Process and installation for treating an organic effluent

Abstract

In this method for treating an organic effluent comprising a solid fraction and a liquid fraction, the following steps are implemented: - dehydration of the effluent in a dehydration module (6) to produce a dehydrated effluent having a dryness greater than or equal to 10%, - digestion of the dehydrated effluent in an anaerobic digester of the dry process type (10) to produce biogas and a digestate, and - drying of the digestate to produce a cake having a dryness greater than or equal to 50%. Figure for the abstract: figure 1

Description

Title of the invention: Method and installation for treating an organic effluent

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

[0002] One way of recovering sewage sludge consists of the production of biogas by anaerobic digestion.

[0003] In the field of wastewater treatment, anaerobic digestion is generally carried out in a liquid process since the effluent often has a very low dryness. Anaerobic digestion in a liquid process makes continuous homogenization possible, promoting digestion and the production of biogas.

[0004] This is one of the main reasons why liquid digestion is more widely used than dry digestion for the treatment of municipal sludge, which poses rheological constraints on the homogenization of the effluent due to its density and viscosity.

[0005] After anaerobic digestion in the liquid process, the digestate is removed from the digester and then subjected to dehydration with the addition of flocculants which make it possible to produce sludge with increased dryness. The sludge thus concentrated is then transported out of the installation for spreading or incineration.

[0006] This installation and the process it allows to be implemented are interesting but nevertheless have several drawbacks. On the one hand, the dehydration of the digestate as presented above does not make it possible to obtain sludge with a dryness as high as would be desired. Therefore, the sludge leaving the installation remains relatively bulky and dense, which complicates its movement to its spreading or incineration site. On the other hand, the flocculants used during dehydration constitute a significant cost in the operation of the installation and the implementation of the effluent treatment process. In addition, the flocculants remain present in the concentrated sludge conveyed from the installation and are not degraded. Therefore, in the case where this concentrated sludge is spread for example for agricultural use, the flocculants are transmitted to the environment and thus contaminate the soil.Even with these drawbacks, it is not easy to reduce the amount of flocculants used because such a reduction would reduce the dryness of the sludge produced, which must be avoided.

[0007] The invention aims in particular to remedy these problems by proposing a method and a sludge treatment installation which make it possible to increase the dryness of the sludge leaving the installation after treatment while reducing the quantity of flocculants used.

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

[0009] - dehydration of the effluent in a dehydration module to produce a dehydrated effluent with a dryness greater than or equal to 10%,

[0010] - digestion of the dehydrated effluent in an anaerobic digester of the dry process type to produce biogas and digestate, and

[0011] - drying the digestate to produce a cake having a dryness greater than or equal to at 50%.

[0012] Thus, mechanical dehydration of the effluent is carried out before the anaerobic digestion step. This dehydration requires fewer flocculants than during dehydration carried out downstream of the digestion due to the required dryness level which is much lower, so that the consumption of flocculants for the implementation of the effluent treatment is reduced. In addition, the drying carried out at the outlet of the anaerobic digester makes it possible to greatly increase the dryness of the digestate and to form particularly compact sludge which does not contain flocculant. This makes it easier to move them outside the installation, which represents a saving on the operating costs of the installation and an improvement in their agronomic properties.The invention thus makes it possible to solve the aforementioned technical problem, in particular by implementing dry anaerobic digestion, which allows dehydration to be carried out upstream of the anaerobic digestion. In other words, the invention could only see the light of day after overcoming the prejudices concerning dry anaerobic digestion, which has the disadvantages presented above. Furthermore, the dehydration carried out upstream of the anaerobic digester makes it possible to reduce the volume of the digester and thus its footprint.

[0013] Advantageously, the effluent dehydrated in the dehydration module has a dryness greater than or equal to 15%.

[0014] Such dryness can be achieved with a moderate quantity of flocculants and makes it possible to accelerate the digestate drying step and / or make it less energy-intensive.

[0015] Advantageously, the dehydration module also produces a liquid output, the liquid output is routed to a wastewater treatment unit.

[0016] It is thus possible to recover the liquid obtained during the implementation of the dehydration of the effluent.

[0017] According to a first embodiment of the invention, the digestate is treated in a hydrothermal carbonization unit.

[0018] Preferably, the digestate is subjected in the hydrothermal carbonization unit, in the presence of water vapor, to a temperature between 170°C and 210°C and to a pressure between 12 and 20 bar. These conditions allow the solubilization of the organic matter and the obtaining of a hydrophobic matrix.

[0019] The drying of the digestate is thus carried out using a controlled and rapid process to implement.

[0020] Preferably, after the step of drying the digestate in the hydrothermal carbonization unit, the digestate is dehydrated in an auxiliary dehydration module.

[0021] Hydrothermal carbonization reduces viscosity and liquefies the sludge and dilutes it due to the addition of water vapor. In other words, hydrothermal carbonization improves the dehydratability of the sludge, and thus forms a thermal conditioning that makes it possible to achieve higher drynesses than those that can be obtained with more conventional conditioning using flocculation with the addition of flocculant. Auxiliary dehydration makes it possible to increase the final dryness of the sludge beyond the desired threshold, the threshold being set here at 50%. The combination of hydrothermal carbonization and auxiliary dehydration thus makes it possible to dry the sludge to more than 50% dryness.

[0022] According to a second embodiment of the invention, the digestate is dried in a solar drying unit or on drying beds.

[0023] The drying of the digestate is thus carried out using particularly economical means. This drying method is particularly suitable if the process is implemented in an area with a temperate, hot or arid climate.

[0024] According to a third embodiment of the invention, the digestate is dried in a biodrying unit.

[0025] The drying of the digestate is thus accompanied by the production of biofuel, which makes it possible to optimize the recovery of the effluent.

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

[0027] - an effluent supply line,

[0028] - a dehydration module comprising an inlet pipe supplied by the effluent supply line and a dehydrated effluent outlet line, the dehydration module being configured to produce a dehydrated effluent having a dryness greater than or equal to 10%,

[0029] - an anaerobic digester of the dry process type comprising an inlet pipe digester connected to the dehydrated effluent outlet pipe, a digestate outlet pipe and a biogas outlet pipe, and

[0030] - drying means configured to dry the digestate to produce a cake having a dryness greater than or equal to 50%.

[0031] Advantageously, the dehydration module is configured to produce a dehydrated effluent having a dryness greater than or equal to 15%.

[0032] Advantageously, the dehydration module further comprises a liquid outlet pipe, the dehydration module being configured to produce a liquid outlet.

[0033] According to a first embodiment of the invention, the drying means comprise a hydrothermal carbonization unit.

[0034] Preferably, the drying means further comprise an auxiliary dehydration module arranged downstream of the hydrothermal carbonization unit.

[0035] According to a second embodiment of the invention, the drying means comprise a solar drying unit or drying beds.

[0036] According to a third embodiment of the invention, the drying means comprise a biodrying unit. Brief description of the figures

[0037] The invention will be better understood on reading the following description given solely by way of example and with reference to the appended drawings in which:

[0038] [Fig.l] is a schematic view of an installation for treating an organic effluent according to a first embodiment of the invention,

[0039] [Fig-2] is a schematic view of an effluent treatment plant organic according to a second embodiment of the invention, and

[0040] [Fig.3] is a schematic view of an installation for treating an organic effluent according to a third embodiment of the invention. Detailed description

[0041] [Fig.l] shows an installation for treating an organic effluent 2 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 sewage treatment plant, to which other various organic wastes may be added in order to carry out co-digestion. The installation 2 comprises an effluent supply pipe 4 for conveying the effluent into the installation 2 for treatment.

[0042] The installation 2 comprises a dehydration module 6 comprising an inlet pipe 6a supplied by the effluent supply pipe 4, a liquid outlet pipe 6b and a dehydrated effluent outlet pipe 6c. The dehydration module 6 makes it possible to dehydrate, or dry, the effluent so as to generate a dehydrated effluent having a dryness greater than that of the effluent entering via the inlet pipe 6a, and for this purpose involves the consumption of flocculants. The dehydration module 6 is for example a device comprising a screw press, a filter belt press, a centrifuge, a filter press or a piston press. Here, the dryness of the dehydrated effluent is greater than or equal to 10%, preferably greater than or equal to 15%. The dehydrated effluent leaves the dehydration module 6 via the dehydrated effluent outlet pipe 6c, while the liquid separated from the effluent leaves the dehydration module 6 via the liquid outlet pipe 6b. The liquid separated from the effluent is discharged from the installation 2 to undergo specific treatment, for example in a wastewater treatment unit 8.

[0043] The installation 2 comprises an anaerobic digester 10, of the dry process type, comprising a digester inlet pipe 10a connected to the dehydrated effluent outlet pipe 6c of the dehydration module 6. The anaerobic digester 10 can be fed with the dehydrated effluent either continuously or in batch. The anaerobic digester 10 makes it possible to produce biogas by allowing the dehydrated effluent to remain there under anaerobic conditions for a period which can be configured according to the installation 2 and the effluent. Since the operating principle of anaerobic digestion is known per se, it will not be described further in the following. The anaerobic digester 10 comprises a digestate outlet pipe 10b, and a biogas outlet pipe 10c. The digestate outlet line 10b allows the dehydrated effluent to be discharged from the anaerobic digester 10 once it has reached its programmed residence time.The biogas outlet line 10c allows the biogas produced by anaerobic digestion to be discharged from the anaerobic digester 10 in order to store it for later use or further transport outside the plant 2. Here, the biogas is stored in a biogas tank 12.

[0044] The installation 2 comprises drying means 14, comprising a drying inlet pipe 14a connected to the digestate outlet pipe 10b, configured to dry the digestate to produce a cake having an increased dryness. In the present embodiment, the drying means 14 comprise a hydrothermal carbonization unit 16 supplied with the digestate coming from the anaerobic digester 10 by means of the drying inlet pipe 14a. The digestate is subjected therein to hydrothermal carbonization to produce biochar, this method being known per se, so that it will not be described further in the following. Preferably, the digestate is subjected in the hydrothermal carbonization unit 16, in the presence of water vapor, to a temperature between 170°C and 210°C and to a pressure between 12 and 20 bar.

[0045] The drying means 14 further comprise an auxiliary dehydration module 18 connected downstream of the hydrothermal carbonization unit 16. The auxiliary dehydration module 18 makes it possible to dehydrate, or dry, the digestate after it has been subjected to hydrothermal carbonization so as to generate a concentrated sludge, or cake, having a dryness greater than or equal to 50%. The module auxiliary dewatering module 18 operates purely mechanically and does not consume any flocculant. This is, for example, a device comprising a screw press, a belt filter, a centrifuge, a filter press or a piston press. The concentrated sludge produced by the auxiliary dewatering module 18 is conveyed out of the installation 2, via a discharge pipe 18a, for subsequent use, for example for spreading or incineration. The discharge pipe 18a thus forms a drying outlet pipe 14b allowing the concentrated sludge to exit the drying means 14 and the installation 2. The liquid extracted from the digestate by the auxiliary dewatering module 18 exits it via an auxiliary outlet pipe 18b. The latter is here connected to the wastewater treatment unit 8.

[0046] Optionally, the drying means 14 comprise a stripping module 20, also called an evaporator, connected to the auxiliary outlet pipe 18b. The stripping module 20 is configured to evaporate gases present in dissolved form in the liquid extracted from the sludge by the auxiliary dehydration module 18. This may be biogas, in which case the extracted biogas is stored in the biogas storage tank 12, and / or nutrients, in which case the extracted nutrients are stored in a tank provided for this purpose, and / or other substances whose stripping makes it possible to reduce the organic pollution in the liquid. The stripping module 20 thus makes it possible to optimize the recovery of the effluent.

[0047] Alternatively, the drying means 14 comprise an upflow anaerobic digester, commonly referred to as an UASB reactor, this acronym designating the English term “Upflow Anaerobic Sludge Blanket”, in place of the stripping module 20. The UASB reactor 21 is connected to the auxiliary outlet pipe 18b. The liquid extracted from the digestate by the auxiliary dehydration module 18 is loaded with easily assimilated carbon. Therefore, digestion of the liquid extracted from the digestate by the auxiliary dehydration module 18 in the UASB reactor 21 makes it possible to produce biogas. This can be stored in the biogas storage tank 12 or consumed within the installation 2 to produce the energy necessary to supply the hydrothermal carbonization unit 16.

[0048] The following table shows an example of operating data for installation 2.

[0049] [Tables 1] Effluent characteristics Unit Primary sludge Biological sludge Flow rate kg / day 6000 4000 Volatile matter (VM) concentration % 70% 75% COD / VM (if known) Ratio 1.75 1.5 Effluent temperature °C 15 Effluent concentration g / L 60 55

[0050] In this first table, COD corresponds to the quantity of easily assimilated carbon contained in the effluent. For these operating data, the following table shows the total consumption of flocculants for three types of treatment installations.

[0051] [Tables2] Type of installation Digestion Flow rate sent to dehydration (kgD S / day) Use of flocculants (kgAM / tD S) Dosage of flocculants (kg AM / day) A (dehydration by dissolved air flotation and gravity dehydration) Dry process 10000 (thickening); 10000 (dehydration) 0 (thickening); 6 (dehydration before digestion) 60 B (dehydration by rapid decantation) Dry process 10000 (thickening); 10000 (dehydration) 2 (thickening); 6 (dehydration before digestion) 80 C (installation of the prior art) Liquid process of 10000 (thickening); 6314 (digestion) 4 (thickening); 8 (dehydration after digestion) 91

[0052] In this second table, installation C corresponds to the state-of-the-art installation such as that presented in the preamble. Installations A and B correspond to installations according to the invention which differ in the method of dehydration of the effluent. In installation A, the effluent is dehydrated with means that consume little flocculants. In installation B, the effluent is dehydrated with means that consume more flocculants but which have the advantage of dehydrating the effluent more quickly, namely a rapid decantation dehydration module.

[0053] It can be seen from reading this second table that installation C represents a consumption of flocculants (for the same quantity of effluent) increased by 51% and 13% respectively compared to installations A and B. These results thus show the unexpected and counter-intuitive effect of reducing the consumption of flocculants permitted by the invention. This effect is counter-intuitive because it could be considered that carrying out the dehydration upstream of the digestion, that is to say almost at the inlet of the installation, would lead to an overconsumption of flocculant, since the effluent at this stage has a very low dryness. This consideration constitutes a prejudice which would dissuade the person skilled in the art, a priori, from considering dry digestion and would thus distance him from the invention.

[0054] [Fig.2] shows an installation for treating an organic effluent 2' according to a second embodiment of the invention. The elements and devices of the installation of this second embodiment identical to the elements and devices of the installation according to the first embodiment bear identical numerical references. In the following, only the elements and devices of the installation 2' which differentiate it from the installation according to the first embodiment previously described will be described.

[0055] In the installation 2', the drying means 14 comprise a drying unit solar or drying beds 22 receiving the digestate from the digester 10 via the drying inlet pipe 14a. In these drying means 14, the digestate is dried passively, i.e. without consuming electrical energy dedicated to drying the digestate. It is thus the wind and / or the sun which allow the digestate to dry. This drying method is particularly suitable if the installation 2' is located in an area with a temperate, hot or arid climate. The digestate is dried until it reaches a dryness greater than or equal to 50%, after which the sludge obtained is discharged from the installation 2' via the drying outlet pipe 14b.

[0056] [Fig. 3] shows an installation for treating an organic effluent 2'' according to a third embodiment of the invention. The elements and devices of the installation of this third embodiment identical to the elements and devices of the installations according to the first and second embodiments bear identical numerical references. In the following, only the elements and devices of the installation 2” which differentiate it from the installations according to the first and second embodiments previously described will be described.

[0057] In the installation 2”, the drying means 14 comprise a biodrying unit 24 receiving the digestate from the digester 10 via the drying inlet pipe 14a. Biodrying is a technology known per se, therefore it will not be described further in the following. The digestate is dried until it reaches a dryness greater than or equal to 50%, after which the sludge obtained is discharged from the installation 2' via the drying outlet pipe 14b.

[0058] The invention is not limited to the embodiments presented and other embodiments will become clear to those skilled in the art. In particular, it is possible to combine drying means from several of the embodiments as described above. List of references

[0059] 2: effluent treatment plant 4: effluent supply line 6: dehydration module 6a: inlet pipe 6b: liquid outlet line 6c: dehydrated effluent outlet pipe 8: wastewater treatment unit 10: anaerobic digester 10a: digester inlet pipe 10b: digestate outlet pipe 10c: biogas outlet pipe 12: biogas tank 14: drying means 14a: drying inlet pipe 14b: drying outlet pipe 16: hydrothermal carbonization unit 18: auxiliary dehydration module 18a: drain pipe 18b: auxiliary outlet line 20: stripping module

[0060] 21: UASB reactor 22: solar drying unit or drying beds 24: biodrying unit

Claims

Claims

1. Method for treating an organic effluent comprising a solid fraction and a liquid fraction, characterized in that it implements the following steps: - dehydration of the effluent in a dehydration module (6) to produce a dehydrated effluent having a dryness greater than or equal to 10%, - digestion of the dehydrated effluent in an anaerobic digester of the dry process type (10) to produce biogas and a digestate, and - drying of the digestate to produce a cake having a dryness greater than or equal to 50%.

2. Method according to the preceding claim, in which the effluent dehydrated in the dehydration module (6) has a dryness greater than or equal to 15%.

3. A method according to any preceding claim, wherein, the dehydration module (6) further produces a liquid output, the liquid output is conveyed to a wastewater treatment unit (8).

4. A method according to any one of claims 1 to 3, wherein the digestate is treated in a hydrothermal carbonization unit (16).

5. Method according to the preceding claim, in which after the step of treating the digestate in the hydrothermal carbonization unit (16), the digestate is dehydrated in an auxiliary dehydration module (18).

6. A method according to any one of claims 1 to 3, wherein the digestate is dried in a solar drying unit or on drying beds (22).

7. A method according to any one of claims 1 to 3, wherein the digestate is dried in a biodrying unit (24).

8. Installation for treating an organic effluent (2; 2'; 2”) comprising a solid fraction and a liquid fraction, characterized in that it comprises: - an effluent supply pipe (4), - a dehydration module (6) comprising an inlet pipe (6a) supplied by the effluent supply pipe (4) and a dehydrated effluent outlet pipe (6c), the module dehydration means (6) being configured to produce a dehydrated effluent having a dryness greater than or equal to 10%, - an anaerobic digester of the dry process type (10) comprising a digester inlet pipe (10a) connected to the dehydrated effluent outlet pipe (6c), a digestate outlet pipe (10b) and a biogas outlet pipe (10c), and - drying means (14) configured to dry the digestate to produce a cake having a dryness greater than or equal to 50%.

9. Installation (2; 2'; 2”) according to claim 8, wherein the dehydration module (6) further comprises a liquid outlet conduit (6b), the dehydration module (6) being configured to produce a liquid outlet.

10. Plant (2) according to claim 8 or 9, wherein the drying means (14) comprise a hydrothermal carbonization unit (16).

11. Installation (2) according to the preceding claim, in which the drying means (14) further comprise an auxiliary dehydration module (18) arranged downstream of the hydrothermal carbonization unit (16).

12. Installation (2') according to claim 8 or 9, in which the drying means (14) comprise a solar drying unit or drying beds (22).

13. Installation (2”) according to claim 8 or 9, in which the drying means (14) comprise a biodrying unit (24).

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