Use of olive mill waste waters in the leather tanning industry

Abstract

Use of olive mill waste waters (OMW) in at least one step of the leather tanning process in the tanning industry is described. Preferably, said olive mill waste waters are used as bactericides, tanning agents, or tanning adjuvant agent and / or antioxidant agents to prevent the formation of chromium (VI) during the retanning step. Advantageously, a tanning composition comprising said olive mill waste waters (OMW) and suitable additives is described.

Description

FIELD OF THE INVENTION

[0001] The invention relates to a process for the requalification of olive oil mill waste waters in the tanning industry.STATE OF THE ART

[0002] Olive oil mill waste waters (OMW) are aqueous solutions of organic substances and minerals, further containing suspended vegetal solid material, resulting from the separation of oily must step. Their composition is extremely variable and depends on several factors, including the type of olive cultivar and their degree of ripening, soil, extraction process, and conservation methodology.

[0003] Olive oil mill waste waters comprise a variety of ingredients, such as organic substances, nitrogenous substances, sugars, tannins, phenolic compounds, polyalcohols, pectins, lipids, minerals, and polyphenols.

[0004] Sugars are the predominant organic substances, especially fermentable sugars, such as glucose (70%), mannitol (14%), fructose (10%), sucrose (5%), galactose (1%), and cellulose.

[0005] Among nitrogenous substances, all amino acids, particularly glutamic acid and proline, were found.

[0006] The pH value, typically comprised between 4 and 6, is determined by the content of organic acids, such as malic, citric, tartaric, succinic, and oxalic. These pH oscillations are attributable to olive variety, maturation period, and storage life.

[0007] The organic fraction of olive oil mill waste waters determines their high polluting power, normally characterized by very high chemical oxygen demand (COD) and biological oxygen demand (BOD5), ranging between 90 grams O 2 / L and 30 grams O 2 / L, respectively, for waters originating from the most modern centrifugal plants, and even 150 grams O 2 / L and 90 grams O 2 / L for waters originating from traditional plants.

[0008] Among other things, during storage in storage tanks, where the waste waters stop for more or less time, the concentration of some easily fermentable organic components may be also significantly reduced by the action of aerobic and anaerobic microorganisms present therein, which are able to decompose them. Even from a microbiological point of view, OMW are in fact a very complex matrix, being therein present a plurality of microorganisms, molds and yeasts reaching even concentration values of 10 5< -10 6< .

[0009] The production and characteristics of olive oil mill waste waters are, therefore, directly related both to the production of olives for olive oil production and to their characteristics, as well as to the extraction process used, and to the subsequent storage conditions and times.

[0010] Generally, 70-100 kg of olive oil mill waste waters are obtained per 100 kg of olives processed.

[0011] Because of their very high pollution potential, and the remarkable volumes produced, it is apparent that olive oil mill waste waters are nowadays a major waste to be disposed of, given the absence of effective treatment methods.

[0012] Various approaches to olive oil mill waste waters treatment, from detoxification processes with fungi to treatment with ozone, and biovalorization strategies, have been attempted over time. None of these systems has ever been proven to be truly effective and industrially applicable, so today olive oil mill waste waters are still disposed of as non-hazardous waste, or by spreading them on farmland. US2010275380 discloses a process for the tanning of hides and skins, characterized in that hides and skins are treated with tanning agents containing deglycosylated iridoids and / or deglycosylated secoiridoids, excluding tanning agents containing genipin.

[0013] In recent years, many efforts have been made to requalify olive oil mill waste waters, converting them from a waste to disposed of into a reusable product, so to obtain the valorization of this waste while reducing its environmental impact. GR20020100071A describes the obtaining of substances with cytotoxic and antioxidant activity from extraction processes starting from olive leaves, olives, olive oil and olive processing waste.

[0014] Ahmed Tafesh et al (Ahmed Tafesh et al: "Synergistic antibacterial effects of polyphenolic compounds from olive mill wastewater", EVIDENCE-BASED COMPLEMENTARY AND ALTERNATIVE MEDICINES: ECAM, HINDAWI PUBLISHING CORPORATION, UNITED STATES, vol. 2011, 1 January 2011, pages 1 -9) and Obied et al (Obied et al: "Bioscreening of Australian olive mill waste extracts: Biophenol content, antioxidant, antimicrobial and molluscicidal activities", FOOD AND CHEMICAL TOXICOLOGY, PERGAMON GB, vol.45, no.7, 19 April 2007, pages 1238-1248) describe the antibacterial properties of compounds extracted from the waters of olive vegetation.

[0015] The cited documents propose long and laborious procedures of extraction, purification and concentration in order to be able to recover reusable components normally present in the waters of olive vegetation, so as to be able to plan their valuable and alternative use for disposal.

[0016] However, although these processes allow a reuse of the vegetation water, they are extremely complicated and expensive, thus being difficult to achieve in a convenient way on an industrial scale.

[0017] The primary aim of the present invention is, therefore, to requalify olive oil mill waste waters resulting from the process of olive processing.SUMMARY OF THE INVENTION

[0018] The above purpose has been achieved by providing for the use of olive oil mill waste waters in the leather tanning industry.

[0019] In particular, and surprisingly, the inventors have found that olive oil mill waste waters could be used in the leather tanning process in at least three different steps in the process of treating animal hides.

[0020] In a first embodiment, the invention therefore relates to the use of olive oil mill waste waters (OMW) as a bactericide in any stage of the tanning process requiring a bactericidal action, preferably in the soaking step.

[0021] In a second embodiment, the invention relates to the use of olive oil mill waste waters (OMW) as a tanning agent or tanning adjuvant agent, in particular in the tanning step.

[0022] Preferably, in said embodiment the use of said olive oil mill waste waters (OMW) occurs in combination with other tanning agents.

[0023] Tanning agents are more preferably selected from the group consisting of a natural tannin, such as tara, mimosa, chestnut, myrobalan, a synthetic tannin, such as hydroxydiphenyl sulphone, a compound belonging to the family of phenolic resins, a compound belonging to the family of naphthalene resins, an aldehyde or a precursor thereof, such as glutaraldehyde, isoxazolidine, oxazolidine, a metal salt, such as a chromium, zirconium, or titanium salt, and an acrylic resin, such as a resin obtained by homopolymerization or radical copolymerization of acrylic acid, methacrylic acid, methyl methacrylate ester, ethyl acrylate ester, acrylamide, preferably salified, such as sodium polyacrylate (PA).

[0024] Even more preferably, in said embodiment, olive oil mill waste waters are used in combination with sodium polyacrylate resin (PA).

[0025] In a third embodiment, the invention relates to the use of olive oil mill waste waters (OMW) as an antioxidant agent to prevent the formation of Chromium (VI) in the retanning step.

[0026] The inventors of the present invention have, in fact, surprisingly found that OMW may be used in at least one step of the leather tanning industry, thus enabling the valorization of a product until now only destined to disposal.DETAILED DESCRIPTION OF THE INVENTION

[0027] The invention therefore relates to the use of olive oil mill waste waters in the leather tanning process in the tanning industry.

[0028] Olive oil mill waste waters (OMW) are waste waters resulting from olive oil processing, and they are obtained during the separation of water from oily must and from plant washing operations.

[0029] Preferably, OMW used in the present invention have a pH comprised between 2 and 8, more preferably between 3 and 5, a suspended solids content comprised between 10,000 and 100,000 mg / L, more preferably between 20,000 and 70,000 mg / L, a dry residue comprised between 1 and 12% by weight, more preferably between 2 and 8%, a total phenols content (as determined according to Folin Ciocalteau method, expressed as gallic acid mg equivalents / L of OMW) comprised between 100 and 15,000, more preferably between 500 and 10,000, a hydroxytyrosol content (as determined by HPLC-UV DAD) comprised between 25 and 5,000 mg / L, more preferably between 500 and 3,000 mg / L.

[0030] More preferably, olive oil mill waste waters used in the tanning process according to the invention are olive oil mill waste waters previously subjected to stabilization methods by treatment with nitrogen. Specifically, said olive oil mill waste waters are enriched in one or more polyphenols, such as hydroxytyrosol, by treatment with nitrogen in the form of a liquid or a gas, preferably a gas.

[0031] Even more preferably, said olive oil mill waste waters previously subjected to stabilization methods by treatment with nitrogen, optionally provide for an earlier step of enrichment with lactic bacteria.

[0032] The tanning process is generally carried out in various ways, depending on the type of raw hides and the type of product to be made.

[0033] In the present invention, when using the expression: "Leather treatment water" means the liquid to be used for hides treatment, to which the various treatment agents are added; its quantity is normally expressed as Kg of liquid compared to Kg of hides to be treated; "Network water" means the water supplied by the normal industrial water system, and typically used as treatment water in the leather tanning process.

[0034] In the present invention, and as it will be apparent from the detailed description below, the treatment water comprises network water and a given percentage of olive oil mill waste waters (OMW), expressed as a percentage by weight based on the total weight of the treatment water.

[0035] The whole tanning process is very complex due to the nature and plurality of chemical-biological reactions occurring in the various steps of treatment. The aim of the tanning process is to provide tanned leather having, at the same time, high hydrothermal stability, good chemical resistance, high resistance to bacterial and enzymatic agents, as well as a number of properties associated with their commercialization, such as softness, flexibility, fullness, firmness, compactness, dyeability, and good physical-mechanical characteristics.

[0036] Therefore, all the chemical agents traditionally used in the various steps of the tanning process should, at the same time, ensure obtainment of the properties pursued in the specific step in which the agent is used, without compromising or altering the delicate reaction balance of the subsequent steps, or compromising the performances obtained after the treatments of the previous steps.

[0037] The soaking step allows to bring the hides back to the original conditions of when just skinned, removing the salt used during storage, cleaning the dirt away and making them adsorb the water lost following the preservation treatment.

[0038] The inventors have surprisingly found that OMW as such, i.e. which did not undergo any purification and / or extraction of components thereof, could be added as bactericides at any stage of the tanning process, particularly during the hides soaking or tanning step, preferably in the soaking step.

[0039] In a first embodiment, the invention therefore relates to the use of olive oil mill waste waters (OMW) as a bactericide in at least one step of the tanning process requiring a bactericidal action, preferably in the soaking step.

[0040] Preferably, hides soaking is performed in a treatment water bath consisting of network water at 25°C, wherein detergents for removing dirt, globular proteins and blood, and biocides are added to prevent bacterial charge formation, which generates malodorous substances and degrades the leather. Surprisingly, and as it will be apparent from the experimental part below, OMW have proved to be excellent bactericides in the environment of use, and have therefore advantageously allowed to significantly reduce, or even replace, the amounts of biocides typically used during the soaking step, such as chlorophenols (chlorocresols, trichlorophenols, etc.), dithiocarbamates (sodium salts), and thiocyanomethylthiobenzothiazoles.

[0041] Typically, in the soaking step, an amount of treatment water, usually network water, comprised between 0.5 kg and 2.5 kg, preferably between 0.7 kg and 1.5 kg, even more preferably of about 1 kg per kg of hides to be treated is used.

[0042] In this embodiment, OMW are used in an amount from 50 to 100% by weight based on the treatment water, preferably in an amount from 75% to 100%, even more preferably in an amount of 100%, i.e. they totally replace the network water.

[0043] In a second embodiment, the invention relates to the use of olive oil mill waste waters (OMW) as a tanning agent, or a tanning adjuvant agent, in the leather tanning step.

[0044] It is known that tanning is a set of operations that allow to obtain a stable crosslinking of the collagen fibers within the dermis of the skin. In the tanning step, stable cross-links among collagen polypeptide chains are formed, with a consequent consolidation that preserves the dermal substance from the degradation processes, and provides tanned leather with mechanical strength and resistance to humidity, temperature, and chemical agents.

[0045] Olive oil mill waste waters (OMW) were found to be both excellent tanning agents and excellent adjuvant agents in the tanning process, as it will be apparent from the experimental part.

[0046] Therefore, in its second embodiment, the invention relates to the use of olive oil mill waste waters (OMW) as tanning agents, or tanning adjuvating agents, in the tanning step of the leather tanning process.

[0047] Advantageously, in the tanning step, an amount of treatment water, expressed as kg of water per kg of dry hides, comprised between 0.5 kg and 2 kg, preferably between 0.8 kg and 1.5 kg, is used.

[0048] In this embodiment, OMW are used in an amount from 50 to 100% by weight based on the treatment water, preferably in an amount from 75% to 100%, even more preferably in an amount of 100%, i.e. they totally replace the network water. Preferably olive oil mill waste waters (OMW) are used in combination with other tanning agents selected from the group consisting of a natural tannin, such as tara, mimosa, chestnut, myrobalan, a synthetic tannin, such as hydroxydiphenyl sulphone, a compound belonging to the family of phenolic resins, a compound belonging to the family of naphthalene resins, an aldehyde or a precursor thereof, such as glutaraldehyde, isoxazolidine, oxazolidine, a metal salt, such as a chromium, zirconium, or titanium salt, and an acrylic resin, such as a resin obtained by homopolymerization or radical copolymerization of acrylic acid, such as sodium polyacrylate, methacrylic acid, methyl methacrylate ester, ethyl acrylate ester, acrylamide, preferably salified, such as sodium polyacrylate (PA), and mixture thereof.

[0049] More preferably, in said embodiment olive oil mill waste waters are used in combination with an acrylic resin, such as the resin obtained by homopolymerization or radical copolymerization of acrylic acid, methacrylic acid, methyl methacrylate ester, ethyl acrylate ester, acrylamide, optionally preferably salified, such as sodium polyacrylate (PA).

[0050] Even more preferably, in said embodiment olive oil mill waste waters are used in combination with sodium polyacrylate resin (PA), obtained by homopolymerization or radical copolymerization carried out in the same OMW, as a reaction medium. Advantageously OMW may also be used as adjuvating agents of tanning agents in tanning processes, preferably in tanning processes not employing chromium (the so-called chromium-free tanning processes).

[0051] According to the invention, therefore, OMW may be used in a composition with other tanning agents in a chromium-free tanning process, such as for example oxazolidine, glutaraldehyde o dihydroxydiphenyl sulphone (DIDS), acting as synergistic compounds of known tanning compounds, as it will be apparent from the experimental part.

[0052] In yet a further aspect, the invention relates to a tanning composition comprising olive oil mill waste waters(OMW), one or more tanning agents selected from the group consisting of a natural tannin selected from the group consisting of:tara, mimosa, chestnut and myrobalan, a synthetic tannin, such as hydroxydiphenyl sulphone, a compound belonging to the family of phenolic resins, a compound belonging to the family of naphthalene resins, an aldehyde or a precursor thereof, such as glutaraldehyde, isoxazolidine, oxazolidine, a metal salt, such as a chromium, zirconium, or titanium salt, and an acrylic resin, such as a resin obtained by homopolymerization or radical copolymerization of acrylic acid, methacrylic acid, methyl methacrylate ester, ethyl acrylate ester, acrylamide, preferably salified, such as sodium polyacrylate (PA), and mixture thereof, and suitable additives.

[0053] Preferably, the invention relates to a tanning composition comprising olive oil mill waste waters (OMW) and at least one acrylic resin obtained by homopolymerization or radical copolymerization of acrylic acid, methacrylic acid, methyl methacrylate ester, ethyl acrylate ester, acrylamide, preferably salified.

[0054] More preferably said acrylic resin is sodium polyacrylate resin (PA).

[0055] Alternatively, said acrylic resin is obtained by homopolymerization or radical copolymerization, carried out in the same OMW as a reaction medium, of acrylic acid, methacrylic acid, methyl methacrylate ester, ethyl acrylate ester, acrylamide, preferably salified, such as sodium polyacrylate.

[0056] Preferably said acrylic resin obtained by polymerization, carried out in the same OMW as a reaction medium, is sodium polyacrylate resin (PA).

[0057] The tanning composition of the invention preferably comprises a ratio, by weight, between OMW and one or more tanning agents selected from the group consisting of a natural tannin selected from the group consisting of:tara, mimosa, chestnut and myrobalan, a synthetic tannin, such as hydroxydiphenyl sulphone, a compound belonging to the family of phenolic resins, a compound belonging to the family of naphthalene resins, an aldehyde or a precursor thereof, such as glutaraldehyde, isoxazolidine, oxazolidine, a metal salt, such as a chromium, zirconium, or titanium salt, and an acrylic resin, such as a resin obtained by homopolymerization or radical copolymerization of acrylic acid, methacrylic acid, methyl methacrylate ester, ethyl acrylate ester, acrylamide, preferably salified, such as sodium polyacrylate (PA), and mixture thereof, in the range from 2 to 20, more preferably in the range from 5 to 18, even more preferably from 10 to 12.

[0058] When OMW are used as synergistic agents of tanning compounds, or as the sole tanning compounds, they also advantageously act as microbiological stabilizers of the leather itself, i.e. as leather preservatives in the tanning step.

[0059] In a third embodiment, the invention relates to the use of olive oil mill waste waters (OMW) as an antioxidant agent to prevent the formation of Chromium (VI) in the retanning step.

[0060] In fact, retanning is another step of the leather tanning process, and it consist in a chemical treatment that allow to improve some leather characteristics. Generally, during retanning, the leather adsorbs further tanning or filling substance, such that it provides the final product with the desired grade of fullness, softness, sweat resistance, and more. Usually tannins, chromium salts or other salts are used as retanning agents, and temperature and humidity conditions can convert Chromium (III) into Chromium (VI), a known toxic and carcinogenic agent, highly aggressive towards biological systems, hence having a high environmental pollution impact. Surprisingly, olive oil mill waste waters resulted to be advantageous antioxidant agents that prevent the formation of Chromium (VI) in the retanning step, as it is apparent from the experimental part reported below.

[0061] Advantageously, in the retanning step an amount of treatment water, expressed as kg of water per kg of dry leather, comprised between 0.5 kg and 2 kg, preferably between 0.8 kg and 1.5 kg is used.

[0062] In this embodiment, OMW are used in an amount from 50 to 100% by weight based on the treatment water, preferably in an amount from 75% to 100%, even more preferably in an amount of 100%, i.e. they totally replace the network water.Experimental Part Example 1: Evaluation of OMW in the process of soaking hides as a bactericide

[0063] In order to verify the OMW antimicrobial effect, 6 soaking tests were performed in parallel on dry hides, comparing the behavior of hides that were subjected to soaking with network water only, network water added with traditional antimicrobial agents, or network water mixed with variable amounts of OMW.

[0064] OMW used in the example had the characteristic shown in the Table 1 below: Table 1pH 4.82Suspended Solids (mg / L) 63815Dry Weight (%) 8.38Total Phenols (mg AG eq / L) 1< 2791.7Hydroxytyrosol (mg / L) 2< 811.59Antioxidant Power (mg AG eq / L) 3< 1315.32

[0065] The tests were as follows: Test 1: Dry hides + 100% (kg hides / kg bath ratio) of network water, at 25°C, rotating in a jar mill for 2 hours (blank test); Test 2: Dry hides + 100% (kg hides / kg bath ratio) of network water added with a commercially available dithiocarbamate antimicrobial agent, at 25°C, rotating in a jar mill for 2 hours; Test 3: Dry hides + 75% (kg hides / kg bath ratio) of network water + 25% OMW, at 25°C, rotating in a jar mill for 2 hours; Test 4: Dry hides + 50% (kg hides / kg bath ratio) of network water + 50% OMW, at 25°C, rotating in a jar mill for 2 hours; Test 5: Dry hides + 25% (kg hides / kg bath ratio) of network water + 75% OMW, at 25°C, rotating in a jar mill for 2 hours; Test 6: Dry hides + 100% (kg hides / kg bath ratio) di OMW, at 25°C, rotating in a jar mill for 2 hours.

[0066] Once the process was completed, the jars were opened and samples of the hides so soaked were left to rest, wet, in a hermetic container (plastic bag) for 24 hours, then they were inspected the following day.

[0067] The hides soaked in water alone (Test 1) already showed significant malodorousness at the jar opening.

[0068] Olfactory evaluations of hides bacterial degradation, assessed based on a numerical index comprised between 0 and 3 (specifically: 0-no smell; 1-mild smell; 2-moderate smell; 3-intense smell) are summarized in Table 2 below: Table 2Test no.Olfactory Evaluation Index132033425160

[0069] As it is apparent, the use of OMW in the process allowed a significant reduction of malodorousness, significant at high OMW concentrations, especially for ratios between network water and OMW of 1:3 or more (Tests 5 and 6), i.e. for OMW concentrations in treatment water ranging from 75% to 100%.

[0070] In particular, the complete replacement of network water with OMW (Test 6), in the soaking process, allowed to obtain the usual performance obtained using a traditional antimicrobial agent dispersed in network water (Test 2).

[0071] In addition, to test the possible hides degradation effect induced by the microbial charge possibly formed, a test typically employed in the tanning industry was carried out, i.e. the evaluation the resistance of the hair present in the hides to pulling.

[0072] This is determined simply by the magnitude of the force applied to pull the hair out the hides by using a qualitative numeric index ranging from 0 to 3 (specifically: 0-practically no applied force, the hair is pulled out with great ease; 1-slight force needed; 2-moderate force needed; 3-high strength needed).

[0073] When a hide is attacked by microorganisms, it undergoes a maceration of the dermis so that hair is easily removed (low pull resistance value).

[0074] The results obtained in the pull test carried out on 6 samples of hides obtained from the above-mentioned soaking processes are shown in Table 3 below. Table 3Hide sample obtained from test no.Pull Resistance Index102330415263

[0075] These test results were absolutely consistent with the results obtained in the evaluation of malodorousness.

[0076] The hides soaked with 50% OMW in the treatment water showed a higher hair pull resistance, more consistent for higher concentrations, and even comparable to that achieved with traditional soaking methods, for 100% OMW concentrations, i.e. when network water was completely replaced by OMW.

[0077] These tests therefore demonstrate the possible and effective use of OMW in the hides soaking step, preferably in concentrations from 50% to 100% by weight, based on the total treatment water used in the soaking step, more preferably from 75% to 100%, even more preferably at concentrations of 100%Example 2: Evaluation of OMW Tanning Capacity

[0078] In order to evaluate the capacity of OMW to tan leather, the Chambort-Jamet standardized method, typically used in the hides and leather industry, which is based on determination of the capacity to adsorb tanning species of a standard hide named Freiberg Hide Powder, purchased by the Forschungs Institut für Leder und Kunststoffbahneng GmbH in Germany, was employed and modified as follows.

[0079] According to the method, the measurement of the tanning capacity, expressed as a percentage, is determined according to the formula: C . C . = C i − C f C i ⋅ 100 where Ci and Cf are the concentrations of the tanning product in the aqueous solution having a tanning effect, initially and after absorption by the standard hide, respectively.

[0080] In order to evaluate the tanning effect of OMW, given they are not a "pure" product, such as commercially available tannins typically used in tannery, a solution having the same total phenols concentration of commercial tannins solutions was employed, at a product concentration of 4 g / L. Such initial total phenols concentration was 2 g / L. The evaluation of the OMW tanning effect was therefore not expressed as a variation in the concentration of the dry residue in the solution, but as a variation in the concentration of the total phenols.

[0081] According to the modified method, the measurement of the tanning capacity, expressed as a percentage, is determined according to the formula: C . C . M . = P i − P f P i ⋅ 100 where Pi and Pf are the total phenols concentrations in the aqueous solution having tanning effect, initially and after absorption by the standard hide. For a comparison, the determination of total phenols was performed also on commercial tannins solutions.

[0082] The measurement of the tanning power results from the determination of the amount of phenols absorbed by the hide. The higher the percentage of absorbed phenols, the greater the tanning power of the agent used.

[0083] The test was performed by preparing samples having the same initial concentration of total phenols (determined by Folin Ciocalteau method) of about 2 g / L, and submitting said samples to tanning treatment with OMW, or with traditional tannins (Tara, Myrobalan, Chestnut, and Mimosa).

[0084] The results obtained in the tests are shown in Table 4 below. Table 4Tanning Agent Phenols absorbed (%) OMW82.94TARA91.71MYROBALAN86.75CHESTNUT90.20MIMOSA91.32

[0085] From the results obtained, it can be seen that OMW exhibited a very high tanning power, of about 83%, i.e. fully comparable to that of vegetable tannins used as a comparison, and traditionally used in vegetal tanning, exhibiting a tanning power ranging between about 87% and 92%.Example 3: Evaluation of OMW as a preservative and tanning adjuvant agent.

[0086] Some leather tanning tests were carried out in order to evaluate the use of OMW as a microbiological stabilizer for hides (to eliminate putrescibility), and as an adjuvant in Chromium-free tanning processes.

[0087] Tests were conducted either by using OMW alone, or OMW in combination with several Chrome-free tanners.

[0088] Each test was carried out on four calfskin hides, in a 50 L volume drum; the 4 hides had an indicative weight of 10 Kg each.

[0089] The evaluated parameters were the control of formation of malodorousness and molds, for the evaluation of the antimicrobial capacity of the baths, and the determination of the contraction temperature Tg, for the evaluation of the tanning activity. It should be considered that this second parameter normally ranges from 70°C to 80°C in the case of tanning processes carried out with tannins or aldehydes.Test 1: Microbiological stabilization

[0090] The hides were picked from the wet work drum and drained from Pickle's bath, and then transferred to the drum containing the tanning bath, made of 100% OMW, based on hides weight, and heated to 30°C. 5% of washed sea salt was added to OMW in order to bring the bath density to about 6°Be. The hides were then made to rotate in the drum for 2 hours, then their thickness was checked, and it resulted to be completely permeated by OMW. As a confirmation of complete permeation across the skin thickness, the bath color was lighter, indicating that the hides had absorbed a certain amount of bath components. Since the fixation process for these types of tanning procedures requires a long time, the hides were rested on a horse for a few days.

[0091] The following tests were performed on the hides so obtained: a) Climate chamber conditioning for 7 days at 40°C and 60% relative humidity. Result: there was no mold or malodorousness formation, and no sign of degradation (putrefaction) of the tissues. b) Determination of contraction or gelatinization temperature (UNI EN ISO 3380 method). Result: Tg = 60°C. Test (a) showed that the OMW treatment was effective in the microbiological stabilization of the hides. Test (b) showed a contraction temperature of 60°C, meaning an interesting tanning activity by OMW. Test 2: Microbiological stabilization and synergistic adjuvant of known tanning agents

[0092] The hides were picked from the wet work drum and drained from Pickle's bath, and then transferred to the drum containing the tanning bath, made of 100% OMW and 5% oxazolidine, previously mixed with OMW and homogenized for 10 minutes. The hides were then made to rotate in the drum for 2 hours, then their thickness was checked, and it resulted to be completely permeated by OMW.

[0093] In parallel, a similar preparation was performed where water was used instead of OMW. After resting the hides on a horse for a few days, the following tests were performed: c) Climate chamber conditioning for 7 days at 40°C and 60% relative humidity. Result: OMW bath had no mold or malodorousness formation, and no sign of degradation (putrefaction) of the tissues. On the contrary, with the water bath the hides were observed to be not as stabilized from the microbiological point of view, with the appearance of molded areas. d) Determination of contraction or gelatinization temperature (UNI EN ISO 3380 method). Result: with OMW bath Tg = 70°C, with water bath Tg = 65°C. Test 2 showed that, in addition to microbiological stabilization, OMW worked in synergy with the oxazolidine tanning agent, enhancing the tanning power of the bath, as evidenced by an increase of 5°C in Tg in the case of OMW bath (Test d). In particular, the OMW presence allowed to obtain a Tg value fully comparable to that of conventional tanning baths with aldehydes or tannins.Test 3: Synergistic adjuvant of known tanning agents

[0094] In this test, we wanted to test the tanning power of OMW, as synergistic adjuvant of known tanning agents, at different OMW concentration values.

[0095] The hides were picked from the wet work drum, and drained from Pickle's bath, and then transferred into two drums containing the tanning baths, made of 100% OMW and 5% oxazolidine, previously mixed with OMW and homogenized for 10 minutes.

[0096] The hides were made to rotate in the drum for 2 hours, then their thickness was checked, and it resulted to be completely permeated by OMW.

[0097] Subsequently, a synthetic dihydroxydiphenyl sulphone (DIDS) tannin was added, in an amount of 40% based on the initial weight of the hides, they were made to rotate slowly for 1h, and then left to automatically rotate overnight.

[0098] In the morning, they were checked again; the tanning agents had been absorbed throughout the thickness of the hide, and hides were well-filled (tanned).

[0099] The pH of the bath was 3.7, an indication that tannins and other tanning compounds had been, at least in part, fixed by the hide.

[0100] After resting the hides on a horse for a few days, the contraction temperature was determined, which resulted to be: e) Bath with 50% OMW, 5% oxazolidine, 40% DIDS: Tg = 74°C f) Bath with 100% OMW, 5% oxazolidine, 40% DIDS: Tg = 77°C

[0101] Test 3 also showed that OMW work synergically with oxazolidine and DIDS tanning agents, as evidenced by the high Tg values obtained, broadly comparable to those obtainable in conventional tanning processes, that increase as the OMW concentration in the tanning bath increases.Test 4: Chromium-free vegetal-like tanning tests with OMW adjuvant in tanning

[0102] Tanning tests were carried out on pickled hides, i.e. acidified until the pH of the tanning bath was at 2.8-2.9.

[0103] Acidification in bath is mandatory for any type of tanning because the alkaline substances used in the previous dehairing phase have to be removed from the hide fibers to modify the hide isoelectric point, in order to aid the entry of the tanning molecules (natural tannins, chromium), which are then reacted with the hide by modifying the isoelectric point again, de facto raising the pH of the bath (and hence of the hides themselves) to about 3.8-4.0.

[0104] The tests were carried out by adding the tanning agents directly into the pickle bath.

[0105] In these tests both OMW and OMW added with sodium polyacrylate resin were used.

[0106] In particular, the two tanning tests I and II described below, containing OMW as such, or OMW with sodium polyacrylate resin, involved the following specific compositions: I) 60% pickle bath + 70% OMW + Acrylic resin sodium polyacrylate II) 60% pickle bath + 70% OMW

[0107] The recipe for the two tests I and II was the following: to pickled hides, OMW were added with the pickle bath (tanning bath); to said bath, 6% by weight of modified glutaraldehyde and 1.5% by weight of long chain alcohol sulfosuccinate were also added, then the jars were made to rotate for 180 minutes.

[0108] Subsequently, 2.0% modified dihydroxydiphenyl sulphone was added and the jars were made to rotate for another 180 minutes.

[0109] Finally, 5.0% of tanning acrylic resin, specifically sodium polyacrylate resin, was added to the bath of Tests I and II, and then they were left to rotate overnight (8 hours).

[0110] The next morning, the tanning baths were drained and a washing with 100% cold water and 1.0% sodium acetate, to clean the hides from any residues of the products used, was performed.

[0111] The hides, wrapped in nylon bags to avoid excessive drying, were then made to rest for a few hours, after which their Tg was measured (contraction temperature, according to UNI EN ISO 3380: 2015).

[0112] A rest under high humidity is needed by hides mainly to homogenize the absorbed chemicals, allowing them to evenly distribute throughout the hide thickness. In this period of time, the completion of chemical reactions, which actually exhaust any residual chemicals that are not yet fixated in the collagen, is allowed.

[0113] All of the above percentages were based on the weight of pickled hides, drained from the pickling bath.

[0114] Tg results (measured three times per each hide) for the three tests were the following: I) Tg= 82, 82, 83°C - Average value: 82°C; II) Tg= 78, 79, 77°C - Average value: 78°C;

[0115] A first observation on these extremely positive test results is based on the consideration that typical Tg values for chromium tanning are of 100°C, a maximum value of 80°C for vegetal tanning (natural tannins), and values typically ranging from 70 to 77°C for aldehydes tanning, whereby it is apparent that the Tg values ranging from 78°C to 82°C obtained with OMW, both containing and not containing acrylic resin, have shown to be broadly in line to those obtainable with other chromium-free tannings, sometimes even superior.

[0116] The use of OMW as a tanning adjuvant allow then to obtain extremely high-performing tanning baths.

[0117] The best Tg values are obtained in Test I, wherein the acrylic resin polymerized in OMW is also present, showing, as in the previous cases, the OMW adjuvant power in the tanning step, when used with other traditional Chromium-free tanning agents.Example 4: Evaluation of the tanning capacity of OMW compositions comprising various amounts of acrylic resins

[0118] The Chambort-Jamet standardized method, as detailed in Example 2 above, was used for the evaluation of the tanning capacity of acrylic resins mixed with OMW. The measurement of the tanning power then results from the determination of the dry matter content of the tanning agent absorbed by the standard hide.

[0119] The test was performed by preparing tanning solutions having all the same initial concentration of dry matter of about 4 g / L.Test 1

[0120] For this test, two different solutions comprising OMW and sodium polyacrylate resin (PA) at two different concentrations were prepared, namely: Mixture 1 - prepared by stirring at room temperature, using a common mechanical or magnetic stirrer, 80% (w / w) of PA with 20% (w / w) of OMW; Mixture 2 - prepared by stirring at room temperature, using a common mechanical or magnetic stirrer, 42% (w / w) PA with 58% (w / w) of OMW.

[0121] The characteristics of the OMW used are shown in Table 5 below. Table 5Sample OMW 1 Characteristics pH4.68Suspended Solids (mg / L)69,025Dry Residue (%)9.09Total Polyphenols (mg / L)2,058.33Hydroxytyrosol (mg / L)514.46Tanning Power (%)79,03

[0122] The characteristics of the sodium polyacrylate resin used are instead listed in Table 6 below. Table 6Resin PA Characteristics Dry Residue (%)30.08pH at 10%5.69Brook Visc. (cPs)8,020Total Polyphenols (mg / L)0Hydroxytyrosol (mg / L)0Tanning Power (%)11.47

[0123] Mixtures 1 and 2, and for comparison the PA resin alone, were used to tan a hide sample to determine also the Tg contraction temperature, according to UNI EN ISO 3380: 2015 method.

[0124] Specifically, the tanning tests were carried out on pickled hides in a jar mill, acidified until the pH of the tanning bath was at 2.8-2.9. The tests were carried out by directly adding a 5% concentration of tanning agents to the pickle bath, and it was made to rotate overnight (8 hours).

[0125] The next morning, the tanning baths were drained and a washing with 100% cold water and 1.0% sodium acetate was performed to clean the hides from any residues of the products used.

[0126] The tests results, listed in Table 7 below, were compiled by comparing a series of properties of interest, such as the tanning power and the Tg value, obtained with sodium polyacrylate resin (PA) alone and with the two Mixtures 1 and 2. Table 7CharacteristicsPAMixture 1Mixture 2Dry Residue (%)30.0825.8817.90pH at 10%5.695.424.95Visc. Brook (cPs)8,0204,2321,274Total Polyphenols (mg / L)04121,164Hydroxytyrosol (mg / L)0103198Tanning Power (%)11.4721.5337.20Tg (°C)677072

[0127] According to these tests, the use of OMW in admixture with sodium polyacrylate resin produces a very significant increase in both tannin power and Tg, compared to what observed for the resin alone. In particular, Mixture 2, characterized by a higher amount of OMW, appears to exhibit the best performance.

[0128] This is an interesting aspect in view of reducing the amount of PA resin in favor of OMW, with significant repercussions both in terms of cost and reduction of environmental impacts related to both the disposal of OMW and the production of PA resin.Test 2

[0129] In order to prepare a more concentrated mixture of acrylic resin and OMW, especially in terms of polyphenols, as it is typically required in the tanning industry, two further mixtures were prepared. Mixture 3 - prepared by stirring at room temperature, using a common mechanical or magnetic stirrer, 30% (w / w) of PA with 70% (w / w) of OMW; Mixture 4 - prepared by stirring at room temperature, using a common mechanical or magnetic stirrer, 20% (w / w) PA with 80% (w / w) of OMW.

[0130] Both mixtures were concentrated on a rotary evaporator at 70°C and 500 mbar until a dry residue of about 40% was obtained.

[0131] Similarly to the previous test, hides were tanned using Mixturex 3 and 4 following the same method, in order to determine the Tg.

[0132] The results of treating the hides with these Mixtures 3 and 4 are shown in Table 8 below, always as a comparison with the results obtained by treatment with the PA resin alone. Table 8CharacteristicsPAMixture 3Mixture 4Dry Residue (%)30.0840.2041.45pH al 10%5.695.575.23Visc. Brook (cPs)8,02032,74644,436Total Polyphenols (mg / L)0479610,290Hydroxytyrosol (mg / L)01,1992,073Tanning Power (%)11.4768.5982.27Tg (°C)677577

[0133] Also in this case, the use of OMW in admixture with sodium polyacrylate resin produced a remarkable increase in tanning power of the solution, and Tg of the hide, which was maximum in the case of Mixture 4 at higher OMW content. From these tests, it can therefore be inferred that OMW have largely demonstrated the ability to increase the tanning power of traditional tanning agents, thus enabling a drastic reduction in the quantities employed in the process, with a significant reduction in costs and environmental impact associated both with OMW disposal and production of chemicals.Test 3.

[0134] Two further OMW and resin mixtures, Mixture 5 and Mixture 6, were prepared, wherein this time the sodium polyacrylate resin was obtained by direct radical polymerization in the same OMW (used as a polymerization medium).

[0135] In particular, two different OMW, referred to as OMW1 and OMW2, were used, whose characteristics are listed in Table 9 below Table 9CharacteristicsOMW 1OMW2pH4.684.71Suspended Solids (mg / L)69,0255,893Dry Residue (%)9.093.13Total Polyphenols (mg / L)2,058.335,283.33Hydroxytyrosol (mg / L)514.461,834.70Tanning Power (%)79.0347.73

[0136] The solution of resin in OMW so obtained, referred to as Mixture 5 (wherein the resin was obtained by direct polymerization in OMW1) and Mixture 6 (wherein the resin was obtained by direct polymerization in OMW2), were used to determine their tanning capacity and to tan hides in order to determine the Tg, according to the methods previously described.

[0137] The results of treating the hides with these Mixtures 5 and 6 are shown in Table 10 below, always as a comparison with the results obtained by treatment with the PA resin alone. Table 10CharacteristicsPAMixture 5Mixture 6Dry Residue (%)30.0838.2041.00pH al 10%5.695.435.43Visc. Brook (cPs)8,0205,0303,486Total Polyphenols (mg / L)011,06712,083Hydroxytyrosol (mg / L)01,8942,861Tanning Power (%)11.4721.0435.99Tg (°C)677072

[0138] These tests also showed an excellent performance, in terms of tanning power, of OMW mixtures, this time characterized in that the resin was obtained by direct polymerization in the OMW.

[0139] This evidence is very important as it allows to hypothesize a reduction in the number of processes required to obtain tanning mixtures of OMW with resins, given the possibility of carrying out the polymerization directly in the same OMW, by reducing both the production costs and the environmental impacts linked to resin preparation.Example 5: Evaluation of the antioxidant capacity of OMW to prevent the formation of Chromium VI in chromium tanned leather

[0140] This test was performed using chromium tanned wet-blue leather, and comparing the results obtained using a standard retanning recipe, employing as a vegetal tannin TARA tannin, and as a fatliquoring agent sulphited fish oil, with the results obtained by using a retanning step wherein the treatment water bath was replaced by OMW (150% based on the weight of the leather) and the vegetable tannin tara was eliminated, in the presence or not of the acrylic resin sodium polyacrylate (PA) obtained by a polymerization process carried out in the same OMW. The whole process of treating the leather is described in Table 11 below.

[0141] In particular, the retanning step was carried out on chromium tanned leather (wet-blue), by developing a typical recipe to obtain a vegetalized-type product, where the leather aspect looks similar to vegetable-tanned leather.

[0142] In the retanning process, sulphited fish oil was used during the fatliquoring step, which, given the high content of unsaturated molecular sites, is a strong promoter of radical ionization processes, triggered by environmental conditions such as temperature and humidity, which lead to mobilization of Chromium III, normally fixated into the collagen fibers of the leather, by oxidation into Chromium VI, which is released and can be determined by spectrophotometry, according to the international method UNI EN ISO 17075. Table 11Process Steps Standard retanning Retanning with con OMW Retanning with OMW + sodium polyacrylate tanning resin Soaking and initial washing200% Water at 35°CThe same as in standard retanningThe same as in standard retanning0.5% Nemolix HH (degreasing agent)1.0% Oxalic acidRotate for 30', then drainWash with 150% water at 35°CRotate for 5', then drainInitial detanning (preparation for retanning); a small amount of chromium fixed to the leather is slightly removed in order to uncover the surface of the leather and to optimally prepare it for penetration of the retanning agents150% water at 35°CThe same as in standard retanningThe same as in standard retanning1.0% Coriamine AS (anti-tearing)Rotate for 10'2.0% Dermoclean (detanning agent)0.5% Sodium formateRotate for 40', then drainWash with 200% cold waterRotate for 5', then drainRetanning30% Cold water30% OMW30% OMW3.0% Suppletan ADF (pretanning agent)3.0% Suppletan ADF (pretanning agent)3.0% Suppletan ADF (pretanning agent)Rotate for 20' 6.0%Rotate for 20'Rotate for 20'Tara (vegetal tannin)5.0% Sodium polyacrylate tanning resin3.0% Suppletan MLR (synthetic tannin)3.0% Suppletan MLR (synthetic tannin)3.0% Suppletan MLR (synthetic tannin)3.0% Coriol X6 (synthetic tannin)3.0% Coriol X6 (synthetic tannin)3.0% Coriol X6 (synthetic tannin)2.0% Filler FD / 10 (Filler)2.0% Filler FD / 10 (Filler)2.0% Filler FD / 10 (Filler)5.0% Dihydroxy diphenyl sulphone (synthetic tannin)5.0% Dihydroxy diphenyl sulphone (synthetic tannin)5.0% Dihydroxy diphenyl sulphone (synthetic tannin)Rotate for 20'Rotate for 20'Rotate for 20'40% Cold water40% Cold water40% Cold water4.0% Suppletan R40 / LT (retanning resin)4.0% Suppletan R40 / LT(retanning resin)4.0% Suppletan R40 / LT (retanning resin)Rotate for 20'Rotate for 20'Rotate for 20'II° retanning step (needed for vegetalized leather)Tara (vegetal tannin) 6.0%3.0% Suppletan MLR (synthetic tannin)5.0% Sodium polyacrylate tanning resin3.0% Suppletan MLR (synthetic tannin)3.0% Coriol X6 (synthetic tannin)3.0% Suppletan MLR (synthetic tannin)3.0% Coriol X6 (synthetic tannin)2.0% Filler FD / 10 (Filler)3.0% Coriol X6 (synthetic tannin)2.0% Filler FD / 10 (Filler)5.0% Dihydroxy diphenyl sulphone (synthetic tannin)2.0% Filler FD / 10 (Filler)5.0% Dihydroxy diphenyl sulphone (synthetic tannin)Rotate for 3 hours, then automatic rotation overnight (Rotate for 10' each hour)5.0% Dihydroxy diphenyl sulphone (synthetic tannin)Rotate for 3 hours, then automatic rotation overnight (Rotate for 10' each hour)Rotate for 3 hours, then automatic rotation overnight (Rotate for 10' each hour)Next morning, wash to remove residues of products not completely absorbed by leather80% Water at 40°CThe same as in standard retanningThe same as in standard retanning0.5% Formic acidRotate for 10', then drain100% Water at 40°C Rotate for 5', then drainFatliquoring100% Water at 45°CThe same as in standard retanningThe same as in standard retanning2.0% CORIOL 727 N (Ester to penetrate throughout thickness)4.0% CORIOL SPR (Sulphated Oil from spermaceti substitutes)5.0% CORIOL D / OR (Sulphited fish oil)2.0% Solidwax 298 / B (Wax) Rotate for 60', then drain and coolWashing and rest on a horse200% Cold water Rotate for 5', then drain 200% Cold water Rotate for 5', then drain RestThe same as in standard retanningThe same as in standard retanning

[0143] The leather thus obtained were left to rest for one day, so as to allow uniform distribution of the products throughout the leather thickness; a number of drying operations were then carried out, involving the use of a vacuum machine, a hot roller machine, and then set out in overhead conveyor for further drying. In this case, however, given that for Chromium VI development the leather would have been subjected to severe humidity and heat conditions, the set out in overhead conveyor was carried out only for half a day, followed by the conditioning procedure for Chromium VI tests.

[0144] In order to aid the formation of Cr VI , the leather obtained was submitted to the following conditioning processes: Conditioning 1: 24 h under UV irradiation 30 cm away from the source (Iron halogenide UV lamp, JELOSIL HG200 L, emitting a 300 W / m 2< , power in the range 320 - 400 nm), T 30°C, relative humidity 55%; Conditioning 2: 24 h in climatic chamber at 60°C, relative humidity 90% (more stressful condition).

[0145] Determination of Cr VI in the leather samples was carried out according to the UNI EN ISO 17075: 2008 method. The results are shown in Table 12 below. Table 12Bath Type Cr VI Concentration (mg / kg leather) Conditioning 1Conditioning 2Standard retanning6.315.68Retanning in 100% OMW bath without TARA0.290.16Retanning in 100% OMW bath without TARA, but with sodium polyacrylate tanning resin0.170.33

[0146] As it can be inferred from Table 12, OMW, both in the presence and in the absence of tanning acrylic resin sodium polyacrylate, excellently inhibited the formation of Chromium VI, in both experimental conditions, thus demonstrating to be highly effective antioxidants for use in the retanning step of the leather tanning process.Example 6: Evaluation of the Cr (VI) abatement capacity in aqueous solution by means of tanning resins polymerized in OMW, with different antioxidant properties.

[0147] This test was carried out using three different types of resins (Resin 1, Resin 2, and Resin 3) obtained by direct polymerization in OMW; the characteristics of these three resins are shown in Table 13 below. Table 13Resin Characteristics Resin 1 Resin 2 Resin 3 Dry (%)35.7035.0633.00pH at 10%5.254.705.04Brook Visc. (cPs)2,8623,1803,547Polyphenols (mg / L) on TQ21,58333,30023,647Antiox. power (mg GA / L)6,89422,31019,490

[0148] To a solution of K 2 Cr 2 O 7 having a Cr(VI) concentration of about 30 mg / L different amounts of the three resins were added, performing multiple tests, so that their initial concentration in said potassium bichromate aqueous solution was of 10%, 5%, and 1% (w / w).

[0149] Subsequently, the percentage of Cr(VI) content abatement, referred to as %red, was assessed, for each test, 1, 3, and 4 hours after the addition of Resins 1, 2, and 3, having initial concentrations of 10, 5, and 1%, to the potassium bichromate solution.

[0150] The results obtained from these tests are shown in Table 14 below. Table 14Resin Resin Conc. (%) After 1h (%red) After 3h (%red) After 4 h (%red) 1 10 1001001002 10 1001001003 10 1001001001 5 92.691001002 5 1001001003 5 1001001001 1 25.6931.4333.192 1 0003 1 47.256.765.49

[0151] As it is apparent from the results shown in Table 14, all resins (1, 2, and 3) polymerized in OMW showed a very high reducing capacity.

[0152] The results further showed that, by using resins having high antioxidant power greater than about 20,000 mgGA / L, as with resin 2, even resin concentrations below 1% are sufficient to achieve a complete reduction of Cr (VI) to Cr (III).

Claims

1. A use of olive mill waste waters (OMW) in at least one step of the leather tanning process in the tanning industry.

2. The use according to claim 1, wherein the olive mill waste waters (OMW) have a pH in the range from 2 to 8, a suspended solids content in the range from 10,000 to 100,000 mg / L, a dry residue in the range from 1 to 12% by weight, a total phenols content (as determined according to Folin Ciocalteau method, expressed as gallic acid mg equivalents / L OMW) in the range from 100 to 15,000, and a hydroxytyrosol content (as determined by HPLC-UV DAD) in the range from 25 to 5,000 mg / L.

3. The use according to claim 1 or 2, wherein the olive mill waste waters are olive mill waste waters previously subjected to stabilization methods by treatment with nitrogen.

4. The use according to any one of claims 1 to 3, wherein said use of olive mill waste waters is as a bactericide in at least one step of the leather tanning process in the tanning industry.

5. The use according to claim 4, wherein said step of the leather tanning process is the soaking step.

6. The use according to claim 5, wherein the olive mill waste waters (OMW) are used in an amount from 50 to 100% by weight based on the total leather treatment water, preferably in an amount from 75% to 100%, even more preferably in an amount of 100%.

7. The use according to claim 4, wherein said step of the leather tanning process is the tanning step.

8. The use according to any one of claims 1 to 3, wherein said use of olive mill waste waters is as a tanning agent, or a tanning adjuvant agent, in the tanning step of the leather tanning process in the tanning industry.

9. The use according to claim 8, wherein the olive mill waste waters (OMW) are used in an amount from 50 to 100% by weight based on the leather treatment water, preferably in an amount from 75% to 100%, even more preferably in an amount of 100%.

10. The use according to claim 8 or 9, wherein the olive mill waste waters (OMW) are used in combination with other tanning agents selected from the group consisting of a natural tannin, such as tara, mimosa, chestnut, myrobalan, a synthetic tannin, such as hydroxydiphenyl sulphone, a compound belonging to the family of phenolic resins, a compound belonging to the family of naphthalene resins, an aldehyde or a precursor thereof, such as glutaraldehyde, isoxazolidine, oxazolidine, a metal salt, such as a chromium, zirconium, or titanium salt, and an acrylic resin, such as a resin obtained by homopolymerization or radical copolymerization of acrylic acid, such as sodium polyacrylate, methacrylic acid, methyl methacrylate ester, ethyl acrylate ester, acrylamide, preferably salified, such as sodium polyacrylate (PA), and mixture thereof.

11. The use according to claim 10, wherein the olive mill waste waters (OMW) are used in combination with sodium polyacrylate (PA).

12. The use according to claims 10 to 11, wherein said acrylic resin is obtained by homopolymerization or radical copolymerization carried out in the same olive mill waste waters (OMW), as a reaction medium.

13. The use according to any one of claims 8 to 12, wherein the step of the leather tanning process in the tanning industry is a chromium-free tanning step.

14. The use according to anyone of claims 1 to 3, wherein said use of olive mill waste waters is as an antioxidant agent to prevent the formation of chromium (VI) in the retanning step.

15. The use according to claim 14, wherein the olive mill waste waters (OMW) are used in an amount from 50 to 100% by weight based on the total leather treatment water, preferably from 75% to 100%, even more preferably in an amount of 100%.

16. A tanning composition comprising olive mill waste waters (OMW), one or more tanning agents selected from the group consisting of a natural tannin selected from the group consisting of: tara, mimosa, chestnut, and myrobalan, a synthetic tannin, such as hydroxydiphenyl sulphone, a compound belonging to the family of phenolic resins, a compound belonging to the family of naphthalene resins, an aldehyde or a precursor thereof, such as glutaraldehyde, isoxazolidine, oxazolidine, a metal salt such as a chromium, zirconium, or titanium salt, and an acrylic resin, such as a resin obtained by homopolymerization or radical copolymerization of acrylic acid, such as sodium polyacrylate, methacrylic acid, methyl methacrylate ester, ethyl acrylate ester, acrylamide, preferably salified, such as sodium polyacrylate (PA), and mixture thereof, and suitable additives.

17. The composition according to claim 16, wherein the olive mill waste waters (OMW) are used in combination with sodium polyacrylate (PA).

18. The composition according to any one of claims 16 to 17, wherein said acrylic resin is obtained by homopolymerization or radical copolymerization carried out in the same olive mill waste waters (OMW), as a reaction medium.

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