Biogas production with headspace vacuum

Abstract

A system for producing biogas comprises a bioreactor, configured to produce the biogas under a reaction condition, including a main body that contains a sludge of a mixture of a biowaste, an alkaline chemical, a pH buffer agent and / or an iron-based additive, and a headspace that contains the produced biogas, a plurality of transducers, configured to measure a headspace pressure through communicating with a PLC for pressure control, installed at a plurality of locations in the bioreactor, and a vacuum pump, configured to extract the produced biogas out of the bioreactor and simultaneously to pump the headspace to a vacuum or a negative pressure, wherein, when the headspace pressure is above a set-point set by the PLC, the vacuum pump is turned on to extract the biogas out of the bioreactor; when the headspace pressure is below the set-point, the vacuum pump is turned off.

Description

BIOGAS PRODUCTION WITH HEADSPACE VACUUMTechnical Field

[0001] The present invention relates to vacuum-driven anaerobic digestion systems and methods for using the same to produce biogas, and, in particular, to single and two stages of vacuum-driven anaerobic digestion systems and methods of using the same to produce CH4 and H2.Background

[0002] Biological anaerobic processes utilize anaerobic microorganisms to treat bioavailable substrates from bio-based wastes. The process may be a single or multi-stage system, such as a two-stage system, depending on the purpose of operation. The single-stage system is generally designed to produce biomethane (biogas) via anaerobic digestion reaction or biohydrogen (biogas) via dark fermentation reaction. The two-stage system is designed to produce biomethane and biohydrogen at the same time by having the dark fermentation bioreactor and the anaerobic digestion bioreactor in series. The produced biomehane and biohydrogen may be extracted and purified to recover renewable energy sources such as heating energy, electricity, biohydrogen, biomethane, and / or biohythane at a post-treatment process. FIG. 7 is a block diagram of an existing two-stage anaerobic digestion system, for example, O-Thong et al., International Journal of Hydrogen Energy XXX (2016) 1-11. As shown, organic waste, i.e., bio-based waste is fed to a 1ststage of dark fermentation bioreactor and H2 and CO2 are produced through a dark fermentation process therein. The produced H2 and CO2 are then separated and biohydrogen H2 is obtained. Effluent from the 1ststage is then fed to a 2ndstage of anaerobic digestion bioreactor and CH4 and CO2 are produced therein via an anaerobic digestion reaction. The produced CH4 and CO2 are then separated and biomethane CH4 is obtained.

[0003] The produced biogas, such as, biomethane or biohydrogen, is released to solid / liquid phase in a dissolved form and is then transferred to gas phase to maintain equilibrium state between two phases. Slightly positive pressure is typically maintained in the headspace of the bioreactor and the transferred biogas is finally vented from the bioreactor to a gas recovery system. The biogas is accumulated in the headspace at the positive pressure condition. According to Le Chatelier’sprinciple, the accumulation of gaseous products in the solid / liquid phase inhibits the biogas generation and can lead to production of undesirable fermentation products such as ethanol, propionate, lactate, butanol and acetone, thereby decreasing the efficiency of a biogas yield in the anaerobic digestion process.

[0004] The typical anaerobic digestion process is operated under partially positive pressure driven by accumulated biogas in headspace. To upgrade the quality of biomethane, some researchers proposed a high-pressure digestion process (Lindeboom et al., “Autogenerative high pressure digestion: anaerobic digestion and biogas upgrading in a single step reactor system”, Water Sci. & Tech., 64(3), pp.647- 653, 2011). The principle of the high-pressure digestion is to increase the methane content in the headspace by dissolving more CO2 in the liquid phase. The composition of biogas in the gas and liquid phases is governed by the applied pressure level because of Henry’s constant’s difference, that is, 0.0016 mol / L / bar for CH4, 0.0318 mol / L / barfor CO2, 0.115 mol / L / bar for H2S, and 62 mol / L / bar for NH3, respectively (Wang et al., “Experimental study on the solubility of natural gas components in water with or without hydrate inhibitor”, Fluid Phase Equilibria, 207(1- 2), pp.143-154, 2003). The technical challenge of the high-pressure digestion is a significant pH drop due to the excessive formation of bicarbonate in the liquid phase (Lemmer et al., “Effects of high-pressure anaerobic digestion up to 30 bar on pH- value, production kinetics and specific methane yield”, Energy, 138, pp.659-667, 2017). It is recommended that the pH of digestate should remain between 6.5 and 7.5.

[0005] A few studies explored vacuum evaporation technology in fermentation processes for decoupling Solids Retention Time (SRT) and hydraulic retention time (HRT) (Okoye et al., “A proof-of-concept experimental study for vacuum-driven anaerobic biosolids fermentation using the IntensiCarb technology”, Water Environ. Res., 94(3), p10694, 2022; and Haroun et al., “Vacuum-enhanced anaerobic fermentation: Achieving process intensification, thickening and improved hydrolysis and VFA yields in a single treatment step”, Water Res., 220, p118719, 2022). The vacuum period, 1.5 or 7 hours depending on the HRT condition, is allocated at the beginning of the process to extract condensate for promoting thickening, fermentation, digestion, and dewatering of biosolids in a single place.

[0006] JP2009532193A to Goschl et al. discloses a device for digesting sludge, in which vacuum in the reactor chamber is used to lower oxygen concentration, insteadof improving a biogas yield.

[0007] LIS20160251587A1 to Nicholas et al. discloses a method and system for removing contaminants in biogas, in which vacuum is mentioned, but it is to increase volatilization rate of ammonia nitrogen, not to improve a biogas yield.

[0008] Clark et al. (“The effect of low pressure and mixing on biological hydrogen production via anaerobic fermentation”, International Journal of Hydrogen Energy, 37 (15), 11504 - 11513, 2012) disclose a headspace pressure below atmospheric levels that does not have a significant effect on gas production.

[0009] Yan et al. (“Influence of acidogenic headspace pressure on methane production under schematic of diversion of acidogenic off-gas to methanogenic reactor”, Bioresource Technology, 245, Part A, 1000 - 1007, 2017) disclose the effect of the headspace pressure in the acidogenic reactor on methane production, in which the two-stage anaerobic digestion system under the positive pressure condition is applied.

[0010] Valero et al. (“Influence of headspace pressure on methane production in Biochemical Methane Potential (BMP) tests”, Waste Management, 48, 193 - 198, 2016) disclose the effect of the headspace pressure on the methane production. This study is a batch biochemical methane potential experiment under positive pressure conditions.

[0011] Mandal et al. (“Improvement of Biohydrogen Production under Decreased Partial Pressure of H2 by Enterobacter cloacae”, Biotechnology Letters, 28, 831 - 835, 2006) disclose the effect of low partial pressure of H2 on hydrogen production in a dark fermentation. A pure culture (enterobacter cloacae) is used and tested under the positive pressure condition.

[0012] Thus, a need remains for improving biogas yields.Summary

[0013] Disclosed is a vacuum-driven process for producing biogas, the process comprising a) feeding a feedstock to a bioreactor forming a sludge of the feedstock therein; b) producing the biogas in the bioreactor under a reaction condition; b) predetermining a set-point of a headspace pressure in the bioreactor; c) extracting the biogas out of the bioreactor by a vacuum pump, and simultaneously pumping the headspace of the bioreactor to a vacuum or a negativepressure; and d) repeating steps a) to c) over a period of a reaction to maintain the vacuum or negative pressure of the headspace constant, wherein, when the headspace pressure is above the set-point, the vacuum pump is turned on to extract the biogas out of the bioreactor; when the headspace pressure is below the set-point, the vacuum pump is turned off, so that the biogas is favorable to produce in the bioreactor under the vacuum or the negative pressure.

[0014] In some embodiments, the feedstock is a mixture of a biowaste, an alkaline chemical, a pH buffer agent and / or an iron-based additive.

[0015] In some embodiments, the vacuum or the negative pressure ranges from - 0.1 to -15 psig.

[0016] In some embodiments, the vacuum or the negative pressure from -0.1 to - 10 psig.

[0017] In some embodiments, the vacuum or the negative pressure from -0.1 to - 5 psig.

[0018] In some embodiments, the bioreactor is an anaerobic digestion bioreactor.

[0019] In some embodiments, the process further comprising producing the biogas of CH4 and CO2; separating the biogas CH4 and CO2 into a CH4-enriched gas stream and a CO2-enriched gas stream; forwarding the CH4-enriched gas stream to a post treatment process; and venting out the CC>2-enriched gas stream, or recycling the CC>2-enriched gas stream back to the anaerobic digestion bioreactor.

[0020] In some embodiments, the reaction condition includes a mesophilic or thermophilic condition of the sludge of the mixture in the anaerobic digestion bioreactor ranging from 25 to 60°C.

[0021] In some embodiments, the reaction condition includes a mesophilic or thermophilic condition of the sludge of the mixture in the anaerobic digestion bioreactor ranging from 30 to 50°C.

[0022] In some embodiments, the reaction condition includes a mesophilic or thermophilic condition of the sludge of the mixture in the anaerobic digestion bioreactor ranging from 30 to 40°C.

[0023] In some embodiments, the reaction condition includes a pH of the feedingmixture before feeding the anaerobic digestion bioreactor ranging from 7 to 11 .

[0024] In some embodiments, the reaction condition includes a pH of the feeding mixture before feeding the anaerobic digestion bioreactor ranging from 8 to 9.

[0025] In some embodiments, the reaction condition includes a pH of the sludge of the mixture in the anaerobic digestion bioreactor ranging from 6 to 9.

[0026] In some embodiments, a retention time of the sludge of the mixture in the anaerobic digestion bioreactor is maintained between 10 days and 40 days.

[0027] In some embodiments, the bioreactor is a dark fermentation bioreactor.

[0028] In some embodiments, the process further comprising producing the biogas of H2 and CO2; separating the biogas H2 and CO2 into a H2-enriched gas stream and a CO2- enriched stream; forwarding the H2-enriched gas stream to a post treatment process; and venting out the CC>2-enriched gas stream.

[0029] In some embodiments, the reaction condition includes a thermophilic or hyperthermophilic condition of the dark fermentation bioreactor ranging from 25 to 80°C.

[0030] In some embodiments, the reaction condition includes a thermophilic or hyperthermophilic condition of the dark fermentation bioreactor ranging from 40 to 60°C.

[0031] In some embodiments, the reaction condition includes a thermophilic or hyperthermophilic condition of the dark fermentation bioreactor ranging from 50 to 55°C.

[0032] In some embodiments, the reaction condition includes a pH of the feeding mixture before feeding the dark fermentation bioreactor ranging from 8 to 10.

[0033] In some embodiments, the reaction condition includes a pH of the feeding mixture before feeding the dark fermentation bioreactor ranging from 8 to 9.

[0034] In some embodiments, the reaction condition includes a pH of the sludge of the mixture in the dark fermentation bioreactor ranging from 5 to 7.

[0035] In some embodiments, the reaction condition including a pH of the sludge of the mixture in the dark fermentation bioreactor ranging from slightly acidic to neutral.

[0036] In some embodiments, a retention time of the sludge of the mixture in thedark fermentation bioreactor is maintained between 10 hours and 3 days.

[0037] In some embodiments, the biowaste is food waste, agricultural waste, sewage sludge and the like.

[0038] In some embodiments, the alkaline chemical is selected from sodium hydroxide, magnesium hydroxide or potassium hydroxide.

[0039] In some embodiments, the pH buffer agent is selected from sodium bicarbonate or a mixture of monopotassium and dipotassium phosphates.

[0040] In some embodiments, the iron-based additive is selected from ferrous iron, iron oxides or iron monomers.

[0041] In some embodiments, the iron-based additive is nano zerovalent iron or zerovalent scrap iron.

[0042] Disclosed is a system for producing biogas, the system comprising a bioreactor, configured to produce the biogas under a reaction condition, including a main body that contains a sludge of a mixture of a biowaste, an alkaline chemical, a pH buffer agent and an iron additive; and a headspace that contains the produced biogas; a plurality of transducers, configured to measure a headspace pressure through communicating with a PLC for pressure control, installed at a plurality of locations in the bioreactor; and a vacuum pump, configured to extract the produced biogas out of the bioreactor and simultaneously to pump the headspace to a vacuum or a negative pressure; wherein, when the headspace pressure is above a set-point set by the PLC, the vacuum pump is turned on to extract the biogas out of the bioreactor; when the headspace pressure is below the set-point, the vacuum pump is turned off, so that the biogas is favorable to produce in the bioreactor under the vacuum or the negative pressure.

[0043] In some embodiments, the system further comprising a gas separation device, configured to separate the produced biogas from CO2, wherein a biogas stream is sent to a post-treatment process and a CO2- enriched stream is recirculated back to the bioreactor.

[0044] In some embodiments, the gas separation device is a gas membrane separation device.

[0045] In some embodiments, the gas separation device is a pressure swing adsorption device.

[0046] In some embodiments, the vacuum or the negative pressure ranges from - 0.1 to -15 psig.

[0047] In some embodiments, the vacuum or the negative pressure ranges from - 0.1 to -10 psig.

[0048] In some embodiments, the vacuum or the negative pressure ranges from - 0.1 to -5 psig.

[0049] In some embodiments, when the bioreactor is an anaerobic digestion reactor, the reaction condition includes a mesophilic or thermophilic condition of the anaerobic digestion reactor ranging from 25 to 60°C.

[0050] In some embodiments, when the bioreactor is an anaerobic digestion reactor, the reaction condition includes a mesophilic or thermophilic condition of the anaerobic digestion reactor ranging from 30 to 50°C.

[0051] In some embodiments, when the bioreactor is an anaerobic digestion reactor, the reaction condition includes a mesophilic or thermophilic condition of the anaerobic digestion reactor ranging from 30 to 40°C.

[0052] In some embodiments, the reaction condition includes a pH of the sludge of the mixture before feeding the anaerobic digestion reactor ranging from 8 to 10.

[0053] In some embodiments, the reaction condition includes a pH of the sludge of the mixture before feeding an anaerobic digestion reactor ranging from 8 to 9.

[0054] In some embodiments, the reaction condition includes a pH of the sludge of the mixture in an anaerobic digestion reactor ranging from 5 to 7.

[0055] In some embodiments, a retention time of the sludge of the mixture in an anaerobic reactor is between 10 days and 40 days.

[0056] In some embodiments, when the reactor is a dark fermentation reactor, the reaction condition includes a thermophilic or hyperthermophilic condition of the dark fermentation reactor ranging from 25 to 80°C.

[0057] In some embodiments, when the reactor is a dark fermentation reactor, the reaction condition includes a thermophilic or hyperthermophilic condition of the dark fermentation reactor ranging from 40 to 60°C.

[0058] In some embodiments, when the reactor is a dark fermentation reactor, the reaction condition includes a thermophilic or hyperthermophilic condition of the darkfermentation reactor ranging from 50 to 55°C.

[0059] In some embodiments, the reaction condition includes a pH of the sludge of the mixture before being fed to a dark fermentation reactor ranging from 7 to 11 .

[0060] In some embodiments, the reaction condition includes a pH of the sludge of the mixture before being fed to a dark fermentation reactor ranging from 8 to 9.

[0061] In some embodiments, the reaction condition includes a pH of the sludge of the mixture in a dark fermentation reactor ranging from 6 to 9.

[0062] In some embodiments, a retention time of the sludge of the mixture in dark fermentation reactor is between 10 hours and 3 days.

[0063] In some embodiments, the biogas is methane.

[0064] In some embodiments, the biogas is hydrogen.

[0065] Disclosed is a vacuum-driven two-stage anaerobic digestion process for producing biogas, the process comprising: a) producing the biogas of H2 and CO2 in a dark fermentation bioreactor under a fermentation condition by feeding a first mixture of a biowaste, an alkaline chemical, a pH buffer agent and / or an iron-based additive to the dark fermentation bioreactor forming a sludge of the first mixture therein; b) extracting the biogas of H2 and CO2 out of the dark fermentation bioreactor by a first vacuum pump, and simultaneously pumping the headspace of the dark fermentation bioreactor to a first vacuum or a first negative pressure; c) forwarding the biogas of H2 and CChto an anaerobic digestion bioreactor producing a biogas of CH4 therein under an anaerobic condition by feeding a second mixture of a waste solid digestate form the dark fermentation bioreactor, the alkaline chemical, the pH buffer agent and the iron-based additive to the anaerobic digestion bioreactor forming a sludge of the second mixture therein; d) extracting the biogas of CH4 and CO2 out of the anaerobic digestion bioreactor by a second vacuum pump, and simultaneously pumping the headspace of the anaerobic digestion bioreactor to a second vacuum or a second negative pressure; e) separating the biogas of CH4and CCMo a CH4-enriched gas stream and a CC>2-enriched gas stream; f) forwarding the CC>2-enriched gas stream from the anaerobic digestion bioreactor back to the anaerobic digestion bioreactor; g) forwarding the CH4-enriched gas stream to a post-treatment process; andh) repeating steps a) to g) over a period of a dark fermentation reaction and an anaerobic digestion reaction to maintain the headspace pressure of each bioreactor constant, wherein, when the headspace pressure of the dark fermentation bioreactor is above a first set-point, the first vacuum pump is turned on to extract the biogas of H2 and CO2 out of the dark fermentation bioreactor; when the headspace pressure of the dark fermentation bioreactor is below the first set-point, the first vacuum pump is turned off, so that the biogas of H2 and CO2 is favorable to produce in the dark fermentation bioreactor under the first vacuum or the first negative pressure, wherein, when the headspace pressure of the anaerobic digestion bioreactor is above a second set-point, the second vacuum pump is turned on to extract the biogas CH4 and CO2 out of the anaerobic digestion bioreactor; when the headspace pressure of the anaerobic digestion bioreactor is below the second set-point, the second vacuum pump is turned off, so that the biogas CH4 and CO2 is favorable to produce in the anaerobic digestion bioreactor under the second vacuum or the second negative pressure.

[0066] In some embodiments, the vacuum-driven two-stage anaerobic digestion process further comprising: separating the biogas H2 and CO2 into a Hh-enriched gas stream and a CO2- enriched stream; forwarding the Fh-enriched gas stream to a post treatment process or recycling to the anaerobic digestion bioreactor; and discharging the CC»2-enriched stream.

[0067] In some embodiments, the vacuum-driven two-stage anaerobic digestion process further comprising: discharging a waste solid digestate from the anaerobic digestion bioreactor; or partially recycling the waste solid digestate from the anaerobic digestion bioreactor back to the first mixture of the dark fermentation bioreactor and discharging the rest of the waste solid digestate.

[0068] In some embodiments, the vacuum-driven two-stage anaerobic digestion process further comprising: discharging a waste solid digestate from the anaerobic digestion reactor.

[0069] In some embodiments, the first and second set-points are the same.

[0070] In some embodiments, the first and second set-points are different fromeach other.

[0071] In some embodiments, the first and second vacuums or the first and second negative pressures range from -0.1 to -15 psig, respectively.

[0072] In some embodiments, the first and second vacuums or the first and second negative pressures range from -0.1 to -10 psig, respectively.

[0073] In some embodiments, the first and second vacuums or the first and second negative pressures range from -0.1 to -5 psig, respectively.

[0074] In some embodiments, the dark fermentation condition includes a thermophilic or hyperthermophilic condition of the dark fermentation bioreactor ranging from 25 to 80°C.

[0075] In some embodiments, the dark fermentation condition includes a thermophilic or hyperthermophilic condition of the dark fermentation bioreactor ranging from 40 to 60°C.

[0076] In some embodiments, the dark fermentation condition includes a thermophilic or hyperthermophilic condition of the dark fermentation bioreactor ranging from 50 to 55°C.

[0077] In some embodiments, the anaerobic condition includes a mesophilic or thermophilic condition of the anaerobic digestion bioreactor ranging from 25 to 60°C.

[0078] In some embodiments, the anaerobic condition includes a mesophilic or thermophilic condition of the anaerobic digestion bioreactor ranging from 30 to 50°C.

[0079] In some embodiments, the anaerobic condition includes a mesophilic or thermophilic condition of the anaerobic digestion bioreactor ranging from 30 to 40°C.

[0080] In some embodiments, the dark fermentation condition includes a pH of the first feeding mixture before feeding the dark fermentation bioreactor ranging from 8 to 10.

[0081] In some embodiments, the dark fermentation condition includes a pH of the sludge of the first mixture in the dark fermentation bioreactor ranging from 5 to 7.

[0082] In some embodiments, the anaerobic condition includes a pH of the second feeding mixture before feeding the anaerobic digestion bioreactor ranging from 7 to 11.

[0083] In some embodiments, the anaerobic condition includes a pH of the sludge of the second mixture in the anaerobic digestion bioreactor ranging from 6 to 9.

[0084] Disclosed is a system for producing biogas, the system comprisinga bioreactor, configured to produce the biogas under a reaction condition, including a main body that contains a sludge of a mixture of a biowaste, an alkaline chemical, a pH buffer agent and / or an iron-based additive; and a headspace that contains the produced biogas; a plurality of transducers, configured to measure a headspace pressure through communicating with a PLC for pressure control, installed at a plurality of locations in the bioreactor; and a vacuum pump, configured to extract the produced biogas out of the bioreactor and simultaneously to pump the headspace to a vacuum or a negative pressures; wherein, when the headspace pressure is above a set-point set by the PLC, the vacuum pump is turned on to extract the biogas out of the bioreactor; when the headspace pressure is below the set-point, the vacuum pump is turned off, so that the biogas is favorable to produce in the bioreactor under the vacuum or the negative pressures.

[0085] In some embodiments, the system further comprising a gas separation device, configured to purify the produced biogas to a purified biogas stream for a post-treatment process and a CC>2-enriched stream for recirculation back to the bioreactor.

[0086] In some embodiments, the vacuum or the negative pressures ranges from -0.1 to -15 psig.

[0087] In some embodiments, the vacuum or the negative pressures ranges from -0.1 to -10 psig.

[0088] In some embodiments, the vacuum or the negative pressures ranges from -0.1 to -5 psig.

[0089] In some embodiments, when the bioreactor is a dark fermentation bioreactor, the reaction condition includes a thermophilic or hyperthermophilic condition of the dark fermentation bioreactor ranging from 25 to 80°C.

[0090] In some embodiments, when the bioreactor is a dark fermentation bioreactor, the reaction condition includes a thermophilic or hyperthermophilic condition of the dark fermentation bioreactor ranging from 40 to 60°C.

[0091] In some embodiments, when the bioreactor is a dark fermentation bioreactor, the reaction condition includes a thermophilic or hyperthermophilic condition of the dark fermentation bioreactor ranging from 50 to 55°C.

[0092] In some embodiments, the reaction condition includes a pH of the feeding mixture before being fed to the dark fermentation bioreactor ranging from 8 to 10.

[0093] In some embodiments, the reaction condition includes a pH of the sludge of the mixture in the dark fermentation bioreactor ranging from 5 to 7.

[0094] In some embodiments, when the bioreactor is an anaerobic digestion bioreactor the reaction condition includes a mesophilic or thermophilic condition of the anaerobic digestion bioreactor ranging from 25 to 60°C.

[0095] In some embodiments, when the bioreactor is an anaerobic digestion bioreactor the reaction condition includes a mesophilic or thermophilic condition of the anaerobic digestion bioreactor ranging from 30 to 50°C.

[0096] In some embodiments, when the bioreactor is an anaerobic digestion bioreactor the reaction condition includes a mesophilic or thermophilic condition of the anaerobic digestion bioreactor ranging from 30 to 40°C.

[0097] In some embodiments, the reaction condition includes a pH of the feeding mixture before feeding the anaerobic digestion bioreactor ranging from 7 to 11 .

[0098] In some embodiments, the reaction condition includes a pH of the sludge of the mixture in the anaerobic digestion bioreactor ranging from 6 to 9.

[0099] Disclosed is a vacuum-driven two-stage anaerobic digestion system for producing biogas, the system comprising: a dark fermentation bioreactor, configured to produce the biogas of H2 and CO2 under a fermentation condition, including a first body that contains a sludge of a first mixture of a biowaste, an alkaline chemical, a pH buffer agent and / or an iron-based additive; and a first headspace that contains the produced biogas; a plurality of a first set of transducers, configured to measure a pressure of the first headspace through communicating with a PLC for pressure control, installed at a plurality of locations in the dark fermentation bioreactor; a first vacuum pump, configured to extract the produced biogas of H2and CO2 out of the dark fermentation bioreactor and simultaneously to pump the first headspace to a first vacuum or a first negative pressure;an anaerobic digestion bioreactor, configured to produce the biogas of CF and CO2 under an anaerobic digestion condition, including a second body that contains a sludge of a second mixture of a waste solid digestate form the dark fermentation bioreactor, the alkaline chemical, the pH buffer agent and the iron-based additive; and a second headspace that contains the produced biogas of CH4 and CO2; a plurality of a second set of transducers, configured to measure a pressure of the second headspace through communicating with the PLC for pressure control, installed at a plurality of locations in the anaerobic digestion bioreactor; a second vacuum pump, configured to extract the produced biogas of CH4 and CO2 out of the anaerobic digestion bioreactor and simultaneously to pump the second headspace to a second vacuum or a second negative pressure; wherein, when the headspace pressure of the dark fermentation bioreactor is above a first set-point, the first vacuum pump is turned on to extract the biogas of H2 and CO2 out of the dark fermentation bioreactor; when the headspace pressure of the dark fermentation bioreactor is below the first set-point, the first vacuum pump is turned off, so that the biogas of H2 and CO2 is favorable to produce in the dark fermentation bioreactor under the first vacuum or the first negative pressure, wherein, when the headspace pressure of the anaerobic digestion bioreactor is above a second set-point, the second vacuum pump is turned on to extract the biogas CH4 and CO2 out of the anaerobic digestion bioreactor; when the headspace pressure of the anaerobic digestion bioreactor is below the second set-point, the second vacuum pump is turned off, so that the biogas CH4 and CO2 is favorable to produce in the anaerobic digestion bioreactor under the second vacuum or the second negative pressure.

[0100] In some embodiments, the vacuum-driven two-stage anaerobic digestion system further comprising a first gas separation device, configured to separate the biogas of H2 and CO2 to a H2-enriched gas stream and a CO2-enriched stream, wherein the H2-enriched gas stream is sent to post-treatment processes and the CO2-enriched gas stream is either vented out or recirculated to the anaerobic digestion bioreactor for producing CH4.

[0101] In some embodiments, the vacuum-driven two-stage anaerobic digestion system further comprisinga second gas separation device, configured to separate the biogas of CH4 and CO2to a CH4-enriched gas stream and a CC>2-enriched gas stream, wherein the CH4-enriched gas stream is sent to the post-treatment processes and the CO2- enriched gas stream is recirculated back to the anaerobic digestion bioreactor for producing CH4.

[0102] In some embodiments, the first gas separation device being a pressure swing adsorption (PSA).

[0103] In some embodiments, the second gas separation device being a gas membrane separation device.Brief Description of the Drawings

[0104] For a further understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:FIG. 1 is a scheme that operates to produce CH4in an anaerobic digestion bioreactor with -0.1 to -15 psig of a low headspace vacuum condition in accordance with an exemplary embodiment of the present invention;FIG. 2 is a scheme that operates to produce H2in a dark fermentation bioreactor with -0.1 to -15 psig of a low headspace vacuum condition in accordance with an exemplary embodiment of the present invention;FIG. 3A is a block diagram of a scheme that operates to produce CH4 in a two-stage anaerobic digestion system with -0.1 to -15 psig of a headspace vacuum condition in each bioreactor in accordance with an exemplary embodiment of the present invention;FIG. 3B is a block diagram of a scheme that operates to produce CH4 in a two-stage anaerobic digestion system with -0.1 to -15 psig of a headspace vacuum condition in each bioreactor in accordance with another exemplary embodiment of the present invention;FIG. 3C is a block diagram of a scheme that operates to produce CH4 in a two-stage anaerobic digestion system with -0.1 to -15 psig of a headspace vacuum condition in each bioreactor in accordance with another exemplary embodiment of the present invention;FIG. 3D is a block diagram of a scheme that operates to produce CH4 in a two-stageanaerobic digestion system with -0.1 to -15 psig of a headspace vacuum condition in each bioreactor in accordance with another exemplary embodiment of the present invention;FIG. 3E is a block diagram of a scheme that operates to produce CF and H2 in a two-stage anaerobic digestion system with -0.1 to -15 psig of a headspace vacuum condition in each bioreactor in accordance with another exemplary embodiment of the present invention;FIG. 3F is a block diagram of a scheme that operates to produce CH4 and H2 in a two-stage anaerobic digestion system with -0.1 to -15 psig of a headspace vacuum condition in each bioreactor in accordance with another exemplary embodiment of the present invention;FIG. 4 is a block diagram of a lab setup that operates to produce CH4 and H2 in a two-stage anaerobic digestion system with 0 to -2 psig vacuum condition in the headspace of the 1ststage dark fermentation bioreactor and -1 to -5 psig vacuum condition in the 2ndstage anaerobic digestion bioreactor in accordance with another exemplary embodiment of the present invention;FIG. 5 is a result of biohydrogen production with a headspace vacuum in a dark fermentation bioreactor in the two-stage anaerobic digestion system as shown in FIG. 4;FIG. 6 is a result of biomethane production with a headspace vacuum in an anaerobic digestion bioreactor in the two-stage anaerobic digestion system as shown in FIG. 4; andFIG. 7 is a block diagram of an existing two-stage anaerobic digestion system.Description of Preferred Embodiments

[0105] Disclosed are systems and methods for biogas production in a bioreactor with vacuum or a negative pressure in the headspace of the bioreactor. The disclosed are vacuum-driven anaerobic digestion processes. More specifically, the disclosed are vacuum-driven single-stage, two-stage or more than two-stage anaerobic digestion processes. The produced biogas may be a mixture of methane and carbon dioxide and / or a mixture of hydrogen and carbon dioxide. The disclosed methods require a relative low energy input for the bioreactor headspace vacuum comparing to pressurization that improves biogas production yields. A suitable vacuum pump provides supplemental energy for post-processes to removecontaminants, such as moisture, CO2 volatile organic carbons, siloxane, hydrogen sulfide, and the like. A net energy balance from the disclosed vacuum-driven biogas production process is positive compared to the conventional processes. The disclosed methods and systems apply vacuum in the headspace of biogas production bioreactors, instead of a positive pressure, which improves physical transfer of dissolved biogas to gas phase according to Henry’s law and stimulates biogas production yields to achieve an energy-positive biogas production process by reducing a partial pressure of the biogas in the headspace of the bioreactor.

[0106] The disclosed systems and methods provide vacuum-driven anaerobic processes in bioreactors to treat bio-based waste or biowaste, e.g., food waste, agricultural waste, sewage sludge, etc. Anaerobic microorganisms convert organic components contained in the bio-based waste into biogas including methane, hydrogen, carbon dioxide, and / or hydrogen sulfide. The produced raw biogas in the headspace of the bioreactor is extracted and purified to recover renewable energy sources such as heating energy, electricity, biohydrogen, biomethane, and / or biohythane. The disclosed systems maintain vacuum (e.g., -0.1 to -15 psig) in the headspace to enhance the transfer of dissolved biogas into the headspace of the bioreactor, thus improving the efficiency of the biogas production compared to conventional non-vacuum systems (i.e. , positive pressure in the headspace). Furthermore, the vacuum energy used in the disclosed processes provides supplemental energy for purification and digestate mixing. Here, since the produced raw biogas is extracted from the headspace of the bioreactor and pressurized, the pressure from the produced raw biogas may give a driving force for gas separation (biogas purification or upgrading) devices. Permeate CC>2-enriched gas may be recirculated back to the reactor and diffused to mix digestate.

[0107] In the disclosed methods and systems, biogas production processes may be performed in a single-stage or a multiple-stage configuration, such as a two-stage configuration, depending on the type of final biogas products. Organic compounds in bio-based wastes, such as food waste, agricultural waste, or sewage sludge, are converted to methane, hydrogen, and carbon dioxide via four reactions: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The single-stage process includes a dark fermentation or an anaerobic digestion. Hydrolysis and acidogenesis take place in the dark fermentation process to produce biohydrogen and carbon dioxide. The above four reactions take pace in series as a chain reaction in ananaerobic digestion process, and biomethane and carbon dioxide are produced as final products.

[0108] On the other hand, a two-stage process includes a 1ststage of a dark fermentation bioreactor and a 2ndstage of an anaerobic digestion bioreactor. The two-stage process produces biohydrogen and carbon dioxide in the 1ststage of the dark fermentation bioreactor and biomethane and carbon dioxide in the 2ndstage of the anaerobic digestion bioreactor. The produced biomehane and biohydrogen may be extracted and purified or separated to recover renewable energy sources such as heating energy, electricity, biohydrogen, biomethane, and / or biohythane at a posttreatment process.

[0109] The disclosed single-stage process may be an anaerobic digestion process or a dark fermentation process. Both processes operates at a vacuum of - 0.1 to -15 psig, preferably -0.1 to -10 psig, more preferably -0.1 to -5 psig, in the headspace of an anaerobic digestion bioreactor or a dark fermentation bioreactor using a vacuum device that may be any existing suitable vacuum devices to generate the vacuum in the headspace. A plurality of pressure transducers are installed at multiple locations in the anaerobic digestion bioreactor and the dark fermentation bioreactor to monitor headspace pressure of both bioreactors, respectively. A programmable logic controller (PLC) is connected with the transducers to control the headspace pressure. A mesophilic or thermophilic condition between 25 and 60°C, preferably between 30 and 50°C, more preferably between 30 and 40°C, gives a favorable condition for the anaerobic digestion process. A thermophilic or hyperthermophilic condition between 25 and 80°C, preferably between 40 and 60°C, more preferably between 50 and 55°C, gives a favorable condition for the dark fermentation process. Alkaline and pH buffer agents may be add to a feedstock stream of a bio-based waste or the bioreactor to adjust an initial pH of the bio-based waste and maintain a favorable condition for acidogenic or methanogenic microorganisms. Iron-based additives such as ferrous element or zerovalent iron may be added to the feedstock stream of the bio-based waste or the bioreactor to stimulate electron transfer between ferredoxin and hydrogenase for hydrogen production in the dark fermentation process and for methane production in the anaerobic digestion process. The produced biogas in the headspace of the bioreactor is vented from the top of the bioreactor and transferred to post-treatmentprocesses. When a gas membrane separation process or other adsorption processes is used to separate or purify the produced biogas in the dark fermentation bioreactor, a permeate gas or liquid stream which is enriched in CO2 may be discharged as waste stream. When a gas membrane separation process is used to purify the produced biogas in the anaerobic digestion bioreactor, a permeate gas stream which is enriched in CO2 may be recirculated back to the anaerobic digestion reactor, or to an algal photo-bioreactor, or to a hydrogenotrophic reactor for additional biomethane production.

[0110] Anaerobic microorganisms are retained in a solid / liquid digestate suspension or attached on a fixed media carrier for biogas production. The disclosed two-stage process may include a dark fermentation process and an anaerobic digestion process in series. Similar to the single-stage process, both processes operates at a vacuum or a negative pressure of -0.1 to -15 psig, preferably -0.1 to - 10 psig, more preferably -0.1 to -5 psig, in the headspace of a dark fermentation bioreactor and an anaerobic digestion bioreactor using a vacuum device that may be any existing suitable vacuum devices to generate the vacuum or the negative pressure therein. A plurality of pressure transducers are installed at multiple locations in the anaerobic digestion bioreactor and the dark fermentation bioreactor to monitor headspace pressures of both bioreactors. A programmable logic controller (PLC) is connected with the transducers to control the headspace pressures of both bioreactors. A mesophilic or thermophilic condition between 25 and 60°C, preferably between 30 and 50°C, more preferably between 30 and 40°C, gives a favorable condition for the anaerobic digestion process in the anaerobic digestion bioreactor. A thermophilic or hyperthermophilic condition between 25 and 80°C, preferably between 40 and 60°C, more preferably between 50 and 55°C, gives a favorable condition for the dark fermentation process in the dark fermentation bioreactor. Alkaline and pH buffer agents may be add to a feedstock stream of a bio-based waste or the dark fermentation bioreactor to adjust an initial pH of the bio-based waste and maintain a favorable condition for acidogenic or methanogenic microorganisms. Iron-based additives such as ferrous element or zerovalent iron may be added to the feedstock stream of the bio-based waste or the bioreactor to stimulate electron transfer between ferredoxin and hydrogenase for hydrogen production in the dark fermentation process. The produced biogas of H2 and CO2 inthe headspace of the dark fermentation bioreactor is vented from the top of the dark fermentation bioreactor and transferred to either directly to the anaerobic digestion bioreactor for producing biomethane or separated or purified through a gas separation device in which biohydrogen is vented to post-treatment processes and CO2 is vent out. When a gas membrane separation process or other adsorption processes is used to separate or purify the produced biogas in the dark fermentation bioreactor, a permeate gas or liquid stream which is enriched in CO2 may be discharged as waste stream. When a gas membrane separation process is used to purify the produced biogas in the anaerobic digestion bioreactor, a permeate gas stream which is enriched in CO2 may be recirculated back to the anaerobic digestion bioreactor, or to an algal photo-bio reactor, or to a hydrogenotrophic reactor for additional biomethane production. A part of the effluent of the 2ndstage bioreactor are wasted to control the concentrations of anaerobic microorganisms. Similar to the 1ststage process, additional alkaline and the pH buffer agents may be fed into the 2ndstage anaerobic digestion bioreactor to adjust an initial pH of the effluent from the 1ststage and maintain a favorable condition for methanogenic microorganisms. Ironbased additives such as ferrous element or zerovalent iron may also be added to the feedstock stream of the bio-based waste or the bioreactor to stimulate electron transfer between ferredoxin and hydrogenase for methane production in the anaerobic digestion process. A thermophilic or hyperthermophilic condition ranging from 25 to 80°C, preferably from 40 to 60°C, more preferably from 50 to 55°C, gives a favorable condition for the 1ststage of dark fermentation process. On the other hand, for the 2ndanaerobic digestion process a mesophilic or thermophilic condition ranging from 25 to 60°C, preferably from 30 to 50°C, more preferably from 30 to 40°C, is desirable for methane production.The produced biogas is released into gas phase and released to the headspace of the bioreactors. A slight negative pressure, such as -0.1 to -15 psig, is applied to maintain low partial pressures of gaseous byproducts in the headspace using a vacuum pump. A post-treatment processes such as adsorption filter / membrane separation, knock-out drum, stripper tower, condenser, and distillation system are required to remove carbon dioxide and other contaminants such as siloxane, hydrogen sulfide, moisture, volatile organic carbon or the like. The waste carbon dioxide stream generated from the gas separation process may be recycled back tothe 2ndstage bioreactor of the two-stage process or the anaerobic digestion bioreactor of the single-stage anaerobic digestion process for additional methane production.

[0111] The following are exemplary embodiments of the disclosed vacuum-driven biogas production systems, methods and processes.

[0112] In one embodiment, a single-stage anaerobic digestion with headspace vacuum, alkaline chemicals and pH buffer agents, supplemental iron addition and CO2-enriched gas recirculation or discharge for biomethane production. FIG. 1 is a block diagram of this embodiment that operates to produce methane with -0.1 to -15 psig of a headspace vacuum or a negative pressure condition in the single-stage of an anaerobic digestion bioreactor. As shown, bioreactor 102 may be a single-stage anaerobic digestion bioreactor. A bio-based waste or biowaste, stored in a feedstock tank, before being fed to bioreactor 102, may be mixed with alkaline chemicals and pH buffer agents to maintain the mixture pH level between 7 and 11 , preferably, between 8 and 9. The bio-based waste or biowaste may be fed to bioreactor 102 continuously or manually. The alkaline chemicals may be sodium hydroxide, magnesium hydroxide or potassium hydroxide or the like. The pH buffer agents may be sodium bicarbonate or a mixture of monopotassium and dipotassium phosphates or the like. Iron-based additives is also mixed with the bio-based waste. The ironbased additives may be ferrous iron, iron oxides or iron monomers such as nano zerovalent iron or zerovalent scrap iron. Then, a feedstock of the mixture of biobased waste, alkaline chemicals, pH buffer agents and iron-based additives is fed to bioreactor 102 forming a sludge of the mixture therein. The pH level of the sludge of the mixture in bioreactor 102 may range from 6 to 9, preferably neutral. A temperature of the sludge of the mixture or digestate content in bioreactor 102 is maintained between 25 and 60°C, preferably between 30 and 50°C, more preferably between 30 and 40°C, for a mesophilic or thermophilic condition. A retention of the sludge of the mixture in bioreactor 102 may be maintained between 10 days and 40 days. Biogas of CH4 and CO2 is then generated from bioreactor 102. The produced biogas of CH4 and CO2 is extracted using vacuum pump 104, thereby, maintaining a vacuum or a negative pressure in the headspace of bioreactor 102. Vacuum pump 104 pumps the biogas of CH4 and CO2 out of bioreactor 102 and sends the biogas of CH4 and CO2 to gas separation device 110. Vacuum pump 104 may be a positivedisplacement pump, such as reciprocating pumps and rotary pump or other suitable vacuum devices such as kinetic pump at large applications. The extracted biogas of CH4 and CO2 is then separated using gas separation 110. Gas separation 110 separates the produced biogas of CF and CO2 into a CFU-enriched stream and a CC>2-enriched stream. Gas separation 110 may be a gas membrane separation device for separating CH4 and CO2 producing a permeate gas stream that is mostly CO2 and a retentate gas stream that is mostly CH4. Gas separation 110 may be other gas separation device, such as a pressure swing adsorption (PSA), water scrubbing, organic solvent scrubbing, chemical scrubbing, cryrogenic separation or the like. Afterwards, the CO2-enriched stream is recirculated back to bioreactor 102 for generating the biogas of CH4 or discharged out of the system; the CH4-enriched stream is sent to post-treatment processes; and a waste solid digestate generated from bioreactor 102 is discharged. PLC 106 is connected (dashed line) with vacuum pump 104 and vacuum transducers 108 mounted in the headspace of bioreactor 102 to control the headspace pressure of bioreactor 102 while producing the biogas of CH4and CO2 from bioreactor 102. Here, vacuum transducers 108 are installed at multiple locations in the headspace of bioreactor 102 to measure the headspace pressure and communicate with PLC 106 for pressure control. A vacuum set-point may be predetermined with PLC 106. When the headspace pressure is above the vacuum set-point, PLC 106 turns vacuum pump 104 on to take the produced biogas out of bioreactor 102. Vacuum pump 104 stops when the headspace pressure is below the vacuum set-point, so that the biogas is favorable to produce in the bioreactor under the vacuum. This cyclic control is repeated over a period of the reaction, thus making the headspace pressure (i.e., vacuum or negative pressure) constant. The headspace pressure may controlled under -0.1 to -15 psig, preferably under -0.1 to -10 psig, more preferably from -0.1 to -5 psig, through PLC 106 and transducers 108.

[0113] In an alternative embodiment, a single-stage dark fermentation with headspace vacuum, alkaline chemicals and pH buffer agents and supplemental iron addition for biohydrogen production. FIG. 2 is a block diagram of this embodiment that operates to produce hydrogen with -0.1 to -15 psig of a headspace vacuum condition in this single-stage of a dark fermentation bioreactor. As shown, bioreactor 202 may be a single-stage dark fermentation bioreactor or the like. A bio-based waste or biowaste, before being fed to bioreactor 202, may be mixed with alkalinechemicals and pH buffer agents to maintain the mixture pH level between 8 and 10, preferably, between 8 and 9. The bio-based waste or biowaste may be fed to bioreactor 202 continuously or manually. The alkaline chemicals may be sodium hydroxide, magnesium hydroxide or potassium hydroxide or the like. The pH buffer agents may be sodium bicarbonate or a mixture of monopotassium and dipotassium phosphates or the like. Iron-based additives is also mixed with the bio-based waste. The iron-based additives may be ferrous iron, iron oxides or iron monomers such as nano zerovalent iron or zerovalent scrap iron. Then, a feedstock of the mixture of bio-based waste, alkaline chemicals, pH buffer agents and iron-based additives is fed to bioreactor 202 forming a sludge of the mixture therein. The pH level of the sludge of the mixture in bioreactor 202 may range from 5 to 7, preferably a slightly acidic to neutral. A temperature of the sludge of the mixture or digestate content in bioreactor 202 is maintained between 25 and 80°C, preferably between 40 and 60°C, more preferably between 50 and 55°C, for a thermophilic condition. A retention of the sludge of the mixture in bioreactor 202 may be maintained between 10 hours and 3 days. Biogas of H2 and CO2 is then generated from bioreactor 202. The produced biogas of H2 and CO2 is extracted using vacuum pump 204, thereby, maintaining a vacuum or a negative pressure in the headspace of bioreactor 202. Vacuum pump 204 pumps the biogas of H2 and CO2 out of bioreactor 202 and sends the biogas of H2 and CO2 to gas separation device 210. Vacuum pump 204 may be a positive displacement pump, such as reciprocating and rotary pumps or other suitable vacuum devices such as kinetic pump at large applications. The extracted biogas of H2 and CO2 is then separated using gas separation 210. Gas separation 210 separates the produced biogas of H2 and CO2 into a H2-enriched stream and a CC>2-enriched stream. Gas separation 210 may be a pressure swing adsorption (PSA), water scrubbing, organic solvent scrubbing, chemical scrubbing, cryogenic separation or the like for separating H2 and CO2 producing CO2-enriched stream that is mostly CO2 and a H2-enriched stream that is mostly H2. Gas separation 210 may be other gas separation device, such as a gas membrane separation device.Afterwards, the CO2-enriched stream is vent out; the H2-enriched stream is sent to post-treatment processes; and waste solid digestate generated from bioreactor 202 is discharged. PLC 206 is connected (dashed line) with vacuum pump 204 and vacuum transducers 208 mounted in the headspace of bioreactor 202 to control theheadspace pressure of bioreactor 202 while producing the biogas of H2 and CO2 from bioreactor 202. Here, vacuum transducers 208 are installed at multiple locations in the headspace of bioreactor 202 to measure the headspace pressure and communicate with PLC 206 for pressure control. A vacuum set-point may be predetermined with PLC 206. When the headspace pressure is above the vacuum set-point, PLC 206 turns vacuum pump 204 on to take the produced biogas out of bioreactor 202. Vacuum pump 204 stops when the headspace pressure is below the vacuum set-point, so that the biogas is favorable to produce in the bioreactor under the vacuum. This cyclic control is repeated over a period of the reaction, thus making the headspace pressure (i.e. , vacuum or negative pressure) constant. The headspace pressure may controlled under -0.1 to -15 psig, preferably under -0.1 to - 10 psig, more preferably from -0.1 to -5 psig, through PLC 206 and transducers 208.

[0114] In another alternative embodiment, a two-stage anaerobic digestion system having a 1ststage and a 2ndstage bioreactors with headspace vacuum in each bioreactor, alkaline chemicals and pH buffer agents and supplemental iron addition in each bioreactor, recirculation of a CO2-enriched gas stream from the 2ndstage and recirculation of a biogas H2 and CO2 from the 1ststage for CH4 production. FIG. 3A is a block diagram of this embodiment that operates to produce CH4 under - 0.1 to -15 psig of a headspace vacuum condition in each bioreactor. As shown, the two-stage anaerobic digestion system includes bioreactor 302 that is the 1ststage dark fermentation bioreactor and bioreactor 312 that is the 2ndstage anaerobic digestion bioreactor. A bio-based waste or biowaste, before being fed to bioreactor 302, is mixed with alkaline chemicals and pH buffer agents to maintain feedstock pH level between 8 and 10, preferably, between 8 and 9. The bio-based waste or biowaste may be fed to bioreactor 302 continuously or manually. The alkaline chemicals may be sodium hydroxide, magnesium hydroxide or potassium hydroxide or potassium hydroxide or the like. The pH buffer agents may be sodium bicarbonate or a mixture of monopotassium and dipotassium phosphates or the like. Iron-based additives is also mixed with the bio-based waste. The iron-based additives may be ferrous iron, iron oxides or iron monomers such as nano zerovalent iron or zerovalent scrap iron. Then, a feedstock of the mixture of bio-based waste, alkaline chemicals, pH buffer agents and iron-based additives is fed into bioreactor 302 forming a sludge of the mixture therein. Bioreactor 302 may be a single-stage dark fermentation bioreactor. A temperature of the sludge of the mixture or digestatecontent in bioreactor 302 is maintained between 25 and 80°C, preferably between 40 and 60°C, more preferably between 50 and 55°C, for a thermophilic condition. The pH level of the sludge of the mixture in bioreactor 302 ranges from 5 to 7. A retention time of the sludge of the mixture in bioreactor 302 is maintained between 10 hours and 3 days. A biogas of H2 and CO2 is then generated from bioreactor 302. The produced biogas of H2 and CO2 is extracted using vacuum pump 304, thereby, maintaining a vacuum or a negative pressure in the headspace of bioreactor 302. Vacuum pump 304 extracts the biogas of H2 and CO2 out of bioreactor 302 and sends the biogas of H2 and CO2 to bioreactor 312 for generating the biogas of CH4, simultaneously, vacuum pump 304 pumps the headspace of bioreactor 302 to a vacuum. Vacuum pump 304 may be a positive displacement pump, such as reciprocating and rotary pumps or other suitable vacuum devices such as kinetic pump at large applications. An effluent from bioreactor 302, the 1ststage dark fermentation, is used for a feedstock of the 2ndstage anaerobic digestion bioreactor 312. The feedstock may be a mixture of the effluent, alkaline chemicals, such as sodium hydroxide, magnesium hydroxide or potassium hydroxide or the like, and pH buffer agents, such as sodium bicarbonate, a mixture of monopotassium and dipotassium phosphates or the like, to maintain the feedstock pH level between 7 and 11 , preferably, between 8 and 9, for bioreactor 312. The preferred pH range of digestate in the anaerobic digestion process is 6 to 9. Iron-based additives is also mixed with the feedstock. The iron-based additives may be ferrous iron, iron oxides or iron monomers such as nano zerovalent iron or zerovalent scrap iron. Then, the feedstock of the mixture of the effluent, alkaline chemicals, pH buffer agents and iron-based additives is fed to bioreactor 312 forming a sludge of the mixture therein. The feedstock may be fed to bioreactor 312 continuously or manually. The pH level of the sludge of the mixture in bioreactor 312 may range from 6 to 9. A temperature of the sludge of the mixture or digestate content in bioreactor 312 may be maintained between 25 and 60°C, preferably between 30 and 50°C, more preferably between 30 and 40°C, for a mesophilic or thermophilic condition in bioreactor 312. A retention time of the sludge of the mixture in bioreactor 312 may be maintained between 10 days and 40 days. A biogas of CH4 and CO2 is then generated from bioreactor 312. Vacuum pump 314 extracts the biogas of CH4 and CO2 out of bioreactor 312 and sends the biogas of CH4 and CO2 to gas separation 316. Here gas separation 316may be a membrane gas separation device that separates CH4 and CO2 to a permeate gas stream that is a CC>2-enriched gas stream and a retentate gas stream that is a CH4-enriched gas stream. Afterwards, the CH4-enriched gas stream is forwarded to the post-treatment processes; the CC>2-enriched gas stream is recirculated back to bioreactor 312 or discharged out of the system. The produced biogases from bioreactors 302 and 312 are extracted using vacuum pumps 304 and 314, respectively, to maintain vacuum in the headspaces of bioreactors 302 and 312. Once again, the CO2-enriched gas stream from bioreactor 312 is recirculated back to bioreactor 312 for producing CH4 or discharged out of the system (not shown). The biogas of H2 and CO2 from bioreactor 302 is sent to bioreactor 312 for producing CH4 as well. The CH4-enriched gas stream produced from bioreactor 312 is sent to the post-treatment processes. A waste solid digestate generated from bioreactor 312 is discharged. PLC 306 is connected with vacuum transducers 308, 318 and vacuum pump 304, 314 to control the headspace pressures of bioreactors 302 and 312, respectively, while producing the biogas of H2 and CO2 from bioreactor 302 and the biogas of CH4 and CO2 from bioreactor 312. Vacuum transducers 308 and 318 are installed at multiple locations in the headspaces of bioreactors 302 and 312, respectively, to measure the headspace pressures of bioreactor 302 and 312, respectively, and communicate with PLC 306 for pressure control therein. A vacuum set-point for each bioreactor may be predetermined with PLC 306. When the headspace pressure in bioreactor 302 is above a vacuum set-point for bioreactor 302, PLC 306 turns vacuum pump 304 on and the produced biogas of H2 and CO2 is extracted out of bioreactor 302. Vacuum pumps 304 stops when the headspace pressure is below the vacuum set-point for bioreactor 302, so that the biogas H2 and CO2 is favorable to produce in bioreactor 302 under the vacuum. This cyclic control is repeated over a period of a dark fermentation reaction, thus making the headspace pressure in bioreactors 302 constant. Similarly, when the headspace pressure in bioreactor 312 is above a vacuum set-point for bioreactor 312, PLC 306 turns vacuum pump 314 on and the produced biogas of CH4 and CO2 is extracted out of bioreactor 312. Vacuum pumps 314 stops when the headspace pressure is below the vacuum set-point for bioreactor 312, so that the biogas of CH4and CO2 is favorable to produce in bioreactor 312 under the vacuum. This cyclic control is repeated over a period of an anaerobic digestion reaction, thus making the headspace pressure in bioreactors 312 constant. The vacuum set-points for bothbioreactors may be the same or may be different but both vacuum set-points are within pressure operation ranges for both bioreactors, respectively. The headspace pressures in both bioreactor 302 and 312 are controlled, respectively, under -0.1 to - 15 psig, preferably under -0.1 to -10 psig, more preferably under -0.1 to -5 psig, through PLC 306 and transducers 308 and 318.

[0115] In another alternative embodiment, a two-stage anaerobic digestion system having a 1ststage and a 2ndstage bioreactors with headspace vacuum, alkaline chemicals and pH buffer agents and supplemental iron addition in each bioreactor, recirculation of a CC>2-enriched gas stream from the 2ndstage, recirculation of a biogas H2 and CO2 from the 1ststage, and partially recirculation of a waste solid digestate from the 2ndstage for CH4 production. FIG. 3B is a block diagram of this embodiment that operates to produce CH4 under -0.1 to -15 psig of a headspace vacuum condition in each bioreactor. The difference between FIG. 3A and FIG. 3B is in FIG. 3B a waste solid digestate generated from bioreactor 312 is partially recirculated back to the bio-based waste that is fed to bioreactor 302 or directly injected into bioreactor 302 and partially discharged, whereas in FIG. 3A the waste solid digestate generated from bioreactor 312 is totally discharged.

[0116] In another alternative embodiment, a two-stage anaerobic digestion system having a 1ststage and a 2ndstage bioreactors with headspace vacuum, alkaline chemicals and pH buffer agents and supplemental iron addition in each bioreactor, recirculation of a H2-enriched gas stream from the 1ststage and recirculation of a CC>2-enriched gas stream from the 2ndstage for CH4 production. FIG. 3C is a block diagram of this embodiment that operates to produce CH4 under - 0.1 to -15 psig of a headspace vacuum condition in each bioreactor. The difference between FIG. 3A and FIG. 3C is in FIG. 3C gas separation 310 is added and the produced biogas of H2 and CO2 from bioreactor 302 is then separated by gas separation 310 forming a H2-enriched gas stream that is forwarded to bioreactor 312 and a CC>2-enriched stream that is vent out. More specifically, after vacuum pump 304 extracts the biogas of H2 and CO2 out of bioreactor 302 while pumping the headspace of bioreactor 302 to a vacuum, the extracted biogas of H2 and CO2 is sent to gas separation device 310, where the extracted biogas of H2 and CO2 is then separated or purified by gas separation 310. Here gas separation 310 may separate the produced biogas of H2 and CO2 into a CC»2-enriched stream that is mostly CO2 and a H2-enriched gas stream that is mostly H2. Here gas separation 310 may be amembrane-based separation device or a non-membrane-based separation device, which separates H2 and CO2 producing the Hh-enriched gas stream and the CO2- enriched stream. More specifically, gas separation 310 may be a gas membrane separation device, a pressure swing adsorption (PSA), water scrubbing, organic solvent scrubbing, chemical scrubbing and cryogenic separation.

[0117] In another alternative embodiment, a two-stage anaerobic digestion system having a 1ststage and a 2ndstage bioreactors with headspace vacuum, alkaline chemicals and pH buffer agents and supplemental iron addition in each bioreactor, recirculation of a H2-enriched gas stream from the 1ststage, recirculation of a CC>2-enriched gas stream from the 2ndstage, and partially recirculation of the waste solid digestate from the 2ndstage to the 1ststage for CH4 production. FIG. 3D is a block diagram of this embodiment that operates to produce CH4 under -0.1 to - 15 psig of a headspace vacuum condition in each bioreactor. The difference between FIG. 3D and FIG. 3C is in FIG. 3D the waste solid digestate generated from the 2ndstage anaerobic digestion bioreactor 312 is partially recirculated back to the bio-based waste for feeding to the 1stdark fermentation bioreactor 302 or directly injected into bioreactor 302 and partially discharged, whereas in FIG. 3C the waste solid digestate generated from the 2ndstage anaerobic digestion bioreactor 312 is totally discharged.

[0118] In another alternative embodiment, a two-stage anaerobic digestion system having a 1ststage and a 2ndstage bioreactors with headspace vacuum, alkaline chemicals and pH buffer agents and supplemental iron addition in each bioreactor, recirculation of a CO2-enriched gas stream from the 2ndstage for CH4 production. FIG. 3E is a block diagram of this embodiment that operates to produce CH4 under -0.1 to -15 psig of a headspace vacuum condition in each bioreactor. The difference between FIG. 3E and FIG. 3C is in FIG. 3E the H2-enriched gas stream from gas separation device 310 is forwarded to the post-treatment processes like the CH4-enriched gas stream from gas separation device 316. Whereas in FIG. 3C, the H2-enriched gas stream from gas separation device 310 is recirculated to bioreactor 312 for CH4 production.

[0119] In another alternative embodiment, a two-stage anaerobic digestion system having a 1ststage and a 2ndstage bioreactors with headspace vacuum, alkaline chemicals and pH buffer agents and supplemental iron addition in each bioreactor, recirculation of a CO2-enriched gas stream from the 2ndstage for CH4production. FIG. 3F is a block diagram of this embodiment that operates to produce CH4 under -0.1 to -15 psig of a headspace vacuum condition in each bioreactor. The difference between FIG. 3F and FIG. 3E is in FIG. 3F the waste solid digestate generated from the 2ndstage anaerobic digestion bioreactor 312 is partially recirculated back to the bio-based waste for feeding to the 1stdark fermentation bioreactor 302 or directly injected into bioreactor 302 and partially discharged, whereas in FIG. 3E the waste solid digestate generated from the 2ndstage anaerobic digestion bioreactor 312 is totally discharged.Examples

[0120] The following non-limiting examples are provided to further illustrate embodiments of the invention. However, the examples are not intended to be all inclusive and are not intended to limit the scope of the inventions described herein. The following non-limiting examples applied the disclosed methods of the vacuum- driven anaerobic digester operation for biogas production.Example 1 . Two-stage anaerobic digestion system design and operation

[0121] A lab-scale two-stage anaerobic digestion system shown in FIG. 4 was operated to treat synthetic food waste and to produce biogas. As shown, the two- stage anaerobic digestion system includes bioreactor 402 that is the 1ststage dark fermentation bioreactor and bioreactor 412 that is the 2ndstage anaerobic digestion bioreactor. Bio-based waste or biowaste, which is synthetic food waste herein, before being fed to bioreactor 402, is mixed with alkaline chemicals and pH buffer agents to maintain a feedstock pH level between 8 and 9. The alkaline chemicals may be sodium hydroxide, magnesium hydroxide or potassium hydroxide or potassium hydroxide or the like. The pH buffer agents may be sodium bicarbonate or a mixture of monopotassium and dipotassium phosphates or the like. Iron-based additives is also mixed with the feed bio-based solid waste. The iron-based additives may be ferrous iron, iron oxides or iron monomers such as nano zerovalent iron or zerovalent scrap iron. Then, a feedstock of the mixture of bio-based waste, alkaline chemicals, pH buffer agents and iron-based additives is fed into bioreactor 402 forming a sludge of the mixture therein. Bioreactor 402 may be a single-stage dark fermentation. A temperature of the sludge of the mixture or digestate content inbioreactor 402 is maintained between 50 and 55°C, for a thermophilic or hyperthermophilic condition. The pH level of the sludge of the mixture in bioreactor 402 may range from 5 to 7. A retention of sludge of the mixture in bioreactor 402 is maintained between 10 hours and 3 days. A biogas of H2 and CO2 is then generated from bioreactor 402. The produced biogas of H2 and CO2 is extracted using vacuum pump 404, thereby, maintaining a vacuum or a negative pressure in the headspace of bioreactor 402. Vacuum pump 404 pumps the biogas of H2 and CO2 out of bioreactor 402 and sends the biogas of H2 and CO2 to biogas collector 410. Vacuum pump 404 may be a positive displacement pump, such as reciprocating and rotary pumps or other suitable vacuum devices such as kinetic pump at large applications. An effluent of bioreactor 402, the 1ststage dark fermentation, is used for a feedstock of the 2ndstage anaerobic digestion bioreactor 412. The feedstock may be mixed with alkaline chemicals, such as sodium hydroxide, magnesium hydroxide or potassium hydroxide or the like, and pH buffer agents, such as sodium bicarbonate, a mixture of monopotassium and dipotassium phosphates or the like, to maintain the feedstock pH level between 8 and 9 for bioreactor 412. Iron-based additives is also mixed with the feedstock. The iron-based additives may be ferrous iron, iron oxides or iron monomers such as nano zerovalent iron or zerovalent scrap iron. Then, the feedstock of the mixture of the effluent, alkaline chemicals, pH buffer agents and iron-based additives is fed to bioreactor 412 forming a sludge of the mixture therein. The pH level of the sludge of the mixture in bioreactor 412 may range from 6 to 9. A temperature of the sludge of the mixture or digestate content may be maintained between 30 and 40°C, for a mesophilic or thermophilic condition in bioreactor 412. A retention of the sludge of the mixture in bioreactor 412 may be maintained between 10 days and 40 days. Biogas of CH4 and CO2 is generated from bioreactor 412. Vacuum pump 414 pumps the biogas of CH4 and CO2 out of bioreactor 412 and sends the biogas of CH4 and CO2 to biogas collector 420. The produced biogases from bioreactors 402 and 412 are extracted using vacuum pumps 404 and 414, respectively, to maintain vacuum in the headspaces of bioreactors 402 and 412. A PLC system was connected with vacuum transducers and vacuum pumps to control the headspace vacuum while producing biogas from the bioreactors. Here, PLC 406 is connected with vacuum transducers 408, 418 and vacuum pump 404, 414 to control the headspace pressures of bioreactors 402 and 412, respectively, whileproducing the biogas of H2 and CO2 from bioreactor 402 and the biogas of CH4 and CO2 from bioreactor 412. Vacuum transducers 408 and 418 are installed at multiple locations in the headspaces of bioreactors 402 and 412, respectively, to measure the headspace pressures of bioreactor 402 and 412 and communicate with PLC 406 for pressure control therein. A vacuum set-point for each bioreactor may be predetermined with PLC 406. The vacuum set-points for both bioreactors may be the same or may be different from each other but both vacuum set-points are within pressure ranges of operations for both bioreactors. When the headspace pressures are above a vacuum set-point in either bioreactor 402 or 412 or both, PLC 406 turns vacuum pump 404 and / or 414 on to take the produced biogas out of bioreactor 402 and / or 412. Vacuum pumps 404 and / or 414 stop when the headspace pressure is below the vacuum set-point in either bioreactor 402 or 412 or both, so that the biogases are favorable to produce in both bioreactors under the vacuum. Vacuum pumps 404 and 414 was a peristaltic pump. This cyclic control is repeated over a period of dark fermentation reaction and anaerobic digestion reaction, respectively, thus making the headspace pressure in both bioreactors 402 and 412 constant, respectively. The headspace pressure in bioreactor 402 is controlled under 0 to -2 psig through PLC 406 and transducers 408. The headspace pressure in bioreactor 412 is controlled under -1 to -5 psig through PLC 406 and transducers 418.

[0122] Table 1 summarizes the composition of the synthetic food waste. Mixed food was stirred overnight and next day filtered out large chunks using the wire mesh (20 mesh, aperture 0.9 mm). Remaining portion was mixed with a prepared 0.1 M potassium pH 8 buffer solution. Micronutrients, 1 N NaOH solution, and a partial waste solid digestate from the 2ndstage bioreactor 412 were added into the pH- buffered food waste to make feedstock for the 1ststage dark fermentation bioreactor 402. Table 2 and Table 3 show the composition of micronutrients and the characteristics of the final feedstock. The same NaOH solution and micronutrients were added to the 1ststage bioreactor effluent and the adjusted effluent from the 1ststage was fed into the 2ndstage anaerobic digestion bioreactor. Supplemental Fe and Mg solutions were added to the both 1stand 2ndstage feedstock solutions to stimulate the microbial activities associated with biogas production. The volume ratios of Fe and Mg solutions to the feedstock were 0.899 and 0.090 for the 1ststage bioreactor and 0.083 and 0.083 for the 2ndstage bioreactor, respectively. The recirculation rate of the 2ndstage effluent was 0.05.Table 1. Food water compositionTable 2. Micronutrient and supplemental nutrient compositionTable 3. Feedstock / bioreactor effluent characteristics

[0123] Digestates from each bioreactor were removed and then feedstock was filled into the bioreactors under the strict anaerobic condition by purging inside the bioreactor with N2 gas. After filling feedstock, the headspace pressure was equalized to atmospheric pressure level by quickly releasing filled-N2 gas. The produced biogas was collected into a glass bottle and its volume was quantified indirectly using a known water displacement method. A pH-4 buffer solution was used as the displacement solution to avoid CO2 dissolution into aqueous phase. The collected biogas was periodically used for a micro-GC analysis for the biogas composition. The hydraulic retention times (HRT) were 2 days for the 1ststage dark fermentation bioreactor and 15 days for the 2ndstage anaerobic digestion bioreactor, respectively, which were the same as sludge retention time (SRT) of each bioreactors. The organic loading rates (OLR) were 14.22 and 0.79 g TVS / L d for the dark fermentation and anaerobic digestion bioreactors. The average temperatures of the bioreactors were 54.3°C for the thermophilic dark fermentation reaction and 40.1 °C for the mesophilic or thermophilic anaerobic digestion reaction.

[0124] There were a total six pressure conditions to investigate the effect of vacuum on the biogas production (see Table 4). Phase 1 was a control condition where biogas was normally produced without vacuum. The headspace vacuum of the anaerobic digestion bioreactor was gradually increased from Phase 2 to 4 to monitor the bio-CH4 production over time. Then, the headspace vacuum was applied to the 1ststage dark fermentation bioreactor during Phase 5 and 6 to see the effect of vacuum on the bio-H2 production. Referring to FIG. 4, a PLC system was connected with vacuum transducers and peristaltic pumps to control the headspace vacuum while producing biogas from the bioreactors. Vacuum transducers were installed at multiple locations in the headspaces of bioreactors 402 and 412, respectively, to measure the headspace pressure and communicate with the PLC system for the pressure control. When the headspace pressure is above apredetermined vacuum set-point, the PLC turns on the peristaltic pump to take the produced biogas out of the bioreactor. The peristaltic pump stops when the vacuum pressure is below the set-point. This cyclic control is repeated over a period of the reaction phase, thus making the headspace vacuum constant.Table 4. Headspace pressure conditionExample 2. Analytical methods

[0125] A spectrophotometer was used to measure chemical oxygen demand (COD), TN and ammonia. Total chemical oxygen demand (TCOD) was defined as COD of entire sample, whereas soluble COD (SCOD) were defined as COD of filtrate through filters with a nominal pore size of 0.45 pm (GN-6 Metricel Filter, Pall). Total solids (TS) and total volatile solids (TVS) were determined according to Standard Methods (APHA et al., 2005). The bioreactor temperature was monitored using thermocouples installed inside the bioreactor and pH values of feedstock and effluent were measured using electrodes (Versa Star Pro, Thermo Scientific, Vernon Hills, IL).

[0126] The biogas composition in the collectors 410 and 420 was analyzed using a micro-GC (Micro GC 3000, Inficon). Helium was supplied as a carrier gas at 40 psig and customized calibration gas (35% H2, 35% CH4, and balanced CO2) was used for the calibration work. The oven temperature was programmed as follows: sample inlet temperature was 90°C, injector temperatures were 90°C for both channel A and B, column temperatures were 95°C for channel A and 65°C for channel B. The channel A detected CH4 and H2 and the channel B detected CH4 and CO2. The collected bulk biogas was injected into the micro-GC using a vacuumpump. The calibration gas was used to validate the micro-GC performance at the beginning of the measurement. Aliquot of gas samples was 100 - 200 mL per a single run. At least three measurements were carried out to take the average gas composition of the collected volume. The analytical duration of the run was 3 minutes.

[0127] The dark fermentation metabolites (acetate, butyrate, lactate and formate) were measured via high pressure liquid chromatography (HPLC) (Shimadzu) using an organic acid column HPX-87H (Bio-Rad). The HPLC was operated at 15°C with 0.5 mL / min 5 mM H2SO4 mobile phase with a refractive index detector (RID). The total analytical duration of the run was 90 minutes.Example 3. Anaerobic digestion system performance3.1. Biogas composition and fatty acid production

[0128] The disclosed vacuum-driven two-stage anaerobic digestion system as shown in FIG. 4 was applied. The strength of organic loading was gradually increased during the acclimation phase (~ 2 months). The value of organic loading rate (OLR) for the dark fermentation bioreactor ranged between 13.5 - 16.4 g TVS / L d for a steady operation period. The anaerobic digestion bioreactor received a relatively stable organic loading because of a high dilution effect (long retention time) of feedstock in the bioreactor.

[0129] The same feed digester sludge was used as a seed culture for the both bioreactors. During the acclimation phase, the TVS concentration of the dark fermentation bioreactor (i.e., 1ststage of the bioreactor, dark fermentation bioreactor) gradually increased under a high OLR condition, whereas the TVS concentration of the anaerobic digestion bioreactor (i.e., 2ndstage of the bioreactor, anaerobic digestion bioreactor) slowly decreased under a low and stable OLR condition. The overall profile of TVSs in the both bioreactors was stable for the entire test period except for a short organic shocking event from the 58thto 64thdays of operation.

[0130] Table 5 summarizes the gas composition (%, v / v) in the vacuum-driven two-stage anaerobic digestion system. The average biogas composition in the 1ststage dark fermentation bioreactor was 57.1 % H2 and 42.9% CO2. On the other hand, the 2ndstage anaerobic digestion bioreactor produced 76.6% CH4 and 23.4% CO2 on average. The 2ndstage anaerobic digestion bioreactor received only 11 % of the effluent from the 1ststage dark fermentation bioreactor as feedstock because the2ndstage anaerobic digestion bioreactor was intentionally under-designed at a limited space inside a fume hood. For the correction of the actual biomethane gas volume, it was assumed that all dark fermentation effluent is used for the 2ndstage anaerobic digestion bioreactor to produce biomethane. The volume flux ratio of biomethane to biohydrogen was 3.47 on average. When the both biogas streams are mixed to make biohythane, the final composition of the gas mixture was estimated to be 12.8% H2, 59.5% CH4, and 27.7% CO2 without the CO2 removal or 17.7% H2 and 82.3% CH4 with the CO2 removal respectively.Table 5. Biogas composition in dark fermentation and anaerobic digestion bioreactors

[0131] The dark fermentation bioreactor produces volatile fatty acids (VFAs) as a result of dark fermentation. The dominant VFAs are formic, acetic, butyric, and lactic acids, which may be used as valuable compounds for industrial applications including chemicals, bioplastics, biofuels and wastewater treatment. In this practice, the two-stage anaerobic digestion process produced consistently butyric acid over the entire period of operation. There was no distinct change in the production rate of butyric acid after applying the headspace vacuum inside the bioreactor. Formic acid was a minor compound, but slightly increased from 0.53 g / L to 0.71 g / L with increasing the headspace vacuum (see Table 6). There was a spike event on the 100th - 105th day in the lactic acid profile. The high concentration of lactic acid in the bioreactor resulted in a temporary reduction in the production of acetic and butyric acids. The acetic acid concentration gradually decreased over time except for the lactic acid spike event. The dark fermentation bioreactor was operated at 57°C for afavorable condition of thermophilic hydrogen producing bacteria. The production of lactic acid was maintained at a low range during the system operation except for the spike period.Table 6. Biogas composition in dark fermentation and anaerobic digestion bioreactors3.2. Biohydrogen and biomethane production improvement

[0132] The disclosed vacuum-driven two-stage anaerobic digestion system as shown in FIG. 4 was applied. The partial pressure of hydrogen affects two enzymatic activities in the dark fermentation. NADH:ferredoxin oxidoreductase (NFOR) catalyzes the formation of reduced ferredoxin (Fdred) (Equation 1) and Pyruvate:ferredoxin oxidoreductase (PFOR) oxidizes pyruvate to form acetyl-CoA and reduced ferredoxin (Equation 2). Hydrogen gas is produced by hydrogenases that reduce protons using electrons from reduced ferredoxin (Equation 3).Puruvate + Co A + Fdox-> CoA + Fdrea+ CO2(Equation 1) NADH + FdoxNAD++ Fdred(Equation 2) 2H++ FdredH2+ Fdox(Equation 3)

[0133] The NFOR activity is inhibited at > 60 Pa (0.0087 psi) of partial hydrogen pressure and PFOR is active at hydrogen concentration up to 3x104Pa (4.35 psi). It has been known this was a first evaluation of the effect of vacuum on the biohydrogen production from food waste using mixed culture. The headspace vacuum was applied to reduce the partial pressure of dissolved hydrogen gas in the digestate of the 1ststage dark fermentation bioreactor. The production yield of biohydrogen was improved with increasing the headspace vacuum as shown in FIG. 5. The range of the hydrogen yield was 226 - 277 L H2 / kg Sadded on average as shown in FIG. 5, which was higher than existing results on the dark fermentation of food waste. Supplemental iron was added to feedstock to promote the enzymatic activity in this practice. Iron is involved in the synthesis of ferredoxin andhydrogenase, thereby the addition of iron component may accelerate electron transfer between ferredoxin and hydrogenase. Ferrous chloride was added manually in feedstock in this study. In a view of the process development, zero valent iron (ZVI) may be a feasible process for the food waste pretreatment. For example, the ZVI column generates Fe2+and OH' ions that may be used to make iron-enhanced basic feedstock for the improvement of dark fermentation.

[0134] To our knowledge, this is the first investigation to maintain a headspace vacuum for promoting biogas production or improving the yield of the biogas production. The headspace vacuum application is a different approach from vacuum evaporation. The mechanism of the headspace vacuum application is to facilitate the gas transfer to the headspace from the digestate phase by lowing the partial pressure of methane according to Henry’s law and in turn to stimulate the biological production of biogas in the digestate phase by removing the dissolved biogas to the headspace from the digestate phase according to Le Chatelier's principle. The operating vacuum condition is maintained for an entire period of operation except for the feeding time. The operating vacuum condition may be maintained constant for the entire period of operation except for the feeding time. There is minimal evaporation of water out of the vacuum-driven bioreactor. Because the biogas temperature rapidly decreases prior to the peristaltic vacuum pump, most of the water vapor is condensed back to the sludge in the bioreactor.

[0135] The headspace vacuum increased stepwise as explained in Table 4. The biomethane yield was increased with the headspace vacuum application in the 2ndstage anaerobic digestion bioreactor as shown in FIG. 6. There was no significant impact of the 1ststage vacuum on the 2ndstage anaerobic digestion activity. The maximum biomethane yield was 820 L CHVkg VSadded at -2 psig headspace pressure for both dark fermentation (1stbioreactor) and anaerobic digestion bioreactors (2ndbioreactor), which was better than the existing results, to our knowledge.

[0136] The above Examples show that the vacuum-driven two-stage anaerobic digestion process may be a feasible solution for the biogas valorization.

[0137] In summary, the disclosed vacuum-driven two-stage anaerobic digestion process improves the biogas production from the Examples that show above. Introducing the headspace vacuum may be a simple and powerful way to improve the dark fermentation and anaerobic digestion.• The vacuum-driven two-stage anaerobic digestion process produced a constant amount of butyric acid (6.5 - 7.7 g / L) as a valuable resource from the 1ststage dark fermentation bioreactor;• The average biogas compositions were 57.1 % H2 and 42.9% CO2 in the 1ststage dark fermentation bioreactor and 76.6% CH4 and 23.4% CO2 in the 2ndstage anaerobic digestion bioreactor;• The headspace vacuum gave a positive impact on the improvement of the two- stage anaerobic digestion process performance;• The maximum biogas yields were 277 L kg VSadded and 820 L CHU / kg VSadded on average, which was higher than existing results.

[0138] It is noticed not all methane produced from anaerobic digestion is recovered in the above Examples using the two-stage anaerobic digestion process and the dissolved methane in digester effluent is released into the environment. Theoretical calculation showed that about 45% of methane produced in the anaerobic digester system operating at 30°C would be present in its dissolved form (Liu et al., “Dissolved methane: a hurdle for anaerobic treatment of municipal wastewater”, Environmental Science & Technology 2014 48 (2), 889-890). This dissolved methane is an extra energy source that may be utilized as electricity, heating, biomethane, or biohythane. The vacuum-driven anaerobic digestion process not only enhances the biological methane production but also improves the physical recovery of the dissolved methane present in digestate before discharging out of the system.

[0139] Another technical benefit of the vacuum-driven two-stage anaerobic digestion process is a simple integration with gas separation process for the biogas upgrading. The vacuum system may provide a driving force for biomethane purification. It is true that the digester structure should be made of strong materials that may withstand vacuum conditions. However, the improvement of the biogas production may compensate for the increased investment costs associated with the digester construction. Therefore, the disclosed herein provides methods and systems for applying vacuum in the headspace of the biogas production bioreactors, instead of the positive pressure, which improves the physical transfer of dissolved biogas to the gas phase and stimulate the biological biogas production yield to achieve the energy-positive biogas production process by reducing the partialpressure of biogas in the bioreactor headspace.

[0140] Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment of the invention. The appearances of the phrase "in one embodiment" and / or “in alternative embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.”

[0141] As used in this application, the word “exemplary” is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the word exemplary is intended to present concepts in a concrete fashion.

[0142] "Comprising" in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e. , anything else may be additionally included and remain within the scope of “comprising”). “Comprising” is defined herein as necessarily encompassing the more limited transitional terms "consisting essentially of" and “consisting of”; “comprising” may therefore be replaced by "consisting essentially of" or “consisting of” and remain within the expressly defined scope of “comprising”.

[0143] “Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actors in the absence of express language in the claim to the contrary.

[0144] Additionally, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or”. That is, unless specified otherwise, or clear from context, “X employs A or B” is intended to mean any of the natural inclusive permutations. That is, if X employs A; X employs B; or X employs both A and B, then “X employs A or B” is satisfied under any of the foregoing instances.

[0145] In addition, the articles “a” and “an” as used in this application and the appended claims should generally be construed to mean “one or more” unless specified otherwise or clear from context to be directed to a singular form.

[0146] As used herein, “about” or “around” or “approximately” in the text or in a claim means ±10% of the value stated.

[0147] The term “ambient temperature” refers to an environment temperatureapproximately 18°C to approximately 25°C.

[0148] As used herein, the abbreviation “RT” means room temperature or a temperature ranging from approximately 18°C to approximately 25°C.

[0149] “Range” or “ranging” may be expressed herein as from about one particular value, and / or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and / or to the other particular value, along with all combinations within said range. Any and all ranges recited herein are inclusive of their endpoints (i.e. , x=1 to 4 or x ranges from 1 to 4 includes x=1 , x=4, and x=any number in between), irrespective of whether the term “inclusively” is used.

[0150] It will be understood that many additional changes in the details, materials, steps, and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above and / or the attached drawings.

[0151] While embodiments of this invention have been shown and described, modifications thereof may be made by one skilled in the art without departing from the spirit or teaching of this invention. The embodiments described herein are exemplary only and not limiting. Many variations and modifications of the composition and method are possible and within the scope of the invention. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims which follow, the scope of which shall include all equivalents of the subject matter of the claims.

Claims

What is claimed is:1 . A vacuum-driven process for producing biogas, the process comprising a) feeding a feedstock to a bioreactor forming a sludge of the feedstock therein; b) producing the biogas in the bioreactor under a reaction condition; c) predetermining a set-point of a headspace pressure in the bioreactor; d) extracting the biogas out of the bioreactor by a vacuum pump, and simultaneously pumping the headspace of the bioreactor to a vacuum or a negative pressure; and e) repeating steps a) to d) over a period of a reaction to maintain the vacuum or negative pressure of the headspace constant, wherein, when the headspace pressure is above the set-point, the vacuum pump is turned on to extract the biogas out of the bioreactor; when the headspace pressure is below the set-point, the vacuum pump is turned off, so that the biogas is favorable to produce in the bioreactor under the vacuum or the negative pressure.

2. The vacuum-driven process of claim 1 , wherein the feedstock is a mixture of a biowaste, an alkaline chemical, a pH buffer agent and / or an iron-based additive.

3. The vacuum-driven process of claim 1 , wherein the vacuum or negative pressure ranges from -0.1 to -15 psig.

4. The vacuum-driven process of claim 1 , wherein the bioreactor is an anaerobic digestion bioreactor, the process further comprising producing the biogas of CH4 and CO2; separating the biogas CH4 and CO2 into a CH4-enriched gas stream and a CC>2-enriched gas stream; forwarding the CH4-enriched gas stream to a post treatment process; and venting out the CC>2-enriched gas stream, or recycling the CC>2-enriched gas stream back to the anaerobic digestion bioreactor.

5. The vacuum-driven process of claim 4, wherein the reaction condition includes a mesophilic or thermophilic condition of the sludge of the mixture in the anaerobic digestion bioreactor ranging from 25 to 60°C.

6. The vacuum-driven process of claim 4, wherein the reaction condition includes a pH of the sludge of the mixture in the anaerobic digestion bioreactor ranging from 6 to 9.

7. The vacuum-driven process of claim 4, wherein a retention time of the sludge of the mixture in the anaerobic digestion bioreactor is maintained between 10 days and 40 days.

8. The vacuum-driven process of claim 1 , wherein the bioreactor is a dark fermentation bioreactor, the process further comprising producing the biogas of H2 and CO2; separating the biogas H2 and CO2 into a H2-enriched gas stream and a CO2- enriched stream; forwarding the H2-enriched gas stream to a post treatment process; and discharging the CC>2-enriched stream.

9. The vacuum-driven process of claim 8, wherein the reaction condition includes a thermophilic or hyperthermophilic condition of the dark fermentation bioreactor ranging from 25 to 80°C.

10. The vacuum-driven process of claim 8, wherein the reaction condition includes a pH of the sludge of the mixture in the dark fermentation bioreactor ranging from 5 to 7.11 . The vacuum-driven process of claim 8, wherein a retention time of the sludge of the mixture in the dark fermentation bioreactor is maintained between 10 hours and 3 days.

12. A vacuum-driven two-stage anaerobic digestion process for producing biogas, the process comprising: i) producing the biogas of H2and CO2 in a dark fermentation bioreactor under a fermentation condition by feeding a first mixture of a biowaste, an alkaline chemical,a pH buffer agent and / or an iron-based additive to the dark fermentation bioreactor forming a sludge of the first mixture therein; j) extracting the biogas of H2 and CO2 out of the dark fermentation bioreactor by a first vacuum pump, and simultaneously pumping the headspace of the dark fermentation bioreactor to a first vacuum or a first negative pressure; k) forwarding the biogas of H2 and CChto an anaerobic digestion bioreactor producing a biogas of CH4 therein under an anaerobic condition by feeding a second mixture of a waste solid digestate form the dark fermentation bioreactor, the alkaline chemical, the pH buffer agent and the iron-based additive to the anaerobic digestion bioreactor forming a sludge of the second mixture therein; l) extracting the biogas of CH4 and CO2 out of the anaerobic digestion bioreactor by a second vacuum pump, and simultaneously pumping the headspace of the anaerobic digestion bioreactor to a second vacuum or a second negative pressure; m) separating the biogas of CH4 and CChto a CH4-enriched gas stream and a CO2-enriched gas stream; n) forwarding the CO2-enriched gas stream from the anaerobic digestion bioreactor back to the anaerobic digestion bioreactor; o) forwarding the CH4-enriched gas stream to a post-treatment process; and p) repeating steps a) to g) over a period of a dark fermentation reaction and an anaerobic digestion reaction to maintain the headspace pressure of each bioreactor constant, wherein, when the headspace pressure of the dark fermentation bioreactor is above a first set-point, the first vacuum pump is turned on to extract the biogas of H2 and CO2 out of the dark fermentation bioreactor; when the headspace pressure of the dark fermentation bioreactor is below the first set-point, the first vacuum pump is turned off, so that the biogas of H2 and CO2 is favorable to produce in the dark fermentation bioreactor under the first vacuum or the first negative pressure, wherein, when the headspace pressure of the anaerobic digestion bioreactor is above a second set-point, the second vacuum pump is turned on to extract the biogas CH4 and CO2 out of the anaerobic digestion bioreactor; when the headspace pressure of the anaerobic digestion bioreactor is below the second set-point, the second vacuum pump is turned off, so that the biogas CH4 and CO2 is favorable to produce in the anaerobic digestion bioreactor under the second vacuum or the second negative pressure.

13. The vacuum-driven two-stage anaerobic digestion process of claim 12, further comprising: separating the biogas H2 and CO2 into a H2-enriched gas stream and a CO2- enriched stream; forwarding the Hh-enriched gas stream to a post treatment process or recycling to the anaerobic digestion bioreactor; and discharging the CC>2-enriched stream.

14. The vacuum-driven two-stage anaerobic digestion process of claim 12, further comprising: discharging a waste solid digestate from the anaerobic digestion bioreactor; or partially recycling the waste solid digestate from the anaerobic digestion bioreactor back to the first mixture of the dark fermentation bioreactor and discharging the rest of the waste solid digestate.

15. The vacuum-driven two-stage anaerobic digestion process of claim 12, wherein the first and second vacuums or the first and second negative pressures range from -0.1 to -15 psig, respectively.

16. The vacuum-driven two-stage anaerobic digestion process of claim 12, wherein the dark fermentation condition includes a thermophilic or hyperthermophilic condition of the dark fermentation bioreactor ranging from 25 to 80°C.

17. The vacuum-driven two-stage anaerobic digestion process of claim 12, wherein the anaerobic condition includes a mesophilic or thermophilic condition of the anaerobic digestion bioreactor ranging from 25 to 60°C.

18. The vacuum-driven two-stage anaerobic digestion process of claim 12, wherein the dark fermentation condition includes a pH of the sludge of the first mixture in the dark fermentation bioreactor ranging from 5 to 7.

19. The vacuum-driven two-stage anaerobic digestion process of claim 12,wherein the anaerobic condition includes a pH of the sludge of the second mixture in the anaerobic digestion bioreactor ranging from 6 to 9.

20. A system for producing biogas, the system comprising a bioreactor, configured to produce the biogas under a reaction condition, including a main body that contains a sludge of a mixture of a biowaste, an alkaline chemical, a pH buffer agent and / or an iron-based additive; and a headspace that contains the produced biogas; a plurality of transducers, configured to measure a headspace pressure through communicating with a PLC for pressure control, installed at a plurality of locations in the bioreactor; and a vacuum pump, configured to extract the produced biogas out of the bioreactor and simultaneously to pump the headspace to a vacuum or a negative pressure; wherein, when the headspace pressure is above a set-point set by the PLC, the vacuum pump is turned on to extract the biogas out of the bioreactor; when the headspace pressure is below the set-point, the vacuum pump is turned off, so that the biogas is favorable to produce in the bioreactor under the vacuum or the negative pressure.21 . The system of claim 20, further comprising a gas separation device, configured to purify the produced biogas to a biogas stream for a post-treatment process and a CC>2-enriched stream for recirculation back to the bioreactor.

22. The system of claim 20, wherein the vacuum or the negative pressure ranges from -0.1 to -15 psig.

23. The system of claim 20, wherein, when the bioreactor is a dark fermentation bioreactor, the reaction condition includes a thermophilic or hyperthermophilic condition of the dark fermentation bioreactor ranging from 25 to 80°C.

24. The system of claim 20, wherein the reaction condition includes a pH of the sludge of the mixture in the dark fermentation bioreactor ranging from 5 to 7.

25. The system of claim 20, wherein, when the bioreactor is an anaerobic digestion bioreactor the reaction condition includes a mesophilic or thermophilic condition of the anaerobic digestion bioreactor ranging from 25 to 60°C.

26. The system of claim 20, wherein the reaction condition includes a pH of the sludge of the mixture in the anaerobic digestion bioreactor ranging from 6 to 9.

27. A vacuum-driven two-stage anaerobic digestion system for producing biogas, the system comprising: a dark fermentation bioreactor, configured to produce the biogas of H2 and CO2 under a fermentation condition, including a first body that contains a sludge of a first mixture of a biowaste, an alkaline chemical, a pH buffer agent and / or an iron-based additive; and a first headspace that contains the produced biogas; a plurality of a first set of transducers, configured to measure a pressure of the first headspace through communicating with a PLC for pressure control, installed at a plurality of locations in the dark fermentation bioreactor; a first vacuum pump, configured to extract the produced biogas of H2 and CO2 out of the dark fermentation bioreactor and simultaneously to pump the first headspace to a first vacuum or a first negative pressure; an anaerobic digestion bioreactor, configured to produce the biogas of CH4 and CO2 under an anaerobic digestion condition, including a second body that contains a sludge of a second mixture of a waste solid digestate form the dark fermentation bioreactor, the alkaline chemical, the pH buffer agent and the iron-based additive; and a second headspace that contains the produced biogas of CH4 and CO2; a plurality of a second set of transducers, configured to measure a pressure of the second headspace through communicating with the PLC for pressure control, installed at a plurality of locations in the anaerobic digestion bioreactor; a second vacuum pump, configured to extract the produced biogas of CH4and CO2 out of the anaerobic digestion bioreactor and simultaneously to pump the second headspace to a second vacuum or a second negative pressure; wherein, when the headspace pressure of the dark fermentation bioreactor is above a first set-point, the first vacuum pump is turned on to extract the biogas of H2 and CO2 out of the dark fermentation bioreactor; when the headspace pressure of the dark fermentation bioreactor is below the first set-point, the first vacuum pump is turned off, so that the biogas of H2 and CO2 is favorable to produce in the dark fermentation bioreactor under the first vacuum or the first negative pressure, wherein, when the headspace pressure of the anaerobic digestion bioreactor is above a second set-point, the second vacuum pump is turned on to extract the biogas CH4 and CO2 out of the anaerobic digestion bioreactor; when the headspace pressure of the anaerobic digestion bioreactor is below the second set-point, the second vacuum pump is turned off, so that the biogas CH4 and CO2 is favorable to produce in the anaerobic digestion bioreactor under the second vacuum or the second negative pressure.

28. The vacuum-driven two-stage anaerobic digestion system of claim 27, further comprising a first gas separation device, configured to separate the biogas of H2 and CO2 to a H2-enriched gas stream and a CO2-enriched stream, wherein the Hh-enriched gas stream is sent to post-treatment processes and the CO2-enriched gas stream is either vented out or recirculated to the anaerobic digestion bioreactor for producing CH4.

29. The vacuum-driven two-stage anaerobic digestion system of claim 27, further comprising a second gas separation device, configured to separate the biogas of CH4 and CC>2 to a CF -enriched gas stream and a CC>2-enriched gas stream, wherein the CH4-enriched gas stream is sent to the post-treatment processes and the CO2- enriched gas stream is recirculated back to the anaerobic digestion bioreactor for producing CH4.

Images