ES2813446A2 - Method and system for the anaerobic treatment of organic waste fluids - Google Patents

Abstract

The invention relates to a method for the anaerobic treatment of organic waste fluids, which uses an anaerobic tank (1) with a slurry blanket from which a supernatant (4) and a first current of gas (5) are extracted; and at least one filtration module (6) in which a gas (8) is used to create the filtering conditions for a current to be filtered (7), and from which a reject current (10) is extracted. The method is characterised in that it comprises the operation of mixing the reject current (10) and the supernatant (4), and holding the mixture in an intermediate tank (11), with gas outlet, the following being extracted from the intermediate tank (11): a refeeding current (3), having been purged of gas, which is supplied at the bottom of the anaerobic tank (1); the current to be filtered (7), which is supplied to the filtration module (6); and a second current of gas (12).

Description

DESCRIPTION

Procedure and system for the anaerobic treatment of organic waste fluids

Technical area of the invention

The invention relates to a process for the anaerobic treatment of organic waste fluids, especially suitable for wastewater and fluid suspensions with a high content of oils and fats of natural origin.

The invention also relates to a system for the anaerobic treatment of organic waste fluids, especially suitable for the implementation of said process.

Background of the invention

It is known that anaerobic wastewater treatment has undoubted advantages: the production of biogas allows the energy recovery of the residual matter to be treated, the generation of sludge is low and does not require energy for the necessary aeration in aerobic processes.

The low growth rate of anaerobic microorganisms makes it necessary to use retention devices and active biomass accumulation. In fact, one of the main problems in the application of anaerobic processes is the loss of biomass, a problem that is known to be aggravated in the treatment of fluids with a high content of lipid materials, which justifies their prior separation.

The retention of the biomass can be carried out by different methods depending on the growth mode of the microorganisms: adhering to filling materials, or forming aggregates that remain suspended in the fluid to be treated.

In the case of suspended biomass systems, self-aggregation is a basic requirement that when it is not satisfactory, or when it is altered by a change in operating conditions, it causes immediate and sometimes irreversible damage to the process. For the retention of biomass in suspended growth systems, a wide range of devices have been used, the effectiveness of which is determined by the difference between the density of the water and the biomass aggregates. The feasibility of these devices generally it is conditioned by the formation of flocs or granules of suitable size and density for separation by sedimentation.

The most widely used suspended biomass anaerobic reactors are anaerobic contact systems and ascending sludge bed or mantle reactors, known by the acronym UASB for Upflow anaerobic Sludge Blanket, and its variants.

UASB reactors are characterized by the fact that the water circulates upwards through the sludge; and the effluent is collected by the upper part in which there is a gas-solid-liquid separation system. In this type of reactors, as in its variants, the key is to achieve aggregates with adequate sedimentation qualities that are not carried away by the upward flow of both the water and the biogas that is generated.

The treatment of wastewater with a high content of oils and fats, characteristics of meat industries, processing of fried products or wool washing machines, among others, raises difficulties that are widely described in the technical literature.

It is apparently contradictory that, since the methanogenic potential of lipids is significantly higher than that of any other natural organic compound, wastewater treatment plants with a high content of oils and fats incorporate pretreatment stages for degreasing, which represents a significant waste in biogas production and an increase in sludge management costs.

Problems of (a) irreversible inhibition of acetoclastic and methanogenic bacteria have been described due to alteration of the permeability of their cell membranes, (b) limitations to the transfer of matter, of the substrate of the solution to the biomass and of the product generated by the biomass to water, associated with the adsorption of long-chain fatty acids to bacterial aggregates, (c) continued loss of small biomass flocs, (d) irreversible loss of biomass due to massive flotation of sludge due to degassing defects, and ( e) clogging of biogas collection systems associated with the formation of surface crusts, among others. Different studies have been carried out to evaluate the importance of each of these individual problems, from which it follows that the severity and consequences largely depend on the type of biomass and reactor configuration. It has been found that the thickness of the fat layer adsorbed to the biomass can reach several tenths of a millimeter in granules of only 2-3 mm and how the presence of long chain fatty acids hinders the adhesion to the biomass granules of the suspended p-oxidant microorganisms, which makes them susceptible to being carried away with the effluent.

One method to retain biomass flocs is the use of external separation devices and return to the anaerobic reactor. Systems are known based on the combination of full-mix anaerobic digesters with external ultrafiltration units in tangential flow tubular membranes operated at low transmembrane pressure.

From patent ES 2524522 a process has been disclosed for the anaerobic treatment of organic waste fluids characterized by their high content of oils and fats, capable of solving the aforementioned problems. This procedure uses an upflow anaerobic tank in which the biomass is completely retained in an external filtration tank, by means of submerged membrane modules for the separation of the treated water with the return of the biomass and undigested materials from the residual fluid to the tank. anaerobe.

The fluid to be treated enters the anaerobic tank through the lower part and circulates homogeneously in an upward direction through the biomass that is mostly settled. The effluent is a supernatant that overflows from the top of the anaerobic tank and is directed towards the filtration tank.

In the filtration stage, one or more modules of submerged, flat or hollow membranes are used. This type of membrane requires agitation in order to reduce the deposition of biomass and any other type of particles throughout the filtration process. This agitation is produced by intense bubbling of an oxygen-free gas, preferably by recirculation of the biogas generated in the anaerobic biological process. The gas stream introduced into the filtration module induces an upward flow of the liquid phase and suspended biomass in it, due to the effect known as gas-lift, which overflowing from said filtration tank is recirculated to the lower part of the anaerobic tank by means of vertical conduction. This recirculation mode minimizes the shear forces characteristic of other pumping methods. On the other hand, the bubbling used in the filtration module provides suitable conditions for the anaerobic process of beta-oxidation of long-chain fatty acids originated in the hydrolysis of oils and fats.

The reject stream leaving the filter tank contains a mixture of sludge and biogas. The fact of recirculating this rejection stream towards the anaerobic tank allows the recovery of the biomass lost with the supernatant that overflows from the anaerobic tank.

However, this procedure has the drawback that the biogas contained in the reject stream, when returning by recirculation to the anaerobic tank, produces unwanted turbulence with the consequent loss of the essential stratification of the sludge blanket.

Another drawback is that the possible agglomerations or flocs of fat and biomass that manage to escape from the anaerobic tank with the overflow of the supernatant and that circulate towards the filtration module, can cause clogging or serious damage to the filtration membranes.

On the other hand, it is desirable to have a suitable solution to recover the biomass that escapes with the supernatant from the anaerobic tank and return it through the recirculation stream without altering the stratification of the sludge blanket of the anaerobic tank, and without limiting the flow rate in the filtration stage. That is, it is of interest to be able to separately control the recirculation of the reject stream from the filtration module to the anaerobic tank and the flow in the filtration module.

On the other hand, it is of interest that the solution can be implemented in a structurally simple way, saving energy costs and reducing operating costs.

Explanation of the invention

In order to provide a solution to the problems raised, a procedure is disclosed for the anaerobic treatment of organic waste fluids, especially suitable for waste fluids containing oils and fats, which uses an anaerobic tank with a blanket of sludge from which it is extracted a supernatant and a first gas stream; and at least one filtration module in which a gas is used to provide the filtering conditions of a stream to be filtered, and from which a reject stream is extracted.

The procedure is characterized in that it comprises the operation of mixing the stream of rejection of the filtration module and the supernatant extracted from the anaerobic tank, and contain the mixture in an intermediate tank, with gas outlet, being extracted from the intermediate tank - a re-feed stream purged of gas that is supplied to the bottom of the anaerobic tank,

- the stream to be filtered that is supplied to the filtration module, and

- a second gas stream.

Thanks to the operability of the intermediate tank, the method of the invention has the following advantages:

• It is important to note that the mixture of the supernatant and the rejection stream in the intermediate tank produces great turbulence due to the high speed reached by the rejection stream, which helps to break possible agglomerations / flocs of fat and biomass, generated from intermediate products of anaerobic digestion, which could leave the anaerobic tank with the supernatant by overflow. Consequently, the particulate matter (eg biomass) that has been disaggregated can settle by going by gravity towards the lower part of the intermediate tank where it will be recirculated as a feed stream towards the anaerobic tank. Therefore, said flocs are prevented from penetrating the filtration module, thus minimizing fouling and the possible risk of failure of the filtration module.

• The intermediate tank acts as a degassing unit, separating the gas bubbles contained in the reject stream and extracting a gas-free re-feed stream that recirculates to the anaerobic tank. In this way, unlike the procedures known in the state of the art, the gas contained in the reject stream is not injected into the anaerobic tank, thus avoiding the turbulence generated by the bubbles and the consequent loss of stratification. and anaerobic tank performance.

• The intermediate tank offers the possibility of splitting the necessary recirculation of the rejection stream from the filtration module to the anaerobic tank into two different recirculation streams that can be controlled separately to optimize the stratification of the sludge in the anaerobic tank, On one side; and the maintenance of a flow rate, for example cross-flow, over the filtration module that favors filtration, on the other hand. Therefore, the flow in the filtration module can be as high as desired without compromising the stratification of the anaerobic tank, and in the same way, the recirculation of the flow from the anaerobic tank can be adjusted as desired to maintain an optimal rate of rise in the anaerobic tank, without limiting the flow rate in the filtration stage.

• The intermediate tank can operate as a “buffer”. Its level can vary between that corresponding to the height of the supernatant inlet of the anaerobic tank and that of the recirculation outlet of the fluid to be filtered to the filtration module. The volume between the two levels allows the filtration flow of the filtration module to be maintained in optimal conditions despite fluctuations in the flow of the residual fluid to be treated, as well as the performance of short-term maintenance operations on the filtration module. , between 2 and 6 hours, such as cleaning with chemical reagents without interrupting the feeding of the biological process.

Consequently, in a variant, the method comprises the operation of regulating the flow of the re-feed stream that enters the anaerobic tank and the flow of the stream of fluid to be filtered that enters the filtration module, independently and differently. regime.

The use of the intermediate tank allows to have these two separate cycles, and therefore it is possible to adjust the speeds of each flow independently, improving the flexibility of the system.

Preferably, the first gas stream generated in the anaerobic tank, the second gas stream separated by the intermediate tank, or a mixture of these is at least partially recirculated towards the filtration module, obtaining a gas flow that is used in the filtration module to ensure the filtering conditions of the stream to be filtered.

It is therefore contemplated to use totally or partially the first gas stream; totally or partially the second gas stream; or totally or partially both gas streams, for which they can be mixed before being used in the filtration module. In addition, the fact of using the own gas extracted from the anaerobic tank and / or the intermediate tank allows reduce production costs, since it is not necessary to have another external source supplying an oxygen-free gas in the filtration module.

According to a characteristic of the invention, the reject stream and the supernatant enter through a zone above the intermediate tank, arranged at a level above the outlet of the stream to be filtered from said intermediate tank. In this way, the turbulence generated by the mixing of the reject stream and the supernatant takes place in the upper part of the buffer tank. This allows the particulate matter, as a result of the breakdown of the agglomerations / flocs of fat and biomass, to settle gradually towards the bottom of the intermediate tank, and on the other hand, it allows the separated gas to accumulate in the upper part of the tank. intermediate.

Preferably, the mixing of the supernatant and the reject stream is carried out inside the intermediate tank, so that the reject stream enters the intermediate tank at a level that allows said mixture to be kept slightly below the level of the supernatant overflow, and in turn at a level slightly above the level of the liquid contained in its interior to avoid the formation of foam due to bubbling or splashes generated by the force with which the rejection stream falls on the surface of the intermediate tank liquid.

Alternatively, the reject stream and supernatant can be mixed prior to entering the reject stream into the buffer tank.

Preferably, the stream to be filtered exits through a lateral zone midway up the intermediate tank. In this way, the extracted stream to be filtered is less dense, since turbulence is relatively low in said mid-altitude area, thus minimizing the concentration of particulate matter that will have already settled towards the bottom of the intermediate tank.

Advantageously, the re-feed stream exits through a lower lateral zone of the intermediate tank, arranged at a level below the outlet of the stream to be filtered from said intermediate tank. Consequently, the particulate matter that has escaped from the anaerobic tank with the supernatant and that has managed to settle when descending along the intermediate tank can be returned to the anaerobic tank through its lower part.

Optionally, the stream to be filtered is subjected to a solids filtration operation before being supplied to the filter module. This additional solids filtering operation makes it possible to retain any precipitates of fats and salts that may have escaped from the intermediate tank together with the stream to be filtered, since the size of said precipitates could cause blockages and serious damage to the filtration module.

According to another aspect, the invention also relates to a system for the anaerobic treatment of organic waste fluids, especially for waste fluids containing oils and fats, comprising

- an upflow anaerobic tank with a sludge blanket, provided with an inlet for a fluid tributary to be treated; a re-feed input for a re-feed stream; an overflow supernatant outlet; and means for collecting a first gas stream; Y

- at least one filtration module with means for supplying a fluid stream to be filtered and a gas stream; at least one permeate outlet; and an output of a reject stream.

The system is characterized in that it comprises an intermediate tank containing a mixture of the supernatant extracted from the anaerobic tank and of the rejection stream of the filtration module, the intermediate tank being provided with means for the output of a second gas stream and of al minus two other outputs of which

- one is an outlet that communicates with the filtration module through which the fluid stream to be filtered can circulate; Y

- another is an outlet that communicates with the re-feeding inlet of the anaerobic tank through which the re-feeding fluid stream can circulate.

Advantageously, the system comprises first drive means to force the re-feed stream to recirculate and control the rate of rise in the anaerobic tank, and second drive means to regulate the flow rate of the fluid to be filtered entering in the filtration module, both drive means being capable of being independently governed by means of control to be able to operate with different speed regimes between the anaerobic tank and the intermediate tank on the one hand, and between the intermediate tank and the module of filtration on the other hand. Said drive means can be, for example, hydraulic pumps.

Preferably, the system comprises a gas conduction line that conducts the first and second gas streams, said line being provided with a by-pass bypass that communicates with the filtration module, provided to recirculate at least partially the first and / or second gas streams, obtaining a gas flow that is used in the filtration module to provide the filtering conditions of the fluid stream to be filtered.

Preferably, the gas used in the filtration module is mixed with the fluid stream to be filtered by means of a diffuser. In this case, the gas used is injected through a diffuser located at the inlet of the filtration module, in a lower area of the same, where it will meet the stream of fluid to be filtered that also enters through the lower part of the filter module. filtration. The diffuser allows gas to be injected with a specific bubble size.

More preferably, the diffuser used is a coarse bubble diffuser with perforations preferably between 1 and 4 mm.

Alternatively, the possibility is also envisaged that said gas and the fluid stream to be filtered are mixed before said mixture is introduced into the lower part of the filtration module.

According to another characteristic of the invention, the anaerobic tank comprises in its head space, above the level of the supernatant overflow, a zone for collecting the gas generated. Likewise, the intermediate tank comprises in its headspace an accumulation zone for the separated gas.

According to an embodiment of the invention, the two gas collection zones of the anaerobic tank and the accumulation zones of the intermediate tank are independent, so that the first and second gas streams are extracted through respective outlets connected in parallel with said line. gas conduction.

According to another embodiment of the invention, the two gas collection zones of the anaerobic tank and the accumulation of the intermediate tank are interconnected, so that the mixture obtained from the first and second gas streams is extracted through a single outlet that communicates with said gas conduction line.

Optionally, the system comprises at least one solids filter arranged between the intermediate tank and the filtration module. In this way, the retention of possible fat and salt precipitates that may have escaped from the intermediate tank together with the stream to be filtered is guaranteed, to avoid blockages and serious damage to the filtration module.

According to another characteristic of the invention, the system comprises a compressor provided to regulate the flow rate of the gas stream used in the filtration module.

Preferably, the intermediate tank is constituted by a vertically oriented container with a height such that its upper level equals or exceeds the highest of the levels of the anaerobic tank and the filtration module, and with a diameter such that it allows the downward flow rate in the lower part of the intermediate tank is suitable for the decantation of gas-free flocs.

The fact of using a particularly high intermediate tank facilitates, on the one hand, the degassing of the reject stream, preventing the separated gas from entering the anaerobic tank through the re-feeding stream; and on the other hand, it facilitates the sedimentation of the particles from the breakdown of the agglomerations / flocs of fat and biomass that may have escaped with the supernatant and that will be returned to the anaerobic tank by the re-feed current.

According to a preferred embodiment, the at least one filtration module is a module of multitubular microfiltration or ultrafiltration membranes.

According to another preferred embodiment, the system comprises at least two filtration modules arranged vertically and coupled in series or in parallel.

Depending on the system requirements, it is possible to arrange the filter modules horizontally or vertically. It has been proven that the optimal way is to operate by arranging the modules vertically and in parallel, since lower energy consumption is obtained.

Brief description of the drawings

The attached drawings illustrate, by way of non-limiting example, some preferred embodiments of the system for the anaerobic treatment of organic waste fluids from the invention. In these drawings:

Fig. 1 is a flow chart of the system of the invention according to a first preferred embodiment;

Fig. 2 is a flow diagram of the system according to another embodiment, in which the mixing of the supernatant and the reject stream is carried out before entering said mixture into the intermediate tank;

Fig. 3 is a flow diagram of the system according to another embodiment, in which the gas collection spaces of the anaerobic tank and the accumulation spaces of the intermediate tank are interconnected with each other;

Fig. 4 is a flow chart of the system according to another embodiment, in which two filter modules connected in series are employed;

Fig. 5 is a flow diagram of the system according to another embodiment, in which two filter modules connected in parallel are used;

Fig. 6 is a flow chart of the system according to another embodiment, devoid of the solids filter; Y

Fig. 7 is a schematic view of a multitubular membrane filtration module comprising a gas diffuser arranged in the lower part thereof, also showing the position of the respective fluid inlets and outlets.

Detailed description of the invention

Next, a system for the anaerobic treatment of organic waste fluids is described that uses a process according to the invention, tested on a pilot scale in the treatment of wastewater from two food industries dedicated, the first, to the preparation of precooked dishes, and the second to the production of potato and corn snacks.

Wastewater is composed of individual discharges that are produced discontinuously mainly in the cleaning operations of equipment in production lines, frying tanks and rooms. After roughing and screening operations, the discharge has a high organic content, with a highly variable Chemical Oxygen Demand (COD), between 2800 and 43000 mg / L. Wastewater has a high content of slowly biodegradable materials and a high adsorption capacity that cause alterations in conventional anaerobic treatment processes, specifically oils and fats, with concentrations between 360 and 12000 mg / L, often being the majority component. The concentration of nutrients, oscillates widely depending on the type of dish prepared, from very low levels 75 mg N / L and 60 mg P / L for snacks and vegetable burgers, up to 470 mg N / L and 80 mg P / L in the discharges generated in the preparation of squid . For optimal maintenance of biological activity, nutrient-deficient waters are added with adequate amounts of urea and phosphoric acid, adding sodium hydroxide to control pH and maintain adequate levels of alkalinity that provide stability to the processes. anaerobic biologicals.

According to a preferred embodiment shown in figure 1, the system for the anaerobic treatment of organic waste fluids, especially for waste fluids containing oils and fats, comprises:

- an upflow anaerobic tank 1 with a sludge blanket, provided with an inlet for a tributary 2 of fluid to be treated; a re-feed input for a re-feed stream 3; an overflow of a supernatant 4; and means for collecting a first biogas stream 5;

- at least one filtration module 6 with means for supplying a stream of fluid to be filtered 7 and a stream of gas 8, in the example of biogas as will be explained later; at least one outlet of a permeate 9; and an output of a reject stream 10;

- an intermediate tank 11 containing a mixture of the supernatant 4 extracted from the anaerobic tank 1 and the reject stream 10 from the filtration module 6, the intermediate tank 11 being provided with means for the outlet of a second stream of biogas 12 and of at least two other exits from which

- one is an outlet that communicates with the filtration module 6 through which the fluid stream to be filtered 7 can circulate; Y

- Another is an outlet that communicates with the re-feeding inlet of the anaerobic tank 1 through which the re-feeding fluid stream 3 can circulate.

The upward flow anaerobic tank 1 is stratified so that the biomass concentration is significantly higher in its lower zone, which allows reducing biomass losses in its upper outlet, and improving its acclimatization and its performance in the degradation of oils and fats.

The anaerobic tank 1 has a layer of partially settled sludge through the circulating the affluent 2, in this case the wastewater to be treated, and the re-feeding current 3 of the intermediate tank 11 in an upward direction, being possible to regulate its speed between 0.5 and 2 m / h in order to maintain the gently agitated sludge blanket and favor the release of biogas from the microbial flocs, minimizing the entrainment of biomass with the flow of water. The anaerobic tank 1 has a volume that allows it to maintain an adequate residence time for the correct development of the anaerobic biological process, between 12 and 48 hours depending on the concentration of organic matter and the proportion of oils and fats in the wastewater.

The affluent 2 of fluid to be treated can be introduced through the lower zone of the anaerobic tank 1. Generally, this feed inlet is carried out through at least one conduit with direct inlet in the lower part of the anaerobic tank 1, as shown in figure 1.

Alternatively, it is also envisaged that the inflow 2 inlet is made through at least one submerged pipe that enters through the upper part of the anaerobic tank 1 and leads the affluent 2 to its lower zone. In this way, it is possible to feed the anaerobic tank 1 in an "upflow" manner without the risk of having to stop the process in case of clogging of pipes due to accumulation of fat, since it is easier to clean this feed pipe, an operation that only needs to be removed. the piping for inspection or maintenance, you have to empty the anaerobic tank 1 to access a direct inlet conduit to the bottom of the anaerobic tank.

As shown in figure 1, from the upper part of the anaerobic tank 1 the supernatant 4 passes by overflow to the intermediate tank 11 where it mixes with the reject stream 10 of the filtration module 6, causing a strong agitation that favors that those flocs of fats and biomass that, by flotation, have been dragged by the supernatant 4, release the biogas and settle in the lower part.

The mixing of the supernatant 4 and the reject stream 10 is carried out inside the intermediate tank 11, so that the reject stream 10 enters the intermediate tank 11 at a level such that it allows said mixture to be slightly maintained. below the level of the overflow of supernatant 4, and in turn at a level slightly above the level of the liquid contained therein to avoid the formation of foam due to bubbling or splashes generated by the force with which the rejection stream falls on the surface of the intermediate tank liquid.

Alternatively, according to another variant shown in Figure 2, the mixing of the supernatant 4 and the reject stream 10 could be carried out before said mixture entering the intermediate tank 11.

With reference to Figures 1 and 2, a first pump 13 forces the recirculation of the re-feed stream 3 in the direction of the anaerobic tank 1, which makes it possible to maintain an adequate ascending speed and returns the decanted solids, free of biogas, to the mud blanket. Said re-feed stream 3 is drawn from the lower part of the intermediate tank 11.

The re-feed fluid stream 3 is introduced towards the lower part of the anaerobic tank 1, being able to use a direct inlet pipeline in the lower part of the anaerobic tank 1, as can be seen in Figures 1 and 2. According to Another variant, in an analogous manner, could use a submerged pipe that enters through the upper part of the anaerobic tank 1 and conducts said stream of re-feeding fluid 3 to its lower zone.

In the examples, the intermediate tank 11 is constituted by a vertically oriented container with a height such that its upper level equals or exceeds the highest of the levels of the anaerobic tank 1 and of the filtration module 6, and with a diameter such that it allows the downward flow rate in the lower part of the intermediate tank 11 is adequate for the decantation of the gas-free flocs.

The level at which the outlet of the stream to be filtered 7 is placed in the intermediate tank 11, makes it possible to differentiate in the intermediate tank 11 two zones: an upper zone A, in which an intense mixture of the supernatant 4 is produced that leaves the anaerobic tank 1 with reject stream 10; and a lower zone B, in which the fluid circulates slowly towards the outlet of the return to the anaerobic tank 1, in a downward direction favoring the recirculation of the flocs that left the anaerobic tank 1 with the supernatant 4 and those rejected by the filtration module 6.

The system comprises a gas conduction line 14 that collects and conducts a biogas mixture 8 formed by the mixture of the first 5 and second 12 biogas streams for use, for example, as fuel in a boiler. In addition, said line 14 is equipped with a bypass bypass that communicates with the filtration module 6, designed to recirculate at least partially said biogas mixture 8 which is used in the filtration module 6 to ensure the conditions of filtering the fluid stream to be filtered 7. Likewise, a compressor 15 provided to regulate the flow rate of the gas stream 8 is used.

The anaerobic tank 1 comprises in its headspace, above the level of the supernatant overflow 4, a collection zone for the biogas 5 generated by the decomposition of organic matter. Likewise, the intermediate tank 11 comprises in its head space an accumulation zone for the separated biogas 12.

According to the embodiments shown in Figures 1 and 2, each biogas collection areas of the anaerobic tank 1 and the accumulation areas of the intermediate tank 11 are independent, so that the first 5 and second 12 gas streams are extracted through respective outlets connected in parallel with said gas conduction line 14.

According to another embodiment shown in figure 3, the two biogas collection areas of the anaerobic tank 1 and the accumulation of the intermediate tank 11 are interconnected through a conduit 12 ', so that the mixture 5' obtained from the first 5 and second 12 streams of biogas is extracted through a single outlet that communicates with said gas outlet line 14.

As can be seen in Figures 1 to 3, the intermediate tank 11 is connected in series with recirculation to the filtration module 6. A second pump 16 drives the fluid stream to be filtered 7, formed by the mixture of supernatant 4 and the reject stream 10, towards the filtration module 6.

The filtration module 6 chosen in these examples is a microfiltration or ultrafiltration multitubular membrane module.

In the case of using two or more filtration modules, these can be installed according to different arrangements depending on the needs of the system. Figure 4 shows an embodiment with two filtration modules 6 arranged vertically and in series; while in figure 5 another embodiment is shown with two filtration modules 6 arranged vertically and in parallel, this last embodiment providing a lower energy consumption.

In any case, the ultrafiltration membranes used are vertically oriented and have a pore size of 30 nm, whose individual tubes have diameters between 5.2 and 8 mm, and a filtration surface of 4.1 m2. The combination of the pressure exerted by the flow of the mixture and the effect of a suction pump (not illustrated) communicated with the external face of the membranes causes filtration, extracting a permeate 9 that circulates through the permeate tank (not represented). Intermittently, after 15 - 60 minutes the filtration stops and a pump is activated that takes water from the lower part of the permeate tank to reintroduce it into the membrane module or modules 6 in the opposite direction to that of filtration, producing a backwash , for 30 - 60 seconds, to control the fouling of the membranes, which makes it possible to maintain the transmembrane pressure below 600 mbar. To maintain adequate tangential flows, and to avoid the deposition of solids on the surface of the membranes, the second recirculation pump 16 can be regulated independently, without affecting the recirculation of the anaerobic tank 1.

The intermediate tank 11 offers the possibility of splitting the necessary recirculation of the reject stream 10 from the filtration module 6 to the anaerobic tank 1 into two different recirculation streams that can be controlled separately to optimize the stratification of the sludge in the anaerobic tank 1, on the one hand, and the maintenance of a cross flow velocity over the filtration module 6 that favors filtration, on the other hand.

For this, the first 13 and the second 16 pumps can be controlled independently by means of control means to be able to operate with different speed regimes between the anaerobic tank 1 and the intermediate tank 11 on the one hand, and between the intermediate tank 11 and the module. filter 6 on the other hand.

Therefore, the cross-flow in the filtration module 6 can be as high as desired without compromising the stratification of the anaerobic tank 1. In the same way, the recirculation of the flow of the anaerobic tank 1 can be adjusted as desired to maintain a velocity. optimal lift in anaerobic tank 1.

The use of intermediate tank 11 makes it possible to have these two separate cycles, and therefore it is possible to adjust the speeds of each flow independently, improving the flexibility of the system.

On the other hand, the intermediate tank can operate as a "buffer". Its level can vary between that corresponding to the height of the supernatant inlet 4 of the anaerobic tank 1 and that of the recirculation outlet of the fluid to be filtered 7 to the membranes of the filtration module 6. The volume between both levels allows the flow of filtration of the membranes in optimal conditions despite the flow fluctuations of the wastewater stream 2 to be treated, as well as the performance of short-term membrane maintenance operations, between 2 and 6 hours, such as cleaning with chemical reagents without interrupting the feeding of the biological process.

According to a preferred embodiment shown in figure 7, the gas 8 used in the filtration module 6 is mixed with the fluid stream to be filtered 7 by means of a diffuser 6a arranged at the inlet of the filtration module 6, in a lower zone of the same, where it will meet the stream of fluid to be filtered 7 that also enters through the lower part of the filtration module 6. The diffuser 6a allows the gas 8 to be injected with a specific bubble size, being able to use a bubble diffuser for this thick with perforations, for example, between 1 and 4 mm.

Alternatively, the possibility is also envisaged that said gas 8 and the fluid stream to be filtered 7 are mixed before said mixture is introduced into the lower part of the filtration module 6, as represented in Figures 1 to 6.

The confluence before or in the filtration module 6 of the recirculated biogas mixture 8 that is driven by the compressor 15 and the fluid stream to be filtered 7, coming from the intermediate tank 11, forms a liquid-gas mixture that circulates through perforated membrane tubes in which thick bubbles, between 1 and 3 mm, are generated, which rise generating a turbulent current over the membranes that helps to keep the filtration surface clean. The injected biogas 8 flow can be regulated between 0.2 and 1.0 m3 / h m2 with respect to the filtration surface. The biogas together with the rejection of the filtration leaves the filtration module 6 at its upper part, the biogas being again separated from said rejection stream 10 in the intermediate tank 11. Although not represented, a small amount of biogas that passes through the membrane is sent together with the permeate 9 to the closed permeate tank where it is separated from it and is collected at the top for its return to the headspace of the anaerobic tank 1.

The formation of the liquid-gas mixture in the lower part of the filtration module 6 also reduces the average density of the fluid, which causes an updraft, known as gas-lift, which contributes to reducing the energy consumption of the aforementioned second pump 16 that drives the stream of fluid to be filtered 7 to the filtration module 6.

Additionally, the system can comprise a solids filter 17 arranged between the intermediate tank 11 and the filtration module 6, in order to retain possible precipitates of fats and salts that may have escaped from the intermediate tank 11 along with the stream to be filtered 7 The mesh size of the solids filter 17 must be chosen according to the type of filtration module 6 that is used; If a tubular membrane module is used, a solids filter should be chosen whose mesh size is smaller than the diameter of the membrane tubes, for example between 2 and 4 mm, to avoid plugging in the membrane tubes.

However, another embodiment of the system is also envisaged in which said solids filter 17 is not included, as illustrated in figure 6. In this case, the fluid stream to be filtered 7 enters directly into the filtration module. 6.

The system provides optimal biogas production, obtaining effluents of high biological and physicochemical quality with complete sludge retention and low sludge generation. It has been proven that the system allows to achieve organic matter removal yields of 85-95%, with a biogas production of 460-620 L / kg COD with a methane content of 72-78% on a dry basis.

Claims (21)

1. Procedure for the anaerobic treatment of organic waste fluids, especially suitable for waste fluids containing oils and fats, which uses an anaerobic tank (1) with a mud blanket from which a supernatant (4) and a first gas stream are extracted (5); and at least one filtration module (6) in which a gas (8) is used to provide the filtering conditions of a stream to be filtered (7), and from which a rejection stream (10) is extracted, characterized The procedure because it comprises the operation of mixing the rejection stream (10) from the filtration module (6) and the supernatant (4) extracted from the anaerobic tank (1), and containing the mixture in an intermediate tank (11), with outlet of gas, being extracted from the intermediate tank (11) - a re-feed stream (3) purged of gas that is supplied to the bottom of the anaerobic tank (1), - the stream to be filtered (7) that is supplied to the filtration module (6), and - a second gas stream (12). 2. Procedure for the anaerobic treatment of organic waste fluids, according to claim 1, characterized in that the first gas stream (5) generated in the anaerobic tank (1), the second gas stream (12) separated by the intermediate tank ( 11), or a mixture of these is at least partially recirculated towards the filtration module (6), obtaining a gas flow (8) that is used in the filtration module (6) to ensure the filtering conditions of the stream to filter (7). 3. Procedure for the anaerobic treatment of organic waste fluids, according to claim 1 or 2, characterized in that it comprises the operation of regulating the flow of the re-feed current (3) that enters the anaerobic tank (1) and the flow of the stream of fluid to be filtered (7) entering the filtration module (6), independently and at different rates. 4. Process for the anaerobic treatment of organic waste fluids, according to any one of the preceding claims, characterized in that the reject stream (10) and the supernatant (4) enter the intermediate tank (11) through an upper zone (A), arranged at a level above the outlet of the stream to be filtered (7) from said intermediate tank (11). 5. Procedure for the anaerobic treatment of organic waste fluids, according to claim 4, characterized in that the mixture of the supernatant (4) and the rejection stream (10) is carried out inside the intermediate tank (11), of so that the reject stream (10) enters the intermediate tank (11) at a level such that it allows said mixture to be kept slightly below the level of the supernatant overflow (4), and in turn at a level slightly above of the level of the liquid contained in its interior. 6. Procedure for the anaerobic treatment of organic waste fluids, according to claim 4, characterized in that the reject stream (10) and the supernatant (4) are mixed before said mixture enters the intermediate tank (11). 7. Process for the anaerobic treatment of organic waste fluids, according to any one of the preceding claims, characterized in that the stream to be filtered (7) exits through a lateral zone at the middle of the intermediate tank (11). 8. Process for the anaerobic treatment of organic waste fluids, according to any one of the preceding claims, characterized in that the re-feed stream (3) exits through a lower lateral zone (B) of the intermediate tank (11), arranged at a level below the outlet of the stream to be filtered (7) from said intermediate tank (11). 9. Process for the anaerobic treatment of organic waste fluids, according to any one of the preceding claims, characterized in that the stream to be filtered (7) is subjected to a solids filtering operation before being supplied to the filtering module (6). 10. System for the anaerobic treatment of organic waste fluids, especially for waste fluids containing oils and fats, comprising - an upward flow anaerobic tank (1) with a sludge blanket, provided with an inlet for a tributary (2) of fluid to be treated; a re-feed input for a re-feed stream (3); an overflow of a supernatant (4); and means for collecting a first gas stream (5); Y - at least one filtration module (6) with means for supplying a stream of fluid to be filtered (7) and a stream of gas (8); at least one permeate outlet (9); and an output of a reject stream (10); the system being characterized in that it comprises an intermediate tank (11) where contains a mixture of the supernatant (4) extracted from the anaerobic tank (1) and the reject stream (10) of the filtration module (6), the intermediate tank (11) being provided with means for the output of a second stream of gas (12) and at least two other outlets from which - one is an outlet that communicates with the filtration module (6) through which the fluid stream to be filtered (7) can circulate; Y - Another is an outlet that communicates with the re-feeding inlet of the anaerobic tank (1) through which the re-feeding fluid stream (3) can circulate. 11. System for the anaerobic treatment of organic waste fluids, according to claim 10, characterized in that it comprises first drive means (13) to force recirculation of the re-feed current (3) and control the rate of rise in the anaerobic tank (1), and second drive means (16) to regulate the speed of the flow of the fluid to be filtered (7) entering the filtration module (6), both drive means (13,16) being capable of be governed independently by means of control means to be able to operate with different speed regimes between the anaerobic tank (1) and the intermediate tank (11) on the one hand, and between the intermediate tank (11) and the filtration module (6) on the other hand. 12. System for the anaerobic treatment of organic waste fluids, according to claim 10 or 11, characterized in that it comprises a gas conduction line (14) that conducts the first (5) and the second (12) gas streams, said line (14) equipped with a bypass bypass that communicates with the filtration module (6), intended to recirculate at least partially the first (5) and / or the second (12) gas streams, obtaining a gas flow (8) that is used in the filtration module (6) to provide the filtering conditions of the fluid stream to be filtered (7). 13. System for the anaerobic treatment of organic waste fluids, according to any one of claims 10 to 12, characterized in that the gas (8) used in the filtration module (6) is mixed with the fluid stream to be filtered (7) by means of a diffuser (6a). 14. System for the anaerobic treatment of organic waste fluids, according to any one of claims 10 to 13, characterized in that the anaerobic tank (1) it comprises in its headspace, above the level of the supernatant overflow (4), a zone for collecting the gas (5) generated; and in that the intermediate tank (11) comprises in its headspace an accumulation zone for the separated gas (12). 15. System for the anaerobic treatment of organic waste fluids, according to claim 14, characterized in that the two gas collection areas of the anaerobic tank (1) and the accumulation areas of the intermediate tank (11) are independent, so that the first (5 ) and second (12) gas streams are extracted through respective outlets connected in parallel with said gas conduction line (14). 16. System for the anaerobic treatment of organic waste fluids, according to claim 14, characterized in that the two gas collection areas of the anaerobic tank (1) and the accumulation areas of the intermediate tank (11) are interconnected, so that the mixture (5 ') obtained from the first (5) and second (12) gas streams is extracted through a single outlet that communicates with said gas conduction line (14). 17. System for the anaerobic treatment of organic waste fluids, according to any one of claims 10 to 16, characterized in that it comprises at least one solids filter (17) arranged between the intermediate tank (11) and the filtration module (6) . 18. System for the anaerobic treatment of organic waste fluids, according to any one of claims 10 to 17, characterized in that it comprises a compressor (15) provided to regulate the flow rate of the gas stream (8) used in the filtration module ( 6). 19. System for the anaerobic treatment of organic waste fluids, according to any one of claims 10 to 18, characterized in that the intermediate tank (11) is constituted by a vertically oriented container with a height such that its upper level equals or exceeds the highest of the levels of the anaerobic tank (1) and of the filtration module (6), and with a diameter such that it allows the downward flow velocity in the lower part of the intermediate tank (11) to be adequate for the decantation of the free flocs Of gas. 20. System for the anaerobic treatment of organic waste fluids, according to any one of claims 10 to 19, characterized in that the at least one filtration module (6) is a module of multitubular microfiltration or ultrafiltration membranes. 21. System for the anaerobic treatment of organic waste fluids, any one of claims 10 to 20, characterized in that it comprises at least two filtration modules (6) arranged vertically and coupled in series or in parallel.