FR2679547A1 - Method for water purification and reactor for making use of this method - Google Patents

Abstract

Method for water purification, in which the aqueous effluents are delivered into a reactor (1) containing supports (2) on which a biofilm can grow, and thereby bring about the desired conversion of the impurities. The supports are plastic particles which have a large specific surface and a relative density between 0.90 and 1.20. The supports with the biofilm are held in suspension in the water in a reactor for the aerobic, anoxic and anaerobic purification of the water. The invention also relates to a reactor in which this method of purification can be employed.

Description

 The present invention relates to a method for purifying water and to a reactor for implementing this method.

 Various methods are already known for purifying waste water, for example, mechanically, by sedimentation or filtration; chemically, by adding chemical compounds and by treatment with a gas such as, for example, ozone or chlorine. In addition, it is also known to treat water biologically, that is to say by bringing it into contact with a culture of bacteria which will cause the desired conversion of pollutants. To a large extent, currently, all of these methods are more or less combined in existing installations.

 The present invention relates more particularly to the problems relating to biological purification using bacterial cultures.

 The biofilm, which will be discussed later, should be understood as representing a layer of bacterial culture in which the bacteria can be aerobic, anoxic or anaerobic, depending on the nature of purification sought.

 Biological purification methods are first used for the treatment of wastewater, but they can also be applied to the purification of water used in aquacultures or for drinking. The present invention can be implemented in all fields in which biological methods can be used for the purification of water and wastewater, in particular in aerobic biological processes in which the contents of the reactor are subjected to oxygenation and stirred by air injection but it can also be applied to anaerobic processes in which the contents of the reactor are not aerated but simply kept under agitation, either mechanically or hydrodynamically.

 Biological methods are widely used for the purification of polluted water. Traditionally, biological methods have been used to reduce the content of organic matter in water, but, especially in recent years, biotechnological methods have been used to remove ammonium (nitrification), remove nitrogen by denitrification and remove phosphorus.

 A distinction must be made between aerobic and anaerobic processes. In aerobic processes, microorganisms need oxygen, while microorganisms used in anaerobic processes must be found in an environment devoid of oxygen. Most of the purification plants existing in the world operate on the basis of aerobic processes but there is a growing interest in anaerobic processes, in particular in relation to the elimination of nitrogen and the purification of water. -industrial valves rich in organic matter.

 A distinction must also be made between biobubble systems and biofilm systems. In the biobubble system, microorganisms float in water in aggregates around the sludge particles in a bioreactor. In aerobic sludge systems, active sludge systems, the sludge particles are separated from the water and returned to the bioreactor to keep the amount of biomass as high as possible.

 In biofilm systems, microorganisms grow on fixed surfaces of the bioreactor. The biofilm grows thicker as microorganisms proliferate and some portions of the biofilm may come off and a new biofilm may reform. As the biofilm is fixed and the water flows on its surface, there is no need to turn over the biomass to obtain the best possible exploitation of microorganisms.

Later, there was a clear trend to replace sludge systems with biofilm systems. The main reasons for this change are as follows:
at. The biomass per unit volume can be considerably higher, with the consequence that the bioreactor will be much smaller in volume.

 b. Biofilm reactors can withstand large load variations as well as in the composition of the water to be treated, which makes the methods using biofilms more flexible and less sensitive than the methods using active sludge.

 vs. If, for any reason, there is a break in the functioning of the biological process, no dramatic consequence is to be feared in the methods using biofilms unlike what happens in the methods using active sludge because the concentration mud outside the bioreactor is much lower.

 Biofilm reactors existing to date are based on different systems such as biorotors (rotating biological contact surface), conventional filters and fluidized bed reactors. Examples of conventional filters can be found in GB patent 2 197 308 in patent EP 301 237 and in patent FR 73 17859 in which the reactor is filled with elements which are fixed. There are also biofilters in which the support medium for the biofilm is immersed and aeration of the volume of water is provided, but these systems are based on fixed solid supports inside the reactor or on foam rubber elements or analog which is left floating on the surface of an active sludge reactor.

Active sludge systems (sludge systems) have the drawback that it can be difficult to obtain sufficient control of the separation of the sludge with the result of unwanted sludge losses and therefore serious consequences for the container.

 Another obvious drawback of these systems is that the volume of the reactor will always be very large since the quantity of biomass per unit volume inside the reactor remains low.

 However, compared to traditional biofilm systems (conventional biorotors and filters), active sludge systems have the advantage of using open bioreactors which cannot, in any case, become clogged or clogged.

 The biggest disadvantage of biorotor systems is that they are based on a prefab biorotor, which makes the device extremely inflexible. We have always encountered big mechanical problems with biorotors and when one of them breaks, it is difficult to adapt another from another system. It must however be recognized that there are many examples of biorotor devices reconstructed in biofilter reactors, but this time in a system using fixed filters.

 The main drawback with the traditional biofilter using conventional filters into which water is poured onto the support material of the biofilm is that the oxygenation takes place by natural ventilation and that the volume of the reactor becomes relatively large.

Another considerable drawback of this system is that the quantity of oxygen brought into the implementation of the process cannot be adjusted to that which is required by the biological process and which obviously depends on the organic load of the water. . It is generally known that in these conditions, traditional biofilters have a poorer purification action for a given organic matter load, per unit area, than the other methods using biofilms.

 Another method is that which uses so-called submerged biofilters. The principle of this biofilter is that a fixed biofiltrant material is immersed in the reactor while the biomass is oxygenated by aeration.

The growth surface of the submerged biofilter is fixed and generally consists of flakes or flakes of corrugated plastic assembled to form cubes which are stacked on top of each other either as bricks in the construction of a wall, or at otherwise randomly stacked; one can also use granules which are, of course, fixed during the operation of the biofilter. The main disadvantage of fixed, submerged biofilters is that access to the underside of the biofilter is extremely difficult. If the biofilter becomes clogged on its underside, or if the ventilation air inlets placed under the biofilter become blocked, the entire biofilter must be removed from the reactor to clean it. A problem of another nature was also encountered when all of the elements of the biofilter began to float following a partial clogging with the formation of large air pockets in the material of the biofilter.

 Another system is what is called a fluidized bed bioreactor. This type of reactor is filled with sand and the water sent, by pumping, from the bottom of the device to its top at a speed sufficient to fluidize the sand. Biofilm grows on grains of sand; in this system, there is a very large biomass per unit volume of the reactor because the specific growth surface for the biofilm is very large.

 The drawback of this system, on the other hand, results from the very large organic load in volume which results therefrom. Consequently, aerobic systems cannot receive, per unit of volume, the supply of the quantity of oxygen sufficient to replace the oxygen consumed by the biomass. In practice, there is yet another problem, that of separating the biofilm from the grains of sand since the latter are extremely small (in general of a size of between 0.4 and 0.6 mm). In addition to all that has been explained above, there are other systems which may be situated intermediate between the various traditional systems described. In most of these systems, the aim is to increase the biomass per unit volume of the bioreactor by forming a biofilm.

 Most of these alternative systems are based on something that lies between a biofilm system and an active sludge system, the sludge recovered in the separation tank, after the operation, then being returned to the bioreactor to create a culture mud, in addition to biofilm culture.

This type of system is unsatisfactory because:
at. The concentration of mud in the separation tank, after the purification operation, becomes very high which is likely to cause significant risks for the container due to lack of mud;
b. The mud particles introduce a new organic strain to the biofilm, a fact which has been demonstrated in various research projects.

 A very important drawback of systems based on biomass growing on and in small foam rubber dice is that these dice float on the surface of the reactor so that only the surface of the water constitutes the possible exchange zone: this results in very little effective contact between the biomass and its substrate. Another very important drawback of this system resides in the fact that it has been shown that the biomass only grows on the surface of the cells and not at all on the cells of the latter as was thought. This is a consequence of the fact that, the biofilm landing on the external surface, closes its cells with water and substrates.

 According to the invention, it has been found that the most significant drawbacks of the various systems described above could be avoided while retaining the most important part of the advantages attached to each of them.

 In the present method of water purification, a new type of biofilm carrier is used which can be used in a bioreactor in which the appropriate bioorganisms can grow on the carrier.

As a result, according to the present invention, there is a method for purifying water in which the waste water is brought into a reactor containing supports on which a biofilm can grow and thereby bring about the desired conversion. impurities. This method is characterized by the use of supports which are particles of plastic material having:
at. A surface which is at least 1.5 and more particularly, at least 2 times, larger than the external surface of a corresponding, solid element, having the same dimensions;
b. A density between 0.90 and 1.20, normally rather between 0.92 and 0.98 and preferably between 0.92 and 0.96;
vs. An external surface protected against external wear of the biofilm during use;
d. A shape that allows easy passage of water over the biofilm, the supports with the biofilm being kept in suspension and moving in water in a reactor equipped with supply and outlet ports and, optionally, means of brewing.

 In general, the size of the support will be a question of adaptation to the problem posed and a suitable size of support will correspond to elements having a linear dimension between 0.2 and 3 cm and, preferably, between 0.5 and 1.5 cm. However, it should be noted that the essential characteristic is that these supports can be kept in suspension in the reactor, and, provided that this condition is satisfied, dimensions other than those indicated above can be envisaged.

 In practice, the support will be made of a non-hard plastic so that it does not wear on contact with other supports or with that of the reactor and its equipment. Since this is a plastic which must be used essentially as a support for a film of bacteria, it is particularly advantageous to use recycled plastic for the manufacture of these supports.

 There is no particular constraint with regard to the shape of the supports provided that they have a large surface area per unit of weight and that they respect the range of density indicated above in order to be able to be kept in suspension. . Supports suitable for this use may, for example, be sections of tubes which have internal partition walls oriented in the direction of the generator. A biofilm layer of the culture of the desired bacteria will form on the external and internal surfaces of the tube and the partition walls. In general, there can be as many partitions as it is physically possible to put to increase the surface, it being understood that one must make sure that the openings between the different internal partitions do not become so small they may clog quickly. When the support is in the form of a tube section with internal partitions, the external walls may preferably have a concave curvature so that the external wall is subjected to less contact with the other supports or with the different surfaces of the reactor during operation. Under these conditions, the biofilm existing on the external wall of the support will be less degraded. A tube usable for the manufacture of its supports may for example have internal partitions arranged in a cross. Alternatively, these internal partitions can be arranged in honeycomb but one can also use any other arrangement which also provides a large surface area and a passage that is sufficiently easy for liquids. It is also possible to use particles having a rough surface, for example, granules even though they would have a smaller surface than the sections of tubes described above.

 Suitably, the support can be a section of an extruded tube having internal partition walls oriented longitudinally and fins oriented in the same way on the external surface. The reason why such a support is particularly advantageous is that it is very easy to manufacture, unlike a support that would be manufactured by other possible methods such as, for example, molding, where each support must be molded individually . In extrusion, on the contrary, the tube is produced continuously and cut, on demand, into sections of appropriate dimensions. All the partitions will therefore be oriented longitudinally so that whatever the dimensions of the sections, the cross section will always be the same.

 In addition to having internal partitions, it has been found more particularly advantageous that the tube also has fins on its external face; this is a form which lends itself perfectly to extrusion and leads to a support having contact surfaces both inside and outside. By doing so, it will easily be understood that a particularly large contact surface with little material, in general plastic material, is obtained in relation to the initial surface of the tube. In the same way as for the internal surfaces of the tube, the portions of external surfaces close to the fins extruded from the external surface of the tube will be protected against mechanical wear of the biofilm during the operation.

 A satisfactory type of support comprising fins is shown, in cross section, in FIG. 1. In side view, this same support resembles a rectangle. It is practically the simplest form to design. Another form is shown in Figure 2 in which the tube has a square section and is equipped with several internal partitions. A modification of this embodiment is shown in FIG. 3 in which, both the internal partitions and the external partitions, project from the circumference of the tube to constitute the said fins. As shown in Figure 1, such fins do not necessarily have to be in the extension of internal or external partitions but can be independent fins placed between those which, for example, have been shown in the figure 3.

 The support is used in reactors for the purification of water by introducing an appropriate quantity of supports into the reactor and the water which must be purified is treated by the latter with the biofilm which is supported and grows on the supports, which brings about the desired conversion of pollutants. In practical terms, a reactor is used comprising a feed at its base and an outlet for purified water at its top; however, such localization is not essential, in particular if appropriate means of mixing and circulation of fluids are used. The reactor is equipped, as appropriate, with filtering means so that the mesh dimensions are less than the smallest diameter of the supports. This is to prevent the media from leaving the reactor. The supports can be easily pumped both to load them in the reactor and to evacuate them and the maintenance operations do not impose a stop of the operation.

 The supports, their use, the reactor and the method according to the invention constitute a system which, compared to the various systems already known and described above, have several advantages: - The volume of the reactor is completely open and the surface on which the biofilm believes is constituted by solid and non-porous particles circulating inside the reactor because these particles have a density close to 1.

- The bioreactor can be completely closed and the supports immersed, which allows optimal contact between the impurities of the water and the microorganisms carried by the supports, as well as a complete control of the odor problems linked to certain agents used in the process.

- The bioreactor can be oxygenated by aeration, which allows an appropriate adaptation between consumption and oxygen supply. Thus, the organic load can be adjusted in relation to what is consumed by the biomass.

 The system according to the present invention has the same advantage as active sludge systems, in that the reactor is open and, consequently, cannot become clogged. In addition, the reactor can have virtually any shape.

 A great advantage of the present system compared with the other biofilm systems is that the existing active sludge systems can be easily reconstructed so that a system according to the present invention can be adapted to it in existing installations based on the principle of active sludge; such a reconstruction would be extremely complicated for other biofilm systems.

 The difference between the present system and the submerged biofilter systems described above mainly lies in the fact that the growth surface of the biofilm in the present system is in permanent circulation in the bioreactor due to the turbulence generated by aeration or by hydrodynamic forces, while the growth surface for submerged biofilters, as stated above, is fixed and normally consists of flakes of corrugated plastic glued together in the form of cubes which are either placed in regular stack as to form a wall, either arranged randomly or can consist of granules arranged randomly in the reactor but remaining fixed during the operation of the biofilter.

 In the present system, clogging of the biofilter medium is not possible since this biofilter medium is not fixed but moves with the fluid streams inside the bioreactor. If the aeration device of the reactor becomes blocked, it would be very easy to remove the biofilter medium simply by pumping. In the same way, the bioreactor would, afterwards, also be recharged by pumping.

 When the bioreactor is used in anaerobic processes, that is to say without aeration, the biofilter medium is subjected to continuous or sporadic agitation, for example, by a propeller agitator or by a circulation pump. Under these conditions, the chances of clogging are low, unlike what happens when a fixed biofilter is used, a system in which the risk of clogging in anaerobic operation is quite significant.

The contents of the reactor can also be heated to accelerate the reaction in anaerobic processes.

 In the present system, the desired surface per unit of volume can be chosen and, therefore, the oxygen supply can be adjusted to adapt exactly to the oxygen consumption observed. The oxygen supply can be adjusted so that one can simply use air instead of pure oxygen for oxygenation. The particles on which the biofilm believes are comparatively large enough, they do not tend to fall but to circulate and are kept in circulation, so that the density of the particles can be chosen independently of the amount of water crossing the reactor.

 In the system according to the present invention, the sludge does not normally have to be returned to the bioreactor with the intention of increasing the biomass. However, this does not prevent it from being able to be used if, for example, the system is used in existing active sludge installations.

 One of the particular objects of the invention is to obtain a higher rate of disintegration of the substrate per unit volume of the reactor than that which is generally obtained in competing systems, and consequently, to achieve lower operating costs per unit of disintegrated weight of the substrate.

 This can be obtained by allowing the biofilm to grow on the supports, according to the present invention, placed in a reactor which is crossed by the water which is intended to be purified.

 If an aerobic biological process is to be carried out in the bioreactor, the contents of the reactor will be aerated. By aeration, the supports are largely stirred in the volume of the reactor and an excellent contact is thus obtained between the biofilm which grows on the supports and the substrate contained in the wastewater.

 If an anaerobic process has to be carried out in the bioreactor, the contents of the reactor are not aerated. Sufficiently large mixing of the reactor contents is then ensured by mechanical means by a propeller agitator or by a circulation pump.

 Normally, the supports will be maintained in the reactor enclosure during the exit of the purified water, thanks to the presence of filters whose light is smaller than the weakest section of the supports. In certain specific applications, for example the elimination of phosphorus by biological means, it would be possible to let the supports leave the reactor at the same time as the purified water, to separate them later and to return them to the reactor, this in this particular case is made so that the biofilm can grow on the supports successively under aerobic and anaerobic conditions.

 The reactors can be in the form of completely closed prefabricated elements, both for aerobic and anaerobic processes. This makes it possible to completely control the odor problem of certain agents produced in the reactor. Whether the reactor is used in aerobic or anaerobic mode, the gases produced during the process are captured and eliminated. In the aerobic process, the gases produced are essentially CO2 and small quantities of other gases which are allowed to escape into the open air after having possibly carried out a deodorization operation. In anaerobic processes, the gases produced essentially consist of methane and CO2 with smaller amounts of other gases. This biogas has a high calorific value and, therefore, can, if desired, be used for energy production.

 If the invention is used to reconstruct existing purification installations, the reactor will normally be opened since it is possible, in this case, to use the existing basins (for example the aeration tanks in active sludge installations).

 The quantity of supports in the reactor will vary according to the field of application and the volume of reactor available. Normally, this quantity will be such that in an empty tank the supports occupy in volume between 30 and 70% of the reactor capacity. However, this quantity can be adapted to the load of substrates that the reactor is intended to process. So the quantity will be determined according to the oxygenation capacity of the reactor.

 The three most important parameters to set when deciding on the dimensions of the bioreactor are the volume of the reactor, the number of supports per unit of volume and the oxygen supply to be supplied (in the case of an aerobic reactor) .

 The reactor itself can be constructed of different suitable materials, but the prefabricated enclosed reactors will be made of steel or reinforced plastic reinforced with fiberglass, while the open reactors will instead be constructed of concrete or steel.

 The biofilm sludge can be separated downstream of the bioreactor by any suitable technique for separating the particles such as, for example, sedimentation, flotation, filtration and techniques using membranes.

 As described in general above, the bioreactor can be used in all purification techniques based on the biological degradation of the substance which is to be eliminated.

However, the fields of use can be the following: * Elimination of organic matter from used water by aerobic reaction.

* Elimination of organic substances from concentrated sewage by anaerobic reaction.

* Elimination of ammonium by oxidation to nitrite then to nitrate by aerobic reaction (nitrification).

* Elimination of nitrogen by reduction of nitrites and nitrates into nitrogen by anaerobic (or anoxic) reaction - denitrification reaction.

* Elimination of phosphorus by an aerobic / anaerobic reaction.

The invention provides the following advantages in the purification of wastewater: * Compared to traditional installations currently existing, the bioreactor according to the invention requires a smaller reactor volume to eliminate a given unit of weight of pollutants (organic materials, ammonium , etc.) since the biomass per unit volume is higher.

 * In its prefabricated embodiment, the reactor according to the invention is normally closed in order to obtain better control of odorous gases than in traditional installations.

* In the aerobic embodiment, there is a better possibility of adapting, than in traditional systems, the supply of oxygen as a function of the oxygen demand.

* Due to the large contact surface between the biomass and the air supplied

This makes it possible to reduce the demand for air and, consequently, lowers the energy operating cost for the present reactor compared to the systems operating with active sludge.

* The reactor will be approximately the same shape whether for aerobic or anaerobic operation. Therefore, it is easy to transform an aerobic system into an anaerobic system and vice versa. This is a particular advantage for systems which implement both aerobic and anaerobic mode, for example systems for the biological elimination of nitrogen and phosphorus.

* Compared to submerged biofilters with fixed growth surfaces for biofilm, the growth support for biofilm considered here is much easier to remove from the reactor tank, which simplifies cleaning, inspection and maintenance both reactor tank than the aeration system and reduces the risk of clogging of the surfaces supporting the growth medium.

* Existing biological purification installations operating with active sludge can very easily increase their capacities without changing reactors when they use the system according to the invention.

A simple reactor has been represented in FIG. 4 where we see a reactor 1 which is a cylinder comprising supports 2 for the biofilm. The outlet of purified water 5 from the reactor is provided with filtration means 3. The water to be treated is sent by a pipe to the inlet 4 of the reactor located at the base and the gases generated during the reaction are evacuated by a tube 6 provided at the top of the reactor. The formation of foam is avoided by means of a spraying system 7 capable of projecting sprayed water onto the surface.

FIG. 5 represents a reactor equipped with means 8 for blowing air at its base of the reactor, this air being supplied by a pipe 9. This reactor is intended for aerobic operation.

Figures 6 and 7 show reactors equipped with stirring means for anaerobic operation but, apart from this device, they are similar to the reactors shown in Figure 1. In Figure 6, the stirring means consist of a propeller agitator 10 driven by a motor, while in FIG. 7, these agitating means consist of a circulation pump 11 associated with a pipe 12.

Claims (10)

1. Method for the purification of the water in which the waste water is brought into a reactor containing supports on which a biofilm can grow to bring about the desired conversion of impurities, characterized by the use of supports which are particles of matter plastic with:  a) a surface which is at least 1.5 times larger than the external surface of the corresponding element, full, having the same dimensions;  b) a density of between 0.90 and 1.20, normally rather between 0.92 and 0.98 and preferably between 0.92 and 0.96;  c) an external surface protected against external wear of the biofilm during use;  d) a shape which allows easy passage of water over the biofilm; in which the supports with the biofilm are kept in suspension and in movement in water in a reactor equipped with a supply and outlet orifice and optionally with stirring means. 2. Method according to claim 1, characterized by the use of supports having a surface which is at least 2 times greater than the external surface of the corresponding element, solid, having the same dimensions. 3. Method according to one of claims 1 or 2, characterized by the use of supports having linear dimensions between 0.2 and 3 cm and preferably between 0.5 and 1.5 cm. 4. Method according to one of claims 1 to 3, characterized by the use of supports which have been prepared from a non-hard plastic, optionally from a recycled plastic material and which are in the form of sections of tube with internal partitions. 5. Method according to one of claims 1 to 4, characterized by the use of supports which are sections of extruded plastic tube. 6. Method according to claim 5, characterized by the use of supports which are sections of extruded tube having separate partitions oriented longitudinally with respect to the tube inside its circumference and outside the fins thereof externally oriented longitudinally. 7. Reactor (1) for aerobic, anoxic and anaerobic purification of water, comprising an inlet orifice (4), outlet orifices (5,6) and, optionally, stirring means (10), characterized by the fact that it contains a large number of supports (2) for biofilm which are used in the process according to one of claims 1 to 6, the volume of the supports in an empty reactor representing 30 to 70% of the volume of the reactor. 8. Reactor according to claim 7, characterized in that it comprises filtering means (3) for separating the supports from the liquid at the outlet orifice (5). 9. Reactor according to one of claims 7 or 8 for the aerobic purification of water, characterized in that it comprises means (8) blowing air which is supplied with air through an inlet air (9). 10. Reactor according to one of claims 7 or 8 for the anaerobic purification of water, characterized in that it comprises stirring means consisting of a propeller agitator (10) or a circulation pump (11 ).