / or nitrogen and / or phosphorus content of the effluent with a reduced solids content leaving the first treatment stage. During this stage, organic matter, nitrogen compounds and / or phosphorus compounds are assimilated or decomposed by aerobic and / or anaerobic and / or anoxic bacteria. It is therefore a biological treatment stage, most often implemented in free-culture reactors (so-called "activated sludge" process).

[0009] The third stage of tertiary treatment is designed to further clean the water when it is discharged into a sensitive ecosystem or for reuse. This stage may involve the removal of phosphorus and / or micropollutants and / or disinfection and / or filtration.

[0010] The most common biological process used in the secondary treatment stage to remove nitrogen compounds from wastewater generally involves nitrification followed by denitrification (N / DN) processes, i.e. the conversion of ammonia into nitrogen.

[0011] Nitrification is an aerobic oxidation reaction, requiring active aeration, in which a specialized group of autotrophic bacteria oxidizes ammoniacal or ammonium nitrogen, denoted as NH4 or NH4 + , in:

[0012] • nitrous nitrogen, also known as nitrite, NO2 or NO2",

[0013] • then nitric nitrogen, also known as nitrate, NO3 or NOs'.

[0014] Denitrification consists of anoxic reduction, during which a specialized group of heterotrophic bacteria (which can be anaerobic) couple the oxidation of organic substrates to the reduction of nitrates either to nitrous oxide (N2O) or to nitrogen gas (dinitrogen, N2).

[0015] At the outlet of the nitrification / denitrification stage, the nitrogen content must comply with limits depending on the regulations, which can significantly impact the design of the treatment to be carried out. Typically, the lower the nitrogen limit content, the longer the nitrification stage must be and the more aeration it requires (high oxygen demand). Low nitrogen contents are also difficult to achieve by a biological denitrification treatment, which also requires large quantities of biodegradable carbon, often from an external supply when biodegradable carbon is not available in sufficient quantity in the effluent to be treated compared to the quantity of oxidized nitrogen to be denitrified.

[0016] Sludge retention time (or SRT) is one of the key parameters for the design of the treatment chain because autotrophic nitrifying bacteria have a lower growth rate than heterotrophic denitrifying microorganisms, and must be maintained in the system to achieve efficient nitrification. This growth rate also decreases with operating temperature. Thus, low temperatures and / or low nitrogen discharge limits require prolonged aeration as well as an increase in the age of the sludge produced, the sludge retention time and the hydraulic retention time, which determines the sizing of the units. While a long sludge retention time allows good development of autotrophic nitrifying bacteria, the amount of biomass formed under these conditions is reduced: the system is said to be operating at low load.Conversely, a short sludge retention time limits, or even prevents, the development of nitrifying bacteria, and consequently denitrification, but allows the quantity of biomass formed to be increased: we say that the system operates under high load.

[0017] However, the methanogenic potential of aeration sludge from nitrification treatment in prolonged aeration ("low load" operation) is lower than that of sludge in "high load" operation, which results in lower anaerobic digestion performance during its subsequent treatment and therefore lower biogas production. Thus, it is necessary to work with "young" sludge to have a higher methanogenic potential and increase biogas production, but, under these conditions, nitrogen treatment is low, or even non-existent.Strict nitrogen discharge limits result not only in high construction costs due to high sludge retention times and hydraulic retention times, but also in high operating costs due to, among other things, reagent addition (addition of carbonates to increase alkalinity during autotrophic nitrification, addition of methanol as an external carbon source during heterotrophic denitrification, addition of a phosphate source in case of nutrient deficiency), high oxygen demand (high aeration) and high pump flows (internal and external recirculation).

[0018] As the wastewater industry moves toward resource recovery and carbon use reorientation, maximizing energy recovery in biogas becomes essential to improve the energy balance of wastewater treatment facilities. However, the current design paradigm for the nitrification / denitrification stage of the wastewater treatment chain seems incompatible with increasingly stringent nitrogen discharge limit regulations and the goal of maximizing energy recovery through biogas production. Indeed, to achieve low nitrogen discharge limits, sufficient carbon must be left at the outlet of primary treatment to achieve sufficient denitrification in secondary treatment, which limits the possibility of capturing carbon in the sludge from primary treatment for biogas production.

[0019] In particular, for a primary treatment stage with determined carbon capture performance, a downstream secondary treatment producing activated sludge at low load has a lower methanogenic potential than a downstream secondary treatment producing activated sludge at high load (also called "High Rate Activated Sludge" or "HRAS"), in which the carbon is not treated by a reactor carrying out biological nitrification. Indeed, at low load, for a long sludge age, heterotrophic bacteria do not have enough food, so they consume from their reserves (endogenous respiration), which reduces the methanogenic potential. Whereas at high load, there is no nitrification, and therefore no need to work at a long sludge age, the sludge produced is therefore younger and sludge production is greater as is its methanogenic potential.

[0020] The limitation of carbon recovery at the outlet of primary treatment and the lower methanogenic potential of sludge from nitrification of secondary treatment therefore do not allow energy recovery to be maximized by biogas production while reducing the nitrogen content at the outlet of secondary treatment.

[0021] Finally, current treatments involve numerous intermediate steps to achieve the electron transfer that allows the reduction of nitrogen compounds to nitrogen: the conversion of electricity into blown air (via a booster) for aeration of the environment, the transfer of oxygen (20% of the air) to the environment (wastewater), the transfer of oxygen from the environment (wastewater) to the bacteria, the conversion of oxygen by the bacteria to oxidize ammonium ions into nitrites / nitrates by electron transfer. At each of these steps, there is a loss of energy due to the conversion, making the process more energy-intensive than necessary.

[0022] Finally, biological nitrogen treatments are complex to control since the performance of these treatments is controlled indirectly: the activity of the bacteria is typically regulated by the dissolved oxygen content in the medium, itself controlled by an air injection setpoint and consequently the flow rate of the blower. These treatments are also only effective after a period of biomass growth. Furthermore, during the biological nitrification stage at low load, filamentous bacteria can be produced, which can cause malfunctions (in particular loss of sludge settleability causing a leak of suspended matter at the clarification outlet) due in particular to swelling and foam formation.

[0023] There is therefore a need for wastewater treatment that maximizes both biogas production during sludge treatment and nitrogen pollution treatment, without adding reagent(s), or with reduced quantities of reagent(s), regardless of the carbon content of the wastewater to be treated (i.e., regardless of the C / N ratio). There is also a need for wastewater treatment that is simpler to implement and control.

[0024] Summary of the invention

[0025] A first subject of the invention relates to a method for treating wastewater containing nitrogen in the form of ammonium ions and carbonaceous matter, said method comprising:

[0026] (a) a wastewater treatment step to produce a first effluent having a reduced carbon content and a second effluent having an increased carbon content,

[0027] (b) a step of treating at least a portion of the first effluent to produce a third effluent having a reduced nitrogen content, and,

[0028] (c) an anaerobic digestion step of the second effluent to produce biogas and digestate.

[0029] According to the invention, step (b) of said method is carried out without implementing biological nitrification under aerobic conditions and comprises at least one electro-oxidation step during which at least part of the ammonium ions contained in the first effluent are oxidized into nitrites and / or nitrates, and / or into nitrogen.

[0030] This particular sequence of steps, and in particular the use of at least one step of nitrogen removal by electro-oxidation, allows a less complex implementation of the treatment process compared to a biological treatment of nitrogen by nitrification / denitrification. Indeed, the control of the electro-oxidation step can be carried out directly by controlling the applied current density and the flow rate of the effluent to be treated, and does not require a biomass growth period.

[0031] This sequence also allows for less expensive implementation than a biological denitrification treatment process in that the use of chemicals is reduced and the overall energy demand is lower since the electro-oxidation treatment requires fewer operating hours to achieve total or partial nitrification (reduced hydraulic retention time) and fewer intermediaries for electron transfers and biogas production is favored. Electro-oxidation is also a treatment with less risk of malfunction compared to a biological nitrification treatment (no risk of malfunction due to the production of filamentous bacteria, foam formation or sludge swelling).Electro-oxidation also makes it possible to achieve very low nitrogen levels in the treated water without being limited by an initial carbon content and makes it possible to reduce the production of nitrous oxide, a reaction intermediate in the biological reactions of ammoniacal nitrogen oxidation and nitrate reduction, which is a greenhouse gas.

[0032] To the extent that the presence of carbon in the first effluent is not necessary for the operation of step (b), step (a) of wastewater treatment can be carried out under conditions in which the carbon content present in the second effluent is maximized, which makes it possible to optimize the production of biogas by anaerobic digestion during step (c).

[0033] Advantageously, the treatment step (a) may comprise at least one step of treatment of the carbonaceous material chosen from a physical treatment step (a1), optionally preceded by a physicochemical treatment step (a2), and a biological treatment step (a3, a4) of the carbonaceous material.

[0034] Physical and / or physicochemical treatment makes it possible to reduce the content of solids, organic matter likely to flocculate and possibly phosphorus in the wastewater to be treated, thereby reducing the carbon content of the first effluent.

[0035] The use of physico-chemical treatment upstream of physical treatment makes it possible in particular to increase the treatment speed, which leads to the use of smaller and therefore less expensive installations.

[0036] Advantageously, the physical treatment step may be chosen from a decantation step, a flotation step and a filtration step, and the physicochemical treatment step may be chosen from a coagulation-flocculation step, a flocculation step alone and an electrocoagulation step followed by flocculation, or a combination of these steps.

[0037] The treatment step (a) may comprise at least one biological treatment step (a3) ​​of the carbonaceous matter, in particular carried out under conditions unfavorable to nitrification. This involves treating (i.e. eliminating) only the biodegradable soluble carbonaceous matter (i.e. the non-particulate carbonaceous matter) from the effluent to be treated in order to produce a first effluent with a reduced carbonaceous matter content and a second effluent with an increased carbonaceous matter content.

[0038] Biological treatment of carbonaceous matter can be carried out in anoxic or oxic conditions.

[0039] In one embodiment, step (b) may comprise a step of total electro-oxidation of at least a portion of the ammonium ions to dinitrogen.

[0040] In another embodiment, step (b) may comprise a step of partial electro-oxidation (bl) of at least a portion of the ammonium ions into nitrates and / or nitrites.

[0041] Advantageously, at least a portion of the effluent produced by said partial electrooxidation step (bl) can be sent to the treatment step (a), upstream or in an anoxic biological treatment step (a3) ​​of the carbonaceous material from step (a). The recirculation of a portion of the partially oxidized effluent in such an anoxic biological treatment step (a3) ​​makes it possible to maximize denitrification and carbon removal to achieve production of sludge (forming the second effluent) with a high methanogenic potential.Indeed, the recirculation in the anoxic biological treatment (a3) ​​step (a) allows the reduction of the carbon present in the wastewater entering the biological treatment step (a3), and possibly of the carbon present in the part of the partially oxidized effluent recycled in this step (a3) ​​made biodegradable by the partial electro-oxidation step, while providing the oxygen necessary for this reduction through the nitrates / nitrites produced during the partial electro-oxidation step (bl), which makes it possible to reduce the need for aeration of said biological carbon treatment step (a3).

[0042] In a variant, step (b) may comprise the partial electro-oxidation step (bl) in which a portion of the ammonium ions are oxidized to nitrates and / or nitrites, for example where only half of the ammonium ions are oxidized to nitrates and / or nitrites, followed by a step (b3) of anoxic biological treatment by oxidation of the ammonium ions by anaerobic autotrophic bacteria (Anammox step). The partial electro-oxidation step (b2) is then incomplete.

[0043] In the case where the incomplete partial electro-oxidation step (bl) is followed by this Anammox step (b3), at least part of the effluent produced by this step (b3) can be returned to the treatment step (a), upstream or in an anoxic biological treatment step (a3) ​​of the carbonaceous material from step (a).

[0044] An advantage of the Anammox step (b3) at the output of the incomplete partial oxidation step (bl) is to oxidize the remaining ammonium ions while reducing the amount of energy required compared to a step (b) implementing only one or more electro-oxidation steps.

[0045] In another variant, step (b) may comprise the step of partial electro-oxidation (bl) of at least a portion, preferably all, of the ammonium ions into nitrates and / or nitrites followed by a step of total electro-oxidation (b2) of at least a portion, preferably all, of the nitrates and / or nitrites into dinitrogen.

[0046] In yet another variant, step (b) may comprise the step of partial electro-oxidation (bl) of a portion of the ammonium ions (for example half) into nitrates and / or nitrites followed by a step of anoxic biological treatment by oxidation of the ammonium ions by anaerobic autotrophic bacteria (b3, Anammox step) then a step of total electro-oxidation (b2) of at least a portion, preferably all, of the nitrates and / or nitrites into dinitrogen. The step of total electro-oxidation (b2) of the nitrates and / or nitrites into dinitrogen aims to complete the elimination of total nitrogen and to reach the total nitrogen limit targeted or regulated by legislation.Preceding this total electro-oxidation step (b2), the Anammox step (b3) aims to oxidize a portion of the ammonium ions, thereby reducing the energy consumption required for the total electro-oxidation step (b2) of nitrates and / or nitrites to nitrogen and reducing the overall energy consumption used during step (b) for the removal of nitrogen. Advantageously, during the electro-oxidation step (b), (bl) or (b2), electrolysis of water present in the effluent can occur resulting in the production of dihydrogen at the cathode and dioxygen at the anode. The dihydrogen and / or dioxygen can then be recovered, and optionally the dioxygen can be sent to the treatment step (a), upstream or in a biological treatment step (a3) ​​of the carbonaceous material from step (a).

[0047] Advantageously, the treatment method may further comprise a treatment step (d) of the third effluent produced by the treatment step (b) to produce a fourth effluent, this treatment step (d) comprising at least one treatment chosen from a treatment for removing suspended matter, a treatment for removing phosphorus compounds, a treatment for removing micropollutants (in particular organic and / or metallic pollutants) and a treatment for removing microorganisms.

[0048] The third effluent treatment step (d) aims to further clean the water so that it meets discharge standards when discharged into a sensitive ecosystem or for reuse.

[0049] Advantageously, the treatment method may further comprise a control step in which:

[0050] (11) determining a quantity of nitrogen present in the third effluent or the fourth effluent and in at least one effluent to be extracted chosen from the first effluent, the effluent from the stage of partial electro-oxidation of ammonium ions and the effluent from stage (b3) of anoxic biological treatment by oxidation of ammonium ions by anaerobic autotrophic bacteria, then,

[0051] (12) determining a quantity of the at least one effluent to be extracted in order to reach a limit nitrogen content in the third or fourth effluent and extracting said quantity of the at least one effluent to be extracted and mixing it with the third or fourth effluent, and / or

[0052] (13) determining a quantity of the effluent from the partial electro-oxidation step (bl) of the ammonium ions to be sent to an anoxic biological treatment step (a3) ​​of step (a), this quantity corresponding to a nitrate and / or nitrite content necessary for an anoxic biological treatment for the elimination of carbonaceous matter, and sending said quantity of this effluent to the anoxic biological treatment step (a3) ​​of step (a).

[0053] The control step comprising the sequence (il) (i2) aims to reduce the quantities of effluent treated by step (b) while respecting the standards for nitrogen discharge in the third effluent, which makes it possible to reduce the dimensions of the installations and / or the treatment costs. The control step comprising the sequence (il) (i3) makes it possible to optimize the recirculation of the effluent produced by the partial electro-oxidation step (bl) so as to provide the necessary quantity of nitrates / nitrites for the reduction in step (a3) ​​of the carbon initially contained in the wastewater. Step (il) may in particular also comprise a determination of the quantity of carbonaceous matter contained in the wastewater to be treated entering step (a), and during step (i3), the quantity of effluent can be determined according to its nitrate and / or nitrite content and the carbonaceous matter content of the wastewater to be treated.The control step comprising the sequence (il) (i3) also makes it possible to optimize the quantity of nitrogen to be treated by the total electro-oxidation step (b2) by minimizing it. The control step can implement the two sequences of steps (il) (i2) and (il) (i3) or only one of the two.

[0054] Advantageously, the treatment method may further comprise at least one treatment step (e) of at least a portion of a liquid fraction of the digestate produced by the digestion step (c), this treatment step being chosen from a step (eO) of treatment by electrocoagulation, a step of treatment by electro-oxidation (el) during which at least a portion of the ammonium ions contained in said liquid fraction are oxidized into nitrites and / or nitrates, and / or into nitrogen, a step (e2) of anoxic biological treatment by oxidation of the ammonium ions by anaerobic autotrophic bacteria (Anammox step) and the succession of the two steps (el) (e2) preceded or not by step (eO).

[0055] Advantageously, the electrocoagulation treatment step (eO) may comprise a sub-step of struvite precipitation by electrochemical dissolution of a sacrificial anode comprising magnesium, coupled with a sub-step of separation of the precipitated struvite.

[0056] The purpose of this treatment step (e) is to treat the nitrogen contained in the liquid fraction of the digestate produced by the ammonium-rich digestion step (c), which in particular makes it possible to return the treated liquid fraction to the main wastewater supply line of the process, in step (a) or upstream of step (a).

[0057] Another subject of the invention relates to a wastewater treatment plant containing nitrogen in the form of ammonium ions and carbonaceous matter, in particular for implementing the method according to the invention. The treatment plant according to the invention comprises:

[0058] - a first wastewater treatment unit comprising a wastewater supply pipe, a first pipe for discharging a first effluent having a reduced carbon content and a second pipe for discharging a second effluent having an increased carbon content,

[0059] - a second treatment unit, comprising a supply pipe connected to the first pipe of the first treatment unit, a discharge pipe for a third effluent having a reduced nitrogen content, the second treatment unit comprising at least one reaction zone for electro-oxidation treatment and being devoid of an aerobic biological treatment reaction zone,

[0060] - a third anaerobic digestion treatment unit comprising a feed pipe connected to the second pipe of the first treatment unit, a biogas discharge pipe and a digestate discharge pipe. Advantageously, the first unit may comprise at least one reaction zone chosen from a physical treatment reaction zone, optionally coupled to a physicochemical treatment reaction zone, and a biological treatment reaction zone.

[0061] Advantageously, the second processing unit can comprise:

[0062] - at least one first electro-oxidation treatment reaction zone and at least one second reaction zone chosen from an electro-oxidation treatment reaction zone and a non-aerated biological treatment reaction zone, each second reaction zone being connected to a first reaction zone by a pipe for discharging the effluent leaving the first reaction zone, or

[0063] - at least one first reaction zone for electro-oxidation treatment, at least one second non-aerated biological treatment reaction zone and at least one third reaction zone for electro-oxidation treatment, each second reaction zone being connected to a first reaction zone by a pipe for discharging the effluent leaving (in particular produced by) the first reaction zone, each third reaction zone being connected to a second reaction zone by a pipe for discharging the effluent leaving the second reaction zone.

[0064] Advantageously, the treatment installation may comprise a recirculation pipe connecting an outlet of the at least one first electro-oxidation treatment reaction zone or of the at least one second non-aerated biological treatment reaction zone to an inlet of a biological treatment reaction zone of the first unit.

[0065] Advantageously, the second treatment unit may comprise at least one discharge pipe from an electro-oxidation reaction zone chosen from a dihydrogen discharge pipe and a dioxygen discharge pipe. The treatment installation may further comprise at least one other treatment unit chosen from:

[0066] - a fourth treatment unit comprising a supply pipe connected to a discharge pipe of the second treatment unit and a discharge pipe for a fourth effluent, and comprising at least one reaction zone chosen from a reaction zone for treatment of removal of suspended matter, a reaction zone for treatment of removal of phosphorus compounds, a reaction zone for treatment of removal of micropollutants, a reaction zone for treatment of removal of microorganisms,

[0067] - a fifth treatment unit comprising a feed pipe connected to a discharge pipe for a liquid fraction of a digestate from the third unit and a discharge pipe for an effluent, optionally connected to the feed pipe of the first unit, and comprising at least one reaction zone chosen from an electrocoagulation treatment reaction zone, an electro-oxidation treatment reaction zone, a non-aerated biological treatment reaction zone by oxidation of ammonium ions by anaerobic autotrophic bacteria (Anammox reaction zone), and these last two reaction zones preceded or not by an electrocoagulation treatment reaction zone, an outlet of the electro-oxidation treatment reaction zone being connected to an inlet of the non-aerated biological treatment reaction zone.

[0068] The electrocoagulation treatment reaction zone may in particular comprise an electrochemical reactor equipped with a sacrificial anode comprising magnesium and a solid-liquid separation device.

[0069] Advantageously, the treatment installation can be equipped with a control system comprising at least one device for determining a content of ammonium and / or nitrate and / or nitrite ions, at least one fluid displacement device, and a control unit configured to:

[0070] - determine a quantity of nitrogen present in the third or fourth effluent and in at least one effluent to be extracted chosen from the first effluent, the effluent from the at least one first electro-oxidation treatment reaction zone and the effluent from the at least one non-aerated biological treatment reaction zone of the first unit, then

[0071] - determining a quantity of the at least one effluent to be extracted in order to reach a limit nitrogen content in the third or fourth effluent and controlling the at least one fluid displacement device in order to extract said quantity of the at least one effluent to be extracted and to mix it with the third or fourth effluent, and / or

[0072] - determining a quantity of the effluent from the at least one first electro-oxidation treatment reaction zone to be sent to a non-aerated biological treatment reaction zone of the first treatment unit, this quantity corresponding to a nitrate and / or nitrite content necessary for a non-aerated biological treatment for the elimination of carbonaceous matter and controlling the at least one fluid displacement device in order to send said quantity of this effluent to said non-aerated biological treatment reaction zone of the first unit.

[0073] Definitions / Abbreviations

[0074] Anammox: ANaerobic AMMOnium OXidation (Anaerobic oxidation of ammonium) In this reaction, under anoxic conditions (in the absence of oxygen), ammonium is converted into nitrogen gas using nitrites as an electron acceptor. This reaction is carried out in the presence of anaerobic autotrophic bacteria (they do not need free or dissolved O2).

[0075] BMP: methanogenic potential, corresponds to the maximum quantity of methane produced by a compound during its anaerobic degradation. It is generally expressed by the volume (NmL) of methane produced per gram of volatile substrate matter.

[0076] HRT (hydraulic retention time): Hydraulic residence time.

[0077] N / DN: Nitrification / Denitrification.

[0078] Refractory carbon: carbon that cannot be broken down by purifying biomass in wastewater treatment processes because the compound is too complex.

[0079] Total electro-oxidation stage: stage in which total electrochemical oxidation of the species present is carried out (total oxidation of ammonium ions to dinitrogen, total oxidation of nitrites / nitrates to dinitrogen).

[0080] Partial electro-oxidation step: step in which a partial electrochemical oxidation of the species present is carried out, e.g. ammonium ions into nitrates and / or nitrites.

[0081] An electro-oxidation step (total or partial) is incomplete when only part of the ions present in the effluent are oxidized.

[0082] By biological treatment in anoxic conditions, we mean a treatment carried out in an environment to which no CH is supplied (without aeration) but where oxygen is available in the medium in the combined form, for example, of nitrates, sulfates or other. Conversely, a treatment in oxic conditions is carried out in an environment to which oxygen O2 is supplied.

[0083] Biological treatment can be carried out in fixed cultures where the bacteria grow in the form of a biofilm on the surface of a support material or in free cultures (also called activated sludge) where the bacteria grow freely in the enclosure (flocs). In the case where the biological treatment is in free cultures (or "activated sludge"), it then includes a step of separation of the bacterial culture (i.e. sludge) from the treated liquid effluent. The sludge is generally returned to the biological treatment. The separation can be carried out in a clarifier (decanter) or in filters using microfiltration or ultrafiltration membranes. In the case where a succession of biological reactors in free cultures are installed (succession of several biological treatments), the separation of the biomass can be carried out downstream of the last reactor.

[0084] "Wastewater" means urban wastewater of mainly domestic origin, but some of which may be of industrial origin, or industrial wastewater, particularly from the food industry or any other industry producing effluents containing carbon and nitrogen. Preferably, the wastewater will be municipal urban wastewater.

[0085] Struvite is an ammonium-magnesium-phosphate salt with the chemical formula NH4MgPO4• 6 H2O.

[0086] Description of figures

[0087] Other features and advantages of the invention will emerge from reading the description given below of several particular embodiments of the invention, given for informational purposes but not as a limitation, with reference to the appended drawings in which: [Fig. 1] Figure 1 is a schematic representation of the treatment installation according to one embodiment of the invention.

[0088] [Fig. 2] Figure 2 is a schematic representation of the treatment installation according to a first alternative embodiment of the invention.

[0089] [Fig. 3] Figure 3 is a schematic representation of the treatment installation according to a second alternative embodiment of the invention.

[0090] [Fig. 4] Figure 4 is a schematic representation of the treatment installation according to a third alternative embodiment of the invention.

[0091] [Fig. 5] Figure 5 is a schematic representation of the treatment installation according to a fourth alternative embodiment of the invention.

[0092] [Fig. 6] Figure 6 is a schematic representation of the treatment installation according to a fifth alternative embodiment of the invention.

[0093] Detailed description of the invention

[0094] Process

[0095] The method according to the invention is a method for treating wastewater containing nitrogen in the form of ammonium ions (NH4 + ) and carbonaceous matter, in other words a process for removing nitrogen and carbonaceous matter from wastewater. This wastewater often contains suspended particles which can also be removed by the process according to the invention.

[0096] The nitrogen present in the wastewater is mainly in the form of ammonium ions, however, it can also be present in the form of organic nitrogen which will ultimately be transformed by the bacteria into ammonium ions by an ammonification reaction. The wastewater treatment process comprises a step (a) of treating the wastewater producing a first effluent having a reduced carbon content and a second effluent having an increased carbon content, a treatment step (b) comprising at least one step of electro-oxidation of at least a portion of the first effluent to produce a third effluent having a reduced nitrogen content and a step (c) of anaerobic digestion of the second effluent to produce biogas and a digestate.

[0097] Step (a) is thus a step of eliminating carbonaceous matter while step (b) is a step of eliminating nitrogen and step (c) a step of producing biogas. The alternative configurations of the different steps of the process presented below can be combined depending on the treatment objective decided.

[0098] Step (a) of wastewater treatment

[0099] Step (a) of wastewater treatment produces a first effluent having a reduced carbon content compared to the wastewater to be treated and a second effluent having an increased carbon content compared to the wastewater to be treated.

[0100] This treatment step (a) is a first treatment step which may comprise at least one treatment step chosen from a physical treatment step (al), optionally preceded by a physicochemical treatment step (a2), and a biological treatment step (a3, a4), or a combination of these steps.

[0101] The physical treatment step (al) can be chosen from a decantation step, a flotation step or a filtration step. The purpose of step (al) is to remove solid particles. The physical treatment step (al) thus produces sludge corresponding to the second effluent with an increased carbon content and an effluent corresponding to the first effluent with a reduced carbon content.

[0102] The physicochemical treatment step (a2) generally includes a coagulation (or electrocoagulation) step followed by a flocculation step. This type of treatment is typically carried out in the presence of chemical reagents, for example a coagulant and / or a flocculant. The coagulant can be added to the water to be treated or be formed in situ (electrocoagulation). The flocculant is usually added to the coagulated water. A flocculation step alone is also possible.

[0103] The physico-chemical treatment step (a2) is implemented upstream of step (a1) when it is present. The addition of a flocculant, or a coagulant and a flocculant, upstream of a physical treatment makes it possible to improve the separation of the first and second effluents, also by eliminating colloidal organic matter that can flocculate, and to accelerate the process, which will make it possible to reduce the size of the installations. Preferably, the physical treatment step (a1) is a settling step with or without a prior physico-chemical treatment step (a2), in particular with or without prior coagulation / flocculation.

[0104] Treatment step (a) may also include at least one step (a3) ​​of biological treatment of the carbonaceous material.

[0105] Biological treatment step (a3) ​​can be carried out under anoxic or oxic conditions suitable for removing the carbonaceous matter present in the treated wastewater. Treatment under anoxic conditions makes it possible to remove the biodegradable carbonaceous matter by denitrification, i.e. without the addition of oxygen by aeration. When the biological treatment is under anoxic conditions, a portion of the effluent produced by step (b) containing nitrates and / or nitrites is returned upstream of step (a) or in step (a), upstream or in step (a3) ​​of biological treatment under anoxic conditions, which makes it possible to remove the residual nitrogen during the biological treatment of the carbon. Treatment under oxic conditions makes it possible to remove the residual biodegradable carbonaceous matter.The sequence of anoxic then oxic conditions makes it possible to limit the air input for the overall elimination of carbon, and thus optimize the energy requirements of said biological treatment step (a3).

[0106] Typically, biological treatment step (a3) ​​is implemented under conditions limiting nitrification, i.e. using a low sludge age (low sludge retention time). The purpose of this step is to treat only the carbonaceous matter and not the nitrogen present in the wastewater. To this end, the person skilled in the art may choose suitable conditions by controlling in particular one or more of the following parameters: sludge residence time (in particular chosen as a function of the temperature of the medium), hydraulic retention time, aeration, oxygen supply, etc. In particular, a high-load activated sludge process may be used to implement biological treatment step (a3) ​​or a denitrifying biofilter.

[0107] The biological treatment step (a3) ​​thus produces sludge which corresponds to the second effluent having an increased matter content and an effluent corresponding to the first effluent having a reduced carbon content.

[0108] Preferably, biological treatment step (a3) ​​is carried out under anoxic conditions. Optionally, biological treatment step (a3) ​​is carried out in two parts, the first being under anoxic conditions and the second under oxic conditions. Carrying out the biological treatment under anoxic conditions and then under oxic conditions will make it possible to treat the residual biodegradable carbon and produce sludge with a high methanogenic potential.

[0109] The treatment step (a) may also comprise at least one step (a4) of anaerobic biological treatment in free cultures of the carbonaceous material to advantageously biologically treat (eliminate) part of the phosphorus contained in the effluent. In the case where step (a4) is followed by step (a3), these steps are carried out in free cultures. Then, step (a3) ​​comprises a separation step carried out for example in a clarifier. The separated activated sludge is then returned to step (a4).

[0110] In a first embodiment, processing step (a) may comprise:

[0111] - at least one step (al) of physical treatment or (a2) of physico-chemical treatment, or the succession of at least one step (a2) of physico-chemical treatment and at least one step (al) of physical treatment (such as for example a step of decantation or filtration with or without coagulation / flocculation) prior,

[0112] - followed by at least one step (a3) ​​of biological treatment to eliminate the carbonaceous matter present in the effluent from the physical treatment step (al) or the physico-chemical treatment step (a2).

[0113] The sludge from the different steps (a1), (a2), (a3) ​​can then be combined to form the second effluent having an increased carbon content. In a second embodiment, the treatment step (a) can comprise:

[0114] - at least one step (al) of physical treatment or (a2) of physico-chemical treatment or the succession of at least one step (a2) of physico-chemical treatment and at least one step (al) of physical treatment,

[0115] - followed by at least one step (a4) of anaerobic biological treatment to eliminate the phosphorus present in the effluent,

[0116] - then at least one step (a3) ​​of biological treatment to eliminate the carbonaceous matter present in the effluent under anoxic and / or oxic conditions.

[0117] The sludge from the physical treatment steps (a1) and / or (a2) physicochemical treatment steps and the biological treatment steps (a3) ​​(a4) are advantageously combined to form the second effluent having an increased carbon content.

[0118] The person skilled in the art may choose step (a), and in particular one or more of the steps (a1), (a2), (a3), (a4) previously described, in the usual manner, depending on several factors, in particular the size of the plant, the quality of the wastewater to be treated and in particular its nitrogen and carbon content. He may also choose appropriate implementation conditions for this step (a) to maximize the carbon content present in the second effluent.

[0119] Step (b) of electro-oxidation treatment

[0120] The first effluent from treatment step (a) is then treated by electro-oxidation to produce a third effluent having a reduced nitrogen content. During step (b), at least a portion of the ammonium ions contained in the first effluent are oxidized to nitrites and / or nitrates, and / or to nitrogen.

[0121] Electro-oxidation, also called anodic oxidation or electrochemical oxidation, is an advanced oxidation process. Electro-oxidation can be direct or indirect. Electro-oxidation is direct when electron exchanges occur directly between the surface of an electrode and ions (ammonium ions) adsorbed on the surface of the electrode. Electro-oxidation is indirect when electrons are exchanged using intermediate oxidants. These oxidants can be generated on the surface of an electrode by electron transfer to ions present in water (chlorides, sulfates) or directly to water (water electrolysis). The main oxidants generated on the surface of an electrode in wastewater are hydroxyl radicals (HO ), sulfate radicals (SO ), hypochlorous acid (HC 1 O), or ozone (O 3 ) or hydrogen peroxide (H 2 O 2 ).For example, the treatment of ammonium ions can be achieved by oxidation of ammonium ions to nitrates and / or nitrites at the anode, while the reduction of nitrites and / or nitrates to N2 takes place at the cathode. Alternatively, ammonium ions can be directly oxidized to N2 at the anode. Electrolysis of water results in the production of H2 (gas) at the cathode and the production of O2 (gas) at the anode. This electrolysis can occur whether the electro-oxidation is partial or total.

[0122] Electro-oxidation is a well-known process that will not be described in further detail. Electro-oxidation is typically controlled by varying the current density applied between the electrodes as well as the flow rate of effluent entering the electro-oxidation chamber (residence time).

[0123] During this electro-oxidation step, organic matter, in particular refractory carbonaceous matter, and / or organic micropollutants still present can be eliminated by partial or total indirect electro-oxidation, which makes it possible to achieve low carbon levels in the treated water.

[0124] In addition, the reaction intermediates (intermediate oxidants), which are by nature non-selective oxidants, have an action on microorganisms, which makes it possible to achieve partial, or even total, disinfection of the treated water.

[0125] Step (b) may comprise, or consist of, a step of total electro-oxidation of the ammonium ions to dinitrogen.

[0126] Alternatively, step (b) may comprise, or consist of, one or more electro-oxidation steps, for example a step of partial electro-oxidation of at least a portion of the ammonium ions into nitrates and / or nitrites followed by a step of total electro-oxidation of at least a portion of the nitrates and / or nitrites into dinitrogen, optionally with an intermediate Anammox step.

[0127] In one embodiment, step (b) of nitrogen removal may thus comprise a step of partial electro-oxidation (bl) of at least part of the ammonium ions into nitrates and / or nitrites.

[0128] At the outlet of this partial electro-oxidation step (bl) at least part of the effluent produced can be sent to treatment step (a). Recycling part of the effluent produced will optimize the energy requirement for carbon reduction when treatment step (a) contains a biological treatment step (a3) ​​in anoxic conditions.

[0129] Recycling is advantageously carried out upstream or in step (a3) ​​of biological treatment of the carbonaceous material. Preferably, this recycling is carried out upstream or in step (a3) ​​of biological treatment under anoxic conditions. The presence of nitrites and / or nitrates in this recycling makes it possible to completely or partially eliminate the aeration usually necessary to reduce the carbon by a non-nitrifying biological treatment.

[0130] The step of partial electro-oxidation (bl) of the ammonium ions can then be followed by a step of total electro-oxidation (b2) of at least a part of the nitrates and / or nitrites to dinitrogen. During step (b2), at least a part of the nitrates / nitrites are reduced to dinitrogen, however, a part of the ammonium ions not oxidized during step (bl) can also be oxidized during this step.

[0131] The step of total electro-oxidation (b2) of at least part of the nitrates and / or nitrites aims to complete the elimination of total nitrogen and to reach a limit content of total nitrogen predetermined by the operators or by the discharge standards. This limit can be verified by direct measurement of the content of ammonium ions and / or nitrates and / or nitrites with appropriate sensors and / or analyzers.

[0132] In a third alternative embodiment, step (b) may comprise the step of partial electro-oxidation (bl) of a portion of the ammonium ions followed by a step (b3) of biological treatment by oxidation of at least a portion of the ammonium ions by anaerobic autotrophic bacteria, also called Anammox, and optionally followed by a step of total electro-oxidation (b2) of at least a portion of the nitrates and / or nitrites to dinitrogen. The Anammox treatment step (b3) is thus coupled with the step of partial electro-oxidation (bl) of the ammonium ions to nitrates / nitrites. At the end of the partial oxidation step (bl), only a portion of the ammonium ions have been oxidized. Anammox treatment step (b3) involves anaerobic autotrophic bacteria that consume ammonium ions and nitrites to produce N2 without the need for oxygen and biodegradable carbon.Therefore, at the input of the Anammox treatment step (b3), both ammonium ions and nitrites are required, which are provided by the incomplete partial oxidation step (bl). The combination of steps (bl) and (b3) makes it possible to reduce the overall energy consumption for nitrogen removal, in particular, the optional step of total electro-oxidation (b2) of nitrates / nitrites located downstream then not having to oxidize all the ammonium ions not treated by step (bl) and all the nitrate / nitrite ions formed by step (bl).

[0133] This implementation also allows better and stable control of the NH4 / NOx ratio at the Anammox inlet because the partial electro-oxidation can be controlled with the applied current density. The partial electro-oxidation step (bl) can thus be implemented in such a way as to obtain at the outlet an effluent having a nitrite / ammonium ion concentration ratio favoring its treatment by the Anammox bacteria. This ratio is for example 0.8 to 1.8, preferably 1.1 to 1.5 gN / gN.

[0134] However, Anammox treatment step (b3) may not achieve a high ammonium ion removal rate, which would lead to a breakthrough of ammonium and nitrite ions at the treatment outlet, which must be converted into nitrates or even partially into N2 to comply with a strict discharge standard. It is then preferable to follow Anammox treatment step (b3) with step (b2) of total electro-oxidation of at least a portion of the remaining nitrates and / or nitrites and / or ammonium to nitrogen. Optionally, whatever the embodiment of the invention and in particular of step (b), the H2 gas produced at the cathode during the electrolysis of water which may occur during the electro-oxidation step (b), and in particular during the steps of partial electro-oxidation (bl) of the ammonium ions into nitrite / nitrate and / or electro-oxidation (b2) of at least a portion of the nitrates and / or nitrites into dinitrogen, may be recovered in order to be used.

[0135] Optionally, the O2 gas produced at the anode during the electrolysis of water which may occur during the electro-oxidation step (b), and in particular during the steps of partial electro-oxidation (bl) of the ammonium ions into nitrite / nitrate and / or total electro-oxidation (b2) of at least part of the nitrates and / or nitrites into nitrogen may be recovered in order to be used, or reinjected into the step (a3) ​​of biological treatment for the elimination of carbonaceous matter to reduce the energy requirement when this is carried out partially or totally in oxic conditions.

[0136] Optional step (d) of the third effluent produced by step (b)

[0137] Optionally, treatment step (b) is followed by a treatment step (d) of the third effluent produced. Treatment step (d) of the third effluent may comprise at least one treatment chosen from a treatment for removing suspended matter, a treatment for removing phosphorus compounds, a treatment for removing micropollutants, a treatment for removing microorganisms.

[0138] The suspended matter removal treatment step may be a physical or physicochemical treatment as described in step (a). It may be, for example, a settling, filtration or flotation step with or without, preferably with, prior coagulation / flocculation.

[0139] The treatment step for removing phosphorus compounds may be a physical or physicochemical treatment of the type previously described with the addition of a coagulant supplied via a chemical reagent or by electrocoagulation with the aim of removing the phosphorus compounds.

[0140] The treatment step for eliminating micropollutants or microorganisms may comprise at least one treatment chosen from electrocoagulation, advanced oxidation such as ozonation or electro-oxidation or by injection of a strong oxidant (e.g. ferrate), an adsorption step on activated carbon, disinfection for example using oxidants (e.g. chlorine, peracids), ultraviolet rays, peracids or chlorine.

[0141] The third effluent treatment step (d) is designed to further clean the water when it is discharged into a sensitive ecosystem or for reuse. Treatment step (d) produces a fourth effluent with a nitrogen and carbon content, micropollutant and / or microorganism content that meets specifications set by the operator or legislation, and which can be discharged into nature or reused.

[0142] Optional control step

[0143] The treatment method may also comprise a step of controlling step (a) of wastewater treatment and step (b) of electro-oxidation treatment. The control step makes it possible to control the quantity of first effluent entering the treatment step (b) and / or one of steps (b1), (b2) (b3) of step (b). Alternatively or in combination, the control step makes it possible to control the quantity of the effluent produced during the partial electro-oxidation step (b1) of the ammonium ions which can be recycled in step (a).

[0144] In the control step, firstly (il) a quantity of nitrogen present in the third effluent or the fourth effluent and in at least one effluent to be extracted chosen from the first effluent of step (a), the effluent from the partial electro-oxidation step (bl) of the ammonium ions and the effluent from step (b3) Anammox is determined. The quantity of nitrogen can be measured directly by a sensor for measuring the quantity of ammonium ions and / or nitrates / nitrites. The quantity of nitrogen can also be measured indirectly by ammonia analyzers.

[0145] Then (i2) a quantity of the at least one effluent to be extracted is determined to achieve a limit nitrogen content in the third effluent or the fourth effluent and said quantity is extracted from the at least one effluent to be extracted and mixed with the third effluent or the fourth effluent. For this purpose, the quantity of nitrogen measured in the third or the fourth effluent can be used to determine the quantity of the at least one effluent to be extracted so that combined with the third or the fourth effluent, the total quantity of nitrogen in the latter does not exceed a limit nitrogen content. The permitted limit nitrogen content, in particular ammonium ions, can be chosen according to the nitrogen discharge limits authorized by legislation.Once the quantity of effluent to be extracted has been calculated based on the measurements taken on the third or fourth effluent and at least one effluent to be extracted, this quantity is extracted to be injected into the third or fourth effluent.

[0146] During the step of controlling the recycling of a portion of the effluent produced during the partial electro-oxidation step (bl) of the ammonium ions, a quantity of the effluent from the partial electro-oxidation step (bl) to be sent to the biological treatment step of step (a) is determined (i3). This quantity can be determined by measurement with nitrate / nitrite / ammonia sensors or by analyzers. This quantity corresponds to a nitrite and / or nitrate content making it possible to eliminate the carbonaceous matter from the wastewater entering said biological treatment step (a3). In particular, provision may be made to determine a quantity of carbonaceous matter from the wastewater in order to determine the quantity of effluent to be recycled. This quantity of effluent is then sent to a biological treatment step (a3) ​​of step (a), preferably when this step is carried out under anoxic conditions.Recycling also reduces the energy required for use in the second electro-oxidation stage (b2).

[0147] Anaerobic digestion step (c)

[0148] The second effluent from treatment step (a) is treated by an anaerobic digestion step (c) producing biogas and digestate.

[0149] Anaerobic digestion or methanization corresponds to a cascade of biochemical reactions allowing methanogenic bacteria to convert the organic matter present in a digester into biogas, mainly a mixture of carbon dioxide and methane. The remaining materials are called digestate. The conditions for implementing this step (c), in particular temperature, pH and residence time, can be advantageously chosen to maximize biogas production.

[0150] Anaerobic digestion step (c) may also include, or be followed by, a liquid-solid separation step of the digestate to separate the digestate into a solid fraction and a liquid fraction. This separation step may be a dehydration step of the digestate producing a solid fraction (dehydrated sludge) and a liquid fraction, such as a centrifugation or filtration step. Optionally, at least one pretreatment step of the second effluent upstream of anaerobic digestion step (c) may be implemented to increase its yield. This pretreatment step may be chosen from a chemical, mechanical, biological and thermal pretreatment step.

[0151] The chemical pretreatment step can be acid or base hydrolysis or advanced oxidation. During the chemical pretreatment step, the sludge can be heated to a temperature below 100°C.

[0152] The mechanical pretreatment step can be an ultrasonic, microwave or electrokinetic disintegration step.

[0153] The biological pretreatment step is, for example, a fermentation / hydrolysis step under mesophilic (30 - 42°C) or thermophilic (45 - 70°C) conditions with a residence time of around 1 to 3 days.

[0154] Finally, the thermal pretreatment step can be a thermal hydrolysis process (THP). The thermal hydrolysis process (THP) is a process consisting of heating the sludge to a temperature generally between 140°C and 180°C with a treatment time of 30 minutes to 60 minutes.

[0155] Optionally, a post-treatment step can also be performed at the outlet of the anaerobic digestion treatment step (c) of the second effluent. Indeed, the digested sludge resulting from the anaerobic digestion step (c) contains a large amount of non-biodegradable organic matter that can be used for additional energy production. The post-treatment step is typically a hydrothermal carbonization (HTC) process. This process typically operates at temperatures between 180°C and 280°C for a period of time ranging from a few minutes to several hours in a non-oxidizing atmosphere. The wet dewatered sludge is treated with pressurized steam and the process produces a solid carbon fraction and a liquid fraction. The liquid fraction can be returned to the anaerobic digestion step (c) to increase biogas production.

[0156] Optional additional treatment step (e) of the liquid fraction of the digestate produced by the

[0157] In one embodiment, step (c) of anaerobic digestion of the second effluent may be followed by an additional treatment step (e) of at least a portion of the liquid fraction of the digestate produced by digestion step (c). The effluent produced by the additional step (e) may then be returned to the input stream of the process to carry out recycling of the effluent. Recycling this stream makes it possible to optimize the process by treating the wastewater as much as possible and thus reducing the load contained in these returns.

[0158] The additional treatment step (e) may be chosen from a step (eO) of treatment by electrocoagulation, a step (el) of treatment by electro-oxidation, a step (e2) of biological treatment by oxidation of ammonium ions by anaerobic autotrophic bacteria (Anammox treatment) and the succession of these last two steps (el) (e2) preceded or not by step (eO). The solid fraction of biomass produced by this Anammox step (e2) may be returned as input to the digestion step (c).

[0159] The additional step (e) allows the treatment of a liquid fraction rich in ammonium ions in order to reduce its nitrogen content so that it can be redirected into the main water line at the inlet of step (a) or in step (a). Preferably, the liquid fraction treatment step of the additional step (e) is an electro-oxidation step which is a total oxidation of the ammonium ions to nitrogen. In this configuration, the electro-oxidation would also oxidize dissolved carbon and should not be affected by variations in the capture rate of the sludge pretreatment process. The high temperature and high nitrogen load also provide favorable conditions for the kinetics of the electro-oxidation.

[0160] Alternatively, the liquid fraction treatment step of the additional step (e) is an electro-oxidation step followed by an Anammox treatment step. In this case, the electro-oxidation step is an incomplete partial electro-oxidation step of ammonium ions to nitrates / nitrites. This implementation leads to a better and stable control of the NH4 / NOx ratio as explained in the description of step (b). The energy balance is also improved because only a part of the liquid fraction of the digestate undergoes electro-oxidation which reduces the associated energy requirement.

[0161] Optionally, the additional step (e) comprises a step (eO) of treatment by electrocoagulation comprising a first sub-step of precipitation of the phosphorus and ammonium contained in the liquid fraction of the digestate in the form of struvite by the implementation of electrocoagulation by sacrificial anode comprising magnesium, coupled with a sub-step of separation of the struvite formed which may be for example a filtration, a decantation. This step (eO) has the advantage of reducing the quantity of nitrogen to be oxidized by the subsequent steps, while producing a resource (struvite) with high agronomic added value (fertilizer), without the addition of external chemical reagent because the magnesium to be provided for the precipitation comes from the electrocoagulation.

[0162] Installation Description

[0163] With reference to Figure 1, the installation 100 for treating wastewater containing nitrogen mainly in the form of ammonium ions and carbonaceous matter comprises a first wastewater treatment unit 110 adapted to the implementation of step (a) of the method, a second electro-oxidation treatment unit 120 adapted to the implementation of step (b) of the method, and a third anaerobic digestion treatment unit 130 adapted to the implementation of step (c) of the method. The first wastewater treatment unit 110 is configured to be fed by a feed pipe 1 with wastewater containing nitrogen partly in the form of ammonium ions and carbonaceous matter and to produce a first effluent having a reduced carbonaceous matter content discharged into a first discharge pipe 2 and a second effluent having an increased carbonaceous matter content discharged into a second discharge pipe 3.The first treatment unit 110 may be a physical, physicochemical or biological treatment unit. For this purpose, it may comprise one or more treatment reaction zones, chosen from a filtration reaction zone, a decantation reaction zone, a flotation reaction zone, a coagulation reaction zone, a flocculation reaction zone, an electrocoagulation reaction zone and a biological treatment reaction zone (with aeration for treatment in oxic conditions or without aeration for treatment in anoxic conditions). In particular, one or more reaction zones may be provided connected in parallel and / or in series for the implementation of each physical (a1), physicochemical (a2) or biological (a3, a4) treatment step.

[0164] A reaction zone may include a reactor or a treatment enclosure. When the reactor or treatment enclosure implements biological treatment in free cultures, the reaction zone may then include a separation system by decantation (clarifier) ​​or by filtration.

[0165] In the example of Figure 1, the first unit 110 may comprise one or more biological treatment reaction zones, for example a single sequential reactor also called SBR reactor (Sequential Batch Reactor), a reactor with free suspended cultures in continuous feed (activated sludge) or Biofilter reactor (reactor using thin and regularly renewed biological films) or several separate reactors, in particular with recirculation between them. The use of separate reactors allows continuous treatment of the effluents. In all cases, the invention is not limited by the number of reactors used, in particular several reactors each operating according to aerated / non-aerated cycles may be provided, or several continuous feed reactors or successive reactors comprising SBRs and continuous feed reactors may be provided.The first unit 110 when it contains an aerobic biological treatment reaction zone will be sized so as not to carry out biological nitrification.

[0166] The second electro-oxidation treatment unit 120 is configured to be supplied with first effluent via a supply line 4 connected to the first discharge line 2 of the first treatment unit 110 and to discharge via a discharge line 5 a third effluent having a reduced nitrogen content. For this purpose, the second unit 120 comprises at least one electro-oxidation reaction zone for implementing the at least one electro-oxidation step. In the example of FIG. 1, the second treatment unit 120 comprises one or more electro-oxidation reaction zones connected in series and / or in parallel. The reaction zone(s) may in particular implement total or partial oxidation. The second unit 120 is furthermore devoid of an aerobic biological treatment reaction zone.The second unit 120 may also comprise a pipe 37 for discharging oxygen and a pipe 38 for discharging hydrogen from an electro-oxidation reaction zone.

[0167] The first discharge line 2 and the supply line 4 are also connected, here by a valve 30, to an optional bypass line 13 (bypass line). The discharge bypass line 13 is connected to the discharge line 5 of the third effluent.

[0168] The third anaerobic digestion treatment unit 130 comprises a feed pipe 6 connected to the second pipe 3 of the first treatment unit 110, a biogas discharge pipe 7 and a digestate discharge pipe 8. The third unit 130 may comprise one or more anaerobic digestion reaction zones, in particular connected in series and / or in parallel.

[0169] Figures 2 and 3 show alternative configurations of the installation, in particular for the implementation of step (a) of wastewater treatment. Figures 2 and 3 show respectively a wastewater treatment installation 200 and 300 comprising a first wastewater treatment unit 110, a second electro-oxidation treatment unit 120 and a third anaerobic digestion treatment unit 130. These units 120 and 130 are not modified compared to Figure 1, the numbering remains the same for these units and the pipes concerned.

[0170] With reference to Figure 2, the first wastewater treatment unit 110 comprises a unit 111 for physical and / or physicochemical treatment of the wastewater capable of implementing steps a1) or a2) + a1) and a biological treatment unit 112 capable of carrying out step a3) of the method. The biological treatment unit 112 comprises one or more reaction zones. If the biological treatment is carried out in free cultures, then the last reaction zone of the unit comprises a biomass separation system such as a clarification chamber, not shown in the figure.

[0171] The physical and / or physicochemical treatment unit 111 is supplied via the wastewater supply pipe 1 and comprises a discharge pipe 9 for part of the second effluent and a discharge pipe 11 for the produced effluent.

[0172] The biological treatment unit 112 comprises a supply line 12 connected to the discharge line 11 of the unit 111. It is also connected to the first discharge line 2 of the first effluent and comprises a second discharge line 3 of a portion of the second effluent. The first discharge line 2 is connected to the supply line 4 of the second unit 120 and optionally to a bypass line 13, as described with reference to FIG. 1. Finally, the unit 112 can receive the oxygen circulating in the line 37 described with reference to FIG. 1.

[0173] Figure 3 differs from Figure 2 by the addition of an intermediate unit 113 between units 111 and 112 capable of implementing step a4) of the method by means of one or more biological treatment reaction zones. Unit 113 is an anaerobic biological treatment unit comprising a feed line 15 connected to the discharge line 11 of unit 111 and a discharge line 16 connected to the feed line 12 of unit 112. Unit 112 also comprises a sludge recirculation line 34 connected to the anaerobic biological treatment unit 113. The recirculation line 34 is present only if the biological treatments of units 112 and 113 are carried out in free cultures.

[0174] Figures 4 and 5 show alternative configurations of the installation, in particular for the implementation of step (b) of treatment by electro-oxidation. Figures 4 and 5 show a wastewater treatment installation 400 and 500 respectively comprising a first wastewater treatment unit 110, a second electro-oxidation treatment unit 120 and a third anaerobic digestion treatment unit 130. These units 110 and 130 are not modified compared to Figure 1, the numbering remains the same for these units and the pipes concerned.

[0175] With reference to Figure 4, the second electro-oxidation treatment unit 120 comprises a unit 121 for partial electro-oxidation treatment of ammonium ions into nitrate / nitrite ions capable of implementing step b1) of the method and a unit 122 for total electro-oxidation treatment of nitrate / nitrite ions into nitrogen capable of implementing step b2) of the method. Each unit 121, 122 comprises one or more electro-oxidation reaction zones.

[0176] The partial electro-oxidation treatment unit 121 is supplied with first effluent via the supply line 4 connected to the discharge line 2 of the unit 110, and comprises a discharge line 17 for an outlet effluent from step b1). The discharge line 17 is connected, here by a valve 30, 32 to at least one line, here three lines: a supply line 18 of the unit 122, an optional recirculation line 20 for a portion of the effluent produced upstream of the unit 110 and a bypass line 19 for a portion of the effluent produced. The bypass line 19 is connected to the bypass line 13 and to the discharge line 5 for the third effluent. The optional recirculation pipe 20 preferentially returns the effluent produced upstream of the treatment unit 112 implementing step a3) of the first treatment unit 110 (not shown in figure 4).The total electro-oxidation treatment unit 122 comprises a supply line 18 connected to the discharge line 17 of the unit 121 and the third effluent that it produces leaves via the discharge line 5.

[0177] Figure 5 differs from Figure 4 by the addition of an intermediate unit 123 located between units 121 and 122 and capable of implementing step (b3) of the method. Unit 123 comprises one or more non-aerated biological treatment reaction zones. Unit 123 is an Anammox treatment unit and comprises a feed line 21 connected to the discharge line 17 of unit 121 and a discharge line 22 connected to the feed line 18 of unit 122 and to the bypass line 19. The feed line 21 of unit 123 is also connected to a bypass line 33 for a portion of the first effluent from the discharge line 2 of unit 110.

[0178] The installation shown in Figure 5 has a fourth optional unit 124 capable of implementing the treatment step (d) of the third effluent. The treatment unit 124 comprises a supply pipe 23 connected to the discharge pipe 5 of the unit 122 and a discharge pipe 24 for the fourth effluent. The fourth unit may comprise one or more treatment reaction zones connected in series and / or in parallel.

[0179] Figure 6 shows an alternative configuration of the treatment facility, in particular downstream of the anaerobic digestion unit. Figure 6 shows a wastewater treatment facility 600 comprising a first wastewater treatment unit 110, a second electro-oxidation treatment unit 120 and a third anaerobic digestion treatment unit 130. These units 110, 120 and 130 are not modified compared to Figure 1, the numbering remains the same for these units and the pipes concerned, except for unit 130 which here comprises a discharge pipe 8' for a liquid fraction of the digestate produced.

[0180] With reference to Figure 6, the installation comprises a fifth unit 131 capable of implementing the additional step e) of the method.

[0181] The fifth unit 131 comprises an optional electrocoagulation treatment unit 132 capable of carrying out step e0), an electro-oxidation treatment unit 133 capable of carrying out step e1) and an optional Anammox treatment unit 134 capable of carrying out step e2). Each unit may comprise one or more suitable treatment reaction zones connected in series and / or in parallel.

[0182] The unit 132 comprises a supply line 25 connected to the discharge line 8' of the unit 130 and a discharge line 26 for the produced effluent.

[0183] In this embodiment, the third unit 130 comprises a liquid-solid separation system (not shown) for separating the digestate into a solid fraction and a liquid fraction. The liquid fraction is then discharged via the discharge pipe 8'. Note that this liquid-solid separation system could be external to the third unit 130 and located between it and the fifth unit 131.

[0184] The unit 133 comprises a supply line 27 connected to the discharge line 26 of the unit 132 and a discharge line 28 for the produced effluent.

[0185] The unit 134 comprises a supply pipe 35 connected to the discharge pipe 28 of the unit 132 and a discharge pipe 36 for the produced effluent sending this effluent into the wastewater supply pipe 1.

[0186] The treatment installations 100, 200, 300, 400, 500, 600 previously described may also comprise a process control system for implementing steps (i1), (i2) and (i3) of the control step. With reference to FIGS. 1 to 6, the control system comprises at least one device 29 for determining a content of ammonium and / or nitrate and / or nitrite ions, at least one fluid displacement device 30 and a control unit 31.

[0187] The control unit 31 is configured to implement:

[0188] - step (il) from the measurements received from the at least one determination device 29,

[0189] - step (i2) from the quantities determined in step (ii) and by controlling the at least one fluid displacement device 30,

[0190] - step (i3) from the quantities determined in step (ii) and by controlling the at least one fluid displacement device 30.

[0191] The at least one determination device 29 may be one or more sensors for the nitrogen content of the effluents. The at least one device 29 may be installed at the level of the discharge pipe 5 of the third effluent or the discharge pipe 24 of the fourth effluent and at the level of at least one pipe chosen from the first discharge pipe 2 of the first effluent having a reduced carbon content, the discharge pipe 17 of the effluent leaving the partial electro-oxidation step and optionally the discharge pipe 22 of a portion of the effluent produced by the Anammox step b3).

[0192] The quantities measured by the at least one determination device 29 are sent to the control unit 31 which will calculate the quantity of fluid to be extracted through the bypass pipes and control the movement of this quantity. The control unit 31 may comprise a computer, or more generally at least one processor or any other type of digital calculator. The control unit 31 may also comprise a plurality of separate processors or digital calculators, forming different means of the device, cooperating with each other.

[0193] The at least one fluid displacement device 30 may comprise one or more valves, for example a three- or four-way valve, the third way of which leads to the bypass pipe 13 and optionally to the bypass pipe 19. The at least one device 30 may also comprise one or more volumetric pumps (or any pump associated with a frequency converter and a flow meter to adjust the flow rate) capable of taking a calculated quantity of fluid to be extracted. The bypass pipes connected to the valves also form displacement devices within the meaning of the invention.

[0194] With reference to figures 4 and 5, for controlling the recycling of the effluent from the first electro-oxidation treatment unit, the control system comprises a device 29 for determining a content of ammonium and / or nitrate and / or nitrite ions, a fluid displacement device 32 and the control unit 31.

[0195] The determination device 29 may be a sensor for the nitrogen content of the effluents. It is here installed at the level of the discharge pipe 17 of the effluent leaving the partial electro-oxidation treatment unit 121.

[0196] The measured quantity is sent to the control unit 31 which will calculate the quantity of fluid to be recycled corresponding to a nitrate and / or nitrite content necessary for an anoxic biological treatment to eliminate carbonaceous matter. The control unit 31 also controls the movement of the quantity of fluid to be recycled.

[0197] The fluid displacement device 32 may be a three-way or four-way valve, one of the ways of which leads to the recirculation pipe 20. The device 32 may also be a volumetric pump (or any pump associated with a frequency converter and a flow meter) capable of taking the calculated quantity to be recycled.

[0198] The different embodiments presented with reference to figures 1 to 6 can be combined depending on the treatment objective decided.

Claims

Claims) A process for treating wastewater containing nitrogen in the form of ammonium ions and carbonaceous matter, said process comprising: (a) a wastewater treatment step to produce a first effluent with a reduced carbonaceous matter content and a second effluent with an increased carbonaceous matter content, (b) a treatment step of at least part of the first effluent to produce a third effluent with a reduced nitrogen content, and, (c) an anaerobic digestion step of the second effluent to produce biogas and digestate; said process being characterized in that treatment step (b) is carried out without the implementation of biological nitrification under aerobic conditions and comprises at least one electro-oxidation step during which at least a portion of the ammonium ions contained in the first effluent are oxidized to nitrites and / or nitrates, and / or to dinitrogen. ) Wastewater treatment process according to claim 1, characterized in that treatment step (a) comprises at least one carbonaceous matter treatment step selected from a physical treatment step (a1), optionally preceded by a physico-chemical treatment step (a2), and a biological carbonaceous matter treatment step (a3, a4).) Wastewater treatment process according to claim 2, characterized in that the physical treatment step (a1a) is selected from a settling step, a flotation step, and a filtration step, and the physicochemical treatment step (a2) is selected from a coagulation-flocculation step, a flocculation-only step, and an electrocoagulation-flocculation step. ) Wastewater treatment process according to any one of claims 1 to 3, characterized in that step (b) comprises a step of total electro-oxidation of at least a portion of the ammonium ions to dinitrogen. ) Wastewater treatment process according to any one of claims 1 to 3, characterized in that step (b) comprises a step of partial electro-oxidation (b1) of at least a portion of the ammonium ions to nitrates and / or nitrites. ) Wastewater treatment process according to claim 5, characterized in that step (b) comprises: (i) the partial electro-oxidation step (bl) in which a portion of the ammonium ions are oxidized to nitrates and / or nitrites, followed by an anoxic biological treatment step (b3) by oxidation of the ammonium ions by anaerobic autotrophic bacteria, or (ii) the partial electro-oxidation step (bl) in which at least part of the ammonium ions are oxidized to nitrates and / or nitrites, followed by a total electro-oxidation step (b2) of at least part of the nitrates and / or nitrites to dinitrogen, or (iii) the partial electro-oxidation step (bl) in which part of the ammonium ions are oxidized to nitrates and / or nitrites, followed by an anoxic biological treatment step (b3) by oxidation of ammonium ions by anaerobic autotrophic bacteria and then a total electro-oxidation step (b2) of at least part of the nitrates and / or nitrites to dinitrogen. 7) Wastewater treatment process according to claim 5 or 6 characterized in that at least a part of the effluent produced by said partial electro-oxidation step (bl) or by the anoxic biological treatment step (b3) is sent to the treatment step (a), upstream or in an anoxic biological treatment step (a3) ​​of the carbonaceous matter of the step (a). 8) A wastewater treatment process according to any one of claims 1 to 7, characterized in that during the electro-oxidation step (b), (bl) or (b2), an electrolysis of water present in the effluent occurs resulting in the production of dihydrogen at the cathode and dioxygen at the anode and in that the dihydrogen and / or dioxygen are recovered, and optionally the dioxygen is sent to the treatment step (a), upstream or in a biological treatment step (a3) ​​of the carbonaceous matter of the step (a). 9) A wastewater treatment process according to any one of claims 1 to 8, characterized in that it further comprises a treatment step (d) of the third effluent produced by the treatment step (b) to produce a fourth effluent, this treatment step (d) comprising at least one treatment selected from a treatment for the removal of suspended matter, a treatment for the removal of phosphorus compounds, a treatment for the removal of micropollutants, a treatment for the removal of microorganisms. 10) A wastewater treatment process according to any one of claims 1 to 9, characterized in that it further comprises a control step in which: (11) a quantity of nitrogen present in the third or fourth effluent and in at least one effluent to be extracted selected from the first effluent, the effluent from the partial electro-oxidation step (bl) of ammonium ions and the effluent from the anoxic biological treatment step (b3) by oxidation of ammonium ions by anaerobic autotrophic bacteria, is determined (12) a quantity of at least one effluent to be extracted is determined to achieve a limit in nitrogen content in the third or fourth effluent, and said quantity of at least one effluent to be extracted is extracted and mixed with the third or fourth effluent, and / or (13) a quantity of the effluent from the partial electro-oxidation step (bl) of ammonium ions is determined to be sent to an anoxic biological treatment step (a3) ​​of step (a), this quantity corresponding to a nitrate content and / or nitrites necessary for an anoxic biological treatment for the removal of carbonaceous matter, and said quantity of this effluent is sent to the anoxic biological treatment step (a3) ​​of step (a). ) Wastewater treatment process according to any one of claims 1 to 10, characterized in that it further comprises at least one treatment step (e) of at least a part of a liquid fraction of the digestate produced by the digestion step (c), this treatment step being selected from an electrocoagulation treatment step (eO), an electro-oxidation treatment step (el) during which at least a part of the ammonium ions contained in said liquid fraction are oxidized to nitrites and / or nitrates, and / or to dinitrogen, an anoxic biological treatment step (e2) by oxidation of ammonium ions by anaerobic autotrophic bacteria and the succession of the two steps (el) (e2) preceded or not by step (eO).) Wastewater treatment process according to claim 11, characterized in that the electrocoagulation treatment step (eO) comprises a substep of struvite precipitation by electrochemical dissolution of a sacrificial anode comprising magnesium, coupled to a substep of separation of the precipitated struvite. ) Wastewater treatment plant (100, 200, 300, 400, 500, 600) containing nitrogen in the form of ammonium ions and carbonaceous matter, comprising:. - a first wastewater treatment unit (110) comprising a wastewater supply line (1), a first discharge line (2) for a first effluent with a reduced carbonaceous matter content and a second discharge line (3) for a second effluent with an increased carbonaceous matter content, - a second treatment unit (120), comprising a supply line (4) connected to the first line (2) of the first treatment unit (110) and a discharge line (5) for a third effluent having a reduced nitrogen content, the second treatment unit comprising at least one electro-oxidation treatment reaction zone and being devoid of an aerobic biological treatment reaction zone, - a third anaerobic digestion treatment unit (130) comprising a feed line (6) connected to the second line (3) of the first treatment unit (110), a biogas discharge line (7) and a digestate discharge line (8). ) Treatment installation according to claim 13 characterized in that the first unit comprises at least one reaction zone selected from a physical treatment reaction zone, optionally coupled to a physico-chemical treatment reaction zone, and a biological treatment reaction zone. ) Processing installation according to claim 13 or 14, characterized in that the second processing unit comprises: - at least one first reaction zone for electro-oxidation treatment and at least one second reaction zone chosen from an electro-oxidation treatment reaction zone and a non-aerated biological treatment reaction zone, each second reaction zone being connected to a first reaction zone by a discharge pipe for the effluent exiting the first reaction zone, or - at least one first electro-oxidation treatment reaction zone, at least one second non-aerated biological treatment reaction zone, and at least one third electro-oxidation treatment reaction zone, each second reaction zone being connected to a first reaction zone by a discharge line for the effluent produced by the first reaction zone, each third reaction zone being connected to a second reaction zone by a discharge line for the effluent exiting the second reaction zone. ) Treatment installation according to claim 15, characterized in that it comprises a recirculation line connecting an outlet of at least one first electro-oxidation treatment reaction zone or of at least one second non-aerated biological treatment reaction zone to an inlet of a biological treatment reaction zone of the first unit.) Processing installation according to any one of claims 13 to 16, characterized in that it comprises at least one other processing unit selected from:. - a fourth treatment unit (124) comprising a supply line connected to a discharge line (5) of the second treatment unit and a discharge line for a fourth effluent, and comprising at least one reaction zone selected from a reaction zone for the removal of suspended solids, a reaction zone for the removal of phosphorus compounds, a reaction zone for the removal of micropollutants, a reaction zone for the removal of microorganisms, - a fifth treatment unit (131) comprising a feed line connected to a discharge line (8') of a liquid fraction of a digestate from the third unit and a discharge line (36) of an effluent, optionally connected to the feed line (1) of the first unit (110), and comprising at least one reaction zone selected from among an electrocoagulation treatment reaction zone, an electro-oxidation treatment reaction zone, a non-aerated biological treatment reaction zone by oxidation of ammonium ions by anaerobic autotrophic bacteria, and these last two reaction zones preceded or not by a treatment reaction zone by electrocoagulation, an outlet of the electro-oxidation treatment reaction zone being connected to an inlet of the non-aerated biological treatment reaction zone. ) Treatment installation according to any one of claims 13 to 17, characterized in that it is equipped with a control system comprising at least one device for determining the content of ammonium and / or nitrate and / or nitrite ions, at least one fluid displacement device, and a control unit configured to: - determine the quantity of nitrogen present in the third or fourth effluent and in at least one effluent to be extracted chosen from the first effluent, the effluent from at least one first electro-oxidation treatment reaction zone and the effluent from at least one non-aerated biological treatment reaction zone of the first unit, then - determine a quantity of at least one effluent to be extracted to achieve a limit in nitrogen content in the third or fourth effluent and order at least one fluid displacement device to extract said quantity of the at least one effluent to be extracted and mix it with the third or fourth effluent, and / or - determine a quantity of the effluent from at least one first electro-oxidation treatment reaction zone to be sent to a non-aerated biological treatment reaction zone of the first treatment unit, this quantity corresponding to a nitrate and / or nitrite content necessary for a non-aerated biological treatment of carbonaceous matter removal and order at least one fluid displacement device in order to send said quantity of this effluent to said non-aerated biological treatment reaction zone of the first unit (HO).

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