U.S. patent application number 14/320781 was filed with the patent office on 2014-10-23 for three stage, multiple phase anaerobic digestion system and method.
The applicant listed for this patent is Advanced Bio Energy Development LLC. Invention is credited to William C. Stewart.
Application Number | 20140315292 14/320781 |
Document ID | / |
Family ID | 41610752 |
Filed Date | 2014-10-23 |
United States Patent
Application |
20140315292 |
Kind Code |
A1 |
Stewart; William C. |
October 23, 2014 |
Three Stage, Multiple Phase Anaerobic Digestion System and
Method
Abstract
A three stage, multiple phase anaerobic digestion system and
method designed to separate the biological phases, optimize
microbial activity in each phase, and significantly increase system
reliability and energy production. The system physically separates
the biological phases of anaerobic digestion based on particle
size, particle density, and solubility of metabolic products. The
system allows a complex multi-phased biological system to develop
without the need for excessive control or operator
intervention.
Inventors: |
Stewart; William C.;
(Caldwell, ID) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Advanced Bio Energy Development LLC |
Caldwell |
ID |
US |
|
|
Family ID: |
41610752 |
Appl. No.: |
14/320781 |
Filed: |
July 1, 2014 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13056841 |
Jan 31, 2011 |
8765449 |
|
|
PCT/US2009/052433 |
Jul 31, 2009 |
|
|
|
14320781 |
|
|
|
|
61085252 |
Jul 31, 2008 |
|
|
|
Current U.S.
Class: |
435/294.1 |
Current CPC
Class: |
C12M 23/58 20130101;
C12M 45/06 20130101; C12P 1/00 20130101; Y02E 50/343 20130101; Y02E
50/30 20130101; C12P 5/023 20130101; C12M 21/12 20130101; C12M
21/04 20130101; B09C 1/10 20130101; C02F 3/286 20130101; C02F 3/34
20130101; C12M 25/18 20130101 |
Class at
Publication: |
435/294.1 |
International
Class: |
C12M 1/00 20060101
C12M001/00 |
Claims
1. A system for the anaerobic digestion of a raw feedstock
containing raw organic solids, said system comprising: a hydrolysis
tank where partial hydrolysis of the feedstock takes place, wherein
the pH of the feedstock in said hydrolysis tank has a pH of 5.5 to
6.5, wherein the temperature of said feedstock in said hydrolysis
tank is maintained between 24.degree. C. to 35.degree. C., wherein
after a predetermined period of time, supernatant is drawn off said
hydrolysis tank, wherein carbon dioxide (CO.sub.2) and hydrogen gas
(H.sub.2) formed in said hydrolysis tank are collected; an upflow
fluidized bed reactor having a top and a bottom, said bottom for
receiving the drawn off supernatant from said hydrolysis tank, said
upflow fluidized bed reactor providing for hydrolysis and
acidogenesis of said supernatant, wherein the pH of the supernatant
in said upflow fluidized bed reactor has a pH of 5.5 to 6.5,
wherein the temperature of said supernatant in said upflow
fluidized bed reactor is maintained between 24.degree. C. to
35.degree. C., wherein said supernatant exits in said upflow
fluidized bed reactor at said top as filtered supernatant, wherein
carbon dioxide (CO.sub.2) and hydrogen gas (H.sub.2) formed in said
upflow fluidized bed reactor are collected; a heating means, said
heating means configured for heating said filtered supernatant to
at least 30.degree. C.; a packed bed reactor where methanogenic
bacteria create methane gas (CH.sub.4), said packed bed reactor
having an inlet end and an outlet end; and a pH adjusting means,
said pH adjusting means configured for adjusting the pH of the
filtered supernatant to maintain a pH of 7.8 to 8.5 in said packed
bed reactor; wherein said filtered supernatant is transferred into
said packed bed reactor at or adjacent said inlet end, wherein said
packed bed reactor includes a biological filter media, wherein said
carbon dioxide (CO.sub.2) and hydrogen gas (H.sub.2) collected from
said hydrolysis tank and said upflow fluidized bed reactor are
injected into said filtered supernatant within said packed bed
reactor, said filtered supernatant, said injected carbon dioxide
(CO.sub.2) and said injected hydrogen gas (H.sub.2) for digestion
by said methanogenic bacteria, said packed bed reactor having a
discharge port through which filtered effluent is discharged from
said packed bed reactor, wherein said methane gas produced in said
packed bed reactor is drawn off and stored.
2. The system of claim 1, wherein said packed bed reactor has an
upper portion and a lower portion, said upper portion containing
low angle anaerobic digester media, and said lower portion
containing high angle anaerobic digester media, wherein said
filtered supernatant flowing through said lower portion then
through said upper portion.
3. The system of claim 2, wherein said heating means is configured
for heating said filtered supernatant to a temperature of
30.degree. to 38.degree. C.
4. The system of claim 3, wherein said collected carbon dioxide
(CO.sub.2) and hydrogen gas (H.sub.2) are injected into said
filtered supernatant within said packed bed reactor at a location
spaced from said inlet end of said reactor.
5. The system of claim 2, wherein said heating means is configured
for heating said filtered supernatant to a temperature of
49.degree. to 57.degree. C.
6. The system of claim 5, wherein said collected carbon dioxide
(CO.sub.2) and hydrogen gas (H.sub.2) gasses are injected into said
filtered supernatant within said packed bed reactor at a location
spaced from said inlet end of said reactor.
7. The system of claim 1, wherein said collected carbon dioxide
(CO.sub.2) and hydrogen gas (H.sub.2) are injected into said
filtered supernatant within said packed bed reactor at a location
spaced from said inlet end of said reactor.
8. The system of claim 1, wherein said heating means is configured
for heating said filtered supernatant to a temperature of
30.degree. to 38.degree. C.
9. The system of claim 1, wherein said heating means is configured
for heating said filtered supernatant to a temperature of
49.degree. to 57.degree. C.
10. The system of claim 1, wherein carbon dioxide and hydrogen gas
formed in said hydrolysis tank are collected via a raw gas
collection system.
11. The system of claim 10, wherein said raw gas collection system
fluidly connects said hydrolysis tank and said upflow fluidized bed
reactor to said packed bed reactor.
12. A system for the anaerobic digestion of a raw feedstock
containing raw organic solids, said system comprising: a hydrolysis
tank where partial hydrolysis of the feedstock takes place, wherein
the pH of the feedstock in said hydrolysis tank has a pH of 5.5 to
6.5, wherein the temperature of said feedstock in said hydrolysis
tank is maintained between 24.degree. C. to 35.degree. C., wherein
after a predetermined period of time, supernatant is drawn off said
hydrolysis tank, wherein carbon dioxide (CO.sub.2) and hydrogen gas
(H.sub.2) formed in said hydrolysis tank are collected; an upflow
fluidized bed reactor having a top and a bottom, said bottom for
receiving the drawn off supernatant from said hydrolysis tank, said
upflow fluidized bed reactor providing for hydrolysis and
acidogenesis of said supernatant, wherein the pH of the supernatant
in said upflow fluidized bed reactor has a pH of 5.5 to 6.5,
wherein the temperature of said supernatant in said upflow
fluidized bed reactor is maintained between 24.degree. C. to
35.degree. C., wherein said supernatant exits said upflow fluidized
bed reactor at said top as filtered supernatant, wherein carbon
dioxide (CO.sub.2) and hydrogen gas (H.sub.2) formed in said upflow
fluidized bed reactor are collected; a packed bed reactor where
methanogenic bacteria create methane gas (CH.sub.4), said packed
bed reactor having an inlet end and an outlet end; a heating means,
said heating means configured for heating said filtered supernatant
to at least 30.degree. C.; and a pH adjusting means, said pH
adjusting means configured for adjusting the pH of the filtered
supernatant to maintain a pH of 7.8 to 8.5 in said packed bed
reactor, wherein said filtered supernatant is transferred into said
packed bed reactor at or adjacent said inlet end, wherein said
packed bed reactor includes a biological filter media, wherein said
carbon dioxide (CO.sub.2) and hydrogen gas (H.sub.2) collected from
said hydrolysis tank and said upflow fluidized bed reactor are
injected into said filtered supernatant within said packed bed
reactor, said filtered supernatant, said injected carbon dioxide
(CO.sub.2) and said injected hydrogen gas (H.sub.2) for digestion
by said methanogenic bacteria, said packed bed reactor having a
discharge port through which filtered effluent is discharged from
said packed bed reactor, wherein said methane gas produced in said
packed bed reactor is drawn off and stored; wherein said collected
carbon dioxide (CO.sub.2) and hydrogen gas (H.sub.2) are injected
into said filtered supernatant within said packed bed reactor at a
location spaced from said inlet end of said reactor.
13. The system of claim 12, wherein said packed bed reactor has an
upper portion containing low angle anaerobic digester media and a
lower portion containing high angle anaerobic digester media,
wherein said filtered supernatant flowing through said lower
portion then through said upper portion.
14. The system of claim 13, wherein said heating means is
configured for heating said filtered supernatant to a temperature
of 30.degree. to 38.degree. C.
15. The system of claim 13, wherein said heating means is
configured for heating said filtered supernatant to a temperature
of 49.degree. to 57.degree. C.
16. The system of claim 12, wherein said heating means is
configured for heating said filtered supernatant to a temperature
of 30.degree. to 38.degree. C.
17. The system of claim 12, wherein said heating means is
configured for heating said filtered supernatant to a temperature
of 49.degree. to 57.degree. C.
18. The system of claim 12, wherein carbon dioxide and hydrogen gas
formed in said hydrolysis tank are collected via a raw gas
collection system.
19. The system of claim 18, wherein said raw gas collection system
fluidly connects said hydrolysis tank and said upflow fluidized bed
reactor to said packed bed reactor.
20. A system for the anaerobic digestion of a raw feedstock
containing raw organic solids, said system comprising: a hydrolysis
tank where partial hydrolysis of the feedstock takes place, wherein
the pH of the feedstock in said hydrolysis tank has a pH of 5.5 to
6.5, wherein the temperature of said feedstock in said hydrolysis
tank is maintained between 24.degree. C. to 35.degree. C., wherein
after a predetermined period of time, supernatant is drawn off said
hydrolysis tank, wherein carbon dioxide (CO.sub.2) and hydrogen gas
(H.sub.2) formed in said hydrolysis tank are collected; an upflow
fluidized bed reactor having a top and a bottom, said bottom for
receiving the drawn off supernatant from said hydrolysis tank, said
upflow fluidized bed reactor providing for hydrolysis and
acidogenesis of said supernatant, wherein the pH of the supernatant
in said upflow fluidized bed reactor has a pH of 5.5 to 6.5,
wherein the temperature of said supernatant in said upflow
fluidized bed reactor is maintained between 24.degree. C. to
35.degree. C., wherein said supernatant exits said upflow fluidized
bed reactor at said top as filtered supernatant, wherein carbon
dioxide (CO.sub.2) and hydrogen gas (H.sub.2) formed in said upflow
fluidized bed reactor are collected; a packed bed reactor where
methanogenic bacteria create methane gas (CH.sub.4), said packed
bed reactor having an inlet end and an outlet; a heating means,
said heating means configured for heating said filtered supernatant
to at least 30.degree. C.; and a pH adjusting means, said pH
adjusting means configured for adjusting the pH of the filtered
supernatant to maintain a pH of 7.8 to 8.5 in said packed bed
reactor, wherein said filtered supernatant is transferred into said
packed bed reactor at or adjacent said inlet end, wherein said
packed bed reactor includes a biological filter media, wherein said
carbon dioxide (CO.sub.2) and hydrogen gas (H.sub.2) collected from
said hydrolysis tank and said upflow fluidized bed reactor are
injected into said filtered supernatant within said packed bed
reactor, said filtered supernatant, said injected carbon dioxide
(CO.sub.2) and said injected hydrogen gas (H.sub.2) for digestion
by said methanogenic bacteria, said packed bed reactor having a
discharge port through which filtered effluent is discharged from
said packed bed reactor, wherein said methane gas produced in said
packed bed reactor is drawn off and stored, wherein said collected
carbon dioxide (CO.sub.2) and hydrogen gas (H.sub.2) are injected
into said filtered supernatant within said packed bed reactor at a
location spaced from said inlet end of said reactor, wherein said
packed bed reactor has an upper portion containing low angle
anaerobic digester media and a lower portion containing high angle
anaerobic digester media, wherein said filtered supernatant flowing
through said lower portion then through said upper portion, wherein
carbon dioxide and hydrogen gas formed in said hydrolysis tank are
collected via a raw gas collection system, and wherein said raw gas
collection system fluidly connects said hydrolysis tank and said
upflow fluidized bed reactor to said packed bed reactor.
Description
BACKGROUND OF THE INVENTION
[0001] Anaerobic digestion refers to both a natural microbial
process, which takes place in the absence of oxygen, and, an
engineered process, which utilizes the microbial process. Both
produce methane gas (CH.sub.4) as an end product. Anaerobic
digestion is of great interest today due to its potential as a
renewable energy source.
[0002] There is much confusion regarding the use of the terms
"stage" and "phase" in the anaerobic digestion literature. Numerous
authors have used the terms interchangeably. However, as used
herein, the term "phase" is used to refer to biological steps in
the anaerobic digestion process, whereas the term "stage" refers to
an engineered physical entity (e.g., tank, container) used to
contain the microbial phases.
[0003] The term "feedstock," also referred to as influent, refers
to liquid and solid material fed into a an anaerobic digester,
including but not limited to dairy manure/waste, municipal and
industrial waste water sludge, organic material, biomass waste,
biodiesel production waste, ethanol production waste, and food
processing waste.
[0004] Anaerobic digestion is a complex process, mediated by a
diverse array of microorganisms in the absence of oxygen. During
anaerobic digestion, these microorganisms digest organic matter and
produce methane gas as an end product. The complexity of the
anaerobic microbial community is illustrated by data identifying
over 9,000 active species in wastewater sludge digesters (Curtis
(2002)).
[0005] Anaerobic digestion has been described as a three phase
process (Geradi (2003)), a four phase process (Schink (1992);
Deublein and Steinhauser (2008); Khanal (2008)), a five phase
process (Liu and Ghosh (1997)), and a nine phase process (Pohland
(1992)). These great variations in how the literature defines the
number of phases present clearly indicates the complexity of the
microbial systems involved.
[0006] Most recently, a four-phase process, constituting a food
chain, has been generally accepted as a working model (Deublein and
Steinhauser (2008); Khanal (2008)). These four phases consisting of
a Hydrolysis Phase, an Acidogenesis Phase, an Acetogenic Phase, and
a Methanogenesis Phase.
[0007] The Hydrolysis Phase is the first phase. The Hydrolysis
Phase involves the digestion of complex carbohydrates, proteins and
lipids into simpler substrates such as sugars, amino acids and
fatty acids. It is analogous, in many ways, to the functions
carried out by the stomach in mammalian digestive systems.
Hydrolysis bacteria include both facultative anaerobic
microorganisms (able to live under aerobic as well as anaerobic
conditions) and strictly anaerobic microorganisms. Hydrolysis
bacteria tend to be highly resistant to environmental fluctuations
such as temperature and pH changes, thrive in an acidic
environment, have high reproductive rates and growth rates, and are
not usually adversely affected by toxins and heavy metals which may
be present in the feedstock. Since the hydrolysis step is required
to treat raw particulate matter, it often is a rate-limiting step
in the anaerobic process due to the difficulty of digesting these
often complex substrates (Sanders, et al. (2000); Zeeman and
Sanders (2001); Sanders (2002); Gomec, et al. (2003); Gosh (1985)).
Improved mixing and particulate disruption approaches can go far to
minimizing this potential limiting problem (Sanders, et al. (2000);
Palmowski, et al. (2003)), as has been shown in a recent report on
the effect of optimizing sludge digester mixing (Marx, et al.
(2007)).
[0008] The Acidogenesis Phase is the second phase in the anaerobic
food chain. The Acidogenesis Phase involves another group of both
facultative and strictly anaerobic bacteria that, utilizing the
simple substrates provided by the hydrolysis bacteria, metabolize
these secondary compounds into water soluble organic acids,
alcohols, and carbon dioxide and hydrogen gas (Britz, et al.
(1994); Yu, et al. (2003)). One study identified two hundred and
eighty eight (288) different strains of acidogenic microbes in four
anaerobic digesters in South Africa (Britz, et al. (1994)),
illustrating the complexity of this phase.
[0009] The Acetogenic Phase is the third phase in the anaerobic
food chain. In the Acetogenic Phase, homoacetogen bacteria utilize
the products produced by the prior Acidogenesis Phase acidogens.
The homoacetogen bacteria produce water-soluble acetate, an
important precursor to methane formation (Deublein and Steinhauser
(2008); Khanal (2008)).
[0010] The Methanogenesis Phase is the fourth and final phase. The
Methanogenesis Phase results in the production of methane gas
(CH.sub.4). Methane producers are not true bacteria, but belong to
an ancient group of microorganisms termed the Archaea. Recent
evidence indicates that methanogens were active 3.5 billion years
ago (Uneno, et al. (2006)). There are numerous species of
methanogens capable of metabolizing a variety of low molecular
weight water-soluble organics and gases. Methanogens are among the
most strictly anaerobic organisms known, their growth being
inhibited by the presence of even extremely small amounts of
oxygen. Methanogens also are slow in reproducing, prefer a basic
pH, and tend to be negatively affected by potential toxins such as
heavy metals, solvents, pesticides and herbicides. Methanogens are
also adversely affected by relatively small changes in
environmental factors, such as pH and temperature. Most of the
reputation of anaerobic digesters for instability, measured by the
cessation of biogas production, can be traced to a failure of the
methanogen populations.
[0011] The natural biological processes described above have been
used extensively in an engineered application for over 100 years,
long before the intricate biological relationships were understood.
Said application has been almost exclusively at wastewater
treatment plants for the stabilization and volume reduction of
sludges. The production of energy has not been the primary goal of
these systems. There are approximately 16,000 individual anaerobic
digestion tanks operating in the United States alone. These tanks
range in size from several hundred thousand gallons to several
million gallons.
[0012] The vast majority are single stage systems where the four
biological phases are forced to operate in a single tank. This
creates numerous operational problems.
[0013] First, the hydrolysis bacteria and acidogenic bacteria
(acidogens) have pH optimums of 5.5 to 6.5; whereas the
methanogenic bacteria (methanogens) have pH optimums of 7.8 to 8.2
(Khanal, 2008). This presents challenges with using a single stage
reactor (digestion tank) because hydrolysis begins immediately when
the raw organic feedstock enters the digestion tank. Hydrolysis
causes a rapid drop in pH as acidic products such as organic acids
are rapidly produced. This acidic pH in turn inhibits the growth
and metabolic activity of the methanogens.
[0014] To counteract this, a buffering agent (e.g., lime) must be
added to the digestion tank to raise the pH to 7.8 to 8.5, the
optimum pH for methane (CH.sub.4) production. This pH adjustment
must be estimated and performed manually because the quantity of
buffering agent required will depend upon multiple factors,
including, but not limited to, the feed rate and the chemical
characteristics of the undigested organics in the feedstock. Due to
the size of these reactors, substantial quantities of buffering
agent are needed to adjust the pH. Since the hydrolysis phase is
facilitated by acid conditions, raising the pH to satisfy the
requirement of the methanogens can inhibit the rate of hydrolysis,
making operation of the digester a precarious balancing act
requiring trained and alert operators. No matter how skilled the
operator is, effectively combining efficient digestion and energy
production has been virtually impossible in such a conventional
digester.
[0015] Second, methanogenic organisms are slow reproducers and do
not compete well for attachment space with the more robust and
aggressive hydrolysis and acidogenic populations.
[0016] Third, in order to achieve the higher temperatures favored
by methanogens, the contents of the entire digestion tank must be
heated via a heating means (e.g., heater) to 30.degree. C. to
38.degree. C. for mesophilic operation or 49.degree. C. to
57.degree. C. for thermophilic operation, at which latter
temperature range the highest rates of methane (CH.sub.4)
production are achieved. Due to the large tank sizes typically
used, these elevated temperatures require the utilization of
significant amounts of energy (to heat the digestion tank), often
reducing the net energy output of the anaerobic digestion system by
as much as fifty percent (50%) or more.
[0017] Fourth, heavy metals or other toxins introduced into the
single reactor with the feedstock come into immediate and direct
contact with the environmentally sensitive methanogens. This is a
frequent contributor to digester problems and reduction or
cessation of methane (CH.sub.4) production.
[0018] Fifth, each time digested solids are discharged from the
single digestion tank, a portion of the valuable, but slowly
reproducing, methanogens, which are attached to the solid
particles, are also lost.
[0019] Sixth, methane gas (CH.sub.4) produced by conventional
anaerobic digesters has a high carbon dioxide (CO.sub.2) content,
often totaling 30 to 40 percent or more. For this reason, it has a
lower BTU value than natural gas, and is referred to as "biogas."
Carbon dioxide is a food source for methanogens, and thus the
presence of CO.sub.2 in the biogas is an indication of conversion
inefficiency in single stage and two stage anaerobic digesters.
[0020] Seventh, these operational challenges require a highly
trained and attentive operational staff to properly operate
conventional digesters. Such staff is in short supply.
[0021] The above items are the main reason why anaerobic digestion
has not progressed more widely as a reliable source of renewable
energy.
[0022] In an attempt to solve these problems, various multiple
stage reactor configurations, using two or more separate tanks,
have been proposed. Two stage reactor designs attempt to isolate
the hydrolysis/acidogenesis phase in the first tank, and the
methanogenic phase in a second tank. This is based on the
well-established fact that the food for the methanogens is
water-soluble.
[0023] In addition, three-stage and even four-stage reactor
configurations have been proposed. However, none of these have
solved the operational sensitivity problems, nor have they
significantly increased biogas yields or biogas purity as evidenced
by the low numbers of full-scale multi-stage installations which
have been constructed. Single stage digesters are still the
norm.
[0024] As a potential source of renewable energy, anaerobic
digestion has a number of distinct advantages over other biofuels,
such as ethanol or biodiesel.
[0025] First, it produces energy from existing waste organics
(e.g., animal manure, municipal solid waste, food processing waste,
wastewater treatment sludge, process sludge from such industries as
ethanol production, biodiesel production, and paper mills). There
are enormous quantities of these waste organics readily
available.
[0026] Second, in deriving energy from these waste organics,
anaerobic digestion also performs a significant role in ground
water protection, odor control, and greenhouse gas reduction.
[0027] Third, anaerobic digestion can be used to produce energy
from biomass crops.
[0028] Fourth, anaerobic digestion does not require energy
intensive drying prior to digestion.
[0029] Fifth, there is a large, albeit inefficient, pre-existing
installed base of single stage digesters, for instance it has been
estimated that there are 12,000 to 16,000 individual digester tanks
in the United States and over 20,000 in Europe. This installed base
provides engineering and operational expertise on construction,
operation, safety and utilization issues for the produced methane
gas (CH.sub.4). Additionally, the installed base is ripe for
retrofitting with technological enhancements aimed at increasing
methane gas (CH.sub.4) production.
SUMMARY OF THE DISCLOSURE
[0030] This disclosure describes a three stage, multiple phase
anaerobic biotechnology process designed to (1) significantly
simplify the operational requirements, (2) significantly increase
the reliability, and (3) significantly increase the organic
degradation and methane gas production rates. The ultimate design
goal of this invention is to make anaerobic digestion a reliable
and profitable source of methane gas as a renewable energy
source.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a schematic representation of one embodiment of a
three stage, multiple phase anaerobic digestion system.
[0032] FIG. 2A is a schematic representation of a second embodiment
of a three stage, multiple phase anaerobic digestion system.
[0033] FIG. 2B is a schematic representation of a third embodiment
of a three stage, multiple phase anaerobic digestion system.
[0034] FIG. 3 is a partial schematic representation of a fourth
embodiment of a three stage, multiple phase anaerobic digestion
system.
[0035] FIG. 4 is a perspective view of Applicant's proprietary
horizontal plate microbial support media.
[0036] FIG. 5 is a plan view of Applicant's proprietary horizontal
plate microbial support media.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] While the invention is susceptible to various modifications
and alternative constructions, certain illustrated embodiments
thereof have been shown in the drawings and will be described below
in detail. It should be understood, however, that there is no
intention to limit the invention to the specific form disclosed,
but, on the contrary, the invention is to cover all modifications,
alternative constructions, and equivalents falling within the
spirit and scope of the invention as defined herein.
[0038] In the following description and in the figures, like
elements are identified with like reference numerals. The use of
"e.g.," "etc," and "or" indicates non-exclusive alternatives
without limitation unless otherwise noted. The use of "including"
means "including, but not limited to," unless otherwise noted.
[0039] Disclosed is a three stage, multiple phase anaerobic
digestion system and method. The preferred embodiment of the
anaerobic digester is a three-stage process designed to separate
the biological phases, optimize microbial activity in each phase,
and significantly increase system reliability and energy
production. The system physically separates the biological phases
of anaerobic digestion based on particle size, particle density,
and solubility of metabolic products. The system allows a complex
multi-phased biological system to develop without the need for
excessive control or operator intervention.
[0040] A schematic of the preferred embodiment of the anaerobic
digester process is shown in FIG. 1. FIG. 1 showing the three
stages (Stages I, II and III). Multiple systems could comprise each
stage, for instance as shown in FIGS. 2A and 2B.
[0041] FIG. 2A shows a second embodiment. FIG. 2A showing a single
Stage I, a single Stage II, and a single Stage III, all in a series
configuration. This type of implementation may be best for smaller
systems.
[0042] FIG. 2B shows a third embodiment. FIG. 2B showing dual Stage
I and Stage III, with a single Stage II. This embodiment is used
with alternating mixing and feed from dual Stages I to single Stage
II, and alternating feed from single Stage II to the dual Stage
III's for larger systems.
[0043] Stage I takes place in the primary (first) digestion tank 1.
It is preferably similar in size and configuration to that used in
conventional single stage anaerobic digestion systems. Stage I is
operated as a complete mix reactor wherein a feed pump 2 injects
raw feedstock into the Stage I digestion tank 1. The primary
digestion tank 1 is provided with at least one mixing means 3
(e.g., mechanical mixer, internal hydraulic jet nozzles, external
chopper pumps) for mixing the contents of the primary digestion
tank 1 and for causing rapid size reduction of the raw organic
solids contained in the feedstock. This mixing and size reduction
preferably uses the Liquid Dynamics Jetmix.RTM. systems.
[0044] Stage I is optimized for mechanical size reduction of the
incoming organic solids and extensive but incomplete microbial
hydrolysis. Stage I is operated with full power mixing interspersed
with shorter settling periods of either quiescent operation (no
mixing) or reduced power mixing. This is to allow the denser solids
to settle while the less dense but still incompletely hydrolyzed
suspended and colloidal solids and water soluble compounds are
drawn off as supernatant from the upper surface of the Stage I
digester at a liquid outlet and pumped through a lower (first)
conduit 4 to Stage II at a second digestion tank inlet.
[0045] Unlike a conventional anaerobic digester, no pH adjustment
or buffering agent is required in Stage I, allowing the prevailing
acidic conditions (pH of 5.5 to 6.5) to increase the rate of
particle disruption and hydrolysis. Since pH adjustment in Stage I
is not required, influent feed rates of the feedstock and types of
feedstocks can be varied without adversely affecting methane
production, as would occur in a conventional digester.
[0046] Methane (CH.sub.4) production in Stage I is suppressed by
the dominance of the hydrolysis bacteria, the aggressive mixing,
and the acidic pH. Gases produced in Stage I are primarily carbon
dioxide (CO.sub.2) and hydrogen (H.sub.2). These gasses, collected
at a first raw gas outlet, are piped to the Stage III reactor via a
raw gas collection system (e.g., upper conduit 11) to be used as
additional food for the methanogens in Stage III.
[0047] Toxins and heavy metals in the feedstock are retained in the
Stage I reactor through chemical binding and bioremediation
effects, and thus do not adversely impact the methanogens in Stage
III. Digested sludge, which falls to the bottom of the Stage I tank
due to increased density, is periodically removed via a drain 14.
Digested sludge can also be periodically removed from the Stage II
and/or Stage III tanks.
[0048] Since the activity of the methanogens is suppressed in Stage
I, it is no longer necessary to operate the Stage I reactor at the
elevated temperatures required to optimize methane production. The
Stage I reactor can be heated to lower operating temperatures
(preferably 24.degree. C. to 34.degree. C.) than that required for
the methanogens. This increases net energy production of the
overall system, particularly for thermophilic operation.
[0049] Stage II takes place in the secondary (second) digestion
tank 5 which is operated as an upflow fluidized bed reactor. The
fluidized bed entraps the fine suspended and colloidal solids
captured from the Stage I supernatant. The pH in the secondary
digestion tank, like the primary digestion tank, being 5.5 to 6.5.
Upflow velocity in Stage II is adjusted to that which will minimize
overflow of the suspended solids to the Stage III reactor. Further,
a settling plate 6 at the top of the Stage II reactor further
minimizes carryover of solids into the Stage III reactor. This
settling plate can be any commercially available (e.g., tube
settlers, settling plates). The Applicant's proprietary horizontal
plate microbial support media 20, shown in FIGS. 4 and 5, modified
to serve as a settling surface, is the preferred settling
plate.
[0050] The Stage II fluidized bed is biologically active,
entrapping suspended and colloidal solids, and completing the
hydrolysis phase by metabolizing the remaining fine suspended and
colloidal solids from Stage I, and continuing the acidification
phase. Gases released in the Stage II reactor 5, collected at a
second raw gas outlet, consisting primarily of carbon dioxide
(CO.sub.2) and hydrogen (H.sub.2), are piped to the Stage III
reactor 10 via a raw gas collection system (e.g., upper conduit
11). The overflow effluent (filtered supernatant) from Stage II
exits via a liquid outlet and is then pumped to the base of Stage
III (at a liquid inlet) via the overflow (second) conduit 7.
[0051] While being pumped from Stage II to Stage III (via the lower
conduit 7 to the third digestion tank inlet), the filtered
supernatant flows through a heater means (e.g., heat exchanger) 8
to raise the temperature to 30.degree. to 38.degree. C. (85.degree.
to 100.degree. F.), the optimum temperature for mesophilic
operation, or 49.degree. to 57.degree. C. (120.degree. to
135.degree. F.), the optimum temperature for thermophilic operation
(preferred). At this time, the pH of the Stage II supernatant is
also adjusted to 7.8 to 8.5 (optimum pH for methane production) via
a pH adjusting means (e.g., in-line pH probes, automated,
computer-controlled chemical feed pumps, static mixer 9).
[0052] Stage III is a packed bed (fixed film) reactor 10 (the
"third" digestion tank). The packing material can be various types
of random or vertical sheet media, including cross-flow or tubular
media. However, for optimization of methane production, the
preferred media is the applicant's proprietary horizontal plate
microbial support media 20 (shown in FIGS. 4 and 5).
[0053] It is preferred that two different zones be created within
the Stage III reactor 10, namely an upper media bed portion 16 and
a lower media bed portion 15. The lower portion of the Stage III
reactor preferably containing high angle anaerobic digester media,
the height or extent varying with the specific application. The
angle will be variable, but the preferred angle is 60 degrees from
the horizontal.
[0054] The upper portion of the Stage III reactor contains a second
digester media, preferably contains low angle anaerobic digester
media, the height or extent varying with the specific application.
The angle will be variable but the preferred angle is 45 degrees
from the horizontal. This use of a reduced angle increases the
available surface area for attachment of the methanogen community.
Alternatively, applicant's proprietary horizontal plate microbial
support enhanced surface area media (shown in FIGS. 4 and 5) can be
applied in the upper media bed to increase surface area.
[0055] The preferred horizontal plate anaerobic digester media in
Stage III is used to combine hydraulic and biological
characteristics which maximize methane production. Hydraulically,
the anaerobic digester media induces constant mixing, remixing, and
flow splitting at low upflow velocities to insure maximum contact
of the liquid-borne substrates with the microbial community
attached to the media. Biologically, the anaerobic digester
horizontal media provides a microbial film on the upper impingement
surface to ensure agglomeration and capture of residual colloidal
solids. These agglomerated particles then drop to the lower surface
of the media where the final acidification and acetogenesis and
takes place, forming water soluble acetate, and other water soluble
organics suitable as food for methanogens. This media permits the
additional development of multiple phases in the Stage III reactor
10.
[0056] The methane bacteria permanently attach to the upper surface
of the anaerobic digester media where they have maximum exposure to
food and are isolated from direct ecological competition with
residual hydrolysis and acidogenesis microorganisms on the lower
surface. The provision of a solid and permanent attachment surface
for the methanogens prevents loss (washout) of these slow growing
microorganisms, maximizing the stability and energy productivity of
the system.
[0057] The raw gas (carbon dioxide (CO.sub.2) and hydrogen
(H.sub.2)) from Stages I and II (carried via the upper conduit 11
(raw gas collection system)) preferably enter the Stage III reactor
at the base of the upper media bed (at the raw gas inlet). This
configuration protects the acetogenic phase microorganism in the
lower media bed from elevated concentrations of hydrogen gas which
could limit their activity and the production of acetate, an
important food source for the methanogenic microorganisms. The
carbon dioxide (CO.sub.2) and hydrogen (H.sub.2) gases from Stages
I and II, as they move upward through the upper media bed, provide
an additional food source for the attached methane-producing
microorganisms, thereby reducing the carbon dioxide (CO.sub.2) in
the final biogas product.
[0058] During the Stage I active mixing phase, when no new
feedstock is being pumped into Stage III, supernatant will
recirculate from the top to bottom (piping not shown in FIG. 1) of
Phase III to ensure stable pH and temperature and more complete
uptake of substrate and production of methane. However, the entire
process can be operated as either a continual flow process or a
semi-continual flow process.
[0059] When new feedstock is pumped into Stage I, a like amount of
liquid (filtered effluent) will preferably exit Stage III via a
discharge conduit (effluent outlet) 12. Due to contact in the
fluidized bed of Stage II and the packed bed of Stage III, this
discharge liquid will require relatively little additional
treatment prior to discharge (e.g., application to land
(irrigation)).
[0060] Methane gas (CH.sub.4), produced in Stage III, is piped off
the top of the Stage III reactor via a methane outlet (gas conduit)
13 where it is collected and stored.
[0061] It is preferred that flocculant (e.g., ferric chloride
(FeCl.sub.3)) be added to the supernatant, for instance at call out
17 in FIG. 1. The purpose of the flocculant being to assist in the
flocculation and/or precipitation of phosphorus from the
supernatant, thereby decreasing the formation of mineral deposits,
such as struvite (ammonium magnesium phosphate) within the second
and third digester tanks and associated equipment. While the
preferred location of injection of the ferric chloride is before
Stage II, it could be injected at Stage I or before both Stages I
and II.
[0062] There are a number of benefits to various embodiments of the
present invention. First, the hydrolysis and acidification phases
are separated from the methanogenic phase through a three-stage
process based on particle size, particle density, and intermediate
product solubility factors. This reduces the potential for process
failure and increases the rate of energy production. The
methanogens in Stage III are protected from changes in pH,
temperature and the effect of toxins and/or heavy metals that exist
in the earlier stages and are provided with ideal conditions of pH
and temperature to optimize energy production. The methanogens in
Stage III are provided with ideal environmental conditions to
maximize methane gas production.
[0063] Second, the quantity and cost of pH control chemicals is
significantly reduced because pH control is only required in the
smaller Stage III tank. As a result, the system permits automated,
computer-controlled monitoring and adjustment to the optimum pH
levels required for methane production. This also reduces operator
attention requirements and the possibility of operator error. In
Stage I, allowance of a lower pH increases the rate of particulate
size reduction and microbial hydrolysis, thereby further
benefitting the efficiency of the system. The reduced demand for pH
adjusting chemicals also permits economical use of sodium
bicarbonate as a preferred pH control chemical in Stage III.
[0064] Third, maintaining thermophilic temperatures in Stage III is
the preferred mode of operation. The present anaerobic digester
system significantly reduces heating requirements associated with
conventional thermophilic digestion in that only the smaller Stage
III reactor(s) are heated to the thermophilic temperatures
(49.degree. to 57.degree. C.) required for increased methane
(CH.sub.4) production. The present anaerobic digester system also
eliminates odor and waste solids dewatering problems associated
with conventional thermophilic operations. The waste sludges from
Stage I, which operates at lower temperatures, do not exhibit the
increased odor formation and poor dewatering characteristics
associated with conventional single-stage anaerobic digesters
operated at thermophilic temperatures.
[0065] Fourth, there is a significant reduction of loss (washout)
of methanogens due to provision of permanent attachment media sites
in the Stage III reactor for the methanogens to attach to,
increasing process stability and energy production rates.
[0066] Fifth, Applicant's anaerobic digester system utilizes stock
equipment (e.g., tanks, chopper pumps, mixers, heat exchangers,
solids handling equipment, pH and temperature adjustment monitors
and controls), further increasing process reliability.
[0067] Sixth, the anaerobic digester system significantly reduces
operational and operator skill requirements due to semi-automatic
operation. Operators are still required for feeding solids
(feedstock) into the Stage I reactor and removing digested sludge,
but operator requirements for estimating and adjusting pH and
temperature are eliminated.
[0068] Seventh, methane (CH.sub.4) gas produced by the Stage III
reactor will be of a significantly higher BTU content with less
carbon dioxide (CO.sub.2) than that produced from conventional
digesters. This is due to the method of piping carbon dioxide
(CO.sub.2) and hydrogen (H.sub.2) gases produced in Stages I and II
to the Stage III fixed film reactor which will increase conversion
of these gases to methane (CH.sub.4).
[0069] Eighth, the present anaerobic digester system will permit
increased rates of solids digestion in Stage I, will improve waste
solids dewatering characteristics, will reduce odors, and will
reduce final disposal requirements and costs.
[0070] Ninth, the invented anaerobic digester system can be both
applied to new construction and used to retrofit pre-existing
anaerobic digester (single stage) systems to improve energy
production rates in the latter.
[0071] Tenth, due to the combination of design factors in the
anaerobic digester system, including separation of phases, use of
the horizontal microbial attachment media, reduced heating
requirement for thermophilic operation, increased stability at
thermophilic temperatures, and reduced potential for operator error
and operator skill levels, embodiments of the invented anaerobic
digester three stage system should at least double, and potentially
triple, the net energy output as compared to conventional single
tank designs.
[0072] Referring now to FIG. 3, shown is an alternative embodiment
of a Stage III rector. Since carbon dioxide (CO.sub.2) is a food
source for methane producing bacteria, Stage III can also be used
as a unique reactor (without Stages I and II) to biologically
transform carbon dioxide (CO.sub.2) from stack gases and other
point sources of carbon dioxide (CO.sub.2) entering the atmosphere
into methane (CH.sub.4) gas which can be used as a fuel. In the
case of stack gases, the air stream 18 carrying the CO.sub.2 and
other pollutants will preferably be first combined into a liquid
carrier (e.g., water, wastewater), possibly under slight pressure,
and then be pumped through Stage III to transform the CO.sub.2 into
methane (CH.sub.4) gas.
[0073] Example implementation. Injecting raw feedstock into a
primary digestion tank; mixing the contents of the primary
digestion tank mechanically, thereby reducing the size of the raw
organic solids contained in the feedstock; holding feedstock in the
digestion tank for a predetermined period of time to provide for
extensive but incomplete microbial hydrolysis; ceasing mixing to
allow for settling of denser solids; drawing off a supernatant from
the upper portion of the contents of the primary digestion tank;
pumping said supernatant to a second digestion tank; collecting
gases from the first digestion tank and transmitting said gasses to
the base of a third digestion tank; draining undigested material
from the bottom of the first digestion tank; maintaining the first
digestion tank and second digestion tank at a temperature of
24.degree. C. to 35.degree. C.; operating the second digestion tank
as an upflow fluidized bed reactor; adjusting upflow velocity to
that which will minimize overflow of the suspended solids to the
third digestion tank; utilizing a settling plate at the top of the
second digestion tank to minimize carryover of solids into the
third digestion tank; collecting gases from the second digestion
tank and transmitting said gases to the base of the third digestion
tank; pumping effluent from the second digestion tank to the base
of the third digestion tank; heating said supernatant to a
temperature of 30.degree. to 38.degree. C. or 49.degree. to
57.degree. C. before injection into said third digestion tank;
adjusting the pH of the supernatant to 7.8 to 8.5 using in-line pH
probes, automated, computer-controlled chemical feed pumps and a
static mixer before injection into the third digestion tank;
operating said third digestion tank as a packed bed (fixed film)
reactor; creating two different zones within the third digestion
tank, namely an upper portion containing low angle or enhanced
surface area anaerobic digester media and a lower portion
containing high angle anaerobic digester media; utilizing gases
from the first and second digestion tanks in the third digestion
tank as an additional food source for the attached methane
producing microorganisms; discharging liquid from said third
digestion tank generally equal to the amount of new feedstock
pumped into said first digestion tank; and collecting and storing
methane gas produced in said third digestion tank.
[0074] First example embodiment. A method for the anaerobic
digestion of a raw feedstock containing raw organic solids, said
method comprising: transferring a quantity of said feedstock into a
first digestion tank; mixing and chopping said feedstock to reduce
the size of the raw organic solids contained in said feedstock;
holding said feedstock in said first digestion tank for a
predetermined period of time to provide for at least partial
microbial hydrolysis of said feedstock, wherein said feedstock has
a pH of 5.5 to 6.5; ceasing the mixing of said feedstock and
allowing the settling of solids from a supernatant; drawing off a
portion of said supernatant from said first digestion tank and
transferring said drawn off portion to a second digestion tank;
collecting first gases from said first digestion tank and
transmitting said first gasses to a third digestion tank;
maintaining the temperature of said supernatant in said first
digestion tank between 24.degree. C. to 35.degree. C.; operating
said second digestion tank as an upflow fluidized bed reactor, said
reactor having a top and a bottom, wherein supernatant pumped from
said first digestion tank enters said second digestion tank at said
bottom and exits said second digestion tank at said top as filtered
supernatant, wherein said feedstock has a pH of 5.5 to 6.5;
adjusting the upflow velocity of the supernatant moving through
said second digestion tank to minimize suspended and colloidal
solids in said filtered supernatant; drawing off a second portion
of said filtered supernatant from said second digestion tank;
collecting second gases from said second digestion tank and
transmitting said second gases to said third digestion tank;
maintaining the temperature of said supernatant in said second
digestion tank between 24.degree. C. to 35.degree. C.; heating said
filtered supernatant to a mesophilic temperature or a thermophilic
temperature; adjusting the pH of the filtered supernatant to
maintain a pH of 7.8 to 8.5 in said third digestion tank;
transferring said second portion of filtered supernatant to said
third digestion tank; operating said third digestion tank as a
packed bed reactor having an upper portion containing a second
digester media, where the second digester media is selected from
the group consisting of low angle anaerobic digester media and
enhanced surface area media, and a lower portion containing high
angle anaerobic digester media, said filtered supernatant flowing
through said lower portion then through said upper portion;
injecting said first and second gasses into said third digestion
tank upper portion; discharging filtered effluent from said third
digestion tank; and collecting and storing methane gas produced in
said third digestion tank.
[0075] Second example embodiment. A three stage, multiple phase
anaerobic digestion system for the anaerobic digestion of a
feedstock, said system comprising: a first digestion tank in which
the hydrolysis of said feedstock begins, said first digestion tank
having a top and a bottom, said first digestion tank including a
mixing means for mixing said feedstock, said first digestion tank
having a gas outlet adjacent said top for allowing gasses within
said first digestion tank to be collected, and a liquid outlet
adjacent said top for allowing a supernatant to be removed from
said first digestion tank; a first conduit connecting said first
digestion tank liquid outlet to a second digestion tank inlet
thereby allowing said supernatant to be conveyed from said first
digestion tank to said second digestion tank; a second digestion
tank in which the hydrolysis of said feedstock completes and the
acidogenesis of said feedstock takes place, said second digestion
tank having a top and a bottom, said second digestion tank operated
as an upflow fluidized bed reactor, said second digestion tank
including said second digestion tank inlet adjacent said bottom, a
gas outlet adjacent said top for allowing gasses within said second
digestion tank to be collected, and a liquid outlet adjacent said
top for allowing a filtered supernatant to be removed from said
second digestion tank; a second conduit connecting said second
digestion tank liquid outlet to a third digestion tank inlet
thereby allowing said filtered supernatant to be conveyed from said
second digestion tank to said third digestion tank; a heater means
connecting with said second conduit; a pH adjusting means
connecting with said second conduit, said pH adjusting means for
adjusting the pH of the filtered supernatant to 7.8 to 8.5; a third
digestion tank in which the methanogenesis of said feedstock takes
place, said third digestion tank having a top and a bottom, said
third digestion tank including said third digestion tank inlet
adjacent said bottom, a methane outlet adjacent said top for
allowing methane within said third digestion tank to be collected,
and a liquid outlet adjacent said top for allowing a filtered
effluent to be removed from said third digestion tank, wherein said
third digestion tank is operated as a packed bed reactor, said
third digestion tank comprising an upper portion containing a
second digester media, where the second digester media is selected
from the group consisting of low angle anaerobic digester media and
enhanced surface area media, and a lower portion containing high
angle anaerobic digester media, said third digestion tank further
comprising a raw gas inlet above said high angle anaerobic digester
media but below said low angle anaerobic digester media for
injection of raw gas into said upper portion; and a raw gas
collection system connecting said first and second digestion tank
gas outlets with said third digestion tank's raw gas inlet.
[0076] Third example embodiment. A system for the anaerobic
digestion of a raw feedstock containing raw organic solids, said
system comprising: a hydrolysis tank where partial hydrolysis of
the feedstock takes place, wherein the pH of the feedstock in said
hydrolysis tank has a pH of 5.5 to 6.5, wherein the temperature of
said feedstock in said hydrolysis tank is maintained between
24.degree. C. to 35.degree. C., wherein after a predetermined
period of time, supernatant is drawn off said hydrolysis tank,
wherein carbon dioxide (CO.sub.2) and hydrogen gas (H.sub.2) formed
in said hydrolysis tank are collected; an upflow fluidized bed
reactor having a top and a bottom, said bottom for receiving the
drawn off supernatant from said hydrolysis tank, said upflow
fluidized bed reactor providing for hydrolysis and acidogenesis of
said supernatant, wherein the pH of the supernatant in said upflow
fluidized bed reactor has a pH of 5.5 to 6.5, wherein the
temperature of said supernatant in said upflow fluidized bed
reactor is maintained between 24.degree. C. to 35.degree. C.,
wherein said supernatant exists said upflow fluidized bed reactor
at said top as filtered supernatant, wherein carbon dioxide
(CO.sub.2) and hydrogen gas (H.sub.2) formed in said upflow
fluidized bed reactor are collected; a heating means for heating
said filtered supernatant to at least 30.degree. C.; a pH adjusting
means for adjusting the pH of the filtered supernatant to maintain
a pH of 7.8 to 8.5 in said third digestion tank; and a packed bed
reactor where methanogenic bacteria create methane, said packed bed
reactor having an inlet end and an outlet end, wherein said
filtered supernatant is transferred into said packed bed reactor at
or adjacent said inlet end, wherein said packed bed reactor
includes a biological filter media, wherein said carbon dioxide
(CO.sub.2) and hydrogen gas (H.sub.2) collected from said
hydrolysis tank and said upflow fluidized bed reactor are injected
into said filtered supernatant within said packed bed reactor, said
filtered supernatant, said injected carbon dioxide (CO.sub.2) and
said injected hydrogen gas (H.sub.2) for digestion by said
methanogenic bacteria, said packed bed reactor having a discharge
port through which filtered effluent is discharged from said packed
bed reactor, wherein methane gas (CH.sub.4) produced in said packed
bed reactor is drawn off and stored.
[0077] While there is shown and described the present preferred
embodiment of the invention, it is to be distinctly understood that
this invention is not limited thereto but may be variously embodied
to practice within the scope of the following claims. From the
foregoing description, it will be apparent that various changes may
be made without departing from the spirit and scope of the
invention as defined by the following claims.
[0078] The purpose of the Abstract is to enable the public, and
especially the scientists, engineers, and practitioners in the art
who are not familiar with patent or legal terms or phraseology, to
determine quickly from a cursory inspection, the nature and essence
of the technical disclosure of the application. The Abstract is
neither intended to define the invention of the application, which
is measured by the claims, nor is it intended to be limiting as to
the scope of the invention in any way.
[0079] Still other features and advantages of the claimed invention
will become readily apparent to those skilled in this art from the
following detailed description describing preferred embodiments of
the invention, simply by way of illustration of the best mode
contemplated by carrying out my invention. As will be realized, the
invention is capable of modification in various obvious respects
all without departing from the invention. Accordingly, the drawings
and description of the preferred embodiments are to be regarded as
illustrative in nature, and not as restrictive in nature.
* * * * *