WO2012104330A1

WO2012104330A1 - Plant and process for treating a liquid effluent comprising a methane fermentation, a biological treatment, a digestion of methane-fermented sludges and of biological sludges, and a methane fermentation of digested sludges

Info

Publication number: WO2012104330A1
Application number: PCT/EP2012/051636
Authority: WO
Inventors: Thierry Arnaud
Original assignee: Veolia Water Solutions & Technologies Support
Priority date: 2011-02-01
Filing date: 2012-02-01
Publication date: 2012-08-09
Also published as: CN103796959A; FR2970961B1; BR112013018301A2; FR2970961A1; AR085102A1; MX2013008738A; CN103796959B

Abstract

The invention relates to a process for treating liquid effluent, said process comprising a step of methane fermentation of said effluent within a methane fermenter (32) producing biogas, methane-fermented sludges and a methane-fermented effluent, a step of biological treatment of said methane-fermented effluent within a biological treatment zone (34) producing biological sludges and a treated effluent, a step of extraction of thickened biological sludges from said biological treatment zone (34), a step of anaerobic digestion of at least one portion of said methane-fermented sludges and of said thickened biological sludges within a digestor (44) producing biogas and digested sludges, a step of recirculating at least one portion of said digested sludges to said methane fermenter (21), and a step of extracting at least one portion of the digested sludges.

Description

SYSTEM AND METHOD FOR TREATING A LIQUID EFFLUENT COMPRISING METHANIZATION, BIOLOGICAL TREATMENT, DIGESTION OF METHANIZED SLUDGE AND BIOLOGICAL SLUDGE, AND METHANIZATION OF DIGESTED SLUDGE

1. Field of the invention

The field of the invention is that of the treatment of effluents with a view to their purification.

The invention more specifically finds application particularly in the field of treatment of urban or industrial wastewater effluents.

2. Prior art and its disadvantages

Urban or industrial effluents are conventionally treated for their purification before being discharged into the natural environment.

A technique commonly used to purify this type of effluent 10 consists, with reference to FIG. 1, in conveying it in pretreatment means 11. These pretreatment means 1 1 generally comprise screening means, desanding means, de-oiling means, and a UASB methanization reactor (for Upflow Anaerobic Sludge Blanket in English). The implementation of these pretreatment means 1 1 leads to the production of primary sludge 12, flared biogas and a pretreated effluent. The latter, depending on the type of upstream treatment, is routed to biological treatment means 13 comprising for example an aerobic activated sludge reactor, a biofilter, biodisks, lagoons ... The effluent from the biological treatment means 13 is introduced into a decanter 14. The implementation of the decanter 14 allows to produce a treated effluent 15 which can be routed to a subsequent treatment or rej éé in the natural environment. It also leads to the production of biological sludge. A first portion 16 of the biological sludge is recirculated upstream of the biological treatment means 1 3 to regulate sludge concentration and possibly in the UASB reactor. A second portion 17 biological sludge is directed with the primary sludge 12 to treatment means 18 whose implementation leads to thicken or dehydrate sludge. Sludge treated 19 are then generally oriented towards agronomic valorization sectors.

The implementation of a UASB technique is advantageous insofar as it allows cutting down, at low energy cost, about 50 to 80% of the COD contained in the effluent to be treated.

This technique nevertheless has some disadvantages.

The efficiency of such a technique, in terms of COD reduction, drops significantly when the temperature of the effluent to be treated is below 17 to 18 ° C. In addition, given the high flows of effluent to be treated, it is impossible, at least without incurring unreasonable energy expenditure, to heat them to bring them to a temperature at which the yield would be better. Therefore, the implementation of such a technique, although it allows in principle a satisfactory and low-cost removal of COD, is not possible in the regions of the world in which the temperature of treat is continuously or seasonally below 15 ° C. To maintain satisfactory yields at temperatures of less than 15 ° C, the hydraulic residence time of the effluents in the UASB reactor should be increased to more than 22 hours. The volume of the UASB reactor should then be multiplied by 2.5, which calls into question the economic viability of this technique for the treatment of effluents whose temperature is below 15 ° C.

Another disadvantage of this technique of the prior art lies in the fact that its yield in terms of nitrogen and phosphorus reduction is almost zero. As a result, the ratio between the carbon concentration and the nitrogen concentration of an effluent treated at the outlet of the methanizer (or UASB reactor) is strongly unbalanced, conventional denitrification techniques by anoxic zone not allowing to break a such imbalance. Therefore, the implementation of such a technique in areas of the world in which the regulations in terms of nitrogen discharge are particularly severe is difficult or impossible. Furthermore, it is estimated that 15 to 50% of the biogas formed during the methanization of an effluent to be treated escapes outside the methanizer with the treated effluent and is released into the atmosphere downstream of the methanizer. Part of the biogas formed is not valued, but is instead released into the atmosphere where it participates in the greenhouse effect. As a result, such a technique benefits from an unfavorable carbon footprint which tarnishes its image on the market for purification techniques.

Another disadvantage of this technique of the prior art is that it generates the production of a relatively large volume of sludge.

3. Objectives of the invention

The invention particularly aims to overcome these disadvantages of the prior art.

More precisely, it is an object of the invention to provide, in at least one embodiment, a technique for treating liquid effluent by methanation, which makes it possible to reduce the production of sludge compared to the techniques of the prior art.

Another object of the invention is to implement such a technique that allows, in at least one embodiment, to increase the production of biogas compared to the techniques of the prior art.

The invention also aims to provide, in at least one embodiment, such a technique that can be implemented in many regions of the world, including that in which the temperature of the effluent to be treated is continuously or seasonally below 15 ° C.

The invention also aims to provide, in at least one embodiment, such a technique that can be implemented in a relatively small footprint and is simple, reliable and inexpensive.

4. Presentation of the invention

These objectives, as well as others that will appear later, are achieved by means of a liquid effluent treatment process whose temperature is between 5 and 15 ° C, which according to the invention comprises: a step of methanization of said effluent within a methanizer producing biogas, methanized sludge and a methanized effluent; a step of biological treatment of said methanized effluent within a biological treatment zone producing biological sludge and a treated effluent;

a step of extracting thickened biological sludge from said biological treatment zone;

a step of anaerobic digestion of at least a portion of said methanized sludge and said thickened biological sludge in a digester producing biogas and digested sludge;

a step of recirculating at least a portion of said sludge digested in said methanizer;

a step of extracting at least a portion of the digested sludge.

Thus, the invention is based on a completely original technique for treating a liquid effluent comprising anaerobic digestion of the effluent within a methanizer, a biological treatment of the methanized effluent from the methanizer, a sludge extraction. thickened biologics from biological treatment, anaerobic digestion of at least a portion of the methanized sludge and thickened organic sludge, and recirculation of at least a portion of the sludge digested to the methanizer.

In Mahmoud, N. and Al (2003) Anaerobic sewage treatment in one-stage UASB and a combined UASB-digester system. Seventh International Water Technology Conference Egypt, 307-322, and Mahmoud, N. (2008) High strength sewage treatment in a UASB reactor and an integrated UASB-digester system. Bioresource Technology 99, 7531-7538, the authors provide, with reference to Figure 2, to treat an effluent 20 by introducing it into a UASB type methanizer 21 to produce a treated effluent 22, primary sludge 23 and biogas 24 The primary sludge 23 is introduced into a CSTR 26 type digester whose implementation makes it possible to produce biogas and digested sludge 27. The digested sludge 27 is then reintroduced into the methanizer 21. whereas this implementation may make it possible to increase the reduction in the COD content of the effluent to be treated, to reduce the volume of sludge produced, to improve the stability and the decantability of the sludge produced, to increase the production of biogas. The attention of the reader is however drawn to the fact that the effectiveness of such an implementation to treat effluents at low temperature, that is to say at a temperature below 15 ° C, remains to be demonstrated.

During the development of the technique according to the invention, the inventors have connected, under the cover of confidentiality, the methanizer 1 1 of a conventional installation according to the prior art illustrated in FIG. 1 with a digester of CSTR type according to the teachings of N. Mahmoud.

They then, always under cover of confidentiality, carried out tests to verify the effectiveness of such an implementation. They then realized that a facility of this type required the implementation of a digester whose volume is so important, that its implementation is ultimately uncompetitive in particular to investment and energy because of that the biogas produced is insufficient to heat the digester.

The inventors have, under these conditions, sought a technical solution that would make it possible to implement, at low temperature, an effluent treatment technique incorporating an anaerobic digestion, the high temperature implementation of which is known to have the advantage of enabling remove the COD content of the effluent to be treated satisfactorily.

The inventors then thought to recirculate at least a portion of the biological sludge in the digester, and realized that this implementation led, quite surprisingly, including:

- significantly reduce the volume of the digester;

reduce sludge production;

increase the production of biogas.

Recirculating at least a portion of the biological sludge in the digester is contrary to the prior art of a person skilled in the art who, in an installation of the type illustrated in FIG. 1, never employs a digester for treat biological sludge whereas anaerobic digestion is already implemented during the anaerobic digestion.

The implementation of a technique according to the invention makes it possible to reduce the volume of the digester. Indeed, the integration of the technique of N. Mahmoud in an installation of the prior art according to Figure 1 leads to implement an installation according to Figure 4, the volume of the digester is 1.3 times lower than that of the methanizer, while the implementation of the technique according to the invention can lead to implement a digester whose volume is between 3 and 9 times lower than that of the methanizer.

The implementation of this technique reduces the overall production of sludge. In fact, the sludge produced is digested in two different modes. Thus, some sludge that is insensitive to one of the modes of digestion, will be more to the other. In addition, recirculation of sludge to the methanizer allows the latter to be inoculated with active anaerobic biomass from the digester. This biomass, which is active, requires no maturation time to act within the methanizer. This recirculation therefore contributes to increasing the yield of the anaerobic digestion.

The use of the technique according to the invention thus makes it possible to increase the solubilization of the organic fraction of the sludge, which contributes, on the one hand, to reducing sludge production and, on the other hand, to increasing the production of sludge. biogas.

The particulate organic fraction of the liquid effluents to be treated is difficult to kill by psychrophilic methanization at low temperature. The solubilization of this organic fraction is improved by the supply of mesophilic anaerobic sludge from the digester. This makes it possible to overcome the yield limits of low temperature methanation.

The technique according to the invention thus makes it possible to effectively treat an effluent whose temperature is between 5 and 15 ° C.

According to an advantageous characteristic, said digestion is of the mesophilic or thermophilic type. According to another advantageous characteristic, said methanation is of the psychrophilic or mesophilic type.

The modes of operation of digestion and methanation can thus be chosen advantageously depending on the nature of the effluent to be treated. If the effluent to be treated is an urban waste water, methanation will be preferentially psychrophilic and mesophilic digestion, whereas if the effluent to be treated is an industrial wastewater, methanation and digestion will be preferentially mesophilic. The thermophilic regime will be applied only to sludge digesters and according to local parameters such as effluent temperature and / or implantation constraints. This diet usually reduces the volume of the digester. It is however more energy consuming.

According to a preferred embodiment, said biological treatment step comprises an aerobic biological treatment step.

The aerobic biological treatment may for example implement activated sludge, a biofilter, biodisks, a sequenced biological reactor (SBR) ...

Anaerobic digestion reduces approximately 50 to 70% of the COD of the effluent to be treated. Aerobic biological treatment removes the residual COD contained in the methanized effluent in order to reach acceptable discharge standards for the natural environment. Thus, downstream of the biological treatment, the total reduction of the COD and the MES is greater than 95%, the reduction of the MES being obtained by separating the biological sludge from the treated effluent.

The biological treatment may also make it possible to eliminate at least part of the nitrogen (for example by thorough nitrification) and / or phosphorus contained in the effluent to be treated and which has not been removed by the methanation.

In a preferred variant, said biological treatment step comprises a biological treatment step of anoxic type.

The implementation of such anoxic biological treatment step can eliminate at least a portion of the nitrogen and / or phosphorus contained in the effluent to be treated that has not been removed by the biological treatment. aerobic.

Phosphorus can also be removed at least partly by physico-chemical route by addition of ferric chloride or equivalent reagent.

The biological treatment zone will include means for separating the biological sludge from the treated effluent. These means may for example include a decanter, a membrane block, a float, filter discs, etc.). Biological sludge separated from the treated effluent can typically be thickened before being extracted. As an example, in the case of a settling tank, the thickened organic sludge will be extracted in the lower part of the settling tank, in the case of a membrane block, the biological sludge will be collected by backwashing ... the sludge thickening allows to preconcentrate them before recirculating them.

A process according to the invention preferably comprises a step of recirculating at least a portion of said thickened biological sludge upstream of the biological treatment zone which will comprise, for example, a biologic reactor under aerobic or anoxic conditions (for example a denitrification reactor). It can therefore be airy or not.

A method according to the invention advantageously comprises a step of controlling the recirculation flow rate of digested sludge in said methanizer, a step of controlling the recirculation rate of the methanized sludge in said digester, and a step of controlling the extraction rate of said sludge. digested.

The recirculation and extraction fines will preferably be controlled to maintain the sludge concentration in said methanizer and digester respectively between 10 and 100 gMVS / L and 30 and 100. gMVS / L according to the temperature of said effluent to be treated. The abbreviation MVS stands for Volatile Suspended Matter.

Such an implementation can treat an effluent whose temperature is between 5 and 15 ° C in a small footprint by maximizing the production of biogas and decreasing the production of sludge. The present invention also relates to an effluent treatment installation, said installation comprising:

a methanizer comprising an inlet for said effluent, a biogas outlet, a methanized sludge outlet and a methanized effluent outlet;

biological treatment means comprising an inlet for said methanized effluent, and a biological sludge outlet;

a treated effluent outlet;

an exit of thickened organic sludge;

digestion means comprising a first inlet cooperating with methanized sludge recirculation means, a second inlet of thickened biological sludge, a biogas outlet and a digested sludge outlet;

means for recirculating at least a portion of said sludge digested in said methanizer;

means for extracting at least a portion of said digested sludge.

Such an effluent treatment plant preferably comprises means for regulating said recirculation means and extraction means.

According to advantageous variants, said methanizer may be of the UASB or HUSB or AnMBR type, that is to say an anaerobic membrane bioreactor or Anaerobic Membrane Bio Reactor in English).

5. List of figures

Other characteristics and advantages of the invention will appear more clearly on reading the following description of a preferred embodiment, given as a simple illustrative and nonlimiting example, and the appended drawings, among which:

Figures 1 and 2 illustrate diagrams of two types of liquid effluent treatment plant according to the prior art;

FIG. 3 illustrates the diagram of an effluent treatment plant liquid according to the invention;

FIG. 4 illustrates the schematic of an effluent treatment plant comprising a UA SB methaneurizer and a communicating C SR stelige;

FIG. 5 illustrates the diagram of a variant of an installation according to the invention implementing an AnMBR type methanizer in place of a UASB methanizer.

6. Description of an embodiment of the invention

6.1. Recall of the principle of invention

The general principle of the invention is based on a completely original technique for treating a liquid effluent comprising a methanization of effluent within a methanizer, a biological treatment of methanized effluent from the methanizer, an extraction of thickened biological sludge. from the biological treatment, anaerobic digestion of at least a portion of the methanized sludge and thickened organic sludge, and recirculation of at least a portion of the sludge digested to the methanizer.

The implementation of such a technique allows:

to treat an effluent whose temperature is between 5 and 15 ° C in a compact installation,

to reduce the production of sludge, and

to increase the production of biogas.

6.2. Example of a liquid effluent treatment plant according to the invention

With reference to FIG. 3, an embodiment of a treatment plant for a liquid effluent according to the invention is presented.

As shown in this FIG. 3, such an installation comprises a pipe for supplying an effluent to be treated 30. This pipe 30 opens at the inlet of pre-treatment means.

The pretreatment means comprise screening means, grit removal and deoiling 31, and a methanizer 32. In this embodiment, the methanizer 32 is of the type of expanded anaerobic sludge (or UASB for Upflow Anaerobic Sludge Blanket in English language) intended to operate in psychrophilic mode. In a variant, it can operate in mesophilic mode. In another variant, it may be a type of methanizer expanded hydrolyzed sludge bed (or HUSB for Hydrolysis Upflow Sludge Blanket in English).

The methanizer 32 houses a triphasic separator. It comprises a biogas outlet 321 which is connected to processing, storage and recovery means (not shown) whose implementation can allow the production of heat and electricity. The methanizer 32 also comprises a methanized effluent outlet 33 and a methanized sludge outlet 322.

The outlet of methane effluent 33 is connected to the inlet of a biological reactor 34 with activated biological sludge. In variants, other biological treatment techniques may be implemented, for example fixed culture technologies such as Biostyr®, MBBR AnoxKaldnes® or other fixed or hybrid cultures, membrane or disk bioreactor, biological reactor sequential...

This biological reactor 34 houses aeration means (not shown). It comprises an outlet which is connected by a pipe 35 to the inlet of a settler 36.

The settler 36 comprises a treated effluent outlet 37 and a thickened biological sludge outlet 38. In a variant, other liquid / solid separation means may be used, such as, for example, aeroflotation means, means filtration ...

The outlet 38 is connected by a pipe to a T-shaped connection means 39.

This T-shaped connection means 39 is connected to a pipe 40 which opens into the outlet 33 upstream of the biological reactor 34. The T-shaped connection means 39 is also connected to a pipe 41 which opens into a pipe means. thickening or dewatering of sludge organic.

These means of thickening or dehydration 42 comprise in this embodiment a thickener. In variants, they may for example include a dewatering device, pressing, centrifugation ...

The means for thickening or dewatering biological sludges comprise a thickening juice outlet (not shown) which opens upstream of the biological reactor 34. It also comprises a thickened biological sludge outlet which is connected by a pipe 43 to the entry of an anaerobic digester 44.

The anaerobic digester 44 is of the permanently stirred type (or CSTR for

Continuously Stirred Tank Reactor in English) and is expected to work in mesophilic mode. In a variant, it can operate in thermophilic mode. It comprises a biogas outlet 441 which is connected to the processing, storage and recovery means. It further comprises a digested sludge outlet which is connected to the methanizer 32 via a pipe 45. A digested sludge extraction pipe 47 is connected to the pipe 45.

The outlet of methanized sludge 322 is connected to the digester 44 via a pipe 46.

The biogas treatment means may be of the desiccant, desulfurization or siloxane removal type. The biogas storage means may be of the compression type to allow the supply of a boiler or a cogenerator. The recovery means may include a boiler or a cogenerator.

The biogas produced within the methanizer 32 and the digester 44 comprises approximately 70% methane and 30% carbon dioxide. It can be used to produce the heat necessary to heat the digester 44 and the electricity used for example to implement the effluent treatment plant. In case of presence of troublesome products (sulphide of hydrogens, siloxanes ...), it will be able to be the subject of a specific treatment before being valorized. In case of large production, it may be exported outside installation.

This installation also comprises a regulation system. This regulation system comprises measuring means:

the temperature of the effluent to be treated;

the pH of the effluent to be treated;

the flow of biogas from the methanizer 32 and the digester 44;

the concentration of COD at the inlet of the methanizer 32 and at the inlet of the digester 44;

measuring the flow of sludge entering the methanizer 32 and the digester 44.

The control system comprises valves and / or variable flow rate pumps 48, 49 and 50 which respectively make it possible to modify the flow rate of methanized sludge introduced into the digester 44, to modify the flow rate of digested sludge introduced into the methanizer 32 and to modify the flow of digested sludge extracted from the installation.

The control system also comprises control means to which the measuring means and the valves 48, 49, 50 are connected.

To function optimally, that is to say to maximize the production of biogas, to minimize the production of sludge, including for effluents to be treated with a temperature between 5 and 15 ° C:

the mass load applied to the methanizer 32 must be between

0.05 and 0.6 Kg COD / KgMVS / day;

the mass load applied to the digester 44 must be between

0.075 and 0.225Kg COD / KgMVS / day;

the pH of the effluent to be treated must be between 6.5 and 7.5.

The abbreviation MVS stands for Volatile Suspended Matter.

On the basis of the data measured by the measuring means and the constraints mentioned above, the control means control the opening of the valves 48, 49, 50 to adjust the flow rates for recirculation and extraction of sludge in such a way that the concentration of sludge in the digester 44 is included between 30 and 100g MVS / L and that the concentration of sludge in the methanizer 32 is between 10 and 100g MVS / L.

When the temperature of the effluent falls, the activity of the biomass within the methanizer 32 decreases. The activity of the anaerobic biomass in the digester 44 must then be increased to reduce the mass load. Recirculations of methanized sludge to the digester 44 and sludge digested to the methanizer 32 are then increased while the extraction of digested sludge is reduced.

The extraction of the sludge from one or both reactors 32, 44 makes it possible to adapt the mass loads in each reactor so that they remain in the abovementioned intervals when the recirculation of sludge between the reactors does not occur. does not compensate for sudden changes in temperature and incoming charge.

The methanogenic activity of anaerobic biomass is directly dependent on temperature, as shown in the table below:

Note (1) Extrapolated values from data between 20 and 25 ° C.

The objective of the regulations is to maintain the methanizer and the digester under optimal conditions of methanogenic activity in order to reduce the production of sludge and increase the production of biogas.

The digester is always regulated at the optimum temperature, but not the methanizer. Therefore, it must be inoculated continuously with biomass from the digester in order to benefit from optimal methanogenic activity.

The lower the effluent temperature at the inlet of the methanizer, the more the anaerobic biomass recirculation from the digester will have to be important to compensate for the drop in temperature (ie the decrease in methanogenic activity). Conversely, the higher the temperature of the effluent to be treated, the lower the recirculation of anaerobic biomass from the digester.

However, these variations in sludge recirculation between the methanizer and the digester must be carried out in compliance with the mass loads applied in order to avoid organic overload or excess sludge in each of the reactors. It is for this reason that the extraction rate of the sludge in excess is also regulated according to the fluctuations of the previously mentioned parameters.

6.3. Example of a method for treating a liquid effluent according to the invention

The treatment of an effluent by means of an installation as described above is to convey via the pipe 30 this effluent to the pretreatment means 31 in such a way that it is rid of sands and oils that 'it contains. The desalted and deoiled effluent is then introduced into the UASB type reactor 32 in which it undergoes anaerobic digestion.

For the treatment of urban wastewater, the methanizer 32 will operate in psychrophilic mode, at ambient temperature of the waste water without heating, with a hydraulic residence time of the effluents of between 2 and 15 hours, advantageously between 2 and 12 hours, to ensure their methanisation.

For the treatment of industrial wastewater, the methanizer 32 will operate in mesophilic mode, at a temperature of around 37 ° C., with a sludge hydraulic residence time of a few hours depending on the volume load applied, which is generally between 5 and 30 ° C. Kg COD / m ³ / d.

This methanisation leads to the production of biogas that is stored, treated and upgraded, and a pre-treated effluent.

The methanized effluent is introduced into the biological treatment means 34 in which it undergoes a biological treatment. This biological treatment leads to the production of biological sludge. The biological sludge is conveyed to the decanter 36, the implementation of which allows the production of a treated effluent 37 and thickened organic sludge 38.

The thickened organic sludge is partly recirculated at the inlet of the biological treatment means 34. This recirculation makes it possible to regulate conventionally the concentration of sludge in the biological treatment means 34. The remainder of the thickened biological sludge is introduced into the means sludge treatment 42. The biological sludge is dehydrated and / or thickened before being introduced into the reactor 44 of the CSTR type.

The thickened organic sludge is mixed in this reactor 44 with the sludge from the methanizer 32. This mixture of sludge undergoes anaerobic digestion inside the digester 44.

For the treatment of urban and industrial wastewater, the digester 44 will generally operate in mesophilic mode at a temperature between 35 and 37 ° C, with a sludge hydraulic residence time of between 15 and 20 days.

This digestion leads to the production of biogas that is stored, processed and upgraded, and digested sludge.

Part of the digested sludge is recirculated in the methanizer 32 via line 45. The other part is extracted from the installation via line 47.

6.4. testing

6.4.1. Validation in terms of biogas production, sludge and plant size

Tests have been conducted, under the seal of confidentiality, to validate the effectiveness of an effluent treatment technique according to the invention.

The characteristics of the effluent treated during these tests were as follows:

Flow: 250000 m ³ / d;

COD concentration: 500 mg / L;

DB0 concentration ₅ : 245 mg / L; MES concentration: 240 mg / L;

NGL (global nitrogen): 40 mg / L;

PT (total phosphorus): 10 mg / L;

Temperature 20 ° C.

A first series of tests consisted in treating such an effluent in an installation such as that illustrated in FIG. 4. This installation differs from that illustrated in FIG. 1 because the methanized sludge 12 leaving the methanizer 11 are recirculated in a CSTR 40 type digester whose implementation leads to the formation of biogas 42 and digested sludge which are partly recirculated 43 in the methanizer 11 and partly extracted 41. In this case, a portion of the biological sludge thickened and some of the digested sludge is extracted directly from the facility. There are two sludge extraction points. The pretreatment was of UASB methanization type with a hydraulic residence time of 8.5 hours at 20 ° C, the biological treatment by activated sludge with high load, mesophilic CSTR digestion in recirculation with the UASB reactor.

A second series of tests consisted in treating such an effluent in an installation according to the invention such as that illustrated in FIG. 3. The pretreatment was of the UASB methanization type with a residence time of 8.5 hours at 20 ° C, the biological treatment by activated sludge at high load, mesophilic CSTR digestion treating both the fresh sludge from the decantation and the biological sludge, with implementation of reciruclation between the UASB reactor and the CSTR digester. The volume of the CSTR reactor is 1.3 times lower than that of the UASB reactor.

The results recorded in the following table show that the implementation of the technique according to the invention makes it possible to increase the production of biogas, to reduce the formation of sludge by means of a plant whose reactor volume CSTR is greatly reduced. Parameters Units Tests 1 Tests 2

Primary decanter volume m3 0 0

Activated sludge volume m3 30,000 20,000

UASB volume m3 88,800 88,800

CSTR volume m3 68,000 17,600

CH4 from UASB m3 / d 2,093 2,163

CH4 from CSTR m3 / d 18 157 19 782

CH4 total m3 / d 20 250 21 945

Sludge produced kg / d 33 399 26 666

Recirculation rate

UASB / flow rate% 23 3

Recirculation rate

CSTR / flow rate% 22 0.3

The large size of the CSTR of the installation of Figure 4 makes this technique uncompetitive to investment and energy because the biogas produced is insufficient to heat CSTR.

The results recorded in the following table show that the implementation of the technique according to the invention makes it possible to produce more energy than it consumes.

The decrease in the overall sludge production is partly due to the fact that sludge retention times in the anaerobic phase are cumulative: ie 30 to 60 days in the UASB and 10 to 20 days in the CSTR, so a total sludge residence time under anaerobic conditions of 40 to 80 days over the entire wastewater treatment plant.

It is also explained by the fact that the regulation of the recirculation of anaerobic sludge between the two UASB and CSTR reactors allows:

- to maintain in each anaerobic reactor, the optimal mass load as a function of T ° C fluctuations and incoming charges in each reactor;

to adapt the sludge residence times in the reactors according to the seasonal fluctuations of temperatures and incoming charges in the treatment plant;

to ensure better hydrolysis of organic matter (those which are not hydrolysed in the UASB, are in the digester);

to ensure a better yield from the UASB which is regularly inoculated with active anaerobic biomass from the digester; to provide improved efficiency of the CSTR that receives, (continuously or sequenced), biological sludges from the aerobic finishing treatment stage and the UASB reactor.

Maintaining optimum biogas production, while treating up to 70% of the COD entering the plant anaerobically, significantly reduces the power consumption related mainly to the supply of the aerators of the treatment stage. aerobic biology. This implies that the implementation of a technique according to the invention generates a low power consumption which leads to an energy balance that can be positive.

6.4.2. Validation in terms of regulation

Tests were carried out to verify that the regulation proposed by the invention was effective.

These tests consisted in treating in an installation according to the invention an effluent having the following characteristics:

Flow: 250000 m ³ / d;

COD concentration: 500 mg / L;

DB0 concentration ₅ : 245 mg / L;

MES concentration: 240 mg / L;

NGL (global nitrogen): 40 mg / L;

PT (total phosphorus): 10 mg / L; Temperature 20 ° C.

In order to observe what happens when the incoming charge increases, an effluent with the following characteristics was then treated without and then with the implementation of the recirculation control:

- Flow rate: 250000 m ³ / d;

COD concentration: 1000 mg / L;

DB0 concentration ₅ : 491 mg / L;

MES concentration: 435 mg / L;

NGL (global nitrogen): 40 mg / L;

- PT (total phosphorus): 10 mg / L;

Temperature 20 ° C.

The operating instructions of the UASB reactor at 20 ° C were as follows:

Hydraulic residence time: 8.5 hours;

- pH between 6.5 and 7.5;

mass load (Cm) approximately equal to 0.5 Kg COD / KgMVS / d.

The operating instructions of the CSTR reactor at 20 ° C. were as follows:

Hydraulic residence time: 20 days;

- pH between 6.5 and 7.5;

mass loading (Cm) of between 0.1 and 0.15 Kg COD / KgMVS / d.

The results recorded in the following table show that if it is desired to maintain the operating instructions (in particular the mass load and the pH in order to avoid acidification and a loss of efficiency of the anaerobic reactors) while the mass load has doubled, the concentration biomass in the CSTR reactor increases to about 150 gMVS / L. This concentration of MVS is not permissible in such a reactor and would cause in the short term a risk of clogging of the hydraulic lines and problems of internal mixing.

The implementation of the recirculation regulation according to the invention In this case, it is possible to reduce the recirculation flow rate between the CSTR and the UASB and to increase the sludge extraction rate of the CSTR from 200 to 400 m ³ / h. This makes it possible to return to an acceptable MVS concentration in the CSTR while maintaining the control instructions for the reactors.

6.5. Advantages

The technique according to the invention allows:

to reduce the formation of sludge;

to increase the production of biogas;

ensure an energy-independent implementation (which is impossible, for example, when the installation illustrated in figure 4 is implemented because of the large size of the CSTR reactor and the high rates of recirculation sludge required);

to implement an installation comprising a CSTR digester and a small UASB methanizer (the ratio between the volume of the CSTR and the volume of the UASB is between 3 and 9, and preferably between 4.5 and 5.5, which renders the technique according to the invention economically viable);

to implement low rates of sludge recirculation;

regulate the recirculation of sludge to adapt the mass load to seasonal variations (load fluctuation during the day, temperature change between day and night, between different periods of the year)

to accelerate the start-up phases due to the continuous inoculation of the UASB reactor into anaerobic bacteria;

to limit the toxicity of sulphate-reducing bacteria and the emissions of H ₂ S.

6.6. variants

As indicated above, a technique according to the invention can alternatively implement a UASB or HUSB type methanizer.

For information, the average residence time of the effluents to be treated is about 2.5 hours in a HUSB type reactor, and about 8 hours in a UASB type reactor. The average yield in terms of total COD abatement is about 43% within a HUSB, and about 50% and can reach 70% optimum operation within a UASB. The average return in terms of SSM reduction is about 83% in a HUSB, and more than 80% in a UASB. The means for aerobic treatment of methanized effluents leaving a UASB are less bulky than those used to treat methanized effluents from a HUSB. Particulate COD is partially solubilized and converted to volatile fatty acids (VFA) in a HUSB. It is partially solubilized and transformed into biogas within a UASB. In other words, the denitrification of methanized effluents from a HUSB is better than that of methanized effluents from a UASB.

Certainly, a HUSB methanizer produces little biogas. It nevertheless transforms a portion of the COD into AGV, the presence of which in the methanized effluents from the HUSB tends to improve their subsequent denitrification. In addition, considering the recirculation between the two anaerobic reactors, some of the AGV is still transformed into biogas. In the end, the implementation of a HUSB methanizer in substitution of a UASB reactor in a technique according to the invention leads to produce a globally satisfactory amount of biogas. In addition, the volume of a HUSB methanizer is about three times lower than that of a UASB methanizer, which may be of economic interest.

Both the implementation of a UASB reactor and that of a HUSB reactor in a technique according to the invention is therefore interesting.

With reference to FIG. 5, a variant of an installation according to the invention is presented. Only the differences between this variant and the embodiment previously described are detailed here.

According to this variant, the methanizer 32 UASB or HUSB is replaced by a methanizer type AnMBR (Anaerobic Membrane Bio Reactor). This methanizer 32 thus consists of an anaerobic membrane bioreactor. Such anaerobic membrane bioreactor may for example be that marketed by the Applicant under the name MEMTHA E ®.

As shown in this FIG. 5, such a methanizer 32 comprises an anaerobic bioreactor 320 as well as a membrane separation unit 323.

The bioreactor 320 comprises an effluent inlet to be treated in which a pipe for supplying an effluent to be treated, possibly pre-treated beforehand, 51 opens. It also comprises a biogas outlet 321, an outlet for methanized sludge 322 and an outlet for effluent 324.

The effluent outlet 324 opens at the inlet of the membrane separation unit 323.

This membrane separation unit 323 houses micro or ultrafiltration filtration membranes of mineral type, for example ceramic, or organic. It comprises a methanized effluent outlet 33 which opens into the biological treatment zone 34. In this case, methanized effluent is a permeate. It also comprises a retentate outlet connected to a recirculation pipe 325, which opens into the bioreactor 320.

The outlet of methanized sludge 322 from the bioreactor 320 is connected to a pipe 46 which opens into the digester 44.

The bioreactor 320 further comprises a digested sludge inlet into which a pipe 45 opens, the inlet of which is connected to the outlet of the digester 44.

The treatment of an effluent by means of an installation according to the variant which has just been described will now be presented. Only the main differences between this treatment and that implementing an installation according to the embodiment described above will be detailed here.

In this case, the desalted and deoiled effluent from the pretreatment means 31 is introduced into the bioreactor 320 of the methanizer 32. It undergoes anaerobic digestion.

For the treatment of urban wastewater, the bioreactor 320 will operate in psychrophilic mode, at ambient temperature of the wastewater without heating, with a hydraulic residence time of the effluents of between 2 and 15 hours and advantageously between 2 and 12 hours to ensure their methanation.

For the treatment of industrial wastewater, the bioreactor 320 will operate in mesophilic mode, at a temperature of around 37 ° C, with a sludge hydraulic residence time of a few hours depending on the applied volume load which is generally between 5 and 30 ° C. Kg COD / m ³ / d.

The methanized effluent generated during this methanation is introduced into the membrane separation unit 323, the implementation of which leads to the production of a methanized permeate and a concentrate.

The concentrate is recirculated in the bioreactor 320 via line 325. The effluent or methanized permeate is introduced via line 324 into the biological treatment means 34 in which it undergoes a biological treatment.

Methanized sludges from the bioreactor 320 are introduced into the digester 44 via the pipe 46.

The sludge mixture present in the digester 44 undergoes anaerobic digestion.

For the treatment of urban and industrial wastewater, the digester 44 will generally operate in mesophilic mode at a temperature between 35 and 37 ° C, with a sludge hydraulic residence time of between 15 and 20 days. Digested sludge from digester 44 is introduced into bioreactor 320 via line 45.

The conventional treatment of an effluent by methanization type AnMBR does not produce a good result when the temperature thereof is less than 15 ° C. The implementation of the technique according to the invention allows an efficient treatment by anaerobic digestion of the AnMBR type of an effluent at low temperature, that is to say whose temperature is less than or equal to 15 ° C. and in particular between 5 ° C and 15 ° C.

Claims

A liquid effluent treatment process whose temperature is between 5 and 15 ° C, said method comprising:

a step of methanization of said effluent in a methanizer (32) producing biogas, methanized sludge and a methanized effluent; a step of biological treatment of said methanized effluent within a biological treatment zone (34) producing biological sludge and a treated effluent;

a step of extracting thickened biological sludge from said biological treatment zone (34);

a step of anaerobic digestion of at least a portion of said methanized sludge and said thickened biological sludge in a digester (44) producing biogas and digested sludge;

a step of recirculating at least a portion of said sludge digested in said methanizer (21);

a step of extracting at least a portion of the digested sludge.

2. Method according to claim 1, characterized in that said digestion is of the mesophilic or thermophilic type.

3. Method according to any one of claims 1 or 2, characterized in that said anaerobic digestion is of psychrophilic or mesophilic type.

4. Method according to any one of claims 1 to 3, characterized in that said biological treatment step comprises an aerobic biological treatment step.

5. Method according to claim 4, characterized in that said biological treatment step comprises a biological treatment step of anoxic type.

6. Method according to any one of claims 1 to 5, characterized in that it comprises a step of recirculation of at least a portion of said thickened biological sludge upstream of said biological treatment zone (34).

7. Method according to any one of claims 1 to 6, characterized in that it comprises a step of controlling the recirculation flow rate of digested sludge in said methanizer (32), a methanized sludge recirculation flow control step in said digester (44), and a step of controlling the extraction flow rate said digested sludge.

8. Method according to claim 7, characterized in that said recirculation and extraction flow rates are controlled so as to maintain the concentration of sludge in said methanizer (32) and in said digester (44) respectively between 10 and 100 gMVS / L and 30 and 100 gMVS / L depending on the temperature of said effluent to be treated.

9. Effluent treatment plant, said installation comprising:

a methanizer (32) comprising an inlet for said effluent, a biogas outlet (321), a methanized sludge outlet (322) and a methanized effluent outlet (33);

biological treatment means (34) comprising an inlet for said methanized effluent, and a biological sludge outlet;

a treated effluent outlet (37),

an outlet for thickened organic sludge (38);

digestion means (44) comprising a first inlet cooperating with means for recirculating (46) methanized sludge, a second inlet for thickened biological sludge, a biogas outlet (441) and a digested sludge outlet (47);

means for recirculating (45) a part of said sludge digested in said methanizer (32);

extracting means (47) for at least a portion of said digested sludge.

10. effluent treatment plant according to claim 9, characterized in that it comprises means for regulating said recirculation means (46, 45) and extraction means (47).

11. Installation according to any one of claims 9 or 10, characterized in that said methanizer is of the UASB or HUSB type.

12. Installation according to any one of claims 9 or 10, characterized in that said methanizer is of AnMBR type.