Production Process for Producing Biogas by Means of Anaerobic Co-Digestion

Abstract

A production process for producing biogas by means of anaerobic co-digestion which includes (a) preparing hydrolyzed biomass from organic solid waste by means of a thermal hydrolysis treatment of waste at between 1.5 and 4.5 bar and between 120 and 160? C., generating raw biomass from which foreign matter is separated, giving rise to clean hydrolyzed biomass with at least 90% organic matter, a volatile solids to total solids ratio of at least 0.6 and at least 5% total solids; (b) mixing the clean hydrolyzed biomass with sludge from a wastewater treatment plant (WWTP), generating a mixture with a concentration of solids of less than 30% which is conditioned to a concentration of less than 20%; and (c) a step of wet anaerobic digestion in a digester at between 25? C. and 40? C. or between 50? ? C. and 60? C. and during an HRT of between 12 and 30 days, generating biogas and a digestate.

Claims

1-6. (canceled)

7. A production process for producing biogas by means of anaerobic co-digestion, characterized in that it comprises: (a) a first step of preparing hydrolyzed biomass from organic solid waste is selected from a group consisting of the organic fraction of municipal solid waste and assimilable waste, the organic waste from the selective collection of municipal solid waste and any combination thereof, wherein said first step in turn comprises: i. a first sub-step comprising a thermal hydrolysis treatment of the organic solid waste, wherein said thermal hydrolysis treatment is carried out at a pressure comprised between 1.5 and 4.5 bar and a temperature of between 120 and 160? C., resulting in raw biomass; ii. a second sub-step is then carried out for the post-treatment of the raw biomass, separating foreign matter and resulting in clean hydrolyzed biomass, characterized in that it comprises a percentage of organic matter of at least 90% by weight, a volatile solids to total solids weight ratio of at least 0.6 and a total solids content of at least 5% by weight; (b) a second step of mixing between 5% and 65% by weight of the clean hydrolyzed biomass obtained in the previous step with between 35% and 95% by weight of sludge from a wastewater treatment plant (WWTP), giving rise to a mixture with a concentration of solids of less than 30% by weight, wherein said mixture is then subjected to a conditioning process until achieving a concentration of solids of less than 20% by weight; (c) a third step of wet anaerobic digestion of the mixture obtained in the previous step in at least one digester, giving rise to biogas and a digestate, wherein said anaerobic digestion is carried out under mesophilic conditions of between 25? C. and 40? C. or thermophilic conditions of between 50? ? C. and 60? ? C. and during a hydraulic retention time (HRT) of between 12 and 30 days.

8. The process according to claim 7, wherein said process comprises an additional step of utilizing biogas by means of cogeneration, use in boilers for generating heat or use for biomethane production by means of a purification or upgrading process.

9. The process according to claim 7, wherein said process comprises an additional step of utilizing the digestate by means of agricultural use as a biofertilizer.

10. The process according to claim 7, wherein the organic solid waste is the organic fraction of municipal solid waste and wherein said process comprises a prior step of separating bulky foreign matter, understanding as such matter having a size greater than 80 mm and/or of separating ferrous metals by means of at least one magnetic separator.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0057] The terms FIG., FIGS., FIGURE, and Figures are used interchangeably in the specification to refer to the corresponding figures in the drawings.

[0058] To complement the present description, the following FIGURE is attached as an integral part thereof:

[0059] FIG. 1 shows a comparative graph of the theoretical production of biogas versus the production of biogas obtained by means of the process object of the invention.

DESCRIPTION OF THE INVENTION

[0060] A particular embodiment of the invention is described below for the purpose of demonstrating the advantages of the claimed process described above. Particularly, said particular embodiment was carried out based on organic solid waste originating from the selective collection of MSW in the city of Madrid, the composition of which is shown in the following table:

TABLE-US-00001 TABLE 1 Composition of the OSW used in the process Fraction Percentage (% by weight, wet base) Food waste* 63.7 Garden and pruning waste 8.7 LWC (Lightweight Containers) 8.0 Plastic 0.7 Paper/cardboard 3.8 Celluloses 1.4 Textiles 2.6 Glass 3.0 Other waste 8.1 *Fruit and vegetable waste, meat and fish waste, egg shells, shells from shellfish and dried fruits and nuts or other food waste, infusions, coffee grounds, etc.
Said waste was subjected to the following steps: [0061] a prior step of preparing waste by means of separating bulky foreign matter carried out in a trommel with a mesh size of 80 mm; [0062] a step of preparing hydrolyzed biomass from the organic solid waste obtained in the previous step. In turn, said step comprised: [0063] a first sub-step of thermal hydrolysis treatment at a pressure of 4 bar and at a temperature of 150? C. for a time of 20 minutes; [0064] a second sub-step of eliminating lightweight foreign matter by means of using a depacker, as well as eliminating heavy foreign matter by means of a sand trap.

[0065] At the end of this step, a substrate or clean hydrolyzed biomass with an organic content of 98% by weight was obtained. Said substrate was used in the co-digestion tests described below.

[0066] Particularly, to carry out said tests, two blanks characterized in that they comprised only anaerobic sludge (ANS) and four samples with a variable aerobic sludge (AES) and clean hydrolyzed biomass ratio were prepared. Particularly, the prepared samples were the following:

TABLE-US-00002 TABLE 2 Description of the samples, % by weight Mixture WWTP sludge (AES) Hydrolyzed biomass Sample 1 (AES) 100.0% 0% Sample 2 (93:7) 93.0% 7.0% Sample 3 (88:12) 88.0% 12.0% Sample 4 (80:20) 80.0% 20.0%

TABLE-US-00003 TABLE 3 Mass (g) of each of the samples Mass Mass Mass ANS AES Sample 2 Sample 3 Sample 4 Mixture mass mass (93:7) (88:12) (80:20) Blank 1 338.4 0 0 0 0 Blank 2 338.4 0 0 0 0 Sample 1 338.4 161.6 0 0 0 Sample 2 (93:7) 338.4 0 150.2 0 0 Sample 3 (88:12) 338.4 0 0 143 0 Sample 4 (80:20) 338.4 0 0 0 132.8

[0067] The main properties of each of the samples are summarized in the following table, wherein the methods of measuring the different parameters were the following: [0068] Total solids (TS): APHA 2540 B [0069] Volatile solids (VS): APHA 2540 E [0070] COD: APHA 5220 D [0071] COD sol.: APHA 5220 D [0072] VFAs: APHA 2310 B

TABLE-US-00004 TABLE 4 Main characteristics of the samples of the invention AES Hydrolyzed ANS Parameter Unit sludge biomass sludge Total solids (TS) % weight 5.94 9.35 4.21 Volatile solids (VS) % weight 2.89 6.02 1.38 COD ppm 69750 86375 18650 COD sol ppm 5800 24175 705 VFAs mg/I CaCO.sub.3 5237 13251 141

[0073] Water was added to each of the samples until achieving a total weight of 500 g. Each sample was prepared in triplicate.

[0074] The prepared samples were subjected to an anaerobic digestion process at a temperature of 35? C. No nutrients or buffers were added.

[0075] Next, an analysis of the obtained results was carried out, and these results are shown in the following tables:

TABLE-US-00005 TABLE 5 Result of TS and VS elimination Start End Start End Sample % TS % TS % VS % VS Blank 0.6 0.54 0.42 0.28 Sample 1 1.26 0.92 0.8 0.5 Sample 2 (93:7) 0.94 0.81 0.81 0.41 Sample 3 (88:12) 0.96 0.88 0.79 0.43 Sample 4 (80:20) 1.18 1.02 0.92 0.57

[0076] It has therefore been demonstrated that the percentage of VS/TS increases considerably when using hydrolyzed biomass originating from the organic fraction of the MSW as a substrate of the digester. Particularly, the percentage of VS/TS was 69% in the blanks (AES), 75% in sample 1 (ANS) and 80 to 85% in samples 2 to 4, which confirms the increase in biodegradability of the mixtures when feeding the digester with hydrolyzed biomass.

TABLE-US-00006 TABLE 6 Result of BOD and VFA elimination Start End Start End BODt BODt Total VFAs Total VFAs Sample (mg/I) (mg/I) (mg/I) (mg/I) Blank 4350 3785 259 26.5 Sample 1 10367 6178 2221.7 15.7 Sample 2 (93:7) 10620 4690 2842.3 34.4 Sample 3 (88:12) 10533 5290 2232.2 18.6 Sample 4 (80:20) 13100 7332 2564.8 31.3

[0077] The preceding results demonstrate that even when the amount of VS is the same, the COD increases with the presence of hydrolyzed biomass, as occurs with the VFAs, which confirms higher biodegradability of the samples.

TABLE-US-00007 TABLE 7 Specific CH.sub.4 production results by eliminated BOD Increase* I CH.sub.4/kg Production eliminated Mixture (I CH.sub.4/kg eliminated BOD) BOD* Increase Sample 1 (control) 48.5 0 0% Sample 2 (93:7) 112.6 64.1 132.2% Sample 3 (88:12) 108.6 60.1 123.9% Sample 4 (80:20) 213.8 165.3 340.8% *Represents the increase in the production of CH.sub.4 of each of the samples with respect to sample 1

[0078] The results demonstrate the synergy achieved as a result of the mixture of the hydrolyzed biomass obtained by means of a process of hydrolysis of the organic fraction of the MSW, with the sludge from a WWTP. Particularly, the process object of the invention achieves a non-linear production in the generation of biogas, achieving with 20% hydrolyzed biomass fed into the digester an increase of 340.8% in the specific production of methane per kilogram of COD eliminated and of 213.8% per kilogram of COD fed in. It has also been demonstrated that the use of hydrolyzed biomass in the mixture fed into the digester in which the anaerobic digestion process takes place increases the degree of hydrolysis and, therefore, biodegradability.

[0079] In addition to the improvement in the production of biogas, an analysis of its methane (CH.sub.4) content was carried out, as shown in the following table:

TABLE-US-00008 TABLE 8 Percentage (by volume) of methane Mixture % vol. (CH4) Sample 1 (control) 63.3 Sample 2 (93:7) 64.6 Sample 3 (88:12) 65.1 Sample 4 (80:20) 67.4

[0080] The preceding results demonstrate the improvement in the quality of the obtained biogas by increasing the percentage of hydrolyzed biomass used in the digester.

[0081] Lastly, for the purpose of demonstrating the synergy obtained as a result of the process object of the invention, a comparison was carried out between the production of biogas obtained by means of the claimed process with respect to that which would theoretically be obtained by the summation of the biogas produced by the sludge from the treatment plant and the biogas produced by the hydrolyzed biomass. The obtained results are shown in FIG. 1. Specifically, as shown in said FIGURE, the real production of biogas (continuous line) is 50% higher with respect to the maximum theoretical production (discontinuous line) equivalent to the theoretical sum of the production of biogas obtained from both substrates independently (i.e., without mixing them). This is due to the improvements derived from the addition of the hydrolyzed biomass to the sludge from the treatment plant. Particularly, the following aspects are improved: [0082] the organic carbon makeup, whereby increasing the C/N ratio (generally sludge from the treatment plant is rich in nitrogen and deficient in carbon); [0083] and the makeup in micronutrients, such as Zn, Co, Fe, K, or P.

[0084] The fact that the real production of biogas in co-digestion is 50% higher than the theoretical maximum demonstrates the synergy that exists when mixing sludge with hydrolyzed biomass.