WO2007063198A1

WO2007063198A1 - Method and device for treating microorganism-containing effluents

Info

Publication number: WO2007063198A1
Application number: PCT/FR2006/002544
Authority: WO
Inventors: Chrystelle Langlais
Original assignee: Degremont
Priority date: 2005-11-28
Filing date: 2006-11-20
Publication date: 2007-06-07
Also published as: FR2893935B1; FR2893935A1

Abstract

A method for treating effluents, in particular waters containing pathogenic or non-pathogenic microorganisms with the aid of a membrane bioreactor (R) for biologically purifying by using a compatible biomass consists in forming a loop (B) for treating the bioreactor mixed liquor in such a way that the level of concentration of microorganisms therein is reduced, wherein said treatment consists in carrying out a solid/liquid separation (12) with a cut-off threshold selected in such a way that the microorganisms, in particular targeted by the method, remain in a liquid phase (L) and a purifying biomass is retained in a solid phase (S), in carrying out a disinfecting and/or sterilising treatment (D) of the liquid phase and in returning the thus treated liquid phase to the bioreactor.

Description

AT

METHOD AND PLANT FOR TREATING EFFLUENTS CHARGED WITH MICROORGANISMS

The invention relates to a process for the treatment of effluents charged with microorganisms, pathogenic or not, using a membrane bioreactor performing biological purification with a suitable biomass, and separation by filtration on membrane membranes. said biomass and treated effluent.

This process can be used with any effluent requiring biological treatment, including surface water, seawater, urban wastewater and industrial effluents.

The membranes used in membrane bioreactors are micro-, ultra-, nanofiltration or even reverse osmosis membranes.

The membranes can be integrated in the or one of the biological reactors (in the case of submerged membranes, or integrated system). According to another possibility, the membranes constitute an entity exclusively dedicated to filtration (so-called separate or external system). In this case the membranes are either immersed in a pool exclusively dedicated to filtration (submerged membranes), or assembled on a block (or skid) filtration (crankcase membranes). The medium contained in the bioreactor is generally designated by the expression "mixed liquor", and consists of a liquid phase called "interstitial liquid" and a solid phase consisting of the purifying biomass composed of flocs.

Membranes constitute real physical barriers, and membrane filtration induces, intrinsically, a phenomenon of concentration within the bioreactor of little or no biodegradable materials present in the effluent to be treated. Depending on the cutoff threshold of the membrane, the materials thus retained and concentrated within the bioreactor are, for example, suspended materials such as fibrous waste as envisaged in WO 03/002468.

The invention is related to the illumination of a problem in the case of the treatment of effluents loaded with microorganisms, pathogenic or not, by such membrane bioreactors. It was generally thought that the microorganisms present in the effluents were mainly bound to the purifying biomass and were in the solid phase of the mixed liquor of the bioreactor. Contrary to this opinion, certain microorganisms, pathogenic or not, can be found essentially in the interstitial fluid of the "mixed liquor". However, the mixed liquor, and therefore the interstitial liquid, are subjected to aeration phases during which air bubbles are blown into the bottom of the bioreactor. Such air bubbles bursting on the liquid surface of the bioreactor can cause bioaerosols. If the interstitial fluid becomes heavily loaded with microorganisms, especially pathogens, bioaerosol from such a liquid may constitute a health and environmental risk depending on the level of contamination in microorganisms.

The invention aims, above all, to provide a solution to this problem, and to control levels of contamination pathogenic microorganisms or not in the interstitial liquid of a mixed liquor of a membrane bioreactor.

According to the invention, the method for treating effluents, in particular water, loaded with microorganisms, pathogenic or not, using a membrane bioreactor is characterized in that, in order to reduce the level of concentration of microorganisms in the bioreactor, a bioreactor mixed liquor treatment loop is established, which treatment comprises:

a solid / liquid separation step with a chosen cutoff threshold such that the microorganisms, in particular those targeted by the process, remain in the liquid phase, whereas the purifying biomass remains in the solid phase,

a treatment for disinfecting and / or sterilizing the liquid phase, the mixed liquor flow rate subjected to the disinfection treatment being determined to at least cancel the phenomenon of concentration of microorganisms intrinsic to the bioreactor,

and a restitution of the liquid phase thus treated in the bioreactor.

The concentration of MES (suspended solids) in the liquid phase is preferably less than 0.5 g / L.

The processing loop may be independent and exclusively dedicated to the purpose of the present invention.

As a variant, in the case of a separate system, the treatment loop may be disposed in a branch on an existing recirculation line of the mixed liquor between a filtration cell and a biological reactor.

The treatment loop may comprise a sludge treatment station comprising the solid / liquid separation step followed, for the solid phase, of a sludge reduction reduction treatment step.

The solid phase, consisting in particular of the purifying biomass, can be returned to the bioreactor without further treatment.

Alternatively, the solid phase can undergo a conventional treatment called reduction of RPB sludge production. RPB treatment consists, for example, of applying biomass to physical and / or mechanical, and / or chemical, and / or biological, and / or thermal stress to stimulate cell lysis of the scavenger biomass for the purpose of reducing sludge production. In this case, the solid fraction can be restored, in whole or in part, in the bioreactor after the so-called "stress" or "solubilization" treatment phase. This RPB treatment can induce partial hygienization of biological sludge.

Preferably, the cut-off threshold for the solid / liquid separation step is at least 0.1 μm; the particles whose size is below the cut-off point, in particular 0.1 μm, are maintained in the liquid phase. The solid / liquid separation step may be carried out by any means implementing a known method, for example: sieving, flotation, centrifugation, decantation, filtration on canvas, separation on membrane, provided that the target microorganisms are maintained in the liquid phase of the solid / liquid separation. The disinfection treatment of the liquid phase can implement any known method in particular:

- physical or mechanical treatment: UV, ultrasound, pressure shock, and / or

chemical treatment: any non-oxidizing or oxidizing biocide such as chlorine, ozone, chlorine dioxide, and / or

- biological treatment such as enzymatic oxidation or hydrolysis, and / or

- heat treatment.

The choice of treatment will be made on a case by case basis depending on the sensitivity to said treatment of the target microorganism (s) concerned.

The invention also consists of an effluent treatment plant, in particular of water, loaded with microorganisms, pathogenic or not, comprising a membrane bioreactor, characterized in that it comprises, to reduce the level of concentration. in microorganisms in the bioreactor, a bioreactor mixed liquor treatment loop comprising:

a solid / liquid separation means with a chosen cutoff threshold such that the microorganisms, in particular those targeted by the treatment, remain in the liquid phase, whereas the purifying biomass remains in the solid phase,

a treatment means for disinfecting and / or sterilizing the liquid phase, and a means of restoring the liquid phase thus treated in the bioreactor.

The invention consists, apart from the arrangements described above, in a certain number of other arrangements which will be more explicitly discussed below with regard to exemplary embodiments described in detail with reference to the appended drawings, but which are in no way limiting. On these drawings:

Fig. 1 is a diagram of an effluent treatment plant loaded with microorganisms implementing a method according to the invention.

Fig. 2 is a diagram of a variant of the installation and FIG. 3 is a diagram of another variant of the installation.

Referring to Fig. 1 can be seen the diagram of a biological purification plant, of the separated or external system type, comprising a bioreactor R formed by the assembly of a biological reactor 1 and a filtration cell 2 constituted by a dedicated basin filtration in which membranes are immersed 3.

The raw effluent to be treated arrives via a pipe 4 in the biological reactor 1. The treated effluent is constituted by the permeate leaving the membranes 3 and discharged through an outlet pipe 5.

The biological reactor 1 comprises, in the usual way, a suitable biomass, formed of bacteria, to ensure a biological purification of the raw effluent. Aeration means 6, in particular constituted by perforated tubes, are provided in the bottom of the reactor 1 to allow oxygen to be blown, for example in the form of air bubbles, into the reactor. The medium (not shown) contained in the bioreactor R constitutes a mixed liquor comprising a liquid phase called "interstitial liquid", and an essentially solid phase constituted by the purifying biomass composed of flocs, that is to say bulky flakes and decanted.

The mixed liquor filtration cell 2 is fed by a connecting line 7 between the reactor 1 and the cell 2. Aeration means 8 are provided in the bottom of the filtration cell 2. These means 8 are constituted for example by perforated tubes for blowing air bubbles into the cell 2.

According to FIG. 1, the filtration cell 2 is shown separately from the biological reactor 1. Alternatively, the cell 2 could be located inside the reactor 1. In addition, several filter cells 2 could be provided.

An outlet 9 disposed in the bottom of the filtration cell 2 is connected to a recirculating branch 10 of the mixed liquor in the reactor 1. line 11 leaving the branch 10 can extract biological sludge produced in excess at a flow rate Qw.

As a result of the effectiveness of the filtration membranes 3, the permeate leaving the line 5 is freed, depending on the type of membrane, not only of suspended materials such as fibrous waste, colloids, salts, but also micro -organisms including both fungi, protozoa, helminths, viruses and prions.

Some of these microorganisms are found mainly in the interstitial liquid, constituting the liquid phase of the mixed liquor, and not in the biomass of this same mixed liquor.

This results in a phenomenon of intrinsic or physical concentration of microorganisms in the interstitial fluid. The air bubbles, blown by the aeration means 6 and 8, escaping into the atmosphere cause particles of the interstitial liquid in the form of a bioaerosol which may constitute a health risk when the concentration of microorganisms , especially pathogens, becomes too important.

In order to control the level of microorganisms in the interstitial liquid, according to the invention, a treatment loop B of the mixed liquor is established, this treatment in the loop comprising: a solid / liquid separation step using a solid / liquid separation means 12 with a chosen cutoff threshold such that the microorganisms remain in the liquid phase L, whereas the purifying biomass remains in the solid phase S,

a disinfection and / or sterilization treatment D of the liquid phase, the mixed liquor flow rate subjected to the disinfection treatment being determined to at least cancel the intrinsic or physical microorganism concentration phenomenon in the bioreactor,

and a restitution of the liquid phase thus treated in the bioreactor, at the level of the biological reactor 1. A line 13 makes it possible to withdraw the mixed liquor at the bottom of the cell 2 to bring it to the solid / liquid separation means 12. The solid fraction S can be returned without treatment to the reactor 1 via a return line 14.

The liquid fraction L, consisting of the interstitial fluid containing the microorganisms, is directed by a line 15 on a means 16 of disinfection-sterilization. The concentration of MES (suspended solids) in the liquid fraction is preferably less than 0.5 g / L. The cut-off threshold of the separation means 12 is advantageously at least 0.1 μm, so that the particles, in particular the microorganisms, whose size is smaller than this cutoff threshold, remain in the liquid phase L directed to the disinfection-sterilization means 16. It can also be said that the retention size of the separation means 12 is at least 0.1 μm, so that particles larger than 0.1 μm are retained. with the solid phase.

The separation means 12 can implement any known method, for example: sieving, flotation, centrifugation, decantation, filtration on canvas, separation on membrane, provided the targeted microorganisms are maintained in the liquid phase L discharged by the pipe 15.

The means 16 for the disinfection treatment of the liquid phase can implement any known method, in particular:

- physical or mechanical treatment: UV ultraviolet rays, ultrasound, pressure shock and / or

- chemical treatment: any non-oxidizing or oxidizing dioxide such as chlorine, ozone, chlorine dioxide, and / or

biological treatment such as enzymatic oxidation or hydrolysis, and / or heat treatment.

The disinfected liquid phase is returned via a pipe 17 to the reactor 1.

The operation of the installation results from the previous explanations.

The raw effluent to be treated arrives via the pipe 4 in the reactor 1 to undergo the treatment exerted by the bacteria of the biomass. The mixed liquor passes into the filtration cell 2; the permeate is extracted from the membranes 3 by line 5 and constitutes the treated effluent. This treated effluent, generally water is substantially ¹ free of microorganisms because the cutoff of the filtration membrane 3 is of the order of 40 nm (nanometers) to 10 nm, below the microorganisms dimensions.

A fraction of the mixed liquor is recirculated in the reactor 1 by the pipe 10, while a flow Qw of biological sludge produced in excess is extracted through the pipe 11.

A fraction of the mixed liquor extracted via the pipe 13 is sent to the separation means 12. The solid phase S coming from the separation means 12 is returned to the reactor 1. The targeted microorganisms remain in the liquid phase L and are evacuated through line 15 to medium 16 of disinfection-sterilization of liquid phase.

Since the disinfection-sterilization step is carried out exclusively on the so-called "interstitial liquid" fraction, the yield of the operation is high.

On the other hand, if the disinfection-sterilization treatment had been applied directly to the effluent to be treated arriving via line 4, the disinfection performance would have been reduced: the ultraviolet penetration would have been very low, the oxidant consumption ( chlorine, ozone ...) would have been raised. By a treatment in line 4, it would be extremely difficult, if not impossible, to have a selective action of disinfection or sterilization vis-à-vis the oxidation of organic pollution, for example. In addition, a disinfection treatment on the effluent arriving via line 4 would not control a possible multiplication of microorganisms interstitial fluid within the reactor itself. For such reasons, the technical and economic efficiency of such a treatment would not be realistic industrially.

According to the embodiment of FIG. 1, the treatment loop B is independent of the recirculation 9, 10 of the mixed liquor, that is to say that the loop B does not have common routing and processing means with the recirculation loop. According to the variant of FIG. 2, in a separate system which comprises a biological reactor 1, a separate membrane filtration cell 2, and a recirculation loop 10 of the mixed liquor between the two structures, a treatment loop B1 is advantageously disposed in branch on the existing recirculation line 10 of the mixed liquor. For this, a pipe 18 is connected to the pipe 10 to bring the separation means 12 a fraction of the mixed liquor.

Fig. 3 illustrates another variant according to which the processing loop B2 comprises a station E sludge treatment. The station E comprises the solid / liquid separation means 12a disposed on the pipe 11 for discharging excess biological sludge, and a station E1 for RPB treatment of the solid fraction from the means 12a, and discharged after RPB treatment by a pipe 19. The treated solid fraction leaving the line 19 may be returned wholly or partly to the biological reactor 1. The liquid fraction from the separation means 12a is directed through line 15 to the disinfection means 16, and then is returned by the pipe 17 at the top of the biological reactor 1.

The operation of the means 12, 12a and 16 of an installation according to the invention may be continuous or discontinuous depending on the health risk identified. A discontinuous operation can be achieved, for example, by providing a valve on line 13 or 18; when the valve is closed the disinfection-sterilization treatment is not implemented.

The determination of the mixed liquor flow rate to be subjected to the disinfection treatment can be carried out as follows.

We denote by "y" the ratio of the flow Qdésinf. mixed liquor directed to the disinfection treatment, the Qalim flow of effluent to be treated arriving via the pipe 4. y is therefore equal to the ratio of the flow Qdésinf. in line 13 or 18 at Qalim flow. (y = Qdisinf / Qalim.) The term "x2" denotes the removal efficiency of the disinfection-sterilization treatment. x2 is equal to the number of microorganisms eliminated per unit volume compared to the number of microorganisms before treatment. A total elimination corresponding to x2 = 1.

SRT, or sludge age, is equal to the quotient of the biomass volume, ie the reactors 1 + 2, expressed in m ³ , by the flow rate Qw expressed in m ³ / h of sludge extracted by the pipe 11.

HRT, or hydraulic retention time in the reactor, is equal to the ratio of the biomass volume, ie the 1 + 2 reactors, to the Qalim feed rate. According to the present invention, with a continuous operation, the volume concentration factor FCV in microorganisms between the effluent to be treated and the interstitial liquid is given by the following relation:

SRT FCV Xractor = #

Xeffluent (x2.y.SRT) + HRT

with:

Xreactor: concentration of parameter X (X = microorganism targeted) within the interstitial fluid of the bioreactor

Xeffluent: concentration of parameter X in the effluent to be treated # meaning "approximately"

From this relation, for a desired concentration Xrefactor, knowing Xeffluent, x2, SRT and HRT, we can deduce the value of y. In function:

the targeted microorganism X in the effluent to be treated,

- its Xeffluent concentration in the effluent to be treated,

the level of acceptable contamination of the said microorganism to control the health and environmental risk at the level of the bioreactor,

x2 disinfection yield - sterilization of the treatment for the targeted microorganism,

and the operating conditions of the bioreactor (SRT and HRT), it is possible to determine the interstitial fluid flow rate necessary and sufficient to maintain an acceptable level of contamination on a case-by-case basis, for example equal to the concentration of target micro-organisms of Peffluent. to be treated so as to cancel the phenomenon of intrinsic concentration of membrane bioreactors. By targeting the disinfection treatment on the exclusively called fraction

"Interstitial liquid", the invention has many advantages.

The invention makes it possible to apply the disinfection treatment to a phase

- partially free from suspended solids - whose pollution, particularly soluble, has already been reduced by biological treatment, which makes it possible to ensure the efficacy and the necessary selectivity of said treatment on the target micro-organisms as illustrated by the examples presented below.

Under these conditions, for a fixed level of contamination, the size of the facility (ie capex) and operating costs (ie opex) will be reduced accordingly.

Another advantage of the present invention is that it does not necessarily imply a stress of the purifying biomass which can result in a deterioration of the quality of the interstitial fluid and the excretion of exopolymers capable of inducing membrane clogging. . The invention also makes it possible to avoid stimulating the cell lysis of the microorganisms producing a release of biological materials in the reaction medium and reducing the filtration capacity of the membranes.

EXAMPLES Example 1

The first example concerns a small community (tourist complex) of 3500 inhabitants whose effluents are treated by Membrane bioreactor, whose effluent is quite representative of the average quality of an urban effluent, namely:

MeS: 160 mg / l

COD: 380 mg O ₂ / L BOD ₅ : 190 mgO ₂ / L

Ntotal: 28 mg / L

Ptotal: 7.5 mg / L

The operating parameters of the membrane bioreactor are as follows:

Average daily flow: 525 m ³ / d

Age of sludge: 35 days = SRT

Average hydraulic retention time: 10 h = HRT

The hypothesis of an epidemic dysentery that would affect on average, over a long period, 1% of the population (carriers), with an infectious agent identified as the protozoa entamoeba hystolitica.

During this outbreak, the average effluent concentration at the inlet of the treatment plant increased from 2.1 cysts / L (cysts per liter) to 98 cysts / L (cysts per liter).

Given the size of the cysts (10 to 15 μm), these are completely retained by the membranes. It is important to underline that, thanks to the implementation of the membranes, throughout the epidemic period, the treated effluent will remain free of contamination: no cyst will be released into the receiving environment of the treated effluent. which, in itself, constitutes a first control of the health risk.

In contrast, the cysts are concentrated in the bioreactor by a factor of about 84 (SRT / HRT), that is:

- approximately 176 cysts / L, in the absence of an epidemic, - up to 8,232 cysts / L observed during the epidemic period.

By their behavior, these cysts are indeed almost exclusively concentrated in the interstitial fluid.

In order to control the health risk associated with this crisis situation, it is decided to quickly set up a treatment loop according to the present invention with the objective of being deemed necessary and sufficient to maintain a concentration within the bioreactor at most equal to that observed outside the epidemic period, ie 176 cysts / L. This objective is equivalent to setting an FCV concentration factor of 1.8 which implies the treatment according to the present invention of an interstitial fluid flow equivalent to 50-55% of the average feed rate treated by the station.

To do this, a skid (block) flotation is implemented according to the present invention to extract 12m ³ / h of interstitial fluid from the mixed liquor of the bioreactor, the equivalent of 50 to 55% of the average flow rate treated by the station.

This interstitial liquid is subjected to an ozonation treatment.

Following the implementation of the treatment according to the present invention under the conditions described above, a rapid decrease in the level of contamination of the interstitial liquid would be observed. This level of contamination would stabilize at 168 cysts / L according to the target.

Example 2

We now consider the hypothesis of an urban wastewater effluent from an agglomeration of 15,000 inhabitants located in a geographical area that is characterized by a natural endemic contamination of salmonella-type bacteria.

This waste effluent is treated using a membrane bioreactor whose operating parameters are: Average flow: 1500 m ³ / d

SRT: 25d

HRT: 7h

The average Salmonella concentration in the raw fluff is

4.5.10 ⁵ NPP / 100mL (milliliters) fully retained within the bioreactor due to the use of ultrafiltration membrane clarification. It is recalled that the abbreviation NPP means "most probable number".

Under the operating conditions set forth above, the theoretical concentration factor is 85.7, which is a theoretical interstitial fluid concentration of 3.85 × 10 ⁷ NPP / 10OmL. In fact, and before the implementation of the present invention, the concentration of salmonella observed in the interstitial fluid would be about

3.8.10 ⁶ NPP / 100mL. This observation is correlated with an abatement of about 1 log, related to the biological process (predation and / or transfer in the solid phase, ie biological floc). The reduction of 1 log corresponds to a division by 10 of the salmonella concentration. The health objective related to the implementation of the present invention is to maintain within the interstitial liquid a concentration at most equal to that of the raw effluent in order to cancel the concentration effect related to the use of the membranes. .

Given the operating parameters of the membrane bioreactor

(SRT, HRT),

- of the observed natural reduction (approximately 1 log), it appears that to maintain within the interstitial fluid a concentration of at most 4.5 × 10 ⁵ MPN / 100 ml, the concentration factor must be reduced to at most 10, taking into account account of the abatement related to the biological process. To do this, a centrifugation clarification phase would have been set up to extract 6.25 m ³ / h of interstitial fluid from the bioreactor, ie the equivalent of 10% of the average flow rate treated on the station.

Said interstitial liquid is then subjected to a UV disinfection treatment.

Under the operating conditions set out above, the salmonella contamination rate of the bioreactor interstitial fluid would then have stabilized at a level of 4.3.10 ⁵ NPP / 100mL, equivalent to the level of the raw effluent to be treated. in accordance with the objective set. Under these conditions, the health risk related to the rate of contamination of the interstitial fluid is not higher locally than that induced by the contamination of the raw effluent. The present invention makes it possible to perfectly control the concentration effect initially induced by the use of the membranes.

Claims

1. A method for treating effluents, in particular water, loaded with microorganisms, pathogenic or not, using a membrane bioreactor performing biological purification with a suitable biomass, characterized in that, for to reduce the level of microorganism concentration in the bioreactor a treatment loop (B; B1; B2) of the bioreactor mixed liquor is established, which treatment comprises: - a solid / liquid separation step (12, 12a) with a chosen cutoff threshold such that the microorganisms, in particular those targeted by the process, remain in the liquid phase (L), whereas the purifying biomass remains in the solid phase (S),

a disinfection and / or sterilization treatment (D) of the liquid phase, the mixed liquor flow rate subjected to the disinfection treatment being determined to at least cancel the phenomenon of intrinsic microorganism concentration in the bioreactor,

and a restitution of the liquid phase thus treated in the bioreactor.

2. Method according to claim 1, characterized in that the ratio "y" of the flow Qdésinf. mixed liquor directed to the disinfection treatment, Qalim flow of effluent to be treated arriving through the pipe is determined by the relationship

SRT #Requirter

Xeffluent (x2.y.SRT) + HRT

in which :

Xrefactor refers to the concentration of parameter X (X = microorganism targeted) within the interstitial fluid of the bioreactor

Xeffluent: refers to the concentration of parameter X in the effluent to be treated

# means "about"

"X2" refers to the removal efficiency of the disinfection-sterilization treatment, SRT refers to the sludge age

HRT is the hydraulic retention time in the reactor

3. Method according to claim 1 or 2, characterized in that the treatment loop (B) is independent of the recirculation of the mixed liquor.

4. Method according to claim 1 or 2, characterized in that, in the case of a separate system, the treatment loop (B1) is disposed in shunt on an existing recirculation line (10) of the mixed liquor between a filtration cell (2) and a biological reactor (1).

5. Method according to one of claims 1 to 4, characterized in that the solid fraction (S) from the solid / liquid separation step (12) is returned without treatment in the bioreactor (R).

6. Method according to one of claims 1 to 4, characterized in that the solid fraction (S) resulting from the solid / liquid separation step (12) is subjected to a reduction treatment of sludge production before to be returned to the bioreactor (R).

7. Method according to claim 1, characterized in that the treatment loop (B2) comprises a station (E) sludge treatment comprising the solid / liquid separation step (12a) followed, for the solid phase (S) of a sludge reduction treatment step (E1).

8. Method according to one of the preceding claims, characterized in that the cutoff threshold for the solid / liquid separation phase (12, 12a) is at least 0.1 microns.

9. Method according to one of the preceding claims, characterized in that the solid / liquid separation phase is carried out by at least one of the following processes: sieving, flotation, centrifugation, decantation, filtration on canvas, separation on membrane, subject to maintaining targeted microorganisms in the liquid phase of the solid / liquid separation.

10. Method according to one of the preceding claims, characterized in that the disinfection treatment of the liquid phase is constituted by at least one of the following treatments:

- physical or mechanical treatment: ultraviolet rays, ultrasound, pressure shock, and / or chemical treatment: any non-oxidizing or oxidizing biocide such as chlorine, ozone, chlorine dioxide, and / or

11. Method according to one of the preceding claims, characterized in that the processing loop (B, B1, B2) is provided for continuous or discontinuous operation.

12. Installation for treating effluents, in particular water, loaded with microorganisms, pathogenic or non-pathogenic, comprising a membrane bioreactor performing biological purification with a suitable biomass, for carrying out a process according to one of the following: of the preceding claims, characterized in that, to reduce the level of microorganism concentration in the bioreactor, the plant comprises a treatment loop (B; B1; B2) of the mixed liquor of the bioreactor, which treatment loop comprises:

a solid / liquid separation means (12, 12a) with a chosen cutoff threshold such that the microorganisms, in particular those targeted by the treatment, remain in the liquid phase (L), whereas the purifying biomass remains in the solid phase (S),

a means for disinfecting and / or sterilizing (D) the liquid phase,

and a means for restoring (17) the liquid phase thus treated in the bioreactor.

13. Installation according to claim 12, characterized in that the treatment loop (B) is independent of the recirculation of the mixed liquor.

14. Installation according to claim 12, characterized in that, in the case of a separate system, the treatment loop (B1) is disposed in shunt on a pipe (10) of existing recirculation of the mixed liquor between a cell of filtration (2) and a biological reactor.

15. Installation according to claim 12, characterized in that it comprises a station (E) sludge treatment comprising the solid / liquid separation means (12a) followed, for the solid phase (S), a station ( E1) treatment for reduction of sludge production.