Method for treating wastewater
Patent Information
- Application Number
- EP2023702868
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-02-09
- Filing Date
- 2023-02-07
- Publication Date
- 2024-12-18
AI Technical Summary
Current wastewater treatment processes face high energy consumption and inefficiencies, particularly in biological treatment stages, due to imbalanced carbon/nitrogen ratios and complex setups, which hinder effective nitrogen removal and overall performance.
A wastewater treatment process involving decantation with coagulants/flocculants, followed by simultaneous nitrification and denitrification with integrated clarification in a single biological basin, and anaerobic digestion, reducing energy consumption by minimizing dissolved oxygen levels and eliminating the need for recirculation pumping, while optimizing sludge volume and energy production.
This approach significantly reduces energy consumption, enhances biogas production, and simplifies the treatment process, making it cost-effective and efficient for treating urban wastewater with improved carbon/nitrogen balance and reduced environmental impact.
Smart Images
Figure 1.1
Abstract
Description
[0001] Description
[0002] Title: Wastewater treatment process
[0003] Technical field
[0004] The present invention relates to methods for treating wastewater.
[0005] Prior art
[0006] Water is oligotrophic by nature. Wastewater is no longer oligotrophic; it is loaded with elements that precipitate the harmful phenomenon of eutrophication of natural environments. It is therefore important to treat wastewater before discharging it into nature, in particular by physical, chemical, or biological treatment, or a combination of these. Wastewater treatment is carried out mainly by a biological treatment step. Microorganisms contained in biological sludge degrade soluble and particulate pollution, in particular carbon and nitrogen pollutants. The biological sludge is then separated from the liquid phase. The result is, on the one hand, treated water that can be discharged into the environment, or sent for tertiary treatment, and, on the other hand, biological sludge, a part of which, rich in microorganisms, is returned to the inlet of the biological basin and the other part of which is treated.There are two distinct processes in wastewater treatment: on the one hand, a wastewater treatment process and on the other, a process for treating the sludge from the wastewater treatment. Both processes require a significant amount of energy, particularly due to the aeration stage of the biological basin required for effective wastewater treatment in the wastewater treatment process and the dynamic thickening and dewatering stages of the sludge treatment process.
[0007] For the treatment of wastewater effluents, particularly urban effluents, an important area for improvement is therefore the reduction of energy consumption in the sectors, in order to reduce the energy impact of wastewater treatment.
[0008] It is known to integrate an anaerobic digestion step into the sludge treatment process, allowing the production of biogas directly on the plant site. The biogas can then be used for various purposes as an energy source. On-site reuse reduces the plant's energy impact. The quantity of biogas produced depends in particular on the amount of carbon pollution captured during an initial primary settling stage of the wastewater effluent. The latter can be pushed into the settling tank, in particular by the addition of coagulants and / or flocculants. However, excessive carbon capture during primary settling creates an imbalance in the carbon / nitrogen (C / N) ratio, which results in poor nitrogen removal during biological treatment, which negatively impacts overall treatment performance.Effluent with an unbalanced carbon / nitrogen (C / N) ratio cannot be properly treated by a standard biological treatment process.
[0009] Certain state-of-the-art biological treatment methods make it possible to treat such effluents.
[0010] Application WO2012019310 discloses a wastewater treatment process involving biological treatment with bacteria called Anammox, which are capable of removing pollutants from an effluent with a highly unbalanced C / N ratio in a low oxygenated environment, thereby significantly reducing pond aeration costs. However, the use of a specific population of bacteria such as Anammox bacteria makes this process relatively complex to implement.
[0011] Patent applications FR 2 643 065 and FR 2 516 071 describe biological treatment processes of the “activated sludge” type comprising two separate volumes or zones: one for nitrification under aerobic conditions and one for denitrification under anoxic conditions. Description of the invention
[0012] There is a need to further improve wastewater treatment processes, in particular to benefit from a process with a reduced or even zero overall energy impact, in particular a process that is easy to implement, allowing the biological treatment of any type of urban wastewater at low cost.
[0013] Summary of the invention
[0014] The invention aims to meet this need, and it achieves this, according to a first of its aspects, by means of a wastewater treatment process, comprising the steps of:
[0015] (a) settling of wastewater by adding coagulants and / or flocculants to the wastewater, delivering at the outlet a primary effluent and a primary sludge, the primary sludge comprising at least 50% of the suspended solids of the wastewater,
[0016] (b) main biological treatment of the “activated sludge” type of the primary effluent, comprising the implementation of a simultaneous nitrification and denitrification process with integrated clarification, delivering at the outlet a treated effluent and a biological sludge, (c) thickening of the primary sludge and the biological sludge delivering at the outlet a thickened mixed sludge and a thickening filtrate,
[0017] (d) anaerobic digestion of the thickened mixed sludge delivering biogas and digested sludge at the outlet,
[0018] (e) dehydration of the digested sludge delivering at the outlet a dehydrated sludge and a dehydration filtrate, and,
[0019] (f) biological treatment of “activated sludge” type returns from the dehydration filtrate, comprising the implementation of a simultaneous nitrification and denitrification process with integrated clarification delivering at the outlet a treated filtrate effluent and a biological filtrate sludge.
[0020] By "a process of simultaneous nitrification and denitrification" we mean that nitrification and denitrification, which usually take place separately, either by alternating phases of aeration and absence of aeration in the same basin, or in two different basins or even in two different zones of the same basin, take place here in the same basin and under substantially the same conditions, in particular the same concentration of dissolved oxygen.
[0021] By "integrated clarification", we mean that the clarification is included in a continuous loop in the hydraulic circulation time of the activated sludge type biological treatment, in particular without recovery by internal pumping of the sludge settled by the clarification.
[0022] Such a process makes it possible to reduce the energy consumption of the biological treatment by reducing the dissolved oxygen concentration in the biological basin through a simultaneous nitrification and denitrification process, and also thanks to an integrated clarification which makes it possible to do without a forced recirculation stage, in particular by pumping, of the biological sludge, which represents an energy saving. In addition to the energy savings achieved at the biological stage, it also makes it possible to reduce the overall energy consumption of the treatment plant by increasing the quantity of biogas produced in the digester by increasing the quantity of primary sludge.
[0023] Such a process makes it possible to optimize the volume of the works while having a simple, efficient treatment with reduced energy balance and cost.
[0024] The fact that the clarifier is integrated also reduces the footprint of the treatment system. Preferably, the primary sludge contains at least 50% of the suspended solids in the wastewater, better, at least 70%. Suspended solids are quantified according to the MES (suspended solids) parameter, according to the NFT-90-105 standard, or ISO 11923 or equivalent. This allows for an increase in the volume of primary sludge for digestion, which improves the overall energy balance of the process.
[0025] Preferably, the primary sludge contains at least 20%, and preferably at least 35%, of the BOD5 (5-day biochemical oxygen demand) of the wastewater. The BOD5 is assessed according to NFT-90-103 or ISO 5815-1 or equivalent.
[0026] Coagulants and / or flocculants can be organic or inorganic, synthetic or natural.
[0027] Preferably, the primary effluent has a C / N ratio (mass content of carbon relative to the mass content of nitrogen) less than or equal to 12, better still less than or equal to 10. The mass content of carbon is quantified by the COD, expressed in mg / L of oxygen (O2), and measured according to the NFT-90101 or ISO 15705 standard or equivalent. The mass content of nitrogen is quantified by the NGL (otherwise called Global Nitrogen), expressed in mg / L of nitrogen, the sum of the contents of the following nitrogenous forms: nitrites (NO2), measured according to the NF EN ISO 10304-1 standard or equivalent, and nitrates (NOs'), measured according to the NF EN ISO 10304-1 standard or equivalent, Kjeldahl nitrogen (NTK), itself corresponding to the sum of ammoniacal nitrogen (NH3 / NH4 +) and organic nitrogen contained in the water, both measured according to standard NF-EN-25663 or ISO 5663 or equivalent. Such a ratio is characteristic of an imbalance between carbon and nitrogen which reduces the performance of the primary effluent treatment for a usual biological treatment with separate nitrification and denitrification and separate clarification.
[0028] Preferably, the primary biological treatment takes place in a single biological basin comprising an aeration zone and a clarification zone between which a mixed liquor circulates in a continuous circulation loop without phase interruption, the primary effluent being introduced into the mixed liquor circulating between the clarification zone and the effluent aeration zone.
[0029] Preferably, the main biological treatment takes place without internal pumping of mixed liquor or biological sludge between the clarification zone and the aeration zone.
[0030] Alternatively, the primary biological treatment may take place with internal pumping of the mixed liquor or biological sludge between the clarification zone and the aeration zone, including internal pumping operating at a flow rate substantially lower than the primary effluent inflow, preferably operating at a flow rate less than 50% of the primary effluent inflow.
[0031] Preferably, the concentration of the biological sludge in the clarification zone is substantially homogeneous throughout said zone.
[0032] Preferably, the concentration of the biological sludge in the mixed liquor is homogeneous and substantially identical in the aeration zone and in the clarification zone.
[0033] The main biological treatment involves continuous aeration of the mixed liquor in the aeration zone.
[0034] Continuous aeration may be achieved by a floor aeration system covering at least a portion of the bottom of the aeration zone, the aeration system comprising a network of air diffusers for aerating the mixed liquor present in the aeration zone. The air diffusers may be distributed uniformly over at least a portion of the bottom of the biological pond, in particular in regularly spaced lines.
[0035] Preferably, the biological basin comprises the following successive zones: a primary effluent inlet zone, the aeration zone, in particular comprising the aeration system, a degassing zone, and the clarification zone, and the mixed liquor is circulated in said successive zones in a hydraulic loop, without phase interruption.
[0036] Preferably, the concentration of biological sludge (total suspended solids (MLSS)) during the main biological treatment, in particular in the mixed liquor present in the basin, is greater than or equal to 6,000 mg / L, better between 6,000 and 8,000 mg / L.
[0037] Preferably, the average dissolved oxygen concentration in the mixed liquor contained in the aeration zone is less than 0.5 mgCL / L, better between 0.2 and 0.4 mgCL / L, even better between 0.3 and 0.4 mgCL / L. Such a dissolved oxygen concentration, lower than in systems with separate nitrification and denitrification, makes it possible to reduce the energy requirement of the process.
[0038] Preferably, the clarification is lamellar. The clarification zone may comprise a lamellar clarifier. Preferably, the process does not require a prior culture step for the microorganisms.
[0039] Preferably, the main biological treatment is devoid of the addition of a specific bacterial population, and in particular the main biological treatment is devoid of Anammox-type bacteria.
[0040] Preferably, the main biological treatment is carried out without any particular temperature regulation.
[0041] Preferably, the primary biological treatment is performed in a biological treatment device as described in US patent application 2017 / 0096354 incorporated herein by reference.
[0042] Preferably, the thickening of biological and primary sludge is carried out in a sludge tank.
[0043] Preferably, the digested sludge comprises at least 4% dry solids by mass.
[0044] Preferably, the digestion step (d) comprises a heat exchange circuit between the digested sludge and the thickened mixed sludge. This makes it possible to reduce the energy consumption of the assembly by using the heat from the digested sludge to heat the incoming thickened sludge.
[0045] Preferably, the biological treatment of the returns comprises one or more of the characteristics described above in relation to the main biological treatment.
[0046] Preferably, the biological treatment of returns is identical to the main biological treatment.
[0047] The filtrate leaving the dehydration can be sent to the input of the main biological treatment, the biological treatment of the returns and the main biological treatment then taking place simultaneously in the same biological basin.
[0048] Alternatively, the biological treatment of the returns is separate from the main biological treatment, in particular takes place in a biological basin different from that of the main biological treatment. The biological treatment of the returns can deliver at the outlet a liquid product which is sent to the inlet of the main biological treatment. The biological treatment of the returns can deliver at the outlet a biological sludge of the filtrate which is mixed with the biological sludge resulting from the main biological treatment. Preferably, the method comprises a step of energy recovery of at least a portion of the biogas comprising the heating of the digestion sludge and / or at least one energy conversion process, in particular chosen from: i) The purification of the biogas into biomethane for its use as biofuel or for injection into the natural gas network, ii) cogeneration, iii) the drying of the dehydrated sludge, and / or iv) the incineration of the dehydrated sludge.
[0049] Preferably, the effluent treated at the outlet of the process complies with European Directive No. 91-271 and Water Framework Directive No. 2000-60, with the French decree of July 21, 2015 relating to collective sanitation systems and non-collective sanitation installations, with the exception of non-collective sanitation installations receiving a gross organic pollution load of less than or equal to 1.2 kg / d of BOD5, which makes it suitable for discharge into the natural environment without additional treatment.
[0050] Brief description of the drawings
[0051] The invention may be better understood by reading the detailed description which follows, non-limiting examples of its implementation, and by examining the appended drawings, in which:
[0052] [Fig 1] Figure 1 is a block diagram representing an exemplary wastewater treatment process according to the invention,
[0053] [Fig 2] Figure 2 is a partial and schematic top view of an example of a biological basin according to the invention,
[0054] [Fig 3] Figure 3 illustrates in a partial and schematic way in section the aeration system in operation of Figure 2,
[0055] [Fig 4] Figure 4 is a partial and schematic sectional view of an example of a lamellar clarifier integrated into the biological basin of Figure 3,
[0056] [Fig 5] Figure 5 is a block diagram representing a variant of the wastewater treatment process according to the invention,
[0057] [Fig 6] Figure 6 is a block diagram representing a variant of the wastewater treatment process according to the invention,
[0058] [Fig 7] Figure 7 is a block diagram representing the biogas energy recovery stage of the wastewater treatment according to the invention. Detailed description
[0059] Figure 1 illustrates an example of a method 1 for treating wastewater according to the invention. In the example considered, the method 1 comprises a primary settling step 10 of the wastewater E, delivering at the outlet a primary effluent 12 and a primary sludge 15 making it possible to extract a portion of the suspended solids from the wastewater E. The primary effluent 12 and the primary sludge 15 obtained are then treated separately as illustrated. The primary effluent 12 is biologically treated in the main biological treatment 20 delivering at the outlet a treated effluent 22 and a biological sludge 25. The primary sludge 15 and the biological sludge 25 are treated jointly by a sludge treatment 30 described below.
[0060] The main biological treatment 20 makes it possible in particular to eliminate nitrogen pollution and the rest of the carbon pollution from the effluent 12. Certain details of the main biological treatment 20 are described below and illustrated in Figures 2 to 4.
[0061] The primary settling 10 comprises the addition of reagents 105 promoting settling in a decanter 100 receiving the wastewater E. The reagents are coagulants and / or flocculants injected into the wastewater effluent E. The addition of such reagents 105 promotes the settling of the suspended matter in the wastewater E. The primary sludge 15 obtained by this primary settling step 10 contains more than 50%, better still more than 70%, of the suspended matter in the wastewater E. The primary effluent 12 obtained by this primary settling step 10 has a BOD5 reduced preferably by at least 20%, or even by at least 35% compared to that of the wastewater E. The BOD5 is for example measured according to the NET-90-103 standard, the ISO 5815-1 standard, or equivalent. The suspended solids (SS) captured are for example quantified according to the NET-90-105 standard, or ISO 11923, or equivalent.
[0062] The primary effluent 12 thus obtained has an unbalanced carbon / nitrogen ratio, in particular less than 12. The measurement of the mass carbon content of the primary effluent 12, represented by the COD, is carried out according to the NET-90101 standard, the ISO 15705 standard or equivalent. It is expressed in mg O2 / L. The mass content of nitrogen is quantified by the NGL (otherwise called Global Nitrogen), expressed in mg / L of nitrogen, the sum of the contents of the following nitrogen forms: Kjeldahl nitrogen (NTK) corresponding to the sum of ammoniacal nitrogen (NtL / NHC) and organic nitrogen contained in the water, measured according to standard NE-EN-25663 or ISO 5663 or equivalent, nitrites (NO2), measured according to standard NF EN ISO 10304-1 or equivalent, and nitrates (NOs'), measured according to standard NF EN ISO 10304-1 or equivalent.
[0063] The main biological treatment 20 is carried out within a biological basin 200, illustrated in FIG. 2. The biological basin 200 is preferably as described in patent application US 2017 / 0096354.It comprises: an effluent mixing zone 210 of the primary effluent 12 comprising an opening 205 connected to the decanter through which the primary effluent 12 enters, a first zone 220 for hydraulically speeding up the effluent on which the effluent mixing zone 210 has an opening 215, an aeration zone 230 where nitrification and denitrification of the primary effluent 12 takes place, separated from the first hydraulic speeding zone by a wall 225 at one of its ends, a first degassing zone 235 separated from the aeration zone 230 by a wall 225 at the other end of the aeration zone 230, a second degassing zone 240, open to the first degassing zone by an opening 237, and a clarification zone 250, in particular lamellar, making it possible to obtain the treated effluent 22 recoverable on the surface and the biological sludge 25 circulating at the bottom towards zone 210.
[0064] The portion of the circulating biological sludge 25, rich in bacteria, facilitates biological treatment of the effluent 12 by free culture, that is to say without the need to add a specific bacterial population or fixation media for them. It also makes it possible to reduce the carbon load of the inlet effluent, which facilitates treatment and allows the treatment of a primary effluent 12 having an unbalanced C / N ratio.
[0065] The main biological treatment 20 is of the “activated sludge” type.
[0066] In the biological basin 200, the mixed liquor, formed from the mixture of the primary effluent 12 and the biological sludge, circulates according to the arrows indicated in FIG. 2, passing successively through the inlet zone 210, the first hydraulic speed-up zone 220, the aeration zone 230, the first degassing zone 235, the second degassing zone 240 and the clarification zone 250 and again in the inlet zone 210. The circulation of the mixed liquor in the biological basin is maintained permanently throughout the basin 200, in particular by means of a system blowing air into the liquid in the zone 220. Preferably, the mixed liquor circulates at a speed such that the biological sludge is kept in suspension in the mixed liquor without precipitating.
[0067] Such a circulation speed makes it possible to maintain a low concentration of pollutants in the liquid. Indeed, the higher the circulation speed, the more the primary effluent 12 is diluted. This also improves the treatment of the primary effluent 12 by increasing the quantity of microorganisms mixed with it.
[0068] The aeration zone 230 comprises an aeration system 232 illustrated in FIG. 2. The latter extends into the bottom of the biological basin 200 and comprises longitudinal air diffusers 236, regularly spaced, forming in the aeration zone 230 an aeration mat 236 at the bottom of the basin. Such a system makes it possible to maintain substantially uniform oxygenation of the liquid throughout the aeration zone 230.
[0069] The diffusers 236 are configured to form columns of bubbles 237 of substantially identical sizes, as illustrated in FIG. 3.
[0070] The average dissolved oxygen (DO) concentration obtained in the aeration zone is between 0.2 and 0.4 mg / L. This low oxygenation rate has several advantages detailed below, including good bacterial growth.
[0071] Nitrification and denitrification of the primary effluent 12 take place within the aeration zone 230.
[0072] Conventional methods of biological wastewater treatment generally involve a nitrification phase, followed by a denitrification phase, either alternating over time or in different basins.
[0073] During the classic nitrification stage, the NELC ammonium present in the effluent is transformed in the presence of oxygen by enzymatic oxidation into nitrite NO2' (nitritation reaction) which is subsequently converted into nitrate NO3' (nitration reaction), under the influence of so-called autotrophic bacteria.
[0074] During the classic denitrification stage, nitrate NOi is reduced to nitrite NO2' which is subsequently eliminated by transformation into nitrogen N2, in the presence of carbon elements, under the influence of so-called heterotrophic bacteria.
[0075] In the process of the present invention, the low oxygenation of the basin creates an oxygen gradient in the flocs from the outside towards the center of the latter, the surface of the flocs being more oxygenated than the center, which allows nitrification on the surface and denitrification in the center of the flocs, these two reactions taking place simultaneously at the level of the same floc.
[0076] Preferably, the clarifier 250 is lamellar, which makes it possible to reduce the surface area required for settling.
[0077] The clarifier 250 comprises, for example, several layers of effluent / sludge separation cells, in particular, as illustrated in FIG. 4, a plurality of inclined parallel tubes 252 carried by inclined lamellae 254 held by an openwork support platform 256. This structure makes it possible to improve the sedimentation of the biological sludge 25. All of the sedimented biological sludge is kept in circulation in the basin by the hydraulic drive at the bottom of the clarifier. Elements 258 allow a homogeneous distribution of the sludge along the length of the clarification section.
[0078] The treated water is recovered from the surface of the clarifier by chutes 259.
[0079] The sizes of the lamellae 252 and tubes 254, the spacing between the lamellae 252 and tubes 254, and the angle of inclination are calculated according to known state-of-the-art physical laws to optimize the gravity discharge speed of the settled sludge, while avoiding accumulation of sludge.
[0080] A portion of the biological sludge 25 may be extracted from the biological basin 200 to be sent with the primary sludge for treatment.
[0081] As illustrated in Figure 1, the method comprises a step 70 of thickening the biological sludge 25 and the primary sludge 15 before digestion 40, in particular in a sludge tank 700, delivering at the outlet a thickened mixed sludge 72 and a thickening filtrate 75.
[0082] The method 1 comprises a step of anaerobic digestion 40 of the thickened mixed sludge 72 in an anaerobic digester 400, delivering at the outlet a digested sludge 42 and producing biogas 45.
[0083] The digested sludge 42 is subsequently dehydrated during a dehydration step 50 in a dehydration device 500 delivering at the outlet a dehydration filtrate 52 and a dehydrated sludge 55. The dehydration filtrate 52 is subjected to a biological return treatment step 60, in particular identical to the main biological treatment 20, in a biological basin 600 delivering at the outlet a treated filtrate effluent 62 and a biological filtrate sludge 65. The treated effluent from the biological return treatment 60 is directed to the main biological treatment line 20. The treated effluent 22 from the main biological treatment 20 can be returned to the natural environment, or sent to a tertiary treatment, for compliance with European Directive No. 91-271 and Water Framework Directive No. 2000-60.
[0084] The thickening filtrate 75 from step 70 can be returned with the wastewater effluent E to the inlet of the primary settling tank 10, which makes it possible to capture the suspended matter it contains. The thickened mixed sludge 72 delivered at the outlet of the thickening step 70 is sent to the anaerobic digester 400 as described previously.
[0085] Biological treatment of returns 60 can be implemented in different ways.
[0086] In an exemplary implementation, and as illustrated in Figure 5, the dehydration filtrate 52 is sent to the inlet of the biological basin 200 with the primary effluent 12, the biological treatment of the returns 60 then corresponds to the main biological treatment 20. It is for example added to the effluent 12 before it enters the basin 200.
[0087] In the variant illustrated in Figure 6, the dehydration filtrate 52 is treated during step 60 in a basin 600 different from the biological basin 200 of the effluent 12. The biological treatment of the returns can be substantially identical to that carried out in the basin 200, that is to say of the activated sludge type, and comprising, among other things, simultaneous nitrification and denitrification within a weakly aerated zone, and clarification integrated into the basin.
[0088] In this example, the biological treatment of the returns 60 delivers at the outlet a treated effluent 62, which is for example returned with the effluent 12 at the inlet of the main biological treatment 20. A biological sludge of the filtrate 65 is also delivered at the outlet, which can be mixed with the biological sludge 25 from the main biological treatment 20, as illustrated in figure 7.
[0089] The method 1 comprises for example a step 70 of energy recovery of the biogas 45 produced by the anaerobic digester in the sludge digestion step 40. As illustrated in FIG. 7, the biogas can be used to heat the thickened sludge 72 in step 81 during the anaerobic digestion 40. The biogas 45 can further, or as an alternative to heating the mixed sludge, assist in step 82 in the drying or incineration of the dehydrated sludge 55, and / or be used for numerous energy applications 84, for example to produce electricity 840, biomethane 842, biofuel 844, or any other type of application suitable for the use of biogas.
[0090] Of course, the invention is not limited to the examples which have just been described. For example, other methods may complement the addition of flocculants or coagulants during the primary settling 10 to increase the capture of suspended matter; for example, a lamellar clarifier or any other suitable device may be used.
[0091] The areas of the biological basin 200 may be of various shapes and sizes, and distributed differently than in the example described above.
[0092] The air diffusers 236 may be distributed in the biological basin 200 in any arrangement which allows uniform aeration of the zone 230.
Claims
Demands 1. A process (1) for the treatment of wastewater, comprising the steps of: (a) settling (10) of wastewater by adding coagulants and / or flocculants (105) to the wastewater, delivering at the outlet a primary effluent (12) and a primary sludge (15), the primary sludge (15) comprising at least 50% of the solids suspended in the wastewater, (b) main biological treatment (20) of the "activated sludge" type of the primary effluent (12), comprising the implementation of a simultaneous nitrification and denitrification process with integrated clarification, the clarification being included in a continuous loop in the time of hydraulic circulation of the biological treatment of the "activated sludge" type, delivering at the outlet a treated effluent (22) and a biological sludge (25), the nitrification and denitrification taking place in the same basin and under substantially the same conditions, (c) thickening (70) of the primary sludge (15) and the biological sludge (25) delivering at the outlet a thickened mixed sludge (72) and a thickening filtrate (75), (d) anaerobic digestion (40) of the thickened mixed sludge (72) delivering at the outlet biogas (45) and digested sludge (42), (e) dewatering (50) of the digested sludge (42) yielding at the outlet a dewatered sludge (55) and a dewatering filtrate (52), and, (f) biological treatment of the returns (60) of the "activated sludge" type of the dewatering filtrate (52), comprising the implementation of a simultaneous nitrification and denitrification process with integrated clarification, the clarification being included in a continuous loop in the time of hydraulic circulation of the biological treatment of the "activated sludge" type, delivering at the outlet a treated effluent of filtrate (62) and a biological sludge of filtrate (65), the nitrification and denitrification taking place in the same basin and under substantially the same conditions.
2. A process according to claim 1, wherein the primary sludge (12) comprises at least 50%, better, at least 70%, of the solids suspended in the wastewater and / or at least 20%, better, at least 35%, of the BOD5 of the wastewater.
3. A process according to any one of the preceding claims, wherein the primary effluent (15) has a C / N ratio less than or equal to 12, preferably less than or equal to 10.
4. A method according to any one of the preceding claims, wherein the main biological treatment (20) takes place in a single biological basin (200) comprising an aeration zone (230) and a clarification zone (250) between which a mixed liquor circulates in a continuous circulation loop without phase interruption, the primary effluent (12) being introduced into the mixed liquor circulating between the clarification zone and the effluent aeration zone.
5. A method according to the preceding claim, wherein the main biological treatment (20) takes place without internal pumping of the mixed liquor or biological sludge (25) between the clarification zone (250) and the aeration zone (230).
6. A method according to claim 4, wherein the main biological treatment (20) takes place with internal pumping of the mixed liquor or biological sludge (25) between the clarification zone (250) and the aeration zone (230), in particular internal pumping operating with a flow rate substantially lower than the incoming primary effluent flow rate, preferably operating with a flow rate less than 50% of the incoming primary effluent flow rate.
7. A method according to any one of claims 4 to 6, wherein the main biological treatment (20) comprises the continuous aeration of the mixed liquor in the aeration zone (230), in particular by a floor aeration system (232) covering at least a part of the bottom of the aeration zone (230) comprising an array of air diffusers (236) for aerating the mixed liquor present in the aeration zone (230).
8. A method according to any one of claims 4 to 7, wherein the average dissolved oxygen concentration in the mixed liquor contained in the aeration zone (230) is less than 0.5 mgC / L, better between 0.2 and 0.4 mgC / L, even better between 0.3 and 0.4 mgCWL.
9. A process according to any one of the preceding claims, wherein the biological sludge concentration during the main biological treatment (20) is greater than or equal to 6000 mg / L, preferably between 6000 and 8000 mg / L.
10. A method according to any one of the preceding claims, wherein the integrated clarification is lamellar.
11. A method according to any one of the preceding claims, wherein the biological treatment of the returns (60) is identical to the main biological treatment (20), in particular takes place in the same biological basin (200).
12. A process according to any one of the preceding claims, wherein the digestion filtrate (52) exiting the dehydration (50) is sent to the input of the main biological treatment (20).
13. A method according to any one of claims 1 to 10, wherein the biological treatment of the returns (60) is separate from the main biological treatment (20), in particular takes place in a biological basin (600) different from that of the main biological treatment (20).
14. A process according to any one of the preceding claims, comprising an energy recovery step of at least a portion of the biogas (80) including heating the digestion sludge (81) and / or at least one energy conversion process, in particular selected from: (i) The purification of biogas for use as biofuel or for injection into the natural gas network (844), (ii) Cogeneration (840), (iii) Drying of dewatered sludge (82), and / or (iv) Incineration of dewatered sludge (842).
15. A process according to any one of the preceding claims, wherein the treated effluent (22, 62) complies with European Directive No. 91-271 and Water Framework Directive No. 2000-60, with the French decree of 21 July 2015 relating to collective sanitation systems and non-collective sanitation installations, with the exception of non-collective sanitation installations receiving a raw organic pollution load less than or equal to 1.2 kg / day of BOD5, which makes it suitable for discharge into the natural environment without additional treatment.