BACKGROUND OF THE INVENTION Contemporary industrial activities generate gaseous effluents containing a multitude of chemical compounds and contaminants which interfere with the equilibrium of elements in nature and affect the environment at different levels. Acid rain, the greenhouse effect, smog and the deterioration of the ozone layer are examples that speak volumes about this problem. Reduction of noxious emissions is therefore not surprisingly the subject of more and more legislation and regulation. Industrial activities and applications which must contend with stricter environmental regulatory standards in order to expect any long-term commercial viability will turn more and more to biological and environmentally safe methods. Consequently, there is a real need for new apparatuses and methods aimed at the treatment of gaseous waste or effluents.

There already exists a vast array of technologies aimed at the separation and recovery of individual or mixed gases and a number of different biological methods is known to treat gaseous waste or effluents : bacterial degradation (JP 2000-287679; JP 2000-236870), fermentation by anaerobic bacteria (WO 98/00558), photosynthesis through either plants (CA 2, 029,101 A1 ; JP 04-190782) or microorganisms (JP 03-216180). Among the more popular are those gained through the harnessing of biological processes such as peat biofilters sprinkled with a flora of microorganisms in an aqueous phase, or biofilter columns comprising immobilized resident microorganisms (Deshusses et a/. (1996) Biotechnol. Bioeng. 49,587-598).

Although such biofilters have contributed to technological advances within the field of

gaseous waste biopurification, the main drawbacks associated with their use are their difficult maintenance and upkeep, lack of versatility, as well as time consuming bacterial acclimation and response to perturbation periods (Deshusses et al.).

A number of biological sanitation/purification methods and products is known to use enzymatic processes, coupled or not to filtration membranes (US 5, 250,305 ; US 4,033, 822; JP 63-129987). However, these are neither intended nor adequate for the cleansing of gaseous waste or effluents. The main reason for this is that, in such systems, contaminants are generally already in solution (US 5, 130, 237; US 4,033, 822; US 4,758, 417; US 5,250, 305; WO 97/19196 ; JP 63-129987). Efficient enzymatic conversion and treatability itself of gaseous waste or effluents in liquids therefore depend on adequate and sufficient dissolution of the gaseous phase in the liquid phase. However, the adequate dissolution of gaseous waste or effluents into liquids for enzymatic conversion poses a real problem which constitutes the first of a series of important limitations which compound the problem of further technological advances in the field of gas biopurification.

Although triphasic « Gas-Liquid-Solid » (GLS) reactors are commonly used in a large variety of industrial applications, their utilization remains quite limited in the area of biochemical gas treatment (US 6,245, 304; US 4,743, 545). Also known in the prior art are the GLS bioprocesses abundantly reported in the literature. A majority of these concerns wastewater treatment (JP 09057289). These GLS processes are characterized in that the gaseous intake serves the sole purpose of satisfying the specific metabolic requirements of the particular organism selected for the wastewater treatment process. Such GLS treatment processes are therefore not aimed at reducing gaseous emissions.

As previously mentioned, these systems are neither intended nor adequate for the treatment of gaseous waste or effluents. An additional problem associated with the use of these systems is the non retention of the solid phase within the reactor.

Biocatalysts are in fact washed right out of the reactors along with the liquid phase.

Different concepts are, nonetheless, based on this principle for the reduction of gaseous emissions, namely carbon dioxide. Certain bioreactors allow the uptake of

C02 by photosynthetic organisms (JP 03-216180) and similar processes bind C02 through algae (CA 2,232, 707; JP 08-116965; JP 04-190782; JP 04-075537).

However, the biocatalyst retention problem remains largely unaddressed and constitutes another serious limitation, along with gaseous effluent dissolution, to further technological advancements.

The main argument against the use of ultrafiltration membranes to solve this biocatalyst retention problem is their propensity to clogging. Clogging renders them unattractive and so, their use is rather limited for the retention of catalysts within reactors. However, a photobioreactor for medical applications as an artificial lung (WO 92/00380; US 5,614, 378) and an oxygen recovery system (US 4, 602, 987; US 4,761, 209) are notable exceptions making use of carbonic anhydrase and an ultrafiltration unit.

The patent applications held by the assignee, C02 Solution Inc., via Les Systèmes Envirobio Inc. (EP 0 991 462 ; WO 98/55210; CA 2,291, 785) propose a packed column for the treatment of carbon dioxide using immobilized carbonic anhydrase.

Carbonic anhydrase is a readily available and highly reactive enzyme that is used in other systems for the reduction of carbon dioxide emissions (US 4, 602,987 ; US 4, 743, 545; US 5, 614, 378; US 6, 257, 335). In the system described by Trachtenberg for the carbonic anhydrase treatment of gaseous effluents (US 6,143, 556; CA 2,222, 030), biocatalyst retention occurs through a porous wall or through enzyme immobilization. However, important drawbacks are associated with the use of enzyme immobilization, as will be discussed below.

Other major drawbacks are associated with the use of enzymatic systems. One of these stems from systems where enzymatic activity is specifically and locally concentrated. This is the case with systems where enzymes are immobilized at a particular site or on a specific part of an apparatus. Examples in point of such systems are those where enzymes are immobilized on a filtration membrane (JP 60014900008A2; US 4,033, 822; US 5,130, 237; US 5,250, 305; JP 54-132291; JP 63-129987 ; JP 02-109986 ; DE 3, 937,892) or even, at a gas-liquid phase boundary (WO 96/40414 ; US 6,143, 556). The limited surface contact area

obtainable between the dissolved gas substrate, the liquid and the enzyme active site poses an important problem. Hence, these systems generate significantly greater waste of input material, such as expensive purified enzymes, because the contact surface with the gaseous phase is far from optimal and limits productive reaction rates. Therefore, as mentioned previously, overcoming the contact surface area difficulty should yield further technological advances.

Other examples of prior art apparatuses or methods for the treatment of gas or liquid effluents are given in the following documents: CA 2, 160,311 ; CA 2, 238,323 ; CA 2,259, 492; CA 2,268, 641; JP 2000-236870; JP 2000-287679; JP 2000-202239; US 4,758, 417; US 5,593, 886; US 5,807, 722; US 6,136, 577; and US 6,245, 304.

SUMMARY OF THE INVENTION An object of the present invention is to provide an apparatus that is distinct from and overcomes several disadvantages of the prior art bioreactor for the treatment of gas effluent, as will be discussed in detail below.

Another object is to provide a process for the biocatalytic treatment of gases, which is more efficient with respect to the reaction rate of the reactants.

In accordance with the present invention, that object is achieved with a process characterized in that it comprises a step of contacting a gas phase, containing a particular gas to be treated, to spray droplets of liquid phase containing biocatalysts.

More particularly, the present invention proposes a process for the biocatalytic treatment of gases, comprising the steps of: a) contacting a gas phase, containing a gas to be treated, to a liquid phase containing a reactant capable of absorbing and/or chemically reacting with the gas, thereby producing a spent liquid phase containing at least one reaction product and a treated gas phase substantially free of the gas, such step being performed in the presence of biocatalysts suitable for catalyzing the chemical reaction between the gas and the reactant, the process being characterized in that:

- it comprises, prior to step a) of contacting, the step of mixing the biocatalysts to the liquid phase; and - the step of contacting comprises the step of spraying droplets of the liquid phase containing the biocatalysts.

The liquid phase may be aqueous or non aqueous and the gas to be treated is usually soluble in the liquid. Contact between the gas and liquid phases results in the absorption of the gas to be treated and thus to its extraction from the gas phase.

The use of droplets of liquid containing the biocatalysts, sprayed into the reaction chamber, enables to increase the gas-liquid interface, thereby allowing a high mass transfer rate of the gas to be treated from the gas phase to the liquid phase. These conditions of high mass transfer enable to transform the gas with a maximum reaction rate.

Then, the dissolved gas is transformed in presence of appropriate biocatalysts into one or more products. The role of biocatalysts is to accelerate the transformation reaction of the dissolved gas in the liquid phase environment. In addition to biocatalysts, the liquid phase may contain reactants required for the transformation of the dissolved gas or for a reaction with one or more reaction products of the dissolved gas. Depending on the reactants and biocatalysts present in the liquid phase, products are in a soluble or solid form. Reaction products are preferably further treated to give useful products to be used in other applications or to be disposed of.

Absorption and biocatalytic transformation of the gas take place in the reaction chamber of a spray absorber bioreactor. The gas phase, containing the selected gas to be treated, is fed into the reaction chamberwhere it is contacted to spray droplets of a liquid phase containing the biocatalysts. Droplets are obtained using atomizers.

The gas phase in contact to the liquid phase has one or more components absorbed in the liquid phase. Then, the dissolved gas is transformed because of biocatalysts activity and presence of reactants, if required. The gas phase is almost free of one or more components and exits the reaction chamber purified. The liquid phase

containing the biocatalysts and reaction products (dissolved and/or solid) exits from the reaction chamber. In accordance with a preferred aspect of the invention, the gas is further treated to remove droplets in suspension or to decrease its humidity.

Hence, the present invention is also directed to a biocatalytic treatment unit for the biocatalytic treatment of gases, comprising: - a bioreactor comprising a reaction chamber having a liquid inlet for receiving a liquid, a gas inlet for receiving a gas to be treated, a liquid outlet for discharging a spent liquid and a gas outlet for releasing a treated gas; the treatment unit being characterized in that it comprises; a mixing unit for mixing biocatalysts to the liquid ; means for conveying the liquid from the mixing unit to the liquid inlet of the reaction chamber; and the liquid inlet comprises at least one atomizer mounted within the reaction chamber to spray droplets of liquid containing the biocatalyst.

Biocatalysts are usually very costly. Therefore, it is preferable to recycle the spent liquid phase containing biocatalysts and reactants. However, recycling requires that reaction products be removed from the liquid phase. Therefore, in accordance with a preferred aspect of the invention, the process further comprises the steps of: b) removing from the spent liquid phase the at least one reaction product, thereby obtaining a recycled liquid phase free of the reaction product and containing the biocatalysts ; and c) recycling the recycled liquid phase to step a) of contacting.

In accordance with that aspect of the invention, the treatment unit further comprises a separation unit in fluid communication with the liquid outlet for removing the reaction products contained in the spent liquid. The separation unit has a first liquid outlet for discharging a liquid fraction substantially free of the reaction products and a

second outlet for discharging the liquid fraction containing the reaction products.

Conveying means are provided for conveying the fraction substantially free of the reaction products to the mixing unit.

Removal assures that a maximum mass transfer rate of the gas from the gas phase to the liquid phase is achieved at each pass of the liquid phase in the reaction chamber of the spray absorber bioreactor. Removal of dissolved reaction products is preferably obtained using membrane processes such as ultrafiltration, microfiltration processes and/or ion exchange and/or adsorption processes. Removal may also be obtained by first precipitating the reaction products with appropriate reactant (s) and then by removing the particles using separation processes. Solid particles originating from solid reaction products or subsequently precipitated products are preferably removed by using separation processes such as settling, filtration or expression.

Moreover, agents facilitating removal of particles, such as coagulants or flocculants or filter aids, may be added to the liquid effluent prior to particle removal units or to the liquid phase entering the spray absorber bioreactor.

A biocatalyst is a biological entity, which can transform a substrate in one or more products. The biocatalysts used in the process are preferably enzymes, cellular organelles (mitochondrion, membranes), animal, vegetal or human cells. The biocatalysts can be used free or immobilized. Immobilization is preferably the result of fixation to a solid support, entrapment inside a solid matrix. Immobilization can also be obtained by using intermolecular binding of biocatalysts molecules or structures. In all these cases, the solid support, the solid matrix and the intermolecular binding must be in the form of micro particles sufficiently small to pass through the atomizer with the liquid phase.

Depending on patterns of spray, droplet size, uniformity of spray, turndown ratio and/or power consumption, available atomizers may fall in three categories: pressure nozzles, two-fluid nozzles or rotary devices. However, any other type of atomizer may be used.

Membrane processes for removal of dissolved products or solid particles may include the use of flat or tubular membranes. Those membranes are preferably involved in

different modules such as plate-and-frame, spiral-wound, tubular capillary and hollow fiber module. The operation of those modules may be dead-end or cross-flow (co- current, countercurrent, cross-flow with perfect permeate mixing and perfect mixing).

Those filtration units may be used in a single-stage or in a multi-stage process in a single-pass system or a recirculation system.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic representation of a spray absorber bioreactor according to a preferred embodiment of the present invention.

Figure 2 is a schematic flow chart of a second preferred embodiment of the process according to the present invention.

Figures 3a and 3b are schematic flow charts of a third preferred embodiment of the process according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to figure 1, a gas phase containing C02 is fed to a reaction chamber (1) of a spray absorber bioreactor. The gas phase is preferably fed at the bottom (2) of the reaction chamber (1) of the spray absorber bioreactor. The aqueous liquid phase containing biocatalysts is preferably fed at the top of the reaction chamber (3) through atomizers (4) where the liquid phase forms droplets. Additional atomizers may be found at the side (4) of the reaction chamber. In this preferred embodiment, the gas phase flows upward and contact spray droplets of the liquid phase. During the contact, C02 is absorbed and then transformed into bicarbonate and hydrogen ions. This transformation is catalyzed by a biocatalyst accelerating C02 transformation. The biocatalyst is preferably the enzyme carbonic anhydrase but may be any biological catalyst enabling CO2 transformation. C02 transformation reaction is the following : C02 +H20 °"" HCO +H+ Equation 1

This reaction is natural. It is at the basis of C02 transportation and removal phenomenon in the human body and in most living beings. Reaction products are ions in solution in the aqueous liquid phase (6). The treated gas phase (5) exits at the top of the spray absorber bioreactor and is almost free of CO2. However, droplets of the liquid phase may remain in suspension in the purified gas exiting the bioreactor (5). An additional equipment, such as a mist eliminator, is thus preferably added to the spray absorber bioreactor to remove those droplets. Both gas and liquid phases are preferably further treated for proper disposal or reuse.

It is well known that under specific conditions, bicarbonate and/or carbonate ions may react with some cations (Na+, Ca2+, Mg2+, Ba2+) to precipitate. For example, if the liquid phase contains calcium ions and the pH is around 9,0, bicarbonate and carbonate ions are present because of natural physicochemical equilibrium reactions. These carbonate ions may react with calcium ions (see Eq. 2 below), for example, by adding Ca (OH) 2 to the solution, thus leading to the formation of solid particles of calcium carbonate in the liquid phase. Consequently, bicarbonate ions transform into carbonate ions, because of equilibrium between both species. Those newly formed carbonate ions will then lead to the formation of calcium carbonate. In this case, an agitation device is preferably required to facilitate discharge of solid particles. Moreover, the bottom of the bioreactor preferably has a conical shape.

C032-+ Ca2+=> CaC03 Equation 2 Biocatalysts are usually costly material. Therefore, it is interesting to recycle the liquid phase containing the biocatalysts. Figure 2 shows a process implying steps described previously in figure 1. However, for the recycling of the liquid phase, additional considerations have to be made. COs removal process is based on absorption, thus removal of the reaction products (HCO3-and H+) is required in order to maintain optimal conditions for C02 mass transfer. In this case, bicarbonate and hydrogen ions are soluble. Bicarbonate ions are preferably removed in a product removal process (11) such as membrane separation processes (ultrafiltration, nanofiltration), ion exchange or adsorption units separately or in combination.

Referring now to figure 2, the liquid phase containing biocatalysts and rich in reaction products (8) is fed to a product removal process (11). Two effluents (12,10) are generated: one (12) rich in reaction product, bicarbonate ions and one (10) containing a very low level of bicarbonate ions. This latter liquid phase is preferably enriched in fresh liquid phase (9) or biocatalysts to compensate for possible loss of those two components. Acid or alkali may also be added to the liquid phase in order to control pH of the liquid phase. pH control may also be done inside the product removal process. In the present application, alkali such as NaOH might be added to the liquid phase to neutralize protons produced during the biological transformation of C02 (Equation 1). The resultant liquid phase (7) enters at the top of the bioreactor.

The effluent (12) is preferably further treated for proper disposal or use in another process.

Turning now to figures 3a and 3b, when solid particles are present in the liquid phase (14) because of a precipitation reaction (Eq. 2), solid particles have to be removed for recycling of the liquid phase. The particles removal process (15) may consist of one or more separation units, as shown in figure 3a. Separation is preferably performed by settling (by gravity, centrifugal force, heavy media, flotation, magnetic force) and/or filtration (on screens or on filters (by gravity, pressure, vacuum or <BR> <BR> centrifugal force) ) and/or expression (batch presses or continuous presses (screw<BR> presses, rolls or belt presses) ). Moreover, agents facilitating removal of particles such as coagulants or flocculants or filter aids may be added to the liquid effluent (16) prior to or in the particle removal process or to the liquid phase entering the spray absorber bioreactor (13). The liquid phase (18) exiting the particles removal process is preferably supplemented in fresh liquid phase and/or biocatalysts (9).

Moreover, acid or alkali may also be added to liquid phase for pH control. The liquid phase (13) is then fed to the bioreactor. Solid particles (17) obtained may further be treated or be disposed of.

The separation unit may, in a particular case, be integrated to the bioreactor. Figure 3b shows a process where separation by gravity is integrated to the bioreactor. In this particular case, an agitation device is preferably added at the bottom of the bioreactor and the bottom of the bioreactor preferably has a conical shape.

Example An experiment was conducted to validate the concept of a spray absorber for removal of CO2. The process diagram of the spray absorber for the test was similar to the one shown in figure 1. The spray absorber consists of a column having a 7,5 cm diameter and a 70 cm height. A pressure nozzle atomizerwas mounted within the reaction chamber. Five litres of a 12 mM Tris solution with 20 mg carbonic anhydrase per litre of solution were pumped into the spray absorber at a flow rate of 1,5 I/min. Carbonic anhydrase was used free within the Tris Solution. The Tris solution is a buffer consisting of 2-amino-2-hydroxymethyl-1, 3-propanediol. The solution was used in a closed loop operation, until the Tris solution was saturated with dissolved COs. The gas flow rate was 6,0 g/min at a C02 concentration of 52000 ppm. Gas and solution were at room temperature. The pressure inside the spray absorber was set at 5 psig.

The results obtained showed that the C02 contained in the gas phase was removed at a rate of 2,3 x 10-3 mol of C02/min.

These results were compared with the ones obtained from experiments conducted with a conventional bioreactor using a reaction chamber filled with carbonic anhydrase immobilized on rashig supports. The following table provides a comparison of these results. Parameters Spray absorber Prior art packed according to the bioreactor invention Concentration of COz in 52000 pm 50000 ppm the gas phase Liquid flow rate 1,5 I/min 0,5 I/min Biocatalyst Carbonic anhydrase free Carbonic anhydrase in liquid immobilized in the bioreactor Gas flow rate 6,0 g/min 1,5 g/min Ratio of liquid flow rate to 0, 251/g 0, 5 I/g gas flow rate Mass of enzyme within the Less than 100 mg* 275 mg reactor Removal rate of C02 2, 3 x 10-3 mol of C02/min 1,3 x 10-' mol of C02/min

* Five liters of Tris solution contains 100 mg. However, for the absoption of C02, only a portion of the enzyme ends in the reaction chamber.

These results show that the process according to the invention is surprisingly more efficient than the process known in the prior art, since 1) at all times, the enzyme participating in the removal of CO2 is less, therefore the removal of C02 requires less enzyme, and 2) even though the ratio of liquid flow rate to gas flow rate is lower with the process according to the invention, the removal rate of CO2 is greater. Indeed, it is well known for a person skilled in the art that in a process for absorbing a gas, if that ratio decreases, the performance of the process also decreases. In other words, the performance of the packed bioreactor would have been even less if the ratio of liquid flow rate to gas flow rate used had been 0,25 I/g, as for the experiments with the spray absorber of the invention.

Although the present invention has been explained hereinabove by way of preferred embodiments thereof, it should be understood that the invention is not limited to these precise embodiments and that various changes and modifications may be effected therein without departing from the scope or spirit of the invention.