ES2977138A1 - Method and system for the production and purification of volatile fatty acids from an organic waste source - Google Patents

Abstract

A method (100) for the production and purification of volatile fatty acids (VFAs) comprising fermenting (10) organic waste (1) under anaerobic and controlled acidity conditions to produce a first fermented product (12); subjecting at least a portion of the liquid fraction (23) of the first fermented product (12) first to at least one filtration operation (30) and then to at least one concentration operation (40), to obtain respective reject (30b, 40b) and VFA concentrate (40a) streams; and the recovery and purification of the VFAs from the VFA concentrate stream (40a) by a liquid-liquid extraction operation (50) followed by at least one staged distillation operation (60), further comprising the fermentation (70) under anaerobic conditions of at least part of the solid fraction (22) obtained from the first fermentate (12) and of at least one reject stream (30b, 40b) from the filtration and/or concentration operations to produce biogas (82) and a second fermentate (72).

Description

DESCRIPTION

Method and system for the production and purification of volatile fatty acids from an organic waste source

Field of invention

The invention relates to a method and a system for the production of volatile fatty acids, in particular for their production and purification from an organic waste source, such as sewage sludge or urban biological waste.

Background of the invention

Anaerobic digestion (AD) is a process that breaks down organic matter with the help of microorganisms in an oxygen-free environment. AD is carried out by different microbial communities that break down organic matter in several successive and interdependent stages.

Spontaneous AD results in methane-rich biogas. Biogas is generally used for heat and electricity generation. However, new developments in the anaerobic digestion process are allowing the production of other intermediate metabolites including carboxylic acids such as volatile fatty acids (VFAs). Currently, the conventional route for industrial VFA production relies on petroleum-derived chemicals. As a less common production route, biosynthesis of VFAs from organic waste offers an alternative to methods of VFA production from non-renewable sources.

However, the use of a wide variety of wastes as substrate, the wide variety of types of microbial consortia used and the need to concentrate and purify these intermediate products make the recovery of VFAs via DA energy inefficient.

Furthermore, to produce products such as VFAs, which are intermediate metabolites, strategies must be adopted to reduce their subsequent transformation into methane.

Both problems, high energy consumption and the necessary inhibition of methanogenesis, represent an important barrier to achieving high rates of conversion, industrialization, yield and concentration of the product of interest.

According to the scientific consensus exemplified by the critical review conducted by Wainaina S, Lukitawesa, Kumar Awasthi M, Taherzadeh MJ. "Bioengineering of anaerobic digestion for volatile fatty acids, hydrogen or methane production: A critical review. Bioengineered”. 2019 Dec;10(1):437-458. doi: 10.1080/21655979.2019.1673937. PMID: 31570035; PMCID: PMC6802927, the golden rule to maximize the accumulation of VFAs in a fermenter is not only to optimize their production, but also to minimize their consumption, that is, to reduce the conversion of VFAs into methane. This reduction of the conversion of VFAs into methane can be achieved through strategies whose purpose is to eliminate methanogenic microorganisms in the fermentation process. These strategies can be the organic overload of the system, the reduction of the hydraulic retention time in the fermenter below the reproduction time of methanogenic microorganisms, the application of acidic or alkaline pH or the addition of certain chemicals that act as inhibitors of methanogenic microorganisms.

On the other hand, due to the huge number of variables involved in anaerobic digestion, there is still no broad consensus on the conditions that maximize VFA production. While pH is among the most important variables, depending on other variables such as the nature of the organic waste, the retention time in the reactor, the pH control methods and the microbial consortia used, the maximum yield pH can vary significantly, such as between slightly acidic and basic conditions (5 pH to 11 pH). Examples of these cases can be found in the following scientific articles:

Dahiya S, Sarkar O, Swamy YV, et al. Acidogenic fermentation of food waste for volatile fatty acid production with co-generation of biohydrogen. Bioresour Technol. 2015;182:103-113.

Jiang J, Zhang Y, Li K, et al. Volatile fatty acids production from food waste: effects of pH, temperature, and organic loading rate. Bioresour Technol. 2013;143:525-530.

Zhang B, Zhang LL, Zhang SC, et al. The influence of pH on hydrolysis and acidogenesis of kitchen wastes in two-phase anaerobic digestion. Environ Technol. 2005;26(3):329-340.

Wang K, Yin J, Shen D, et al. Anaerobic digestion of food waste for volatile fatty acids (VFAs) production with different types of inoculum: effect of pH. Bioresour Technol. 2014;161:395-401.

As far as patent literature is concerned, document US 2021/309549 discloses a method of converting an organic waste stream comprising fermenting the organic waste stream, exemplified as brewery wastewater, performed with a solids retention time of about 3 days to about 20 days and/or a hydraulic retention time of no more than 7 days at a pH of 7.5 or less, to generate an organic product comprising one or more VFAs and/or alcohols, specifically comprising one or more of lactic, acetic, butyric and propionic acids, wherein the lactic acid is present in a greater amount than the other organic acids.

The conversion method described provides that organic acids and/or alcohols can be recovered from the reactor effluent using ion exchange resins, adsorbents, membrane filtration, electrodialysis, electrodeionization or any combination thereof. Examples of suitable membrane filtration methods that can be used are, for example, ultrafiltration, nanofiltration and reverse osmosis.

This conversion method favours the methanogenesis process, as it is carried out at pH 7.5, but by applying a short sludge retention time in the bioreactor, the complete transformation of organic matter into methane is prevented. In addition, it contemplates the aeration of the organic waste influent to the reactor, which eliminates methanogens from the microbial community.

The use of water-insoluble organic waste for fermentation in water is known from US 2012149076, which is touted as particularly advantageous because it allows for different residence times in the reactor for the solid fraction and the liquid fraction of the organic waste. The separation process of the solid fraction and the liquid fraction referred to above will be carried out by means of, for example, filters and presses.

According to US 2012149076, although low concentrations of VFA are easily achieved, the technical difficulty lies in achieving an economically viable recovery of VFA due to the large quantities of water that must be removed from the liquid fraction in which the VFA are dissolved. US 2012149076 proposes using the combination of reverse osmosis followed by evaporation as a method of VFA recovery. Liquid-liquid extraction is discouraged because the attractive extractants for the most common VFAs are miscible in water, having to be separated later from the water or being lost in the process.

For higher performance in the production of VFAs, US 2012149076 contemplates the use of two reactors in series, a first low-performance reactor fed with a high concentration of solid fraction with a short stay in the reactor that once fermented is filtered and subjected to a separation stage to obtain a first liquid stream rich in VFAs and a solid fraction that is used to feed a second reactor, in this case with higher performance and with a lower concentration of solids, which once fermented will be filtered and subjected to a separation stage to obtain a second liquid stream rich in VFAs that will be mixed with the first liquid stream obtained from the first reactor.

From document US10662447, a fermentation method is known in which the VFA produced are in their salt form, for which the fermentation occurs in a basic medium pH>8. For the recovery of the VFA from a liquid stream obtained, the basicity of the solution is a drawback since the VFA must be in their protonated form to proceed to their recovery by liquid-liquid extraction. To this end, document US10662447 proposes subjecting this liquid stream to a high-pressure flow of CO2 in order to produce carbonate or bicarbonate that acidify it to reset the VFA to their protonated form.

Most fermentation technologies for VFA production use techniques to reduce methanogenesis through culture conditions (US 2021/309549), pH conditions that hinder the growth of methanogenic microorganisms (US10662447) or substrate concentration and reactor retention time conditions that result in minimal methane production (US2012149076).

However, all of these methods have drawbacks such as having to pre-treat bacterial cultures, using high amounts of chemicals to regulate pH, producing low concentrations of product whose refining is very energy-intensive, or generating waste from the fermentation process that must undergo successive steps to complete its recovery cycle (composting, etc.).

Summary of the invention

In order to solve the above-described drawbacks, a method for producing and purifying volatile fatty acids (VFAs) from an organic waste source is disclosed, which includes a step a) comprising fermenting organic waste under anaerobic conditions to produce a first fermented product; a step b) comprising separating the first fermented product into a liquid fraction and a solid fraction; a step c) comprising subjecting at least a portion of the liquid fraction from step b) first to at least one filtration operation and then to at least one concentration operation, to obtain respective reject and VFA concentrate streams; and a step d) comprising recovering and purifying VFAs from the VFA concentrate stream by means of a liquid-liquid extraction operation followed by at least one stepwise distillation operation.

In essence, the method is characterized in that the fermentation of step a) is carried out under controlled acidity conditions, with a pH between 5 and 6, preferably between 5.1 and 5.4; and because the method also comprises a step e) which comprises the fermentation under anaerobic conditions of at least part of the solid fraction of step b) and of at least one reject stream of step c) to produce biogas.

It should be noted that the acidity conditions in the fermentation in step a) are conditions that, according to the state of the art, do not maximize the production of VFAs. However, among other aspects, they prevent the production of other compounds of lesser value such as lactic acid and minimize the amount of chemical compounds that need to be added to maintain the pH at the desired value.

In order to take full advantage of this method, according to a preferred form of the invention, the fermentation in step a) is carried out in a first reactor with organic waste with an organic matter concentration of between 60 and 120 g COD/L and with a percentage of solid matter preferably between 5% and 15%; and with a hydraulic retention time (HRT) of between 2 and 6 days.

At the time of conception of the present invention, this procedure is considered suboptimal because the operating conditions applied (pH, hydraulic retention time) do not imply the maximum conversion of organic waste to VFAs nor do they minimize the amount of solid waste obtained after fermentation.

The combined selection of these operating parameters also results in a solid fraction obtained in step b) that would be considered too active to be directly discarded, which also entails the need to carry out further steps for its conditioning, which is a clear disincentive for the expert to have considered it.

The inventors have overcome these prejudices by considering the anaerobic fermentation process holistically so that methane production, previously considered a drawback, is synergized in a second fermentation process with VFA production. In this way, VFA production is not considered as a process whose yield must be maximized, but as a process that allows obtaining a significant production of VFA under the best conditions to minimize the costs of their subsequent purification. Through the second stage of biogas production, the solid fraction obtained in step b), still too rich in biodegradable organic matter to be directly disposed of, is subjected to a second fermentation process aimed at methane production.

This procedure will allow a first fermentation obtained from step a) to be separated into a liquid fraction with dissolved VFA and a solid fraction with sufficient methanogenic potential to be subjected to a respective second fermentation, in a second reactor, in this case to produce a biogas rich in methane. This biogas produced in this second fermentation represents a source of heat and/or electrical energy for the concentration operations and subsequent recovery and purification of VFA, in favour of an overall energy balance that makes the entire method profitable and beneficial.

Advantageously, the reject stream or streams obtained by subjecting at least part of the liquid fraction of step b) to at least one filtration operation and which have a significant concentration of soluble organic matter not transformed into VFAs (carbohydrates, proteins, etc.), may be used in the second fermentation to be transformed into biogas. In addition, this stream or streams provide the solid fraction obtained after the first fermentation with part of the humidity and heat necessary to be subjected to the aforementioned second fermentation, further improving the overall performance of the method.

This preferred form of the invention is therefore characterised by the combination of the following measures:

- pH control.

Although it is known that the maximum VFA production yield can be obtained at a slightly acidic or slightly alkaline pH, the invention aims to operate the reactor under suboptimal conditions (pH between 5 and 6) to reduce the costs of chemicals for pH control and the costs for the recovery and purification of the VFA.

Note that to counteract the overload effect and to prevent the acidity from decreasing, the pH must be controlled, although this can be done in known ways, for example, by adding a base such as NaOH, without harming the potential of the invention.

In addition to the above, this particular pH control makes it possible to avoid the production of caproic acid in the first reactor. This acid has a boiling point (208°C) that is close to some solvent candidates that can be used for the liquid-liquid extraction operation in step d) and can, consequently, represent a drawback due to the difficulty in separating it from the potential solvents to be used.

Likewise, the liquid-liquid extraction operation referred to above in step d) has its optimum pH under acidic conditions, so carrying out the first fermentation at slightly acidic values allows minimizing the costs of adding additives to reduce the pH of the AGV concentrate stream.

- Selection of HRT.

Although it is known that the maximum concentration of VFAs can be obtained with a HRT of 15-20 days, the invention aims to operate the reactor in which the first fermentation takes place under suboptimal HRT conditions (2-6 days), in order to, among others, minimize the volume and cost of said reactor.

- Certain concentration of organic matter.

Although a low concentration of organic matter allows for a higher VFA production yield, a more diluted stream is obtained, which is more expensive to purify. Thus, an organic matter concentration between 60 and 120 g COD/L/d and a HRT between 2 and 6 days implies a high rate of organic loading in the reactor where the first fermentation takes place (15-60 g COD/Ld) and the obtaining of a VFA stream with a concentration between 30 and 40 g/L.

Step b) will allow the first fermentation to be separated into a liquid fraction and a solid fraction. This operation can be carried out, for example, by centrifuges or by a filter press in a known manner.

In one embodiment of the invention, this separation is carried out by press filtration since advantageously a higher degree of dryness can be achieved in the solid fraction and therefore a greater recovery of VFAs.

In one embodiment of the invention, coagulants and/or flocculants are added to the first fermentation during step b) since advantageously a greater degree of dryness in the solid fraction can thus be achieved and therefore a greater recovery of VFAs.

In a preferred form, the coagulant and/or flocculant comprises cationic polymers, since unlike other coagulants/flocculants, such as metal salts, they do not promote the precipitation of VFAs.

In step c), the liquid fraction obtained in step b) is subjected to at least one filtering operation and one concentration operation.

Advantageously, the pH established in step a) allows compounds such as phosphate salts, ammonium salts and other cations such as calcium or magnesium to pass through the filtering and concentration operations without accidental precipitation. This is particularly convenient since these compounds follow the same path as the VFA stream up to the liquid-liquid extraction operation. After liquid-liquid extraction, the VFAs are removed from the aqueous phase and transferred to the solvent, which causes an increase in the pH of the soluble phase which, together with the high concentration of these ions, facilitates the recovery of ammonium and phosphate in the form of precipitates.

In the filtration operation, one or more reject streams will be produced that will contain organic components of a solid, colloidal and soluble nature not transformed into VFAs.

At least one reject stream, rich in organic components, will be fermented in step e) together with the solid fraction generated in step b). Advantageously, this contributes to rehydrating the solid fraction intended to be fermented in step e) as well as utilizing the organic content of the reject stream.

In one embodiment, step c) comprises two filtration stages: a first stage of ultrafiltration followed by a second stage of nanofiltration, in series.

In the concentration operation of step c), a first stream of VFA concentrate and at least one reject stream will be obtained, which will be mainly water and colloidal and soluble organic compounds.

Advantageously, the reject stream from the concentration operation will be used in step e) to hydrate the solid fraction resulting from step b), intended to be fermented in step e).

Advantageously, the concentration operation will also concentrate the previously described compounds other than VFAs such as phosphate, ammonium and other cation salts such as calcium or magnesium in ionic form. This will not only facilitate their separation in the liquid-liquid extraction operation but will also facilitate the recovery of ammonium and phosphorus salts, which is favoured at high concentrations of these compounds.

Also advantageously, the concentration operation in step c) prior to the liquid-liquid extraction operation in step d) allows a very notable reduction in the quantity of acid needed to lower the pH of the solution and of the solvent needed to recover the VFAs from the liquid phase in which they are dissolved due to the increased concentration of VFAs in the solution and the fact that the pH of the concentration decreases. This translates into a notable saving of heat energy, of up to 70-80% and a decrease in the reagents used to adjust the pH of the solution to that appropriate for liquid-liquid separation compared to alternatives that do not contemplate a concentration operation.

In one embodiment, the at least one concentration operation is by reverse osmosis. In this case the reject stream from the concentration operation is the reverse osmosis clarification.

Advantageously, the performance of reverse osmosis in a method according to the invention exceeds the performance of concentration operations that the state of the art can contemplate performing, such as dialysis.

In one embodiment the reverse osmosis concentration operation uses polyamide polymeric membranes.

In the liquid-liquid extraction operation, it is contemplated to use a light solvent partially soluble in water, one candidate being ethyl acetate.

In the liquid-liquid extraction operation, it is also contemplated to use a heavy solvent not soluble in water, one candidate being ethyl octanoate.

In any case, the stage-wise distillation operation comprises, according to a variant of the invention, a first distillation stage from which a VFA mixture stream is obtained, lacking the solvent used for the liquid-liquid extraction operation; the method comprising subjecting this VFA mixture stream to a distillation cascade to separate different types of VFA, at least acetic, propionic, butyric and valeric.

Brief description of the figures

Figure 1 is a flow chart of a method of producing and purifying VFAs according to an embodiment of the invention;

Figure 2 is a more detailed flow diagram of the concentration, liquid-liquid extraction and distillation operations according to a variant of the method of the invention;

Figure 3 is a more detailed flow diagram of the concentration, liquid-liquid extraction and distillation operations according to another variant of the method of the invention; and Figure 4 is a complete flow diagram of a method of production and purification of VFAs, according to an embodiment of the invention.

Detailed explanation of the invention

The present invention is illustrated by means of two examples using respective sources of organic matter. Specifically, Example 1 refers to a variant of the implementation of the method of the invention that starts with urban biowaste; while Example 2 refers to another variant of the implementation of the method of the invention that starts with sludge from a sewage treatment plant.

The flowchart in Figure 1 illustrates steps that are common to both Examples 1 and 2.

With reference to this Figure 1, a step a) comprises a first fermentation 10. This first fermentation 10 is a fermentation of the organic waste 1 from which the VFAs will be obtained, which first fermentation is carried out under anaerobic conditions to produce a first fermented product 12. This first fermentation 10 is carried out in a first reactor which we will refer to later.

A subsequent step b) comprises the separation 20 of the first fermentation 12 into a liquid fraction 23 and a solid fraction 22. As shown in Figure 1, these liquid 23 and solid 22 fractions will follow different courses. This separation can be carried out by separation means which are exemplified below in the form of a filter press.

Following the flow of Figure 1, on the one hand, at least a part of the liquid fraction 23 obtained from the first fermentation 12 is first subjected to at least one filtration operation 30, preferably with two cascade filtration stages as will be described with reference to Examples 1 and 2, and then to at least one concentration operation 40. From the filtration operations 30 and concentration 40, respective reject streams 30b and clarification streams 40b will be obtained and, as a result of the concentration operation 40, a VFA concentrate stream 40a will also be obtained. The filtration and concentration operations can be carried out with respective filtration and concentration means, which are exemplified below.

This will be followed by a step d) of recovery and purification of the VFAs from the VFA concentrate stream 40a comprising a liquid-liquid type extraction operation 50 followed by at least one distillation operation 60, preferably with several distillation stages in cascade as will be described with reference to Examples 1 and 2, to produce essentially pure VFAs.

In the context of this description, by essentially pure we mean products with a percentage by weight of the valued product, in this case a VFA, greater than 95%.

The concentration operations 40 and the overall step d), which includes the extraction operations 50 and the distillation operations 60, are energy-intensive actions.

The method of the invention comprises actions that, among other aspects, allow this drawback to be overcome. One aspect of these actions comprises taking measures so that the first fermentation 10 of step a) is carried out under controlled acidity conditions, between 5 pH and 6 pH, even though this selection is not associated in the literature as that which maximizes the production of VFAs. Another aspect of these actions comprises, as shown in the diagram in Figure 1, a step e) which comprises a second fermentation 70. This second fermentation 70 is a fermentation under anaerobic conditions of at least part of the solid fraction 22 of the first fermentate, obtained in the aforementioned step b), and also of at least one reject stream 30b and/or 40b obtained in the filtration 30 and concentration 40 operations, respectively, to produce a biogas 82 and a second fermentate 72. This second fermentation is carried out in a second reactor which will also be referred to later.

The biogas 82 may be valorised outside the system or may be transformed into heat or electrical energy in a power generation operation 80, as the dashed line in Figure 1 is intended to illustrate, to be used in the concentration operations 40 and/or especially in one of the stages of the distillation operation in step d).

Example 1:

The flowchart that follows the implementation of this Example 1 is illustrated in Figure 4, which we will rely on in this section for a better explanation.

Table 1 provides a characterization of the source of organic matter used, in the form of urban bio-waste obtained from the selective collection of municipal organic waste.

Table 1: Characterization of the urban biowaste used in Example 1.

As a preliminary step to step a), bio-waste 1 was taken to a crusher 15, where water was also added until reaching a solids content of 8%. Thus, a mass flow rate of 1000 kg/d of bio-waste was crushed and diluted until reaching a flow rate of 2500 kg/d.

The mixture was then taken to the first reactor 11, where step a) was carried out, specifically the first fermentation, selecting a hydraulic retention time (HRT) of 5 days and a temperature of approximately 37°C, that is, under mesophilic temperature conditions. To maintain the pH between 5 and 6 in the reactor, it was necessary to add 20 kg of NaOH (50%). The colony of microorganisms was naturally selected from a mixed microbial culture from an industrial anaerobic digester of sewage sludge.

From this first fermentation the first fermented product 12 was obtained, which was subjected in step b) to a separation operation, specifically being taken to a filter press 21. This first fermented product 12 had the properties shown in Table 2.

Table 2: Characterization of the first fermented effluent from the first reactor, in Example 1.

To promote solid-liquid separation in filter press 21, a dose of 1000 ppm FLOPAM FL 4820 coagulant and a dose of 1500 ppm FLOPAM EM 640 L flocculant were added.

The filter press 21 allowed obtaining a solid fraction 22 and a liquid fraction 23 of the first fermentation 12, whose characteristics are detailed in Table 3.

Table 3: Characterization of the solid fraction and the liquid fraction obtained from the first fermentation, in Example 1.

The total liquid fraction 23 produced was used to carry out step c), and was subjected to a two-stage filtration operation: a first stage of ultrafiltration 31 and a second stage of nanofiltration 32, in cascade. From the first stage of ultrafiltration 31 a first reject stream 31b and a first clarified stream 31a were obtained. This first clarified stream was subjected to the second stage of nanofiltration 32 producing a second reject stream 32b and a second clarified stream 32a. These filtrations were carried out at a pressure of between 25 and 35 bar with a flow of approximately 40 L/m2h. The products and rejects of this filtration operation are shown in Table 4.

Table 4: Characterization of the ultrafiltration and nanofiltration stages of the filtration operation of the liquid fraction of the first fermentation, in Example 1.

The second clarified stream 32a was subjected to a concentration operation by double reverse osmosis step 41 at a pressure between 40 and 50 bar, with a flow rate of about 20 L/m2h. In this example, a clean water permeate flow rate of about 1221 kg/d was produced, with a reject of about 523 kg/d, so that the VFA concentration increased from 21.2 to 77 g/L and their volume was reduced by 70%. By maintaining the pH at an acidic value, the undesired precipitation of ammonium and/or phosphorus salts was avoided, since the ammonium and phosphate concentrations rose to about 4.5 and 1.1 g/L, these concentrations being favourable for obtaining precipitates such as struvite.

In Example 1, the reject streams 31b, 32b from the filtration operation 30 and the clarification streams 40b from the concentration operation 40 were used to provide soluble organic matter, hydrate and partially heat the solid residue from the first fermentation to maximize biogas production in the second fermentation.

In any case, the concentration operation completed step c) to proceed to the next step d) consisting of the recovery and purification of the VFAs from the VFA concentrate stream 40a.

For this purpose, the VFA concentrate stream 40a was subjected to a liquid-liquid extraction operation 50. In Example 1, ethyl acetate was used as solvent.

Before proceeding with the liquid-liquid extraction operation 50, however, the pH of the VFA concentrate stream was modified by adding 0.35 kg/d of HCl, in order to reduce the pH values to a target value close to 3. Two streams were obtained from this extractor, a first stream rich in ethyl acetate and VFA 51a and a second stream rich in water and ethyl acetate 51b, all as illustrated in greater detail in Figure 2.

From the first stream rich in ethyl acetate and VFA 51a, the cascade distillation operation 60 will be started. This distillation operation is also illustrated in greater detail in Figure 2.

Since ethyl acetate is a partially water-soluble product, two distillation steps 52, 61 were performed in the present Example 1 for the recovery of ethyl acetate after liquid-liquid extraction 51. The first distillation step 61 had the purpose of separating the first stream rich in ethyl acetate and VFAs 51a into two fractions, the first fraction 61a consisting of a mixture of VFAs and the second fraction 55 consisting of ethyl acetate.

As for the second stream rich in water and ethyl acetate 51b, it was also subjected to a recovery stage by distillation 52 that generated a stream rich in ethyl acetate 55 and an aqueous stream.

The recovered ethyl acetate stream will be used in successive liquid-liquid extraction operations.

As a final step for the refinement of the VFAs, the latter was subjected to a distillation cascade 62, 63, 64 to separate the different types of VFAs, in this case acetic 101, propionic 102, butyric 103 and valeric 104, obtaining the profile in Table 5.

Table 5: Characterization of the VFAs obtained in Example 1.

On the other hand, as is characteristic of the method, the solid fraction 22 of the first fermentation obtained by means of the filter press 21 was used as influent of a second reactor 71, in Example 1 together with the reject streams 31b and 32b of the ultrafiltration stages 31 and nanofiltration 32. In Example 1, to the 460 kg of solid fraction of the first fermentation were added 100 kg/d of reject water from the UF, 190 kg/d of reject water from the NF and also 1221 kg/d of the clarification from the osmosis 40b, to reach a suitable degree of humidity in this influent.

In this second reactor 71, a second fermentation takes place. For this second fermentation, a HRT of 20 days was selected and it was produced at approximately 55°C, i.e. under thermophilic temperature conditions. Under these conditions, a sanitized digested product is obtained that can be used directly in agriculture without the need for any post-treatment.

From this second fermentation, the second fermented product 72 was obtained, whose properties are shown in Table 6, and a biogas 82.

Table 6: Characterization of the second fermentation, effluent from the second reactor, in Example 1.

By mixing the different reject streams from the filtration operation stages, the pH is adjusted to a value suitable for the biogas production process. The microbial consortium adapts to the operating conditions by natural selection from a mixed culture from an anaerobic digester.

The second fermented product 72 was conditioned, specifically directed to a second filter press 76 using 1000 ppm of coagulant and 1500 ppm of the same flocculants used in the separation stage of the first fermented stream, such that the solid fraction 77 obtained was compatible with agricultural uses; while the liquid fraction 78 was subjected to a return treatment operation 79, known per se, to be discarded, although it could be mixed with the aqueous stream obtained from the distillation operation 50 to recover ammonium and phosphorus salts from both streams simultaneously.

In any case, the biogas 82 produced in the second reactor 71, in this case a CH4-rich biogas 82, was partially fed to a boiler 81 for heat energy generation and the remainder to a cogeneration engine to produce electricity.

In Example 1, part of the heat energy produced in boiler 81 could be effectively used for the distillation operations 52, 61, 62, 63, 64 referred to above in step d) of recovery and purification of the VFAs. The electrical energy produced was used to cover the energy needs of the other units (pumps, agitators, etc.), and a surplus could be produced that could be injected into the network.

The energy balance obtained in Example 1 is shown in Table 7. From the waste streams of the different processes, 78 m3/d of biogas 82 were obtained, with a methane concentration of 60%. 25% of this biogas was specifically destined to the aforementioned cogeneration engine, from which 1.6 kW of electrical energy and 1.4 kW of heat energy were obtained. The remaining 75% was destined to boiler 81, from which another 12.6 kW of heat energy were obtained.

Table 7: Balance of the HEAT energy consumed and generated in Example 1.

Table 8: Balance of electrical energy consumed and generated in Example 1.

Example 2:

Example 2 differs from Example 1 in that the source of organic matter is sewage sludge and that ethyl octanoate is used as a solvent in the liquid-liquid extraction operation.

A characterization of the source of organic matter used follows in Table 9.

Table 9: Characterization of the sewage sludge used in Example 2.

In this case, it was not necessary to transfer the sludge to a crusher before step a).

Instead, the sludge was taken to a hydration unit to adjust its humidity and then to the first reactor 11, where step a) was carried out and specifically the first fermentation, selecting a hydraulic retention time (HRT) of 5-6 days and at an approximate temperature of 37°C, that is, under mesophilic temperature conditions. To maintain the pH between 5 and 6 in the reactor, it was necessary to add 15 kg of NaOH (50%). The microorganism colony was naturally selected from a mixed microbial culture from an industrial anaerobic digester of sewage sludge.

The first fermented 12 had the properties shown in Table 10.

Table 10: Characterization of the first fermented effluent from the first reactor, in Example 2.

To promote solid-liquid separation in filter press 21, a dose of 200 ppm FLOPAM FL 4820 coagulant and a dose of 1500 ppm FLOPAM EM 640 L flocculant were added.

The filter press 21 allowed obtaining a solid fraction 22 and a liquid fraction 23 of the first fermentation 12, whose characteristics are detailed in Table 11.

Table 11: Characterization of the solid fraction and the liquid fraction obtained from the first fermentation, in Example 2.

Similarly to Example 1, the entire liquid fraction 23 produced was used to carry out step c), and was subjected to a filtration operation in two stages: a first stage of ultrafiltration 31 and a second stage of nanofiltration 32, in series. The products and rejects of this filtration operation are shown in Table 12.

Table 12: Characterization of the ultrafiltration and nanofiltration stages of the filtration operation of the liquid fraction of the first fermentation, in Example 2.

The second clarified stream 32a was subjected to a concentration operation, by double reverse osmosis step 41, under conditions similar to Example 1.

The VFA concentrate stream 40a was subjected to a liquid-liquid extraction operation 50, using ethyl octanoate as solvent in this case; and to the cascade distillation operation 62 to 64 shown in this case in Figure 3 to separate the different types of VFA, in this case acetic 101, propionic 102, butyric 103 and valeric 104, obtaining the profile in Table 13.

It should be noted that before proceeding with the liquid-liquid extraction operation 50, the pH of the VFA concentrate stream was modified by adding 0.30 kg/d of HCl, in order to reduce the pH values to a target value close to 3.

Table 13: Characterization of the VFAs obtained in Example 2.

On the other hand, as is characteristic of the method, the solid fraction 22 of the first fermentation obtained by means of the filter press 21 was used as influent of a second reactor 71, in Example 2 also together with the reject streams 31b and 32b of the ultrafiltration 31 and nanofiltration 32 stages. In Example 1, 114 kg/d of UF reject water, 217 kg/d of NF reject water and also 1462 kg/d of the osmosis clarification 40b were added to the 470 kg of solid fraction of the first fermentation, in order to achieve a suitable degree of humidity in this influent.

From this second fermentation, the second fermented product 72 was obtained, whose properties are shown in Table 14, and a biogas 82.

Table 14: Characterization of the second fermentation, effluent from the second reactor, in Example 2.

The energy balance obtained in Example 2 is shown in Table 15. From the waste streams of the different processes, 61 m3/d of biogas 82 were obtained, with a methane concentration of 60%. 20% of this biogas was used for a cogeneration engine, from which 1.1 kW of electrical energy and 0.9 kW of heat energy were obtained. The remaining 80% was used for boiler 81, from which another 10.9 kW of heat energy were obtained.

Table 15: Balance of the HEAT energy consumed and generated in example 2.

Table 16: Balance of electrical energy consumed and generated in example 2.

Claims (18)

1. - A method (100) for the production and purification of volatile fatty acids (VFAs) from an organic waste source (1), with the steps: a) comprising the fermentation (10) of organic waste (1) under anaerobic conditions to produce a first fermented product (12), b) comprising the separation (20) of the first fermented product (12) into a liquid fraction (23) and a solid fraction (22), c) comprising subjecting at least a portion of the liquid fraction (23) from step b) first to at least one filtration operation (30) and then to at least one concentration operation (40), to obtain respective reject streams (30b, 40b) and VFA concentrate (40a), and d) comprising the recovery and purification of the VFAs from the VFA concentrate stream (40a) by a liquid-liquid extraction operation (50) followed by at least one stepwise distillation operation (60) to obtain essentially pure VFAs (101, 102, 103, 104), the method being characterized because, The fermentation (10) of step a) is carried out under controlled acidity conditions with a pH between 5 and 6 and why The method also includes the step e) comprising the fermentation (70) under anaerobic conditions of at least part of the solid fraction (22) obtained in step b) and of at least one reject stream (30b, 40b) from step c) to produce biogas (82) and a second fermented product (72). 2. A method according to claim 1, characterized in that the fermentation (10) of step a) is carried out in a first reactor (11) fed with organic waste (1) with an organic matter concentration of between 60 and 120 g COD/L and with a solid matter percentage of between 5% and 15%; and because the hydraulic retention time (HRT) of the organic waste in the first reactor (11) is between 2 and 6 days. 3. - A method according to claims 1 or 2, characterized in that the fermentation (10) of step a) is carried out under conditions of controlled acidity with a pH between 5.1 and 5.4. 4. - A method according to any one of the preceding claims, characterized in that the concentration operation (40) of step c) is carried out by reverse osmosis (41). 5. A method according to the preceding claim, characterized in that the reverse osmosis (41) uses polyamide polymeric membranes. 6. - A method according to any one of the preceding claims, characterized in that it comprises acidifying the VFA concentrate stream (40a) obtained in step c) prior to the liquid-liquid extraction operation (50) of step d). 7. - A method according to the preceding claim, characterized in that the VFA concentrate stream (40a) is acidified so that the liquid-liquid extraction operation (50) occurs at a pH lower than 4. 8. - A method according to the preceding claim, characterized in that the liquid-liquid extraction operation (50) occurs at a pH between 2.5 and 3.5. 9. A method according to any one of the preceding claims, characterized in that the filtration operation (30) of step c) comprises subjecting the liquid fraction (23) of the first fermented product (12) to at least one ultrafiltration stage (31), obtaining a first stream containing rejected solids (31b) and a first clarified stream (31a); and subjecting said first clarified stream (31a) to a nanofiltration stage (32), obtaining a second stream containing rejected solids (32b) and a second clarified stream (32a). 10. - A method according to any one of the preceding claims, characterized in that the organic waste (1) being bio-waste, prior to step a) the method comprises a previous step of conditioning the bio-waste by grinding and hydration. 11. - A method according to any one of the preceding claims, characterized in that in the separation operation (20) of step b) a dose of coagulant and/or flocculant is added to the first fermentation (12). 12. A method according to the preceding claim, characterized in that the dose of coagulant and/or flocculant comprises cationic polymers. 13. - A method according to any one of the preceding claims, characterized in that in the liquid-liquid extraction operation (50) a light solvent partially soluble in water is used, obtaining - a reject stream (51b) rich in water and light solvent, and poor or lacking in VFAs, which is subjected to a recovery stage by distillation (52) to generate an aqueous stream and a stream rich in light solvent (55) that is reused in a subsequent liquid-liquid extraction operation; and - a stream rich in ethyl acetate and VFAs (51a) which is subjected to the stage-by-stage distillation operation (60). 14. A method according to the preceding claim, characterized in that the light solvent partially soluble in water is ethyl acetate. 15. A method according to any one of claims 1 to 12, characterized in that a heavy solvent not soluble in water is used in the liquid-liquid extraction operation (50). 16. - A method according to the preceding claim, characterized in that the heavy solvent not soluble in water is ethyl octanoate. 17. A method according to any one of the preceding claims, characterized in that the stepwise distillation operation (60) comprises a first distillation stage (61) from which a VFA mixture stream is obtained, lacking the solvent used for the liquid-liquid extraction operation (50); and in that the method comprises subjecting this VFA mixture stream to a distillation cascade (62, 63, 64) to separate different types of VFA, at least, acetic (101), propionic (102), butyric (103) and valeric (104). 18. - A system for the production and purification of volatile fatty acids (VFAs) from an organic waste source, comprising: a first reactor (11) for the fermentation of organic waste under anaerobic conditions to produce a first fermented product (12), separation means (21) for separating the first fermented product (12) into a liquid fraction (23) and a solid fraction (22), means of filtration (31, 32) of at least a part of the liquid fraction (23) and means of concentration (41) of the filtered liquid fraction, from which respective reject and/or clarified streams (31b, 32b, 40b) and a VFA concentrate stream (40a) are obtained, a liquid-liquid separator for recovering VFAs from the VFA concentrate stream (40a) followed by a staged distillation apparatus (62, 63, 64) for obtaining essentially pure VFAs (101, 102, 103, 104), the system further comprising a second reactor (71) for the fermentation under anaerobic conditions of at least part of the solid fraction (22) obtained from the first fermentation and at least one of the reject or clarified streams (31b, 32b, 40b) to produce biogas (82) and a second fermentation (72).