Method and facility for treating an organic effluent for the production of biogas

Abstract

The invention relates to a method for treating an organic effluent comprising a solid fraction and a liquid fraction, the method comprising the following steps: - digesting the organic effluent in a main anaerobic digester (6) at a temperature of between 5°C and 35°C to produce biogas, a main effluent and a main digestate; - degassing the main effluent in a main degassing module (10) to extract biogas present in dissolved form in the main effluent and produce a degassed main effluent; - biologically treating the degassed main effluent in a biological treatment unit (12) to produce a biological sludge; and - digesting the biological sludge in a secondary anaerobic digester (26) at a temperature of between 20°C and 65°C to produce biogas, a secondary effluent and a secondary digestate.

Description

Process and installation for the treatment of an organic effluent for the production of biogas

[0001] The invention relates to the treatment of an organic effluent. More specifically, the invention relates to a process and an installation for treating an organic effluent for the production of biogas.

[0002] In the water treatment sector, anaerobic digestion is typically reserved for larger plants, generally serving over 30,000 population equivalents (PE), due to the low return on investment. This is explained by the significant capital expenditures, also known as CAPEX, required for the construction of the biogas plants that equip such facilities. Furthermore, these installations typically lead to modifications in the main treatment process, more commonly referred to as "mainstream," by favoring the installation of primary settling tanks. Indeed, sludge from primary treatment is much more biodegradable and methanogenic than biological sludge.However, these modifications not only entail additional investment expenditures, but also an increase in operating costs, also known as OPEX, due to the more complex operation of these stations: different types of sludge to manage, odor production, higher risks related to the presence of hydrogen sulfide (H2S), to name just a few examples.

[0003] On the other hand, anaerobic digestion on the main, or mainstream, effluent is typically dependent on the temperature of the effluent and is generally reserved for hot or tropical climates.

[0004] In addition to complicating the treatment process, sludge treatment by methanation does not utilize the soluble carbon in the effluent arriving at the plant. However, approximately 30% of the readily assimilable carbon, commonly referred to as Chemical Oxygen Demand (COD), is in soluble form. It is also more than 80% biodegradable, whereas the particulate COD found in primary sludge is only 50 to 60% biodegradable.

[0005] Furthermore, due to Henry's Law, some of the biogas is present in dissolved form in the effluent leaving the digester and remains in this form at the digester outlet. This portion of biogas thus remains in the effluent and is ultimately released into the atmosphere, representing a loss of potential.

[0006] Furthermore, mainstream anaerobic digestion depletes the effluent of readily assimilable carbon. Carbon is an essential compound used for biological denitrification of the effluent, which is necessary to convert some of the nitrates in the effluent into nitrogen to comply with the NGL10 standard (as it stands at the date of this application). Consequently, performing anaerobic digestion under such conditions would require the addition of carbon to the installation specifically for denitrification. This carbon represents an additional cost for implementing the process, which is best avoided.

[0007] The invention aims in particular to remedy these problems by proposing a process and a sludge treatment installation which makes it possible to increase the quantity of biogas produced per quantity of effluent.

[0008] To this end, the invention provides a process for treating an organic effluent comprising a solid fraction and a liquid fraction, the process implementing the following steps:

[0009] - Digestion of organic effluent in a main anaerobic digester at a temperature between 5°C and 35°C to produce biogas, a main effluent and a main digestate,

[0010] - Degassing of the main effluent in a main degassing module to extract biogas present in dissolved form in the main effluent and produce a degassed main effluent,

[0011] - biological treatment of the main degassed effluent in a biological treatment unit to produce biological sludge, and

[0012] - digestion of biological sludge in a secondary anaerobic digester at a temperature between 20°C and 65°C to produce biogas, a secondary effluent and a secondary digestate.

[0013] Thus, anaerobic digestion is combined with degassing of the main effluent, allowing for the recovery of some of the biogas remaining in dissolved form after anaerobic digestion in the main digester. It is further combined with secondary anaerobic digestion performed on the main effluent from the main digestion process after degassing. This secondary anaerobic digestion, which can be described as "sidestream" (a term that can be translated here as secondary feeding), is carried out with an effluent flow rate that is significantly lower than that of the main digestion process. It is therefore possible to perform the secondary digestion at a higher temperature since heating the effluent at this stage is less energy-intensive, given that the effluent flow rate in the sidestream is lower than in the main digester. Consequently, it is possible to produce more biogas during this secondary digestion.It is thus understood that the degassing of the main effluent and secondary anaerobic digestion make it possible to increase the production of biogas for the same quantity of organic effluent involved in the implementation of the process, regardless of the value of the mainstream flow rate.

[0014] Advantageously, an additional step is implemented after the biological treatment step, of thickening the biological sludge to produce a thickened biological sludge.

[0015] This reduces the mass of biological sludge entering the secondary anaerobic digester, which in turn reduces the digester's footprint and therefore its manufacturing cost.

[0016] Advantageously, an additional degassing step of the secondary digestate is implemented in a secondary degassing module to extract biogas present in dissolved form in the secondary digestate and produce a degassed secondary digestate.

[0017] This further increases biogas production for the same quantity of effluent involved in implementing the process.

[0018] Advantageously, part of the organic effluent bypasses the main anaerobic digester and the main degassing module to be fed into the biological treatment unit.

[0019] This makes readily available carbon (COD), contained in the organic effluent, available for biological treatment, for example, to denitrify the effluent and the main digestate. Consequently, the need for a carbon feedstock specifically dedicated to biological treatment is avoided. In other words, an additional operating cost for the process is prevented.

[0020] Advantageously, an additional dehydration step is implemented on the degassed secondary digestate to produce a "cake" and a liquid, the latter being fed into the main anaerobic digester. "Cake" is an Anglo-Saxon term that can be translated into French as "gâteau".

[0021] A recirculation loop is thus implemented, returning the secondary digestate to the main anaerobic digester in order to extract even more biogas from this secondary digestate. This increases the biogas production yield of the process.

[0022] Advantageously, the biogas produced in the main anaerobic digester, and / or the main degassing module and / or the secondary anaerobic digester is treated in a gas scrubber to extract hydrogen sulfide, the extracted hydrogen sulfide being introduced into the biological treatment unit in the form of hydrogen sulfide.

[0023] Hydrogen sulfide (H2S) is recovered in the form of hydrogen sulfide (HS) - which is available for supply to the biological treatment unit. Hydrogen sulfide is used as an electron donor to carry out biological dephosphatation and denitrification as described in application FR2992639A1, and / or biological nitrification using hydrogen sulfide as an electron donor instead of oxygen as described in application WO2008132296A3. This reduces the amount of consumables required to carry out the biological treatment.

[0024] Advantageously, the biological treatment of the degassed main effluent includes a degassed main effluent clarification step.

[0025] This clarification step allows for the separation of some of the water from the biomass contained in the main degassed effluent, thereby reducing the flow rate of the main degassed effluent while increasing the biomass concentration in said effluent. This ensures that the sidestream effluent flow rate is lower than the mainstream effluent flow rate.

[0026] The invention also provides for an installation for the treatment of an organic effluent comprising a solid fraction and a liquid fraction, the installation comprising:

[0027] - a pipe supplying organic effluent,

[0028] - a main anaerobic digester, configured to digest an effluent at a temperature between 5°C and 35°C, comprising a main digester inlet pipe connected to the organic effluent feed pipe, a main effluent outlet pipe and a main biogas outlet pipe,

[0029] - a main degassing module comprising a main degassing inlet pipe connected to the main effluent outlet pipe, a main degassed biogas outlet pipe and a main degassed effluent outlet pipe,

[0030] - a biological treatment unit comprising a biological treatment inlet pipe, connected to the degassed effluent outlet pipe, and a biological treatment outlet pipe, and

[0031] - a secondary anaerobic digester, configured to digest an effluent at a temperature between 20°C and 65°C, comprising a secondary digester inlet pipe connected to the biological treatment outlet pipe, a secondary digestate outlet pipe and a secondary biogas outlet pipe.

[0032] Advantageously, the installation further includes a thickening module comprising a thickening inlet pipe connected to the biological treatment outlet pipe and a thickening outlet pipe connected to the secondary digester inlet pipe. In other words, the thickening module is located between the biological treatment unit and the secondary anaerobic digester, considering the flow of biological sludge exiting the biological treatment unit.

[0033] Advantageously, the installation further includes a secondary degassing module comprising a secondary degassing inlet pipe connected to the secondary digestate outlet pipe, a secondary degassed biogas outlet pipe and a secondary degassed digestate outlet pipe.

[0034] Advantageously, the installation also includes a bypass line connecting the organic effluent supply line to the biological treatment inlet line.

[0035] Advantageously, the installation further includes a dewatering module comprising a dewatering inlet line, connected to the secondary digestate outlet line, a cake outlet line and a liquid outlet line connected to the main digester inlet line.

[0036] Advantageously, the installation further includes at least one gas scrubber connected, on the one hand, to the main biogas outlet line and / or to the main degassed biogas outlet line and / or to the secondary biogas outlet line and, on the other hand, to the biological treatment unit.

[0037] Advantageously, the biological treatment unit includes a clarification unit. Brief description of the figures

[0038] The invention will be better understood upon reading the following description, given solely by way of example and made with reference to the accompanying drawings in which:

[0039] is a schematic view of an organic effluent treatment plant according to an embodiment of the invention,

[0040] is a histogram illustrating the gross biogas production of several organic effluent treatment facilities,

[0041] is a histogram illustrating the net biogas production of several organic effluent treatment facilities,

[0042] is a graph illustrating the evolution of oxygen consumption at the installation as a function of the use of a bypass pipe, and

[0043] is a graph illustrating the evolution of the volume of sludge produced by the installation of laen as a function of the use of its bypass pipe. Detailed description

[0044] A treatment plant for organic effluent 2 is shown according to a first embodiment of the invention. The effluent comprises a solid fraction and a liquid fraction; for example, sewage sludge from an urban and / or industrial wastewater treatment plant, to which other various organic wastes may be added for co-digestion. The plant 2 includes an organic effluent supply line 4 for conveying the effluent into the plant 2 for treatment.

[0045] Installation 2 comprises a main anaerobic digester 6, configured to digest effluent at a temperature between 5°C and 35°C, including a main digester inlet pipe 6a connected to the organic effluent feed pipe 4. The main anaerobic digester 6 can be continuously fed with the dewatered effluent. The main digester 6 produces biogas by allowing the effluent to remain in it under anaerobic conditions for a duration that can be set according to the installation 2 and the effluent. The operating principle of anaerobic digestion being known, it will not be described further below. The main digester 6 includes a main effluent outlet pipe 6b, a main digestate outlet pipe (not shown), and a main biogas outlet pipe 6c.The main effluent outlet 6b allows the main effluent to be discharged continuously from the main digester 6, so that the effluent remains in the digester for an average residence time which is a function of the organic effluent feed rate into the main digester 6. The main biogas outlet 6c allows the biogas produced by anaerobic digestion to be discharged from the main anaerobic digester 6 for storage for later use or subsequent transport outside the installation 2. Here, this biogas is at least partially conveyed to a biogas treatment line 8.

[0046] Installation 2 includes a main degassing module 10 comprising a main degassing inlet line 10a connected to the main effluent outlet line 6b, a main degassed biogas outlet line 10c, and a main degassed effluent outlet line 10b. The main degassing module 10 is fed with the main effluent from the main anaerobic digester 6 for degassing. This degassing process transforms some of the biogas present in dissolved form, typically methane, into a gaseous form that can be easily recovered. The main degassing module 10 includes a vacuum device operating with an agitator and a gaseous vent. The degassed main effluent is discharged from the main degassing module 10 via the main degassed effluent line 10b.The biogas from the degassing of the main digestate is evacuated from the main degassing module 10 by the main degassed biogas outlet line 10c to be at least partly conveyed into the biogas treatment line 8.

[0047] Installation 2 includes a biological treatment unit 12 comprising a biological treatment inlet line 12a, connected to the main degassed effluent outlet line 10b. The biological treatment unit 12 is fed with the main degassed effluent from the main degassing module 10 in order to perform biological treatment of the main degassed effluent. To this end, the biological treatment unit 12 includes an anaerobic dephosphatation unit 14, an anoxic denitrification unit 16, a nitrification and decarbonation unit 18, and a clarification unit 20. These four units are arranged in series, and the nitrification and decarbonation unit 18 includes a nitrate recirculation loop 18a connected to the anoxic denitrification unit 16. The operation of these four units being known per se, it will not be described further below.The biological treatment of the main degassed effluent in the biological treatment unit 12 produces a biological sludge, which exits the biological treatment unit 12 through a biological treatment outlet line 12b.

[0048] Installation 2 includes a recirculation loop 21 connecting the biological treatment outlet line 12b to the main digester inlet line 6a. This recirculation loop 21 allows biological sludge to be returned to the main anaerobic digester 6 in the event that the production of biological sludge is too high for further treatment in installation 2.

[0049] Installation 2 includes a bypass line 22 connecting the organic effluent feed line 4 to the biological treatment inlet line 12a. The bypass line 22 allows a quantity of organic effluent to bypass both the main anaerobic digester 6 and the main degassing module 10. In this way, the readily assimilable carbon contained in the effluent is not transformed or extracted as biogas. This carbon is used to feed the biological treatment unit 12 for biological treatment and the production of biosludge. The ratio of the effluent flow rate through the bypass line 22 to the total flow rate of the effluent feed line 4 varies according to the carbon requirements of the biological treatment unit 12. This variation can be continuous or occur at regular intervals.

[0050] Installation 2 includes a thickening module 24 comprising a thickening inlet pipe 24a connected to the biological treatment outlet pipe 12b. The thickening module 24 thickens the biological sludge by enriching its solid fraction, thus generating thickened biological sludge with a higher dry solids content than the biological sludge entering through the thickening inlet pipe 24a. The thickening module 24 is, for example, a device comprising a screw press, a belt filter, a centrifuge, a filter press, or a piston press. The thickened biological sludge exits the thickening module 24 through a thickening outlet pipe 24b, while the liquid separated from the biological sludge exits the thickening module 24 through the liquid outlet pipe (not shown).The liquid separated from the biological sludge is discharged from installation 2 for specific treatment, for example in a wastewater treatment unit.

[0051] Installation 2 includes a secondary anaerobic digester 26, configured to digest effluent at a temperature between 20°C and 65°C, comprising a secondary digester inlet line 26a connected to the thickening outlet line 24b of the thickening module 24. The secondary anaerobic digester 26 can be continuously fed with thickened biosludge. The secondary digester 26 produces biogas by allowing the thickened biosludge to remain in it under anaerobic conditions for a duration that can be set according to the requirements of Installation 2 and the thickened biosludge. The secondary digester 26 includes a secondary digestate outlet line 26b and a secondary biogas outlet line 26c. The secondary digestate outlet line 26b allows the thickened biosludge to be discharged from the secondary anaerobic digester 26 once it has reached its programmed retention time.The secondary biogas outlet line 26c allows the biogas produced by the anaerobic digestion of the secondary anaerobic digester 26 to be removed for storage for later use or further transport outside the installation 2. Here, this biogas is at least partly conveyed into the biogas treatment line 8.

[0052] In order for the secondary anaerobic digester 26 to operate at a temperature between 20°C and 65°C, the thickened biosludge is heated until it reaches this temperature. This heating is achieved by consuming a portion of the biogas produced during anaerobic digestion in the secondary anaerobic digester 26. As an alternative embodiment, the thickened biosludge in the secondary anaerobic digester 26 can be heated using heating equipment specifically designed for this purpose.

[0053] Installation 2 includes a secondary degassing module 28 comprising a secondary degassing inlet line 28a connected to the secondary digestate outlet line 26b, a secondary degassed biogas outlet line 28c, and a secondary degassed digestate outlet line 28b. The secondary degassing module 28 is fed with secondary digestate from the secondary anaerobic digester 26 for degassing. This degassing process transforms some of the biogas present in dissolved form, typically methane, into a gaseous form that can be easily recovered. The secondary degassing module 28 includes a vacuum device operating with an agitator and a gaseous vent. The degassed secondary digestate is discharged from the secondary degassing module 28 via the secondary degassed digestate line 28b.The biogas from the degassing of the secondary digestate is evacuated from the secondary degassing module 28 via the secondary degassed biogas outlet line 28c to be at least partially conveyed into the biogas treatment line 8.

[0054] Installation 2 includes a dewatering module 30 comprising a dewatering inlet line 30a connected to the degassed secondary digestate line 28b. The dewatering module 30 dewaters, or dries, the degassed secondary digestate to produce a cake with a higher dry solids content than the degassed secondary digestate entering through the dewatering inlet line 30a. The dewatering module 30 is, for example, a device comprising a screw press, a belt filter, a centrifuge, a filter press, or a piston press. The cake exits the dewatering module 30 through a cake outlet line 30b, while the liquid separated from the degassed secondary digestate exits the dewatering module 30 through a liquid outlet line 30c.The liquid separated from the degassed secondary digestate is recycled into the main digester inlet line 6a and there is mixed with the organic effluent from the organic effluent feed line 4.

[0055] Installation 2 comprises a main gas scrubber 32 connected, on the one hand, to the main biogas outlet line 6c and the main degassed biogas outlet line 10c and, on the other hand, to the biological treatment unit 12. The biogas is scrubbed in the main gas scrubber 32 to extract hydrogen sulfide (H₂S). The extracted hydrogen sulfide is introduced in the form of hydrogen sulfide (HS₂). - , in the biological treatment unit 12 for the needs of nitrification. The biogas depleted in hydrogen sulfide exits the main gas scrubber 32 to join the biogas treatment line 8.

[0056] Installation 2 comprises a secondary gas scrubber 34 connected, on one side, to the secondary biogas outlet line 26c and, on the other side, to the biological treatment unit 12. The biogas is scrubbed in the secondary gas scrubber 34 to extract hydrogen sulfide (H₂S). The extracted hydrogen sulfide is introduced in the form of hydrogen sulfide (HS₂). - , in the biological treatment unit 12 for the purposes of nitrification. The biogas depleted in hydrogen sulfide exits the secondary gas scrubber 34 to join the biogas treatment line 8. The main gas scrubber 32 and secondary gas scrubber 34 include, for example, scrubbing towers.

[0057] A histogram illustrating the gross biogas production of several organic effluent treatment plants is shown. This histogram does not take into account the heating requirements for heating the effluent inside the secondary anaerobic digester operating at a temperature between 20°C and 65°C. From left to right, the first two columns correspond to a prior art installation with a single anaerobic digester, respectively with (first column) and without (second column) the use of a primary clarifier located upstream of the anaerobic digester. The third column corresponds to installation 2 according to the invention in which 75% of the organic effluent from the organic effluent feed line 4 passes through the bypass line 22 (and 25% of the organic effluent is fed into the main anaerobic digester 6).The fourth column corresponds to installation 2 according to the invention, in which 50% of the organic effluent from the organic effluent feed line 4 passes through the bypass line 22 (and 50% of the organic effluent is fed into the main anaerobic digester 6). Within the third and fourth columns, the lower part represents the gross biogas production by the secondary anaerobic digester 26, and the upper part represents the gross biogas production by the main anaerobic digester 6. Production is expressed in MWh-t / year on the left-hand y-axis and as a relative increase compared to the second column on the right-hand y-axis.

[0058] We observe that with 50% of the effluent passing through the bypass pipe 22 according to the invention, the installation 2 produces more biogas than the prior art installations, with an increase of about 30% compared to the first column.

[0059] A histogram illustrating the net biogas production of several organic effluent treatment plants is shown. This histogram takes into account the biogas consumed to heat the effluent inside the secondary anaerobic digester operating at a temperature between 20°C and 65°C. From left to right, the first two columns correspond to a prior art installation with a single anaerobic digester, respectively with (first column) and without (second column) the use of a primary clarifier located upstream of the anaerobic digester. The third column corresponds to installation 2 according to the invention, in which 75% of the organic effluent from the organic effluent feed line 4 passes through the bypass line 22 (and 25% of the organic effluent is fed into the main anaerobic digester 6).The fourth column corresponds to installation 2 according to the invention, in which 50% of the organic effluent from the organic effluent feed line 4 passes through the bypass line 22 (and 50% of the organic effluent is fed into the main anaerobic digester 6). Within the third and fourth columns, the lower part represents the net biogas production by the secondary anaerobic digester 26, and the upper part represents the net biogas production by the main anaerobic digester 6. Production is expressed in MWh-t / year on the left-hand y-axis and as a relative increase compared to the second column on the right-hand y-axis.

[0060] It is observed that from 25% of the effluent fed into the main anaerobic digester 6, the total net biogas production of the installation 2 is higher than that of prior art installations, with an increase of approximately 68% (compared to the first column) when 50% of the effluent is fed into the main anaerobic digester 6. Furthermore, from 25% of the effluent fed into the main anaerobic digester 6, the latter produces more biogas than the secondary anaerobic digester 26 within the installation 2.

[0061] A graph illustrating the evolution of oxygen consumption in plant 2 as a function of the use of bypass pipe 22 is shown, expressed here as a percentage of effluent fed into the main anaerobic digester 6. Oxygen consumption represents one of the most significant cost items in the operation of an organic effluent treatment plant; therefore, it is advantageous to reduce this consumption as much as possible. In the graph, consumption is expressed in kg / day on the left-hand y-axis (bottom curve) and as a percentage relative to a reference on the right-hand y-axis (top curve).

[0062] It is observed that oxygen consumption is reduced by 10% as soon as 25% of the effluent from the organic effluent feed line 4 passes through the main anaerobic digester 6, and that oxygen consumption is reduced by 20% as soon as 50% of the effluent from the organic effluent feed line 4 passes through the main anaerobic digester 6. The invention thus makes it possible to reduce the oxygen consumption of the installation 2 and therefore its operating cost.

[0063] A graph has been shown illustrating the evolution of the mass of sludge produced by installation 2 as a function of the use of the bypass pipe 22, here expressed as a percentage of effluent fed into the main anaerobic digester 6. In general, a reduction in the mass of sludge is advantageous since it makes it easier to move this sludge and to reduce the sizing of certain elements of installation 2, for example the thickening module 24. On the graph, the mass of sludge is expressed in kg / day on the left ordinate axis (bottom curve) and as a percentage relative to a reference on the right ordinate axis (top curve).

[0064] It is observed that the volume of sludge is reduced by 15% as soon as 25% of the effluent from the organic effluent feed line 4 passes through the main anaerobic digester 6, and that the volume of sludge is reduced by 24% as soon as 50% of the effluent from the organic effluent feed line 4 passes through the main anaerobic digester 6. The invention thus makes it possible to reduce the volume of sludge produced by the installation 2.

[0065] The invention is not limited to the embodiments shown and other embodiments will be obvious to a person skilled in the art.

[0066] It is possible to add iron to the biological effluent or to the inlet of the clarification unit.

[0067] Secondary digestion can be preceded by thermal hydrolysis, or it can be carried out in two phases, in which case it is commonly referred to as "advanced digestion." Furthermore, secondary digestion can be performed in a single stage, in a multi-stage process, or with pre / post thermal treatment. List of references

[0068] 2: Effluent treatment plant 4: Organic effluent feed line 6: Main anaerobic digester 6a: Main digester inlet line 6b: Main effluent outlet line 6c: Main biogas outlet line 8: Biogas treatment line 10: Main degassing module 10a: Main degassing inlet line 10b: Main degassed effluent outlet line 10c: Main degassed biogas outlet line 12: Biological treatment unit 12a: Biological treatment inlet line 12b: Biological treatment outlet line 14: Anaerobic dephosphatation unit 16: Anoxic denitrification unit 18: Nitrification and decarbonation unit 18a: Nitrate recirculation loop 20: Clarification unit 21: Recirculation loop 22: Bypass line 24: Thickening module 24a: Thickening inlet line 24b: Outlet line thickening26: anaerobic digestersecondary 26a: digester inlet pipe secondary 26b: digestate outlet pipe secondary 26c: biogas outlet pipe

[0069] 28: Secondary degassing module 28a: Secondary degassing inlet pipe 28b: Secondary degassed digestate outlet pipe 28c: Secondary degassed biogas outlet pipe 30: Dewatering module 30a: Dewatering inlet pipe 30b: Cake outlet pipe 30c: Liquid outlet pipe 32: Main gas scrubber 34: Secondary gas scrubber

Claims

A process for treating an organic effluent comprising a solid fraction and a liquid fraction, characterized in that it carries out the following steps: - digestion of the organic effluent in a main anaerobic digester (6) at a temperature between 5°C and 35°C to produce biogas, a main effluent and a main digestate, - degassing of the main effluent in a main degassing module (10) to extract biogas present in dissolved form in the main effluent and produce a degassed main effluent, - biological treatment of the degassed main effluent in a biological treatment unit (12) to produce a biological sludge, and - digestion of the biological sludge in a secondary anaerobic digester (26) at a temperature between 20°C and 65°C to produce biogas, a secondary effluent and a secondary digestate. A process according to the preceding claim, implementing an additional step, after the biological treatment step, of thickening the biological sludge to produce a thickened biological sludge. A method according to any one of the preceding claims, implementing an additional step of degassing the secondary digestate in a secondary degassing module (28) to extract biogas present in dissolved form in the secondary digestate and produce a degassed secondary digestate. A process according to any one of the preceding claims, wherein a portion of the organic effluent bypasses the main anaerobic digester (6) and the main degassing module (10) to be fed into the biological treatment unit (12). A process according to any one of the preceding claims, comprising an additional step of dehydrating the degassed secondary digestate to produce a cake and a liquid, the latter being fed into the main anaerobic digester (6). A process according to any one of the preceding claims, wherein the biogas produced in the main anaerobic digester (6), and / or the main degassing module (10) and / or the secondary anaerobic digester (26) is treated in a gas scrubber (32, 34) to extract hydrogen sulfide, the extracted hydrogen sulfide being introduced into the biological treatment unit (12) in the form of hydrogen sulfide. A process according to any one of the preceding claims, wherein the biological treatment of the degassed main effluent includes a step of clarifying the degassed main effluent. An organic effluent treatment plant (2) comprising a solid fraction and a liquid fraction, characterized in that it comprises: - an organic effluent feed line (4), - a main anaerobic digester (6), configured to digest an effluent at a temperature between 5°C and 35°C, comprising a main digester inlet line (6a) connected to the organic effluent feed line (4), a main effluent outlet line (6b) and a main biogas outlet line (6c), - a main degassing module (10) comprising a main degassing inlet line (10a) connected to the main effluent outlet line (6b), a main degassed biogas outlet line (10c) and a main degassed effluent outlet line (10b), - a biological treatment unit (12) comprising a biological treatment inlet line (12a), connected to the degassed effluent outlet line (10b),and a biological treatment outlet line (12b), and a secondary anaerobic digester (26), configured to digest an effluent at a temperature between 20°C and 65°C, comprising a secondary digester inlet line (26a) connected to the biological treatment outlet line (12b), a secondary digestate outlet line (26b) and a secondary biogas outlet line (26c). Installation (2) according to claim 8, further comprising a thickening module (24) comprising a thickening inlet pipe (24a) connected to the biological treatment outlet pipe (12b) and a thickening outlet pipe (24b) connected to the secondary digester inlet pipe (26a). Installation (2) according to claim 8 or 9, further comprising a secondary degassing module (28) comprising a secondary degassing inlet line (28a) connected to the secondary digestate outlet line (26b), a secondary degassed biogas outlet line (28c) and a secondary degassed digestate outlet line (28b). Installation (2) according to any one of claims 8 to 10, further comprising a bypass line (22) connecting the organic effluent supply line (4) to the biological treatment inlet line (12a). Installation (2) according to any one of claims 8 to 11, further comprising a dehydration module (30) comprising a dehydration inlet line (30a), connected to the secondary digestate outlet line (26b), a cake outlet line (30b) and a liquid outlet line (30c) connected to the main digester inlet line (6a). Installation (2) according to any one of claims 8 to 12, further comprising at least one gas scrubber (32, 34) connected, on the one hand, to the main biogas outlet line (6c) and / or to the main degassed biogas outlet line (10c) and / or to the secondary biogas outlet line (26c) and, on the other hand, to the biological treatment unit (12). Installation (2) according to any one of claims 8 to 13, wherein the biological treatment unit (12) comprises a clarification unit (20).

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