FR2684094A1 - Process for the treatment of aqueous effluents by bioassisted tangential ultrafiltration - Google Patents

Abstract

The process is intended for treating aqueous effluents, especially industrial aqueous residues originating from mechanical workshops. These effluents consist of an aqueous phase and of a polluting charge comprising dissolved matter, colloidal matter and possibly suspended matter. After the effluents have been collected they are subjected to a tangential filtration on a membrane capable of separating a filtrate from a retentate. According to the invention the process additionally comprises a biological biodegradation pretreatment or cotreatment consisting in converting, by biological assimilation and/or enzyme action, the dissolved and colloidal phases of the effluent to be treated into a particulate phase essentially made up of bacteria or of aggregates of bacteria, the pretreated effluent being next subjected to filtration (ultrafiltration, nanofiltration or even reverse osmosis) on the membrane; the porosity of the membrane is chosen so as to retain the bacterial particulate phase thus formed, so as to concentrate the polluting charge in the retentate and to deliver a depolluted filtrate which may be reusable; and, after filtration, the retentate thus concentrated is removed or recycled. The biological pretreatment or cotreatment is carried out especially by a biomass comprising a bacterial complex consisting essentially of nonpathogenic specific Pseudomonas with the addition of an enzyme complex and of a nutrient charge.

Description

Process for treating aqueous effluents
by bioassisted tangential ultrafiltration
The present invention relates to a method for treating aqueous effluents by tangential ultrafiltration.

 Tangential ultrafiltration consists in passing the effluent to be filtered through a membrane with very low porosity, generally made of ceramic material, in order to extract a filtrate (also called permeate), which leaves a retentate (also called concentrate) formed of elements that have not passed through the membrane. The two essential characteristics of tangential ultrafiltration are due to the fact that, on the one hand, the effluent to be treated is circulated so as to pass through the filter cell parallel to the membrane over the entire extent of the latter ( tangential flow) and that, on the other hand, the effluent to be treated circulates under pressure in the filtering cell, in order to force the establishment of a transmembrane flow (which is therefore oriented perpendicular to the tangential flow). The tangential flow, which is a grazing laminar flow, makes it possible in particular to carry out the self-cleaning of the membrane, the pores of very small dimensions of which would be, in the absence of such a flow, immediately closed by the particles in suspension.

 Here and in the following, the term ultrafiltration will be understood in the broad sense, that is to say that this term will apply equally to ultrafiltration proper, in the strict sense (corresponding to typical pore dimensions between 10 and 500 nm, and which meets most of the conceivable applications), than with nanofiltration (pores from 1 to 10 nm) or even with reverse osmosis which, in addition to the actual filtration of suspended matter, provides a function additional elimination of soluble salts.

 Tangential ultrafiltration, despite its proven effectiveness, however has a number of drawbacks which have so far limited its use to restricted, specific areas.

 First, a number of physical considerations limit the effectiveness of this process.

 Firstly, because of the fineness of the pores, the treatment rate per unit area is low, which means that large membrane areas have to be provided, greatly reducing the cost of the installation.

 In addition, depending on the particulate composition of the pollutant load and taking into account the distribution of the pore size, which can be quite spread out, it is possible to quickly plug certain channels of the membrane, either by mechanical deformation of the particles under the effect of pressure, either by clogging of the pores with particles of practically identical size; under these conditions, the transmembrane flow (which is proportional to the area representing, on the pore distribution curve, the set of unplugged pores) which was already relatively small, further decreases, hence the need to carry out a frequent unclogging of the membrane pores.

This unclogging, which is carried out mechanically by operating in back pressure, results in a significant additional cost of use since it can only take place after the filter has stopped and the treatment itself has been interrupted.

 Another series of drawbacks results from the phenomenon known as concentration polarization due to the fact that liquids, depending on their rheological qualities and the level of their polluting load, more or less quickly form a gel on the surface of the membrane when they receive a pressure, which thereby limits the transmembrane flow and can go as far as blocking the filtration process. It is therefore necessary, on the one hand, to increase the membrane surface in proportion to the increase in the transmembrane mass transfer sought - this in order to remain below the polarization threshold - and, on the other hand, here again, provide regular mechanical unclogging to eliminate the surface gel that could have formed on the membrane.

 For these reasons, the tangential ultrafiltration process has so far been limited to noble applications, essentially food applications such as the production of cheeses (the element retained being the retentate) or fruit juice (the element preserved being the filtrate).

 In the industrial field, tangential ultrafiltration has received few applications; one can however cite its use for the reprocessing of effluents containing hydrocarbons which one wishes to recover (for example cutting oils with high performances), these hydrocarbons being recovered in the filtrate and the retentate being rejected or recycled for additional treatment .

 One of the aims of the present invention is to extend the field of tangential ultrafiltration to much more diverse applications, in particular the large-scale treatment of industrial effluents, by proposing an original way of overcoming the aforementioned drawbacks linked to clogging of pores and the phenomenon of polarization of concentration on the membrane.

 As will be seen, the invention can in particular be applied (this example being of course in no way limiting the possibilities of the invention) to the treatment of industrial waste water such as the effluents produced in garages, vehicle parking lots engine, mechanical workshops, maintenance workshops, etc., which are all typically effluents produced in large quantities, the aqueous phase of which is present in high proportion, but whose level of polluting load nevertheless requires those who produce them treat them before dumping them down the drain.

 The origin of this wastewater is, for example, washing vehicles, cleaning mechanical parts, local floors, maintenance pits, etc., using low or high pressure water jets, the preparation and degreasing of surfaces by degreasing baths, often hot, using washing machines, all common operations in garages or mechanical workshops.

 Regarding the nature of the pollutant load, this is generally quite complex, both from a physical and chemical point of view.

It can contain:
suspended matter (particles of the order of micro)
meter and above), more or less dense but easy to separate
rer, such as sand, dust, paper, mud, etc.,
colloidal matter (particles of the order of 10 nm), included
ant in particular emulsions of hydrocarbons (aromatic
or aliphatic), mineral grease, drain oil,
diesel lost on the charging stations, net residues
toying, etc., of hydroxylated cyclic compounds (phenols, or even
polyphenols), etc. ; these colloidal materials are, of course, separate
effluent sands by filtering, but a
long residence time, which means that, in practice, this type
polluting load is not economically filterable,
- solubilized materials (solvents, halogenated derivatives,
heavy metals in the form of cations, cyanides, nitrites,
nitrates, phosphates, etc.); this type of charge, by definition,
cannot be separated by filtration, except in the specific case
reverse osmosis filtering.

 In practice, there are relatively few economic means to deal effectively with this type of pollution.

 Generally, it is planned to carry out pollution retention works, called gravity separators or hydrocarbon traps, in which the immiscible phase of the effluents (mainly consisting of hydrocarbons) is collected after a certain period of stay, and sent to a center. specialized treatment; the rest of the effluent is discharged to the sewer (in fact, given its residual level of physical pollution (presence of abrasive particles) and chemical (presence of a subsidiary colloidal phase, pH and redox potential likely to cause a corrosion), it is not suitable for reuse, and therefore discharged into the sewer.

 R is also recognized that, during the emptying of retention structures, in particular oil traps, a large quantity of little polluted water is mixed during pumping with the stored charge, which results in a significant additional treatment cost by compared to the actual load to be eliminated (the increased volume increasing the cost of transport to the treatment center, as well as the cost of treatment in this center).

 In addition, the cleaning and pumping of gravity separators must be relatively frequent, and these various installations are both bulky (footprint, in particular), complex and costly to manage (pH regulation, recovery of floating materials, etc. ).

One of the aims of the present invention is, in particular, to overcome all these drawbacks by allowing continuous treatment of industrial waste water, practically without storage, with:
- a high concentration of the pollutant load (hence a decrease
the cost of its emptying and its reprocessing by the
specialized center),
- a significant reduction in the size of the milking facilities
is lying,
- a possibility to reuse water (which constitutes the major
part of the volume of the effluent) after treatment and this, so
practically direct, delivering water that is both
depolluted (in particular, devoid of hydrocarbons), not abra
sive (absence of particles) and non-corrosive (neutral pH,
whatever the variations in the pH of the effluent received, and
low redox potential), and therefore zero discharge to the sewer; account
given the importance of the water content of the charged fluid, it
it is indeed economically interesting to separate the phases
so that you only have to remove the pollutants
very, and thus be able to reuse the filtrate after having separated the
two phases by filtration.

 However, the existing methods currently cannot meet these requirements, nor can the tangential filtration process described above, which will allow the colloids and the fraction soluble in the filtrate to pass, or (if there is a very low threshold of cut) will only allow the soluble fraction to pass but with a very low transmembrane flow.

 The main idea of the invention consists in transforming the colloidal charges and the soluble charges (or, at the very least, the major part of the latter) into a particulate phase, easily filterable.

This transformation of state (from soluble to particulate or from colloidal to particulate) is carried out, in a characteristic manner of the invention, by biological route, thanks to a pretreatment or co-treatment by bacterial route (digestion with a biomass, always very simple and economical to use) and possibly enzymatic.

 In addition, digestion with biomass modifies the viscosity of the medium and therefore significantly modifies the polarization threshold of the membrane; the transmembrane flow can then be multiplied in ratios from ten to twenty compared to filtration without biological treatment, which makes it possible to increase the filtration flow without having to increase the filtration surface.

 More specifically, the process of the invention, which is a process for treating aqueous effluents, in particular industrial waste water from mechanical workshops, these effluents consisting of an aqueous phase and a polluting load comprising solubilized materials, colloidal materials and, possibly, suspended materials, in which, after having collected the effluents, they are subjected to a tangential filtration on a membrane suitable for separating a filtrate from a retentate is characterized, according to invention: in that it further comprises a biological pretreatment or biodegradation consisting in transforming, by biological assimilation and / or enzymatic action, the solubilized and colloidal phases of the effluent to be treated into a particulate phase essentially formed of bacteria or of bacteria aggregates, the pretreated effluent then being subjected to filtration on the membrane; in that the porosity of the membrane is chosen so as to retain the bacterial particulate phase thus formed, so as to concentrate the pollutant load in the retentate and to deliver a depolluted filtrate which may be reusable; and in that, after filtration, the retentate thus concentrated is removed or recycled.

 Thus, by the action of the bacterial biomass, a colloidal pollution (colloids of the order of 10 nm typical, which cannot be economically filtered) is transformed into a particulate pollution consisting of bacteria whose dimensions are generally of the order of 1 to 5 Rm, these bacteria being able moreover to gather in flocs (dimensions of the order of 50 to 200 Rm), and therefore very easily filterable, with a high flow rate and without significant risk of blockage of the pores of the membrane.

 Very advantageously, the biological pretreatment or co-treatment is carried out by a biomass comprising a bacterial complex consisting essentially of specific non-pathogenic Pseudomonas, this biomass preferably being added beforehand with an enzymatic complex consisting essentially of hydrolases from the family of esterases and carbohydrases , as well as a nutritional burden.

 The filtration implemented is generally an ultrafiltration, but can also be a nanofiltration or even reverse osmosis (which also makes it possible to eliminate the soluble salts).

0
We will now explain in more detail the process implemented by the invention.

 We can consider that, to reduce a given pollution, bacteria have two perfectly defined modes of action: assimilation (protoplasmic synthesis) and enzymatic hydrolysis.

With regard to assimilation, as soon as all the constituent elements of the biomass are present in the culture medium, the bacteria do not reproduce duplication, the synthesis of a new biomass being carried out from a substrate to from which the bacteria will feed. We can schematize this process by the following chemical balance (aerobic route): n (Ca, Hb, OC, Nd) + m (02) -> p (C5H7O2N) + q (H2O) + r (CO2), (Ca, Hb, OC, Nd) representing the composition of the substrate to be degraded, and
C5H702N being the generic formula representative of the bacterial biomass.

 Bacterial growth continues until the substrate is depleted. By an appropriate choice of bacteria, practically any substrate can be made biodegradable and thus pass from a solubilized or colloidal state to a particulate state made up of bacteria or aggregates of bacteria, easily separable on a membrane.

 The second phenomenon involved, enzymatic hydrolysis, alters long chains which cannot be assimilated in the state, particularly hydrocarbons (aromatic or aliphatic), which are generally not directly assimilable, especially when their large number of atoms. carbon represents too high a binding energy to overcome by simple direct assimilation.

 By breaking these molecules by hydrolysis, we modify their steric hindrance and their molecular weight so as to allow the crossing of the cytoplasmic membranes of bacteria.

 To hydrolyze these hydrocarbons, whether aromatic or aliphatic, it is possible to use selected bacteria so that they can grow on the substrate to be treated and above all, that they are capable of excreting the necessary enzymes (hydrolases ), making it possible to cut the polymers into shorter chains, or even into monomers.

 We use a mixture of appropriate strains of bacteria capable of maintaining hydrolysis and above all of directing metabolism. The biomass consists in particular of P. sp. Non-pathogenic (specific pseudomonas), family from which one can choose biotypes, for example P. Stutzeri, adapted to the digestion and hydrolysis of the hydrocarbons present in the effluents to be treated, and taking into account the pH and temperature conditions of this effluent.

 Note, however, that the purpose of this hydrolysis is not, here, to reduce the size of the molecules so that they cross the membrane more quickly - on the contrary, it is a matter of retaining the polluting load and concentrating it in the retentate. In practice, hydrocarbons tend, even at very low cutoff thresholds, to pass from the effluent to the filtrate - which would cause, in the absence of biological pretreatment, a filtrate much more concentrated in hydrocarbons than the effluent basic - it is essential that the hydrocarbons are hydrolyzed and assimilated in a manner that is both rapid and sensitive.

 Therefore, to initiate the necessary hydrolysis reactions and accelerate the development of biomass metabolism, an enzyme complex containing hydrolases, in particular of the family of esterases and carbohydrases, is advantageously added to the biomass, for example the marketed complex under the name SPECIFLOR HYDROCARBURES by LOBLAL, department of the company SOGEVAL, in Laval, France (SPECIFLOR is a registered trademark of the company SOGEVAL).

Finally, it is necessary to add to the biomass an additional charge containing at the same time nutritive elements in which the effluent is deficient (essentially nitrogenous nutrients), cofactors and growth factors of the biomass. Such an additional charge is, for example, the product sold under the name EXIFLOR by LOBIAL, department of the company SOGE
VAL, in Laval, France (EXIFLOR is a registered trademark of the company
SOGEVAL). This nutritional load contains all the elements necessary for the maintenance of the biomass metabolism.

 It will be noted here that, depending on the circumstances, the biological treatment can be either a pre-treatment or a co-treatment (that is to say a treatment concomitant with filtration), all other things unchanged.

 Optionally, during the pre-treatment or co-treatment stage, the water is oxygenated enough (air bubbling) to allow the maintenance of aerobic reactions.

 After reacting the industrial waste water and the enzymatic-bacterial complex described above, the effluent is filtered, which gives, on the one hand, a decontaminated filtrate (possibly reusable) and a retentate concentrated in pollutant load which can be evacuated (but with an overall volume much lower than what would have been without biological pretreatment) or recycled for additional concentration. The retentate, that is to say the pollutant load captured during filtration, can be concentrated until total elimination of the free aqueous phase by installing a loop on the membrane circuit.

 Note that this way of proceeding is exactly the opposite of that used in the known technique, mentioned above, of filtering hydrocarbons such as cutting oils by tangential ultrafiltration where, unlike the invention, the hydrocarbons are collected in the filtrate and the retentate is cleaned up.

 Furthermore, it is interesting to note that the biological treatment according to the invention allows effective depollution of the heavy metals contained in industrial waste water. The effluent is indeed often loaded with heavy metals such as lead, zinc, mercury and iron, present in the form of divalent cations.

As the cell walls of bacteria carry many negative charges due to the presence of ligants such as the orthophosphate ion, the carboxyl ion, the hydroxyl ion, etc. and molecules such as chitins, chitosans and glycans, there is created between negative charges (metal cations) and positive charges (cell walls of bacteria) a natural attraction resulting in rapid adsorption (of the order of half -hour), independent of temperature but reversible (it is not an assimilation), of these metal cations. Specific non-pathogenic Rhizopus strains are thus known which can capture up to 9.6% of their weight (in dry extract) of heavy metals, all metals combined; these strains are selected according to the particular application envisaged, according to the nature of the cations to be captured and the physicochemical conditions of the medium where they will develop, and they are incorporated into the basic bacterial reactive mixture.

0
We will now give the result values obtained by implementing the method of the invention.

 After reacting the industrial waste water and the enzymatic-bacterial complex described above, the effluent is filtered at different porosities, and the level of hydrocarbons in the filtrate is measured.

 For pores of 100 nm, in the absence of any biological pretreatment, there is in the filtrate a concentration of hydrocarbons increased by 190% (140% with pores of 50 nm), due to the propensity of the hydrocarbons to cross the membrane, as explained above.

 If, on the contrary, the biological pretreatment according to the invention is applied to the effluent, before tangential ultrafiltration, only 122% of the initial concentration is found in the filtrate if a residence time of one hour is provided, 44.5% if a residence time of 24 hours is foreseen and 0% if a residence time of 36 hours is foreseen (all the hydrocarbons having then been assimilated or hydrolysed).

 As regards the phenols, in the absence of pretreatment, 185% of the initial concentration of the effluent is found in the filtrate, while, with a pretreatment of 6 hours, only 51% of this initial concentration is found, and 0% after 24 hours.

 The yield on the suspended solids is 100%, since the measurement of the suspended solids during the control tests is carried out by means of a reference filter with a porosity of 0.7 or 1.2 μm, which represents a cutoff threshold much higher than that of the membranes used.

 As regards the flow rate, and therefore the efficiency of the filter, this is increased in considerable proportions compared to an ultrafiltration without pretreatment due to the double action of biological pretreatment: on the one hand, the modification of the polarization threshold of the membrane, on the other hand the profound modification of the rheological behavior of the effluent.

 The transmembrane flow can thus be increased by a factor of five for membranes with a high cut-off threshold (200 nm and 500 nm) and by a factor of ten for membranes with lower porosity (50 nm and 100 nm), the flow rate going from 15 1111 / m2 to 150 lXm2 and can even reach 300 l1h / m2.

 In other words, for equal performance, large volumes of effluent can be treated with membranes of small dimensions and therefore at a reasonable cost.

 In addition, as the polarization threshold of concentration of the membrane is pushed back, the frequency of unclogging is considerably reduced, which allows longer work of the loop; finally, the membrane being less soiled, the frequency and the duration of the washes are reduced.

 It is further noted that the filtrate obtained has particularly advantageous physicochemical characteristics which make it suitable for immediate reuse without further treatment; the overall volume of water consumed by the workshop or garage is reduced accordingly, resulting in substantial savings.

More specifically, the aqueous phase extracted from the effluent is:
free of hydrocarbons (as well as phenols, metals
heavy, etc.), as indicated above,
- non-abrasive, because free of its suspended particles
sion: with a filtering at 0.1 llm, the water quality is very known
higher than that of generally prescribed standards, which impo
feels the absence of particles larger than
0.7 Rm or 1.2 Clam, depending on the case,
- chemically neutral from the point of view of its acidity: in mo
yenne, the pH of the filtrate outlet is around 7.2 + 0.2,
therefore practically neutral, despite variations in pH
effluent inlet which can range from 7 to 11 (i.e. 4 orders of
logarithmic size); there is thus a buffer effect
important on pH,
chemically neutral from the point of view of redox:
the water having been oxygenated to maintain the biomass, its po
redox tential is positive, so it is not corrosive
for most common applications.

Claims (7)

 and in that, after filtration, the retentate thus concentrated is removed or recycled.  in that the porosity of the membrane is chosen so as to retain the bacterial particulate phase thus formed, so as to concentrate the polluting load in the retentate and to deliver a depolluted filtrate which may be reusable,  in that it further comprises a biological pre-treatment or co-treatment of biodegradation consisting in transforming, by biological assimilation and / or enzymatic action, the solubilized and colloidal phases of the effluent to be treated into a particulate phase essentially formed of bacteria or aggregates of bacteria, the pretreated effluent then being subjected to filtration on the membrane,  characterized process:  1; A process for treating aqueous effluents, in particular industrial waste water from mechanical workshops, these effluents consisting of an aqueous phase and a pollutant load comprising solubilized materials, colloidal materials and, optionally, suspended matter, process in which, after having collected the effluents, they are subjected to a tangential filtration on a membrane suitable for separating a filtrate from a retentate,  2. The method of claim 1, wherein the biological pretreatment or co-treatment is carried out by a biomass comprising a bacterial complex consisting essentially of specific non-pathogenic Pseudomonas.  3. The method of claim 2, wherein the biomass is previously added with an enzyme complex consisting essentially of hydrolases of the family of esterases and carbohydrases.  4. The method of claim 2, wherein the biomass is previously added with a nutritional load.  5. The method of claim 1, wherein the filtration implemented is an ultrafiltration.  6. The method of claim 1, wherein the filtration implemented is a nanofiltration.  7. The method of claim 1, wherein the filtration implemented is a reverse osmosis.