WO1999021800A1

WO1999021800A1 - Method and device for adjusting the permeate flow rate in membrane bioreactors for water treatment

Info

Publication number: WO1999021800A1
Application number: PCT/FR1998/002217
Authority: WO
Inventors: Karl Glucina; Jacques Manem; Vincent Urbain
Original assignee: Suez Lyonnaise Des Eaux
Priority date: 1997-10-29
Filing date: 1998-10-15
Publication date: 1999-05-06
Also published as: FR2770210A1; FR2770210B1; AU9632798A

Abstract

The invention concerns a method for adjusting the treated water flow rate in membrane bioreactors, consisting in eliminating nitrates by biological process, adsorbing suspended matters on powdered activated carbon, these two steps being simultaneously carried out in said bioreactor (1), and disinfecting by ultra-filtration on its membranes (2), the retentate being recycled in the bioreactor (1). The invention is characterised in that it consists in pressurizing the ultra-filtration membrane inner compartment, in response to a decrease in the permeate flow rate, that is the treated water, below a predetermined set value, said pressurization, increasing the permeate flow rate. The invention also concerns an adjusting device for implementing the method.

Description

Method and device for regulating the permeate flow in membrane bioreactors for water treatment.

The present invention relates to a method for regulating the flow rate of water treated in membrane bioreactors.

We know that human activity is responsible for water pollution by dissolved compounds such as ammonia and nitrates, potentially pathogenic microorganisms as well as organic phytosanitary products such as pesticides. Depending on the concentration of pollutants and according to quality standards, in the field of drinking water production in particular, it is sometimes necessary to set up one or more specific processes for each of the compounds to be eliminated. Thus, the ammoniacal ozone as well as the nitrates can be eliminated by biological or physicochemical route. On the other hand, pesticides, organic matter of natural or anthropogenic origin (for example pesticides), pathogenic microorganisms (protozoa, bacteria and viruses) as well as suspended matter (turbidity) can only be eliminated by processes physico-chemical.

As seen above, the elimination of nitrogen can be carried out either biologically or physicochemically. The state of the art according to these techniques will be recalled below. I - Elimination of nitrogen

A - Biological processes:

These methods use microorganisms in the form of a suspension in the case of activated sludge or biofilms in the case of fixed culture procedures. For example, the latter type of process has been used since the 1980s for the treatment of nitrogen in the production of drinking water. Such a process known under the trade name of "NITRAZUR" (registered trademark) is described in particular in the Water Technical Memento, 9th edition, edition of the Cinquantenaire - 1989 -Tome 2 - pages 740 - 742.

The principle of biological processes consists in concentrating reactors from the natural environment in a bioreactor by placing them in an environment favorable to their growth, thus allowing them to transform the desired pollutant. Thus, nitrifying bacteria oxidize ammoniacal nitrogen (NH4) to nitrate (NO3) in the presence of oxygen (nitrification), while denitrifying bacteria transform nitrates into molecular nitrogen (N2) in the absence of oxygen (denitrification ). When denitrifying bacteria are used for the production of drinking water, it is necessary to provide a source of carbon and energy such as ethanol. Indeed, these denitrifying bacteria use a source of carbon and energy of organic origin, unlike nitrifying bacteria which use CO2 as a source of carbon and ammonia as a source of energy.

B - Physico-chemical processes:

Various physicochemical processes are known which can also make it possible to purify waters with too high a content of nitrogenous compounds. The techniques currently available on an industrial scale are electrodialysis, ion exchange, nanofiltration and reverse osmosis respectively. In the case of nitrates, as for most other pollutants, the fundamental difference compared to biological processes is that the physicochemical processes only assure a separation between water and undesirable compounds, without real elimination of pollutants but a transfer to a more concentrated phase which must then itself be treated before release into the natural environment.

It - Elimination of organic matter

The organic matter contained in waste water can be eliminated biologically as is the case in treatment plants. On the other hand, the organic matter present in natural waters is hardly biodegradable and it can only be eliminated by physico-chemical means. Likewise, organic micro-pollutants such as pesticides are also not very biodegradable and must therefore be eliminated by the same techniques.

These elimination processes are based on the action of strong oxidants such as ozone and hydrogen peroxide (process known under the name "PEROZONE" (registered trademark), or on adsorption by activated carbon in grains or powder, or on separation techniques such as nanofiltration or reverse osmosis. Ultrafiltration can also be used but in combination with powdered activated carbon because the cutoff threshold of the ultrafiltration membrane is too high and a high proportion of molecules with low molecular mass passes through said membrane. This process is known under the name "CRISTAL" (registered trademark), and it is described in particular in FR-A-2 628 337 and 2 696 440 Oxidative treatments cause degradation of organic matter into smaller molecules or even CO2 when the doses applied are sufficient. III - Elimination of suspended matter and microorganisms.

Apart from disinfection techniques such as ozone and chlorination which make it possible to inactivate or even destroy microorganisms, only separation techniques such as ultrafiltration make it possible to retain suspended matter and at the same time to ensure total disinfection of the treated water.

The removal of suspended matter can also be achieved by using less sophisticated techniques such as sand filters, but which do not constitute an absolute barrier for microorganisms.

The known methods recalled above have a certain number of drawbacks.

Thus, with the exception of reverse osmosis, which removes both organic matter and ions such as ammonia and nitrates, these processes are rather specific for one type of pollutant. Thus, when the water to be treated contains a set of undesirable products or present in too high a concentration, it is necessary to provide a combination of treatments. This is the case of the "NITRAZUR" process mentioned above which must be followed:

- a sand filter to retain bacteria from the bioreactors as well as suspended matter brought in by the raw water and;

- a granular activated carbon filter if pesticides must also be eliminated. Finally, if the flow-piston hydraulics of the fixed-culture reactors of the "NITRAZUR" type mentioned above have an advantage in terms of the progress of the biological reactions, it does not always ensure optimal conditions of contact between the biomass and the substrate: the distribution of water on the surface of the filter is indeed a function of the number of injection points, that is to say of the number of nozzles per m ² of filter floor area.

If reverse osmosis eliminates several pollutants at the same time, its main drawback is linked to the high energy cost due to the high pressure to be used. In addition, and this is a direct consequence of the salt rejection characteristics of the membranes used, the mineral balance of the water is greatly modified which requires remineralization of the treated water.

In the case of nanofiltration, electrolysis and ion exchange, the pollution is only transferred to a more concentrated phase which, as mentioned above, must then be treated before its discharge in the natural environment.

Likewise, adsorption on activated carbon (in grains or in powder) does not allow real elimination of the compounds but achieves a transfer of the pollutants onto a solid phase which must be regenerated, after saturation, for example thermally (to a temperature of about 800 ° C, in a reducing atmosphere).

Recently appeared a new process known under the name of "BIOCRISTAL" (registered trademark). This process is described in particular in the publication: "membrane bioreactor: a new treatment tool" by Vincent Urbain, Raymond Benoit and Jacques Manem, AWWA Journal - May 1996 pages 75 to 86. This process is based on the principle of membrane bioreactors known as the name "BRM" (registered trademark) and described in the publication "Membrane Bioreactors" by Jacques Manem and Ron Sanderson, Chapter 17, Membrane Processes - McGra-HILL - 1996. Such a process combines both elimination of nitrates by biological means (denitrification), adsorption of organic matter on powdered activated carbon (PAC) and disinfection by ultrafiltration. Figure 1 of the accompanying drawings illustrates the block diagram of such a process.

The mixture of denitrifying bacteria, reagents (ethanol and powdered activated carbon) and suspended solids provided by the raw water is delivered to a perfectly mixed bioreactor 1. The reagents are added in proportion to the flow of raw water in bioreactor 1 in which the concentration of suspended matter is controlled by regular extraction. The extraction makes it possible both to renew the used powdered activated carbon and to control the growth rate of denitrifying bacteria. The biological reactions (denitrification) and of adsorption by the activated carbon take place in this bioreactor, from which the suspended matter is pumped (biomass, activated carbon or particles coming from the raw water) in order to be delivered, by a pump 3 with ultrafiltration membranes 2. Filtration is ensured in tangential mode due to the quality of suspended matter present in the liquid to be filtered. The installation comprises a pipe 5 for recirculating the retentate as well as a pipe 4 for evacuating the permeate, that is to say treated water. Backwashing operations are carried out at regular intervals with the treated water and conditioning of the membranes with chlorine. For this purpose, the installation comprises a backwashing pump 5 which is put into action to pump the treated water in order to bring it back under pressure in the reactor 1, the recirculation of the sludge then being stopped.

FIG. 2 illustrates, in the form of a diagram, the reactions implemented in this "BIOCRISTAL" process. Starting from this state of the art, the present invention aims more particularly at controlling filtration performance according to the principle called "interactive regulation", the aim of the invention being in particular to obtain an extension of the production time when the performance filtration are reduced. A reduction in filtration performance can for example occur in the following cases:

- failure of the backwashing procedure;

- evolution of the mixture to be filtered resulting in partial clogging of the ultrafiltration membranes and,

- clogging related to the volume of filtered water and requiring chemical washing of the membranes.

The implementation, according to the invention, of such an interactive regulation allows the operator to continue to ensure the production of water for a sufficient duration before carrying out the intervention necessary for restoring the initial filtration performance.

The subject of the invention is therefore a method for regulating the flow rate of water treated in a membrane bioreactor which comprises elimination of nitrates by biological means, adsorption of the materials in suspension on powdered activated carbon, these two stages being carried out simultaneously in said biological reactor, and disinfection by ultrafiltration on membranes, the retentate being recycled into the bioreactor characterized in that it consists in carrying out a pressurization of the internal compartment of the ultrafiltration membranes, in response to a reduction in the flow rate of the permeate , i.e. treated water, below a predetermined set value, this pressurization causing an increase in the permeate flow.

According to the invention, it is also possible to provide a measurement of the pressure in the predetermined compartment, this pressure measurement being able to be carried out for example at the inlet of the ultrafiltration membranes.

The invention also provides a regulating device for implementing the method as specified above, this device being characterized in that it comprises:

- a flow meter placed on the permeate circuit of the ultrafiltration membranes;

- an automatic valve placed on the membrane retentate circuit and,

- an electronic regulator receiving information transmitted by said flow meter in order to actuate said automatic valve when the permeate flow is lower than said set value, in order to reduce this flow to this set value, by pressurizing the internal compartment of said membranes .

According to a variant of this device, it also comprises:

a second electronic valve placed on the permeate circuit of said membranes and,

- a second electronic regulator receiving the information transmitted by said flow meter in order to actuate the automatic valve placed on the retentate circuit, either when said second automatic valve is open at 100%, or when the water flow rate is less than said value setpoint. According to another embodiment of this device, it further comprises a pressure sensor which is positioned in the retentate circuit, in particular at the entrance to the retentate compartment of the ultrafiltration membranes, in order to comply with a preset pressure setpoint. , displayed in the electronic regulator actuating the automatic valve placed on the retentate circuit.

Referring now to Figure 3 of the accompanying drawings which illustrates schematically a non-limiting embodiment of a device implementing the method of the invention.

As stated above, the invention consists in pressurizing the internal compartment of the ultrafiltration membranes when the flow of treated water decreases below a set value set by the operator. . In fact, during operation, due to clogging of the ultrafiltration membranes, the flow of treated water tends to decrease. By pressurizing, according to the invention, the internal compartment of the membranes, this flow can be restored to its set value.

In Figure 3, there is shown schematically an installation of the type described above with reference to Figure 1. It is therefore an installation implementing the BRM process. In this figure, we find the bioreactor 1 receiving the mixture of raw water with its suspended matter, denitrifying bacteria, reagents (ethanol, activated carbon powder) and an ultrafiltration membrane 2 (of course, it does not s This is only an example, a BRM type installation generally comprising a separator with several membranes such as 2. According to the present invention, there is provided a first automatic valve 9 placed on the retentate circuit of the membrane 2 and a second automatic valve. 10 positioned on the permeate circuit of this membrane. These automatic valves 9 and 10 are coupled to electronic regulation boxes 8 and 11 respectively which control them. There is provided a flow meter 7 placed on the outlet of the permeate 4, that is to say treated water.

The electronic regulator 11 receives the information transmitted by the flow meter 7 and therefore it controls the valve 10. When this valve 10 is open to 100% or when the flow of treated water decreases below a set value, a signal is sent to the electronic regulator 8 which then actuates the valve 8 which closes with a value sufficient to pressurize the retentate compartment of the membrane 2, which has the consequence of increasing the permeate flow rate. The opening of the valve 9 is then controlled by the regulator 8 in order to ensure the flow rate set by the operator.

According to the invention, it is possible to omit the automatic valve 10 and its electronic regulation unit 11, the flow information delivered by the flow meter 7 being directly transmitted to the electronic regulator 8 controlling the automatic valve 9.

Thus, when the flow rate of the treated water is at the set value, the valve 9 controlled by the regulator 8 is adjusted to the requested flow rate. When the flow of treated water (permeate) comes to decrease as a result in particular of a clogging of the membrane 2, the flow meter transmits information either to the valve 10 for more under the control of the regulator 11, or in the event of deletion of this valve 10 and of its regulator, in the regulating box 8, in order to close the automatic valve 9 of a sufficient value to restore the set flow rate by pressurization of the retentate compartment of the membrane 2.

According to the invention, a pressure sensor 12 can be provided, measuring the value of the pressure in the retentate compartment, for example at the inlet of the membrane 2, in order to comply with a pressure setpoint set by the operator as a function of technical constraints related to the equipment used (for example, limit pressure on the use of modules and membranes, maximum pressure setpoint before washing, critical transmembrane pressure setpoint).

It follows from reading the above description that the present invention effectively makes it possible to ensure control and regulation of the performance of membrane bioreactors by interactive regulation making it possible to extend the duration of production in the event of a reduction in filtration performance. in the cases mentioned above.

It remains to be understood that the present invention is not limited to the exemplary embodiments described and / or shown here but that it encompasses all variants thereof which fall within the scope of the scope of the appended claims.

Claims

1 - Process for regulating the flow rate of water treated in a membrane bioreactor, which comprises elimination of nitrates by biological means, adsorption of the materials in suspension on powdered activated carbon, these two stages being carried out simultaneously in said biological reactor (1), and disinfection by ultrafiltration on membranes (2), the retentate being recycled in the bioreactor (1) characterized in that it consists in carrying out a pressurization of the internal compartment of the ultrafiltration membranes, in response to a decrease of the permeate flow, that is to say of the treated water, below a predetermined set value, this pressurization causing an increase in the permeate flow.

2 - Method according to claim 1 characterized in that there is provided a measurement of the pressure in the retentate compartment of the ultrafiltration membranes in order to meet a preset pressure setpoint.

3 - Method according to claim 2 characterized in that said pressure measurement is carried out at the inlet of the ultrafiltration membranes (2).

4 - Regulating device for implementing the method as specified in any one of claims 1 to 3, characterized in that it comprises:

- a flow meter (7) placed on the permeate circuit (4) of the ultrafiltration membranes (2); - an automatic valve (9) placed on the retentate circuit (6) of the membranes and,

- an electronic regulator (8) receiving information transmitted by said flow meter (7) with a view to actuating said automatic valve (9) when the permeate flow is less than said set value, in order to reduce this flow to this value of setpoint, by pressurizing the internal compartment of said membranes.

5 - Device according to claim 4 characterized in that it further comprises:

a second automatic valve (10) placed on the permeate circuit (4) of said membranes and,

a second electronic regulator (11) receiving the information transmitted by said flow meter (7) in order to actuate the automatic valve (9) placed on the retentate circuit, either when said second automatic valve (10) is open to 100%, or when the water flow rate is lower than said set value.

6 - Device according to any one of claims 4 or 5 characterized in that it further comprises a pressure sensor (12) which is positioned in the retentate circuit, in particular at the entrance to the retentate compartment of the ultrafiltration membranes (2), in order to comply with a preset pressure setpoint, displayed in the electronic regulator (8) actuating the automatic valve (6), placed on the retentate circuit.