Technical field

The present invention relates to a method and a plant for the continuous production of hydrogen (H2) and methane (CH4) from zootechnical effluents.

The term "zootechnical effluents" mainly denotes livestock dejections, or a mixture of litter and dejections, with possible addition of waters resulting from washing of shelters, draining liquids and precipitation water.

Zootechnical effluents are a source of considerable environmental impact related to their high content in nutrients, especially nitrogen (N). Environmental impact mainly affects air (through emissions of ammonia and greenhouse gases) and water (through release of phosphorus and of ammonia nitrogen and nitric nitrogen).

Prior Art

At present, it is known that both energy, mainly in the form of methane and/or hydrogen, and products for the market can be obtained from zootechnical effluents through a process of anaerobic digestion (AD). Co-products of AD, mainly consisting of digestate, actually form an interesting raw matter for obtaining high quality fertilisers.

Energy production from effluents generally takes place through degradation of the organic substance under anaerobic conditions, where the organic substance is mainly converted into CH4 and C02. As known, anaerobic degradation can be decomposed into a series of sequential processes, the most important of which are hydrolysis, acidogenesis, acetogenesis and methanogenesis, which are promoted by several microbial groups. The intermediate phase of acidogenesis - acetogenesis is the control key of the whole process of methane production. If such phase is sufficiently slow and pH remains neutral, methane can develop. If on the contrary it is fast and far from neutral conditions, energy is produced in faster manner in the form of hydrogen, but methanogenesis is inhibited. This particular aspect of the whole process results in methanogenesis being a slow and unstable process.

In order to supplement zootechnical effluents with materials rich in carbohydrates, in order to increase CH4 production, energy-giving crops (maize, sorghum, etc.) are used at present: however such crops, being a resource and not a waste, cause a considerable reduction of the advantages of the process.

Moreover, the addition of ingredients (substrates) in the form of dedicated crops in co-digestion with zootechnical effluents may entail risks for the proper development of the process, due to the nature of ingredients themselves. In particular, some particularly acidogenic substrates, even though they increase the yield in methane (the main component of interest in biogas), if added in excessive amount, can give rise to acidosis phenomena and to stops in methane production. Their use is hence to be dosed with care.

Traditionally, CH4 production takes place in single stage plants where H2 production is ephemeral, since H2 being produced is immediately consumed by hydrogenotrophic methanogenic microorganisms. Yet, it is also known that, if the process conditions are directed towards"fermentation in the darkness", H2 production can be favoured to the detriment of CH4 production.

According to WO 2011/017292 Al, H2 and CH4 are biologically produced from biomasses under anaerobic conditions. Production of both gases takes place in a single bioreactor. Switching from H2 production to CH4 production is controlled by correcting moisture: at low moisture values, H2 is produced, whereas, by increasing moisture (> 90%), through the addition of water or leach liquid, the biological transformation is directed towards CH4 production. In this process, the possibility of controlling the pH of the process is provided. The main object of such a solution is producing H2 from wastes with low moisture content.

Agro-food wastes used in traditional single stage AD include, among others, whey and ricotta cheese whey ("scotta"), by-products of the working of cheeses and ricotta cheese.

Scotta is the liquid fraction remaining after ricotta cheese production, obtained by means of a thermal treatment of acidified whey, possibly with the addition of milk and/or cream. Whey is the liquid residual of milk protein coagulation in cheese production; scotta is the liquid residual of whey protein coagulation in ricotta cheese production. Such a definition takes into account standard UNI 10978:2013 "Ricotta fresca - Definizione, composizione, caratteristiche" (Fresh ricotta cheese - Definition, composition and characteristics), and the definition of "dairy product" included in EC Regulation 853/2004, Annex 1, Point 7.2: "Dairy products". Unlike whey, scotta no longer contains proteins and, for this reason, it is also defined as "deproteinised whey". Scotta may be of bovine, ovine or caprine origin. Scotta has a low content in noble constituents, which in any case are present in denatured conditions, as is the case for proteins, or in partly modified conditions, as is the case for lactose.

In traditional single stage AD systems, use of agro-food or agro-industrial wastes rich in fermentable molecules with high energy-giving content (e.g., whey, scotta or glycerol) in co-digestion with the effluents must be limited (at most 10%, see Hublin et al, 2012, Biotechnology and Bioprocess Engineering 17: 1284-1293), since the first demolition thereof results in a strong acidification of the medium, which has an inhibiting effect for the methanogenic microorganisms. Thus, in traditional systems, use of such wastes is reduced

In order to increase biogas production and to produce H2, two-stage plants, where the acidic and methanogenic phases have been separated, have been proposed in the past.

The process proposed in US 2012/0009643 Al comprises two stages, one for H2 production and the other for CH4 production, CH4 being however a by-product. H2 production is the main object, whereas CH4 production is considered as a "disturbance", and the separation of the two phases serves to remove such a disturbing element.

According to JP2006255538 (A), H2 and CH4 are produced in a two-stage plant from food wastes. Also in this case the aim is the decoupled production of H2 and CH4, which are stored in separate tanks.

In JP 10066996 (A) the object is to produce CH4 from organic material, by decoupling H2 and CH4 production in a two-stage system and recycling H2 from the first to the second stage in order to increase CH4 production. This process is however performed under highly thermophilic conditions, with a consequent non-negligible energy consumption in order to maintain the required temperature.

From DE 10 2007 063091 Al a process for producing hydrogen and methane in a two-stage plant is known, where hydrogen and methane are separately used.

From EP 2 246 436 Al a two-stage semi-continuous process for hydrogen and methane production in two bioreactors under mesophilic conditions is known. The first bioreactor produces hydrogen, whereas the second bioreactor produces methane. Hydrogen from the first bioreactor is introduced into the second bioreactor. This document discloses a semi-continuous process using mixtures consisting of organic fractions of solid urban waste, which, as is well-known, have a high concentration of dry substance. The presence of high percentages of dry substance requires dilution with vegetation waters from olive oil production.

From DE 10 2010 043630 Al a plant for the treatment of biological wastes is known, which comprises two bioreactors, one of which is intended for hydrogen production and the other is intended for methane production under mesophilic conditions. Hydrogen is transferred from the first reactor to the second reactor. This solution uses column bioreactors, which typically determine a variable concentration depending on the level in the column.

From DE 10 2008 027850 a plant is known comprising two bioreactors for hydrogen and methane production, respectively. The plant can operate under mesophilic conditions. This document concerns very small size plants, which are transportable by road or by rail and which are unsuited to agro-food industries, like dairy factories. Moreover, the plant disclosed uses a substrate with a high organic charge and hence requires mixing with water.

It is an object of the invention to overcome the drawbacks of the prior art by providing a method for the continuous production of hydrogen (H2) and methane (CH4) from zootechnical effluents, in a two-stage plant, which is faster and which does not necessarily require use of crops.

It is another object of the invention to provide a two-stage plant for carrying out the method in continuous manner.

It is a further, but not the last object of the invention to provide a method and a plant of the above kind, which is cheap to be constructed and managed, allows high yields in effluent transformation into H2 and CH4 and therefore is suitable for large-scale industrial implementation, in particular for use of wastes from dairy factories.

Description of the invention

The above and other objects of the invention are achieved by the method and the plant as claimed in the appended claims.

According to the invention, the following conditions are simultaneously adopted in the method of producing H2 and CH4:

- recycling H2 produced in the first stage in order to increase CH4 production in the second stage and to reduce the partial pressure of H2 in the head space of the first bioreactor, by maintaining vacuum conditions;

- operating under mesophilic conditions, with considerable savings in management costs with respect to operation under thermophilic conditions;

- automatically monitoring and controlling the most important process parameters, such as pH, temperature, pressure and level of the fermentation matrix.

According to the invention, the phases of hydrolysis, acidogenesis and acetogenesis take place in a first bioreactor, under conditions such as to promote H2 production, whereas the phase of methanogenesis takes place in a second bioreactor, which is fed with the fermented broth resulting from the first bioreactor. Always according to the invention, H2 produced in the first bioreactor is removed from the fermentation matrix by creating vacuum conditions in said first bioreactor. The vacuum condition in the first bioreactor, i.e. the condition of subatmospheric pressure, is an essential condition of the invention. Actually, in this manner, the partial pressure of hydrogen in the first bioreactor is advantageously kept at a minimum. Should the hydrogen pressure increase, fermentation would become unstable, hydrogen would no longer be produced and lactic acid would be prevailingly produced. Moreover, pH would tend to become so low as to block the fermentation reaction. In a system without vacuum control, therefore, it would be necessary to intervene very frequently on pH for buffering purposes, but this would not be sufficient to direct fermentation towards the production of liquid metabolites useful for methanogenesis.

Advantageously, according to the invention, it is possible to obtain CH4 from co- digestion of zootechnical effluents with high percentages of agro-industrial wastes rich in fermentable molecules with high energy-giving content, which wastes include a substance containing carbohydrates and consisting of scotta or a mixture of scotta and glycerol. On the contrary, in the traditional AD, co-digestion is possible only if the percentage of such wastes is kept low, typically below 10%, otherwise the concentration of volatile fatty acids would increase, with block of methanogenesis.

Mixing zootechnical effluents with organic wastes rich in carbohydrates, and in particular with scotta, has proven particularly advantageous since it increases methane production even when said effluents consist of the only liquid fraction of the zootechnical effluents. Indeed, at present, there is a widespread tendency to separate zootechnical effluents into their solid and liquid fractions to be destined for separate workings. The solid fraction is generally destined to compost production, whereas the liquid fraction can be used for biogas production. Yet, the liquid fraction of zootechnical effluents is considerably poorer in carbon than the effluents as such, since part of the organic C is removed in the solid fraction. In the known plants, therefore, the yields of biogas production starting from the liquid fraction are considerably lower than the yields attainable from sludge as such. Moreover, according to the present state of the art, the supply of organic substrates rich in carbohydrates is to be limited in order to avoid sudden increases in acidity with block of methanogenesis. On the contrary, thanks to the invention, and in particular to the constant control of the parameters temperature, pH, level of the fermentation matrix and pressure, it is possible to conveniently exploit both the liquid fraction of zootechnical effluents, and sludge as such, thereby obtaining high methane productions in the second bioreactor.

Moreover, advantageously, with the method according to the invention, digestion time is considerably reduced, at least by 30%, with respect to the time attainable in traditional single stage AD, starting from non-digested raw matter.

A further advantage of the method according to the invention is the maintenance of mesophilic conditions, that is of temperatures comprised between about 30°C and 40°C, which are far lower than temperatures required by thermophilic processes, typically of at least 55 - 60°C, which require high energy consumptions for maintaining the temperature.

Advantageously, according to an aspect of the invention, use of selected microbial consortia allows attaining higher yields in biogas.

According to an aspect of the invention, the two-stage method allows using, in co- digestion, high percentages of agro-food or agro-industrial wastes rich in fermentable molecules with high energy-giving content, which wastes include a substance containing carbohydrates and consisting in whey, scotta or a mixture of scotta and glycerol, without creating problems in the methanogenesis and, instead, with a considerable increase in CH4 production with respect to what attainable with AD of the only effluents.

According to a main aspect of the invention, recycling the H2 produced in the first stage or bioreactor into the second stage allows further increasing CH4 production, while eliminating at the same time the need to manage H2 leaving the plant. Actually, since the methanogenic microorganisms produce CH4 mainly from acetate, but also from H2+C02, if the H2 produced in the first stage is transferred into the second stage, the result will be a further increase in CH4 production. The latter aspect is important since it determines a better industrial applicability of the method and the plant according to the invention. Tests for "batch" systems have shown that, by recycling the H2 produced in the first stage into the second stage bioreactor, it is possible to obtain an increase in methane production. Brief Description of the Figures

The invention will be described hereinafter with reference to some preferred embodiments, given by way of non limiting examples with reference to the accompanying drawings, in which:

- Fig. 1 is a block diagram of a plant for producing H2 and CH4 according to the invention;

- Fig. 2 is a table reporting the conditions of the main parameters of the process for hydrogen production, in a preferred embodiment:

- Fig. 3 is a graph of the response surface for H2 production; - Fig. 4 is a table containing the optimum ranges of percentages of volatile solids (VS) from sludge, scotta and glycerol, for H2 production;

- Fig. 5 is a table containing comparison data between single stage ("S") and two-stage ("D") treatment.

Description of a Preferred Embodiment

Reference will be made to Fig. 1, where plant 1 according to a preferred embodiment of the invention has been schematically shown. According to this preferred embodiment of the invention, said plant mainly comprises a first bioreactor Bl for producing H2 and a second bioreactor B2 for producing CH4. Moreover, the plant preferably comprises a first tank SI for preparing the feeding mixture for the plant, a second tank S2 containing a buffering solution for re-establishing the pH in the first bioreactor Bl and a third tank S3 containing a buffering solution for re-establishing the pH in the second bioreactor B2. In the alternative, the feeding mixture for the plant and the buffering solutions can arrive from sources different from tanks SI, S2, S3, for instance sources external to the plant, through suitable feeding circuits.

When the composition of the material to be treated allows so, according to a variant of the invention it is also possible to provide for adjusting the pH in the first bioreactor to a value close to the pH value to be maintained in the second bioreactor, thereby making pH correction in the second bioreactor superfluous. This is generally possible in case of high substrate volumes, allowing attaining a uniform pH value in the continuously stirred mass.

In a further variant of the invention, the pH is only corrected along the path from the first to the second bioreactor, in which case a single source of buffering solution will be sufficient.

According to the construction scheme illustrated, the plant further comprises pumps PI, P2, P3 for conveying liquids and possibly crumbled solid bodies, pumps P4, P5 for conveying gases, pumps P6, P7 for liquids, e.g. membrane pumps, and a heater/cooler HC associated with a circuit for heating/cooling the plant, schematically shown in dashed lines in the Figure. Furthermore, the plant preferably comprises a system for controlling and measuring the parameters temperature, pressure, level of the fermentation matrix and pH for managing the plant so as to ensure in automatic manner the desired conditions for H2 and CH4 production. The control system includes an electronic control unit associated with sensors arranged to generate signals representing the values taken by said parameters, and means for adjusting the percentages of the ingredients of the feeding mixture. In the illustrated embodiment, given by way of example, bioreactor Bl for producing H2 consists of an insulated and thermostatised stainless steel tank with a height- to-diameter ratio equal to 2. According to an advantageous aspect of the invention, bioreactor Bl preferably is a CSTR ("Continuous Stirred Tank Reactor") bioreactor. Bioreactor Bl is preferably equipped with a flanged cover, the flange of which is preferably obtained so as to be under liquid seal in normal operation conditions of the plant. This feature allows limiting leakages of gas, in particular of H2 produced in bioreactor Bl . Moreover, an inclined vane stirrer is preferably incorporated into bioreactor Bl . More preferably, stirring of the material contained in bioreactor Bl is obtained by means of a circulation pump, which can coincide with pump P2 provided for transferring the fermentation broth from bioreactor Bl to bioreactor B2. To this end, a valve VI allows sending the fermentation broth, which preferably comes out from the bottom of bioreactor Bl, to bioreactor B2, or introducing again the fermentation broth into bioreactor Bl, preferably at the top. Advantageously, the size of bioreactor Bl will be chosen so as to ensure a suitable retention time as far as the formation of H2 and CH4 is concerned.

Always with reference to the illustrated embodiment, bioreactor B2 for producing CH4 consists of an insulated and thermostatised stainless steel tank with a height-to- diameter ratio equal to 2. According to an advantageous aspect of the invention, bioreactor B2 preferably is a CSTR ("Continuous Stirred Tank Reactor") bioreactor. Bioreactor B2 is preferably equipped with a flanged cover, the flange of which is preferably obtained so as to be under liquid seal in normal operation conditions. This feature allows limiting leakages of gas, in particular of CH4 produced in bioreactor B2. Moreover, an inclined vane stirrer is preferably incorporated into bioreactor B2. More preferably, stirring of the material contained in bioreactor B2 takes place through a circulation pump P3. Preferably, the size of bioreactor B2 is chosen so as to ensure a suitable retention time as far as the formation of CH4 and C02 is concerned. According to a preferred aspect of the invention, the volume of bioreactor B2 is about 10 times the volume of bioreactor Bl .

According to the invention, bioreactors Bl and B2 preferably are CSTR reactors, as stated above. In CSTR reactors, continuous stirring of the fermentation broth advantageously allows maintaining a homogeneous temperature and composition and avoids clotting. Non-homogenous composition and clotting are typical drawbacks of column reactors generally used in prior art plants. Moreover, CSTR bioreactors Bl and B2 are preferably stirred by recycling the fermentation broth by means of pumps, for instance single screw pumps. Tank SI preferably is a steel tank that is insulated, thermostatised and stirred by means of an inclined vane stirrer. The tank is possibly arranged for demolition of solid substrates and has an input IN for the ingredients of the feeding mixture for the plant. Preferably, tanks S2, S3 containing the buffering solutions capable of re-establishing the proper pH in bioreactors Bl and B2, respectively, also are two steel tanks. A pump PI is provided for transporting the biomass from tank SI to bioreactor Bl .

Pumps PI, P2 and P3, being to ensure the transportation of the biomass, which is prevailingly in liquid condition but could include crumbled suspended solids, will preferably be screw pumps.

Pumps P4 and P5 are two pumps for gases and are used for evacuating the H2 produced in bioreactor Bl and the CH4 produced in bioreactor B2, respectively, and therefore they must be suitable for the quality of the transported gas. Pump P4 will be capable of generating a subatmospheric pressure, preferably comprised between 0.10 and 0.99 ata, inside bioreactor Bl .

Pumps P6 and P7 are two pumps suitable for conveying a buffering solution, present in tanks S2 and S3, respectively, to bioreactors Bl and B2 and, according to a preferred embodiment of the invention, their operation is related to the control of pH in bioreactors Bl and B2, as it will become apparent from the continuation of the description.

Heater/cooler HC is a heating/cooling apparatus for maintaining the process temperatures in bioreactors Bl and B2 and for maintaining an adequate temperature in tank SI, such that fermentation of the biomass contained therein is not promoted. Said apparatus HC can advantageously be an apparatus using water, of commercial type.

In the method according to a preferred embodiment of the invention, zootechnical effluents are introduced into the first tank SI where preparation of a feeding mixture takes place. Preferably, tank S I will have such a capacity as to enable a charge at least every fifth day and to ensure at the same time the continuity of the whole process of H2 and CH4 production. According to the invention, zootechnical effluents destined for the plant could be for instance sludge, coming from breeding farms of different kinds: for cattle, pigs, etc.

According to a main aspect of the invention, said sludge is mixed with agro-food wastes, in order to obtain a feeding mixture for the plant suitable for the production of H2 and CH4 in a two-stage plant, i.e. a plant where H2 and CH4 are produced in two different stages or bioreactors.

According to the invention, said agro-food wastes will be rich in fermentable molecules with high energy-giving content, coming from substances rich in carbohydrates, in particular, but not exclusively, sugars such as lactose, said substances mainly including whey, scotta or a mixture of said scotta and glycerol.

Reference will be made to the table in Fig. 2, where the conditions and the experimentally obtained average values (with the respective standard deviations) of the main parameters of the hydrogen production process in bioreactor Bl : hydrogen content in the biogas (%), volume production of hydrogen (ml), yield (ml H2 / g VS) and pH at the end of the fermentation. The hydrogen content varies from minimum values, obtained by using 100% Volatile Solids (VS) from zootechnical effluents in the form of sludge as feeding mixture or substrate, to maximum values obtained by utilising scotta as the only substrate (100% VS from scotta). It will be appreciated that the pH at the end of the fermentation has an opposite behaviour and ranges from a minimum of 4.45±0.02 obtained with scotta only to a maximum of 6.57±0.01 obtained with sludge only. Such results confirm on the one hand scotta as the preferred substrate for hydrogen production, and on the other hand the high buffering power of sludge.

Referring now to Fig. 3, there is reported the average value of the maximum production of hydrogen predicted by the model, which value oscillates about 458 ml hydrogen (range of the optimum response surface: 432 to 484 ml H2), together with the experimentally determined maximum, equal to 459.57±36.41 ml H2, obtained by mixing 66.6%) VS from scotta with 33.3% VS from sludge (Fig. 2). It will be appreciated that the optimum response surface further shows the possibility of mixing zootechnical effluents and agro-food wastes, i.e. the substrates for feeding the plant according to the invention, in different combinations, depending on the availability of the individual substrates, while keeping H2 production within a sufficiently wide optimum range. Referring to Fig. 4, as far as scotta is concerned, the maximum percentage of VS from scotta allowing remaining inside said optimum response surface is 92%, with 8% sludge. Glycerol contribution, on the contrary, is more limited, with a maximum VS percentage equal to 28%, together with 16%) for sludge and 56%> for scotta. Lastly, as far as sludge is concerned, the optimum combination for H2 production can reach 54% VS from sludge together with 46% VS from scotta. The latter, also thanks to its good industrial availability, is the preferred type of substrate for bio-conversion into hydrogen and hence it can be suitably used for diluting sludge to be disposed.

Advantageously, according to the invention, it is possible to operate in very flexible way with the above-mentioned substrates, depending on the needs and the amounts of substrates to be disposed. This is very important, taking into account that the availability of the different substrates can also change over the seasons and that plants installed in different enterprises can have different availability and/or amounts of substrates.

In a preferred embodiment of the invention, the feeding mixture is stirred at a speed of about 100 rpm and maintained at a temperature between 2 and 15°C, in order to avoid solidification of the liquid in the pipes in cold periods, and aerobic fermentation, with loss of properties useful for the process and generation of unpleasant smells, in hot periods. The liquid mixture thus obtained, having a density between 1 and 1.2 kg/1 and containing at most some crumbled solid, forms the preferred feedstock for the whole process.

Said feeding mixture, during plant operation, is sent into bioreactor Bl, into which hydrogenogenic microorganisms (starter Ml in Fig. 1) are also introduced. According to this preferred embodiment of the invention, inside bioreactor Bl, the fermentation of said feeding mixture is caused and the hydrolysis, acidogenesis and acetogenesis processes are promoted under conditions of anaerobiosis, darkness, temperature between 30 and 40°C, preferably between 35 and 38°C, and a pressure below atmospheric pressure, preferably between 0.10 and 0.99 ata and more preferably between about 0.68 and 0.82 ata.

Filling can be introduced into bioreactor Bl in order to promote the settlement and the growth of microorganisms. The apparent filling volume considered as optimum is half the total volume of the filled portion of the bioreactor (biomass plus filling). Moreover, the filling material will preferably be a porous, e.g. quartzeous, material.

The processes taking place in bioreactor Bl essentially produce, after a retention time of about 24 to 48 h, an almost equimolecular gaseous mixture of C02 and H2 and a fermentation broth that, as it will become apparent from the following description, forms the feedstock for bioreactor B2.

According to this embodiment of the invention, the estimated daily production of H2 will be comprised between 5 and 8 volumes of H2 per each volume of biomass present in bioreactor Bl . Anyway, the volumes of H2 produced are strictly dependent on the substrate composition.

The fermented broth resulting from the first bioreactor Bl is sent, by means of pump P2, to bioreactor B2 where methanogenic digestion takes place in the presence of microorganism consortia, preferably selected (starter M2 in Fig. 1) and acting in synergy, for producing CH4 (acetoclastic methanogens and other microorganisms). Advantageously, according to the described configuration, it has been possible to experimentally obtain a reduction of the retention time by about 30% with respect to the time required by a single stage system.

Referring to Fig. 5, the results relating to the second stage methanogenesis, i.e. obtained in bioreactor B2, with 70% sludge and 30% scotta have been compared with the results obtained with sludge only in a single stage treatment "U": The standard deviation has been reported between brackets. As it can be appreciated, hydrogen H2 has been produced only when sludge (LI) was co-digested with scotta (SCO). In the case of single stage "U", hydrogen production from sludge plus scotta (LI+SCO) has inhibited the production of methane CH4. After seven days incubation, H2 was no longer present in the biogas from the treatments. In traditional AD ("U"), sludge alone has produced no H2, but only CH4. For a same incubation time, the LI+SCO mixture has produced, besides H2, almost twice a volume of CH4 in a two-stage treatment "D" with respect to sludge alone in a single stage treatment "U". After two weeks, in a decreasing phase of CH4 accumulation, the differences diminished, but an advantage in CH4 production from LI+SCO was maintained over LI (+56%). For attaining this result, reinoculation of the fermentation broth coming from the first stage (bioreactor Bl) and directed to the second stage (bioreactor B2) with selected methanogenic consortia was fundamental. Indeed, in the absence of reinoculation, "LI+SCO+ RI" in the table of Fig. 5, start of methanogenesis in two-stage treatment "D", and consequently CH4 production, has been delayed.

This result has lead the authors of the present invention to conclude that the addition of scotta to sludge, in the first stage mixture for producing hydrogen, considerably improves the performance of the methanogenic microorganisms, and consequently the production of methane, in the second stage in a two-stage plant of the type described.

According to a preferred embodiment of the invention, bioreactor B2 is maintained at a temperature between about 30 and 40°C, preferably between 35 and 38°C, under conditions of anaerobiosis, in the darkness, at atmospheric pressure and at a pH comprised between about 6.5 and 7.5, preferably of about 7.

The above conditions result in the discharge, from bioreactor B, of an almost equimolecular gaseous phase of C02 and H2, and of a liquid phase consisting of digestate. In the illustrated example, the discharge of the gaseous phase takes place through the circuit associated with pump P5, and digestate is discharged through line OUT.

The described plant according to this preferred embodiment of the invention is capable of producing about three to six volumes CH4 per volume of biomass transferred from bioreactor Bl to bioreactor B2, assuming that the gas is produced at atmospheric pressure and at a temperature of 0°C. Anyway, the volume of methane produced is strictly related to the content in volatile solids (VS) of the fermented broth.

The initialisation or start-up phase of the plant includes charging bioreactor B2 with the feeding mixture prepared in tank SI; this is followed by the introduction of nitrogen into the same bioreactor B2, to allow producing anaerobiosis conditions. Advantageously, the characteristics of the feeding mixture prepared in tank SI and used in the start-up phase will preferably be the same as those of the mixture that will be used for the continuous operation of the plant.

Stirring is then started in bioreactor B2 and the biomass inside it is brought to the process temperature. The specific starter microorganisms (M2 in Fig. 1) can be introduced in such phase.

Then, it is necessary to wait for a period of time that indicatively can be of at least about twenty days so as to obtain, owing to methanogenesis, a stable digestate, such that it can be removed from the system. At this point, nitrogen is introduced into bioreactor Bl and bioreactor Bl is then charged with the feeding mixture coming from tank SI and is brought to process temperature and pressure.

After introduction of the specific starter microorganisms (Ml in Fig. 1) into bioreactor Bl, the plant is ready for the continuous operation.

Heating and cooling of bioreactors Bl and B2 are ensured by a circuit, preferably a water circuit, which is heated and cooled by means of suitable commercial plants, such as for instance a chiller HC.

According to the invention, the pressure of the gas and the level, the temperature and the pH of the liquid are controlled in bioreactors Bl and B2. The control system preferably comprises a single operator interface with display, on which the process state and parameters are displayed, a logic unit for process control or CPU and the associated analogue and digital I/O modules, which communicate with a software-equipped system via an Ethernet bus

According to the invention, bioreactor Bl is equipped with piezometric level sensors, located in the lower portion (in correspondence of the liquid phase) and in the upper portion (in correspondence of the gaseous phase) of bioreactor Bl . Said sensors detect whether the sludge level inside bioreactor Bl is within the range set by the user (as a percentage of the total height) and generate a signal allowing the control system to act on the flow rate of the sludge leaving the same bioreactor Bl, by increasing or decreasing it depending on whether the minimum of maximum level (for instance, 70 - 80%) has been exceeded.

According to the invention, the temperature of bioreactor Bl producing hydrogen is controlled by acting on the heating/cooling circuit. The temperature measurement is continuously performed inside the sludge and a suitable transmitter sends the temperature signal to the control system. If the measured value is outside the range set by the user, the control system acts on the heater/cooler which sends a suitable amount of a thermo-vector liquid (usually water), at the temperature required for cooling or heating the bioreactor (preferred range 35 to 38°C).

Always according to a preferred embodiment of the invention, the plant is controlled by setting a priority in the heating/cooling of bioreactors Bl and B2 and tank SI . The priority is determined by relation Bl— > B2— > SI . Thus, the temperature of bioreactor B2 producing methane is controlled by acting on the heating/cooling circuit if the temperature of bioreactor Bl producing hydrogen is already within a preset range. In B2, the measurement of temperature is continuously performed inside the sludge and a suitable transmitter sends the temperature signal to the control system. If the measured value is outside the range set by the user, the control system acts on the heater/cooler, which sends a suitable amount of a thermo-vector liquid (usually water) at the temperature required for cooling or heating bioreactor B2 (preferred range 35 to 38°C). According to the same priority rule, the temperature of tank SI for accumulating the feeding mixture is controlled by acting on the heating/cooling circuit if the temperature of bioreactor Bl producing hydrogen and the temperature of bioreactor B2 producing methane are already within a respective preset range. In SI, the measurement of temperature is continuously performed inside the feeding mixture and a suitable transmitter sends the temperature signal to the control system. If the measured value is outside the range set by the user, the control system acts on the heater/cooler, which sends a suitable amount of a thermo-vector liquid (usually water) at the temperature required for cooling or heating tank S 1 (preferred range 2 to 15°C). Preferably, moreover, the temperature ranges can be modified by the user.

Preferably moreover the invention includes the control of the pH of the liquid inside bioreactor Bl producing hydrogen. Said control takes place by acting on the flow rate of a buffering solution contained in a tank S2. The pH measurement is continuously performed by a suitable sensor placed inside the sludge. If the measured value is outside the range set by the user, the control system acts on pump P6 in order to send the suitable amount of buffering solution capable of bringing back the pH to the set-up value (preferably, acid pH comprised between 4 and 5). Values of acid pH higher than 5, e.g. between 5 and 6, are however tolerated.

Preferably the invention also includes the control of the pH of the liquid inside bioreactor B2 producing methane. Said control takes place by acting on the flow rate of a buffering solution contained in a tank S3. The pH measurement is continuously performed by a suitable sensor placed inside the sludge. If the measured value is outside the range set by the user, the control system acts on pump P7 in order to send the suitable amount of buffering solution capable of bringing back pH to the set-up value (preferably, neutral pH, 6.5 to 7.5).

According to the invention, the pressure inside bioreactor Bl producing hydrogen is preferably controlled by acting on the flow rate of gas leaving the bioreactor. The pressure measurement is continuously performed inside bioreactor Bl in its upper portion where gas is present. If the pressure is outside the range set by the user, the control system intervenes by varying the number of revolutions of pump P4, thereby increasing or decreasing the flow rate of gas leaving bioreactor Bl depending on whether the pressure is below or above such a range, in order to bring back the pressure to the set-up value (preferred range 0.68 to 0.82 ata).

According to the invention, also the pressure inside bioreactor B2 producing methane is preferably controlled by acting on the flow rate of gas leaving the same bioreactor. The pressure measurement is continuously performed inside bioreactor B2 in its upper portion where gas is present. The measured value is continuously transmitted and, if the pressure is outside the range set by the user, the dedicated software intervenes by varying the number of revolutions of pump P5, thereby increasing or decreasing the flow rate of the gas leaving bioreactor B2 depending on whether the pressure is below or above such a range, in order to bring back the pressure to the set-up value (preferred range 0.99 to 1.01 ata).

The present invention is also suitable for updating and optimising existing single stage plants, by adding a bioreactor for producing hydrogen, preferably a CSTR bioreactor, where hydrolysis, acidogenesis and acetogenesis take place, which bioreactor is separate and can be decoupled from the methanation reactor to which hydrogen generated in the first reactor is sent.

The invention as described and shown can undergo several changes and modifications lying within the same inventive principle.