METHODS AND SYSTEMS FOR THE PRODUCTION OF LACTIC ACID AND THE RECYCLING OF POLYLACTIC ACID

Abstract

Industrial fermentation is provided for the production of lactic acid from organic waste combined with chemical recycling of polylactic acid, to obtain lactic acid with high yields.

Description

The present invention relates to the dual recycling of organic waste and polylactic acid. In particular, methods and systems are provided for the recycling of organic waste into lactic acid combined with the chemical recycling of polylactic acid (PLA). BACKGROUND OF THE INVENTION Lactic acid fermentation Lactic acid fermentation, the production of lactic acid from carbohydrate sources through microbial fermentation, has gained interest in recent years due to the potential of using lactic acid as a building block in bioplastics manufacturing. Lactic acid can be polymerized to form the biodegradable and recyclable polyester polylactic acid (PLA), which is considered a potential substitute for petroleum-based plastics. PLA is used in the manufacture of various products, including food packaging, disposables, fibers in the textile and hygiene industries, and more. Lactic acid production via fermentation bioprocesses is preferred over chemical synthesis methods for several reasons, including environmental concerns, cost, and the difficulty of generating enantiomerically pure lactic acid through chemical synthesis, which is desirable for most industrial applications. The conventional fermentation process typically relies on anaerobic fermentation by lactic acid-producing microorganisms, which produce lactic acid as the primary metabolic end product of carbohydrate fermentation. For PLA production, the lactic acid generated during fermentation is separated from the fermentation broth and purified through various processes. The purified lactic acid is then polymerized. Lactic acid has a chiral carbon atom and therefore exists in two enantiomeric forms, D- and L-lactic acid. To generate PLA suitable for industrial applications, the D- or L-lactic acid entering the production process must be highly purified to meet the required polymerization specifications. Furthermore, lactic acid bacteria that produce only the L-lactate enantiomer or only the D-lactate enantiomer are typically used to produce a discrete enantiomer (L or D). In currently available commercial processes, the carbohydrate source for lactic acid fermentation is typically a renewable source containing starch, such as corn and cassava root. Additional sources, such as cellulose-rich sugarcane bagasse, have also been proposed. Lactic acid bacteria can generally utilize reducing sugars like glucose and fructose, but they lack the ability to break down polysaccharides such as starch and cellulose. Therefore, to utilize these polysaccharides, the process requires the addition of glycolytic enzymes, usually in combination with a chemical treatment, to degrade the polysaccharides and release reducing sugars. An additional source of carbohydrates for lactic acid fermentation that has been proposed is complex organic waste, such as mixed food waste from municipal, industrial, and commercial sources. Organic waste is advantageous because it is readily available and less expensive compared to other carbohydrate sources for lactic acid fermentation. Mixed food waste typically includes varying proportions of reducing sugars (glucose, fructose, lactose, etc.), starch, and lignocellulosic material. Mixed food waste also contains endogenous D,L-lactic acid (e.g., from dairy products or natural decomposition during transport), one of which must be removed to use the waste as a substrate for producing optically pure lactic acid (L- or D-lactic acid). Document WO 2017 / 122197, assigned to the applicant of the present invention, describes genetically modified, dual-action lactic acid (LA)-utilizing bacteria that secrete polysaccharide-degrading enzymes such as cellulases, hemicellulases, and amylases, useful for processing organic waste to both remove lactic acid present in the waste and degrade complex polysaccharides. Organic waste, such as mixed food waste, is also characterized by significant variability in carbohydrate content, with its composition varying from batch to batch. Some batches may be high in carbohydrates compared to others that are low in carbohydrates. Therefore, lactic acid fermentation processes based on organic waste can result in batch-to-batch yields, and in some cases, very low lactic acid yields are obtained. Polylactic acid (PLA) recycling PLA produced from renewable resources is an alternative to petroleum-based plastics, and its use in manufacturing products such as food packaging is continuously growing. Due to the increasing presence of PLA in end-use products, it is important to ensure that PLA is properly treated after disposal. Unlike thermoplastic resins such as polyethylene, polypropylene, polystyrene, and poly(ethylene terephthalate), PLA is subject to thermal degradation. Consequently, when recycling products containing a mixture of PLA and the aforementioned plastics, it is advisable to separate the PLA to prevent contamination of recycling streams. Recycling options for PLA include landfilling, composting, anaerobic digestion (biogas production), incineration, and chemical recycling into its constituent monomers. Chemical recycling is preferred over other methods because the monomers can be reused in the production of new PLA. One of the common forms of PLA on the market is the acid copolymer PDLLA (poly(DL)lactic acid), predominantly composed of PLLA (made from L-lactic acid) and small amounts of PDLA (made from D-lactic acid). A significant portion of commercially available PLA plastics contain a small amount of PDLA which, upon hydrolysis, releases D-lactic acid. The hydrolyzed material may also contain unknown amounts of D-LA formed by racemization during hydrolysis. Optical purity greater than 99% is generally required for both D-lactic and L-lactic acid. ML / t / ZUZZ / U / 0 / 00 enter the PLA production process. Therefore, PLA recycling processes must address the problem of isomer separation. Chemical separation of the two enantiomers is expensive, generally using liquid or solid enantio-selective membranes or high-performance liquid chromatography (HPLC). Cam, Hyon, and Ikada (1995) Biomaterials, 16(11):833-43, report on the degradation of high molecular weight poly(L-lactide) in an alkaline medium. The study tested the effect of molecular weight and morphology on hydrolytic degradation. The degradation was carried out at 37°C in a 0.01 N NaOH solution. Siparsky, Voorhees and Miao (1998) Journal of Environmental Polymer Degradation, 6(1):31-41, report on the hydrolysis of polylactic acid (PLA) and polycaprolactone (PCL) in aqueous acetonitrile solutions. Xu, Crawford and Gorman (2011) Macromolecules, 44(12):4777-4782, report on the effects of temperature and pH on the degradation of poly(lactic acid) brushes. Chauliac (2013) “Development of a thermochemical process for hydrolysis of polylactic acid polymers to L-lactic acid and its purification using an engineered microbe”, PhD dissertation, University of Florida, UMI Number: 3583516, proposes a process for the post-consumer use of PLA polymers. In this process, thermohydrolysis is the first step, followed by the removal of D-LA from the hydrolyzed material to produce pure L-LA, which could be redirected to the production of the polymer itself. Thermohydrolysis was performed with water in the presence of NaOH. The removal of D-LA from the resulting syrup was achieved using an Escherichia coli lacking the three identified L-lactate dehydrogenases. Wadso and Karlsson (2013) Polymer Degradation and Stability, 98(1):73-78, report on two studies to measure the alkaline hydrolysis enthalpy of polymers containing carboxylic acid esters. Two materials were used: poly(vinyl acetate), PVAc, films and poly(lactic acid), PLA, fibers. Degradation was carried out using sodium hydroxide and potassium hydroxide at 30°C. Elsawy et al. (2017) Renewable and Sustainable Energy Reviews, 79:1346-1352, review the hydrolytic degradation of polylactic acid (PLA) and its compounds. Motoyama et al. (2007) Polymer Degradation and Stability, 92(7): 1350-1358, reports on the effects of the MgO catalyst on the depolymerization of poly-L-lactic acid to L,L-lactide. WO 2015 / 112098 describes a process for manufacturing lactide from plastics containing polylactic acid (PLA-based plastics). The process comprises preparing PLA-based plastics, accelerating the decomposition of polylactic acid in the plastics by alcoholysis or hydrolysis to yield low molecular weight polylactic acid, and thermally decomposing the low molecular weight polylactic acid to yield lactide. The process further comprises minimizing the size of the PLA-based plastics after the preparation step and purifying the lactide after the thermal decomposition of the low molecular weight polylactic acid. US patent 7,985,778 describes a method for decomposing and recovering synthetic resin having an ester linkage in its compositional structure by performing a hydrolysis treatment followed by a collection treatment by separation. In the hydrolysis treatment, an article containing the synthetic resin to be decomposed and recovered is exposed to a steam-filled atmosphere. ML / t / ZUZZ / U ÍOfOO under saturation steam pressure at the treatment temperature at or below the melting point of the synthetic resin. The synthetic resin of the article to be treated is hydrolyzed with steam generated at the treatment temperature to produce a decomposition product before polymerizing into a synthetic resin containing an ester linkage. The collection by separation treatment is a treatment where the decomposition product generated by the hydrolysis treatment is separated into a liquid component and a solid component for individual collection. US patent 8,614,338 describes a method for the stereospecific chemical recycling of a polylactic acid (PLA)-based polymer blend to reform the PLA monomer or one of its derivatives. The method comprises a step of suspending the polymer blend in a lactic acid ester capable of dissolving the PLA fraction, followed by the separation of the lactic acid ester, PLA, and other dissolved impurities, and then the insoluble mixture of other polymers and impurities. The resulting PLA-containing solution is then subjected to a catalytic transesterification depolymerization reaction to form oligoesters. The transesterification depolymerization reaction is then stopped at a specific point, and the residual lactic acid ester is separated.The oligoester thus obtained is then subjected to a cyclization reaction to produce lactide, which will finally be stereospecifically purified to obtain a purified lactide fraction having a mesolactide content of between 0.1% and 40%. US documents 8,431,683 and US 8,481,675 describe a process for recycling a polymer mixture that necessarily contains PLA, comprising grinding, compacting, dissolving in a PLA solvent, removing undissolved contaminant polymers, alcoholysis depolymerization reaction, and purification steps. US patent 8,895,778 describes the depolymerization of polyesters such as post-consumer polylactic acid. Ultrasound-induced implosions can be used to facilitate depolymerization. Post-consumer PLA was exposed to methanol as a suspension medium in the presence of organic or ionic salts of alkali metals, such as potassium carbonate and sodium hydroxide, as depolymerization catalysts to provide high-quality lactic acid monomers with high yield. US patent 2018 / 0051156 describes a method for improving / accelerating the depolymerization of polymers (e.g., those containing hydrolyzable linkages). The method generally involves contacting a polymer comprising hydrolyzable linkages with a solvent and an alcohol to form a polymer mixture in which the polymer is substantially dissolved. This contact is carried out at a temperature equal to or lower than the boiling point of the polymer mixture. A resulting depolymerized polymer can be separated (including, for example, monomers and / or oligomers). Such methods can be carried out under relatively mild temperature and pressure conditions. In some embodiments, the polymer is poly(lactic acid). There remains a need for cost-effective chemical recycling of PLA and successful integration of hydrolyzed PLA into existing LA / PLA production processes. ML / t / ZUZZ / U ZO / OO There is also still a need to improve the yield of lactic acid production, particularly from organic waste. BRIEF DESCRIPTION OF THE INVENTION The present invention provides methods and systems for the highly efficient production of an enantiomerically pure lactate salt, particularly an L-lactate salt, through the combined recycling of organic waste and PLA waste. More particularly, the present invention integrates the production of L-lactate monomers by lactic acid fermentation of organic waste with the chemical hydrolysis of PLA to its constituent monomers (L- and optionally D-lactate monomers). As described herein, the L-lactate monomers produced by fermentation and the lactate monomers produced by chemical hydrolysis of PLA are combined and purified together in a single downstream purification and recovery process to obtain a pure L-lactate salt. In some embodiments, the lactate monomers resulting from the hydrolysis of PLA are combined with L-lactate monomers produced by fermentation after the fermentation is complete.Alternatively, the lactate monomers produced by PLA hydrolysis are supplemented in a lactic acid production reactor where L-lactate monomers are produced by fermenting organic waste. Subsequently, the supplemental lactate monomers and the newly produced L-lactate monomers undergo a single purification process to obtain a pure L-lactate salt. The purified L-lactate salt can then be acidified to L-lactic acid and used in the production of new PLA. As described herein, lactic acid fermentation is carried out in the presence of an alkaline compound that adjusts the pH during fermentation, resulting in a fermentation broth comprising L-lactate monomers and a counterion. PLA hydrolysis is performed using a metal oxide or hydroxide, resulting in a hydrolysis suspension comprising lactate monomers (L- and optionally D-) and a counterion. The alkaline compound used during fermentation and the metal oxide or hydroxide used for PLA hydrolysis according to the present invention produce L-lactate monomers and a counterion, which may be the same or different, each possibility representing a separate embodiment. When the same counterion is used, the L-lactate monomers and counterions can be combined and purified together to obtain a pure lactate salt.When a different counterion is used, at least one of the counterions can be exchanged, resulting in the same counterion, which, along with L-lactate monomers, can then undergo combined purification. In some embodiments, the alkaline compound used during fermentation and the metal oxide or hydroxide used for PLA hydrolysis are the same compound; that is, the same compound is used both for PLA hydrolysis and for pH adjustment in the heater. For example, magnesium hydroxide can be used as the hydroxide for PLA hydrolysis and as the alkaline compound to adjust the pH during fermentation, resulting in lactate monomers and magnesium ions in both the hydrolysis slurry and the fermentation broth, which can be recovered as magnesium lactate. In other embodiments, the compounds are different but produce the same counterion.For example, magnesium hydroxide can be used as a hydroxide for the hydrolysis of PLA, and carbonate of. Magnesium hydroxide can be used as an alkaline compound to adjust the pH during fermentation, resulting in lactate monomers and magnesium ions in both the hydrolysis slurry and the fermentation broth, which can be recovered as magnesium lactate. In still other embodiments, the compounds are different, but one of the counterions is exchanged to produce the same counterion as the other for subsequent purification. For example, sodium hydroxide can be used as the hydroxide for PLA hydrolysis, and magnesium hydroxide can be used as an alkaline compound to adjust the pH during fermentation. The sodium ions in the hydrolysis slurry can then be exchanged with magnesium ions, resulting in lactate monomers and magnesium ions in both the hydrolysis slurry and the fermentation broth, which can be recovered as magnesium lactate salt.In additional modalities, both counterions are exchanged to produce the same counterion for subsequent purification. The present invention advantageously integrates the products of two processes, namely, i) hydrolyzed PLA lactate and i) lactate produced through the fermentation of organic waste, into a single downstream purification process to recover an L-lactate salt, thereby saving in both capital expenditure (CAPEX) and operating expenditure (OPEX). Furthermore, the present invention improves the yield of L-lactic acid production from organic waste. Organic waste, such as mixed food waste, is characterized by significant variability in carbohydrate content, and its composition varies from batch to batch, with some batches being high in carbohydrates compared to others that have a low carbohydrate content. Therefore, lactic acid fermentation processes based on organic waste can result in varying yields from batch to batch, and in some cases, very low lactic acid yields are obtained. The integration of hydrolyzed PLA lactate and lactate produced by organic waste fermentation increases the amount of lactic acid obtained per fermentation cycle, thereby improving the yield and facilitating the reproducibility of lactic acid fermentation processes. The systems and methods of the present invention therefore provide cost-effective recycling of PLA and organic waste and improve the performance of L-lactic acid production. According to the first aspect, a method is provided for producing an L-lactate salt from a combined recycling of polylactic acid (PLA) and organic waste; the method comprises the following steps: (a) hydrolyzing PLA waste with a metal oxide or hydroxide to obtain a PLA hydrolysis suspension comprising L-lactate monomers and a first counterion; (b) fermenting organic waste with a lactic acid-producing microorganism in a heater in the presence of an alkaline compound to obtain a fermentation broth comprising L-lactate monomers and a second counterion, wherein the first and second counterions are the same; or wherein at least one of the first and second counterions undergoes ion exchange, thereby obtaining a first and second counterion that are identical; MA / IZ / ZUZZ / U lOfOS (c) optionally, contacting the PLA hydrolysis suspension from step (a), the fermentation broth from step (b), or a mixture comprising the PLA hydrolysis suspension from step (a) and the fermentation broth from step (b) with a lactic acid-degrading D-enzyme or a microorganism that uses D-lactic acid to remove D-lactate monomers, thereby obtaining L-lactate monomers; and (d) purifying a mixture comprising the PLA hydrolysis suspension from step (a) and the fermentation broth from step (b) or the L-lactate monomers from step (c), thereby obtaining an L-lactate salt. In one modality, step (a) and step (b) are performed in any order or simultaneously, each possibility representing a separate modality. In some embodiments, the PLA hydrolysis slurry is gradually added to the lactic acid heater of step (b) during fermentation. According to these embodiments, the mixture comprising the PLA hydrolysis slurry from step (a) and the fermentation broth from step (b) is obtained by gradually adding the PLA hydrolysis slurry to the lactic acid heater during fermentation. In some embodiments where the PLA hydrolysis slurry is added to the lactic acid fermenter of step (b), step (a) may comprise the metal oxide or hydroxide in excess, thereby eliminating the need to add the alkaline compound to the lactic acid fermenter in step (b) for pH adjustment during fermentation. Advantageously, integrating L-lactate monomers produced by lactic acid fermentation with L-lactate monomers resulting from PLA hydrolysis in a single downstream purification process to obtain a purified L-lactate salt increases the overall L-lactate production yield. This is particularly beneficial for low-carbohydrate organic wastes that cannot achieve high lactic acid titers. In one embodiment, the overall L-lactate production yield increases by 10% or more. In another embodiment, the overall L-lactate production yield increases by 50% or more. In yet another embodiment, the overall L-lactate production yield increases by 100% or more. In certain formulations, the lactate salt is magnesium L-lactate. In particular formulations, the lactate salt is crystalline magnesium L-lactate. In specific formulations, the lactate salt is crystalline magnesium L-lactate dihydrate. In some forms, the metal oxide or hydroxide from step (a) and the alkali compound from step (b) are the same compound. In various forms, the alkali compound of step (b) is a metallic oxide, carbonate, or hydroxide. Each possibility represents a separate form. In certain embodiments, the metal oxide comprises at least one of MgO, CaO, and a mixture or combination thereof. Each possibility represents a separate embodiment. In other embodiments, the carbonate comprises at least one of CaCCL, MgCCL, and a mixture or combination thereof. Each possibility represents a separate embodiment. ML / t / ZUZZ / U ISfOO In certain embodiments, the hydroxide comprises at least one of NaOH, KOH, NH4OH, Ca(OH)2, Mg(OH)2 and a mixture or combination thereof. Each possibility represents a separate embodiment. In one particular embodiment, the hydroxide in step (a) is NaOH and the alkali compound in step (b) is Mg(OH)2. In another particular embodiment, the hydroxide in step (a) and the alkali compound in step (b) are both Mg(OH)2. In several embodiments, the hydrolysis in step (a) is carried out at elevated temperatures in the range of approximately 50°C to approximately 90°C, for example, temperatures in the range of approximately 60°C to approximately 90°C, approximately 70°C to approximately 90°C, approximately 50°C to approximately 80°C, and approximately 50°C to approximately 75°C, inclusive of each value within the specified ranges. Each possibility represents a separate embodiment. In additional modalities, the hydrolysis in step (a) is carried out over a period of time in the range of approximately 1 to approximately 12 hours, inclusive of any value within the specified range. In other modalities, the hydrolysis in step (a) is carried out over a period of time in the range of approximately 12 to approximately 36 hours, inclusive of any value within the specified range. In additional embodiments, step (a) comprises hydrolyzing PLA waste with Mg(OH)2 at a concentration of between approximately 5 and approximately 15% by weight to obtain a PLA hydrolysis suspension comprising L-lactate monomers and a magnesium ion. In other embodiments, hydrolysis in step (a) results in L-lactate monomers and a first counterion in crystalline form. In still other embodiments, fermentation in step (b) results in L-lactate monomers and a second counterion in crystalline form. In additional embodiments, the method for producing an L-lactate salt further comprises the pretreatment of PLA waste before step (a). In specific embodiments, the pretreatment comprises a mechanical pretreatment selected from the group consisting of grinding, chipping, crushing, milling, and a combination thereof. Each of these represents a separate embodiment. In other specific embodiments, the pretreatment comprises extrusion pretreatment. In additional embodiments, the method for producing an L-lactate salt further comprises subjecting the PLA hydrolysis slurry obtained in step (a) to a solid-liquid separation. The solid-liquid separation is intended to remove unhydrolyzed PLA residues or impurities such as other polymers, inert materials, and / or food residues from the slurry. The PLA waste according to the present invention may include impurities and contaminants other than PLA. In some embodiments, the PLA waste is sorted prior to step (a) to increase the amount of PLA in the waste relative to the non-PLA impurities and contaminants. Advantageously, the recycling of PLA according to the present invention is insensitive to the impurities and contaminants present in the PLA waste, including contaminants that cannot be successfully sorted. As described herein, the PLA waste following alkaline hydrolysis is integrated into a downstream purification process of a lactic acid fermentation broth. The downstream purification process simultaneously removes contaminants originating from both the organic waste used as a substrate for fermentation and the PLA waste. In certain forms, PLA residues comprise poly L-lactic acid (PLLA) and poly D-lactic acid (PDLA). In some forms, the organic waste comprises endogenous D-lactic acid, L-lactic acid, or a combination of both. Each possibility represents a separate form. In certain embodiments, where D-lactic acid is formed and / or present, the PLA hydrolysis suspension from step (a), the fermentation broth from step (b), or a mixture comprising the PLA hydrolysis suspension from step (a) and the fermentation broth from step (b) is contacted with an enzyme that degrades D-lactic acid or a microorganism that uses D-lactic acid to remove D-lactate monomers, thereby yielding L-lactate monomers. In particular embodiments, the D-lactic acid-degrading enzyme in step (c) is a D-lactate oxidase. In some categories, organic waste is selected from the group consisting of food waste, municipal waste, agricultural waste, plant material, and a mixture or combination thereof. Each possibility represents a separate category. In other methods, the L-lactate salt obtained is purified by at least one of the following: crystallization, recrystallization, distillation, partitioning, silica gel chromatography, preparative HPLC, and combinations thereof. Each possibility represents a separate method. In additional embodiments, the L-lactate salt obtained is acidified to form L-lactic acid using at least one of hydrochloric acid, hydrobromic acid, phosphoric acid, sulfuric acid, or combinations thereof. Each possibility represents a separate embodiment. In particular embodiments, the L-lactic acid is used for the further formation of polylactic acid. According to a second aspect, a method is provided for producing magnesium L-lactate salt by recycling polylactic acid (PLA) waste; the method comprises the following steps: (a) hydrolyzing PLA waste with a selected base of sodium, potassium and ammonium hydroxide to obtain a PLA hydrolysis suspension comprising L-lactate monomers and a selected counterion of sodium, potassium and ammonium; (b) optionally perform at least one of neutralizing the PLA hydrolysis slurry with an acid and removing the unhydrolyzed PLA residues; and (c) add a magnesium salt to the PLA hydrolysis slurry from step (a) or (b) to precipitate the magnesium L-lactate salt. In some forms, the base is sodium hydroxide. In other embodiments, the hydrolysis in step (a) is carried out at elevated temperatures in the range of approximately 50°C to approximately 90°C, for example, temperatures in the range of approximately 60°C to approximately 90°C, approximately 70°C to approximately 90°C, approximately 50°C to approximately 80°C, and approximately 50°C to approximately 75°C, inclusive of each value within the specified ranges. Each possibility represents a separate embodiment. In still other embodiments, the hydrolysis in step (a) is carried out over a period of time in the range of approximately 1 to approximately 24 hours, inclusive of each value within the specified range. In other embodiments, the hydrolysis in step (a) is carried out over a period of time in the range of approximately 1 to approximately 12 hours, inclusive of each value within the specified range. In certain configurations, the base exceeds the PLA residues. In alternative configurations, the PLA residues exceed the base. In additional embodiments, step (b) is performed and the acid is selected from hydrochloric acid, hydrobromic acid, phosphoric acid, sulfuric acid, and combinations thereof. Each possibility represents a separate embodiment. In one embodiment, step (b) is performed and the acid is sulfuric acid. In various modalities, step (b) is performed and the disposal of unhydrolyzed PLA waste includes solid-liquid separation. In additional embodiments, the magnesium salt in step (c) is added in solid form. In alternative embodiments, the magnesium salt in step (c) is added as an aqueous solution. In further embodiments, the magnesium salt in step (c) is added gradually. In particular embodiments, the magnesium salt in step (c) is magnesium sulfate. In additional embodiments, the magnesium L-lactate salt obtained is further purified. In other embodiments, the magnesium L-lactate salt is combined with magnesium L-lactate salt derived from the fermentation of organic waste followed by further purification. Other objects, features, and advantages of the present invention will become clear from the following description, examples, and drawings. BRIEF DESCRIPTION OF THE FIGURES Figure 1. Schematic illustration of a combined production of lactic acid and recycling of PLA according to certain embodiments of the present invention. Figure 2. Schematic illustration of a combined production of lactic acid and recycling of PLA according to further embodiments of the present invention, wherein Mg(OH)2 is used as an alkaline hydrolysis agent of PLA and as an alkaline pH adjustment compound in the lactic acid fermentation. Figure 3. Schematic illustration of a combined production of lactic acid and recycling of PLA according to further embodiments of the present invention, wherein NH4OH is used as an alkaline hydrolysis agent of PLA and as an alkaline pH-adjusting compound in lactic acid fermentation, and wherein ion exchange is carried out to obtain Mg(LA)2 that can be recovered and purified. Figure 4. Alkaline thermohydrolysis of thin-film PLA bags using magnesium hydroxide. Lactate yield versus wt% of Mg(OH)2. MA / IZ / ZUZZ / U ÍSfOO DETAILED DESCRIPTION OF THE INVENTION The present invention provides methods and systems for combined lactic acid fermentation and polylactic acid (PLA) recycling, to obtain enantiomerically pure L-lactate salts with high yields. The systems and methods provided readily induce the degradation of PLA waste into its constituent monomers, lactic acid (LA), and efficiently recycle the LA monomers in PLA production processes. Although PLA is considered a biodegradable bioplastic, its hydrolysis rate is relatively low in aqueous or alcoholic solutions. Furthermore, PLA degradation occurring in the "open environment" can be considered waste, as the resulting lactic acid is not reused. The present invention not only provides accelerated degradation of PLA waste but also enables sustainable and cost-effective PLA recycling, since the LA monomers obtained from hydrolysis are provided in the form of a lactate salt, which is then combined and integrated into a downstream purification process of lactic acid fermentation broths derived from organic waste. As used here, the term “lactic acid” refers to the hydroxycarboxylic acid with the chemical formula CH3CH(OH)CO2H. The terms lactic acid or lactate (unprotonated lactic acid) may refer to stereoisomers of lactic acid: L-lactic acid / L-lactate, D-lactic acid / D-lactate, or a combination thereof. For most industrial applications, high-purity L-lactic acid monomers are required to produce PLA with suitable properties. Therefore, the methods and systems of the present invention are directed, in particular, to processes for the production of L-lactate salts with high yields, which can then be converted into L-lactic acid suitable for reuse. In particular, the present invention provides for the combined recycling of PLA waste and organic waste. According to the principles set forth herein, PLA recycling is carried out by hydrolysis of PLA waste, and organic waste recycling is carried out by lactic acid fermentation, where both processes result in lactate monomers and the same counterion. It should be understood that when the resulting counterions are different, at least one of the hydrolysis or fermentation products may be subjected to an ion-exchange processing step to obtain the same counterion. The products obtained from PLA hydrolysis and lactic acid fermentation are combined and processed together to obtain a pure lactate salt, preferably a high-yield L-lactate salt. The L-lactate salt can then be purified to obtain a highly purified L-lactate salt with an improved yield.In some forms, the recovered L-lactate salt can be converted into lactic acid and used for the production of new PLA. Before explaining in detail at least one embodiment of the invention, it should be understood that the application of the invention is not limited to the details set forth in the following description or exemplified by the Examples. The invention is susceptible to other embodiments or can be implemented or carried out in various ways. Furthermore, it should be understood that the phraseology and terminology used herein are for descriptive purposes and should not be considered limiting. ML / t / ZUZZ / U / 0 / 00 With reference to the drawings, Figure 1 provides an overview of a combined L-lactic acid production and PLA recycling process according to embodiments of the present invention. Organic waste, such as municipal waste, food waste, and agricultural waste, serves as a substrate for L-lactic acid fermentation by L-lactic acid-producing microorganisms. The organic waste undergoes a biological process that results in the production of L-lactic acid. This biological process includes the enzymatic digestion of the waste using polysaccharide-degrading enzymes (e.g., amylase, cellulase) to break down the polysaccharides present in the waste and release soluble reducing sugars suitable for fermentation. The biological process further includes the fermentation of L-lactic acid by an L-lactic acid-producing microorganism.Due to the formation of L-lactic acid, an endogenous decrease in pH occurs. Thus, the fermentation process is carried out in the presence of an alkaline compound to adjust the pH during fermentation. The alkaline compound neutralizes the pH, resulting in the formation of a counterion, represented in Figure 1 as (X2+) for illustrative purposes. It should be understood that the alkaline compound used may comprise a monovalent cation, resulting in a monovalent counterion in the fermentation broth. Enzymatic digestion and lactic acid fermentation can be performed simultaneously. Alternatively, enzymatic digestion can be carried out before lactic acid fermentation, either in the same reactor where the fermentation takes place or in a different reactor. Each possibility represents a separate modality. The biological process and / or PLA hydrolysis may also include the removal of D-lactate.D-Lactate removal can be performed at the end of fermentation to remove D-lactate present in organic waste, at the end of PLA hydrolysis to remove D-lactate present in PLA waste or generated during hydrolysis, or after mixing the fermentation broth with the PLA hydrolysis slurry. Each option represents a separate method. D-Lactate removal can be performed in the same reactors used for fermentation and hydrolysis or in a different reactor. Each option represents a separate method. PLA waste according to the present invention includes any discarded PLA product, for example, PLA discarded from municipal solid waste (MSW) or industrial / commercial rejects / waste remaining from the production of PLA products. For example, PLA waste products may be obtained from the food industry, medical device industry, automotive industry, furniture industry, and aviation industry. Each possibility represents a separate embodiment. The PLA waste is decomposed by alkaline hydrolysis. A metal oxide or hydroxide, represented in Figure 1 as (X(OH)2) for illustrative purposes, is mixed with PLA waste. It is understood that the metal oxide or hydroxide used may comprise a monovalent cation resulting in a monovalent counterion in the hydrolysis slurry along with L-lactate monomers.When PLA waste includes PLLA and PDLA, the hydrolysis slurry may also contain D-lactate monomers. In some embodiments, the hydrolysis slurry is mixed with the lactic acid fermentation broth after fermentation is complete. In other embodiments, the hydrolysis slurry comprises an excess of the metal oxide or hydroxide, thus creating an alkaline pH, and is gradually added to the lactic acid fermentation process. The alkaline slurry. ML / E / ZUZZ / U / 0 / 00 neutralizes the pH of the fermentation broth, thus eliminating the need to add a separate pH adjusting agent. A mixture of the PLA hydrolysis slurry and the fermentation broth is then processed to obtain purified L-LA salt. The L-LA salt can then be re-acidified and polymerized to form PLA, which is useful in the production of PLA products, thereby completing a PLA decomposition and synthesis cycle. Figure 2 illustrates a combined process according to some embodiments of the present invention, wherein Mg(OH)₂ is used as an alkaline hydrolysis agent for PLA and as an alkaline pH-adjusting compound in lactic acid fermentation. PLA waste is chemically degraded using Mg(OH)₂ to obtain Mg(LA)₂. The organic waste is fermented by an L-lactic acid-producing microorganism in the presence of Mg(OH)₂ to neutralize the pH, resulting in Mg(L-LA)₂. The Mg(LA)₂ obtained from PLA hydrolysis and the Mg(LA)₂ produced by fermentation are combined and acidified to obtain L-lactic acid (LAH) and Mg(OH)₂. The LAH can be used in the synthesis of new PLA. Mg(OH)2 can be reused for pH adjustment and / or PLA hydrolysis in other lactic acid fermentation and PLA hydrolysis processes. Figure 3 illustrates a combined process according to further embodiments of the present invention, wherein NH4OH is used as an alkaline hydrolysis agent for PLA and as an alkaline pH-adjusting compound in lactic acid fermentation, and wherein ion exchange is carried out to obtain Mg(LA)2 that can be recovered and purified. PLA waste is chemically degraded using NH4OH to obtain NH4LA. Organic waste is fermented by an L-lactic acid-producing microorganism in the presence of NH4OH to neutralize the pH, resulting in NH4(LLA). The NH4LA obtained from PLA hydrolysis and the NH4LA produced by fermentation are combined and subjected to ion exchange with Mg(OH)2 to obtain Mg(LA)2 and NH4OH. The Mg(LA)2 can be recovered, purified, and acidified to obtain LAH, which can be used in the synthesis of new PLA.The NH4OH can be reused for pH adjustment and / or PLA hydrolysis in other lactic acid fermentation and PLA hydrolysis processes. Alternatively, the NH4OH can be disposed of by evaporating the ammonia gas. According to certain exemplary embodiments of the present invention, a process is carried out using NaOH as the alkaline hydrolysis agent for PLA and Mg(OH)₂ as the alkaline compound that adjusts the pH in lactic acid fermentation. Ion exchange is performed in the hydrolysis slurry to obtain Mg(LA)₂, which is then combined with the Mg(LA)₂ from the fermentation broth. The Mg(LA)₂ can be recovered, purified, and acidified to obtain LAH for the subsequent formation of PLA. According to the principles of the present invention, the counterion present in the hydrolysis of PLA (first counterion) and / or the counterion present in the fermentation broth (second counterion) are the same counterion or are exchanged for the same counterion. Even in cases where the same counterion is produced during hydrolysis and fermentation, the present invention encompasses embodiments in which the first and second counterions are exchanged to yield a desirable lactate salt. The ion exchange can be carried out as known in the art, for example, using cation exchange resins. Cation exchange resins are negatively charged polymers that can freely exchange associated cations based on differences in their selectivities. ML / / 3103 cations. Suitable cation exchange resins within the scope of the present invention include, but are not limited to, those obtained from commercial sources such as DOWEX™ cation exchange resins. In other embodiments, ion exchange can be carried out by neutralizing the lactate salt using a suitable acid, including, but not limited to, hydrochloric acid, hydrobromic acid, phosphoric acid, sulfuric acid, and combinations thereof, followed by exposure of the resulting lactic acid to a base or salt comprising the desired counterion. Suitable bases include, but are not limited to, sodium, potassium, magnesium, and calcium hydroxides. Each possibility represents a separate embodiment. Currently, the use of magnesium salts selected from MgCl2, MgCCh, MgSO4, Mg3(PO4)2, and the like is preferred to yield magnesium lactate. Each possibility represents a separate embodiment.Other embodiments covered by the present invention include the use of PLA in excess of the metal oxide or hydroxide. According to these embodiments, after the decomposition of the PLA, the excess PLA is separated from the hydrolysis slurry, which is then subjected to ion exchange using a salt as described earlier herein. Advantageously, when PLA is used in excess, neutralization of the lactate salt by an acid is not required after hydrolysis. PLA waste, according to the principles of the present invention, can be sorted prior to use in the method of the present invention to increase the relative amount of PLA in the waste compared to non-PLA materials. Generally, there are three types of PLA polymers: homopolymers, copolymers, and stereocomplexes. Homopolymers are composed of either 100% L-lactic acid (PLLA) or 100% D-lactic acid (PDLA). The common commercial copolymer of PLA is PDLA (poly(DL-)lactic acid), predominantly composed of L-lactic acid, with small amounts of D-lactic acid. Stereocomplex PLA (sc-PLA) is also available, consisting of PLLA and PDLA chains, where the PLLA and PDLA chains are packed together to form a superstructure with improved thermal and mechanical properties compared to PLLA. Therefore, most commercial PLAs include L-lactic acid as a major component, but also D-lactic acid, and thus both isoforms will be present in the hydrolysis suspension after the chemical hydrolysis of PLA. Furthermore, racemization during hydrolysis can contribute unknown and uncontrolled amounts of D-lactic acid. Without being tied to any specific theory or mechanism of action, PLA hydrolysis typically occurs through surface erosion, where ester groups are primarily hydrolyzed on the surface of the waste PLA, proceeding through one or more erosion fronts into the bulk. Surface hydrolysis typically occurs when the kinetics of bond hydrolysis are faster than water diffusion. Consequently, increasing the surface area is desirable, thereby accelerating the hydrolysis process. Suitable methods for increasing the surface area include mechanical pretreatment such as, but not limited to, shredding, chipping, crumbling, and grinding. Each of these represents a separate modality. In additional modalities, the surface area can be increased using enzymatic hydrolysis as a pretreatment prior to chemical hydrolysis.In certain aspects and configurations, the surface area of ​​discarded PLA can be increased using an extrusion pretreatment, optionally accompanied by alkaline hydrolysis. The extrusion pretreatment utilizes a combination of heat, compressive forces, and shear forces that lead to physical alteration and chemical modifications of the material passing through the extruder. Alkaline hydrolysis can be combined with extrusion to improve process efficiency. Extruders that can be used include, but are not limited to, a single-screw extruder, a twin-screw extruder (including co-rotating, counter-rotating, geared, and non-geared extruders), a multi-screw extruder, a piston extruder (which uses a heated cylinder and piston to extrude the feed material), a gear pump extruder (which uses a heated gear pump), and a conveyor extruder.Each possibility represents a separate modality. It should be understood that the aforementioned pretreatments (e.g., extrusion) may also be used as part of the process of the present invention, sequentially following one or more process steps, simultaneously with one or more process steps, or a combination thereof. Each possibility represents a separate embodiment. Embodiments in which the entire process is accompanied by one or more of the aforementioned pretreatments are also contemplated. In some aspects and embodiments, the pretreated PLA is fed into an alkaline suspension comprising a metal oxide or a hydroxide to induce alkaline hydrolysis. Suitable metal oxides within the scope of the present invention include, but are not limited to, MgO, CaO, and combinations thereof. Each possibility represents a separate embodiment. Suitable hydroxides for use within the scope of the present invention include, but are not limited to, NaOH, KOH, NH4OH, Ca(OH)2, Mg(OH)2, and mixtures or combinations thereof. Each possibility represents a separate embodiment. Currently, the use of magnesium hydroxide, resulting in magnesium lactate, is preferred. When magnesium hydroxide is used for hydrolysis, it is typically present at a concentration of approximately 2 to approximately 15% by weight, inclusive, within the specified range.Additional preferred methods include the use of sodium hydroxide for hydrolysis, resulting in sodium lactate, which can then undergo ion exchange using, for example, MgCl₂, MgCO₃, MgSO₄, Mg₃(PO₄)₂, Mg(OH)₂, and similar compounds to yield magnesium lactate. When sodium hydroxide is used for hydrolysis, it is typically present at a concentration of 1 N to approximately 10 N, inclusive of all values ​​within the specified range. Optionally, at least one suitable additive may also be used in conjunction with the metal oxide or hydroxide to accelerate PLA hydrolysis. Additives that may be used to accelerate PLA hydrolysis include, but are not limited to, a phase-transfer catalyst, such as a quaternary ammonium salt selected from benzalkonium chloride, benzyltriethylammonium chloride, methyltricaprylammonium chloride, methyltributylammonium chloride, and methyltrioctylammonium chloride, where each possibility represents a separate modality; or a quaternary phosphonium salt selected from tetrabutylphosphonium bromide, tetraphenylphosphonium chloride, tetraphenylphosphonium bromide, and hexadecyltributylphosphonium bromide, each possibility representing a separate modality. Additional additives that may be used to accelerate PLA hydrolysis include enzymatic catalysts, for example, lipases.The hydrolysis of PLA waste may also involve the use of thermohydrolysis instead of, or in addition to, the alkaline chemical hydrolysis detailed above. Typically, thermohydrolysis is performed at elevated temperatures in the range of approximately 50°C to approximately 90°C, inclusive. Other possible temperatures include approximately 60°C to approximately 90°C, approximately 70°C to approximately 90°C, approximately 50°C to approximately 80°C, and approximately 50°C to approximately 75°C, inclusive. Each of these represents a separate process. The duration of thermohydrolysis can range from approximately 1 to approximately 36 hours, inclusive.Example durations include, but are not limited to, approximately 1 to approximately 12 hours, approximately 12 hours to approximately 36 hours, and any duration in between. Typically, thermohydrolysis is performed for approximately 1 hour, approximately 2 hours, approximately 4 hours, approximately 6 hours, approximately 10 hours, approximately 12 hours, approximately 14 hours, approximately 16 hours, approximately 18 hours, approximately 20 hours, approximately 22 hours, or approximately 24 hours. Each possibility represents a separate modality. If any PLA residue particles remain unhydrolyzed, they can be separated from the hydrolysis slurry, for example, using solid-liquid separation techniques such as filtration or decantation. Each of these methods represents a separate modality. In some modality, lactate monomers in the form of lactate salts crystallize at the end of the hydrolysis step. For example, a PLA hydrolysis slurry can be evaporated and / or cooled to obtain lactate salt crystals. These crystals can then be collected and integrated into the lactic acid production process as described herein. In accordance with the principles of the present invention, a method for producing magnesium L-lactate salt by recycling polylactic acid (PLA) waste is further provided herein. The method advantageously provides chemical hydrolysis of PLA at lower temperatures and shorter durations (e.g., less than 12 hours, less than 10 hours, or even less than 5 hours) and also allows for the decomposition of denser and more compact PLA waste. The PLA waste (which may be pretreated as described herein) is hydrolyzed using a base selected from sodium, potassium, and ammonium hydroxide to obtain a PLA hydrolysis slurry comprising L-lactate monomers and a counterion selected from sodium, potassium, and ammonium. Each possibility represents a separate embodiment.Currently, sodium hydroxide is preferred as the base, yielding a PLA hydrolysis suspension comprising L-lactate and sodium ions. The hydrolysis is typically performed at elevated temperatures in the range of approximately 50°C to approximately 90°C for a duration of approximately 1 to approximately 36 hours, preferably between approximately 1 and approximately 24 hours, inclusive. Exemplary temperature ranges include, but are not limited to, approximately 60°C to approximately 90°C, approximately 70°C to approximately 90°C, approximately 50°C to approximately 80°C, and approximately 50°C to approximately 75°C, inclusive. Each range represents a separate modality. Exemplary hydrolysis durations include, but are not limited to, approximately 1 to approximately 5 hours. ML / t / ZUZZ / U fOfOO approximately 1 to approximately 10 hours, approximately 1 to approximately 12 hours, approximately 1 to approximately 24 hours, approximately 12 to approximately 24 hours, and approximately 12 to approximately 36 hours, inclusive of each value within the specified ranges. In some embodiments, the base exceeds the PLA residues, resulting in a PLA suspension in a pH range of approximately 10 to approximately 14, inclusive of each value within the specified range. In alternative embodiments, the PLA residues exceed the base, resulting in a PLA suspension in a pH range of approximately 7 to approximately 10, inclusive of each value within the specified range. The hydrolysis suspension may then be neutralized using an acid such as, but not limited to, hydrochloric acid, hydrobromic acid, phosphoric acid, sulfuric acid, and combinations thereof.Each possibility represents a separate modality. Additionally or alternatively, the hydrolysis slurry can be subjected to the removal of unhydrolyzed PLA, for example, by solid-liquid separation techniques using, for example, filtration or decantation. Each possibility represents a separate modality. Magnesium salt is then added to induce the precipitation of magnesium L-lactate. The magnesium salt can be added in solid form or as an aqueous solution. Each possibility represents a separate embodiment. Currently, the addition of magnesium salt as an aqueous solution at a concentration ranging from approximately 50 to approximately 500 g / L, inclusive, is preferred. In some embodiments, the aqueous magnesium salt solution is added gradually to the PLA suspension while mixing. Magnesium salts within the scope of the present invention include, but are not limited to, MgCl₂, MgCO₃, MgSCO₄, Mg₃(PO₄)₂, Mg(OH)₂, and the like. Each possibility represents a separate embodiment. Currently, the addition of magnesium sulfate (e.g., magnesium sulfate heptahydrate) is preferred.The magnesium L-lactate salt thus obtained can also be subjected to downstream purification processes with or without magnesium L-lactate salt from the fermentation of a lactic acid. The carbon source for lactic acid fermentation according to the present invention is derived from organic waste. Organic waste suitable for use according to certain embodiments of the present invention includes, but is not limited to, food waste, the organic fraction of municipal solid waste, agricultural waste, plant material, and mixtures or combinations thereof. Each possibility represents a separate embodiment. Food waste according to the present invention encompasses food waste of plant origin. Food waste according to the present invention includes household food waste, commercial food waste, and industrial food waste. Each possibility represents a separate embodiment. Organic food waste may originate from vegetable and fruit waste, plants, cooked food, protein waste, slaughterhouse waste, and combinations thereof.Each possibility represents a separate category. Industrial organic food waste may include factory waste, such as by-products, factory rejects, market returns, or trimmings of inedible food portions (such as peels). Commercial organic food waste may include waste from shopping centers, restaurants, supermarkets, etc. Plant material in accordance with this document. The invention covers agricultural waste and man-made products such as paper waste. Typically, the organic waste comprises endogenous D-lactic acid, L-lactic acid, or L- and D-lactic acid, which originate, for example, from natural fermentation processes, e.g., in dairy products. Lactic acid fermentation is carried out using a lactic acid-producing microorganism. “LA-producing microorganisms,” as used herein, refers to microorganisms that produce lactic acid as the main end metabolic product of carbohydrate fermentation. Currently, microorganisms that produce only L-lactic acid are preferred. LA-producing microorganisms may naturally produce only L-lactic acid or may be genetically modified to produce only L-lactic acid, for example, by removing one or more enzymes involved in the synthesis of the unwanted D-enantiomer. LA-producing microorganisms include various bacteria, including, for example, Lactobacillus and Bacillus species, and fungi. Fermentation is typically carried out in the presence of an alkaline compound, such as a metal oxide, carbonate, or hydroxide, as detailed above. Suitable alkaline compounds include, but are not limited to, MgO, CaO, CaCO3, MgCO4, NaOH, KOH, NH4OH, Ca(OH)2, Mg(OH)2, and mixtures or combinations thereof. Each of these represents a separate method. The alkaline compound is added to adjust the pH of the fermentation broth to a desired value, typically in the range of 5 to 7, inclusive of all values ​​within that range. The alkaline compound also results in the neutralization of L-lactic acid to a lactate salt. During fermentation, the pH in the fermentation broth decreases due to the production of lactic acid, which negatively impacts the productivity of the lactic acid-producing microorganism.Adding bases such as magnesium, sodium hydroxide, potassium hydroxide, or calcium hydroxide adjusts the pH by neutralizing lactic acid, resulting in the formation of a lactate salt. In configurations where the PLA hydrolysis slurry is added to the fermenter during fermentation, the slurry may contain an excess of a metal oxide or hydroxide. The excess metal oxide or hydroxide adjusts the pH in the fermenter, thus eliminating the need to separately add an alkaline compound to adjust the pH during fermentation. The additional advantage of supplementing the fermentation broth with PLA hydrolysis slurry is an increase in the overall L-lactate production yield, which is particularly beneficial for organic waste with a low carbohydrate content. The overall L-lactate production yield typically increases by at least 10%, preferably by at least 30%, 40%, 50%, 60%, 70%, 80%, 90%, or even 100% or more. Each of these increases represents a separate modality. Typically, fermentation is carried out under anaerobic or microaerophilic conditions, using batch, fed-batch, continuous, or semi-continuous fermentation. Each possibility represents a separate embodiment of the present invention. In batch fermentation, carbon substrates and other components are loaded into the reactor, and the product is collected when fermentation is complete. Except for the alkaline compound discussed earlier for pH control, no other ingredients are added to the reaction before it begins. MA / t / ZUZZ / U / 3103 complete. The inoculum size is typically approximately 5-10% of the liquid volume in the reactor. Fermentation is maintained at substantially constant temperature and pH, with the pH maintained by adding the alkaline compound. In fed-batch fermentation, the substrate is fed continuously or sequentially into the reactor without removing the fermentation broth (i.e., the products remain in the reactor until the end of the operation). Common feeding methods include batch feeding, constant feeding, pulse feeding, and exponential feeding. Each of these represents a separate mode. In continuous fermentation, the substrate is added to the reactor continuously at a fixed rate and the fermentation products are extracted continuously. In semi-continuous processes, a portion of the culture is removed at intervals and fresh medium is added to the system. Repeatedly fed batch culture, which can be maintained indefinitely, is also considered a semi-continuous process. Lactic acid fermentation is normally carried out for approximately 1 to 4 days or any amount in between, e.g., 1 to 2 days, 2 to 4 days, or 3 to 4 days, inclusive of each value within the specified ranges. Once fermentation is complete, the wort can be clarified by centrifugation or passed through a filter press to separate solid residues from the fermented liquid. The filtrate can then be concentrated, for example, using a rotary vacuum evaporator. The fermentation broth according to the present invention may contain D-lactic acid from organic waste. Furthermore, D-lactic acid may be present in the PLA hydrolysis suspension, either from degraded PLA or formed by racemization during hydrolysis. D-LA is undesirable in the production of L-LA for polymerization because it results in the formation of more D,D-lactide and mesolactide, which negatively affects the quality of the final PLLA product. When D-LA is formed, the present invention advantageously removes it by employing a D-lactic acid-degrading enzyme or a D-lactic acid-utilizing microorganism in each fermentation broth or hydrolysis suspension alone, or in a combined mixture. Each possibility represents a separate embodiment. Currently, the use of D-lactate oxidase is preferred as a degrading enzyme for D-lactic acid. D-lactate oxidase is an enzyme that catalyzes the oxidation of D-lactate to pyruvate and H₂O₂ using O₂ as an electron acceptor. The enzyme uses flavin adenine dinucleotide (FAD) as a cofactor for its catalytic activity. A D-lactate oxidase according to the present invention is typically a soluble (rather than membrane-bound) D-lactate oxidase. Advantageously, the enzyme acts directly in the fermentation broth to remove D-lactic acid. In some embodiments, the D-lactate oxidase is from the species Gluconobacter sp. In some embodiments, the D-lactate oxidase is from Gluconobacter oxydans (see, for example, GenBank accession number AAW61807). The removal of D-lactate from fermentation broths derived from organic waste using a D-lactate oxidase is described in WO 2020 / 208635 assigned to the Applicant of the present invention. ML / t / ZUZZ / U ÍOfOO Microorganisms that utilize D-lactic acid suitable within the scope of the present invention include, but are not limited to, an Escherichia coli that lacks all three L-lactate dehydrogenases. As used herein, “elimination,” when referring to D-lactic acid / D-lactate, means the reduction to residual amounts so as not to interfere with subsequent processes for the production of L-lactic acid and its subsequent polymerization to poly(L-lactic acid) suitable for industrial applications. “Residual amounts” means less than 1% (w / w) D-lactate, and even more preferably less than 0.5% (w / w) D-lactate, of the total lactate (L+D) in a treated mixture of a fermentation broth at the end of fermentation along with PLA hydrolysis products. In certain specific embodiments, D-lactate elimination is the reduction to less than 0.5% (w / w) D-lactic acid out of the total lactate in a treated mixture of a fermentation broth at the end of fermentation along with PLA hydrolysis products. Depending on additional aspects and methods, L-lactate monomers are further purified. L-lactate monomers can be purified as L-lactate salts. Typically, the purification of L-lactate salts can be carried out by at least one of the following: crystallization, recrystallization, distillation, partitioning, silica gel chromatography, preparative HPLC, and combinations thereof. Each of these methods represents a separate method. Alternatively, a reacidification step can be performed to obtain crude L-lactic acid, followed by purification steps to obtain purified L-lactic acid. Reacidification can be carried out as known in the art, for example, using at least one of hydrochloric acid, hydrobromic acid, phosphoric acid, sulfuric acid, and combinations thereof. Each of these methods represents a separate method. Purification processes can include distillation, extraction, electrodialysis, adsorption, ion exchange, crystallization, and combinations of these methods. Several methods are reviewed, for example, in Ghaffar et al. (2014) Journal of Radiation Research and Applied Sciences, 7(2): 222-229; and López-Garzón et al. (2014) Biotechnol Adv., 32(5): 873-904. Alternatively, one-step recovery and conversion of lactic acid to lactide can be used (Dusselier et al. (2015) Science, 349 (6243): 78-80). In some particular embodiments of the present invention, the metal oxide or hydroxide used for PLA hydrolysis and the alkaline compound used to adjust the pH during fermentation produce a magnesium ion as a counterion. For example, in some embodiments, magnesium hydroxide (Mg(OH)₂) is used for PLA hydrolysis and also for pH adjustment during fermentation. In other embodiments, other cations (e.g., sodium or calcium ions) are used in PLA hydrolysis and during fermentation, which are exchanged for magnesium ions as described above. According to these embodiments, the PLA hydrolysis slurry comprises lactate and Mg²⁺ monomers, and the lactic acid fermentation broth comprises lactate and Mg²⁺ monomers, which can be recovered as magnesium lactate. The magnesium lactate can be obtained in crystalline or amorphous form, each possibility representing a separate embodiment.Any solvate or polymorph of magnesium L-lactate can be recovered, including, in particular, crystalline magnesium L-lactate dihydrate. A particular step-down purification process for purifying magnesium lactate by crystallization is described in a pending patent application, WO 2020 / 110108, assigned hereto MA / IZ / ZUZZ / U / 0 / 00 to the applicant of the present invention. The purification process can be applied to the mixture of the PLA hydrolysis suspension with the fermentation broth after treatment that removes D-lactate monomers, where applicable. The purification process comprises the following steps: - providing a clarified mixture from which insoluble impurities have been removed, wherein the clarification can be carried out in the fermentation broth before or after the step of mixing with the PLA hydrolysis suspension, the clarified mixture comprising magnesium lactate in a soluble form, the mixture being at a temperature between 45°C and 75°C; - Concentrate the clarified mixture to a lactate concentration of 150-220 g / L; - perform at least one crystallization by cooling of the clarified concentrated mixture to obtain magnesium lactate crystals; and - collect the magnesium lactate crystals obtained. In some forms, the mixture is provided at a temperature between 55°C and 65°C. The separation of insoluble impurities may include at least one technique selected from the following: filtration, centrifugation, flotation, sedimentation, flocculation, and decantation. Each option represents a separate method. For example, the separation of insoluble impurities can be carried out using centrifugation and microfiltration. The clarified mixture can be concentrated by evaporation, nanofiltration, reverse osmosis, or a combination thereof. In some methods, the clarified mixture is concentrated to a lactate concentration of 160–220 g / L, for example, 170–220 g / L or 180–220 g / L, inclusive of all values ​​within the specified ranges. At least one cooling crystallization may begin at an initial temperature in the range of 50 to 75°C, inclusive of all values ​​within the specified range. In some embodiments, at least one cooling crystallization may begin at an initial temperature in the range of 50 to 70°C, inclusive of all values ​​within the specified range. In additional embodiments, at least one cooling crystallization may begin at an initial temperature in the range of 50 to 65°C, inclusive of all values ​​within the specified range. The at least one cooling crystallization step may terminate at a second temperature in the range of 10 to 1°C, inclusive of all values ​​within the specified range. In some embodiments, the at least one cooling crystallization step terminates at a second temperature in the range of 6 to 2°C, inclusive of all values ​​within the specified range. The cooling rate of at least one cooling crystallization step can be in the range of 10 to 0.5°C / h, inclusive of all values ​​within the specified range. In some embodiments, the cooling rate is in the range of 5 to 1°C / h, inclusive of all values ​​within the specified range. Before crystallization by cooling, the pH of the concentrated mixture can be adjusted to be in the range of 6 to 7. The magnesium lactate crystals obtained can be separated from the remaining liquid by microfiltration or nanofiltration. The remaining liquid can then be concentrated, followed by at least one further crystallization by cooling, to obtain additional magnesium lactate crystals. ML / t / ZUZZ / U ZO / OO After separation from the liquid, the magnesium lactate crystals can be washed with an aqueous solution or an organic solvent such as ethanol and purified. Further processing of the magnesium lactate crystals may include at least one of the following: extraction, microfiltration, nanofiltration, activated carbon treatment, distillation, drying, and grinding. Each of these represents a separate processing method. As used in this document and in the accompanying claims, the term “approximately” refers to ±10%. As used herein and in the appended claims, the singular forms “a,” “an,” “one,” “the,” and “the” include plural references unless the context clearly indicates otherwise. Thus, for example, the reference to “an alkali compound” includes a plurality of such compounds unless the context clearly indicates otherwise. It should be noted that the term “and” or the term “or” are generally used in their sense that includes “and / or” unless the context clearly indicates otherwise. The following examples are presented to more fully illustrate certain embodiments of the invention. However, they should in no way be construed as limiting the broad scope of the invention. A person skilled in the art can readily devise many variations and modifications of the principles described herein without departing from the scope of the invention. EXAMPLES EXAMPLE 1 Alkaline thermohydrolysis of thin-film PLA bags using magnesium hydroxide The following experiment tested the hydrolysis of thin-film PLA bags from post-consumer waste using magnesium hydroxide. Substrate preparation The PLA bags were manually cut with scissors into small rectangular pieces with dimensions of approximately 3x5 cm. Each piece cut from the PLA bag weighed approximately between 100 and 200 mg. Experimental design To evaluate the amount of Mg(OH)2 needed to achieve complete degradation of PLA products, 5 g of pieces cut from PLA bags were subjected to hydrolysis in 150 ml DW using 0 (as a control), 1, 2.5, 5, 10 and 15 wt% of Mg(OH)2. The reactions were carried out at 90°C for 24h. Procedure'. All reactions were carried out in a 250 mL round-bottom flask fitted with a large, oval-shaped magnetic stir bar. Mg(OH)₂ was added to each flask using a powder funnel to form a milky suspension. The flasks were placed in an oil bath and heated at 90°C for 24 h with stirring. The suspension from each flask was then distilled. The flask was vacuum filtered using a 90 mm Buchner funnel fitted with Whatman 3 filter paper. The residue from the inside of the flasks was washed with DW and passed through the filter. The filtrates were typically semi-oily and were filtered once more through the Mg(OH)₂ filter cakes formed to provide clear, colorless aqueous solutions. Each solution was transferred to a heavy round-bottom flask, and the water was evaporated to dryness using a Rotovap to produce a white solid. After evaporation, the flasks were placed overnight in a vacuum desiccant to remove the residual water. Each flask was reweighed after complete drying, and the total weight of the solids inside the flask was calculated. The solids were then dissolved in DW (250 ml) by stirring for at least 30 minutes at room temperature. The pH and conductivity of the solution were measured with a calibrated instrument.Lactate was measured using an enzyme kit (Lactic Acid Test). Procedure for the control experiment (without Mg(OH)2): The experimental procedure described above was repeated under the same conditions, but without the addition of Mg(OH)₂. After filtration of the PLA residue, the aqueous solution was not evaporated to dryness because solid magnesium dilactate could not be formed. Instead, the total filtrate volume was increased to 250 mL using DW, and pH, conductivity, and lactate measurements were taken. MA / IZ / ZUZZ / U ZO / OO Results'. The results are summarized in Table 1 below and in Figure 4. Table 1 - Hydrolysis of cut pieces from thin film PLA bags by Mg(OH)2 % by weight of Mg(OH)2 Weight of dry “white solid” [g] In 250mL DW Lactate conc. Per kit* [g / L] Weight of lactate in 250mL [g] 100% lactate yield ** [g] Lactate yield PH Cond. [mS] 0 N / A 2.9 238 0.228±0.004 0.057 6.18 0.9% 1 6.32 5.7 3.6 11.5±0.9 2.875 47% 2.5 13.1 9.1 8.4 16.7±0.3 4.175 68% 5 8.24 9.3 8.2 17.8±0.5 4.450 72% 10 12.38 9.4 7.8 18.5±1.2 4.625 75% 15 repeat 9.6 repeat 18.7±0.9 4.675 76% * The lactate concentration provided is an average of three measurements, and the error is the relative standard deviation (RSD). The lactate concentration for the control experiment, where no Mg(OH)2 was added, was measured using a 10x dilution with DW since almost no PLA degradation occurred. **The 100% yield calculation assumed that all the PLA degraded to lactate units. Since each repeating unit of PLA weighs 72 Da and the molecular weight of lactate is 89 Da, the total weight must increase due to the addition of one water molecule after hydrolysis by a factor of 89 / 72 = 1.236. Consequently, 5.0 g of PLA should yield 5.0 x 1.236 = 6.18 g of lactate if all the PLA were completely hydrolyzed. Therefore, according to the previous 15% by weight example, 4.675 g of lactate corresponds to (4.675 / 6.18) x 100% = 76% yield. It is likely that low molecular weight PLA oligomers were still present in the aqueous solution obtained after filtration of the hydrolysis slurry. This was thought to be the reason for the clear solution observed. However, no more than 76% yield was achieved. It appears that 5 wt% Mg(OH)2 provides a similar level of PLA hydrolysis as 10 and 15 wt%, as evidenced by the similar amounts of lactate achieved for these concentrations of Mg(OH)2. Integration with a lactic acid production line: The lactate-containing solution formed as a result of hydrolysis in 5% Mg(OH)2 was successfully added to an acidic lactic acid fermentation broth (pH = 5.5). The pH was increased to pH = 6.8 and the solution was subjected to a downstream processing procedure (DSP) to produce pure magnesium lactate crystals as described in a pending patent application, WO 2020 / 110108, assigned to the applicant of the present invention. EXAMPLE 2 Alkaline thermohydrolysis of PLA granules using sodium hydroxide Fifty grams of PLA granules (Ingeo™ Biopolymer 4032D, NatureWorks LLC.) were added to a 250 ml three-necked flask equipped with a condenser and a thermometer. Fifty milliliters of 5M NaOH were added, and the flask was heated to 80°C. The measured pH was 13.5. After 3.5 hours of rapid degradation, the concentration reached 320 g / L with a minor further increase in lactate concentration over time. After 21.5 hours, the lactate concentration stopped increasing (final concentration of 340 g / L) and the reaction cooled to room temperature. The PLA waste was filtered using a sintered glass funnel to yield a clear solution. The final measured pH was 12.9, which is suitable for further PLA degradation. The solution was neutralized with concentrated H₂SO₄, then 280 mL of magnesium sulfate heptahydrate solution (300 g / L) were added dropwise while stirring. The resulting MgLa₂·2H₂O precipitate was filtered using a sintered glass funnel, washed with acetone, and dried at 80°C to a final weight of 64 g. The filtrate was added dropwise to 500 mL of acetone while stirring, and the mixture was stirred for another hour. The resulting precipitate was filtered using a sintered glass funnel, washed with acetone, and dried at 80°C. Yield: 74% yield. The magnesium lactate precipitate is added to a lactic acid fermentation broth and subsequently subjected to a downstream processing procedure (DSP) to produce pure magnesium L-lactate crystals. The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others may, by applying current knowledge, readily modify and / or adapt such specific embodiments for various applications without undue experimentation and without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be understood within the meaning and range of equivalents of the embodiments described. It should be understood that the phraseology or terminology employed herein is for descriptive purposes and not for limitation. The means, materials, and steps for carrying out the various disclosed functions may take a variety of alternative forms without departing from the invention.

Claims

1. A method for producing an L-lactate salt from a combined recycle of polylactic acid (PLA) and organic waste, the method comprising the steps of: (a) hydrolyzing PLA waste with a metal oxide or hydroxide to obtain a PLA hydrolysis suspension comprising L-lactate monomers and a first counterion; (b) fermenting organic waste with a lactic acid-producing microorganism in a fermenter in the presence of an alkaline compound to obtain a fermentation broth comprising L-lactate monomers and a second counterion, wherein the first and second counterions are the same; or wherein at least one of the first and second counterions is subjected to ion exchange, thereby obtaining a first and second counterion that are identical;(c) optionally, contacting the PLA hydrolysis suspension from step (a), the fermentation broth from step (b), or a mixture comprising the PLA hydrolysis suspension from step (a) and the fermentation broth from step (b) with a D-lactic acid-degrading enzyme or a microorganism that uses D-lactic acid to remove D-lactate monomers, thereby yielding L-lactate monomers; and (d) purifying a mixture comprising the PLA hydrolysis suspension from step (a) and the fermentation broth from step (b) or the L-lactate monomers from step (c), thereby yielding an L-lactate salt.

2. The method according to claim 1, wherein step (a) and step (b) are performed in any order or simultaneously.

3. The method according to claim 1, wherein the mixture comprising the PLA hydrolysis suspension of step (a) and the fermentation broth of step (b) is obtained by gradually adding the PLA hydrolysis suspension to the lactic acid fermenter during fermentation.

4. The method according to any of claims 1 to 3, wherein step (a) is performed at a temperature in the range of approximately 50°C to approximately 90°C.

5. The method according to any of claims 1 to 4, wherein step (a) is performed during a period of time in the range of approximately 1 to approximately 12 hours.

6. The method according to any of claims 1 to 4, wherein step (a) is performed during a period of time in the range of approximately 12 to approximately 36 hours.

7. The method according to any of claims 1 to 6, wherein the metal oxide or hydroxide of step (a) and the alkali compound of step (b) are the same compound.

8. The method in accordance with any of claims 1 to 6, wherein the first and second counterions are different.

9. The method according to claim 8, wherein at least one of the hydroxide suspension of step (a) and the fermentation broth of step (b) are subjected to ion exchange resulting in first and second counterions that are equal.

10. The method according to any of claims 1 to 9, wherein the alkali compound in step (a) is a metal oxide, a carbonate, or a hydroxide.

11. The method according to claim 1 or claim 10, wherein the metal oxide comprises at least one of MgO, CaO and a mixture or combination thereof.

12. The method according to claim 10, wherein the carbonate comprises at least one of CaCO3, MgCO3 and a mixture or combination thereof.

13. The method according to claim 1 or claim 10, wherein the hydroxide comprises at least one of NaOH, KOH, NH4OH, Ca(OH)2, Mg(OH)2, and a mixture or combination thereof.

14. The method according to claim 13, wherein the hydroxide is Mg(OH)2.

15. The method according to any of claims 1 to 14, wherein the lactate salt is magnesium L-lactate salt.

16. The method according to claim 15, wherein the magnesium L-lactate is in crystalline form.

17. The method in accordance with any of claims 1 to 16, further comprising pretreating the PLA waste before step (a).

18. The method according to claim 17, wherein the pretreatment comprises a mechanical pretreatment selected from the group consisting of polishing, chipping, crushing, grinding and a combination thereof.

19. The method according to any of claims 1 to 18, further comprising subjecting the PLA hydrolysis suspension obtained in step (a) to a solid-liquid separation.

20. The method according to any of claims 1 to 19, wherein the organic residue comprises endogenous D-lactic acid, L-lactic acid, or both.

21. The method according to any one of claims 1 to 20, wherein the organic waste is selected from the group consisting of food waste, municipal waste, agricultural waste, plant material and a mixture or combination thereof.

22. The method according to any of claims 1 to 21, wherein the PLA residues comprise poly L-lactic acid (PLLA) and poly D-lactic acid (PDLA).

23. The method in accordance with any of claims 1 to 22, wherein step (c) is performed.

24. The method according to claim 23, wherein the D-lactic acid degrading enzyme in step (c) is a D-lactate oxidase.

25. The method according to any one of claims 1 to 24, wherein the L-lactate salt obtained is purified by at least one of crystallization, recrystallization, distillation, partitioning, silica gel chromatography, preparative HPLC and combinations thereof.

26. The method according to any of claims 1 to 25, wherein the L-lactate salt obtained is acidified to form L-lactic acid for the subsequent formation of polylactic acid. MA / IZ / ZUZZ / U ÍSfOO 27. A method for producing magnesium L-lactate salt by recycling polylactic acid (PLA) waste, the method comprising the steps of: (a) hydrolyzing PLA waste with a base selected from sodium, potassium, and ammonium hydroxide to obtain a PLA hydrolysis slurry comprising L-lactate monomers and a counterion selected from sodium, potassium, and ammonium; (b) optionally performing at least one of neutralizing the PLA hydrolysis slurry with an acid and removing unhydrolyzed PLA waste; and (c) adding a magnesium salt to the PLA hydrolysis slurry from step (a) or (b) to thereby precipitate the magnesium L-lactate salt.

28. The method according to claim 27, wherein the base is sodium hydroxide.

29. The method according to claim 27 or 28, wherein step (a) is performed at a temperature in the range of approximately 50°C to approximately 90°C.

30. The method according to any of claims 27 to 29, wherein step (a) is performed during a period of time in the range of approximately 1 to approximately 24 hours.

31. The method according to any of claims 27 to 29, wherein step (a) is performed during a period of time in the range of approximately 1 to approximately 12 hours.

32. The method in accordance with any of claims 27 to 31, wherein the base is in excess of PLA residues.

33. The method in accordance with any of claims 27 to 31, wherein the PLA residues exceed the base.

34. The method according to any of claims 27 to 33, wherein step (b) is performed and wherein the acid is selected from hydrochloric acid, hydrobromic acid, phosphoric acid, sulfuric acid and combinations thereof.

35. The method according to any of claims 27 to 34, wherein step (b) is performed and wherein the disposal of unhydrolyzed PLA waste comprises solid-liquid separation.

36. The method according to any of claims 27 to 35, wherein the magnesium salt in step (c) is added in solid form.

37. The method according to any of claims 27 to 35, wherein the magnesium salt in step (c) is added as an aqueous solution.

38. The method according to any of claims 27 to 37, wherein the magnesium salt in step (c) is magnesium sulfate.

39. The method according to any of claims 27 to 38, wherein the resulting magnesium L-lactate salt is further subjected to subsequent purification. ML / t / ZUZZ / U / 0 / 00 40. The method according to any of claims 27 to 39, wherein the magnesium L-lactate salt is combined with the magnesium L-lactate salt derived from the fermentation of organic waste followed by further purification.