ES2813446B2 - Procedure and system for the anaerobic treatment of organic waste fluids - Google Patents

Description

DESCRIPTION

Procedure and system for the anaerobic treatment of organic waste fluids

Technical sector 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 putting said procedure into practice.

Background of the invention

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

The low growth rate of anaerobic microorganisms makes it necessary to use retention and accumulation devices for active biomass. 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.

Biomass retention can be carried out by different methods depending on the mode of growth of the microorganisms: adhered 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, causes immediate and sometimes irreversible damage to the process. A wide range of devices have been used for biomass retention in suspended growth systems, the effectiveness of which is determined by the difference between the density of water and 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 anaerobic suspended biomass reactors are anaerobic contact systems and rising sludge blanket or bed reactors, known by the acronym UASB for Upflow anaerobic Sludge Blanket, and their variants.

UASB reactors are characterized in that the water circulates upward through the sludge; and the effluent is collected at the top where there is a gas-solid-liquid separation system. In this type of reactor, as in its variants, the key is to achieve aggregates with adequate sedimentation qualities that are not dragged 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, preparation of fried products or wool washers, among others, poses difficulties widely described in the technical literature.

It is apparently contradictory that, since the methanogenic potential of lipids is significantly greater 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 reduction in biogas production and an increase in sludge management costs.

Problems have been described of (a) irreversible inhibition of acetoclastic and methanogenic bacteria due to alteration of the permeability of their cell membranes, (b) limitations to the transfer of matter, from 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 assess the importance of each of these individual problems, from which it can be deduced that the severity and consequences depend largely on the type of biomass and the configuration of the reactor. It has been found that the thickness of the layer of fat adsorbed to biomass can reach several tenths of a millimeter in granules as small as 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 dragged with the effluent.

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

Patent ES 2524522 has disclosed a procedure 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 treated water with the return of biomass and undigested materials from the residual fluid to the tank. anaerobe.

The fluid to be treated enters the anaerobic tank at the bottom and circulates homogeneously upwards through the biomass, which is mostly sedimented. The effluent is a supernatant that overflows through the upper part of the anaerobic tank and is directed towards the filtration tank.

In the filtration stage, one or more submerged, flat or hollow membrane modules 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 the 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 biomass suspended 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 driving. This recirculation mode minimizes the shear forces characteristic of other pumping methods. On the other hand, the bubbling used in the filtration module provides adequate conditions for the anaerobic process of beta-oxidation of long-chain fatty acids originating from the hydrolysis of oils and fats.

The reject stream leaving the filtration tank contains a mixture of sludge and biogas. The fact of recirculating this reject stream to 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 current, 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 blockages 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 current without altering the stratification of the sludge blanket of the anaerobic tank, and without limiting the flow rate in the filtration stage. In other words, it is of interest to be able to separately control the recirculation of the reject current from the filtration module to the anaerobic tank and the flow in the filtration module.

On the other hand, it is interesting 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 for the anaerobic treatment of organic waste fluids is disclosed, especially suitable for waste fluids containing oils and fats, which uses an anaerobic tank with a layer 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 for 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 current of rejection of the filtration module and the supernatant extracted from the anaerobic tank, and containing 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 current to be filtered that is supplied to the filter module, and

- a second gas stream.

Thanks to the operation 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 reject stream in the intermediate tank produces great turbulence due to the high speed reached by the reject stream, which helps to break up any 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 disintegrated can settle down by gravity towards the lower part of the intermediate tank where it will be recirculated as a refeed current 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 refeed stream that is recirculated to the anaerobic tank. In this way, unlike what happened in 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 performance of the anaerobic tank.

• The intermediate tank offers the possibility of splitting the necessary recirculation of the rejection current from the filtration module to the anaerobic tank in two different recirculation currents that can be controlled separately for the optimization of the sludge stratification in the anaerobic tank, On one side; and the maintenance of a flow speed, for example cross flow, on the filtration module that favors filtration, on the other hand. Therefore, the flow into the filtration module can be as high as desired without compromising the stratification of the anaerobic tank, and in the same way, the flow recirculation of the anaerobic tank can be adjusted as desired to maintain an optimum rate of ascension in the anaerobic tank. 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 from the anaerobic tank and that of the recirculation outlet of the fluid to be filtered to the filtration module. The volume between both levels makes it possible to maintain the filtration flow of the filtration module in optimal conditions despite fluctuations in the flow of the residual fluid to be treated, as well as to carry out short-term maintenance operations on the filtration module. , between 2 and 6 hours, such as cleaning with chemical reagents without interrupting the supply of the biological process.

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

The use of the intermediate tank allows these two cycles to be separated, 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 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 that supplies an oxygen-free gas in the filtration module.

According to a feature of the invention, the reject current and the supernatant enter through an area above the intermediate tank, arranged at a level above the outlet of the current to be filtered from said intermediate tank. In this way, the turbulence generated by the mixing of the reject current and the supernatant takes place in the upper part of the intermediate 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 current is carried out inside the intermediate tank, so that the reject current enters the intermediate tank at a level such that it allows said mixture to be kept slightly below the level of the overflow of the supernatant, and in turn at a level slightly above the level of the liquid contained inside it to avoid the formation of foam due to bubbling or splashes generated by the force with which the reject current falls on the surface of the intermediate tank liquid.

Alternatively, the reject stream and supernatant can be mixed before said mixture enters the buffer tank.

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

Advantageously, the feed-back stream exits through a lower lateral area 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 as it descends 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 filtering operation before being supplied to the filtering module. This additional operation of filtering solids allows retaining possible precipitates of fats and salts that may have escaped from the intermediate tank together with the current 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 an inflow of fluid to be treated; a feedback input for a feedback current; an outlet of a supernatant by overflow; and means for collecting a first gas stream; Y

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

The system is characterized in that it comprises an intermediate tank containing a mixture of the supernatant extracted from the anaerobic tank and the reject stream from the filtration module, the intermediate tank being provided with means for the outlet of a second stream of gas and at least two other exits from which

- one is an outlet that communicates with the filtration module through which the current of fluid 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 current can circulate.

Advantageously, the system comprises first drive means to force the recirculation of the re-feed current and control the rate of rise in the anaerobic tank, and second drive means to regulate the speed of the flow of the fluid to be filtered that enters in the filtration module, both drive means being capable of being governed independently by means of control means to be able to operate with a different speed regime between the anaerobic tank and the intermediate tank on the one hand, and between the intermediate tank and the filtration on the other hand. Said driving 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 equipped with a by-pass bypass that communicates with the filtration module, designed to at least partially recirculate the first and/or second gas streams, obtaining a gas flow that is used in the filtration module to ensure 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 placed at the inlet of the filtration module, in a lower part of it, where it will meet the current of the fluid to be filtered, which also enters through the lower part of the filtration module. filtration. The diffuser allows the 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 provided for said gas and the fluid stream to be filtered to mix before said mixture is introduced into the lower part of the filtration module.

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

According to one embodiment of the invention, the gas collection zones of the anaerobic tank and the intermediate tank accumulation zones 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 gas collection zones of the anaerobic tank and the intermediate tank accumulation zones 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 precipitates of fats and salts that may have escaped from the intermediate tank together with the current to be filtered is guaranteed, to avoid blockages and serious damage to the filtration module.

According to another feature of the invention, the system comprises a compressor designed 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 of the filtration module, and with a diameter such that it allows the downward flow velocity in the lower part of the intermediate tank is suitable for decantation of the floccules free of gas.

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-feed stream; and on the other hand, it facilitates the sedimentation of the particles coming from the rupture of the agglomerations/flocs of fats 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 microfiltration or ultrafiltration multitubular membrane module.

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

Depending on the system requirements, it is possible to arrange the filtration 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 diagram 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 current is carried out before said mixture enters 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 diagram of the system according to another embodiment, in which two filtration modules connected in series are used;

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

Fig. 6 is a flow diagram of the system according to another embodiment, without 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 procedure 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 made up of individual discharges that are produced discontinuously, mainly in the cleaning operations of production line equipment, frying tanks and rooms. After roughing and sieving operations, the discharge has a high organic content, with highly variable Chemical Oxygen Demand (COD), between 2,800 and 43,000 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 12,000 mg/L, often being the majority component. The concentration of nutrients varies widely depending on the type of dish prepared, from very low levels 75 mg N/L and 60 mg P/L for appetizers and vegetable burgers, up to 470 mg N/L and 80 mg P/L in the discharges generated in the preparation of squid rings . For optimal maintenance of biological activity, adequate amounts of urea and phosphoric acid are added to water with a nutrient deficit, adding sodium hydroxide to control the 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 anaerobic tank 1 with upward flow with a blanket of sludge, provided with an inlet for a tributary 2 of the fluid to be treated; a feedback input for a feedback current 3; an outlet of a supernatant 4 by overflow; 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 permeate outlet 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 rejection stream 10 of the filtration module 6, the intermediate tank 11 being provided with means for the output of a second stream of biogas 12 and of at least two other outlets from which

- one is an outlet that communicates with the filtration module 6 through which the current of fluid 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 current 3 can circulate.

The upflow anaerobic tank 1 is stratified so that the concentration of biomass is significantly higher in its lower area, which allows biomass losses to be reduced in its upper outlet, and improves its acclimatization and its performance in the degradation of oils and fats.

Anaerobic tank 1 has a layer of sludge partially settled through the which circulates the influent 2, in this case the wastewater to be treated, and the re-feed current 3 from the intermediate tank 11 in an ascending 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 influent 2 of the fluid to be treated can be introduced through the lower part of the anaerobic tank 1. Generally, this feed input is carried out through at least one conduit with direct entry into the lower part of the anaerobic tank 1, as shown in figure 1.

Alternatively, it is also foreseen that the inlet of the influent 2 is made through at least one submerged pipe that enters through the upper part of the anaerobic tank 1 and that leads the influent 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 the event of blockage of the pipes due to accumulation of fat, since it is easier to clean this feed pipe, an operation that only requires removing the pipeline for inspection or maintenance, having to empty the anaerobic tank 1 to access a direct inlet line 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 strong agitation that favors that those flocs of fat and biomass that, by flotation, have been dragged by the supernatant 4, release the biogas and settle at the bottom.

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

Referring to figures 1 and 2, a first pump 13 forces the recirculation of the re-feed stream 3 towards the anaerobic tank 1, which allows maintaining an adequate rate of ascension and returns the settled solids, free of biogas, to the mud blanket. Said feed-back stream 3 is drawn from the bottom of the intermediate tank 11.

The re-feeding fluid current 3 is introduced towards the lower part of the anaerobic tank 1, being able to use a conduit with direct entry into the lower part of the anaerobic tank 1, as can be seen in figures 1 and 2. According to another variant, in an analogous way, could use a submerged pipe that enters through the upper part of the anaerobic tank 1 and that leads said current of re-feeding fluid 3 to its lower zone.

In the examples, the intermediate tank 11 is made up of 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 descending flow velocity in the lower part of the intermediate tank 11 is adequate for the decantation of the floccules free of gas.

The level at which the outlet of the current to be filtered 7 is located in the intermediate tank 11, allows two zones to be differentiated in the intermediate tank 11: an upper zone A, in which an intense mixture of the supernatant 4 that comes out of 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 descending 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 by-pass bypass that communicates with the filtration module 6, intended to at least partially recirculate said biogas mixture 8 that is used in the filtration module 6 to ensure the conditions of filtering of the fluid stream to be filtered 7. Likewise, a compressor 15 is used to regulate the flow rate of the gas stream 8.

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

In accordance with the embodiments shown in figures 1 and 2, the respective biogas collection zones of the anaerobic tank 1 and accumulation zones 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 respective biogas collection zones of the anaerobic tank 1 and accumulation zones of the intermediate tank 11 are interconnected through a conduit 12', so that the mixture 5' obtained from the first 5 and second 12 biogas streams 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 current of fluid to be filtered 7, formed by the mixture of the supernatant 4 and the reject current 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 figure 5 shows another embodiment with two filtration modules 6 arranged vertically and in parallel, this last embodiment providing 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) connected to the external face of the membranes causes filtration, extracting a permeate 9 that circulates through the permeate tank (not shown). represented). Intermittently, after 15 - 60 minutes, the filtration stops and a pump is activated that takes water from the bottom 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 membrane fouling, which allows the transmembrane pressure to be maintained 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 rejection current 10 from the filtration module 6 to the anaerobic tank 1 into two different recirculation currents that can be controlled separately for the optimization of the sludge stratification in the anaerobic tank 1, on the one hand, and the maintenance of a cross-flow velocity on the filtration module 6 that favors filtration, on the other hand.

For this, the first 13 and second 16 pumps can be governed independently by means of control means to be able to operate with a different speed regime 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 flow recirculation of the anaerobic tank 1 can be adjusted as desired to maintain a constant flow rate. optimal lift in anaerobic tank 1.

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

On the other hand, the buffer 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 maintaining the flow of filtration of the membranes in optimal conditions despite fluctuations in the flow of the wastewater stream 2 to be treated, as well as carrying out short-term maintenance operations on the membranes, between 2 and 6 hours, such as cleaning with chemical reagents without interrupting the biological process feed.

According to a preferred embodiment shown in Figure 7, the gas 8 used in the filtration module 6 is mixed with the fluid stream 7 to be filtered by means of a diffuser 6a arranged at the inlet of the filtration module 6, in a lower area of the same, where it will meet the current of the 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, and a bubble diffuser can be used for this thick with perforations, for example, between 1 and 4 mm.

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

The confluence before or in the filtration module 6 of the recirculated mixture of biogas 8 that is driven by the compressor 15 and the current of fluid to be filtered 7, coming from the intermediate tank 11, forms a liquid-gas mixture that circulates through the perforated tubes of the membranes in which thick bubbles are generated, between 1 and 3 mm, which rise generating a turbulent current on the membranes that contributes to keeping the filtration surface clean. The flow of injected biogas 8 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 through its upper part, the biogas being again separated from said rejection current 10 in the intermediate tank 11. Although not shown, a small amount of biogas passing through the membrane is sent together with the permeate 9 to the closed permeate tank where it is separated from it and collected at the top for return to the head space of the anaerobic tank one.

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 ascending current, known as gas-lift, which contributes to reducing the energy consumption of the aforementioned second pump 16 that drives the current of fluid to be filtered 7 to the filtration module 6.

Additionally, the system may 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 together with the current to be filtered 7 The mesh size of the solids filter 17 must be chosen based on 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 blockages 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 optimum biogas production, while obtaining effluents of high biological and physical-chemical quality with complete sludge retention and low sludge generation. It has been verified that the system allows organic matter removal yields of 85-95% to be achieved, 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 sludge blanket from which a supernatant (4) and a first gas stream are extracted (5); and at least one membrane filtration module (6) in which a gas (8) is used to produce a turbulent mixture with a stream to be filtered (7), and from which a reject stream (10) and a permeate (9), the procedure being characterized in that it comprises the operation of mixing the rejection stream (10) of the filtration module (6) and the supernatant (4) extracted from the anaerobic tank (1), and containing the mixture in a tank intermediate (11), with gas outlet, 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 filter 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 current filtering conditions. 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 the fluid to be filtered (7) entering the filtration module (6), independently and at different rates. 4. Procedure for the anaerobic treatment of organic waste fluids, according to any one of the preceding claims, characterized in that the reject current (10) and the supernatant (4) enter the intermediate tank (11) through an upper zone (A), arranged at a level above the outlet of the current to be filtered (7) of 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 current (10) is carried out inside the intermediate tank (11), of so that the reject stream (10) enters the intermediate tank (11) at a level that allows said mixture to remain slightly below the level of the supernatant overflow (4), and in turn at a level slightly above the level of the liquid contained inside. 6. Procedure for the anaerobic treatment of organic waste fluids, according to claim 4, characterized in that the reject current (10) and the supernatant (4) are mixed before said mixture enters the intermediate tank (11). 7. Procedure 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 mid-height of the intermediate tank (11). 8. Procedure for the anaerobic treatment of organic waste fluids, according to any one of the preceding claims, characterized in that the refeed stream (3) exits through a lower lateral area (B) of the intermediate tank (11), arranged at a level below the outlet of the current to be filtered (7) of said intermediate tank (11). 9. Procedure for the anaerobic treatment of organic waste fluids, according to any one of the preceding claims, characterized in that the current 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 anaerobic tank (1) with upward flow with a blanket of sludge, provided with an inlet for a tributary (2) of fluid to be treated; a feedback input for a feedback current (3); an outlet of a supernatant (4) by overflow; and means for collecting a first gas stream (5); Y - at least one membrane 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 rejection current (10) of the filtration module (6), the intermediate tank (11) being provided with means for the output of a second current of gas (12) and from at least two other outlets from which - one is an outlet that communicates with the filtration module (6) through which the current of fluid 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 current (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 the 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 current of the fluid to be filtered (7) that enters the filtration module (6), both drive means (13,16) being capable of be governed independently by control means to be able to operate with a different speed regime 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 being line (14) equipped with a bypass as a by-pass that communicates with the filtration module (6), designed to at least partially recirculate the first (5) and/or the second (12) gas streams, obtaining a gas flow (8) that is used in the filtration module (6) to ensure the filtering conditions of the fluid current 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 current of fluid to be filtered (7) through 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 head space, above the level of the overflow of the supernatant (4), a collection area for 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 respective gas collection areas of the anaerobic tank (1) and 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 respective gas collection areas of the anaerobic tank (1) and 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 solid 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) designed 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) consists of 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 descending flow velocity in the lower part of the intermediate tank (11) to be adequate for the decantation of the exempt 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 microfiltration or ultrafiltration multitubular membrane module. 21. System for the anaerobic treatment of organic waste fluids, according to 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 parallel.