METHODS AND BIOREACTORS FOR MICROBIAL DIGESTION USING IMMOBILIZED BIOFILMS
20180237734 · 2018-08-23
Assignee
Inventors
Cpc classification
C02F3/2806
CHEMISTRY; METALLURGY
C12M25/16
CHEMISTRY; METALLURGY
B01J8/06
PERFORMING OPERATIONS; TRANSPORTING
Y02E50/30
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
B01J8/0292
PERFORMING OPERATIONS; TRANSPORTING
B01J2208/00876
PERFORMING OPERATIONS; TRANSPORTING
C02F2203/006
CHEMISTRY; METALLURGY
International classification
C12M1/107
CHEMISTRY; METALLURGY
B01J8/06
PERFORMING OPERATIONS; TRANSPORTING
Abstract
The invention relates to methods, devices and inserts for reactors for microbial and anaerobic digestion. In particular, the invention relates to reactors comprising inserts for biofilms, such as methane-producing biofilms, immobilized on a carrier matrix.
Claims
1. An insert comprising one or more baffles (1) defining at least two open compartments (2,3), said one or more baffles comprising one or more open edges (21), thereby when inserted into a tank reactor and when said tank reactor is in operation said one or more open edges define an underflow (22) or an overflow aperture thus forcing a fluid to flow upwardly or downwardly across said underflow or said overflow aperture.
2. An insert (14) according to claim 1, wherein said at least two compartments (6,7) further comprise a continuous closed side wall (4) surrounding said one or more baffles (5), wherein said one or more open edges (23) are displaced in respect to a height (24) of said continuous closed side wall.
3. An insert (14) according to any of the claims 1-2, wherein said one or more baffles (5) are fastened to said continuous closed side wall (4).
4. An insert (14) according to any of the claims 1-3, wherein said continuous closed side wall is a curved wall.
5. An insert (15) according to any of the claims 1-4, wherein said one or more baffles are a plurality of baffles (11,12) and wherein said at least two open compartments are a plurality of open compartments (8,9,10).
6. An insert (15) according to claim 5, wherein said one or more open edges (25, 26) of said plurality of baffles (11,12) are displaced/staggered/shifted in respect to each other, thereby when inserted into a tank reactor and when said tank reactor is in operation said one or more open edges define a plurality of underflow and overflow aperture, thus forcing a fluid to flow from an underflow aperture of a first compartment upwardly towards an overflow aperture of a second subsequent compartment and downwardly towards an underflow aperture of a third subsequent compartment.
7. An insert (20) according to any of the claims 1-6, wherein said plurality of baffles are interconnected baffles (16,17,18,19).
8. An insert (20) according to any of the claims 1-7, wherein one or more of said at least two open compartments (27, 29, 30, 31) defines one or more sections (27, 28) of said insert.
9. An insert according to claim 8, wherein said one or more sections are external (28) or internal (27) sections.
10. An insert according to any of the claims 1-9, wherein said one or more baffles and/or said continuous closed side wall are made from a corrosion resistant and liquid impermeable material.
11. An insert according to any of the claims 1-10, comprising means for supporting biofilms located in said at least two open compartments.
12. An insert according to claim 11, wherein said means for supporting biofilms are a plurality of biofilm carriers suitable for biofilm growth upon exposure to a flow of fluid containing biofilm precursors, said biofilm carrier comprising a three dimensional structure having at least one surface comprising cavities and protrusions thereby providing a rough surface.
13. An insert according to claim 12, wherein said rough surface has a rough surface area Ra between 3 and 6 mm.
14. An insert according to any of the claims 12-13, wherein said rough surface has a minimum valley Rv of 1 mm.
15. An insert according to any of the claims 12-14, wherein said rough surface has a peak depth Rp of 2 mm.
16. An insert according to any of the claims 12-15, wherein said three dimensional structure comprises openings, such as holes throughout said at least one surface.
17. An insert according to any of the claims 12-16, wherein said three dimensional structure is a tubular porous three dimensional structure.
18. An insert according to claim 17, wherein said porous are open porous.
19. An insert according to any of the preceding claims 12-18, wherein said three dimensional structure is or comprises a threaded structure.
20. An insert according to any of the preceding claims 1-19, comprising a biofilm, such as a biofilm comprising one or more different microorganisms adapted to aerobic or anaerobic digestion/fermentation.
21. An insert according to any of the preceding claims 1-20, defining a preferential vertical path along and inside said biofilm carriers, thereby when inserted into a tank reactor and when said tank reactor is in operation a fluid flow substantially parallel to the biofilm formation.
22. A bioreactor comprising: a container having one or more side walls and a bottom wall having an internal surface and a bottom opening; at least two removable open compartments located inside said container; at least one overflow aperture or underflow aperture between said at least two removable compartments; thereby when in operation a fluid flows between said at least two removable compartments downwardly towards said at least one underflow aperture or upwardly towards said at least one overflow aperture.
23. A bioreactor according to claim 22, comprising said insert according to any of the claims 1-21, wherein said at least two removable open compartments are said at least two open compartments defined by said one or more baffles of said insert and wherein said at least one overflow aperture or underflow aperture are said underflow aperture or said overflow aperture defined by said one or more open edges.
24. A bioreactor according to any of the claims 22-23, wherein said one or more side walls of said container are said continuous closed side wall of an insert according to any of the claims 1-21.
25. A bioreactor according to any of the claims 22-24, further comprising, means for forcing, when in operation, a fluid to flow downwardly towards said at least one underflow aperture or upwardly towards said at least one overflow aperture through a preferential path.
26. A bioreactor according to claim 25, wherein said means for forcing a fluid to flow are said biofilm carriers according to any of the claims 12-21.
27. A bioreactor according to any of the claims 22-26, further comprising means for promoting removal of precipitate deposited or located on said internal surface of said bottom wall of said container.
28. A bioreactor according to claim 27, wherein said means for promoting removal of precipitate are one or more rotating means, such as one or more rotating scrapers.
29. A bioreactor according to claim 28, wherein each of said one or more rotating scrapers has a scraping edge and a top edge opposite to said scraping edge.
30. A bioreactor according to claim 29, wherein, when not in motion, said one or more rotating scrapers lay in a position that reduces or avoids short circuiting flow between neighbouring sections.
31. A bioreactor according to any of the claims 29-30, wherein when not in motion each of said one or more rotating scrapers rotating scrapers is located underneath a full baffle delimiting a section.
32. A bioreactor according to any of the claims 29-31, comprising a gap between said top edge of each of said one or more rotating scrapers and a edge of said full baffle, thereby ensuring for correct rotation as well as for reducing and/or avoiding cross-flow between sections.
33. A bioreactor according to any of the claims 29-32, wherein said one or more rotating scrapers are adapted to rotate clockwise or counter clockwise from a resting position underneath a first full baffle, to a second resting position underneath a second full baffle, thereby, one rotation provides scraping of the internal surface of said bottom wall of an entire section.
34. A bioreactor according to any of the claims 29-33, wherein said one or more rotating scrapers are located underneath low baffles or high baffles.
35. A bioreactor according to any of the claims 29-34, wherein said one or more rotating scrapers provide a fluid tight seal between a bottom edge of full baffles and said top edge of said one or more rotating scrapers.
36. A bioreactor according to any of the claims 23-35, wherein said container comprises a bottom chamber defined/located between said internal surface of said bottom wall and a lowest level/part of said insert according to any of the claims 1-21.
37. A bioreactor according to claim 36, wherein said bottom chamber comprises said means for promoting removal of precipitate.
38. A bioreactor according to claim 37, wherein said means for promoting removal of precipitate are adapted to define, when not in operation, static zones within said bottom chamber wherein cross-flow between compartments and said static zones is lower than a desired value, while when in operation, said static zones becomes mixing zones wherein cross-flow between compartments and said static zones is higher than said desired value.
39. A bioreactor according to any of the claims 23-38, wherein a width/size/diameter of said insert is substantially equal to a width/size/diameter of said container.
40. A bioreactor according to any of the claims 23-39, wherein said lowest level/part of said insert is located at a desired distance from said internal surface of said bottom wall.
41. A bioreactor according to any of the claims 23-40, further comprising means for keeping said insert at said desired distance from said internal surface of said bottom wall.
42. A bioreactor according to claim 41, wherein said means for keeping said insert at said desired distance are a plurality of protrusions located on said one or more side walls of said container.
43. A bioreactor according to claim 41, wherein said means for keeping said insert at said desired distance from said internal surface of said bottom wall are a curvature of said bottom wall, said curvature gradually reducing said width/size/diameter of said container, said width/size/diameter defined by said one or more side walls of said container.
44. A bioreactor according to claims 23-43, wherein when in operation, said open edges displaced in respect to each other define a plurality of underflow and overflow apertures whereby fluid flow from an underflow aperture of a first compartment upwardly towards an overflow aperture of a second subsequent compartment and downwardly towards an underflow aperture of a third subsequent compartment.
45. A bioreactor according to any of the claims 25-44, wherein when in operation, said means for forcing a fluid to flow define a preferential flow path upwardly towards an overflow aperture or downwardly towards an underflow aperture of subsequent compartments.
46. A bioreactor according to any of the claims 22-45, wherein when in operation a cross-flow in between not subsequent compartment is lower than a desired value.
47. A bioreactor according to any of the claims 22-46, further comprising means for recirculating a fluid within each of said at least two open compartments.
48. A bioreactor according to any of the claims 22-47, further comprising means for recirculating a fluid in between said at least two open compartments.
49. A bioreactor according to any of the claims 23-48, further comprising means for recirculating a fluid within each sections.
50. A bioreactor according to any of the claims 23-49, further comprising means for recirculating a fluid in between sections.
51. A bioreactor according to any of the claims 47-50, wherein said means for recirculating a fluid are one or more recirculation pumps.
52. A bioreactor according to claim 51, wherein said one or more recirculation pumps are at least in an amount equal to the amount of sections of said insert.
53. A bioreactor according to claim 51, wherein said one or more recirculation pumps are in an amount equal to the amount of compartments of said insert divided by two.
54. A bioreactor according to any of the claims 22-53, wherein said container is a cylindrical tank reactor.
55. A method of operating a bioreactor, said bioreactor according to any of the claims 22-54, said method comprising: feeding said bioreactor with a fluid containing biofilm precursors; conducting digestion of said fluid.
56. A method according to claim 55, wherein said fluid containing biofilm precursors is a feedstock having a COD at least 30.0 gr/L.
57. A method according to any of the claims 55-56, wherein said conducting digestion of said fluid occurs with a hydraulic retention time of 120 hours or less while maintaining a flow velocity of at least 0.0002 m/s and a gas production rate of at least 5.0 Lgas/Ldigester/day in such a manner as to maintain a substantial laminar flow through said biofilm carriers.
58. A method according to any of the claims 55-57, wherein said conducting digestion comprises forcing said fluid to flow between said at least two compartments downwardly towards said at least one underflow aperture or upwardly towards said at least overflow aperture.
59. A method according to claim 58, wherein said forcing said fluid to flow further comprises forcing said fluid to flow through a preferential flow path defined by said plurality of biofilm carriers.
60. A method according to any of the claims 58-59, wherein said forcing said fluid to flow, further comprises recirculating said fluid within each compartments.
61. A method according to any of the claims 58-60, wherein said forcing said fluid to flow, further comprises recirculating said fluid in between compartments.
62. A method according to any of the claims 58-61, wherein said forcing said fluid to flow, further comprises recirculating said fluid within each sections.
63. A method according to any of the claims 58-62, wherein said forcing said fluid to flow, further comprises recirculating said fluid in between sections.
64. A method according to any of the claims 55-63, further comprising: removing precipitate located on said internal surface of said bottom wall of said bioreactor.
65. A system for producing biogas (74), said system comprising: at least one feed tank (75) for feeding bioreactors; one or more interconnected bioreactors (77) according to any of the claims 22-54; at least one effluent tank (76) for collecting effluents from said one or more interconnected bioreactors.
66. A method of converting a Continuously Stirred tank Reactor (CSTR) having an internal surface into a fixed film, fixed orientation, fixed bed anaerobic digestion reactor, said method comprising installing an insert according to any of the claims 1-21 within said CSTR.
67. A method according to claim 66, wherein said installing comprises fastening said one or more baffles to one of more locations of said internal surface of said CSTR.
68. A method according to any of the claims 66-67, wherein said installing comprises, firstly inserting and fitting said insert in said CSTR and secondly installing said plurality of biofilm carriers.
69. A method according to any of the claims 66-68, further comprising growing a biofilm within said insert.
70. A method for performing maintenance of a CSTR modified according to the method of any of the claims 66-69, or a bioreactor according to any one of the claims 22-54, said method comprising: temporarily interrupting a normal operation of said modified CSTR; removing at least part of said insert; re-installing said at least part of said insert.
71. A method according to claim 70, wherein said at least part of said insert is at least one compartment of said insert.
72. A method according to claim 70, wherein said at least part of said insert is at least one section of said insert.
73. A method according to claim 70, wherein said at least part of said insert is one or more biofilm carriers within said compartments of said insert.
74. Use of a bioreactor according to any of the claims 22-54, for producing biogas.
75. Use of a bioreactor according to any of the claims 22-54, for rapid determination of a biomethane potential of a feedstock.
76. Use of a bioreactor according to any of the claims 22-54 to produce a product produced by microbial organisms supported on a biofilm.
77. A method of aerobic or anaerobic digestion of a feedstock in the bioreactor according to any of the claims 22-54 comprising the steps of: feeding the feedstock into the compartments of the bioreactor; digesting the feedstock by passing the feedstock through the compartments of the bioreactor with a retention time sufficient to digest the feedstock.
78. The method of claim 77, wherein the compartments contain biofilm carriers, which have been pre-inoculated to obtain a biofilm with a suitable bacterial consortium.
79. The method of claim 78, wherein the bacterial consortium is a consortium of methane producing bacteria.
80. The method of any of the claims 77-79, wherein the feedstock is mixed between each compartment.
81. The method of any of the claim 77-79, wherein feedstock is feed simultaneously at several compartments across the bioreactor in order to form a feeding gradient.
82. The method of any of the claims 77-81, wherein the feedstock is partly or completely recirculated through the bioreactor.
83. The method of claim 82, wherein the feedstock is partly recirculated between the compartments of the bioreactor.
84. The method of any of the claims 77-83, wherein the feedstock is digested under anaerobic conditions to produce biomethane with a hydraulic retention time of 120 hours or less while maintaining a flow velocity of at least 0.0002 m/s through the bioreactor and/or a gas production rate of at least 5.0 liters/liter digester volume/day in such a manner as to maintain a substantial laminar flow through said biofilm carriers.
85. The method of any of the claims 77-84, wherein the feedstock has a chemical oxygen demand (COD) of at least 20.0 g/L, such as at least 30.0 g/L, at least 35 g/L, at least 40 g/L or at least 50 g/L or wherein the feedstock has a COD of 20-300 g/L, 30-300 g/L, 40-300 g/L, 50-300 g/L, 75-300 g/L, 100-300 g/L, such as 25-250 g/L, 30-200 g/L, 35-150 g/L, 40-150 g/L, 50-150 g/L or such as 20-125 g/L, 30-100 g/L, 30-75 g/L, 30-50 g/L, 35-75 g/L, 40-100 g/L, 50-175 g/L, 50-200 g/L.
86. The method of any of the claims 77-85, wherein the feedstock is digested at a temperature between 30 and 55 C.
87. The method of claim 86 wherein the feedstock is digested at a temperature between 37 and 48 C.
88. The method of any of claims 77-87, wherein the feedstock is digested at a pH between 6.6 and 8.5.
89. The method of claim 88, wherein the feedstock is digested at a pH between 6.8 and 7.4.
90. The method of any of claims 88 and 89, wherein the pH is adjusted by recirculation and/or addition of pH adjusting agents.
91. The method of any of claims 84-90, wherein the retention time is less than 110 hours, such as less than 100 hours, less than 90 hours, less than 80 hours, less than 75 hours, less than 60 hours, less than 50 hours, or less than 40 hours or wherein the retention time is 20-120 hours, 30-120 hours, 40-120 hours, 50-120 hours, 75-120 hours, 100-120 hours, or such as 50-110 hours, 50-100 hours, or 50-75 hours.
92. The method of any of claims 84-91, wherein the flow velocity is at least 0.00025 m/s, such as at least 0.0005 m/s, at least 0.00075 m/s, at least 0.001 m/s, at least 0.0025 m/s, at least 0.005 m/s, or at least 0.0075 m/s or wherein the flow velocity is 0.0002-0.015 m/s, such as 0.0002-0.0125 m/s, 0.0002-0.01 m/s, 0.0002-0.0075 m/s, 0.0002-0.005 m/s, or such as 0.00025-0.01 m/s, 0.0005-0.01 m/s, 0.00075-0.01 m/s, 0.001-0.01 m/s, 0.0025-0.01 m/s, 0.005-0.01 m/s, or 0.0075-0.01 m/s.
93. The method of any of claims 84-92, wherein the gas production rate is at least 6.0 liters/liter digester volume/day, such as 7.0 liters/liter digester volume/day, at least 8.0 liters/liter digester volume/day, 9.0 liters/liter digester volume/day, at least 10.0 liters/liter digester volume/day, such as at least 12.5 liters/liter digester volume/day, at least 15 liters/liter digester volume/day or at least 20 liters/liter digester volume/day wherein the gas production rate is 5.0-20 liters/liter digester volume/day, such as 6.0-20 liters/liter digester volume/day, 7.0-20 liters/liter digester volume/day 8.0-20 liters/liter digester volume/day, 9.0 liters/liter digester volume/day, or 10-20 liters/liter digester volume/day.
94. The method of any of claims 77-93, wherein the feedstock is a biomass.
95. The method of claim 94, wherein the biomass is selected from the group consisting of waste, sewage, manure, or a cellulosic, hemicellulosic, lignocellulosic or starch containing biomass selected from wheat straw, corn stover, sugar cane bagasse, sweet sorghum bagasse, distillery vinasse, or empty fruit bunches.
96. The method of claim 95, wherein the waste is selected from the group consisting of municipal solid waste (MSW), liquefied organic components of MSW, industrial waste, animal waste, plant waste or wastes from abattoirs, restaurants, dairy processing, and tanneries.
97. The method of claim 96 wherein the waste contains a level of total solids greater than 7% (w/w), such as greater than 8% (w/w), greater than 9% (w/w), greater than 10% (w/w), such as 7-20% (w/w), 8-20% (w/w), 9-20% (w/w), 10-20% (w/w), or 15-20% (w/w).
98. The method of any of claims 94-97, wherein the biomass has been subjected at least in part to one or more of hydrothermal pre-treatment, enzymatic hydrolysis and/or aerobic digestion.
99. The method according to any of the claims 84-98, wherein the feedstock is digested under anaerobic conditions to produce biomethane with a hydraulic retention time of 120 hours, or less, such as less than 110 hours, such as less than 100 hours, less than 90 hours, less than 80 hours, less than 75 hours, less than 60 hours, less than 50 hours, or less than 40 hours or wherein the retention time is 20-120 hours, 30-120 hours, 40-120 hours, 50-120 hours, 75-120 hours, 100-120 hours, or such as 50-110 hours, 50-100 hours, 50-75 hours, while maintaining a flow velocity of at least 0.0002 m/s through the bioreactor and/or a gas production rate of at least 5.0 liters/liter digester volume/day in such a manner as to maintain a substantial laminar flow through said biofilm carriers.
100. The method according to any of the claims 84-99, wherein the feedstock is digested under anaerobic conditions to produce biomethane with a hydraulic retention time of 120 hours, or less, while maintaining a flow velocity of at least 0.0002 m/s, such as a flow velocity between 0.0002 m/s to 0.08 m/s, such as between 0.0030 and 0.07, such as between 0.009 and 0.05, such as between 0.015 m/s to 0.045 m/s through the bioreactor and/or a gas production rate of at least 5.0 liters/liter digester volume/day in such a manner as to maintain a substantial laminar flow through said biofilm carriers.
101. The method according to any of the claims 84-100, wherein the feedstock is digested under anaerobic conditions to produce biomethane with a hydraulic retention time of 120 hours, or less, while maintaining a flow velocity of at least 0.0002 m/s through the bioreactor and/or a gas production rate of at least 5.0 liters/liter digester volume/day (L/L/D), such as in between 6.0 L/L/D and 10.0 L/L/D, such as in between 7.0 L/L/D and 9.0 L/L/D, such as at least 8.0 L/L/D.
Description
BRIEF DESCRIPTION OF THE FIGURES
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DETAIL DESCRIPTIONS OF SOME EMBODIMENTS
[0243] In some embodiments, the invention provides a method of anaerobic digestion to biomethane comprising the steps of [0244] introducing a substrate feedstock having COD content at least 30.0 g/L into a fixed film, fixed orientation, fixed bed bioreactor system in which the immobilization matrix is characterized in comprising a plurality of vertically oriented, porous tubular carriers supporting biofilm, and in which mixing zones are provided both above the upper openings and below the lower openings of the tubular carriers, and conducting anaerobic digestion of the feedstock with a hydraulic retention time of 120 hours or less while maintaining a flow velocity of at least 0.0002 m/s and a gas production rate of at least 5.0 liters/liter digester volume/day in such manner as to maintain a substantially laminar flow through the tubular carriers as well as mixing within each of the mixing zones.
[0245] In some embodiments, the invention provides an anaerobic digestion bioreactor comprising a cylindrical tank having a plurality of internal, vertical biofilm carrier compartments defined by baffles or walls made from corrosion resistant and liquid impermeable material that are open at the top, where in each carrier compartment comprises a shortened wall or underflow aperture on one side at the bottom which serves as an opening into another carrier compartment whereby fluid flows can be directed through succeeding compartments, and wherein a plurality of the carrier compartments further comprise a shortened wall or overflow aperture at the top on a side other than that side which contains a shortened wall or underflow aperture at the bottom which serves as an opening into another carrier compartment whereby fluid flows can be directed through succeeding compartments, optionally further comprising a rotable scraper that is adapted to define sealed sections in a sedimentation zone situated beneath the lowest edge of the carrier compartments when in a closed position or to permit removal of sedimented solids when in an open position.
[0246] In some embodiments, the invention provides an insert for converting a continuously stirred tank reactor (CSTR) into a fixed film, fixed orientation, fixed bed anaerobic digestion reactor comprising [0247] interconnected baffles made from corrosion resistant and liquid impermeable material that define a plurality of vertical biofilm carrier compartments that are open at the top, each of which has a shortened wall or underflow aperture on one side at the bottom which serves as an opening into another carrier compartment whereby fluid flows can be directed through succeeding compartments, and most of which have a shortened wall or overflow aperture at the top on a side other than that which contains a shortened wall or underflow aperture at the bottom which serves as an opening into another carrier compartment whereby fluid flows can be directed through succeeding compartments.
[0248] In some embodiments, the invention provides a method of converting a CSTR tank into a fixed film, fixed orientation, fixed bed anaerobic digestion reactor comprising the steps of [0249] assembling an insert of interconnected baffles made from corrosion resistant and liquid impermeable material that define a plurality of vertical biofilm carrier compartments that are open at the top, each of which has a shortened wall on one side at the bottom which serves as an opening into another carrier compartment whereby fluid flows can be directed through succeeding compartments, and most of which have a shortened wall at the top on a side other than that which contains a shortened wall at the bottom which serves as an opening into another carrier compartment whereby fluid flows can be directed through succeeding compartments, [0250] installing the insert within a modified or unmodified CSTR tank, [0251] fitting the carrier compartments defined by the insert with a plurality of porous, tubular carriers either before or after installation in the CSTR tank, and [0252] raising a productive biofilm on the carriers.
[0253] In some embodiments, the invention provides methods and laboratory scale devices for rapid determination of biomethane potential of tested substrates.
[0254] By maintaining very high biogas production rates in fixed film, fixed orientation, fixed bed anaerobic digestion systems, biofilms can be maintained in excellent productive condition without excess accumulation of biomass and associated clogging problems. Typically biogas flows should be maintained at least at 5.0 liters total gas/liter digester volume/day (L/L/D), or at least 6.0 L/L digester volume/day, or at least 7.0, or at least 8.0, or at least 9.0. In order to achieve such high gas production, a processed waste stream typically has high COD content, at least 30.0 g/L, or at least 40.0, or at least 50.0. The range of COD content in the feed stream is typically between 20.0 g/L and 300 g/L. Total gas in this context refers to the mixed product gas comprising both carbon dioxide and methane.
[0255] COD content is determined by the ferrous ammonium sulphate method well known in the art and is expressed in mg/L or g/L.
[0256] High COD/high solids waste streams typically are associated with high content of undissolved solids. A suitable bioreactor should typically be adapted to handle undissolved solids of at least 3.0 g/L, or 5.0, or 7.0, or 8.0, or 10.0, or 15.0, or 20.0, or 25.0, or 30.0, or 35.0, or 40.0, or 45.0, or 50.0, or 55.0, or 60.0.
[0257] One approach to handling anaerobic digestion of feed stream having a high content of undissolved solids in fixed film, fixed orientation, fixed bed systems is through the use of vertically oriented immobilization matrix. Undissolved solids in a vertically oriented matrix simply precipitate along the flow path. In some embodiments, sedimenting particles can be directed into sedimentation zones where particles can be collected harmlessly.
[0258] In some embodiments, the invention provides fixed film, fixed orientation, fixed bed anaerobic digestion bioreactors comprising multiple compartments suitable for containing biofilm carrier matrix, each of which or most of which compartments is associated with a sedimentation zone. Sedimentation zone refers to a free volume situated between the bottom of the bioreactor tank and the lowermost edge of the carrier compartments, which are typically set significantly above the bottom of the tank. In operation, tubular biofilm carriers are typically set within the carrier compartments such that the lower openings of the carriers are situated significantly above the lowermost edge of the carrier compartments. The lowermost edge of the carrier compartments, in turn, are typically set within the bioreactor tank significantly above the physical bottom of the tanktypically between 15 and 500 cm, or between 50 and 1000 cm, depending on the size of the digester.
[0259] In some embodiments, a bioreactor of the invention is equipped with a digester bottom scraper device adapted to transport sediment formed in sedimentation zones at the bottom of the active digester volume into a sludge pump system. Sediments recovered from sedimentation zones can, in this manner, be re-introduced into the digester feed stream. This serves to extend the exposure of undissolved solids to active biomethane-producing microbiology by separating the actual retention time of undissolved solids from the overall hydraulic retention time of the feed input. In other words, undissolved solids that are precipitated from the feed stream can be recirculated without extending an otherwise short hydraulic retention time. This generally improves gas production and is in marked contrast with standard CSTR systems, in which hydraulic retention time applies to the entire feed stream, including dissolved and suspended solids.
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[0262] All the four sections accommodate baffles of similar size.
[0263] In this example, a full baffle 45 has a height 80 of 54 cm, a low baffle 41, i.e. a baffle having an overflow aperture has a height 81 of 48 cm; a high baffle 46, i.e. a baffle having an underflow aperture has a height 82 of 51 cm. The compartments 42 accommodate porous tubular biofilm carrier 44.
[0264] The porous tubular biofilm carrier 44 has a height 83 of 35 cm.
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[0269] The baffle 5 has an open edges 23 that is displaced in respect to a height 24 of the continuous closed side wall 4. When inserted into a reactor the flow follows the path as shown in
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[0272] The open edges 25 and 26 of the two baffles 11 and 12 are displaced in respect to each other so that when a fluid is flowing through the insert it will flow through the underflow aperture defined by open edge 25 and towards and through the overflow aperture defined by open edge 26.
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[0275] Scrapers 54 prevents or reduces short circuiting between neighbouring sections.
[0276] Scraper in section sealing position ensures no passing of liquid or reduces passage of fluids between sections even though sedimentation space is shared.
[0277] Sedimentation zones 55 are located at the bottom of the bioreactor 47.
[0278]
[0279]
[0280] The insert 20 has an inner or internal section 27 and an outer or external section 28. The outer section 28 comprises three open compartments 29, 20 and 31, in between baffles 16 and 18 and defined by baffles 17 and 19.
[0281] The insert 20 forces a fluid inserted, according to the direction of arrow 36, in section 27 to flow downwardly towards the underflow aperture leading to compartment 29 and then upwardly towards the overflow aperture leading to compartment 30. In compartment 30 the fluid flows downwardly towards the underflow aperture leading to compartment 31 and upwardly according to the direction of arrows 34 and 35 back into sections 27 or out of the section and insert according to the direction of arrow 35.
[0282] Fluid flow is directed sequentially through succeeding carrier compartments within each quarter section by a system of overflow and underflow apertures. Re-circulation suction pumps are provided for each quarter section. The pumps are adapted to withdraw fluid from the top of the last compartment within the flow sequence of a quarter section, in which the vertical direction of flow is upward. This removed fluid is then re-introduced to the first compartment within the flow sequence of the quarter section. The recirculation flow can be introduced from above the surface of liquid in this compartment, thereby actively enhancing mixing in the chamber to which the recirculation flow is introduced. Influent feed stream is mixed with recirculation flow. This in turn drives fluid flow through the reactorthe net volume of feed stream introduced drives net flow through the reactor.
[0283] A feed inlet introduces feed stream mixed with recirculating effluent into one quarter-cylinder compartment of the inner section. The curved wall of this compartment is shortened at the bottom, providing an opening for fluid flow into the bottom of a first trapezoidal compartment of the outer section. This shortened wall is one means for achieving an underflow aperture, meaning an opening at the bottom of the compartment that permits fluid flow into a succeeding compartment. Similarly a shortened wall at the top of a compartment is one means for achieving an overflow aperture whereby fluid flow is directed into succeeding compartments. Underflow or overflow apertures may alternatively be simply an opening in an otherwise intact wall. However, this arrangement increases the risk of channelling. In operation, tubular biofilm carriers are typically set in compartments at a level such that the lowermost openings of the tubular carriers are at a height above the underflow aperture (i.e. above the lowermost surface of the shortened wall at the bottom) corresponding to between 2-10 times the diameter of the carriers' primary fluid channel. Similarly, the uppermost openings of the tubular carriers are at a height below the overflow aperture (i.e. below the uppermost surface of the shortened wall at the top) corresponding to between 2-10 times the diameter of the carriers' primary fluid channel. This placement minimizes the risk of channelling as flows enter into or emerge out of the biofilm carriers and further defines mixing zones both above the uppermost openings and below the lowermost openings of the carriers.
[0284] The physical carrier compartments themselves are set within the bioreactor tank at a level such that the lowermost edge of the compartments is above the physical bottom of the bioreactor tank. The open volume beneath the lowermost edge of the carrier compartments defines a sedimentation zone in which sedimenting particles can accumulate. The embodiment shown in
[0285] Feed stream mixed with recirculated liquid enters a first compartment of the inner section, travels in a downward vertical direction through biofilm carriers, then passes through the underflow aperture into a trapezoidal compartment of the outer section. The fluid flow through biofilm carriers in the second compartment is forced into an upward vertical direction. At the top of this second compartment, the fluid flow passes through an overflow aperture into a third compartment. Here again, the fluid flow is forced to change vertical direction into a downward vertical flow through the third compartment. In this manner, the flow is forced into a pattern of alternating downward and upward direction and routed sequentially through each compartment of the reactor until it reaches the last compartment of the sequence, which is fitted with an effluent outlet that is situated at a level intermediate between the top surface of the compartments and the level corresponding to the bottom of the overflow apertures, i.e. the uppermost surfaces of the shortened walls at the top of compartments. This ensures that effluent will be driven out by force of gravity.
[0286] In some embodiments, the flow within each quarter section is continuously recirculated. The volume of feed stream introduced ensures that there will be net displacement sequentially between the quarter sections and out through the effluent outlet, notwithstanding continuous recirculation within each quarter section.
[0287] The region beneath the lowermost openings of the biofilm carriers at the bottom of two compartments which are in fluid communication via an underflow aperture provides a mixing zone. The region above the uppermost openings of the biofilm carriers at the top of two compartments which are in fluid communication via an overflow aperture similarly defines a mixing zone. In some embodiments, mixing is achieved within the mixing zones during operation by the forced change of vertical direction of flow from downward to upward. Because fluid flow through the reactor is achieved without agitation, the flow through the tubular biofilm carriers is substantially laminar. Furthermore, without agitation, undissolved particles will precipitate down the tubular biofilm carriers' primary, vertical fluid channel and into the sedimentation zones.
[0288] The basic fluid flow patterns achieved in one quarter section of the bioreactor shown in
[0289]
[0290]
[0291] In
[0292] In embodiments such as those described in
[0293] In practicing methods of the invention, the carrier matrix used to support biofilm in a fixed film, fixed orientation, fixed bed system is ideally tubular and porous. The term tubular as used herein refers to a structure that defines one or more central channels through which fluid will flow in one direction by the force of gravity when it is placed in an upright, vertical orientation. A tubular matrix can have one or more central channels having an irregular, rectangular or even triangular cross sectional geometry. However, tubular matrix is preferably cylindrical, that is, having one or more central channels having a circular cross sectional geometry. Cylindrical geometry is preferable because the presence of corners in the fluid channel creates pockets of restricted flow. This in turn tends to promote accumulation of biomass and even sediment in the restricted flow areas, which both reduces the active surface of biofilm and also increases the risk of channelling effects.
[0294] The biofilm carrier matrix is preferably porous. The term porous refers to a carrier matrix having openings on the channel-forming surface which may be openings formed between twisting and evaginated surfaces. A smooth surface matrix, for example, the CLOISONYL tubes used by Escudie et al. (2011), permit only one possible direction for biomass accumulation in the biofilmtowards occlusion of the biofilm carrier's fluid channels. In contrast, as illustrated in
[0295]
[0296] The porous biofilm carrier 66 has carrier walls 67 consisting of threaded material. The biofilm 68 attaches to all surfaces of the threads. If the biofilm pointing inwards in the tube is torn, the biofilm attached to the other dimensions of the thread remains attached, thus being able to regenerate the biofilm washed away.
[0297] In some embodiments, a porous biofilm carrier matrix has a total surface area to volume ratio of between 60 m.sup.2/m.sup.3 and 300 m.sup.2/m.sup.3, or between 80 and 200, or between 90 and 150. The total surface area to volume ratio of the carrier matrix is defined by the nominal total volume of the channel-forming matrix, as defined by its outer-most boundaries, and by the exposed surface area of the matrix prior to biofilm accumulation. In some embodiments, the central channel of a porous biofilm carrier matrix as a percentage of cross-sectional area prior to biofilm accumulation is between 40% and 80%, or between 50% and 70%, or between 60% and 65%. In some embodiments, the percentage of void volume of the total volume of a porous biofilm carrier matrix is between 50% and 90%, or between 60% and 88%, or between 72% and 82%. In some embodiments, the tube diameter of a porous, cylindrical biofilm carrier matrix is between 0.030 m and 0.080 m, or between 0.036 and 0.070, or between 0.04 and 0.055.
[0298] Suitable material for use as immobilization matrix may include polyethylene, polypropylene, nylon, ceramics and most other materials that are resistant to acid and alkali corrosion and that will allow for bacterial exopolymer to attach to the carrier.
[0299] In some embodiments, a matrix comprising netting is used in which the netting is formed into a tube and in which the netting defines the outer periphery of the total volume. In some embodiments, the netting is formed by intertwined, extruded polyethylene threads having surface roughness. The roughness of carrier threads promotes microbial adherence as it presents small crevices and holes in which microbes may attach. Netting also renders biofilm resilient to dis-attachment by ordinary shear forces compared with a biofilm carrier having a smooth surface. Where the biofilm carrier is formed from rough netting, high flow velocity is less likely to increase risk of biofilm disruption, and the related risk of clogging. One suitable, commercially available material for use as biofilm carrier formed by netting are the various forms of BIO BLOK provided by EXPO NET, including BIO BLOK 80, BIO BLOK 100, BIO BLOK 150, BIO BLOK 200 and BIO BLOK 300.
[0300] Methods of the invention are practiced using a plurality of vertically oriented, porous, tubular carriers supporting biofilm. In order to develop a biofilm suitable for practicing methods of the invention, start-up and initiation procedures known in the art may be used, including but not limited to those described by Hickey et al. 1991. Cell density of microorganisms within a biofilm formed on a carrier can typically reach levels one order of magnitude higher than can be achieved in CSTR liquid volumes. See Langer et al. (2014). It is advantageous to develop a biofilm having a high relative proportion of Archaea to bacteria. This can typically be achieved by use of high VFA feeding, as described by Hickey et al. 1991, where COD from VFA is at least 20 g/L in the start-up feed stream. It is further advantageous to use a high COD organic load in the start-up feed stream, wherein total COD is at least 30 g/L, or between 35-15o g/L, and wherein organic load is taken to levels of at least 50 g/L digester volume/day. The biofilm advantageously has a relative proportion of methanogenic Archaea relative to bacteria of at least 25%, or at least 30%, or at least 31%, or at least 32%, or at least 33%, or at least 34%, or at least 35%, or at least 36%, or at least 37%, or at least 38%, or at least 39%, or at least 40%, or at least 41%, or at least 42%, or at least 43%, or at least 44%, or at least 45%, or at least 46%, or at least 47%, or at least 48%, or at least 49%, or at least 50%.
[0301] The relative proportion of Archaea to bacteria in the biofilm is determined in a biofilm sample by comparing the products from 16srRNA polymerase chain reaction (PCR) using universal 16s rRNA and Archaea-specific 16s rRNA primers reported by Gantner et al. (2011) in a DGGE gel.
[0302] In the bioreactor which contains the plurality of carriers, there are ideally mixing zones both above the upper openings and below the lower openings of the carriers, where openings refers to the central channel through which fluid flow emerges at the bottom surface of the tubular structure which defines the channel. Mixing zone refers to an open volume in which mixing can be achieved outside the carrier channel volume in which fluid flow should be substantially laminar and, thus, substantially unmixed, except for some back-mixing at the biofilm surface.
[0303] Flow is said to be substantially laminar where the corresponding Reynolds number is 3200 or lower. As is well known in the art, Reynolds number is a dimensionless parameter used to predict flow patterns within defined physical constraints. Reynolds number is calculated from a ratio of inertial forces to viscous forces under defined flow conditions. For example, in the specific case of fluid flow through a pipe, which is analogous to fluid flow through the central channel of a tubular biofilm carrier having cylindrical geometry, the Reynolds number is defined as Q*Dh/vA, where Q refers to volumetric flow in m.sup.3/s, Dh refers to the hydraulic diameter, meaning the effective internal diameter of the channel defined by the tubular carrier, v is the kinematic viscosity in m.sup.2/s (calculated as the ratio of the fluid viscosity in kg/m*s to its density in kg/m.sup.3), and A is the effective cross-sectional area of the internal diameter of the channel in meters (m). It is possible to calculate an upper limit to the possible Reynolds number under any given flow circumstances for anaerobic digestion feed stream by using the kinematic viscosity of water at an appropriate temperature, since this is invariably smaller than the corresponding value for high solids feed streams.
[0304] Generally the flow through tubular carriers will remain substantially laminar so long as the flow velocity through each carrier does not give rise to Reynolds number higher than 3200 Flow is said to be substantially laminar meaning that the flow pattern is expected to be laminar, however, some back mixing may occur as a consequence of biogas production or for other reasons. In order to achieve an even fluid velocity distribution and substantially laminar flow through the tubular carriers, the total carrier cross sectional area will be limited by the chamber dimensions. Flow velocities through systems of the invention are determined by inter-relationships between dimensions of the carrier compartments and capacity of circulation pumps. Bioreactors of the invention typically permit one circulation pump to circulate many compartments. As digester size and digesting capacity increase, the number of carrier compartments increases.
[0305] In some embodiments, control of flow through a bioreactor of the invention can be described as follows. To secure the correct flow in the embodiments shown in
[0306] The capacity of the circulation pumps should ideally be enough to re-introduce the circulation flow into each of the carrier compartments within a quarter-section at least two times per hour. In some embodiments, capacity of recirculation pumps is sufficient to re-introduce circulation flow into each of the carrier compartments within a quarter-section between 2 and 30 times per hour, or between 3 and 20. The required flow velocity and the minimum volume re-introduction requirement define the maximum and minimum circulation pump capacity for any given size of FAD digester. As the feed flow is introduced into one or more of the four quarter-section circulation streams, the feed flow contributes to the overall biofilm carrier flow velocity and should generally be taken into consideration when determining the correct circulation pump flow capacity.
TABLE-US-00001 TABLE 1 Chamber and circulation pump design parameters for FAD digesters Fad Fad Parameter Fad1 Fad2 100 m.sup.3 1000 m.sup.3 Tank r (m) 0.090 0.370 2.000 5.200 Height (m) 0.40 0.54 8.00 12.00 Circulation pump 27.00 300.00 12000.00 20000.00 flow (L/h) Feed flow (L/h) 0.31 2.10 1380.00 13800.00 No. of Chambers 1.00 16.00 16.00 64.00 Tank cross area (m.sup.2) 0.025 0.43 12.56 84.91 Tank Vol. (m.sup.3) 0.010 0.23 100.48 1018.87 Chamber cross area (m.sup.2) 0.025 0.027 0.79 1.33 Chamber Vol. (m.sup.3) 0.010 0.015 6.280 15.920 Tank vol. exchange (h) 0.4 0.8 7.5 30.1 Section volume N/A 0.2 1.9 7.5 exchange (h) Chamber reintroduction 2.7 20.8 2.1 2.1 (1/h) Tank flow velocity (m/s) 2.98E04 1.95E04 2.96E04 1.11E04 Chamber flow velocity 2.98E04 3.12E03 4.73E03 7.08E03 (m/s) Reynolds number in 7 137 208 311 biofilm carrier tubes
[0307] Any size and type of CSTR tank can be fitted with an insert to make a bioreactor of the invention. The dimensions, circulation flows and chamber arrangement will differ and can be adapted to each tank type as described in Table 1. It will be readily understood by one skilled in the art that other schemes for compartmentalization may be used in addition to the quarter-section scheme of embodiments shown in
[0308] It is advantageous to achieve mixing of fluids both before and after they pass through the biofilm carriers. Flow through the central channel of the carriers is typically a plug-flow. This flow will only experience a slight back-mixing within each separate tubular carrier as the plug flow progresses. Each tube will then experience a flow front that is characterized by having a Gaussian distribution of different velocities, with that portion of the liquid flow through the center part of the channel having a higher velocity than that portion of the liquid flow that is in close proximity with the carrier walls, i.e. with the biofilm surface.
[0309] When the flow through each tubular carrier exits the carrier, the periphery of the flow near the biofilm surface will have had a longer dwell time in the tube than that part of the flow in the center of the channel. The periphery flow will thus have had a much better chance of exchanging substrate and products with the biofilm than will the flow in the center of the channel. In order to avoid a situation where the same central flow that exits one carrier enters a subsequent carrier in a compartmentalized bioreactor again in the center of the carrier channel, the flowing fluid should ideally be mixed when passing from one compartment to another. When using an anaerobic digestion reactor of the invention, the direction of vertical fluid flow through biofilm carriers alternates between succeeding biofilm carrier compartments between down and up. The liquid passing through the biofilm carrier in an up/down direction will be transferred to the next compartment via a horizontal movement. Thus when down flow enters a sedimenting zone below the lower openings of the biofilm carriers, the flow will be forced sideways through the sedimentation zone to the volume under the succeeding biofilm carrier chamber. The sideways movement of the flow, whichuntil this point has been verticalachieves a gentle mixing of the liquid prior to its being forced upwards in a plug-flow through a biofilm carrier in a succeeding biofilm carrier chamber.
[0310] Mixing is achieved in mixing zones, and can be accomplished by a variety of different means. In some embodiments, sedimentation zones are themselves mixing zones. In some embodiments, mixing may be achieved by a mixing pump or an agitator. In some embodiments, mixing in some compartments of a bioreactor may be achieved by introducing a feed stream and/or recirculation stream from above the fluid surface, thereby achieving a splashing mixing effect. In some embodiments, mixing is achieved simply by forcing the fluid flow into a volume from which it is forced to change its direction of vertical flow.
[0311] Anaerobic digestion is conducted by means well known in the art, but informed by new results presented here. We have discovered that, using fixed film, fixed orientation, fixed bed systems in which the biofilm was developed using high VFA feed, the anaerobic biofilm is resistant to all manner of phenomena that are normally toxic in CSTR systems, including high salt content, high VFA content, and oxygen exposure. Further, and surprisingly, the operation temperature is in fact readily changeable, notwithstanding prevailing prior belief that anaerobic digestion microbes cannot simply be rapidly shifted from mesophilic (35-42 C.) to thermophilic (49-55 C.) conditions. See e.g. Bouskova et al. (2005) and see Li et al. (2014). Our results demonstrate that in fixed film, fixed orientation, fixed bed systems, such a rapid shift is in fact readily possible.
[0312] In order to achieve very high total gas production rates, the high solids feed stream is typically processed within a short hydraulic retention time (HRT), 120 hours or less, or 100 hours or less, or 75 hours or less.
[0313] Further, to maintain high total gas production, an appropriately fast flow velocity is maintained. Flow velocity as used herein refers to the linear velocity of fluid flow through the tubular biofilm carriers, expressed in meters/second (m/s). Flow velocity can be controlled by a variety of means, as will be readily understood by one skilled in the art. In some embodiments, flow velocity is controlled by the total influent input including both feed stream and recirculation. For example, when methods of the invention are practiced using a reactor that is compartmentalized so as to comprise a plurality of equally sized biofilm carrier compartments which are filled with tubular carriers supporting biofilm, flow velocity can be approximated as follows: ( 1/3600 seconds/hour)*[total input in liters/hour (including feed stream input and recirculation stream)/total digester usable internal volume in liters (which is defined by the total volume of liquid in the digester tank minus the net volume displacement of liquid by the tubular carriers)]*(height of the liquid column in the digester in m)*(total number of biofilm carrier compartments in the digester).
[0314] In some embodiments, for example, when using a small sized reactor as a laboratory scale device for determination of biogas potentials of tested substrates, flow velocity should be maintained at least at 0.0002 m/s or higher. In a 1000 m.sup.3 commercial scale reactor, flow velocity should be maintained at much higher rates at least 0.020 m/s. Typically, flow velocity should be maintained within the range 0.0002 m/s to 0.08 m/s, or between 0.0030 and 0.07, or between 0.009 and 0.05.
[0315] Other embodiments of a bioreactor of the invention may have other shapes of digester chambers. One such alternative chamber shape could be rectangular shaped chambers, round chambers, hexagonal or octagonal chambers. The chambers can take on any shape that both allow for the chambers to occupy the whole of the digester cross section area and prevent sharp flow-slowing corners.
[0316] Sediment typically has between 12-15% by weight dry matter, where dry matter refers to total solids, and typically comprises a substantial component of biologically inert, i.e. undigestible, COD and inorganic dry matter, primarily inorganics that were freed from the feed stream biomass during digestion. Sediment obtained from such systems typically offers good fertilising power in that it contains most of the phosphorous content from the feed stream as well as a high concentration of nitrogen-containing compounds and nutrient salts. When dewatered further, for example, by means of decanting, filtration, evaporation or other means known in the art, sediment obtained from such systems can have between 30-50% dry matter, which reduces handling costs when the material is discarded, transported for use as fertiliser or incinerated.
[0317] A smaller, simpler version of a reactor suitable for practicing methods of the invention can be used as a laboratory scale device for rapid determination of biomethane potential of tested substrates. It is generally accepted by those skilled in the art that biomethane potentials determined in 20-week long laboratory batch tests inevitably overestimate the yields that can actually be achieved in a commercial scale CSTR system. Typically these laboratory figures are deflated by 20% in calculation of commercial expectations. In contrast with batch CSTR tests, however, the fixed film, fixed orientation, fixed bed systems of the invention provide biomethane potential estimates on laboratory scale that very nearly approximate the yields that can be achieved using these systems in commercial scale. Moreover, unlike CSTR batch tests, which are time consuming, biomethane potential tests using systems of the invention can deliver accurate measurements within a single week.
[0318]
[0319] Thus
Example 1. Design of 30 Liter Reactor System
[0320] A 30 L biogas bioreactor system termed Fast Anaerobic Digestion (FAD) system was designed comprising a feed tank, three consecutive anaerobic digesters and an effluent tank. Each of the three consecutive digester tanks was equipped with non-random vertically oriented tubular bacteria carriers, BIO BLOK 300, on which an anaerobic biofilm was attached that conducts anaerobic degradation of organic biomass and subsequent conversion into biogas. Each of the three consecutive digesters had a total liquid volume of 10 L and 6 L of this volume was occupied by biofilm carriers.
[0321] Each of the three consecutive digesters was 20 cm wide. Each of the tubular carriers inside is 20 cm long had an open end diameter of 22 mm and an outer carrier diameter of 32 mm. The digesters were filled with liquid. Over and under the biofilm carriers were app. 5 cm free liquid. Each of the three digesters was equipped with central-shaft mounted propeller agitators in the carrier free liquid over and under the biofilm carriers. Inner diameter of the primary fluid channel defined by the tubular carriers in the absence of biofilm was 2.2 cm.
[0322] The three digesters were mounted at different vertical positions with the first digester mounted highest, the next consecutive digester 25 cm lower than the first digester and the last consecutive digester mounted 25 cm lower than the second digester. The differences in vertical mounting height allowed for liquid to flow from the first digester to the second and third by gravity.
[0323] The liquid level in all three digesters was defined by an effluent pipe above the carriers. When new feed enters the first and highest mounted digester the level in this digester will rise over the effluent pipe level and the excess liquid will leave the digester to enter the second digester which will then experience level elevation and the excess liquid from this digester will then flow to the third and last consecutive digester. From this digester, the excess liquid will flow out of the effluent pipe of the third digester into an effluent holder.
[0324] All three digesters have circulation effluent tubes in the bottom of the digester. From the effluent pipe, the digester content is continuously sucked into a peristaltic circulation pump and returned to the digester through a digester top circulation liquid inlet pipe.
[0325] The circulation flow rate was defined by the wanted flow velocity through the open diameter of the vertically oriented biofilm carriers. The circulating liquid was mixed by the propeller over the biofilm carrier before the liquid flow enters the carrier body through which the flow is a laminar plug-flow. When the circulating liquid leaves the carrier zone it was again be mixed by the agitator propeller under the carriers before repeating the circulating cycle.
[0326] Over the liquid level of the three digesters is a head-space where the produced biogas collects. The produced gas escapes the digester through a plastic tube connected to a gas flow meter with a 10 ml gas resolution. The gas production is logged in the control system.
Internal Digester Flow Pattern
[0327] The internal circulation flow may have at least two functions: [0328] 1. Re-introduction of substrate rich liquid to the bacterial biofilm dwelling in the internal periphery of the biofilm carriers. [0329] By mounting the biofilm carriers with the open end of the tubes in the liquid flow direction, only periphery of the liquid flowing in a laminar plug-flow though the carriers will be in contact with the biofilm attached to the carriers. As the circulation flow velocity is too high for the substrates in the core liquid to diffuse to the bacteria dense biofilm, the substrate not in direct contact with or very close to the biofilm will not meet the fixated biogas bacteria. By mixing the liquid leaving the carrier zone and mixing the liquid entering the carrier zone, a well-mixed re-introduction is secured and during the next plug-flow passage of the carrier zone, the liquid parts that is in close proximity with the biofilm again carry undergradated substrate. [0330] 2. Counteracting debris deposition in the carrier tubes and aiding sedimenting particle transport away from the biofilm carrier zone. [0331] When the digester feed contains sedimenting particles and/or slowly degradable colloid particles, there is a risk that such particles will clog together to form debris and create flow obstacles slowing the liquid flow through the tubular carriers and break up the plug flow. For all biofilm to be well exploited, all tubes must be kept open to the liquid flow. If debris is allowed to settle in a tube, this tube will experience higher flow resistance than other tubes and the liquid will seize to flow through the tube as the liquid always selects the easier flow passage. This will subsequently create dead-zones in the carrier zoneand too high flow velocity in the remaining open tubes reducing the exposure time between substrate and bacteria and risking biofilm rupture by shear force. [0332] The sedimenting particles thus transported through the carrier zone without attaching to the biofilm carriers are allowed to sediment in the bottom of the digester and be sucked into the circulation tube along with the colloid particles to be re-introduced with the circulation stream. This in term keeps the particles suspended and enables passage of sedimenting particles through the whole system and into the effluent tank.
Control and Operation
[0333] The system was operated automatically with pulse-pause and speed control on both feed pump and circulation pumps.
[0334] pH, digester temperature and gas flow were measured and logged on-line and could be accessed and controlled remotely.
[0335] pH, temperature and gas flows along with analysis measurements of VFA (Volatile Fatty Acids), COD (Chemical Oxygen Demand), Nitrogen and cations were used to monitor system health and provide data for test purposes.
Example 2. Flow Tests in the Absence of Biofilm
[0336] In order to verify proper mixing, local circulation and general plug-flow distribution was measured by passing through a pulse of concentrated methylene-blue dye that could be determined with a spectrophotometer after being distributed in the system.
[0337] The RTD analysis provides a mathematical, graphical and vessel wise picture of fluid and particle distribution in the system. For optimum mixing, the total system behaves like a true plug-flow, and each digester as a CSTR notwithstanding, there are plug-flow zones in each digester. If RTD analysis shows that mixing is not optimal, it should point towards an optimal solution.
[0338] The reactors were visually inspected for proper functioning and each of them was filled with 7.5 liters of tap water. The first reactor of the cascade, was injected with a single dose of methylene blue to a final concentration of 0.0058 mM and the absorbance was recorded using a spectrophotometer set at 668 nm (wavelength where methylene blue displays maximum absorption). A constant flow of water was then introduced to the first reactor in series using a peristaltic pump and from the top, in order to have an entire volume displacement inside the reactor in a lapse of 2 hours. During this process, every 5 minutes a sample is taken from the top of each individual reactor and measured in the spectrophotometer. The RTD curve was then plotted to verify if the system has a proper mixing and if the flow occurred as intended. Results were compared with similar experiments in literature.
[0339] The circulation speed was set to 0.45 per Minute. The cuvettes and the spectrophotometric measurements were done soon after the sample was taken from the inside of each reactor.
Example 3. Digester Seeding
[0340] Many residual product biomasses contain the microbes responsible for the anaerobic biogas production from organic matter. When building up the necessary concentration of the different bacteria and Archaea bacteria the reproduction time for all microbes must be respected. In order to minimise the time consumption for the build-up of the wanted biological activity, it is recommendable to start out with a biomass that already have high concentrations of the biogas microbes.
[0341] The best match of microbial composition will be from anaerobic digesters converting a biomass similar to the biomass expected in the fully loaded FAD digester. As the FAD is expected to operate on enzymatically and microbially pre-digested (liquefied) organic fraction of municipal solid waste (MSW) and as no such digester exist, the closest are digesters operated on other types of pre-digested biomass. The Billund, Denmark, biogas digester was selected as a source of seed inoculum since this operates on source separated food waste. Most human consumables have been pre-processed and consist mostly of carbohydrates, fat and meat proteins. This was the closest match to the liquefied organic fraction of municipal solid waste (MSW) that will present the highest concentration of the required microbial consortia.
Inoculation
[0342] Two samples of 20 L each were retrieved from the well-mixed Billund digester. The samples were transported at digester temperature to the FAD digester facility and immediately applied to the FAD digesters. As the inoculum did not fill up the digesters, warm water was added to make the correct digester level.
[0343] When taken out and transported, the methanogens can be expected to stress and become temporarily inactive. In order for the seed methanogens to acclimatise the FAD digesters were left standing for 7 days at an operation temperature of 37 C.
[0344] After the inoculation acclimatisation period had passed, a pulse shock load of 150 ml of liquefied organic fraction (LOF) of MSW (SLR of 0.6 gCOD/Vd) were injected into all digesters to test their livelihood and their readiness for beginning the load-up. The 150 ml LOF was well under the usual load per day for the digester content of the Billund Biogas digester and did not present any danger of overfeeding the inoculum bacteria. The resulting gas and a COD balance analysis indicated that the expected amount of biogas had been produced. When this digester health check was approved and the expected conversion rate had been shown for all the digesters, the digesters were ready for load-up.
Example 4. Building Biofilm on High VFA Feed Stream
[0345] Before commencing the load-up of the FAD digesters, the starting load was determined. When transported, re-located, heated and diluted, the digester content cannot be expected to exert the same efficiency as it did inside the source digester. The Billund Biogas informed about the normal COD load in their digester being app. 3 gCOD/L*d, and the starting load of LOF in the FAD bench scale system was determined to be 'rds (2 gCOD/L*d) for this flow to both gain a fast load-up and respect any process difficulties originating from the transfer to the FAD digesters.
[0346] The biofilm was expected to attach to the carrier and encompass the wanted microbes within a time frame of 8-12 weeks (as known in the art). The sequence will be firstly attachment of different exopolymer excreting rods and cocci followed by a diverse consortium of bacteria families over 5-7 weeks and only followed by methanogenic Archaea during the 10'th to 12'th week of the biofilm inoculation.
[0347] As the biofilm carriers were at all times covered by digester liquid, the biofilm itself could not be directly monitored. During the inoculation the digester liquid was able to convert the fed-in COD like any CSTR digester. Thus, the digester system will be loaded up in the same way a conventional biogas system is loaded up with the COD load increase respecting the growth limitation on the slowest growing microbesthe methanogens. The load-up is performed at an feed-in increase of app. 1.5% per day based on the feed-in during the preceding day.
[0348] During the conventional biogas digester load-up, the methanogen maintenance time of 10-14 days was respected. When the hydraulic load exceeds this and the hydraulic retention time (HRT) falls below 10 days, the conventional biogas digester cannot longer support the necessary reproduction of methanogenic bacteria and they will be flushed out resulting in decreasing biogas forming capacity, increasing VFA concentrations and ultimately seizure of the digester process. When the biofilm has developed correctly attached onto the biofilm carriers, the bacteria consortium in the biofilm will uphold the COD conversion efficiency even though the methanogenic concentration in the digester liquid drops. Consequently, the feed load-up to reach an HRT of 10 days takes at least 12 weeks in order to allow for the biofilm to fully develop.
[0349] When the digester COD conversion remains intact and still increases according to the COD load up, the biofilm has de-facto taken over biogas production and the continuing COD load-up shall no longer respect the reproductive speed of the methanogens in the digester liquid but that of the methanogens engulfed in, or attached to, the now formed biofilm. When the COD load-up continues to even lower HRT's it is proof that the biofilm microbial community is plastic and can be altered after the full formation in a manner that suits the purpose of degrading COD and converting it to methane gas.
[0350] The characteristics of the liquefied organic fraction of MSW used as feed for the biofilm build up are shown in Table 2. The pH was generally approximately 4.0-4.2. Total volatile fatty acids, in particular acetate and lactate, have already been fermented during enzymatic and microbial liquefaction of the organic fraction of municipal solid waste. The total and volatile solids of this LOF feed typically can oscillate between 100-120 gr solids/L and 80-100 gr/L respectively. Total solids is expressed as a percentage w/w.
TABLE-US-00002 TABLE 2 Characteristics of liquefied organic fraction of MSW Feed Characteristics Volatile fatty acids Total Lactic acetic propionic n-butyric iso-valeric n-valeric measured acid acid acid acid acid acid VFA g/L g/L g/L g/L g/L g/L g/L 34.17 8.34 2.55 2.31 0.04 0.49 47.91 Sugar Monomers and Ethanol Glucose Xylose Xylitol Ethanol g/L g/L g/L g/L 0.59 0.17 0.67 7.99 Solids and Chemical Oxygen Demand Total COD Soluble COD Total Solids Volatile solids g/L g/L % % 134.28 81.43 10.83 8.49
[0351]
[0352]
[0353] The COD load-up levels out when the COD conversion no longer responds by increasing the gas production when the COD load increases and the digester responds by increasing the VFA concentration in the digester effluent. In the case of the LOF feed used in biofilm buildup, this point was initially reached at 72 hours hydraulic retention time. However, this was not believed to reflect any underlying metabolic limit of the system but rather technical difficulties arising from pH balance issue using the highly acidic feed.
Example 5. Characterization of Biofilm
[0354] One single tube from a BIO BLOK 300 carrier was removed from the bioreactor described in example 1 after development of biofilm as described in example 4.
[0355] In order to perform tests on the biofilm and describe its properties biopsies of the carrier have been taken from the digesters. The biofilm seems to be easily removable from the carrier by means of high velocity water flushing, etc. When removing the biofilm from the carrier, the biofilm does not loosen in layers but only the film at the shear force point will rub off.
[0356] Cell density of microorganisms within a biofilm formed on a carrier can typically reach levels one order of magnitude higher than can be achieved in CSTR liquid volumes. See Langer et al. (2014). Thus it is expected that high density of methane producing Archaea within biofilm formed on porous tubular carriers in practicing methods of the invention contributes to increased biogas production performance using bioreactors of the invention. Maintaining high cell densities within the biofilm serves to protects the microbial community from process imbalances that would affect a CSTR system. Furthermore, the high organic loads used during the reactor's seeding favours the attachment of dense microbial communities to the biofilm with even higher cellular ratios than at lower organic loads.
[0357] The relative percentage of Archaea to bacteria and approximate cell densities within biofilm removed from biopsies as described can be determined by comparing the products from 16srRNA polymerase chain reaction (PCR) using universal 16s rRNA and Archaea-specific 16s rRNA primers reported by Gantner et al. (2011) in a DGGE gel. In so doing, it will be possible to described the Archaeal communities developed within the biofilm and to understand that ratio between them and the rest of the microorganisms responsible for breakdown of organic compounds until the production of biogas. The first digester chamber of the cascade is expected to have a higher ratio of Archaea than in the subsequent chambers since most of the easily degradable metabolized organics are converted by microbes preferring monomeric sugars, low molecular lipids, etc. in this process step. However it is possible that subsequent digester chambers comprise communities more specialized in degrading larger organic compounds that are converted into biogas. A further study regarding the quantification and determination of overall microbial groups and their distribution in the consecutive chambers can be performed to describe the above mentioned.
Example 6. Rapid Conversion from Mesophilic to Thermophilic ConditionsIncreased Conversion Rate Obviates the Need for pH Adjustment of High Solids, Acidic Feed Stream
[0358] It has previously been believed in the prior art that there exists a clear distinction between mesophilic and thermophilic Archaea producing biomethane. See e.g. Bouskova et al. (2005) and see Li et al. (2014). During the course of the experiments described in example 4, we discovered that the microbial community within the biofilm functions normally throughout the entire temperature range from 37 C. to 53 C. The difference between the temperature regimes is that COD conversion occurs faster at thermophilic temperature andas a consequencethe need for adjusting pH of the acidic LOF feed media disappeared when temperature was from mesophilic (37 C.) to thermophilic (53 C.) conditions. The need for pH adjustment disappeared at higher temperatures because the internal buffer effect from COD conversion into CO.sub.2, CH.sub.4 and NH4+ occurred fast enough to counteract the acidification caused by incoming acidic feedstock.
[0359] Biofilm in the digesters was developed using LOF feed at mesophilic temperature37 C. The temperature was subsequently raised to 52 C. over a very short period of time. This resulted in faster COD turnover and elimination of the need for pH adjustment of the acidic LOF feed. The active biology, immobilized in the biofilm, cannot change very rapidly. Ability to change process temperature up and down between mesophilic and thermophilic range in as short time as three hours indicated that the temperature flexibility is already inherent in the microbial community grown at mesophilic temperature. It is hard to imagine that thermophilic microorganisms won some selective battle during the mesophilic biofilm build-up. This indicates that the same microorganisms, when constrained within a high cell density biofilm, actually have a much greater temperature operation window than was previously believed possible. As a consequence of this previously unknown feature, bioreactors of the invention can operate in either temperature range regardless of the initial load-up temperature.
[0360] In order to document the temperature flexibility of the FAD system developed as described in examples 1, 3 and 4, the system was fed with a constant feed rate of acidic LOF feed at 53 C. until stable gas production was achieved. Temperature of the system at constant feed rate was suddenly changed to 37 C. and maintained until stable gas production was again achieved. The temperature was then restored to 53 C. and maintained until stable gas production was again achieved. The results of this experiment are shown in
Example 7. Long Term Stability with Minimal Requirement for Process Controls
[0361] The FAD system developed as described in examples 1, 3 and 4 was fed with an LOF feedstock having the characteristics shown in Table 3 with a hydraulic retention time of 91 hours for a period of 52 days.
[0362] As shown, the system supports stable operation with minimal need for process controls. Such a stable operation is very beneficial in terms of the determination of both the biomass gas potential and gas production under continuous conditions. With the COD conversion efficiency preserved regardless of the feed-rate and the gas production becoming stable with the feed-in stabilises it is a very strong indicator of the real gas production under continuous conditions. In additionstill with the prerequisite that the COD conversion is unchangedthe total gas production from the first feed-in to the gas production seizes after removal of the feed will show the gas potential of the feed material just as good as if it was performed in a regular batch-test.
TABLE-US-00003 TABLE 3 Characteristics of REnescience C biomass for steady operation test Feed Characteristics Volatile fatty acids Total Lactic acetic propionic n-butyric iso-valeric n-valeric iso-Butyric measured acid acid acid acid acid acid acid VFA g/L g/L g/L g/L g/L g/L g/L g/L 28.228 2.518 0.202 0.072 0 0 0.24 31.26 Sugar Monomers and Ethanol Glucose Xylose Xylitol Ethanol g/L g/L g/L g/L 1.14 3.31 0 2.99 Solids and Chemical Oxygen Demand Total COD Soluble COD Total Nitrogen Total Solids Volatile solids g/L g/L g/L % % 134.28 81.43 1.85 9.43 7.49 Effluent Characteristics Volatile fatty acids Acetic Lactic Propionic iso-Butyric n-Butyric iso-Valeric n-Valeric Formic Total acid acid acid acid acid acid acid acid VFAs g/L g/L g/L g/L g/L g/L g/L g/L g/L 0.082 0.014 0.16 0.00 0 0.011 0 0 0.26 Sugar Monomers and Ethanol Cellobiose Glucose Xylose Arabinose Xylitol Ethanol g/L g/L g/L g/L g/L g/L 0 0 0.00 0.00 0.056 0 Solids and Chemical Oxygen Demand Total Solids Volatile solids Total COD Soluble COD % % g/L g/L 2.67 1.81 18.23 2.8
Example 8. Immunity from VFA Toxicity
[0363] In literature, It has been described that inside an anaerobic digester, the volatile fatty acid (VFAs) concentration begins to be inhibitory over acetate concentrations of 100 mM (6 g/l) and over lower concentrations of the other VFA species (Ahring. B. K, 1994) [0364] Reference: Ahring. B. K. (1994). Volatile fatty acids as indicators of process imbalance in anaerobic digestors. Applied Microbiology Biotechnology.
[0365] During a month period, the system described in examples 1, 3 and 4 was fed with VFA rich (38-45 g/l) and COD rich (100-130 gCOD) LOFMSW (Liquefied Organic Fraction of Municipal Solid Waste) substrate with Hydraulic retention time between 160 and 72 hours. During this period, the VFA concentration inside one digester tanks was measured to be higher than 12 g/l VFAtwice the concentration reported to be inhibitive to the biogas process. The high VFA concentration did not affect the gas production that in every case was higher than 70% total COD reduction as measured in the effluent.
[0366]
Example 9. Immunity from Oxygen Toxicity
[0367] In
A1: Load-Up Substrate 1
[0368] B1: Stable production Substrate 1
C2: Burn-Down Substrate 1
[0369] D: Filter explosion to Oxygen
E: Rehydration of filters with effluent
A2: Load-Up Substrate 2
[0370] B2: Stable production Substrate 2
C2: Burn-Down Substrate 2
[0371] Line 402 shows the Feed in; line 401 the gas produced.
[0372] The system described in examples 1, 3 and 4 was fed with two similar LOF substrates; the first having a total COD load of 107.5 g COD/l and the second 101.7 g COD/l.
[0373] The Load-up periods A1 and A2 consist in a two day operation in which the reactors are fed with increasing amount of substrate up to the stable production load, respectively B1 and B2. For the first substrate, the average biogas production was 88.08 NL/day, with a methane content of 61.8% for an average of 54.44 l CH4/day. This is equivalent to a 80% COD conversion efficiency.
[0374] After the first burn-down, that is C1, the liquid inside the reactor was entirely removed through the recirculation escape in the digester bottom. The digester was then flushed with an amount of tap water (at room temperature) equivalent to twice the volume of the digester. The carriers remained exposed to atmospheric oxygen for 3 days, after which the reactor was filled again with an effluent from a previous experiment, similar to that removed before the air exposure. According to the common knowledge of the fragility to air exposure the operation in D should result in a misbalance to the anaerobic digester, as it has been described that oxygen is toxic and inhibitory in conventional anaerobic digestion processes (Deshai, B 2011) Even though the exposure to the oxygen rich atmosphere should deactivate the anaerobic bacteria, this does not happenpresumably due to the protection provided by the moist biofilm. In the conventional biogas digester, inactivated bacteria leaves the digester with the effluent and is thus removing the digestion power even though the bacteria could regain activation by removal of the oxygen exposure. In the FAD digester the bacteria cannot leave and, as the latter phases clearly show, the bacteria are equally active after re-hydration and load-up of new substrate
[0375] After the rehydration (E) of the filters with effluent, an analogous load-up and stable feed in was performed in the digester. During the phase B2, the average production was 85.55 NL/day, this time with a methane content of 62% for an average of 53.04 L CH4/day. The conversion efficiency was 83%, which is very similar to the COD conversion efficiency before the exposure to oxygen. This observation shows that the system in claim is resilient to air exposure and that the continuity of the process is not compromised by this otherwise damaging operation. Furthermore, both the burn-down phases C1 and C2 were also similar. When the feed-in was stopped, the production ceased between the 5th and the 7th day. In both cases, the production had already decreased more than 50% after the first day without substrate. [0376] (Deshai, B. (2011). Oxygen Effects in Anaerobic DigestionA Review. The Open Waste Management Journal.
Example 10. Feedstock Flexibility
[0377] The system described in examples 1, 3 and 4 was fed with a variety of different feedstocks.
[0378] The system in claim is flexible for operation with high gas production at high organic loading rates with different feedstock. The system has been in continuous operation at lower HTR than 5 days with dissimilar feedstock composed by different sugars, volatile fatty acids and ethanol that can be metabolically transformed in anaerobic digestion processes. The feed-in of the reactor system has been performed continuously alternating among the different feedstock and therefore different organic loading. The productivity of the digester reflects a rapid adaptation to the newly introduced feedstock as the produced biogas following the change corresponds to the potential of each feedstock that had been previously determined.
Thin Stillage
[0379] Thin stillage is a waste water fraction originating from the 2G bioethanol production. Thin stillage is free of large particles as the lignin containing particles have been separated to be used elsewhere. The thin stillage contains mainly oligomering sugars that is challenging for the biogas process as it requires a high hydrolysing power to degrade the oligomers. As it shows in
TABLE-US-00004 TABLE 4 Characteristics of lignocellulosic Thin stillage and Effluent produced Feed Characteristics Volatile fatty acids Acetic Lactic Propionic iso-Butyric n-Butyric iso-Valeric n-Valeric Formic Total acid acid acid acid acid acid acid acid VFAs g/L g/L g/L g/L g/L g/L g/L g/L g/L 4.55 0.30 0.34 0.16 0 0 0 0 5.35 Sugar Monomers and Ethanol Cellobiose Glucose Xylose Arabinose Xylitol Ethanol g/L g/L g/L g/L g/L g/L 0.00 0.34 0.65 0.38 0.00 1.35 Solids and Chemical Oxygen Demand Total Solids Volatile solids Total COD Soluble COD % % g/L g/L 4.53 3.39 52.50 50.30 Effluent Characteristics Total Solids Volatile solids Total COD Soluble COD % % g/L g/L 2.67 1.81 6.05 4.09
Pig Manure
[0380] The pre-treated biomasses of REnescience bioliquid from enzymated MSW and the Thin stillage form enzymated lignocellulosic biomass are both examples of biomasses that is expected to have some content of easily degradable organics that will make them good substances for the gas conversion time in a low HRT immobilised biofilm digester. In contrast, pig manure are normally thought of as a heavy degradable substance altogether as it both does not contain many easily degradables and as only app. 50% of the COD content is convertible to biogas. Consequently, it can be expected that the FAD digester will be challenged by being fed with pig manure. As can be seen in
TABLE-US-00005 TABLE 5 Properties of pig manure from Maabjerg Bioenergy Total Soluble COD COD g/L g/L 59150 21690
[0381] The embodiments and examples shown are illustrative only and not intended to limit the scope of the invention as defined by the claims.
Example Shock Test
[0382] During this experiment, the feed in has been stopped for 15 days and the gas production of the 240 liter reactor was down to zero. In the lapse time of 2 days, a reactor was fed from 0 to nearly 70 liters per day, with a substrate of approximately 90 g/l of COD. The reactor was able to produce biogas without any detrimental effect due to the sudden and elevated amounts of substrate. The feed in continued for additional 5 days after this shock test. The feed in and the respective gas production during this period are depicted in
[0383] This shows the ability of the bioreactor to sustain sudden stops in the feed and strong variation in the amount of feed.
Example of Multiple Feed
[0384] This example shows the possibility of the system to produce biogas at high rates when being fed at a single and in multiple points. This feature can help to distribute the high organic loads between the compartments of the reactor. During the period of 18 days (as shown in
[0385] This provide flexibility to the system as well as optimizing yield of production.
[0386] Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms comprising or comprises do not exclude other possible elements or steps. Also, the mentioning of references such as a or an etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.
REFERENCES
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Items.
[0431] 1. A method of anaerobic digestion to biomethane comprising the steps of [0432] introducing a substrate feedstock having COD content at least 30.0 g/L into a fixed film, fixed orientation, fixed bed bioreactor system in which the immobilization matrix is characterized by comprising a plurality of vertically oriented, porous tubular carriers supporting biofilm, and in which mixing zones are provided both above the upper openings and below the lower openings of the tubular carriers, and conducting anaerobic digestion of the feedstock with a hydraulic retention time of 120 hours or less while maintaining a flow velocity of at least 0.0002 m/s and a gas production rate of at least 5.0 liters/liter digester volume/day in such manner as to maintain a substantially laminar flow through the tubular carriers as well as mixing within each of said mixing zones.
2. An anaerobic digestion bioreactor comprising a cylindrical tank having a plurality of internal, vertical biofilm carrier compartments defined by baffles or walls made from corrosion resistant and liquid impermeable material that are open at the top, where in each carrier compartment comprises a first shortened wall or underflow aperture on one side at the bottom which serves as an opening into another carrier compartment whereby fluid flows can be directed through succeeding compartments, and wherein a plurality of the carrier compartments further comprise a second shortened wall or overflow aperture at the top on a side other than that side which contains said first shortened wall or underflow aperture on one side at the bottom which serves as an opening into another carrier compartment whereby fluid flows can be directed through succeeding compartments, optionally further comprising a rotable scraper that is adapted to define sealed sections in a sedimentation zone situated beneath the lowest edge of the carrier compartments when in a closed position or to permit removal of sedimented solids when in an open position.
3. An insert for converting a continuously stirred tank reactor (CSTR) into a fixed film, fixed orientation, fixed bed anaerobic digestion reactor said insert comprising [0433] interconnected baffles made from corrosion resistant and liquid impermeable material that define a plurality of vertical biofilm carrier compartments that are open at the top, each of which has a shortened wall or underflow aperture on one side at the bottom which serves as an opening into another carrier compartment whereby fluid flows can be directed through succeeding compartments, and most of which have a shortened wall or overflow aperture at the top on a side other than that which contains a shortened wall or underflow aperture at the bottom which serves as an opening into another carrier compartment whereby fluid flows can be directed through succeeding compartments.
4. A method of converting a CSTR tank into a fixed film, fixed orientation, fixed bed anaerobic digestion reactor comprising the steps of [0434] assembling an insert of interconnected baffles made from corrosion resistant and liquid impermeable material that define a plurality of vertical biofilm carrier compartments that are open at the top, each of which has a shortened wall on one side at the bottom which serves as an opening into another carrier compartment whereby fluid flows can be directed through succeeding compartments, and most of which have a shortened wall at the top on a side other than that which contains a shortened wall at the bottom which serves as an opening into another carrier compartment whereby fluid flows can be directed through succeeding compartments, [0435] installing the insert within a modified or unmodified CSTR tank, [0436] fitting the carrier compartments defined by the insert with a plurality of porous, tubular carriers either before or after installation in the CSTR tank, and [0437] raising a productive biofilm on the carriers.
5. A laboratory scale device for rapid determination of biomethane potential of tested substrates adapted to practice the method of claim 1.