FR2822150A1 - Water treatment procedure using plant with biological reactor and filter membrane employs oxygen-lean gas to clean membrane - Google Patents

Abstract

The procedure, comprising stages in which the water is passed through a biological reactor (1) designed to receive a biomass and through at least one filter membrane (3) immersed in the reactor or in a second reactor linked to the first, introducing air or oxygen for aerobic or anoxic phases, recuperating the clarified effluent and cleaning the membrane with an injected oxygen-lean gas. Where two reactors are used at least one of them is closed, and the oxygen-lean gas can be obtained from the top of a closed reactor or a supplementary gas comprising nitrogen, carbon dioxide and oxygen-lean air can be employed. The procedure can also use air fractionated to produce an oxygen-lean fraction used for cleaning the membrane and an oxygen-rich fraction for the reactor.

Description

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Method and installation for water treatment using at least one submerged membrane bio-reactor.

 The invention relates to the field of water treatment for the purpose of purification or for the purpose of drinking water.

 More specifically, the invention relates to the field of bio-reactors with submerged membranes conventionally used in this field.

 According to the prior art, submerged membrane bio-reactors have essentially two configurations.

 According to a first configuration, they comprise a biological basin provided with means for supplying the water to be treated, in which a biomass is developed capable of degrading the carbon and nitrogen pollution of the water passing through the basin. Inside is also provided a set of filtration membranes, intended to be immersed by the water passing through the basin and making it possible to filter it. This device also includes a pipe for discharging purified and filtered water cooperating with pumping means.

 In order to be able to provide ventilation periods and anoxic periods within the basin allowing degradation of nitrogen pollution, the basin is conventionally provided with air supply means.

 According to another configuration, the bio-reactors comprise a first biological tank provided with means for supplying water to be treated and with means for injecting air allowing aerated periods and anoxic periods to be provided therein, and a second basin communicating with the first biological basin in which a set of membranes is provided, connected to means for extracting the filtrate. According to this configuration, the installation further comprises means for recirculating the content of the second tank upstream of the first tank.

 One of the drawbacks of bio-reactors is the tendency of the membranes they integrate to become clogged.

 In fact, these membranes are in contact with a relatively charged medium consisting of water to be treated and biological sludge present in them.

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 To prevent clogging and, if necessary, to unclog the membranes of the bioreactors, it is known to agitate them by injecting air.

 The two known configurations of bio-reactors described above are therefore provided with air injection means near the membranes.

 However, such a solution has the drawback of requiring the supply of large quantities of air in order to obtain an effective result, that is to say an effective and relatively durable unclogging of the membranes. Since the air contains a high proportion of oxygen, this massive injection of air in a continuous periodic manner disturbs the anoxic phases necessary for the denitrification process involved in the degradation of nitrogenous pollutants in the water to be treated.

 In addition, it was found that this air injection led the biomass to generate clogging substances increasing the fouling speed of the membranes.

 The main objective of the present invention is to propose means making it possible to solve these problems.

 More specifically, one of the objectives of the invention is to propose a process for treating water using a bio-reactor using a biomass and at least one submerged membrane making it possible to prevent clogging of the membranes or of unclog membranes very effectively without involving a disturbance in the denitrification process.

 Another objective of the invention is also to propose such a process which results in limiting the natural propensity of the biomass to produce abundant products in the presence of oxygen.

 Yet another objective of the present invention is to provide facilities for the implementation of such a method.

 These various objectives as well as others which will appear subsequently are achieved thanks to the invention which relates to a water treatment process comprising the steps consisting in passing said water through at least one

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 first biological reactor accommodating a biomass and through at least one filtration membrane, said membrane being immersed in said reactor or in a second reactor communicating with said first reactor, to supply air or pure oxygen to said first reactor so as to provide aerobic phases and anoxic phases therein, and to recover the clarified effluent after passage through said membrane, said method also comprising a step of cleaning said membrane by continuously or sequentially injecting a gas near or inside said membrane, characterized in that it consists in using a gas depleted in oxygen to carry out said step of cleaning said membrane.

 The invention therefore proposes to no longer use air to agitate the membranes and prevent clogging thereof or allow their unclogging, but an oxygen-depleted gas.

 It will be noted that in the present description, by oxygen-depleted gas is meant any gas whose natural oxygen content has been lowered by any means.

 Such an oxygen-depleted gas has the advantage of allowing mechanical action on the membranes while providing little or no oxygen to the surrounding biomass and thus preventing disturbance of anoxia and bacterial growth.

 According to a first embodiment, the first and / or the second reactor used is a closed reactor, and the method consists in using the oxygen-depleted gas present in the sky of said closed reactor to carry out the step of cleaning said membrane .

 By using a closed reactor, that is to say a reactor making it possible to provide a space between the level of the effluent present in it and the closing means, said space being commonly called by the man of the art "sky", it is possible to trap in the sky of the reactor the process air having passed through it and having been, during this transit, depleted in oxygen due to the activity of the biomass which consumed it. This oxygen-depleted air is,

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 according to this variant of the invention, routed near the membranes to clean them.

 According to this embodiment of the method, it may be envisaged an additional step consisting in continuously or intermittently injecting a make-up gas containing little or no oxygen into the headspace of the reactor. This injection may be necessary in two cases: - when the air flow rate for the oxygen supply necessary for the biomass in the first tank is less than the air flow rate for mixing the membranes; - in cases where there is a risk of precipitation of mineral salts (CaC03 for example) on the membranes. In this case, it will be beneficial to inject a membrane mixing gas enriched in CO2 which will acidify the surface of the membranes and the liquid medium in the immediate vicinity thereof.

 This make-up gas may in particular consist of nitrogen, carbon dioxide or oxygen-depleted air.

 Advantageously, the method comprises a step consisting in automatically regulating the injection of the make-up gas with respect to a set point relating to the pressure of said gas and / or to the oxygen concentration in the overhead gas.

Such a process has many advantages.

 The gas, since it is depleted in oxygen, makes it possible to minimize the propensity of the biomass to be fixed on the membranes and thus to reduce their clogging power.

 Furthermore, the gas used to clean the membranes being taken from the sky above the reactor, it is relatively hot. Its temperature makes it possible to improve its efficiency in unclogging the membranes or preventing them from clogging.

 Finally, it should be noted that this gas will gradually become charged with CO2, which will limit the mineral deposits on the membranes by acidification of their surface and of the liquid medium near them.

 According to another variant of the method according to the invention, it comprises a step consisting in fractioning the air supplied in the first

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 reactor to obtain an oxygen-enriched fraction and an oxygen-depleted fraction and to use the oxygen-depleted fraction to perform the step of cleaning said membrane and the oxygen-enriched fraction in the biological reactor.

 The invention also relates to installations for implementing the method as described above.

According to a first embodiment of the invention, the installation comprises at least a first reactor intended to receive a biomass and at least one filtration membrane provided in said reactor or in a second reactor communicating with said first reactor, and means for supplying air to said first reactor. and is characterized in that at least one of the reactors is closed and in that it comprises means for conveying the gas present in the sky from the reactor to said membrane
Advantageously, these conveying means include at least one pipe connecting the upper part of the reactor to the lower zone of the membrane, and pumping means provided on said pipe if necessary.

 Preferably, the installation also includes means for injecting a make-up gas into the headspace of the reactor.

 Also preferably, the installation comprises means for automatic regulation of the injection of make-up gas.

 According to another variant of the invention, the installation comprises at least a first reactor intended to receive a biomass and at least one filtration membrane provided in said reactor or in a second reactor communicating with said first reactor, and means for supply of gas to said reactor, characterized in that it comprises fractionation means cooperating with said gas supply means and means for conveying a fraction of said gas to said membrane.

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 According to this embodiment, the installation preferably has a pipe going from the fractionation means to the said membrane and from the pumping means.

- la figure 1 représente un premier mode de réalisation de l'invention ; - la figure 2 représente une variante du premier mode de réalisation montré à la figure 2 ; la figure 3 représente une deuxième mode de réalisation de l'invention ; - la figure 4 représente une variante du troisième mode de réalisation montré à la figure 3. The invention and its various advantages which it presents will be more easily understood thanks to the following description of embodiments with reference to the figures in which:

- Figure 1 shows a first embodiment of the invention; - Figure 2 shows a variant of the first embodiment shown in Figure 2; FIG. 3 represents a second embodiment of the invention; FIG. 4 represents a variant of the third embodiment shown in FIG. 3.

 According to the first embodiment of the invention shown in Figure 1, the membrane bio-reactor installation comprises a reactor 1 closed by a roof provided with means for supplying water to be treated inside. of this reactor 1. The gaseous volume defined between the surface of the water present in the reactor 1 and the roof 1a constitutes the gaseous sky 6 of the reactor 1.

 This reactor 1 is moreover provided with means 4 for supplying air in its lower part.

 These means 4 can operate intermittently so as to provide aeration phases and non-aerated phases (anoxic phases) conducive to the nitrification-denitrification process aimed at reducing the nitrogen pollution present in the water to be treated.

 The installation represented in FIG. 1 also includes a set of membranes 3 immersed in the water present inside the reactor 1 and is provided with means 21 for evacuating the filtered water having passed through these membranes.

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 In accordance with the present invention, the installation also includes means 5 for conveying the gas present in the sky 6 from the reactor 1 at the foot of the membranes 3.

 These means 5 including a pipe 8 going from the sky above the reactor at the foot of the membranes and a pump 7 making it possible to suck the gas present in the sky of the reactor and to dispense it at the level of the membranes so that it prevents clogging of these. these or actually unclog these.

 The operation of the installation shown in Figure 1 is as follows.

 The water to be treated arrives in reactor 1 by the supply means 20.

Thanks to the use of the air supply means 4, this water undergoes an alternation of aeration and non-aeration phase allowing, thanks to a biomass housed inside the reactor 1, to degrade carbon pollution and nitrogen pollution of the water to be treated.

 This water then passes through the membranes of the set of membranes 3 so as to remove the suspended solids therefrom.

 The purified and filtered water is then evacuated by the evacuation means 21.

 The oxygen-depleted gas present in the headspace 6 of the reactor 1 is constituted by the air having passed through the treated water present in the reactor. Part of the oxygen present in the air supplied by the means 4 has in fact been consumed by the biomass. This oxygen-depleted air is conveyed via the pipe 8 and thanks to the pump 7 at the foot of the set of membranes 3 , which makes it possible to clean them.

 In the variant shown in FIG. 2, the installation no longer comprises a single reactor but two reactors, namely, a first reactor 1 in which the means 4 for supplying process air are provided and a second reactor 2 at the inside which the set of membranes 3 is installed. The two reactors 1 and 2 communicate with each other by a conduit 22.

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 Recycling means 23 for the water present in the reactor 2 make it possible to re-route part of this water upstream from the first reactor 1.

 The two reactors 1 and 2 are closed by the same roof 1a.

 According to the present invention, means 5 including a pipe 8 connecting the roof of the reactor 6 at the foot of the membranes of the set of membranes 3 and a pump 7 are provided for conveying the oxygen-depleted gas present in the sky of the reactor 6 at the foot membranes to clean them.

 In the second embodiment shown in FIG. 3, the installation comprises an open reactor 1 provided with means for supplying a gas 4 and a set of membranes 3 immersed in the reactor 1.

 This reactor 1 is equipped with means 20 for supplying the water to be treated and with means 21 for discharging the filtered water in the set of membranes 3.

 According to the present invention, air fractionation means 10 are provided and make it possible to obtain, on the one hand, oxygen-enriched air supplied to the means 4 and, on the other hand, a fraction of oxygen-depleted air. . These fractionation means may be constituted by any means known to those skilled in the art and in particular by sieves, by P. S. A, by gas permeation.

 The oxygen-enriched air is used as process air while the oxygen-depleted air is supplied by means 11 including a pipe 12 and a pump 3 at the bottom of the set of membranes 3 to clean them.

 In the variant shown in FIG. 4, the same means are used with the exception that in this variant, the installation comprises a first reactor 1 in which the gas supply means 4 are provided and a second reactor 2 in which the set of membranes 3 is provided.

 The embodiments of the invention described here are not intended to reduce the scope of these.

Claims (12)

 CLAIMS 1. A method of water treatment comprising the steps of passing said water through at least one first biological reactor accommodating a biomass and through at least one filtration membrane, said membrane being immersed in said reactor or in a second reactor communicating with said first reactor, supplying air or pure oxygen to said first reactor so as to provide aerobic phases and anoxic phases therein, and to recover the clarified effluent after passage through said membrane, said method also comprising a step of cleaning said membrane by continuously or sequentially injecting a gas near or inside said membrane, characterized in that it consists in using an oxygen-depleted gas to proceed during said step of cleaning said membrane. 2. Method according to claim 1 characterized in that at least one reactor is a closed reactor, and in that it consists in using the oxygen-depleted gas present in the sky of said closed reactor to carry out the cleaning step of said membrane. 3. Method according to claim 2 characterized in that it comprises an additional step consisting in continuously or intermittently injecting a make-up gas containing little or no oxygen in the headspace of the reactor. 4. Method according to claim 3 characterized in that said gas

 make-up is chosen from the group consisting of nitrogen (Nz), carbon dioxide (Cûz) and oxygen-depleted air. 5. Method according to any one of claims 3 or 4 characterized in that it comprises a step consisting in automatically regulating the injection of the make-up gas with respect to a set point relating to the pressure of said gas and / or the oxygen concentration in the gaseous sky. 6. Method according to claim 1 characterized in that it comprises a step consisting in fractionating the air supplied to the first reactor to obtain a fraction enriched in oxygen and a fraction depleted in oxygen and in using the fraction depleted in oxygen to proceed to the stage of <Desc / Clms Page number 10>  cleaning of said membrane and the oxygen-enriched fraction in the biological reactor. 7. Installation for implementing the method according to one of claims 2 to 5 comprising at least one first reactor (1) intended to receive a biomass, and at least one filtration membrane (3) provided in said first reactor ( 1) or in a second reactor (2) communicating with said first reactor (3), and means (4) for supplying air into said first reactor (1), characterized in that at least one of the reactors ( 1,2) is closed and in that it comprises means (5) for conveying the gas present in the sky (6) from the reactor (1,2) to said membrane (3) 8. Installation for implementing the method according to claim 7 characterized in that said conveying means (5) include at least one pipe (8) connecting the upper part of the reactor to the lower zone of the membrane and means pumping (7) provided on said pipe (8). 9. Installation according to claim 7 or 8 characterized in that it includes means for injecting a make-up gas into the sky (6) of the reactor (1,2). 10. Installation according to claim 9 characterized in that it comprises means for automatic regulation of the injection of make-up gas. 11. Installation for implementing the method according to claim 6 comprising at least one first reactor (1) intended to receive a biomass and at least one filtration membrane (3) provided in said first reactor (1) or in a second reactor (2) communicating with said first reactor (1), and means for supplying a gas (4) into said reactor, characterized in that it comprises fractionation means (10) cooperating with said means of gas supply (4) and means for conveying (11) a fraction of said gas to said membrane (3). <Desc / Clms Page number 11> 12. Installation according to claim 11 characterized in that it has a pipe (12) going from the fractionation means (10) to the said membrane (3), and pumping means (13) if necessary.