U.S. patent application number 10/013423 was filed with the patent office on 2002-08-01 for biogasification of solid waste with an anaerobic-phased solids-digester system.
This patent application is currently assigned to The Regents of the University of California. Invention is credited to Zhang, Ruihong, Zhang, Zhiqin.
Application Number | 20020102673 10/013423 |
Document ID | / |
Family ID | 22447464 |
Filed Date | 2002-08-01 |
United States Patent
Application |
20020102673 |
Kind Code |
A1 |
Zhang, Ruihong ; et
al. |
August 1, 2002 |
Biogasification of solid waste with an anaerobic-phased
solids-digester system
Abstract
The present invention provides methods for the generation of
methane by a two phase anaerobic phase system (APS) digestion of
organic substrates. Also provided is a device for practicing the
methods of the invention. The APS-digester system is a
space-efficient, high-rate solids digestion system. The
APS-digester system consists of one or more hydrolysis reactors and
one biogasification reactor.
Inventors: |
Zhang, Ruihong; (Davis,
CA) ; Zhang, Zhiqin; (Davis, CA) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER
EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Assignee: |
The Regents of the University of
California
Oakland
CA
|
Family ID: |
22447464 |
Appl. No.: |
10/013423 |
Filed: |
December 11, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10013423 |
Dec 11, 2001 |
|
|
|
09131010 |
Aug 7, 1998 |
|
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Current U.S.
Class: |
435/167 ;
435/289.1 |
Current CPC
Class: |
C12P 5/023 20130101;
C12M 45/06 20130101; Y02E 50/30 20130101; C12M 21/04 20130101; C12M
23/58 20130101; Y02E 50/343 20130101; Y10S 210/92 20130101; C12M
29/00 20130101 |
Class at
Publication: |
435/167 ;
435/289.1 |
International
Class: |
C12P 005/02; C12M
001/00 |
Claims
What is claimed is:
1. A method for methane production by two-phase anaerobic digestion
of solid organic material, said method comprising: (a) incubating
for a first period of incubation, a first mixture comprising said
solid organic material and an aqueous liquid, under anaerobic
conditions, in a first hydrolysis digester having an upper portion
and a lower portion and containing a hydrolysis means therein; (b)
after said first period of incubation, transferring a portion of
said aqueous liquid of said first mixture residing in said lower
portion of said hydrolysis reactor to a methane phase digester to
form a second mixture, said methane phase digester having an upper
portion, a lower portion and a methanogenesis; (c) incubating said
second mixture for a second incubation period during which methane
is generated; (d) transferring a portion of said second mixture
residing in said upper portion of said methane phase digester to
said first hydrolysis phase digester for a third incubation
period.
2. The method according to claim 1, further comprising
intermittently agitating said second mixture.
3. The method according to claim 1, wherein said solid organic
material is a member selected from the group consisting of sewage
sludge, forestry waste, food waste, agricultural waste, municipal
waste and combinations thereof.
4. The method according to claim 1, wherein said solid organic
material comprises agricultural waste.
5. The method according to claim 4, wherein said agricultural waste
comprise rice straw.
6. The method according to claim 1, further comprising collecting
said methane generated in steps (c) through (e).
7. The method according to claim 1, wherein methane is generated in
step (a).
8. The method according to claim 7, further comprising collecting
said methane generated in step (a).
9. The method according to claim 1, wherein said first mixture has
a pH of from about 4.5 to about 6.5.
10. The method according to claim 1, wherein said second mixture
has a pH of from about 6.5 to about 7.5.
11. The method according to claim 1, wherein said hydrolysis means
comprises a bacterial culture.
12. The method according to claim 11, wherein said bacterial
culture is a member selected from the group consisting of
Aerobacter, Aeromonas, Alcaligenes, Bacillus, Bacteroides,
Clostridium, Eschericia, Klebsiella, Leptospira, Micrococcus,
Neisseria, Paracolobacterium, Proteus, Pseudomonas,
Rhodopseudomonas, Sarcina, Serratia, Streptococcus and
Streptomyces, Methanobacterium omelianskii, Mb. formicium, Mb.
sohngenii, Methanosarcina barkerii, Ms. methanica and Mc. mazei and
mixtures thereof.
13. The method according to claim 1, wherein said methanogenesis
means comprises a bacterial culture.
14. The method according to claim 11, wherein said bacterial
culture is a member selected from the group consisting of
Aerobacter, Aeromonas, Alcaligenes, Bacillus, Bacteroides,
Clostridium, Eschericia, Klebsiella, Leptospira, Micrococcus,
Neisseria, Paracolobacterium, Proteus, Pseudomonas,
Rhodopseudomonas, Sarcina, Serratia, Streptococcus and
Streptomyces, Methanobacterium omelianskii, Mb. formicium, Mb.
sohngenii, Methanosarcina barkerii, Ms. methanica and Mc. mazei and
mixtures thereof.
15. The method according to claim 2, wherein said agitating is
carried out for between 30 seconds and 10 minutes every hour.
16. The method according to claim 1, further comprising pretreating
said solid organic material prior to said first period of
incubation by a method which is a member selected from the group
consisting of chemical pretreatment, mechanical pretreatment, heat
pretreatment and combinations thereof.
17. The method according to claim 16, wherein said pretreating is
mechanical pretreatment which is a member selected from the group
consisting of cutting, grinding and combinations thereof.
18. The method according to claim 16, wherein said pretreating is
chemical pretreatment which is a member selected from the group
consisting of bicarbonate treatment, radiation treatment, alkaline
peroxide treatment, ammonia treatment and combinations thereof.
19. The method according to claim 16, wherein said pretreating is
heat pretreatment at a temperature from about 50.degree. C. to
about 120.degree. C.
20. The method according to claim 19, wherein said temperature is
from about 60.degree. C. to about 90.degree. C.
21. The method according to claim 16, wherein said solid organic
material is rice straw and said pretreating comprises: (a) grinding
said rice straw; (b) heating said rice straw; and (c) treating said
rice straw with ammonia.
22. The method according to claim 19, wherein said grinding
produces rice straw sized from about 5 millimeters to about 50
millimeters.
23. The method according to claim 22, wherein said heating is at a
temperature of from about 50.degree. C. to about 120.degree. C.
24. The method according to claim 23, wherein said heating is at a
temperature of from about 60.degree. C. to about 90.degree. C.
25. The method according to claim 22, wherein said treating with
ammonia utilizes an amount of ammonia equal to about 0.5% to about
10% of the total weight of the rice straw.
26. The method according to claim 25, wherein said amount of
ammonia is equal to about 1% to about 5% of the total weight of the
rice straw.
27. The method according to claim 1, wherein a member selected from
the group consisting of said first incubation period, said fourth
incubation period and combinations thereof occur in a hydrolysis
reactor which is not said first hydrolysis reactor.
28. An anaerobic phased solids digester system for methane
production, said system comprising: a first hydrolysis reactor
containing therein a perforated support means separating the
reactor into an upper portion and a lower portion, the upper
portion having a hydrolysis reactor liquid inlet and the lower
portion having a hydrolysis reactor liquid outlet; a
biogasification reactor having a biogasification reactor gas
outlet, an agitating means, an upper portion and a lower portion,
the upper portion having a biogasification reactor liquid outlet
and the lower portion having a biogasification liquid inlet; a
first conduit connecting the hydrolysis reactor outlet to the
biogasification inlet; and a second conduit connecting the
biogasification reactor outlet with the hydrolysis reactor
inlet.
29. The digester system according to claim 28, further comprising
between 1 and 15 additional hydrolysis reactors.
30. The digester system according to claim 29, wherein said
hydrolysis reactors and said methanogenesis reactor are linked in a
manner selected from the group consisting of parallel linking,
series linking and combinations thereof.
31. The digester system according to claim 29, wherein said
hydrolysis reactors are linked in parallel with said methanogenesis
reactor.
32. The digester system according to claim 29, wherein said
hydrolysis reactors are linked in series with said methanogenesis
reactor.
33. The digester system according to claim 29, wherein said
perforated support means is a member selected from the group
consisting of grids, filters, grates, sieves, slats, strainers and
combinations thereof.
34. The digester system according to claim 27 further comprising a
pump operably connected to said first hydrolysis reactor.
Description
FIELD OF THE INVENTION
[0001] This invention relates to improved two phase anaerobic
digestion having separated hydrolysis and biogasification reactors
which convert biomass to desired methane product gas with high
efficiency.
BACKGROUND OF THE INVENTION
[0002] Anaerobic digestion has been known to stabilize sludge and
other predominantly organic materials, and usable product gas, of
varying composition, has been obtained from such anaerobic
digestion processes. The organic feed mixture which provides the
substrate for anaerobic biodegradation can comprise a wide variety
of organic carbon sources, ranging from raw sewage sludge to
municipal refuse, or biomass material such as plants and crop
wastes. The process of anaerobic digestion degrades any of these
organic carbonaceous materials, under appropriate operating
conditions, to product gas which contains the desirable methane
gas.
[0003] Anaerobic digestion uses a consortium of natural bacteria to
degrade and then convert an organic substrate into a mixture of
carbon dioxide and methane. The existing anaerobic digestion
systems for organic substrate digestion can be separated into two
major types, one phase systems and two phase systems. Existing one
phase systems include the batch digester, completely mixed digester
and the plug flow digester. These one phase systems, in which the
organic substrate and the microorganisms are housed together are
easy to operate and of low cost. Completely mixed digesters and
plug flow digesters require continuous handling of feedstock and do
not operate in batch mode. Further, the biogas produced in one
phase systems consists primarily of carbon dioxide in the early
stages of digestion. The high carbon dioxide content of the biogas
is attributable to the slow growth of the methanogenic
microorganisms and their inhibition by high concentrations of
volatile fatty acids (VFAs). In order to reduce the inhibition of
the microorganisms by the VFAs, the two phase digester has been
introduced.
[0004] Separated two phase anaerobic digestion systems have been
found to enhance the conversion efficiency, such as described in
Pohland and Ghosh, Biotechnol. and Bio-eng. Symp. No. 2, 85-106
(1971), John Wiley and Sons, Inc. and by the same authors in
Environmental Letters, 1: 255-266 (1971). A typical two phase
anaerobic digester system comprises an acid phase digester and a
biogasification reactor. The acid phase digester is usually
designed as a solid-bed batch reactor where solid waste is housed
and leached soluble compounds are collected. In the acid first
phase, the microbial population and operating conditions are
selected to promote the conversion of organic carbonaceous
materials to carbonaceous materials of lower molecular weight,
primarily volatile fatty acids. The liquid and solid effluent from
the acid phase is conveyed to a biogasification second phase, where
methanogenic organisms convert the volatile fatty acids to product
gas that is composed primarily of methane and carbon dioxide.
Product gas is removed from the biogasification reactor and
processed, or scrubbed, to separate the methane component that is
drawn off as pipeline gas.
[0005] Two phase anaerobic digestion has been carried out in a
single reactor as taught, for example, by U.S. Pat. No. 4,735,724
which teaches a non-mixed vertical tower anaerobic digester and
anaerobic digestion process which provides passive concentration of
biodegradable feed solids and microorganisms in an upper portion of
a continuous digester volume and effluent withdrawal from the
middle to the bottom portion of the digester, resulting in
increased solids retention times, reduced hydraulic retention times
and enhanced bioconversion efficiency.
[0006] U.S. Pat. No. 4,022,665 discloses certain specific operating
conditions for a two phase anaerobic digestion process, such as
feed rates and detention times, which promote efficient conversion
of organic materials. Additionally, the '665 patent discloses two
separated biogasification reactors, a biogasification reactor I
operated in series with a biogasification reactor II. The
biogasification reactor II receives effluent fluid and/or effluent
gas from biogasification reactor I. A somewhat similar process is
disclosed in U.S. Pat. No. 4,696,746 which teaches a process for
two phase anaerobic digestion with two discrete biogasification
reactors operated in parallel.
[0007] U.S. Pat. No. 3,383,309 teaches that the rate and efficiency
of the anaerobic digestion process, particularly in the methane
forming phase, are increased when hydrogen gas is introduced into
the digester sludge. According to the '309 patent, hydrogen gas is
introduced into both the acid forming and the methane forming
phases, to increase the availability of energy rich "hyper-sludge."
All improvements disclosed in U.S. Pat. Nos. 4,022,665, 4,696,746 3
and 383,309 can be adapted for use according to the improved
process of the present invention and the teachings of that patent
are incorporated herein by reference.
[0008] French Patent No. 78 34240 describes an apparatus for
biogasification which is known in the art as an upflow sludge
blanket reactor. This apparatus utilizes a two-stage digestion
apparatus. The apparatus is designed for and uses continuous
recirculation between the reactors of the two stages. Continuous
recirculation requires a relatively complex apparatus including
filters, pumps and manifolded inlets to disperse the recirculated
liquid stream and to avoid its Ashort circuiting directly to the
outlet of the reactor into which it was just circulated.
Additionally, the continuous recirculation requires two pumps that
must operate continuously. In contrast, the present invention
utilizes intermittent recirculation.
[0009] The sequential batch anaerobic composting (SEBAC) reactor is
a relatively new digestion system. See, Chynoweth et al., Appl.
Biochem. Biotech. 28: 421-32 (1991). The SEBAC system consists of
three reactors. Each reactor operates as a single phase batch
digester. The three reactors are interconnected and operated on a
different digestion schedule, the first being newly started, the
second running in the middle of a digestion and the third running
toward the end of a digestion. When new feedstock is loaded into
the first reactor, the liquid from the third reactor is transferred
to the first reactor to inoculate the feedstock and speed-up the
digestion process.
[0010] A broad range of organic substrates are appropriate
feedstocks for biogasification reactors. An exemplary feedstock is
agricultural waste. Agricultural waste consists mostly of
carbonaceous organic materials and it presents a particularly
attractive renewable source of raw material for the generation of
methane. The use of agricultural waste for this purpose serves a
dual purpose, it produces a useful product and reduces the volume
of agricultural waste which must be disposed of. Many different
types of agricultural waste can be digested utilizing a two phase
anaerobic digestion scheme. The waste from the production of rice
provides a salient example.
[0011] In California, for example, large quantities of rice straw
are produced each year as by-products of rice production. In the
Sacramento Valley alone, 1,452,000 tons of rice straw were produced
in the crop year of 1994-1995 (CARB-CDFA, Progress report on the
phase down of rice straw burning in the Sacramento Valley Air
Basin, Report To The Legislature, California Air Resources Board
and California Department of Food and Agriculture (1995)). Due to
lack of feasible conversion technologies, however, utilization of
these materials for energy production has not become practical for
the agricultural sectors.
[0012] Current methods for disposal of these agricultural residue
materials have caused widespread public concerns with regard to
their environmental impact. In the case of rice and wheat straw
disposal, for example, open field burning is considered as a
practice causing serious air pollution problems, because of the
emissions of smoke and other air pollutants, such as gases,
particles and aerosols.
[0013] Current California legislation (the Connelly-Areias-Chandler
Rice Straw Burning Reduction Act of 1991) mandates the rice growers
to phase down burning of rice straw, requiring a reduction in rice
straw acreage burning to no more than 25% of the planted acreage or
125,000 acres in the Sacramento Valley by the year 2000, whichever
is less. As a result, in 1994-95, about 59% of the rice straw was
burned and 38.4% was disposed of in the fields by soil
incorporation. Off-farm disposal of rice straw as livestock feed
and materials for environmental mitigation and erosion control
counted for only 0.6%. Rice growers are under extreme pressure to
find alternative environmentally friendly methods for straw
disposal and/or utilization. If no other practical straw disposal
alternatives are developed to compensate for the burning phasedown,
rice farmers will be forced to incorporate an estimated 72.9% of
the straw production by the year 2000 to comply with the statutory
rice straw burning phasedown requirements. However, available
research and experience suggest that incorporation rates this high
could potentially cause reduction in crop yield and increase of
foliar disease and possible development of adverse soil
conditions.
[0014] Rice straw is offered as a single relevant example. The
disposal of other solid wastes presents similar problems and new
economical technologies for solid waste disposal and/or utilization
must be developed. Thus, a method for disposing of agricultural and
other wastes which utilized an apparatus of simple design, required
little expenditure of energy to operate and which produced methane
as it reduced the volume of disposable solids would represent a
significant advance. Quite surprisingly the present invention
provides such methods and devices.
SUMMARY OF THE INVENTION
[0015] Anaerobic digestion of solid waste, particularly
agricultural waste, is a promising technique for both generating
energy and reducing the volume of waste which must be disposed of.
The energy generated can be significant. For example, the energy
content of a pound of rice straw is about 6,500 Btu (British
Thermal Units), and the energy stored in the straw by growing crop
each year in the Sacramento Valley is 1.95.times.10.sup.12 Btu.
Thus, it is realistic to consider agricultural waste as a renewable
resource for energy generation.
[0016] Anaerobic digestion is an enhanced biodegradation process
that offers a promising alternative approach for helping solve
problems caused by agricultural waste such as the imminent rice
straw disposal problems in concentrated rice production regions
such as California. Anaerobic digestion uses a consortium of
natural bacteria to degrade and then convert a large portion of
solid waste into biogas, which is a mixture of methane and carbon
dioxide. If captured, biogas can be utilized as a clean fuel for
heat and power generation.
[0017] Anaerobic phase digestion (APS) is a new type of two phase
system. The system employs at least one hydrolysis reactor and a
biogasification reactor. In the APS digester system, carbon
compounds in the organic substrates are liquefied into VFAs in the
hydrolysis reactor. The soluble VFAs produced are transferred to
the biogasification reactor at a controlled rate. This allows the
maintenance of a stable pH level in the biogasification reactor so
that the optimum growth rate of methanogenic bacteria can be
achieved. In a first aspect, the present invention provides a
process for methane production by two-phase anaerobic digestion of
organic material. The process comprises incubating a first mixture
having a solid organic component and an aqueous liquid component,
under anaerobic conditions, in a hydrolysis digester having an
upper portion and a lower portion and containing a hydrolysis means
therein. After a first period of incubation, a portion of the
liquid component of the first mixture residing in the lower portion
of the hydrolysis digester is transferred to a methane phase
digester having an upper portion and a lower portion and a
methanogenesis means therein. In the methane phase digester, the
first mixture is combined with the methanogenesis means to form a
second mixture. The second mixture is incubated for a second period
of time, generating methane. The second mixture is intermittently
agitated, then allowed to remain still for a third period of time.
After the third period of time, a portion of the second mixture
residing in the upper portion of the methane phase digester is
transferred to the hydrolysis phase digester.
[0018] The APS-digester system of the invention has innovative
design features that allow it to handle the solid organic
substrates effectively. The hydrolysis reactor is operated in a
batch or semi-batch mode to ease the handling of solid materials,
and the biogasification reactor operated continuously to maintain
active bacterial culture in the system and to produce biogas at a
relatively constant level. The device used in the system of the
invention is of simple design and is economical to construct and
operate.
[0019] In a second aspect, the present invention provides an
anaerobic phased solids digester system for methane production. The
system comprises a hydrolysis reactor which is separated into upper
and lower portions by a perforated support means. The upper portion
of the hydrolysis reactor has a hydrolysis reactor liquid inlet and
the lower portion has a hydrolysis reactor liquid outlet. The
device further comprises a biogasification reactor. The
biogasification reactor has a biogasification reactor gas outlet
and, optionally, an agitating means. Similar to the hydrolysis
reactor, the biogasification reactor has an upper portion and a
lower portion. The upper portion has a biogasification reactor
liquid outlet and the lower portion has a biogasification liquid
inlet.
[0020] The hydrolysis reactor and the biogasification reactor are
connected via a series of conduits through which liquid from one
reactor can be transferred to another reactor. Thus, the device
also comprises a first conduit connecting the hydrolysis reactor
outlet to the biogasification inlet and a second conduit connecting
the biogasification reactor outlet with the hydrolysis reactor
inlet.
[0021] Other features, objects and advantages of the present
invention and its preferred embodiments will become apparent from
the detailed description that follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a schematic drawing of the anaerobic solids
digester system (APS-digester).
[0023] FIG. 2 is a schematic diagram of the laboratory set-up of
the APS-digester system.
[0024] FIG. 3 displays the daily biogas production at different
pretreatment temperatures.
[0025] FIG. 4 displays the accumulative biogas production at
different pretreatment temperatures.
[0026] FIG. 5 displays the pH variation in the hydrolysis reactor
during the digestion period for different pretreatment
temperatures.
[0027] FIG. 6 displays the pH variation in the biogasification
reactor during the digestion period at different pretreatment
temperatures.
[0028] FIG. 7 displays the daily biogas production for different
physical pretreatment conditions with thermal pretreatment at
90.degree. C.
[0029] FIG. 8 displays the daily biogas production for different
pretreatment conditions without thermal pretreatment.
[0030] FIG. 9 displays the accumulative biogas production of rice
straw for different physical pretreatment.
[0031] FIG. 10 displays the accumulative biogas production for
different physical pretreatment without thermal pretreatment.
[0032] FIG. 11 displays the daily biogas production of rice straw
with different solids loading rates.
[0033] FIG. 12 displays the accumulative biogas production of rice
straw for different solids loading rates.
[0034] FIG. 13 displays the biogas production of a prototype
APS-digester system with two hydrolysis reactors and one
biogasification reactor for digestion of rice straw (chopped and 25
mm).
[0035] FIG. 14 is a schematic diagram of the laboratory set-up of:
(a) batch; and (b) SEBAC systems.
[0036] FIG. 15 is a schematic diagram of the laboratory set-up of:
(a) single batch APS; and (b) multiple batch APS digesters.
[0037] FIG. 16 displays the daily biogas production at different
total solids (TS) loading levels with the APS digester and batch
systems.
[0038] FIG. 17 displays the cumulative biogas production at
different TS lading levels with the APS digester and batch
systems.
[0039] FIG. 18 displays the pH variation at different TS loading
levels with the APS digester and batch systems.
[0040] FIG. 19 displays the methane content of biogas at different
TS loading levels with the APS digester and batch systems.
[0041] FIG. 20 displays the daily and cumulative biogas production
at 75 g/L TS loading with the APS digester and SEBAC systems.
[0042] FIG. 21 displays the pH variation at 75 g/L TS loading with
the APS digester and SEBAC systems.
[0043] FIG. 22 displays the methane content of the biogas at 75 g/L
TS loading with the APS digester and the SEBAC systems.
[0044] FIG. 23 displays the simulated daily biogas production of
the APS digester system with one or twelve hydrolysis reactors.
[0045] FIG. 24 displays the simulated daily biogas production of
the APS digester system with two, three, four, six and eight
hydrolysis reactors.
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS
[0046] Abbreviations and Definitions
[0047] APS, anaerobic phased solids digester; SEBAC, sequential
batch anaerobic composition; TS, total solids, VS, volatile solids;
SRT, solid retention time; HRT, hydraulic retention time.
[0048] As used herein, the term "organic substrate" refers to
carbonaceous feedstock which can be used in the process and device
of the invention to produce methane.
[0049] The terms "biogasification" and "methanogenesis" are used
herein essentially interchangeably
[0050] The present invention provides improved methods for the
anaerobic digestion of waste to produce methane and devices with
which to perform these methods.
[0051] Anaerobic phase digestion (APS) is a new type of two phase
system. The system employs at least one hydrolysis reactor and a
biogasification reactor. In the APS digester system, carbon
compounds in the organic substrates are liquefied into VFAs in the
hydrolysis reactor. The soluble VFAs produced are transferred to
the biogasification reactor at a controlled rate. This allows the
maintenance of a stable pH level in the biogasification reactor so
that the optimum growth rate of methanogenic bacteria can be
achieved. In a first aspect, the present invention provides a
process for methane production by two-phase anaerobic digestion of
organic material. The process comprises incubating a first mixture
having a solid organic component and an aqueous liquid component,
under anaerobic conditions, in a hydrolysis digester having an
upper portion and a lower portion and containing a hydrolysis means
therein. After a first period of incubation, a portion of the
liquid component of the first mixture residing in the lower portion
of the hydrolysis digester is transferred to a methane phase
digester having an upper portion and a lower portion and a
methanogenesis means therein. In the methane phase digester, the
first mixture is combined with the methanogenesis means to form a
second mixture. The second mixture is incubated for a second period
of time, generating methane. The second mixture is optionally
intermittently agitated, then allowed to remain still for a third
period of time. After the third period of time, a portion of the
second mixture residing in the upper portion of the methane phase
digester is transferred to the hydrolysis phase digester.
[0052] The process of the invention can be practiced with any
carbonaceous organic substrate including, but not limited to,
sewage sludge, forestry waste, food waste, agricultural waste,
municipal waste, and the like.
[0053] Municipal waste primarily contains cellulosic products,
particularly kraft paper. It is known that such cellulosics can be
digested as well as the minor amounts of waste protein,
carbohydrates and fat present in municipal waste.
[0054] In a presently preferred embodiment, the organic substrate
consists, at least in part, of an agricultural waste. Agricultural
wastes include both plant and animal wastes. Many types of
agricultural waste can be used in conjunction with the present
invention. Useful agricultural wastes include, but are not limited
to, foliage, straw, husks, fruit, manure and the like.
[0055] The present invention utilizes a separate acid digestion
phase wherein fermentation under anaerobic conditions leads to the
production of aldehydes, alcohols and acids. Methane is also
generated during this phase. The methane can be collected directly
from the hydrolysis phase or it can be routed to the
biogasification phase for later routing to a methane collection
apparatus. The fermentation in the biogasification phase leads to
the production of methane and carbon dioxide. These gases are
collected and they can optionally be passed into a clean up zone
where the methane and the carbon dioxide are separated. The
separator can be any separator known to the art which can separate
gas components primarily of carbon dioxide and methane,
[0056] Both the hydrolysis phase and the methanogenesis phase are
operative over variable pH ranges that are related to the nature of
the organic substrate and the amount of total solids in the organic
substrate. In a preferred embodiment, the acid phase pH is
maintained from about 4.5 to about 6.5. In another preferred
embodiment, the biogasification phase pH is maintained from about
6.5 to about 7.5.
[0057] Any art known hydrolysis or methanogenesis means can be used
in the present invention. These include, but are not limited to
acids, bases, enzymes and combinations of these substances. In a
presently preferred embodiment, the hydrolysis and methanogenesis
means are microorganisms.
[0058] Any active hydrolytic or methane producing mesophilic or
thermophilic anaerobic digestion system can be used in the present
invention. Methane-producing anaerobic systems utilizing acid
forming bacteria and methane-producing organisms, as are well known
to be employed to produce methane from sewage sludge, can be
employed in the practice of the present invention. A review of the
microbiology of anaerobic digestion is set forth in Anaerobic
Digestion, 1. The Microbiology of Anaerobic Digestion, D. F.
Toerien and W. H. J. Hattingh, Water Research, Vol. 3, pages
385-416, Pergamon Press (1969). As set forth in that review, the
principal suitable acid forming species include, species from
genera including, but not limited to, Aerobacter, Aeromonas,
Alcaligenes, Bacillus, Bacteroides, Clostridium, Eschericia,
Klebsiella, Leptospira, Micrococcus, Neisseria, Paracolobacterium,
Proteus, Pseudomonas, Rhodopseudomonas, Sarcina, Serratia,
Streptococcus and Streptomyces. Also of use in the present
invention are microorganisms which are selected from the group
consisting of Methanobacterium omelianskii, Mb. formicium, Mb.
sohngenii, Methanosarcina barkerii, Ms. methanica and Mc. mazei and
mixtures thereof. Other useful microorganisms and mixtures of
microorganisms will be apparent to those of skill in the art.
[0059] A wide variety of substrates are utilized by the methane
producing bacteria, but each species is believed to be
characteristically limited to the use of a few compounds. It is
therefore believed that several species of methane producing
bacteria are required for complete fermentation of the compounds
present in certain organic substrates such as sewage. For example,
the complete fermentation of valeric acid requires as many as three
species of methane producing bacteria. Valeric acid is oxidized by
Mb. Suboxydans to acetic and propionic acids, which are not
attacked further by this organism. A second species, such as Mb.
Propionicum, can convert the propionic acid to acetic acid, carbon
dioxide and methane. A third species, such as Methanosarcina
methanica, is required to ferment acetic acid.
[0060] An operative mixed culture is capable of sustaining itself
indefinitely as long as a fresh supply of organic materials is
added because the major products of the fermentation are gases,
which escape from the medium leaving little, if any, toxic growth
inhibiting products. Mixed cultures generally provide the most
complete fermentation action. Nutritional balance and pH
adjustments can be made as is known in the art to favor hydrolytic
activity
[0061] As discussed in U.S. Pat. No., 4,022,665, issued May 10,
1977 to Ghosh et al., various studies in the art have demonstrated
that a number of acids are converted to methane and carbon dioxide
when such acids are contacted with mixed anaerobic cultures. For
example, the fermentation of acetic, propionic and butyric acids,
as well as ethanol and acetone, all result in the production of
methane and carbon dioxide. Only the ratio of methane to carbon
dioxide changes with the oxidation level of the particular
substrate. Studies in the art have also established that carbon
dioxide can be methanted by the oxidation of hydrogen. It has even
been suggested that methane fermentation of an acid such, as acetic
acid, is a two step oxidation to form carbon dioxide and hydrogen
followed by a reduction to form methane. The net result is the
formation of methane and carbon dioxide. It has also been advanced
that carbon dioxide could be converted to methane in a step-by-step
reduction involving formic acid or carbon monoxide, formaldehyde
and methanol as intermediates. Whatever the actual underlying
mechanism, it is accepted that carbon dioxide can participate in
the methanation process. Applicants provide the above discussion as
useful background and are not binding themselves to any particular
theory of operation.
[0062] Mechanical degradation or chemical treatment of the organic
substrate may be required either to achieve a particle size
appropriate for use in anaerobic digestion according to the
invention or to render the carbonaceous components of the organic
substrate more accessible to the digestion media. Suitable methods
of mechanical degradation are known in the art. Various
pretreatment of the organic substrate can advantageously be used
with the present invention, such as acid or alkaline
hydrolysis.
[0063] The method also contemplates the selective use of
predigestion hydrolysis of the organic substrate before
introduction into the organic phase, as well as post
biogasification hydrolysis of waste removed from the
biogasification phase. The hydrolysis can be conducted as mild acid
or mild alkaline hydrolysis, optionally followed by neutralization
of the added acid or alkali.
[0064] In a presently preferred embodiment, the organic substrate
is rice straw. Previous research has demonstrated the feasibility
of anaerobically digesting a mixture of straw (rice straw and wheat
straw) and other agricultural and food wastes, such as animal
manure, green leaves and molasses, using conventional digestion
reactors fed in batches or semicontinuously (Hills, D. J. and D. W.
Roberts, Agricultural Wastes 3:179-189 (1981); Dar, G. H. and S. M.
Tandon, Biological Wastes 21:75-83 (1987); Adbullah et al., Journal
of Agricultural Sciences 119:255-263 (1992); Somayaji, D. and S.
Khanna, World Journal of Microbiology & Biotechnology
10:521-523 (1994)). The research of Hills and Roberts (1981) showed
that adding either chopped rice straw or chopped wheat straw to
dairy manure enhanced the anaerobic digestion process and increased
the methane production.
[0065] Rice straw is a ligno-cellulosic material mainly composed of
cellulose (37.4%), hemicellulose (44.9%), lignin (4.9%), and
silicon ash (13.1%) (Hills, D. J. and D. W. Roberts, Agricultural
Wastes 3:179-189 (1981)). The straw contains about 0.4% nitrogen
and has a carbon to nitrogen ratio (C/N) of around 75. The proper
range of C/N ratio for anaerobic digestion is 25-35 (Hills, D. J.
and D. W. Roberts, Agricultural Wastes 3:179-189 (1981)).
Therefore, nitrogen needs to be supplemented in order to effect the
anaerobic digestion of rice straw. Nitrogen can be added in
inorganic forms, such as ammonia, or in organic forms such as
organic nitrogen contained in urea, animal manure or food wastes.
But once nitrogen is released from the organic matter, it will
become ammonia (NH.sub.4.sup.+) which is water soluble. Recycling
of nitrogen in the digested liquid will reduce the amount of
nitrogen needed for continuous operation of anaerobic digesters.
Animal manures and food wastes are good nutrient sources if they
are readily available in the areas close to rice straw production.
Nitrogen fertilizer, such as ammonia or urea, is another source of
nitrogen that can be easily added to the straw and may be more
suitable for the areas where handling other types of wastes is not
feasible.
[0066] Thus, in a preferred embodiment, the organic substrate is
supplemented with a nitrogen source. In a further preferred
embodiment, the nitrogen source is a member selected from the group
consisting of urea, animal manure, food waste, inorganic nitrogen
fertilizers and combinations thereof.
[0067] Because of its ligno-cellulosic structure, rice straw is
difficult to biodegrade. Its major component, cellulose, is a
fibrous, water-insoluble substance. It is a linear, unbranched
homopolysaccharide of 10,000 to 15,000 d-glucose units in a
crystalline structure (Lehninger, A. L. et al., Principles of
Biochemistry (2.sup.nd ed.), Worth Publishers, New York, N.Y.
(1993)). Another major component, hemi-cellulose, is also
water-insoluble and consists of a mixture of polymers made up from
xylose, arabinose, glucuronic acid and glucose. Breakdown of
cellulose and hemi-cellulose through the process of chemical
hydrolysis or biodegradation will release simple sugars and make
them available for further conversion into other products, such as
gases in anaerobic digesters. Lignin is a building component for
the cell wall of rice straw and forms the barrier around cellulose
and hemi-cellulose. It is a complex aromatic polymer of
phenylpropane building blocks and is highly resistant to chemical
and biological degradation. Lignin is generally considered not
biodegradable in anaerobic digesters although it can be degraded by
some aerobic microorganisms, such as fungi (Hobson and Wheatley,
1992). The hydrolysis of cellulose can only occur after the lignin
structure is damaged. Enzymes play an important role in
biodegradation of lignocellulosic materials. Cellulases, the
enzymes that help break down celluloses, can convert cellulose into
glucose with little by-products. However, celluloses cannot easily
penetrate through the lignin seal surrounding cellulose fibers, and
therefore, pretreatment of straw, such as treatment with mechanical
grinding and cutting, heat, strong acids or alkaline, are usually
helpful.
[0068] Several works have been published on chemical pretreatment
of rice straw to achieve delignification and hydrolysis of
cellulose. The pretreatment methods that have been explored
include: bicarbonate treatment (Liu, J. X. et al., Animal Feed
Science and Technology 52:131-139 (1994)), radiation (Xin and
Kumakura, Bioresource Technology 43:13-17 (1992)), alkaline
peroxide treatment (Patel and Bhatt, J Chem. Tech. Biotechnol.
53:53-263 (1991)), and ammonia treatment (Sankat and Lauckner,
Canadian Agricultural Engineering 33(2):309-313 (1991)).
[0069] Thus, in a preferred embodiment, the rice straw is
pretreated by a chemical treatment method selected from the group
consisting of bicarbonate treatment, alkaline peroxide treatment,
radiation treatment, ammonia treatment and combinations
thereof.
[0070] The ammonia treatment shows several advantages over the
other treatment, such as the presence of hydroxyl ions as a
delignification factor, a source of nitrogen for biodegradation,
and no separate waste water streams generated from the pretreatment
process. Thus, in a presently preferred embodiment, the rice straw
is treated with aqueous ammonia. In a further preferred embodiment,
the ammonia is present in an amount of from about 0.5% to about
10%, more preferably from about 1% to about 5% relative to the
total weight of solids derived from rice straw.
[0071] Mechanical size reduction of rice straw will also help with
the biodegradation by rupturing the cell walls and making the
biodegradable components more accessible to microorganisms. Thus in
a preferred embodiment, the rice straw is pretreated by a physical
process selected from the group consisting of grinding, cutting,
heating and combinations thereof. In another preferred embodiment,
the rice straw is pretreated using a method comprising grinding the
rice straw to a size from about 5 millimeters to about 50
millimeters. In a further preferred embodiment, the rice straw is
heated to a temperature of from about 50.degree. C. to about
120.degree. C., more preferably from about 60.degree. C. to about
90.degree. C.
[0072] Portions of the liquid component of the digestion mixture
are intermittently exchanged between the hydrolysis digester and
the biogasification digester during the course of the
digestion.
[0073] In a preferred embodiment, an amount of liquid from about
10% to about 50% of a digester's liquid content is exchanged
between the digesters from 1 to 12 times over a 24 hour period,
more preferably from 4 to 6 times in a 24 hour period.
[0074] In a second aspect, the present invention provides an
anaerobic phased solids digester system for methane production. The
system comprises a hydrolysis reactor which is separated into upper
and lower portions by a perforated support means. The upper portion
of the hydrolysis reactor has a hydrolysis reactor liquid inlet and
the lower portion has a hydrolysis reactor liquid outlet. The
device further comprises a biogasification reactor. The
biogasification reactor has a biogasification reactor gas outlet
and an agitating means. Similar to the hydrolysis reactor, the
biogasification reactor has an upper portion and a lower portion.
The upper portion has a biogasification reactor liquid outlet and
the lower portion has a biogasification liquid inlet.
[0075] The hydrolysis reactor and the biogasification reactor are
connected via series of conduits through which liquid from one
reactor can be transferred to another reactor. Thus, the device
also comprises a first conduit connecting the hydrolysis reactor
outlet to the biogasification inlet and a second conduit connecting
the biogasification reactor outlet with the hydrolysis reactor
inlet.
[0076] In a preferred embodiment, the system of the invention
comprises additional hydrolysis reactors. Any number of hydrolysis
reactors can be used in conjunction with the present invention. In
a preferred embodiment, the system utilizes between 1 and 15
hydrolysis reactors, more preferably between 2 and 8 hydrolysis
reactors.
[0077] The hydrolysis reactors and the biogasification reactor can
be linked in any useful arrangement selected from parallel linking,
series linking and combinations thereof. For example, the
hydrolysis reactors can be linked in parallel with the
biogasification reactor. Alternatively, the hydrolysis reactors can
be linked in series with other hydrolysis reactors and this
hydrolysis manifold can be linked to the biogasification. In still
another embodiment, more than one manifold of hydrolysis reactors
can be linked in parallel with the biogasification reactor.
[0078] Any perforated support means known in the art can be used in
the system of the invention. The support means can comprise
structures including, but not limited to, grids, screen, filters,
grates, sieves, slats, strainers and the like. The perforated
support means can be constructed of any material that is
substantially inert under the hydrolysis conditions including, but
not limited to, plastics, metals, resin, composites, graphite, and
the like. Suitable support means configurations and compositions
will be apparent tot hose of skill in the art.
[0079] Any means known in the art for agitating a liquid or
suspension can be used in the system of the invention. Exemplary
means include, but are not limited to, overhead stirrers, gas or
motor driven stirrers, magnetic stirrers, shakers, homogenizers,
sonicators, gas bubbling tubes, ebulliators and the like. Other
useful agitating means will be apparent to those of skill in the
art.
[0080] The solids feedstock, such as rice straw, and a bacterial
culture are contained in the hydrolysis reactor. Each hydrolysis
reactor works with semibatches while the biogasification reactor
produces biogas continuously. In preferred embodiment, digesting
straw, the straw is fed into the hydrolysis reactor from the top of
the reactor in batches or semibatches. After the straw is
continuously hydrolyzed during each batch treatment, the soluble
substances produced in the hydrolysis reactor are transferred
intermittently to the biogasification reactor for continuous biogas
production. The biogasification reactor contains a concentrated
bacterial. After completing a digestion cycle, the digested straw
is removed from the hydrolysis reactor before a new batch of straw
is added.
[0081] The APS-Digester System has innovative design features that
allow it to handle the solid organic substrates effectively. The
hydrolysis reactor is operated in a batch or semi-batch mode to
ease the handling of solid materials, and the biogasification
reactor operated continuously to maintain active bacterial culture
in the system and to produce biogas at a relatively constant
level.
[0082] The operation of the system of the invention will become
apparent by reference to FIG. 2. The principle outlined herein with
reference to this figure is equally applicable to those systems
utilizing additional hydrolysis reactors.
[0083] The organic substrate is fed into the hydrolysis reactor 10
through an inlet or the top of the vessel 1. The organic material
rests on top of perforated support 25. The hydrolysis reactor
contains at least sufficient liquid to wet the organic substrate in
the hydrolysis reactor. After a period of incubation in the
hydrolysis reactor, the liquid containing the hydrolyzed organic
substrate is transferred from the hydrolysis reactor into the
biogasification reactor via first conduit 40. This transfer process
can be assisted by means of a positive drive pump located inside
the hydrolysis reactor, or a negative drive pump located inside the
biogasification reactor or along the conduit 40. The mixture in the
biogasification reactor is optionally intermittently agitated with
an agitating means 70. Following a period of incubation and
digestion in the biogasification reactor, the liquid containing the
digested organic substrate can optionally be recirculated back into
the hydrolysis reactor via second conduit 50. This recirculation
can be assisted by a pump as described above, with the caveat that
the fluid flow is in the opposite direction, thus, the pumping
direction must be similarly shifted. During the period of
incubation in the biogasification reactor, the digesting organic
substrate gives rise to a gaseous methane-containing product which
is vented through the biogasification reactor gas outlet 60. When
the methane generating potential of the organic substrate has been
exhausted, the remaining material is removed through an outlet or
the top of reactor 10.
[0084] In operation, withdrawing the liquid from the bottom of the
hydrolysis reactor prevents disturbing the organic substrate
hydrolysis process proceeding in the upper portion of the
hydrolysis reactor.
[0085] The following examples further define the invention and
should not be construed as further limiting. The contents of all
references, patents and patent applications cited throughout are
expressly incorporated herein by reference.
[0086] The detailed examples which follow illustrate the device and
methods of the invention as applied to the production of biogas
from the digestion of rice straw.
EXAMPLES
[0087] Example 1 illustrates the method and the device of the
invention in conjunction with the digestion of rice straw.
[0088] Example 2 sets forth a comparative study between the device
and the method of the invention and other art recognized methods of
digesting organic substrates. Similar to Example 1, Example 2
utilizes rice straw as an exemplary organic substrate.
Example 1
[0089] 1.1 Experimental Materials and Methods
[0090] Two laboratory scale APS-Digester Systems were used for this
study. One system had one hydrolysis reactor and one
biogasification reactor as shown in FIG. 2, and the other had two
hydrolysis reactors and one biogasification reactor. All the
reactors were made of plexiglas with inside diameters of 4.5
inches. The total and working volumes of each reactor were 5.2 L
and 4.0 L, respectively. The rice straw was processed in batches,
i.e. the system was operated in a batch mode with a retention time
of 24 days. During the digestion, liquid was circulated
intermittently between the hydrolysis reactor and the
biogasification reactor to transport the soluble substances from
the hydrolysis reactor to the biogasification reactor. After each
batch of treatment, the residual solids were removed from the
hydrolysis reactor and a new batch of rice straw was loaded. All
the reactors were heated to a constant temperature of 35.degree. C.
with a circulated and heated water jacket. The hydrolysis reactor
was not mixed while the biogasification reactor was mixed
intermittently (1 minute every 2 hours) by a mechanical mixer.
[0091] Each reactor was connected to a gas collection bag and a
wet-tip gas meter which was used to record the daily biogas
production volume. Gas samples were taken twice a week from the
sampling port on the gas collection line of each reactor and
analyzed for the contents of methane and carbon dioxide using a Gas
Chromatograph (GC) equipped with a thermal conductivity detector
(TCD). The liquid samples were taken from each reactor and measured
for pH using a pH meter to monitor the stability of the reactors.
For each batch digestion, samples of straw before and after
digestion and samples of the reactor contents before and after
digestion were taken and analyzed vent for total solids (TS),
volatile solids (VS), and pH. The analysis procedures of TS and VS
followed the standard methods (APHA, 1992). The reductions of TS
and VS in the straw after digestion were calculated using the mass
balance method. The reductions of TS and VS, daily biogas
production, and total biogas production during the 24 day period
were used to evaluate the performance of the digester system under
different operational conditions. A total of 17 runs were conducted
including three repetitions. All the digestion runs were at a
temperature of 35.degree. C. and a retention time of 24 days. The
biogasification reactor was initially seeded with the sludge taken
from an anaerobic digester in the municipal waste water treatment
plant of Davis, Calif.
[0092] To study the changes of elemental components in the rice
straw during the anaerobic digestion, the solid and liquid samples
of three batch treatments were analyzed for various elements
including nitrogen (N), phosphorus (P), potassium (K), sulfur (S),
calcium (Ca), chloride (Cl), magnesium (Mg), silicon (Si), sodium
(Na) and carbon (C). The chemical analysis was conducted by the
analytical laboratory of Division of Agriculture and Natural
Resources (DANR) at the University of California at Davis (UC
Davis). The changes of the elemental composition of the straw after
the digestion were calculated using the mass balance method.
[0093] Effects of different pretreatment methods, including
physical (mechanical), thermal, and chemical (ammonia) treatment,
on the digestion of rice straw were investigated. The physical
pretreatment included grinding the straw into two lengths (10 mm
and 25 mm) with a hammer mill and chopping the straw into one
length (25 mm) with a cutter. Thermal treatment was conducted by
heating the straw in a pressure cooker for two hours at three
different temperatures (60.degree. C., 90.degree. C. and
110.degree. C.). Tap water was added to the straw in 6 to 1 weight
ratio prior to the thermal treatment. Chemical treatment was
carried out with 58% ammonia hydroxide solution. Only one ammonia
treatment level was used for all the digestion runs. The amount of
ammonia added to the straw for each digestion run was 2% based on
the dry weight of the straw digested. This level was determined
based on the adjustment of C/N ratio of the treated straw to around
25. This level of ammonia treatment was also found to be effective
for increasing the digestibility of rice straw in an in vitro
digestibility study of Sankat and Lauckner (1991). A list of
digestion runs operated under a combination of different
pretreatment conditions are listed in Table 1.
1TABLE 1 Pretreatment Conditions Chemical Solids Physical Size
Thermal Ammonia Loading Run of Straw Temperature (%)* Rate (g/L) 1
25 mm (ground) no treatment 2 50 2 25 mm (ground) 60.degree. C. 2
50 3 25 mm (ground) 90.degree. C. 2 50 4 25 mm (ground) 110.degree.
C. 2 50 5 10 mm (ground) 90.degree. C. 2 50 6 25 mm (ground)
90.degree. C. 2 50 7 25 mm (chopped) 90.degree. C. 2 50 8 whole
90.degree. C. 2 50 9 25 mm (ground) no treatment 2 50 10 25 mm
(chopped) no treatment 2 50 11 whole no treatment 2 50 12 25 mm
(chopped) no treatment 2 50 13 25 mm (chopped) no treatment 2 75 14
25 mm (chopped) no treatment 2 100 *Ammonia addition was % of dry
weight of rice straw loaded into the hydrolysis reactor.
[0094] Three digestion runs (3, 10 and 11) were repeated to
validate the testing procedures used in this study. After finding
the difference between the repetitions was less 5%, all the other
digestion runs were carried out as a single run for each
pretreatment condition in order to save the time for laboratory
operations.
[0095] 1.2 Results
[0096] 1.2a Characteristics of Rice Straw
[0097] Rice straw was collected in bales from a county in northern
California and transported to the laboratory. The characteristics
of raw rice straw are presented in Table 2.
2TABLE 2 C N P K H S TS VS Ash (%) (%) (%) (%) (%) (%) (%) (%) (%)
34.80 0.46 0.09 1.58 4.61 0.14 92.12 79.50 20.50 Note: The contents
of C, N, P, K, H, S, VS and ash were calculated as the percentage
of TS.
[0098] 1. 2b Effects of Thermal Pretreatment
[0099] The temperature used for pretreatment did have a significant
effect on the digestibility of rice straw as shown in Table 3 with
regards to solids (TS and VS) reduction and biogas production. A
higher temperature resulted in higher conversion rates of solids
and higher biogas production. As compared with non-pretreatment,
the TS and VS reductions were increased by 3.4 22.4% and 3.6-22.6%,
respectively, and the biogas yield increased by 2.5-17.5% when
pretreatment temperature varied from 60.degree. C. to 110.degree.
C. The temperature effect was not linear, however. The increase of
solids reduction (15.6%) and biogas production was more significant
when the temperature increased from 60.degree. C. to 90.degree.
C.
[0100] The positive temperature effect may be explained by the
increased chemical reaction rate between the components of rice
straw and ammonia hydroxide which was added prior to heating. More
soluble compounds were released from the straw during the thermal
treatment process at higher temperatures and made available to
subsequent bacterial degradation. This is clearly reflected by the
daily biogas production data as shown in FIG. 3. A higher
pretreatment temperature resulted in higher daily biogas production
rate during the first six days of digestion. FIG. 4 shows the
accumulative biogas production for different pretreatment
temperatures. Higher pretreatment temperatures resulted in more
acid production and lower pH levels initially in the hydrolysis
reactor as shown in FIG. 5. The initial pH was below 6.0 for
90.degree. C. and 110.degree. C. pretreatment. As digestion
progressed and acids were transported to and consumed in the
biogasification reactor, the pH of the hydrolysis reactor was
slowly increased to the neutral level (around 7.0). The pH level of
the biogasification reactor for all pretreatment temperatures was
maintained relatively constant throughout the digestion as shown in
FIG. 6. Therefore, the biogasification reactor provided both
chemical and biological buffering capacities for the hydrolysis
reactor, making the digester system stable in operation.
3TABLE 3 Methane TS VS Content Pretreatment Reduc- Reduc- Total
Biogas Biogas of Temperature tion tion Production Yield] Biogas
(.degree. C.) (%) (%) (L) (L/gVS fed) (%) No treatment 40.6 48.4
63.5 0.40 49.4 60.degree. C. 44.0 52.0 65.3 0.41 49.9 90.degree. C.
59.6 67.6 74.2 0.46 51.4 110.degree. C. 63.0 71.0 75.4 0.47
52.1
[0101] From the biogas production data as shown in FIGS. 3 and 4,
we can see that the digestion process slowed down after two weeks
when the hydrolysis of straw and release of soluble sugars became
the limiting step. About 75-80% of the biogas was produced in the
first two weeks. This implies that if the retention time for a
digestion system is designed to be 14 days instead of 24 days, the
digester size can be reduced by 42% for a sacrifice of 21-25%
biogas production.
[0102] 1.2c Effects of Physical Pretreatment
[0103] Table 4 shows the effects of size reduction of rice straw by
mechanical processing (grinding or chopping) on the solids
reduction and biogas production. Generally speaking, the smaller
the straw particles were, the better the digestion was, i.e. the
more solids reduction was and the higher the biogas yield was.
Grinding yielded the best digestion results, because milling broke
up the cell walls of straw better than chopping alone and made the
inside of the straw more accessible for chemical and biological
breakdown. Such effects of size reduction are clearly shown in the
daily biogas production data (FIG. 7). More soluble sugars were
available in the reactors during the initial nine days, yielding a
higher biogas production rate, if the straw was processed into
smaller particles.
[0104] Size reduction appears to have more significant effects when
combined with thermal pretreatment than without thermal
pretreatment. The biogas yield of ground, 10 mm, thermally
pretreated straw was 0.47 L/g VS fed, which is 17.5% higher than
the yield of thermally pretreated whole straw (0.40 L/gVS fed). If
comparing the ground straw with the chopped straw, we noticed that
the biogas yield of ground, 25 mm, thermally pretreated straw was
0.46 L/g VS fed, 12.2% higher than chopped, 25 mm, thermally
pretreated straw.
[0105] The chopped straw was very close to the whole straw for the
digestion, showing only 2.5% increase in the biogas yield.
Improvement of digester performance by mechanical processing
(milling and chopping) was very small if the straw was not
thermally pretreated.
[0106] The digestion rates of these three straws were very close as
shown in FIG. 8. FIGS. 9 and 10 show the accumulative biogas
production of rice straw for different physical pretreatment
conditions with and without thermal pretreatment.
4TABLE 4 Thermal Biogas Methane Physical Pretreat- TS VS Yield
Content Pretreatment ment Reduction Reduction (L/gVS of Biogas
(Size of straw) (.degree. C.) (%) (%) fed) (%) 10 mm 62.4 69.6 0.47
51.1 (ground) 25 mm 90 59.6 67.6 0.46 50.6 (ground) 25 mm 44.8 60.0
0.41 50.1 (chopped) Whole 43.0 56.4 0.40 50.0 25 mm 40.6 48.4 0.40
49.4 (ground) 25 mm None 37.3 43.8 0.38 50.0 (chopped) Whole 36.3
42.4 0.38 50.5
[0107] 1.2d Effects of Total Solids Loading Rate
[0108] All the results reported as above were obtained from the
digestion runs with the same total solids (TS) loading rate in the
hydrolysis reactor of 50 g/L. Potential of increasing the solids
loading rate was investigated with three levels of TS loading rate,
50 g/L, 75g/L and 100 g/L, with the chopped, 25 mm, not thermally
pretreated straw. A higher solids loading rate means a smaller
digester system for treating a given amount of rice straw.
[0109] Table 5 shows the solids reduction and biogas production for
three solids loading rates. The digester system performed better at
a higher loading rate. When the loading rate was increased from 50
g/L to 100 g/L, the solids reduction and biogas yield increased by
about 10%. The initial concentration of bacterial mass as measured
by the mixed liquor volatile suspended solids (MLVSS) in the
biogasification reactor was controlled at 1.2% for all the three
loading rates. A higher biogas yield at a higher loading rate means
that the capacity of bacteria in the reactors was better utilized
at a higher loading rate. Future research will study the optimum
food to microorganism ratio (F/M) in the system for different
solids loading rates.
5TABLE 5 Methane TS Loading Content of Rate TS Reduction VS
Reduction Biogas Yield Biogas (g/L) (%) (%) (L/g VS fed) (%) 50
35.8 43.8 0.38 50.0 75 37.3 44.9 0.39 49.4 100 40.1 48.4 0.42
50.5
[0110] 1.2e Changes of Elemental Composition of Rice Straw During
Anaerobic Digestion
[0111] Table 6 lists the contents of elemental components in rice
straw before and after digestion as obtained from the digestion run
with ground, 25 mm straw thermally pretreated at 60.degree. C. The
contents of both N and P in the rice straw increased after the
digestion. The N content of straw residue from the digester was
twice as much as the N content of raw straw. This is beneficial for
use of such straw residues as soil amendment because of increased
nutrient contents and reduced carbon contents as compared with raw
rice straw. The contents of all the other elements as listed in the
table decreased after the digestion. The contents of K, Cl, and S
were reduced by 90%, 87%, and 43%, respectively. These three
elements, together with silicon (Si) are the major problematic
elements for combustion of rice straw, causing slagging and fouling
of the boilers (Jenkins, B. M. et al. Biomass and Bioenergy
10(4):177-200 (1995)). Reduction of these elements through
anaerobic digestion will make the straw residues a more desirable
biomass fuel for combustion. Preliminary results of combustion
tests with the straw residue showed that the residue was combusted
successfully without causing fouling problems even when the
combustion temperature reached 1600.degree. C., as compared to the
fact that raw rice straw usually starts to cause the fouling
problems at 1400.degree. C. (Jenkins, B. M., Net Energy Analysis,
EBS 216 Class Handout, University of California at Davis (1997)).
The average heating value of the residues tested was 14.44 MJ/Kg,
as compared to 14.75 MJ/Kg for raw rice straw.
6TABLE 6 Rice N P K Ca Cl Mg S Na C straw (%) (%) (%) (%) (%) (%)
(%) (%) (%) Before 0.457 0.09 1.58 0.24 0.87 0.21 0.028 0.023 34.8
diges- tion After 0.947 0.11 0.16 0.28 0.11 0.14 0.016 0.017 32.7
diges- tion Note: The reported content is percentage of dry matter
(total solids).
[0112] 1.2f Operation of APS-Digester System for Continuous Biogas
Production
[0113] Batch digestion tests reported as above have shown the
feasibility of using the APS-Digester for biogasification of rice
straw with a supplement of nitrogen source, such as ammonia. Batch
digestion is featured with cyclic biogas production. In practical
applications, the APS-Digester system may be designed to use more
than one hydrolysis reactor to couple with the biogasification
reactor so that the space of biogasification reactor can be
utilized more efficiently and the biogas production can be
maintained at a relatively constant level, which is normally
required by the operation of an engine-generator system for
electrical power generation. From the daily biogas production data
of batch digestion runs (FIGS. 3, 7, 8 and 11), we can see that the
digestion rate of each batch of rice straw was slowed down after
about 14 days. Introducing a new batch of feedstock at this time
will sustain the biological activities in the digestion system,
especially in the biogasification reactor, so to keep the biogas
production continuously at a high level. FIG. 13 shows the biogas
production of a prototype APS-Digester System with two hydrolysis
reactors and one biogasification reactor. The system was operated
for 47 days and three batches of rice straw was digested. The straw
was chopped and 25 mm long without thermal pretreatment. We can see
that cyclic variation of biogas production was much damped and the
biogas production was continuous. With proper design of the
operational schemes in terms of feedstock loading and unloading and
retention time, the APS-Digester system will become a viable and
highly efficient anaerobic digestion system for biogasification of
biomass materials such as rice straw.
Example 2
[0114] 2.1 Materials and Methods
[0115] 2.a General
[0116] Two sets of experiments were designed to compare Batch
system with the APS-Digester system using single-batch digestion
and to compare SEBAC system with the APS-Digester system using
multiple-batch digestion, respectively. The schematic diagrams of
three digestion systems used are shown in FIGS. 14 and 15. The
engineering features and operational procedures of individual
systems are described as follows.
[0117] 2.1b Description and Operation of, Anaerobic Digestion
Systems
[0118] All the anaerobic reactors used were made of plexiglass and
had a total and working volume of 5.2 and 4.0 L each, respectively.
All the reactors were maintained at 35.+-.1.degree. C. using heated
circulating water jackets. Each reactor was connected to a gas
collection bag and a wet-tip gas meter, which measured the biogas
production (L) per day. Ammonia hydroxide solution (58%) was added
to rice straw for all the digestion runs to adjust the C/N of rice
straw 25 prior to digestion. Duplicative tests were performed for
all the digestion runs. The data reported in this paper are the
average of duplicate test runs.
[0119] The batch system was operated as a single-batch digestion
system. Three different TS loading levels of 50g/L. 75 g/L. and 100
g/L were tested. The corresponding amount of dry straw used 200 g.
300 g. and 400 g. respectively. The TS loading was defined as the
amount of dry rice straw (g) loaded per unit working volume (L) of
hydrolysis reactor. For each batch digestion, rice straw (chopped
into 1-inch length) was mixed with anaerobic seed sludge collected
from a mesophilic digester as the municipal wastewater plant in
Davis, Calif. The amount of the seed sludge used was determined to
provide biomass equal to 40% of volatile solids (VS) in 200 g of
rice straw on a dry weight basis. Water was added to the reactor to
achieve the final TS concentrations of 5%, 7.5%, and 10% for 50
g/L, 75 g/L, and 100 g/L TS loading levels, respectively. Each
batch digestion proceeded for 24 days.
[0120] The SEBAC system (FIG. 14-b) was operated as a
multiple-batch digestion system. Both reactors in the system are
solid-bed reactors with a perforated steel plate placed in the
lower part of each reactor to allow liquid collection at the bottom
of the reactor. The first batch digestion was started with the
mixture of rice straw and anaerobic seed sludge in the same way as
with the Batch system described above. After 12 days, when the
digestion process in the first batch was established, the second
batch digester was started with the mixture of rice straw and water
which had a TS concentration of 7.5%. Intermittent liquid
circulation (one minute for every two hours) between two reactors
at a constant flow rate of 600 mL/min was initiated as soon as the
second batch was started to allow the inoculum to transfer. After
24 days, the first batch digestion was finished. The residual
solids were taken out and the third batch was carried out in the
same way as the second batch. A total of three batches were
monitored using a digestion period of 42 days. The second and third
batches of digestion were assumed to represent typical operation of
a SEBAC system. The laboratory set up of the SEBAC system is
presented in FIG. 14-b.
[0121] Two types of the APS-Digester system were used. The first
system (FIG. 15-a) had one hydrolysis reactor and one
biogasification reactor. The first system was used to compare with
the Batch system and therefore operated as a single-batch digestion
system. The second system was used to compare with the SEBAS system
and operated as a multiple-batch digestion system. A perforated
steel place was placed in the lower part of each hydrolysis reactor
to allow the liquid collection. With the first system, the
biogasification reactor was initially seeded with anaerobic seed
sludge to provide the Mixed Liquid Volatile Suspended Solid (MLVSS)
of 11,000 mg/L. The hydrolysis reactor was started with rice straw
and water. Liquid was recirculated between the two reactors once
every two hours at a constant flow rate of 600 mL/min. Right after
recirculation, the biogasification reactor was mixed for one minute
and then allowed to react quiescently with biomass settled to the
bottom prior to next recirculation. Three TS loading levels of 50
g/L, 75 g/L and 100 g/L were tested. The second system--the
multiple-batch APS-Digester system was started in the same way as
the single-batch APS-Digester system described above. After 12 days
of operation with the first batch system, the second hydrolysis
reactor loaded with rice straw and water was put in line. The
liquid recirculation and reactor mixing sequence between the second
hydrolysis reactor and the biogasification reactor was the same as
in the first system but with one-hour delay. A TS loading level of
75 g/L was used in the second system to compare with the SEBAC
system. The system operation was monitored for the same length of
time (42 days) as with the SEBAC system.
[0122] Finally, computer simulation was performed for a model
APS-Digester system with a capacity of processing one ton of dry
straw per day to study the variation of daily biogas production as
affected by the number of hydrolysis reactors. One biogasification
reactor was coupled with different numbers of hydrolysis reactors
(one, two, three, four, six, eight, and twelve). The daily biogas
production data from the laboratory test with the TS loading level
of 100 g/L were used in the simulation. Each system was stimulated
for a period of four months with a retention time of 24 days for
each batch digestion. For each simulation, hydrolysis reactors were
started in sequence. For example, for the system with one
biogasification reactor coupled with eight-hydrolysis reactors, the
batch digestion in the hydrolysis reactors was three days apart in
schedule. The daily biogas production (L/day) was calculated for
each simulation.
[0123] 2.1c Analytical Procedure
[0124] Gas samples were taken daily from the sampling port in the
gas collection line of each reactor and analyzed for the contents
of methane (CH.sub.4 and carbon dioxide (CO.sub.2) using a Gas
Chromatography (GC) equipped with a thermal conductivity detector
(TCD). The liquid samples were taken from each reactor and measured
for pH using a pH meter. Before and after the digestion, both
liquid and solid samples from each reactor were taken to analyze
for TS and VS concentrations. The reductions of TS and VS for each
treatment were calculated based on mass balances. The analysis
procedures of TS and VS followed the standard methods (APHA,
1992).
[0125] 2.2 Results
[0126] 2.2a General
[0127] The rice straw used in this study was collected in bales in
northern California and transported to the laboratory. The
characteristics of the rice straw as determined from three
replicates are presented in Table 7. The C/N of rice straw was 76.
Ammonia was therefore added to adjust the C/N to 25, which was
found to be the optimum level for anaerobic digestion (Hills and
Roberts, 1981**).
7TABLE 7 C N P K H S TS VS Ash (%) (%) (%) (%) (%) (%) (%) (%) (%)
34.81 0.46 0.09 1.58 4.61 0.14 92.12 79.50 20.50 .+-.0.44 .+-.0.021
.+-.0.008 .+-.0.024 .+-.0.05 .+-.0.01 .+-.0.89 .+-.0.45 .+-.0.21
Note: The contents of C, N, P, K, H, S, VS, and Ash are expressed
as the percentage of TS.
[0128] 2.2b Comparison of the APS-Digester with the Batch
System
[0129] The daily and cumulative biogas production, methane contents
of biogas, and pH variation for both APS-Digester and batch systems
are presented in FIGS. 16-19. The average methane yield, methane
content of biogas, and reductions of TS and VS are presented in
Table 8. The methane yield with the APS-Digester system increased
from 0.38 to 0.42 L/g VS added with the TS loading was increased
from 50 to 100 g/L whereas the methane yield with the batch system
decreased from 0.37 to 0.05 L/g VS added. The increase of the
methane yield with the APS-Digester system could be explained by
the ability of methanogenic bacteria in the biogasification reactor
to handle a higher organic loading level. The decease of the
methane yield with the batch system might be caused by the
excessive accumulation of VFSs, leading to the rapid drop of pH to
a level (below 6.0) that became inhibitory to the methanogenic
bacteria. Therefore, the APS-Digester system showed advantages over
the batch system by having higher TS and VS reductions, higher
methane content of the biogas and smaller variation of pH during
digestion.
[0130] With the batch system, it should also be noticed that the
daily biogas production rapidly increased shortly after the
digestion was initiated and reached to the maximum on the fourth
day for all three TS loading levels. The biogas produced during the
first four days was essentially carbon dioxide (CO.sub.2). This
indicates that soluble sugars were released quickly during these
initial period. The acetogenic bacteria were responsible for the
acid and CO.sub.2 production and pH decrease. When the TS loading
became too high, such as at 75 g/L, accumulation of VFAs in the
system lead to inhibition of methanogenic bacteria, resulting in
reduced or stopped biogas production. The results showed that the
TS loading in the batch system should be limited to 50 g/L. In
contrast, the APS-Digester did not show VFA inhibition even at the
highest loading level (100 g/L) tested. This was reflected by the
daily and cumulative biogas production as shown in FIGS. 16 and 17.
At higher loading levels (75 g/L and 100 g/L), the daily biogas
production in the APS-Digester system was much higher during the
first several days than in the batch system. The methane content of
the biogas was also much higher.
8TABLE 8 Digester System APS Batch Total Solid Loading (g/L) 50 75
100 50 75 100 Methane Yield (L/g VS 0.38 0.38 0.42 0.37 0.19 0.05
added) Methane Content in Biogas 50.10 49.14 50.60 41.45 37.63
27.72 (%) Total Solid Reduction (%) 37.48 36.59 40.67 35.88 16.56
5.33 Volatile Solids Reduction (%) 43.18 44.28 49.14 47.66 22.51
8.01
[0131] 2.2c Comparison of the APS-Digester with SEBAC Systems
[0132] The daily and cumulative biogas production, methane content
of biogas, and pH variation during digestion are presented in FIGS.
20-22. The pH of the APS-Digester system was measured to be the pH
in the biogasification reactor. The average methane yield, methane
content, and reductions of TS and VS are presented in Table 9. The
two systems achieved similar methane yield and reductions of TS and
VS. However, the biogas produced from the APS-Digester system had
higher methane content (50.22% on average) than the biogas from the
SEBAC biogas production throughout digestion. This is because the
biogasification reactor in the APS-Digester system provided
buffering capacity for the system and better environmental
conditions to methanogenic bacteria, resulting in higher methane
production. Therefore the APS-Digester system is found to be more
advantageous than the SEBAC system in terms of methane production
and process stability.
9 TABLE 9 Digester Performance APS SEBAC Total Solid Loading (g/L)
75.00 75.0 Methane Yield (L/g VS added) 0.34 0.35 Methane Content
(%) 50.22 40.78 Total Solid Reduction (%) 35.66 36.21 Volatile
Solids Reduction (%) 40.68 41.08
[0133] 2.2d Computer Simulation of the APS-Digester System for Best
Design Configurations
[0134] Computer simulation was conducted to analyze the biogas
production profile of the APS-Digester system with one
biogasification reactor coupled with different numbers of
hydrolysis reactors. The predicted daily biogas production with one
biogasification reactor coupled with one, two, three, four, six,
eight, and twelve hydrolysis reactors are presented in FIGS. 23-24
and the predicted average daily biogas production and its variance
are presented in Table 10. The variation of daily biogas production
became smaller with the increase of the number of hydrolysis
reactors. With the processing capacity of 1 ton/day, the daily
biogas production was 365 m.sup.3/day for all the combinations
after a start-up period of 24 days. However, the variations of
daily biogas production decreased from 17.77% to 4.05% and 0.14%
when the numbers of hydrolysis reactors were increased from two to
eight and twelve, respectively. The least variation in daily biogas
production was achieved with one biogasification reactor coupled
with twelve hydrolysis reactors.
10TABLE 10 Num. of Hydrolysis Reactors 1 2 3 4 6 8 12 Ave. Daily
Biogas 365.00 365.00 365.00 365.00 365.00 365.00 365.00
(m.sup.3/day) Standard Deviation (m.sup.3) .+-.179.82 .+-.64.86
.+-.38.23 .+-.24.06 .+-.20.09 .+-.14.78 .+-.0.50 Std. Dev./Average
(%) 49.27 17.77 10.47 6.59 5.50 4.05 0.14
[0135] It is to be understood that the above description and is
intended to be illustrative and not restrictive. Many embodiments
will be apparent to those of skill in the art upon reading the
above description. The scope of the invention should, therefore, be
determined not with reference to the above description, but should
instead be determined with reference to the appended claims, along
with the full scope of equivalents to which the claims are
entitled. The disclosures of all articles and references, including
patent applications and publications are incorporated herein by
reference
* * * * *