LU85141A1 - PROCESS FOR THE PRODUCTION OF METHANE BY ANAEROBIC DIGESTION OF ORGANIC MATERIALS - Google Patents

Abstract

1. Process for the biomethanization of solid or semi-solid organic substrates, in which such a substrate suspended in water is subjected to the action of communities of microorganisms, having the effect of successively extracting water soluble products from the substrate, converting these solubilized products into acids and/or alcohols and methanizing these acids and/or alcohols in two fermentors mounted in series, characterised in that the suspension of organic substrate introduced into the first fermentor (1) is subjected to a solubilization of the organic substances contained therein, to an acidogenesis and to a methanization while the pH is maintained at 6.5 to 8.5, in that a liquid effluent deprived of solid substances and containing the excess of non-methanized acids and/or alcohols is withdrawn from the first fermentor (1) and this liquid effluent is transferred to the second fermentor (2) which is charged with methanogenic microorganisms, in which second fermentor this effluent liquid is methanized, also at a pH of 6.5 to 8.5, in contact with the methanogenic microorganisms, and in that a neutral or alkaline effluent liquid charged with methanogenic microorganisms is withdrawn from the second fermentor (2), and this effluent neutral or alkaline liquid is recycled to the first fermentor, optionally discontinuously.

Description

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PROCESS FOR THE PRODUCTION OF METHANE BY ANAEROBIC DIGESTION OF ORGANIC MATERIALS

The present invention relates to a process for biomethanation of solid or semi-solid organic substrates, in which such a substrate, suspended in water, is subjected to the action of communities of microorganisms successively having the effect of extract water-soluble products from the substrate, transform these solubilized products into acids and methanize these acids in two fermenters connected in series.

The biomethanization of solid or semi-solid organic materials is a known process which uses communities of microorganisms working in symbiosis by anaerobic digestion of the biodegradable fractions of these organic materials.

A first community of microorganisms attacks solid or semi-solid organic matter and transforms this organic matter into water-soluble products, such as sugars, lipids, amino acids, proteins, etc.

These solubilized products are then transformed, by other communities of bacterial microorganisms into volatile fatty acids (such as acetic, propionic and butyric acids) and other products (such as lactate, ethanol, bicarbonate, etc.) which are hydrogenated to acid acetic, hydrogen and bicarbonate. Finally, a final bacterial community transforms acetic acid, hydrogen and bicarbonate into a mixture of methane and carbon monoxide called "biogas".

It is known to produce such a biogas by digestion of solid or semi-solid organic substrates in a single fermenter, but the low growth rate of methanogenic bacteria requires long residence times of the substrate in the fermenter, from 1 15 to 30 days, and therefore the use of large volumes of fermentation.

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There is also known a process for biomethanisa-tion of solid and / or semi-solid organic substrates in two stages (Ghosh et al, Journal WPCF, h7, (1), pages 30-45). In this process, a first fermenter is used, 10 the content of which is constantly stirred and in which operates, at a pH generally between 4 and 6, the extraction of soluble materials and their transformation into volatile fatty acids. After these treatments, the entire content of the first fermenter is transferred to a second fermenter, in which the methanization of grss acids takes place. This two-stage process has the disadvantage of being slow, while requiring the use of two large capacity fermenters.

Another process for the biomethanisation of solid or semi-solid organic materials in two stages is also known (Rijkens, Energy from Biomass, Serie E, edited by Reidel D Publ. Co, Dordrecht, Netherlands, 1981, 1_, 121-125).

In this process, the reactions leading to the formation of acids are carried out in a first "batch" type fermenter, the liquid phase of which is brought into a "up flow" type digester in which biomass accumulates. and "whose liquid effluent is used as dilution liquid for the contents of the" batch "fermenter. When the production of volatile fatty acids is finished in the" batch "fermenter, this is disconnected from the up-flow digester and the latter is itself reconnected to a second "batch" fermenter, while the first "batch" fermenter then becomes a methanization fermenter for volatile fatty acids. This process requires the use of a series of batch fermenters | 3 .

"For a single" up-flow "reactor.

Finally, French patent FR-A-2,443-504 discloses a biomethanization process, in which an organic BUbstrate (pulp and beet waste) is loaded into a first digester, in which said substrate is simply liquefied by hydrolysis to forming a mass containing soluble products (carbohydrates), this mass being sent to a second digester where it is subjected to acidogenesis and anaerobic digestion by anaerobic biomass. From this second digester is withdrawn, after filtration in a separator, a liquid phase containing hydrolyzing enzymes which is returned to the first digester where it ensures the liquefaction of the organic substrate. The incomplete biomethanisation of the substrate in the second fermenter or digester causes an accumulation of a- | s volatile fatty acids in the liquid phase of the system, so that this second digester must be periodically subjected to deconcentration purges, resulting in losses of methanisable fatty acids.

The present invention relates to a process of the type defined in the first paragraph of this specification which offers the advantage of not requiring purges resulting in losses of products which can still be methanized.

The process according to the invention is based on the idea of carrying out hydrolysis (extraction of soluble materials from a solid or semi-solid organic substrate), acidogenesis and anaerobic digestion in a single fermenter and metha -nize in a second fermenter the excess of volatile fatty acids produced in the first fermenter, which has the effect not only of accelerating the production of biogas and increasing the yield, but also of preventing 'accumulation of volatile fatty acids (requiring periodic purges) escaping anaerobic digestion.

. The process according to the invention requires only periodic purges of products which are no longer methanisable, so that its yield is higher than that of the process known from FR-A-2,443,504. In addition, the process according to the invention does not require the passage of the entire substrate in the second digester, only the excess of volatile fatty acids produced in the first digester having to be brought into the second digester.

According to the invention, the suspension of organic substrate loaded in the first fermenter is subjected to a solubilization of the organic materials contained therein, to an acidogenesis and to anaerobic digestion, an acidic liquid effluent is withdrawn from the first fermenter and this liquid effluent is sent acid in the second fermenter charged with methanogenic microorganisms, where this acidic liquid effluent is methanized in contact with methanogenic microorganisms, and finally, an alkaline liquid effluent charged with methanogenic microorganisms is withdrawn from the second fermenter and this alkaline liquid effluent is recycled in the first fermenter .

According to a feature of the invention, the acidic liquid effluent from the first fermenter is withdrawn by overflowing into a conduit opening into the upper part of the first fermenter and bringing this effluent to the lower part of the second fermenter, while withdrawing the alkaline liquid effluent from the second fermenter also by overflowing into a conduit opening into the upper part of the second fermenter and bringing this alkaline liquid effluent to the lower part of the first fermenter.

According to another characteristic of the invention, a mixed liquor periodically withdrawn from the first fermenter containing non-biodegradable solid or semi-solid material, this mixed liquor is filtered to separate this non-biodegradable material and the filtrate is recycled , preferably in the second fermenter.

Other particularities and details of the process according to the invention will emerge from the following description, 35 to 5.

in which reference is made to the accompanying drawing, the single figure of which schematically shows and by way of example only, an installation in which the method according to the invention can be implemented.

5 The attached drawing shows an installation comprising two fermenters 1 and 2 mounted in series. The first fer-liar 1 is equipped with an inlet 3 used to introduce charges of solid organic or semi-solid biodegradable materials, such as algae, garbage, residues due to sewage, etc. , suspended in water. A conduit A terminating in a tube 5 plunging into the first fermenter 1 is used to recycle an effluent from the second fermen-tor 2 there, as described below. The fermenter 1 has an outlet 6 allowing the biogas which forms therein to be removed, this biogas being collected in a collector shown diagrammatically at 32. The fermenter 1 is also equipped with an agitator 7 actuated by a motor 8. A a level located between that of the free lower end of the dip tube 5 and the free level 9 of the aqueous suspension of organic substrate contained in the first fermenter 1, the latter is provided with a withdrawal conduit 10 through which a liquor aqueous containing the non-biodegradable fractions possibly contained in the charges of substrates of the first fermenter are withdrawn as described below. Finally, an inclined T-shaped conduit 11 25 opens out through a branch 12 into the first fermenter, the branch being able to be closed by a plug 13 actuated by * a rod 13 '· The T-conduit 11 is connected by another branch 14 to the second fermenter 2.

The. second fermenter 2, the capacity of which is much lower than that of the first fermenter 1 is provided with an outlet 15 making it possible to sample the liquor from the fermenter, with an outlet 16 making it possible to evacuate the biogas which is absorbed therein , this outlet being able to be connected to the biogas collector 32, of a conduit 17 opening into the upper part of the se-35 |

Icond fermenter 2 for the withdrawal of a liquid effluent as explained below and of a dip tube 18 connected to the branch 14 of the T-duct 11 *

In the conduit 17 are optionally inserted a decanter 19 and a storage tank 20. This conduit 17 is connected to the conduit 4 optionally by means of a metering pump 21 which can be optionally supplied with liquid effluent from the second fermenter 2 by a conduit 22 connected to a storage tank 23. In the conduit 17 10 is also inserted a three-way valve 24 allowing the evacuation of part of the liquid effluent from the second fermenter by an evacuation conduit 25.

The installation also includes a filter 26 to which the draw-off pipe 10 of the first fermenter 1 15 ends, as well as a pipe 27 for bringing the liquid filtrate obtained in the filter 26 into the pipe 11, a storage tank 28 and a metering pump. 29 possibly being inserted into the conduit 27.

The second fermenter 2 can be equipped with a stirrer, such as a magnetic stirrer 30.

Valves 31 are provided on the various conduits shown in the drawing.

The process according to the invention is described below with reference to the installation described above.

In the fermenter 1 containing a biomass of communities of microorganisms allowing liquefaction, aci-dogenesis and methanogenesis, a solid or semi-solid organic substrate in aqueous suspension is introduced via inlet 3 After short-term stirring of the content of the fermenter 1, the liquefaction, acidogenesis and methanization reactions are allowed to take place in the fermenter 1 without the contents being further mixed using the agitator 7. From this fermenter 1, it is withdrawn then continuously an acidic liquid containing an excess of acidic products which have not been methanised in the fermenter 1. This acidic liquid is withdrawn, i "* 7 by overflow or overflow" through the T pipe 11 ·

The conduit 11 brings via its branch 14 and the tube 18 plunging the acid liquid into the fermenter 2, where it is metha-! nized in contact with a biomass made up of a community 5 of methanogenic bacteria previously introduced.

The biogas formed in the fermenter 1 is extracted from it by its outlet 6 and collected in the collector 32. The additional biogas formed in the fermenter 2 at 10 is extracted by its outlet 16 and can be brought into the collector 32. In short, the fermenter 2 is used to anaerobic digestion the fraction of the liquefied and acidified substrate which could not be anaerobic digested in the fermenter 1. This fermenter 2 contains substantially no solid substrate "but only a liquid activim 15 resulting from the liquefaction and of the acidogenesis of the substrate which took place in the fermenter 1. At the upper part of the fermenter 2 is withdrawn by overflow or overflow, in the conduit 17, an alkaline liquid effluent containing substantially no more methanisable components.

20 This alkaline effluent is optionally decanted in the descaler 19 and stored in the storage tank and it is recycled in the fermenter 1 by the conduit 17 connected to the conduit 4 possibly via the metering pump 21. In the event of if necessary, the alkaline liquid effluent is collected, before being recycled in the fermenter 1, in the tank 23, from which it is brought by the conduit 22 in the recycling pump 21 which serves to recycle it in the fermenter 1 by the dip tube 5 with which this fermenter is equipped 1. The alkaline liquid effluent recycled in fermenter 1 is used to adjust the pH there to a value conducive to favor the reactions of liquefaction, acidogenesis and methanization of the substrate in the fermenter 1.

35 | 1 When the substrate has remained in the fermentor 1 for a time sufficient for the major part of the biodegradable organic matter which it contains to have been liquefied, acidified and methanised, a mixed liquor composed of major is drawn from this liar-5 part of non-biodegradable material in aqueous suspension. This mixed liquor drawn off through line 10 is filtered in filter 26, in which the non-biodegradable material is separated from it to be removed and from which the liquid filtrate is brought through line 27 into line 14 supplying the fermenter 2 with acidic liquid, when this filtrate still contains methanisable organic acids. If necessary, the liquid filtrate obtained in the filter 26 is stored in the storage tank 28 and brought, by the metering pump 15 29, into the supply duct 11 of the second fermenter 2, the supply of the fermenter 1 in loads of solid or semi-solid organic substrate are made so as to maintain in this fermenter a substantially constant volume of suspension of this substrate, taking into account the periodic sou-20 draw through the conduit 10 of mixed liquor, the recycling of alkaline effluent from the fermenter 2 in the fermenter 1 through the dip tube 5 and the withdrawal of acid liquid from the fermenter 1 through the T-pipe 11. In the fermenter 1, therefore, continuous biogas formation takes place, while a additional biogas is formed almost continuously in fermenter 2.

If necessary, part of the alkaline liquid effluent withdrawn from the fermenter 2 is evacuated through the evacuation pipe 25 by operating the three-way valve 24.

It can thus be seen that the process according to the invention ensures complete methanisation of the biodegradable materials contained in the starting organic substrate, without a fraction of these biodegradable materials being able to escape this methanisation. Thus, the yield of the process is optimal, the fermenter 2 serving to transform into biogas the 9 liquefied and acidified fractions of the substrate which have not been methanized in the fermenter 1 and which are recovered in the mixed liquor withdrawn from the fermenter 1 and especially in the acidic liquid fraction withdrawn by the con-5 duit T of fermenter 1.

The process according to the invention is illustrated by the following nonlimiting examples · EXAMPLE 1

Hydrodictyon 10 algae of the following composition was used as substrate ï total dry matter (TS): 939 g / kg volatile matter (VS) ï 482 g / kg chemical oxygen demand (DOC): 640 g O ^ / kg.

In the first fermenter 1 with a capacity of 15 2.8 liters containing communities of liquefying, acidifying and methanogenic bacteria, algae loads were introduced at the rate of one feed per day, so as to ensure in the fermenter 1 the following conditions: 0 - average hydraulic residence time (H): 1.04 20 days 6 - average solid residence time (H): 5.0 days - volume load (By) î 16 g COD / liter / day 25 - pH of the fermenter 1: 7.2.

The continuous supply of acidic liquid (pH: 7.2-7.5) from fermenter 2 and the withdrawal of alkaline effluent (pH: 7.5-8) from this fermenter (this alkaline effluent being recycled in fermenter 1 ) took place in such a way as to ensure the following conditions in fermenter 2: - average hydraulic residence time (H): 0.3 days - volume load (By): 7.41 g DOC / liter / day

- PH: 7.5. AT

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I 10

The contents of the two fermenters 1 and 2 have been main- | held at a temperature of 35 * C + 1e C. The overall charge | the installation was 12.5 g DOC / liter / day and the tenq> 8 of each fraction of algae 5 days.

I 'a 5 The overall biogas production obtained under regime was 3.39 1 of gas per liter of volume of fermentable material and per day with an average methane concentration of 66 96. This production of biogas was distributed by as follows: 3. 78 liters of biogas (6596 CH4) 10 per liter per day in the first fermenter 1 and 1.97 liters of biogas (75 96 CH4) per liter per day in the second fermenter 2. The yield economic obtained was 0.18 1 CH / g DOC.

15 EXAMPLE 2

An organic fraction of urban residues of the following composition was used as substrate: - total dry matter (TS) î 247 g / kg - volatile matter (VS): 204 g / kg 20 - chemical oxygen demand (DOC): 352 g / kg

In the first fermenter 1 with a capacity of 2.8 liters containing communities of liquefying, acidifying and methanogenic bacteria, loads of the organic fraction of urban residues were introduced at the rate of one feed per day, so as to ensure the following conditions in fermenter 1: 0 - average hydraulic residence time (H): 1.4 days - average solid residence time (S): 7 30 days - volume load (By): 23.35 g of DOC / liter / day - pH of the fermenter 1 to 7.3.

35 | i 11 i 'j.

The continuous supply of acid liquid to the fermenter 2 and the withdrawal of alkaline effluent from this fermenter 2 (this alkaline effluent being recycled in the fermenter 1) took place so as to ensure the following conditions 5 in the fermenter 2: - average hydraulic residence time (¾) s 0.4 day - volume load (By) î 24.36 g DOC / liter / day - pH î 7.5.

- stirring: 2 seconds per hour · 10 The content of the two fermenters 1 and 2 was kept at a temperature of 35 * C + 1 · C. The overall load of the installation was 18.17 g of COD / liter / day and the residence time of each load of urban organic waste of 7 days.

In operation, 4.95 liters of biogas per liter of volume of fermentable material and per day were obtained. an average methane content of 68 This production of biogas is distributed as follows: 4.74 liters of biogas (65% CH ^) per liter per day in the first 20 fermentor 1 and 5.60 liters of biogas (78% CH4) per liter per day in the second fermenter 2 · The economic yield was therefore in this case 0.18 liters of CH ^ / g of DOC.

The method according to the invention mainly offers the advantage over known methods of allowing a significant increase in the overall volume load of the installation and a spectacular reduction in the residence time of the organic substrate compared to known methods in which the anaerobic digestion of organic substrates is carried out in conventional mixed type fermenters.

Another advantage of the process according to the invention is to allow an optimal transformation into biogas of the biodegradable materials contained in the organic substrates, without significant loss of these biodegradable materials, unlike the known processes in which i inevitably occurs! 12 complete biogas methanation.

In known methods, biomethanisation is only complete in the case of an overload resulting in partial methanisation of the volatile fatty acids formed or in the case of an excessively short residence time resulting in incomplete liquefaction and acidogenesis.

These advantages are made possible mainly by * maintaining an optimal liquefaction pH (approximately 7.0 to 7.3) in fermenter 1, by recycling 10 the alkaline effluent (biocarbonated effluent) from fermenter 2 in the fermenter 1, thanks to the continuous withdrawal of only the volatile non-methanised fatty acids in the first fermenter 1, these acids being methanized in the second fermenter and thanks to the recycling of methanogenic microorganisms 15 with the alkaline effluent in the fermenter 2 from the I fermenter 1.

It is obvious that the invention is not limited to the details described above and that numerous modifications can be made to these details without departing from the scope of the invention.

Thus, the shape of fermenters 1 and 2 can be variable.

In the fermenter 1, the residence time of the solid fraction (organic substrate) can vary between 5 and 25 days and even more, depending on the rate of biodegradability of the substrate. In this same fermenter 1, the hydraulic residence time can be 10 to 0.3 days, depending on the rate of acidogenesis of the substrate. The volume load of the first fermenter can also vary for example between 30 1 and 25 COD / liter / day.

Instead of withdrawing the acidic liquid effluent by overflow or overflow, this withdrawal can be carried out using a pump.

In the second fermenter 2, the hydraulic residence time is also variable and can be between 0.25 and 10 days, while the volume load can be from 1 to 30 g of DOC / liter / day. k Γ 13 • k

In the second fermenter 2, the biomass "that is to say the mass of methanogenic bacteria can be fixed on a support or be free" while the intermittent agitation of the content of this second fermenter is optional! v 5 tative.

The effluent from the second fermenter can be decanted or not, before its recycling in the first fermenter, I, In the event of decantation, the decanted biomass can be returned, in batch, to the first fermenter 1, 10 Before its recycling in the first fermenter, the liquid effluent from the second fermenter can optionally be stored and its physico-chemical characteristics can possibly be modified.

! The mixed liquor drawn off from the first fermenter 1! 15 is advantageously filtered, in order to remove the non-degradable material from the substrate and, if the liquid filtrate still contains volatile methanizable acids, this liquid phase I is advantageously sent to the second fermenter 2.

If the acidic liquid effluent from the first fermenter at 20 is withdrawn by means of a pump, the two fermenters 1 and 2 can obviously be at the same level.

The separation and withdrawal of the acidic liquid effluent from the first fermenter 1 is advantageously carried out by not agitating the content of this fermenter 1 and thus taking advantage of the flotation and decantation of the solid particles. However, any liquid / solid separation system can be placed at the outlet of the acidic liquid effluent from the first fermenter, the contents of which can then be optionally agitated preferably continuously.

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Claims (13)

1 · * 4 It * “I CLAIMS I -! 1, Process for the biomethanisation of solid or semi-solid organic substrates "in which such a substrate suspended in water is subjected to the action of communities of microorganisms having successively the effect of extracting from the substrate water-soluble products, to transform these solubilized products into acids and to methanize these acids in two fermenters connected in series, characterized in that the suspension of organic substrate loaded in the first fermenter is subjected to solubilization of the organic materials contained therein, to acidogenesis and to methanization, in that an acidic liquid effluent is withdrawn from the first fermenter and this acidic liquid effluent is sent to the second fermenter charged with methanogenic microorganisms, where the acidic liquid effluent is methanized in contact with micro- methanogenic organisms, and in that we extract from the second fermenter an alkaline liquid effluent charged with methanogenic microorganisms and we recycle , possibly continuously, this alkaline liquid effluent in the first fermenter " 2. Method according to claim 1, characterized in that the acidic liquid effluent from the first fermenter is drawn off through a conduit opening into the upper part of the first fermenter and bringing this effluent to the lower part of the second fermenter · 3. Process according to either of Claims 1 and 2, characterized in that the alkaline liquid effluent containing methanogenic microorganisms is drawn off from the second fermenter also by a conduit opening into the upper part of the second fermenter and bringing this effluent \ alkaline liquid to the lower part of the first fermenter. 4 "Method according to either of claims 2 and 3, characterized in that the liquid effluents 35 are withdrawn from the first and second fermenter by overflow in the aforementioned conduits. | \ \ I * 15 5. Method according to any one of the claims | previous cations, characterized in that a mixed liquor containing non-biodegradable solid or semi-solid material is periodically withdrawn from the first fermenter, this mixed liquor is filtered to separate this non-biodegradable material and the filtrate is recycled. 6. Method according to claim 5 "characterized * in that the above-mentioned filtrate is recycled to the second farmer, when this filtrate still contains 10 methanizable fatty acids · 7. Method according to claim 1, characterized in that intermittently recycles 1 * alkaline liquid effluent from the second fermenter in the first fermenter. 8. A method according to any one of the preceding claims, characterized in that one introduces peri. new charges of organic substrate suspended in water in the first fermenter and a fraction of alkaline liquid withdrawn from the second fermenter is periodically removed, this fraction corresponding substantially to the amount of liquid introduced into the first fermenter with each new substrate charge. 9. Method according to any one of the preceding claims, characterized in that the I25 content of the first fermenter is homogenized each time a new charge of organic substrate is introduced therein and the above-mentioned mixed liquor is drawn off therefrom. 10. Method according to any one of the preceding claims, characterized in that the content of the second fermenter is intermittently stirred. 11. Method according to any one of the preceding claims, characterized in that the withdrawal of the acid liquid from the first fermenter is interrupted during the introduction of each new charge of substrate into this first fermenter and during each withdrawal of liquor mixed of! I - · "16 this first fermenter. 12. Method according to any one of the preceding claims, characterized in that a container with a capacity greater than 5 than that of the container used as a second fermenter is used as the first fermenter. * 13 · The method of claim 12, characterized in that the ratio of the capacity of the first fermenter to that of the second fermenter is between 1/10 and 10/1. 14. Method according to any one of the preceding claims, characterized in that the substrate charge is renewed in the first fermenter, when the preceding charge introduced into the latter contains substantially no more biodegradable material. 15 15 * Process according to claim 1, characterized in that the alkaline liquid effluent withdrawn from the second fermenter is subjected to decantation and the mass of methanogenic microorganisms separated by decantation is returned, in discon-timij to the first fermenter. 20 I \ 25 30 4t. % 35