EP1874695A2

EP1874695A2 - Method for purifying effluent in an anaerobic reactor

Info

Publication number: EP1874695A2
Application number: EP06743849A
Authority: EP
Inventors: Michel Torrijos; René MOLETTA; Joseph V. Flat N· MF2 Ramani Mithila THANIKAL; Nicolas Bernet
Original assignee: Institut National de la Recherche Agronomique INRA
Current assignee: Institut National de la Recherche Agronomique INRA
Priority date: 2005-04-27
Filing date: 2006-04-27
Publication date: 2008-01-09
Also published as: FR2885126B1; WO2006114552A2; UA89402C2; US20090078648A1; US7670489B2; FR2885126A1; MD20070287A; NO20076095L; CA2606062A1; MD4155B1; WO2006114552A3

Abstract

The invention relates to a method for purifying effluents in an anaerobic reactor (1) in which the micro-organisms are held by supports, said supports forming a bed which is fixed in a part (4, 5) of the reactor. The inventive method is characterised in that it comprises a step wherein the reactor is backwashed once it has been at least partially clogged, by temporarily suspending the supports. Advantageously, the method comprises an initial step for starting the reactor, during which the load of the reactor is increased with a short and constant hydraulic residence time.

Description

Process for the purification of effluent in anaerobic reactor.

The invention relates to a process for purifying effluent in anaerobic reactor. More specifically, the invention relates to a purification process in which the microorganisms are fixed on solid supports and form a fixed bed, or anaerobic filter that can be fluidized demand. The invention also relates to the use of a biological reactor in which the carriers carrying the microorganisms are allowed to organize in a fixed bed during the stationary operating phase. The supports are suspended only during the initial phase of reactor overload (start-up phase) or punctually during the stationary operating phase only once the reactor is clogged.

Currently there are two main modes of biological treatment of wastewater, or effluents, namely aerobic and anaerobic treatments. The invention relates to the purification of wastewater by anaerobic treatment.

The use of aerobic reactors for the treatment of heavily loaded wastewater results in the production of significant amounts of sludge by the microorganisms to clean up the wastewater. Thus, an average of 50% of the pollutants eliminated in the aerobic reactor are converted into sludge, which is another form of pollution that will in turn have to be treated. Aerobic processes are therefore used primarily for effluents with little concentration in pollution. In addition, aerobic reactors require frequent and prolonged aeration, consuming energy.

It is therefore known to use for certain biological treatments wastewater, whose concentration of pollution is high, an anaerobic reactor. The pollution treated in this type of reactor is mainly transformed into energy gas, which can be used later for other applications.

Among the advantages of this mode of depollution of wastewater is the fact that the production of excess sludge is low because the biomass yield, that is to say the kilograms of microorganisms formed per kilogram of COD removed or provided, is weak. Among the disadvantages of this method of depollution of wastewater are the fact that Anaerobic microorganisms have a lower growth rate than aerobic microorganisms and the fact that in a suspended microorganism reactor, it is generally not possible to exceed a microorganism concentration of 15 to 20 g / L. COD is the chemical oxygen demand, that is to say the amount of oxygen necessary to chemically oxidize all the pollutants, in particular the biodegradable organic compounds or not.

Also, to obtain a larger quantity of microorganisms in the reactor, it is known to use special supports which are immersed and immobile in the liquid phase, and on which the microorganisms are fixed. There are two types of media, the so-called "bulk" media and the so-called "ordered" media.

The use of such fixed supports in the reactor makes it possible to retain the biomass in the reactor in the form of a biofilm and thus to increase the quantity of microorganisms in the reactor, which makes it possible to increase the performance of the reactor. By biomass, we mean all the microorganisms used to degrade the pollution contained in the effluent. By biofilm, we mean highly structured cellular formations, in which microbial cells, which serve to degrade the pollutants contained in the effluent, which are included in a complex matrix.

The effluent to be treated flows through the support from the bottom up (upflow) or from the top down (downflow). The effluent is thus in contact with the biomass which carries out the depollution.

The two main disadvantages of such an anaerobic biological reactor are, on the one hand, as for any anaerobic process, the length of the reactor startup phase, which usually lasts between 3 and 6 months, and on the other hand the risk of clogging of the reactor which may cause, after a longer or shorter time, a total blockage of the reactor. In fact, although sludge production by microorganisms is less important in this type of reactor than in aerobic reactors, over time the sludge accumulates on the walls of the supports and in the interstices between the supports and can not not be evacuated. The accumulation of sludge can result in reactor service by blocking it completely.

In the field of anaerobic wastewater treatment, there is known, for example, a biological reactor using fixed supports on whose walls the microorganisms are fixed. The supports are formed of hollow tubes fixed at one end and in an orderly manner in the reactor vessel. The tubes have a diameter of 102.5 mm and are divided in their diameter into fourteen channels in which the microorganisms are fixed and can thus be organized into biofilm.

However, experience shows that with this type of reactor, a progressive clogging inevitably occurs after a few months of operation. Indeed, over time, the sludge accumulates on the walls of the columns and can not be evacuated. This clogging greatly disrupts the operation of the reactor and, over time, the accumulation of sludge can result in the shutdown of the reactor by blocking it completely.

In an alternative manner, it is known to use an anaerobic reactor operating in a fluidized bed, that is to say with permanently movable supports. The microorganisms are fixed on supports of very small sizes maintained in suspension in the reactor. The supports are movable in the reactor and rub against each other and / or with the internal walls of the reactor vessel, tearing the biofilm formed by the microorganisms as soon as it exceeds a certain thickness. The sludge falls into the bottom of the tank where it can be purged or exit with treated effluent. There is thus no risk of clogging of the reactor since the accumulation of sludge in the supports is made almost impossible. The permanent or quasi-permanent mobility of the supports is obtained by the creation of permanent or quasi-permanent turbulence in the reactor.

However, such a reactor has the drawbacks, on the one hand, consume a lot of energy to permanently maintain the suspended media, and on the other hand, to require intensive monitoring of the installation.

An object of the invention is to treat effluents by biological treatment in an anaerobic medium, so as to eliminate a large part of the pollution they contain with excellent performance, without the risk of clogging the reactor, with low costs and low operating time, and with a sharp reduction in the duration of the initial phase of ramp up of the reactor (phase starting). The increase in load is the gradual increase in the load applied over time following the seeding of the reactor, until the nominal load which has been set during the design of the reactor is reached. Another object of the invention is to allow the treatment of all types of effluents, including effluents heavily charged with pollution. A further object of the invention is to promote the growth of biomass in the reactor. The invention also aims to provide a solution for the treatment of effluents, which is easily used directly by polluting industries.

For this purpose, the invention proposes to treat the wastewater to be decontaminated in an anaerobic reactor, in a fixed bed, which can be made mobile according to the needs of the users, during the start-up phase or the unclogging phase of the reactor, in order to optimize its operation. Fixed bed operation results in low costs and ease of operation. The fixed bed is formed of a plurality of supports arranged in bulk in the reactor and on which the microorganisms form a biofilm and accumulate in the supports and in the interstices created between the supports.

At first and during the start-up phase of the reactor (increase in load), the growth of the biofilm on the supports is favored by maintaining short dwell times. The frequent fluidization of the supports can also promote the formation of the biofilm. The aim is to reduce the duration of the rising phase by removing free bacteria from the reactor to force the fixing on the supports.

In a second step (stationary operating phase), the reactor operates in a fixed bed at its nominal load. The retention of interstitial biomass is then favored. Interstitial retention refers to the accumulation of biomass in interstices of supports and between supports. The fluidization device is no longer used and the reactor operates in pure fixed bed. It is in this way that we manage on the one hand to mount high load in a short time, usually less than a month, and on the other hand to achieve very important loads for a fixed bed, it that is, up to 45 kg COD / m³. As fixed-bed reactor wastewater treatment is used, sludge accumulates on, in, and between supports, and begins to clog the reactor. When the reactor is at least partially clogged, it is provided according to the invention to fluidize the supports. The fluidization is temporary and spread over a short time, that is to say of the order of a few minutes or a few hours. During fluidization, the mobility of the supports allows sufficient friction to drop the sludge in the bottom of the tank. The surplus sludge can then be removed from the tank in a conventional manner. The fluidization is then stopped. The carriers become immobile again in the reactor so as to form a fixed bed again. The time flowing between two fluidizations can be relatively long, of the order of several months to several years, depending on the type of media and the load applied. By unclogging the reactor only once it is partially clogged, it does not disturb the biomass, and eliminates the risk of clogging the reactor that could make it unusable. The subject of the invention is therefore a process for purifying an effluent in anaerobic reactor in which the microorganisms are retained by supports, the supports forming a fixed bed in a portion of the reactor, characterized in that it comprises the step of temporarily suspending the supports in the entire reactor or, during the start-up phase, for eliminating the free or trapped microorganisms in or between the supports or, during the stationary operating phase, for unclogging of the reactor once it is at least partially clogged.

Purification means at least partial removal of pollution contained in the effluent at the time of entering the reactor. Suspending consists of making the initially immobile supports mobile in the reactor. The supports can then be distributed throughout the reactor volume and move. Suspending is only temporary insofar as, as soon as the unclogging stops, the supports reorganize into a fixed bed.

While the principle of the invention suggested to the specialists performance close to that of conventional fixed beds, that is to say a maximum volume load of 15 to 20 kg COD / m ³ .j with a purification efficiency of

80%, they found with astonishment that the method according to the invention makes it possible to achieve performances in terms of applied load more than twice higher than those obtained with conventional fixed beds with values of

45 kg of COD / m ³ .j while having a purification efficiency greater than 80%. In addition, in this way the load increase is performed quickly.

The effluent purification process according to the invention comprises the following steps: a) carrying out a reactor overload with a hydraulic residence time (TSH) of less than 48 hours; b) purify the effluent by maintaining the supports in a fixed bed; c) fluidizing the reactor once it is at least partially clogged, by temporarily suspending the supports.

Raising the reactor load means the reactor initiation phase, during which the applied load is regularly increased to the nominal load defined during the design and where it is aimed at the formation of biofilm on the supports. For example, the reactor is loaded up to 20 kg COD / m³, +/- 10%.

This step of loading is characterized by a gradual increase in the COD of the effluent passing through the reactor, with a short hydraulic residence time during the entire stage of increase in load, so as to increase the applied load of the reactor . Preferably, the COD of the effluent to be treated, and therefore the load applied, is increased daily by a constant percentage and between 5 and 15%. For example, the daily increase can be 10% of the previous value. Once the desired volume loading (OLR) has been obtained, that is to say at the end of step a), the reactor can be used in a fixed bed to purify the effluents to be treated optimally. Of course, the reactor is already functional throughout step a), but is not considered to operate under the nominal conditions as defined during the design of the reactor.

Applied charge means the amount of pollution that is treated in the reactor per cubic meter of reactor (applied volume load) or per gram of biomass (applied mass load) per day.

According to examples of implementation of the method according to the invention, it is possible to provide all or part of the following additional characteristics: in step a) the TSH is constant;

in step a) the TSH is between 12 and 36 hours, preferably between 20 and 30 hours, and even more preferably between 22 and 26 hours, more preferably equal to 24 hours.

step a) lasts 35 days, plus or minus 5 days. during step a) the supports are put at least once in suspension; for example, the supports are temporarily suspended for 10 minutes, +/- 2 minutes, every hour; it is also possible to provide a continuous suspension throughout step a).

This regular fluidization of the supports promotes the fixation of the biomass on the supports, that is to say the formation of biofilm, allowing the leaching and the elimination of the free biomass or simply accumulated in the interstices. Indeed, under these conditions, only the biomass physically fixed to the supports in the form of biofilm can be maintained on said supports during fluidization. The biomass that is accumulated / retained in the interstices, it is eliminated in the same way as the biomass in suspension.

- Step b) lasts between 2 and 12 months, preferably between 6 and 9 months, even more preferably 8 months. In a more general manner, the purification step b) is prolonged as long as the reactor is not clogged, and the purification efficiency, for an applied volume load (OLR) corresponding to the nominal load, is satisfactory. Preferably, it is considered that the treatment efficiency is satisfactory when it is at least equal to 80%. As soon as the treatment efficiency decreases beyond threshold value, set by the user, it is unclogging. For example, the threshold value is set at 75% of treatment efficiency.

Step c) unclogging lasts between 15 minutes and 1 hour, plus or minus 10 minutes. Once the reactor unclogged, it ceases the suspending of the supports, so that they reorganize into a fixed bed, can then again proceed to the purification of the effluent according to step b), and so to after.

In a particular embodiment of the method according to the invention, it is possible to use supports which have a density lower than the density of the liquid contained in the reactor, so that they form a fixed bed in the upper part of the reactor. . Conversely it is possible to use supports which have a density greater than the density of the liquid contained in the reactor, so that they form a fixed bed in the lower part of the reactor.

Advantageously, the supports comprise attachment zones on which the microorganisms can be fixed, the attachment zones being arranged to allow a physical retention of said microorganisms. This allows the physical retention of the biomass by trapping inside said supports. For example, the attachment zones comprise fins. The microorganisms can then be fixed on said fins, and form a biofilm, and / or be retained in interstices between the fins and / or between the supports, in which they accumulate.

These supports are preferably extruded or molded and provided with internal fins protected turbulence that may exist within the reactor, said inner fins having a significant attachment surface for microorganisms. These supports make it possible to fix the biomass on a large surface but in addition to retain the biomass in the interstices between the fins and between the supports themselves.

The means for fluidizing the supports may be located in a portion of the reactor without the fixed bed. Likewise, the means for supplying the reactor vessel with effluent can be located in the part of the reactor that does not have a fixed bed. It is also possible to provide homogenization means for distributing the effluent in the assembly of the reactor.

Advantageously, the supports occupy between 40% and 80% of the reactor volume and preferably between 50% and 70%.

The greater the number of supports inside the reactor, the higher the biomass can be and therefore the higher the load applied.

However, there must be in the reactor sufficient space between the fixed bed and at least one wall, lower or upper, of the reactor, to allow resuspension at the request of said supports in the entire volume. of the reactor. By resuspension or fluidization of the supports, it is meant that the supports are disorganized, the fixed bed broken, so that all the supports is movable in the reactor. This allows the declogging of the reactor.

The fluidization can be obtained by operating a fluidization system, such as a pump, capable of creating turbulence inside the reactor, so as to suspend the supports.

It may be advantageous to use a reactor in which the fluidization system is removable and external to the reactor, said fluidization system being connected at least temporarily to pipes to perform the unclogging. By external fluidization system, it should be understood that the fluidization system is not immersed in the reactor volume but is located outside. The fluidization system is therefore not in contact with the effluent and the supports. This facilitates in particular the maintenance of the fluidization system which is in fact easily accessible.

In addition, it is possible to use the same fluidization system for unclogging of several reactors. Indeed, insofar as according to the method of the invention the declogging is performed only rarely and for a short time, the same fluidization system can be used to decolmate alternately different reactors.

The fluidization of the supports in stationary operation, that is to say outside the initialization step, is carried out only once that the reactor is clogged. The time required for clogging may vary from one reactor to another, depending in particular on the type and quantity of supports, the amount of biomass in the reactor and the applied load.

In general, it is possible to adjust the unclogging period, as well as the unclogging time, so as to take into account the quality of the supports, and / or the amount of biomass and / or the load applied. .

The declogging can be periodic, with a period ranging from a few days to a few years depending on the needs. In particular embodiments, the declogging can be performed once a year, or every two years, three years, and so on. Unclogging can also take place irregularly, each time the reactor is more or less clogged and the user wishes to unclog it.

In order to know the level of clogging of the reactor and thus be able to unclog it accordingly, it is possible to provide a system for detecting the level of clogging in the reactor, which measures, for example, pressure drops in the reactor.

Between two successive unclogging, the supports are reorganized into a fixed bed.

The invention also proposes a use of a biological reactor comprising microorganisms retained on supports and fluidization means capable of suspending the supports, characterized in that the reactor is used for the purification of an effluent by treatment. anaerobic, the fluidization means being used temporarily to unclog the reactor, the supports being immobile and forming a fixed bed in the reactor when the fluidization means are not used. For the use according to the invention, the supports occupy for example

40% to 80% of the reactor volume, and preferably 50% to 70%, so that when the temporary fluidization means are used the supports can be distributed throughout the reactor volume, because of the suspension said supports which allows them to be mobile in the reactor volume.

The supports used are for example extruded or molded and provided with fins, the microorganisms being fixed on said fins and / or retained in interstices between the fins and / or between the supports.

In a particular embodiment of the invention, the fluidization means of the biological reactor used comprise a fluidization system capable of creating turbulence inside the reactor, to suspend the supports.

The fluidization system is for example a pump. The pump may be a water pump or a biogas pump.

In another embodiment, the fluidization system comprises a reserve of inert gas, such as a nitrogen cylinder, from which the pressurized gas is injected into the reactor.

The fluidization system may be located outside or inside the reactor. In the case where the fluidization system comprises an external pump, said fluidization system also comprises at least one pipe, said pipe opening into the reactor In another embodiment of the invention, it is possible to create the turbulence mechanically, that is to say by mechanical agitation by any known means.

The invention will be better understood on reading the description which follows and on examining the figures which accompany it. These are presented as an indication and in no way limitative of the invention. The figures represent:

- Figure 1: an example of a biological reactor that can be used for the treatment of effluents according to the invention;

FIG. 2: a second example of a biological reactor that can be used for the treatment of the effluents according to the invention; 3: a representation of the biological reactor according to FIG. 2, in which the supports are fluidized;

- Figures 4A and 4B: a schematic representation of a biomass fixation support, seen from the side (Figure 4A) and seen from above (Figure 4B);

- Figure 5: a graphical representation of the evolution of the hydraulic retention time (TRH) and the amount of pollution (OLR) introduced into the reactor in an exemplary implementation of the treatment according to the invention;

- Figures 6A and 6B: graphical representations of the evolution of the efficiency of the effluent treatment process as a function of the hydraulic retention time (TRH) and the amount of pollution (OLR) introduced into the reactor;

FIG. 7: a graphical representation of the increase in load in a reactor used according to the invention;

FIG. 8: a graphical representation of the evolution of the charges applied in a reactor as a function of the supports used.

FIG. 1 shows a biological reactor 1 capable of operating anaerobically. The reactor 1 comprises a tank 2 in which a plurality of solid supports 3 are arranged in bulk. In the case shown, the supports 3 have a density substantially lower than the density of the water so that all the supports are concentrated in an upper portion 4 of the vessel 2 of the reactor 1. The density of the supports is for example between 0.90 and 1, 2. FIGS. 4A and 4B show an example of support 3 that can be used in reactor 1. Support 3 has a generally cylindrical circular shape with a height h of about 3 cm and a diameter d of between 2.5 and 3, 5 cm. By height h is meant the dimension of the support 3 in the direction parallel to the longitudinal axis of the support. The support 3 is provided with a plurality of rigid fins 11 directed towards the center 12 of the support 3. The fins 11 are all fixed by a first end 15 to a first rigid ring 13 and by a second end 16 to a second ring Rigid 14. The fins 11 form the body of the support 3. The fins 11 are spaced apart from each other so as to provide a space 17 through which the effluent, or the liquid in general, can pass in order to be in contact with the part of the fins 11 directed towards the center 12 of the support 3.

It is of course possible to use all kinds of other supports 3 to fix the microorganisms.

Preferably, the supports 3 used are supports of macroscopic size, with a height h of between 15 mm and 50 mm, and with a diameter of between 10 mm and 50 mm.

In the lower part 5 of the tank 2, devoid of supports 3, are arranged fluidizing means 6 internal, that is to say fully housed in the tank 2. The fluidization means 6 comprise a pump 7 adapted to blow a gas or a liquid in the tank 2. The pump 7 can be actuated and stopped on demand. The lower part 5 of the vessel 2 of the reactor 1 also comprises homogenization means 8. The homogenization means 8 comprise, for example, a turbine provided with blades so as to mix the effluent which enters the vessel 2 at the level of the lower part 5 of said tank 2.

Such homogenization means 8 may be particularly advantageous when the reactor 1 is used to treat highly charged effluents, which otherwise could remain concentrated locally in the lower part of the tank 2.

With the homogenization means 8, the effluent is mixed with the rest of the fluid contained in the vessel 2. The effluent thus passes through the upper part 4 of the vessel 2 containing the fixed bed formed by the supports 3.

A gate located at the outlet of the liquid prevents the supports 3 from leaving the tank 2 through the tank outlet pipe (not shown) through which the treated effluent is discharged.

It is also possible to provide a grid (not shown) at the bottom 5 of the reactor 1, to protect the fluidization means 6 and homogenization 8. The grid then makes it possible to avoid any contact between the fluidization means and supports 3. Thus, even if some supports 3 fall to the bottom 10 of the tank 2, for example when the amount of microorganisms attached to the supports is such that the total density is greater than that of water, said supports 3 can neither be damaged by the fluidization means nor damage them.

It is possible, in another embodiment, to use supports 3 having a density greater than the density of water, so that the fixed bed formed by the supports 3 is located in the lower part 5 of the tank 2. The effluent then enters the tank 2 at the top 4, or the bottom 5, which may also include the homogenization means 8 and the fluidization means 7. In another embodiment, the fluidization means 6 may be located in the part of the reactor 1 comprising the fixed bed.

In the example shown in Figure 2, the fluidization means 6 are only partially internal. The fluidization means 6 comprise a pump 7 external to the tank 2 and an internal pipe system 9. The pump 7 is connected to the pipe system 9 located inside the tank 2. The pipe system 9 comprises a plurality of pipes distributed in the bottom 10 of the tank 2. It is of course possible to provide only a single pipe in the piping system 9. The external pump 7 may be a removable pump. Thus, it is possible to connect the pump 7 to the pipe system 9 temporarily when the user wishes to create turbulence in the tank 2.

In FIG. 3, one can see the biological reactor 1 in which the fluidization means 6 have been activated. The pump 7 blows a liquid or a gas into the tank 2, so that turbulence is created. The turbulence is sufficient to disorganize the fixed bed supports 3. The supports 3 are all suspended in the entire volume of the vessel 2, moving in said tank 2. Thus, the surplus sludge accumulated in the supports 3 can fall into the bottom of the tank, where it can be easily drained.

When the pump 7 is deactivated, that is to say stopped, the turbulence ceases. The supports 3, because of their density, then all go back to the upper part 4 of the tank 2 of the reactor 1 where they stop to form a new fixed bed. One of the disadvantages associated with anaerobic reactors is that the time required to increase the load of such a reactor is very long, since it takes most often between three and six months. During this period of increase in load, the charges applied to the reactor are low, because of the low concentration of biomass. It is therefore not possible to apply from the outset the maximum load defined during the design of the installation to such an anaerobic reactor. A ramp-up phase, characterized by a progressive increase in the load applied to the reactor, is necessary. In the invention, it was desired to reduce the duration of the increase phase in the anaerobic reactor used according to the invention. For this, experiments were conducted, the results of which are shown in Figure 7.

These experiments have shown that if the volume load applied in the reactor is gradually increased, maintaining a short hydraulic residence time, the load can be greatly increased in a very short time. With a residence time of one day, the COD concentration of the effluent to be treated in kg / m ³ is equal to the volumetric load applied in kg / m ³ .

The decoupling applied load / hydraulic residence time is achieved by initially decreasing the concentration of COD in the inlet effluent, that is to say by diluting the latter, and increasing it gradually until reaching the concentration in COD of the raw effluent to be treated. The COD of the effluent to be treated is preferably increased daily by a constant percentage and between 5 and 15%. For example, as shown in FIG. 7, the concentration of COD in the effluent is increased by approximately 10% every day, which makes it possible to pass from a COD of the feed of 0.5 to 20 kg. / m ³ in 35 days. The TSH being 24h during these 35 days, we obtain at the end of this period a

3 applied volume load of 20 kg / m J while maintaining a purification efficiency of 80%. The reactor is then able, after only 35 days, to operate under the nominal conditions defined during the design.

In the example shown in Figure 7, the hydraulic residence time is constant. Of course, it is possible to maintain a short residence time, preferably less than 48 hours, and variable. For example, during the first 10 days of the scaling step, one maintains a

TSH 24 hours, then for the next 10 days a TSH is maintained

36 hours, then we descend the TSH at 30 hours until the 35th day.

During the use of the purification process according to the invention, the applied load can be increased, up to most often greater than or equal to 45 kg COD / m ³ .day, depending on the effluent to treat, while effectively removing more than 80% of the pollution contained in the effluent. Advantageously, during this loading phase, in order to promote the growth of the biomass fixed on the supports in the form of a biofilm, rather than the accumulated biomass retained in the interstices or on the supports without actually being hooked, it is possible to proceed frequent fluidization of the supports. Preferentially, the fluidization is temporary, the supports reorganizing in a fixed bed between each fluidization. It is also possible to keep the supports in suspension during the whole load step. The reactor then operates in a fluidized bed during this step. However, since such a continuous fluidized bed operation requires large amounts of energy, it is preferred to fluidize frequently, for example every hour, but temporarily, for example less than 15 minutes. The development of a biofilm on the surface of the support makes it possible to accumulate microorganisms with very high activity in the reactor, which makes it possible to increase the charge rapidly even if the quantity of biomass in the reactor is not very high. high.

Once the ramp-up phase is complete, the accumulation of biomass is not detrimental to the operation of the reactor and, on the contrary, will make it possible to further increase the quantity of microorganisms in the reactor, which makes it possible to further increase the load applied in stationary operation. It is therefore no longer necessary to carry out frequent fluidizations. On the contrary, the reactor is maintained in fixed bed to promote the accumulation of biomass, in the form of biofilm and its accumulation in interstitial form and inside the supports. The major benefit of moving to fixed bed at the end of the start-up phase is to optimize energy costs by stopping the fluidization pump, as well as reducing the time required for reactor monitoring. In addition, the fixed-bed operation makes it possible to ensure filtration of the effluent and good retention of the solid particles in the reactor thanks to the filtering effect of the supports.

This strategy allows for a considerably accelerated increase in load compared to conventional strategies, in which the progressive increase of the OLR is generally associated with the proportional reduction of the TSH, without dilution of the feed. Thus, to increase the performance of the anaerobic reactor used according to the invention, the purification process according to the invention can be broken down into two main phases, namely:

A first ramp-up phase, between t0 and t + 1month, for example, during which the biomass is fixed on supports in the form of biofilm and the leaching of free biomass and interstitial biomass. For this, the TSH is weak, and the COD gradually increased.

A second stationary operating phase, between t + 1 month and t °°, during which the reactor operates in a fixed bed, with a very spaced unclogging of the supports to evacuate excess biomass. During this second phase, retention is promoted by accumulation of biomass in the interstices of the supports.

An example of use of the reactor 1 described above is now studied in more detail.

Material and method

A reactor having a cylindrical PVC tank is used. The internal diameter of the tank is about 190 mm for a height of 1150 mm. By height means the largest dimension of the vessel, parallel to the longitudinal axis of said vessel. This reactor has a useful volume of about 30 liters.

The reactor is equipped with heating means making it possible to maintain the inside of the tank at a temperature of approximately 35 ^° C.

A feed pipe makes it possible to bring the effluent to be treated inside the tank, at the lower part of said tank. An evacuation pipe, located in the upper part of the tank, makes it possible to evacuate the treated effluent. An overflow system makes it possible to maintain the liquid level at a height of 1000 mm in the tank.

The reactor vessel contains polyethylene supports of generally cylindrical tubular shape. The supports fill about 60% of the volume of the tank. These supports have a density substantially equal to 0.93 and a specific surface area of 320 m ² / m ³ . By specific surface, we mean the surface on which microorganisms are likely to cling to form a biofilm. The supports used have a large size compared to the supports usually used in moving bed, and a relatively small size compared to the supports usually used in fixed bed. More precisely, the supports used have dimensions of approximately 30 to 35 mm in height and approximately 29 mm in diameter.

In order to be able to fluidize on demand the supports organized in fixed bed in the reactor, the reactor is equipped with an internal pump, fixed to the bottom of the tank so as to be immersed in the liquid contained in the tank. The internal pump constitutes the means of fluidization, that is to say unclogging, of the reactor. The flow rate of the pump is about 480 L / h.

The effluent to be treated is distillery vinasse whose total COD is between 10 and 24 g / L with a soluble COD of between 10 and 19 g / L. The initial pH of the effluent is between 4 and 5.5. By initial pH means the pH of the effluent at the time of being introduced into the reactor vessel. Before the depollution treatment in the reactor, the pH is brought to a neutral pH, that is to say about 7.

The anaerobic inoculum used is from another anaerobic reactor that has been used to process distillery vinasse and has been concentrated by decantation. By inoculum is meant groups of bacteria used to seed the reactor vessel.

Unclogging was performed after 101 days of use, by activation of the pump for 15 minutes.

Measurements In order to analyze the reactor performance and effluent clearance quality, the evolution of the amount of biomass on the supports as well as the evolution of the soluble COD are measured.

The amount of biomass on the supports is determined by measuring the dry weight of the supports previously heated at 100 ^° C. for 24 hours. COD is measured conventionally by a colorimetric method (Jirka, 1975).

Observations of reactor operation. In a first step, in order to initialize the effluent treatment process, the reactor is activated so that the time during which the effluent to be treated remains in the reactor, that is to say the time TRH hydraulic retention, either high, and the applied volume load (OLR), that is to say the amount of pollution introduced into the reactor, per m ³ of reactor and per day, is low. Then the TRH was gradually decreased while IOLR was increased, increasing the volume of distillery vinasse introduced into the reactor.

The reactor was used for 180 days. For the reactor to be considered efficient, it is estimated that the

TRH must be as small as possible and IOLR as large as possible.

FIG. 5 shows a graph showing the evolution of TRH and OLR over time in the reactor used.

During a first period, between day 1 and day 81, the OLR is low and remains below 5.6 g COD / L.day. HRT decreases rapidly from 35 days on day 1 to 5 days on day 81.

During a second period, between day 82 and day 101, the HRT increases slightly. The average value of the TRH is 7.7 days, due to insufficient availability of distillery vinasse. The OLR has a slight drop at the same time and is between 1.6 and 2.6 g COD / L. day.

In a third period, between day 102 and day 180, the HRT decreases rapidly to reach a minimum residence time of 0.7 days. The OLR, on the contrary, increases very rapidly to reach values of up to 36 g COD / L.day. For comparison, under identical conditions, a fixed bed anaerobic reactor, containing an ordered carrier such as Cloisonyl, has a much lower OLR, not exceeding 14 g COD / L.day.

Analysis and results

The first operating period is the reactor start-up phase and low charges are applied to avoid organic overload and allow the biomass to accumulate in the reactor. During this first purification period, the purification efficiency is greater than 85%.

During the third period of operation, which is representative of the optimum operation of the reactor, the residence time is around 0.7 days and the applied load 30 g COD / L.day to eliminate more than 85% of the pollution of the effluent. Soluble COD at the outlet of the reactor being less than 5.5 g / L.

In the graphs of FIGS. 6A and 6B, the evolution of the reactor's depollution capacity can be seen. These graphs clearly show that with the process according to the invention it is possible to eliminate more than 80% of the pollution of a highly charged effluent, such as distillery vinasse, with an applied load of at least 30 g COD. /L.day and a residence time in the reactor less than 1 day.

Evolution of the amount of biomass fixed on the supports Samples of four supports are regularly taken from the reactor vessel and dried for 24 hours at 100 ^° C. The samples are then weighed to evaluate the amount of biomass present on the supports. .

The first sample is taken after 66 days of operation of the reactor, that is to say during the first period of operation. The average amount of biomass is 2.5 g per support.

The second sample is taken after 156 days of operation of the reactor, that is to say during the second period of operation. The average amount of biomass is then 3.2 g per support. The biomass thus increased by 30% between day 66 and day 156. The third sample was taken after 180 days, that is to say at the end of the third period of operation. The average amount of biomass is 4.5 g per support. After 180 days of use of the reactor, the total concentration of biomass fixed on the supports in the reactor is about 57 g / l of reactor. The biomass thus increases satisfactorily in the reactor.

This is explained by the large surface area on the supports used and the fact that the supports are organized in a fixed bed, which allows the biomass to attach to supports and accumulate in interstices without disturbing events. With respect to a reactor in which the microorganisms are free in the reactor volume, the concentration of microorganisms is 5 to 6 times higher with a specific activity maintained. Specific activity refers to the amount of COD that can be removed per kilogram of biomass. As a result, it is expected that the reactor's cleaning performance is greater. Reactor Performance The charge applied in the purification process according to the invention is greater than 30 g COD / L, while effectively eliminating more than 80% of the pollution contained in the effluent. In general, for the sake of comparison, the activity conventionally measured in a fixed-bed anaerobic reactor without any possible declogging is about 15 g COD / L.day.

The performance of the reactor used in the invention is directly related to the ability of the microorganisms to attach to the supports and the absence of turbulence within said reactor. Leaving the reactor clogged allows the biomass to grow optimally. Unclogging, which takes place on demand, for example only once the user believes that the clogging can be detrimental, allows to increase the operating time of the reactor and maintain its performance throughout the use.

In order to best determine the performances associated with the specificities of the support chosen during the implementation of the method according to the invention, experiments have been carried out using various supports provided with fins, such that the microorganisms can physically bind to said fins and / or be retained in interstices between the fins and / or between the supports.

During these experiments three inert supports R1, R2 and R3 were used, having different specific surfaces, respectively of 310,

320 and 855 m ² / m ³ Each of the three supports R1, R2 and R3 is in accordance with the support 3 as represented in FIG. 4A. Thus, the supports R1, R2, R3 may be covered with biofilms fixed to said supports, but may also accumulate biomass in the orifices and interstices 17 formed between the fins 11 and the center 12 of said supports.

The results, represented in FIG. 8, show that the performances obtained with the three supports R1, R2, R3 are close. High applied loads, of the order of 20 kg COD / m ³ .day, were obtained after fairly similar operating times, namely about 110 days.

It is deduced from this that the operation of the purification process according to the invention during the fixed bed purification stage, seems based both on the filtering effect of the supports used, which trap the biomass in their interstices, and on the formation of biofilm by physically fixing the biomass on said supports. Indeed, if the only biofilm was responsible for the treatment, the organic load would tend to be proportional to the specific surface.

This makes it possible to explain the surprising performances obtained by the process according to the invention, which makes it possible to obtain an applied load of up to 45 kg COD / m 2 against maximum 20 kg COD / m 2 fixed fixed beds.

Insofar as the industrial dimensions are proportional to the load applied, the method according to the invention therefore allows a strong reduction in the size of the equipment. Thus, in the process according to the invention, once the increase in charge is complete and the reactor operates in a fixed bed, the reactor mainly allows retention by accumulation of the biomass, in addition to the conventional formation of a biofilm . By retention of the biomass is meant its accumulation in the interstices of the supports and / or between the supports. Indeed, in fixed bed, the formation of biofilm leads to a reduction of the specific surface area and the activity of the biomass because of the recovery of the supports by successive layers of biofilm. These successive recoveries may explain the limitation of the charges applied on conventional fixed beds. In the invention, the declogging system makes it possible to evacuate the excess biomass accumulating in the layer of supports. The effect of "biomass retention" by filtration is related to size, shape, hydrodynamics supports and fixed bed use of the reactor. This is therefore different from the so-called conventional biofilters which allow the retention by a stack of supports and not by the supports themselves.

Claims

A process for purifying effluent in anaerobic reactor (1) in which the microorganisms are retained by supports (3), the supports forming a fixed bed in a part (4, 5) of the reactor, characterized in that it comprises the following steps: a) carrying out a load increase of the reactor with a hydraulic residence time (TSH) of less than 48 hours; b) purify the effluent by maintaining the supports in a fixed bed; c) fluidizing the reactor once it is at least partially clogged, by temporarily suspending the supports.

2- purification process according to claim 1, characterized in that during step a) the TSH is constant. 3- purification process according to one of claims 1 to 2, characterized in that during step a) the TSH is between 12 and 36 hours, preferably between 20 and 30 hours, and even more preferably between 22 and 26 hours, more preferably 24 hours.

4- purification process according to one of claims 1 to 3, characterized in that step a) lasts 35 days, plus or minus 5 days.

5. purification process according to one of claims 1 to 4, characterized in that during step a) the reactor is fluidized at least once, by suspending the supports.

6. The purification process according to claim 5, characterized in that the supports are suspended temporarily every hour.

7- purification process according to claim 5, characterized in that the supports are suspended continuously throughout step a).

8- purification process according to one of claims 1 to 7, characterized in that step b) lasts between 2 and 12 months, preferably between 6 and 9 months, even more preferably 8 months.

9- purification process according to one of claims 1 to 8, characterized in that step c) lasts between 15 minutes and 1 hour, plus or minus 10 minutes.

10- purification process according to one of claims 1 to 9, characterized in that repeats n times step b) and c).

11- purification process according to one of claims 1 to 10, characterized in that the supports comprise attachment zones on which the microorganisms can be fixed, the attachment zones being arranged to allow physical retention said microorganisms.

12- purification process according to claim 11, characterized in that the attachment zones comprise fins (11), the microorganisms can be fixed on said fins and / or retained in interstices between the fins and / or between the supports.

13- Method according to one of claims 1 to 12, characterized in that the supports occupy between 40% and 80% of the reactor volume and preferably between 50% and 70%.

14- Method according to one of claims 1 to 13, characterized in that it comprises the step of

- Operate a fluidization system (7) capable of creating turbulence inside the reactor, so as to suspend the supports. 15- A method according to claim 14, characterized in that the fluidization system is an external removable system, said system being connected at least temporarily on at least one pipe (9) opening into the reactor to perform the unclogging.