RELATED PLANT FIELD OF THE INVENTION

The present invention concerns a process for the conversion of the exhausted digestate fraction from end-stage anaerobic digestion into a substrate for further biochemical degradation to produce biogas in the frame of the existing biomethanation processes. More precisely, the invention concerns a thermal process for converting the exhausted digestate fraction from end-stage anaerobic digestion into a carbon- enriched bio-oil.

The invention lies in the field of energy production, through anaerobic digestion of vegetable- and/or animal-derived biomass, committed to the production of high- energy content biogas (biomethanation processes).

BACKGROUND OF THE INVENTION

Biomethanation is known to be a process for converting vegetable- and/or animal- derived organic waste into biogas and manure. This conversion occurs through the use of anaerobic microorganisms that decompose the biodegradable wastes in absence of oxygen.

At the current state of the art, the degradation of organic materials by the action of anaerobic bacteria results in the production of methane and carbon dioxide. Typical organic materials that undergo anaerobic digestion are municipal solid waste (MSW), animal manure, wastewater and organic industrial effluents.

Many organic molecules, such as carbohydrates, proteins, lipids and cellulose are totally or partially biodegraded to gaseous products, such as methane or carbon dioxide, and also into molecules forming microbial cells.

Upon decomposition of the organic matter, it is generally found that a fraction of the digestate, also said inactive digestate fraction or digestate sludge, has not been consumed and represents the main by-product at the end of the process.

The digestate sludge is usually composed of 60 - 80% water by weight, and undigested organic matter, for example lignin and derivatives thereof. In such inactive digestate fraction, traces of phyto-sanitary products are also found, as well as pesticides and other chemicals derived by the agricultural activities for the production of silage and the feeding mixtures for cattle or other animals, which manure is used to produce biogas through anaerobic digestion.

The waste sludge is disposed of different known technologies. A conventional approach consists in the separation of the liquid and solid fractions of the raw sludge, from which the obtained dry matter could be further processed by means of composting or dried for subsequent incineration. The separation is mainly carried out using screw-press separators, filter-press separators or through decantation.

The resulting liquid fraction can be further purified from solid mass residues by means of ultrafiltration or air floatation, while nitrogen stripping and reverse osmosis may be applied to obtain chemicals with high appeal in the context of fertilizers manufacturing.

A problem of the known processes is that they often imply high investment costs and significant energy demand, thus often being not suitable for entities deeply integrated into the energy production chain. Furthermore, the high levels of humidity that are typical of the digestate sludge, require strong dehydration prior to the application of conventional energy valorization approaches, such as incineration and pyro-gasification, which are then strongly discouraged in the frame of digestate disposal.

In consideration of the average size of biomethanation plants (up to 500 m ³ per day), the digestate waste disposal is often associated with critical environmental risks due to the high pollutants concentrations within the inactive digestate fraction.

Environmental hazards arise from the improper dumping of digestate sludge into plots of land that are usually pledged to the production of silage earmarked for direct feed into the anaerobic digestion systems and/or livestock feeding. In consideration of the large amount of digestate generated by biomethanation processes, the disposal of this waste sludge through improper dumping leads to bioaccumulation of pollutants within the soil, resulting into a scenario where remediation may not be practicable.

At the same time, leaking of digestate from the digestate storage tanks and from the digester tanks is responsible of another layer of risk pertaining the long-term usage of anaerobic digestion systems.

It is therefore necessary to devise a process of conversion of the digestate sludge which can overcome at least one of the drawbacks of the known art.

In particular, a scope of the present invention is that of providing a process of conversion of the exhaust digestate fraction from an end-stage anaerobic digestion which allows to convert the digestate in an efficient manner.

Another scope of the present invention is that of providing a process which allows to convert the exhaust digestate in a substrate which can be used for further applications. A further scope of the present invention is that of providing a process that enables to increase the biogas yield of biomethanation without direct modification of the pre existing digester systems.

Another scope of the present invention is that of providing a process which allow the disposal of the digestate sludge with consequent abatement of related management costs.

The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages. SUMMARY OF THE INVENTION

The present invention is set forth and characterized in the independent claims. The dependent claims describe other characteristics of the present invention or variants of the main inventive idea.

In accordance with the aforesaid objectives and purposes, the present invention concerns a process for the conversion of exhausted digestate fraction from an end- stage anaerobic digestion which provides a one-step combination of sub-critical water extraction and selective hydrothermal liquefaction, advantageously carried out at a low temperature.

According to one aspect of the present invention, the process comprises a conversion step during which the exhaust digestate fraction is made to react with sub- critical water in a gas-tight vessel, at a set point temperature of 100°C or more, i.e. at a set point temperature which is equal to or greater than the water boiling temperature. Preferably, the heating step is made also in presence of air inside the vessel. In accordance with embodiments, the water is present at a total concentration of 5%- 80% in volume with respect to the volume of the vessel. With total concentration, it is intended the concentration of water in both the liquid phase and the vapor phase.

Due to the reaction temperature being higher than boiling temperature of water, evaporation thereof generates pressure in the containing vessel, which in turn allows to maintain the water in its liquid state throughout the entire reaction step. This particular state of water is referred to as sub-critical water. The idea at the basis of the invention is that of heating the digestate sludge in a gas-tight environment in presence of sub-critical water, this latter being used as a strong oxidizing agent.

Through this process, the inactive digestate fraction remaining upon biomass degradation at the end-stage of the anaerobic digestion processes is converted into a mixture of organic compounds (bio-oil), composed of organic molecules that have been broken down through low-temperature oxidation in presence of oxygen gas from sub-critical water, and preferably of oxygen gas from air.

The resulting bio-oil acts as an active substrate for biomethanation processes leading to the production of additional high-value biogas fuel with no need to modify the already existing anaerobic digestion plants or vary the composition of the original recipe.

In accordance with embodiments, the reaction is carried out at a pH comprised between 0.1 and 20.

Preferably, the process is carried out in presence of one or more catalysts that permit the molecular breakdown of partially biodegraded components composing the exhausted digestate fraction of biomethanation processes. Such catalysts are selected amongst ammonia, organic compounds containing nitrogen, hydroxides of alkaline metals, alkaline earth and salts derived from the latter, oxides of metals and transition metals, and their combinations.

More preferably the concentration of the catalysts ranges from 0.001% to 100% in weight with respect to the weight of the dry organic precursor mass, i.e. of the exhaust digestate fraction mass.

When a catalyst as above is present, the reaction is carried out in an alkaline environment, i.e. pH is 7 or more.

According to embodiments, the heating step is carried out at a set point temperature comprised between 100°C and 500°C, preferably between 100°C and 400°C, more preferably between 100°C and 374°C.

Preferably, the conversion reaction is carried out at a pressure comprised between 0.1 and 40 MPa.

Advantageously, the treatment time varies from 1 minute to 24 hours when the set point temperature has been reached.

According to possible solutions, prior to the conversion step, the reactants are heated at a predetermined heating rate to reach the set point temperature for the conversion reaction. Preferably, the heating rate is comprised between 0.05 and 20°C/min, more preferably between 0.1 and 10°C/min.

In the above conditions, the yield, considered as the weight percentage of newly synthesized liquid organics in respect to the original dry mass weight, is found to be between 20% and 90%.

ILLUSTRATION OF DESIGNS

These and other features of the present invention will become apparent from the following description of some embodiments provided by way of non-re strictive example, with reference to the accompanying drawings wherein:

- Fig. 1 is a schematic representation of a digestion plant suitable for carrying out the process; and

- Fig. 2 is a schematic representation of a digestion plant as of fig. 1, according to an alternative variant.

To facilitate understanding, the same reference numbers have been used, where possible, to identify identical common elements in the figures. It is understood that elements and characteristics of one embodiment can conveniently be incorporated into other embodiments without further clarifications.

DETAILED DESCRIPTION OF SOME EMBODIMENTS It will be now referred in detail to the possible embodiments of the invention, one or more examples of which are illustrated in the attached drawings as examples and not as limitations. Phraseology and terminology used here is also for description purposes and shall not be considered as limiting.

In fig. 1 is represented a plant 10 for anaerobic digestion of vegetable- and/or animal-derived biomass suitable for carrying out the process of the invention.

The plant 10 comprises a biodigestor 20 wherein the anaerobic digestion is performed. As is known, such anaerobic digestion is carried out by means of anaerobic microorganisms which degrade biomass, such as animal manure, silage and other types of biomass, and produce biogas 21, containing generally carbon dioxide CO2 and methane CFL, with high calorific value (more than 50% of methane).

Due to the incomplete digestion of the biomass, alongside the biogas 21, which is the desired product of the digestion, a fraction of undigested biomass is produced, denominated exhaust digestate or digestate sludge 22, and composed of water and undigested organic matter as mentioned above, in the prior art section. Such exhaust digestate 22 is the organic precursor to be converted through the process of the invention.

In accordance with the embodiment of fig. 1, the exhaust digestate 22 is taken to a homogenization unit 30 in order to undergo a mixing, grinding and homogenization, as well as a pre-heating prior to the thermal treatment provided by the invention. As will be explained in the following, such step may be avoided provided the organic precursor 22 is sufficiently homogenized as retrieved from the biodigestor 20.

The plant 10 also comprises a catalyst reservoir 40 to contain the one or more catalysts to be used for the conversion reaction. The catalysts for the process according to the invention may be, in general, any compound that, when solubilized in water, can generate alkaline environment. Examples of catalysts will be given in the following.

The catalyst is to be loaded in the reservoir 40 and dosed into the homogenization unit 30.

In an alternative configuration, represented in fig. 2, the catalyst reservoir 40 is linked directly to a hydrothermal liquefaction unit 50, comprised in the plant 10, and wherein the heating step in presence of sub-critical water is to be performed. Such hydrothermal liquefaction unit 50 is connected to the homogenization unit 30, more precisely to the exit of the homogenization unit 30, so that it can receive the substrate coming from the homogenization unit 30. The hydrothermal liquefaction unit 50 comprises a gas-tight vessel of any type provided it can be sealed, and it can withstand temperatures above 100°C, in particular temperatures ranged between 100°C and 500°C for periods of time ranging from 1 minute to 24 hours, at a pressure comprised between 0.1 and 40 MPa. Preferably the vessel is also chemically resistant to the catalyst species. The plant 10 also comprises heating means configured to heat at least the hydrothermal liquefaction unit 50 and, if present, the homogenization unit 30.

The output of the hydrothermal liquefaction unit 50 is a crude bio-oil obtained from the heating step of the process in accordance with the invention. The obtained bio-oil has a gas fraction and a liquid fraction. This latter mainly comprises low molecular weight compounds such as short organic acids (formic acid, lactic acid, acetic acid) and their relative structural isomers. It is also found other components such as ketones, aldehydes, aromatics, alcohols, partially degraded fatty acids, amino-acids and by-products of their oxidation and/or condensation.

The gas fraction mainly comprises carbon dioxide CO2 and short-chain volatile hydrocarbons as methane, ethane or the like. Such crude bio-oil, which is digestible by anaerobic microorganisms, is then recirculated in the plant 10 towards the biodigestor 20, to which the hydrothermal liquefaction unit 50 is connected (fig. 1).

Between the hydrothermal liquefaction unit 50 and the biodigestor 20, the plant 10 comprises a catalyst recovery unit 60 wherein the catalyst is extracted from the bio oil exiting from the vessel 50.

The extracted catalyst is then recovered as will be explained in the following, and fed towards the catalyst reservoir 40, to which the catalyst recovery unit 60 is connected (fig. 1). The crude bio-oil 70 exiting from the catalyst recovery unit 60 is then deprived of catalyst, and is recirculated towards the biodigestor 20, as shown in the representation of fig. 1.

It is noted that, in case the catalyst is not contaminated, or made poisonous, during the biomass conversion in the vessel 50, it can be recirculated as well to the biodigestor 20 and salvaged from the exhaust digestate fraction to be used again in the hydrothermal liquefaction unit 50.

The conversion process of the invention, carried out in a plant 10 as described above, takes place after an anaerobic digestion of biomass in the biodigestor 20. As said above, the biodigestor 20 outputs biogas 21, which is the desired reaction product and is fed to another system in order to be used or further treated, and an inactive exhaust digestate 22, which is a side product of the anaerobic digestion.

The exhaust digestate 22 is recirculated in the plant 10, wherein it is used as organic precursor for the conversion process.

In particular, the exhaust digestate 22 is preferably sent to the homogenization unit 30, wherein it undergoes a homogenization step. The homogenization step can comprise, for example, mixing and/or grinding, as well as a pre-heating. As noted above, in case the exhaust digestate 22 exiting from the biodigestor 20 is sufficiently homogenized, the homogenization step can be avoided. In such case, the process can be carried out using the plant 10 illustrated in fig. 2. During this step, it is preferably provided to add the catalyst to the exhaust digestate 22, at the homogenization unit 30.

The species to be used as catalyst are chosen in the group consisting in alkaline metal hydroxides, such as NaOH or KOH, in any available form; alkaline earth hydroxides, such as calcium hydroxide, in any available form; alkaline salts derived from the alkaline metal and earth hydroxides reacted with organic acids or other weak acids, such as acetate and carbonate salts of sodium, potassium and any given alkaline metal or alkaline earth, in any available form; ammonia and organic compounds containing nitrogen and preferably having alkaline properties, such as amines and amides, in any available form; and any metal oxide and/or transition metal oxide that can be used alone and/or in combination with the aforementioned catalysts, not necessarily associated with alkaline behavior, like for example titanium(II) oxide, Zinc(II) oxide; or combination of two or more of such species.

In a preferred embodiment of the process, the catalyst species is chosen between sodium hydroxide NaOH, sodium carbonate NaiCCb, or a combination of both. The catalyst is preferably used at a concentration comprised between 0.001% and 100% in weight with respect to the weight of the exhaust digestate 22. When a catalyst is used, the reaction is to be carried out in an alkaline environment.

In case the homogenization step, in the homogenization unit 30, is avoided, the catalyst can be added to the exhaust digestate directly into the hydrothermal liquefaction unit 50 (fig. 2).

It is also noted that the use of a catalyst in the conversion process is optional.

In case no catalyst is used, alkaline environment is neither provided at the start of the process nor during the treatment or at the end-stage of this latter. This leads to variations of the final output and its product distribution. At pH equal or lower than 7 (neutral or acidic), formation of char-like solids, either as a suspension of particles with very limited size (10 to 500 pm) is observed as a direct consequence of molecular condensation pathways being favored over hydrolysis and other chemical conversion routes that would lead to production of liquid components. The char-lie solids are formed as a combination of poly-cyclic and poly-aliphatic units assembled together and partially oxidized by the effect of subcritical water and the airborne oxygen.

A combination of low temperature (100 °C to 180 °C) and low heating rate (below 2 °C/min), together with the lack of a specific catalyst, leads to formation of even larger amount of char-lie materials within the bio-oil and consequent lesser yield of volatile organic acids, ketones, aldehydes and alcohols.

After the homogenization step, the exhaust digestate 22, together with the catalyst, is fed to the gas-tight vessel of hydrothermal liquefaction unit 50, to carry out the conversion reaction. At the time the exhaust digestate 22 is introduced into the vessel, this latter is not at the set point temperature for the conversion step. The vessel is therefore sealed, and subsequently it is heated at a predetermined heating rate, to be taken to the set point temperature.

It is to be noted that the conversion reaction is a one-step reaction, wherein the exhaust digestate 22 and the catalyst, if present, are heated at a temperature above 100°C, preferably comprised between 100°C and 500°C, more preferably between 100°C and 400°C, even more preferably between 100°C and 374°C in presence of water, while kept enclosed within a confined gas-tight environment inside the sealed vessel. The water can be derived from the natural humidity of the exhaust digestate 22 (typically ranging from 20% to 90% of the weight of the digestate 22), or can be added to the same from external sources.

The water is preferably at a concentration comprised between 5% and 80% by volume with respect to the volume of the vessel.

The applied temperature, which provokes the evaporation of water, generates an increase of pressure inside the vessel 50, which can range between 0.1 and 40 MPa. Due to this increase pressure, a sub-critical phase is obtained wherein water is still liquid above its boiling point, and a part of it is present in form of compressed steam.

At any time, water must be present in any form, being that compressed steam or subcritical liquid, and should not be lower than the 1% of the total confinement volume (defined as the pressure vessel total volume); water quantity is allowed as high as the 100% of the total available volume, and the filling amount is directly related to the pressure generated at a given temperature, with increasing overall pressure at increasing loading of water.

In a variant of the process, it is possible to apply an additional external pressure to further increase the pressure inside the vessel.

In particular, additional pressure can be generated on the system by feeding compressed air prior or during the heating process, as well as once the set point reaction temperature is achieved. Depending on the reaction temperature, compressed air should account for a pressure between 0.1 and 10 MPa in addition to the autogenous pressure derived from heating the water medium in the restraint environment inside the vessel 50.

The presence of additional oxygen provided through compressed air has an effect on the production of components of the bio-oil and distribution of products phase. An increased amount of gaseous products is observed as long as the oxygen volumetric concentration within the reaction environment is above 20%, with increase in the production of short chain organic acids such as acetic acid, lactic acid a formic acid (from 0.1% to 95% in comparison to the reaction performed at atmospheric oxygen concentration). In such case, oxidation reactions occur at higher extent, independently on the volume of water that is present within the reaction environment. The volumetric concentration of methane and other very volatile hydrocarbon compounds (e.g. ethane, propane, butane and their respective isomers) at the end of the process, expressed in consideration of the free headspace volume within the pressure vessel, increases from low percentage (0.1% - 1.5%) to higher percentage (1.5% - 25%). The pH of the reaction environment can be comprised between 0.1 and 20. In case a catalyst as indicated above is used, the reaction as to be carried out at an alkaline pH, i.e. a pH above 7.

As already indicated, such conversion step outputs a bio-oil, the composition of which varies depending upon the conditions of the conversion step, in particular the reaction temperature and the heating rate.

Varying these two parameters has an effect on the ratio of solid, liquid and gaseous products generated by the conversion of the organic precursor 22 into bio-oil. Phase streams are directly related to the heating rate rather than temperature, due to the prevalence of secondary reactions at non-optimum heating rates and possible formation of char, with major effects observed in absence of strong alkaline environment.

On the basis of the total yield of highly carbon- and hydrogen-enriched compounds, it has been found that an optimum heating rate is comprised between 0.05 and 20°C/min, preferably between 0.1 and 10°C/min. The conversion step is performed for a duration time comprised between 1 minute and 24 hours at set point temperature.

It has been observed that short residence time (comprised between 1 minute and 60 minutes) increases yields of liquid components in the form of digestible organic compounds for the anaerobic digestion and biomethanation of the organic substrate. On the contrary, longer residence time (above 1 hour, till 24 hours) is associated with the production of gaseous by-products including carbon dioxide and short-chain volatile hydrocarbons as methane, though in limited percentage with respect to the total mass yield (from 0.1 to 10%).

Through this process, the inactive digestate 22 derived from anaerobic digestion of biomass is converted into a mixture of organic compounds, which act as an active substrate for the production of high-value biogas fuel through anaerobic digestion.

The original molecules of the digestate 22 are broken down through low- temperature oxidation in presence of both oxygen gas from air and sub-critical water, which exerts the function of a strong oxidizing agent. In fact, the physical-chemical properties of sub-critical water highly differ from those observed in standard conditions.

The dissociation constant KW of water increases from 10 ¹⁴ to 10 ¹¹ within a temperature range between 100°C and 350°C, which favors basic and acid catalysis mechanisms of hydrolysis due to the greater amount of free ions (H30 ⁺ , OH ) derived from water dissociation.

The dielectric constant (e) of water, as well as other parameters such as viscosity and surface tension decrease with increasing temperature. Therefore, the solubility of organic compounds with more hydrophobic behavior increases in sub-critical water compared to standard conditions, with significant variation at temperature higher than 200 °C.

Long carbon-chain molecules composing the main biomass fraction undergo thermal cracking with removal of oxygen in the form of H2O (dehydration) and CO2 (decarboxylation), leading to production of a liquid mixture (bio-oil) with high hydrogen-to-carbon (H/C) ratio. Presence of oxidizing agents and occurrence of the reaction at alkaline pH (> 7) promotes the multi-step oxidation of partially degraded molecules and formation of low molecular weight compounds such as short organic acids (e.g. formic acid, lactic acid, acetic acid) and their relative structural isomers. Other components that are found in the bio-oil are ketones, aldehydes, aromatics, alcohols, partially degraded fatty acids and compounds derived from protein degradation, such as amino-acids and by-products of their oxidation and/or condensation.

Hydrolytic depolymerization of macromolecules such as peptides, proteins, polysaccharides and others may lead to occurrence of medium- to low- molecular- weight fragments in the bio-oil mixture. The application of alkaline hydroxides (e.g. NaOH, KOH...) is of particular interest, as they execute the two-fold functionality of salification reagents (leading to formation of derivative alkaline salts) and chelating agent for the acidic gaseous by products like carbon dioxide released during the simultaneous decarboxylation processes. Upon reaction with CO2 molecules, the derivative carbonate salts are formed and the overall pH of the bio-oil is lowered in comparison with the original harsh alkaline conditions.

Transition metals and other oligo-elements that were originally present within the digestate sludge 22 are not oxidized due to the very low temperature applied in the treatment; as such, those components are found in the bio-oil and are supplied to the digester microorganisms that are fed with the produced bio-oil to further improve production of biogas and, in particular, methane derived from biomethanation processes.

The catalysts that are left within the bio-oil mixture at the end of the reaction can be recovered, through the catalyst recovery unit 60 as indicated above.

Alkaline metal hydroxides and alkaline earth hydroxides are likely reacted with carbon dioxide released during the molecular degradation of the digestate fraction, due to decarboxylation reactions; as such, it is then converted to the relative carbonate salts that are found at the end of the process within the bio-oil. Catalysts recirculation is carried out, as the carbonate salts are eligible for their use as primary catalysts in the hydrothermal liquefaction process. The use of hydroxides in any of their form as catalyst for the hydrothermal treatment of the digestate is thus correlated to formation of carbonate salts, which have very similar activity on the degradation of the digestate composing molecules and are either added in the digester systems of the paired biomethanation plants or recovered (through decantation or any suitable mean) to be re-used within the process.

An example of recovery of catalyst will be now described with NaOH and/or NaiCCb as catalyst for the exhaust digestate 22 effluent degradation process.

With such catalysts, the resulting bio-oil 70 appears as a saturated or oversaturated solution of alkaline salts (e.g. sodium acetate, sodium lactate, sodium formiate, sodium propionate), which are distributed as both suspensions and solutes in the water phase of said bio-oil. Depending on the product distribution, which is in turn dependent on the catalyst concentration at the start of the reaction, different techniques are suggested to perform efficient separation of these components to be then recirculated within the system.

Sodium hydroxide may be totally or partially converted into carbonate derivatives due to reaction with carbon dioxide produced in-situ, as a result of molecular decarboxylation pathways that can parallelly occur during the hydrothermal treatment. This observation applies as well to any hydroxide compound, being either formed with alkaline metals or alkaline earth metals, giving the respective carbonate salt upon reaction with free carbon dioxide.

Recovery of the catalyst for all those components that appear as a suspension ca be performed via sedimentation and clarifier, and/or via filtration techniques. Sedimentation refers to any physical water treatment process using gravity to remove suspended solids from water. Clarifiers refer to any settling tanks/basins built with mechanical machineries committed to the continuous removal of solids deposited by sedimentation.

In addition, prior to enter the clarifiers units, coagulation and flocculation reagents (e.g. polyelectrolytes and/or ferric sulfate) can be added to cause the clumping of finely suspended particles into larger and denser clusters, called floes, that settle more quickly and stably. Filtration is referred to as any physical operation that separates solid matter and fluid from a mixture by means of a filter medium.

Recovery of the catalyst for all those components that appear as a solution can be performed via precipitation and/or reverse osmosis. Precipitation is referred to as any process where a solid is generated from a solution, which occurs as the concentration of a compound exceeds its solubility, or in any case where parameters such as temperature and pH are modified or where different solvents are mixed together. Reverse osmosis is referred to as any water purification process that uses a partially permeable membrane to remove ions, unwanted molecules and larger particles from water.

In reverse osmosis, an applied pressure is used to overcome osmotic pressure, a colligative property that is driven by chemical potential differences of the solvent.

The invention will now be described with reference to some specific examples, wherein experimental data is provided in support of the described process and its effects on biomass degradation, as well as the yield of the obtained bio-oil. Biological oxygen demand (BOD) and chemical oxygen demand (COD) are reported to express the content of organics fond at the end-stage of the process, which can be used to determine the total concentration of components of the crude bio-oil. EXAMPLE 1

Upon treatment of exhausted digestate fraction of animal manure (cattle) at 180 °C - 220 °C for 30 minutes (heating rate of 4 °C/min), in presence of sodium carbonate (0.5 - 25 wt% in comparison to the initial dry digestate mass), BOD values are increased from 0.11 - 0.56 mg L ¹ to 6900 - 38740 mg L ¹ ; COD values increases from 65.2 - 93.6 g L ¹ to 74.2 - 95.4 g L ¹ . EXAMPLE 2

Upon treatment of exhausted digestate fraction of animal manure (cattle) at 220 °C

- 260 °C for 30 minutes (heating rate of 4 °C/min), in presence of sodium carbonate (0.5 - 25 wt% in comparison to the initial dry digestate mass), BOD values are increased from 0.15 - 0.48 mg L ¹ to 5800 - 41260 mg L ¹ ; COD values increases from 68.2 - 96.7 g L ¹ to 71.0 - 95.2 g L ¹ .

EXAMPLE 3

Upon treatment of exhausted digestate fraction of animal manure (cattle) at 180 °C

- 220 °C for 30 minutes (heating rate of 6 °C/min), in presence of sodium hydroxide (0.5 - 25 wt% in comparison to the initial dry digestate mass), BOD values are increased from 0.25 - 0.38 mg L ¹ to 9500 - 57300 mg L ¹ ; COD values increases from 59.2 - 94.0 g U ¹ to 63.9 - 101.8 g L ¹ .

EXAMPLE 4

Upon treatment of exhausted digestate fraction of silage and other vegetable biomass at 180 °C - 220 °C (heating rate of 4 °C/min), for 30 minutes, in presence of sodium carbonate (0.5 - 25 wt% in comparison to the initial dry digestate mass), BOD values are increased from 0.31 - 0.68 mg L ¹ to 12800 - 84900 mg L ¹ ; COD values increases from 87.2 - 105.0 g L ¹ to 92.6 - 109.2 g L ¹ .

EXAMPLE 5 Upon treatment of exhausted digestate fraction of silage and other vegetable biomass at 180 °C - 220 °C for 30 minutes (heating rate of 6 °C/min), in presence of sodium hydroxide (0.5 - 25 wt% in comparison to the initial dry digestate mass), BOD values are increased from 0.45 - 0.95 mg L ¹ to 15600 - 109000 mg L ¹ ; COD values increases from 107.6 - 123.4 g L ¹ to 104.9 - 119.6 g L ¹ . All tests have been performed from samples with humidity comparable to that of the original exhausted digestate fraction (60 - 80%).

From the examples above it can be seen that treatment of the exhausted fraction according to the process of the invention triggers an increase in both BOD and COD, which reflects an increase in concentration of both organic and inorganic components in the crude bio-oil.

In particular, during the hydrothermal liquefaction step, previously undigested molecules are broken down into smaller entities and oxidized to be mainly converted into organic acids and other compounds that can be used as substrate by the microorganisms in the biodigestor. In the context of biomethanation, for example, high level of BOD indicates that the anaerobic digestion is well sustained by the presence of digestible molecules, meaning production of methane and carbon dioxide from the converted digestate fraction.

It is clear that modifications and/or additions of parts or phases may be made to the process for conversion of exhausted digestate as described heretofore, without departing from the field and scope of the present invention.

In the following claims, the references in brackets have the only scope of facilitating reading and shall not be considered as limitative factors as far as the scope of protection intended in the specific claims is concerned.