U.S. patent application number 10/161387 was filed with the patent office on 2002-12-19 for anaerobic digestion apparatus methods for anaerobic digestion and for minimizing the use of inhibitory polymers in digestion.
This patent application is currently assigned to Biothane Corporation. Invention is credited to Lanting, Jelte, Murphy, John L. III.
Application Number | 20020192809 10/161387 |
Document ID | / |
Family ID | 23135007 |
Filed Date | 2002-12-19 |
United States Patent
Application |
20020192809 |
Kind Code |
A1 |
Lanting, Jelte ; et
al. |
December 19, 2002 |
Anaerobic digestion apparatus methods for anaerobic digestion and
for minimizing the use of inhibitory polymers in digestion
Abstract
The invention includes an anaerobic solids digestion apparatus
comprising a digester, at least one draft tube; at least one nozzle
and a biogas source; a method for digesting a waste stream in an
anaerobic solids digestion apparatus comprises feeding a waste
stream to a digester; reacting the anaerobically biodegradable
material in the waste stream with anaerobic bacteria in the
digester; introducing a mixed liquor into the digester and mixing
the mixed liquor; and a method for minimizing the use of inhibitory
polymers by concurrently digesting and concentrating the mixed
liquor in the digester.
Inventors: |
Lanting, Jelte; (Sewell,
NJ) ; Murphy, John L. III; (Voorhees, NJ) |
Correspondence
Address: |
AKIN, GUMP, STRAUSS, HAUER & FELD, L.L.P.
ONE COMMERCE SQUARE, SUITE 2200
2005 MARKET STREET
PHILADELPHIA
PA
19103
US
|
Assignee: |
Biothane Corporation
Camden
NJ
|
Family ID: |
23135007 |
Appl. No.: |
10/161387 |
Filed: |
May 31, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60294805 |
May 31, 2001 |
|
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|
Current U.S.
Class: |
435/290.1 |
Current CPC
Class: |
C02F 11/04 20130101;
Y02W 10/10 20150501; C12M 27/20 20130101; Y02W 10/20 20150501; C02F
3/2853 20130101; C12M 29/18 20130101; C02F 2209/06 20130101; Y02E
50/30 20130101; C12M 27/24 20130101; Y02W 10/12 20150501; C02F
3/282 20130101; C12M 21/04 20130101; Y02W 10/23 20150501; C12M
29/24 20130101; Y02E 50/343 20130101; C02F 3/2873 20130101; C02F
2301/106 20130101; C12M 27/00 20130101 |
Class at
Publication: |
435/290.1 |
International
Class: |
C12M 001/00 |
Claims
We claim:
1. An anaerobic solids digestion apparatus comprising: (a) a
digester; (b) a mixing device in the digester capable of directing
a flow of a mixed liquor within the digester; and (c) a shearing
device in communication with a mixed liquor inlet to the digester,
the shearing device being capable of imparting shear to a mixed
liquor within the digester.
2. The anaerobic solids digestion apparatus according to claim 1,
wherein the mixing device comprises a draft tube positioned in the
digester and having an upper inlet and a lower outlet.
3. The anaerobic solids digestion apparatus according to claim 2,
wherein the draft tube has a length measured along a longitudinal
axis of the draft tube of about 50% to about 90% of the digester
liquid depth measured along a longitudinal axis of the
digester.
4. The anaerobic solids digestion apparatus according to claim 2,
further comprising an deflector plate below the lower outlet of the
draft tube and positioned to at least partially block gas from the
lower outlet of the draft tube from entering a liquor outlet of the
digester.
5. The anaerobic solids digestion apparatus according to claim 1,
wherein the shearing device comprises a shearing nozzle.
6. The anaerobic solids digestion apparatus according to claim 5,
wherein the nozzle comprises a gas inlet, a liquid inlet, an
outlet, an interior surface, and a gas tube having an exterior
surface, the gas tube extending from the nozzle gas inlet to the
nozzle outlet, wherein a generally annular space is defined by the
exterior surface of the gas tube and the interior surface of the
nozzle.
7. The anaerobic solids digestion apparatus according to claim 6,
further comprising a biogas source in communication with the gas
inlet of the nozzle.
8. The anaerobic solids digestion apparatus according to claim 7,
wherein the biogas source is a biogas recycle system capable of
removing biogas from a biogas collection area in the digester and
directing it to the gas inlet of the nozzle.
9. The anaerobic solids digestion apparatus according to claim 8,
wherein the biogas recycle system comprises a conduit having a
first end forming an outlet for biogas removed from the gas
collection area and a second end in communication with the gas
inlet of the nozzle.
10. The anaerobic solids digestion apparatus according to claim 7,
wherein the biogas source includes methane and carbon dioxide.
11. The anaerobic solids digestion apparatus according to claim 10,
wherein the biogas source includes nitrogen.
12. The anaerobic solids digestion apparatus according to claim 10,
wherein the biogas source includes a small amount of oxygen
sufficient to modulate the oxidation-reduction potential (ORP) of
the mixed liquor.
13. The anaerobic solids digestion apparatus according to claim 5,
further comprising a recirculation system, wherein the nozzle
further comprises a liquid inlet and the recirculation system is in
fluid communication with a mixed liquor recirculation outlet of the
digester capable of recirculating the mixed liquor from the mixed
liquor recirculation outlet of the digester to the liquid inlet of
the nozzle.
14. The anaerobic solids digestion apparatus according to claim 13,
wherein the recirculation system comprises a pump and a first
conduit having a first end connected to the pump and a second end
in fluid communication with the liquid inlet of the nozzle and a
second conduit with a first end connected to the mixed liquor
recirculation outlet and a second end connected to an inlet of the
pump.
15. The anaerobic solids digestion apparatus according to claim 13,
wherein the recirculation system is capable of providing a liquid
circulation within the digester.
16. The anaerobic solids digestion apparatus according to claim 13,
wherein the recirculation system further comprises a temperature
control mechanism.
17. The anaerobic solids digestion apparatus according to claim 16,
wherein the temperature control mechanism is a heat exchanger.
18. The anaerobic solids digestion apparatus according to claim 16,
wherein the temperature control mechanism is a steam injector.
19. The anaerobic solids digestion apparatus according to claim 1,
further comprising a concentrator in fluid communication with the
mixed liquor inlet of the digester and at least one mixed liquor
outlet of the digester.
20. The anaerobic solids digestion apparatus according to claim 19,
wherein the concentrator comprises a pump and a separator having an
outlet in fluid communication with a the mixed liquor inlet of the
digester.
21. The anaerobic solids digestion apparatus according to claim 20,
wherein the concentrator further comprises a conduit with a first
end in fluid communication with the mixed liquor inlet of the
digester and a second end in fluid communication with an outlet of
the separator.
22. The anaerobic solids digestion apparatus according to claim 20,
wherein the separator is a membrane separator.
23. The anaerobic solids digestion apparatus according to claim 1,
wherein the digester comprises a gas de-entrainment zone in
communication with a mixed liquor outlet of the digester.
24. The anaerobic solids digestion apparatus according to claim 1,
wherein the digester has a solids retention time of about 2 to
about 20 days.
25. The anaerobic solids digestion apparatus according to claim 24,
wherein the digester has a solids retention time of about 6 to
about 12 days.
26. The anaerobic solids digestion apparatus according to claim 1,
further comprising a pH adjustment system (capable of maintaining
the mixed liquor in the digester at a pH of about 6 to about 8.
27. An anaerobic solids digestion apparatus comprising: (a) a
digester comprising a shear source capable of imparting shear to a
mixed liquor within the digester; and (b) a concentrator in fluid
communication with a mixed liquor inlet of the digester and at
least one mixed liquor outlet of the digester, wherein the
concentrator and digester are configured to allow for concurrent
concentration and digestion of a mixed liquor.
28. The anaerobic solids digestion apparatus according to claim 27,
further comprising a draft tube comprising an upper inlet and a
lower outlet and positioned in the digester, wherein the draft tube
is capable of directing a flow of a mixed liquor.
29. The anaerobic solids digestion apparatus according to claim 27,
wherein the shear source is a shearing nozzle for introducing mixed
liquor into the digester.
30. The anaerobic solids digestion apparatus according to claim 29,
wherein the nozzle comprises a gas inlet, a liquid inlet, an
outlet, and an interior surface, the nozzle further comprising a
gas tube having an exterior surface, the gas tube extending from
the nozzle gas inlet to the nozzle outlet, wherein a generally
annular space is defined by the exterior surface of the gas tube
and the interior surface of the nozzle.
31. The anaerobic solids digestion apparatus according to claim 29,
further comprising a biogas source, wherein the nozzle further
comprises a gas inlet and the biogas source is a biogas recycle
system capable of removing biogas from a biogas collection area in
the digester and directing it the gas inlet of the nozzle.
32. The anaerobic solids digestion apparatus according to claim 27,
further comprising a biogas source.
33. The anaerobic solids digestion apparatus according to claim 27,
wherein the concentrator comprises a pump and a separator having an
outlet in fluid communication with a mixed liquor inlet of the
digester.
34. The anaerobic solids digestion apparatus according to claim 33,
wherein the concentrator further comprises a conduit with a first
end in fluid communication with the mixed liquor inlet of the
digester and a second end in fluid communication with an outlet of
the separator.
35. The anaerobic solids digestion apparatus according to claim 33,
wherein the separator is a membrane separator.
36. An anaerobic solids digestion apparatus comprising: (a) a
digester; (b) at least one draft tube positioned in the digester
and capable of directing a flow of a mixed liquor and comprising an
upper inlet and a lower outlet; (c) at least one nozzle comprising
a gas inlet, a liquid inlet, an outlet and an interior surface, the
nozzle further comprising a gas tube having an exterior surface,
the tube extending from the nozzle gas inlet to the nozzle outlet,
wherein a generally annular space is defined between the exterior
surface of the gas tube and the interior surface of the nozzle; and
(d) a biogas source in communication with the gas inlet of the
nozzle.
37. A method for digesting a waste stream in an anaerobic solids
digestion apparatus, the method comprising: (a) feeding a waste
stream comprising anaerobically biodegradable solids to a digester;
(b) reacting the anaerobically biodegradable solids in the waste
stream with anaerobic bacteria in the digester to reduce an amount
of the biodegradable solids, thereby producing a mixed liquor and a
biogas; (c) introducing a mixed liquor to the digester through a
shearing device; and (d) mixing the mixed liquor within the
digester.
38. The method according to claim,37, wherein the shearing device
comprises a nozzle, wherein the mixed liquor flows through the
nozzle in a generally annular space defined by an exterior surface
of a gas flow tube positioned within the nozzle and an interior
surface of the nozzle.
39. The method according to claim 37, further comprising removing a
portion of the biogas from the digester and introducing the portion
of the biogas into a nozzle gas inlet.
40. The method according to claim 37, further comprising
introducing the portion of the biogas and mixed liquor from the
nozzle into a draft tube within the digester and mixing the portion
of the biogas and mixed liquor, thereby inducing internal
circulation within the digester as the biogas and mixed liquor flow
downwardly through the draft tube.
41. The method according to claim 37, further comprising
concentrating the mixed liquor.
42. The method according to claim 41, wherein the steps of reacting
the anaerobically biodegradable solids and concentrating the mixed
liquor occur concurrently.
43. The method according to claim 37, wherein the method minimizes
the need for use of a polymer that inhibits biological activity in
a waste stream.
44. The method according to claim 37, further comprising
maintaining a mixed liquor pH of about 6 to about 8.
45. The method according to claim 37, further comprising
maintaining a mixed liquor temperature of about 80.degree. F.
(25.degree. C.) to about 105.degree. F. (40.degree. C.).
46. The method according to claim 37, further comprising
maintaining a mixed liquor temperature of about 125.degree. F.
(50.degree. C.) to about 145.degree. F. (60.degree. C.).
47. The method according to claim 37, further comprising minimizing
the entrainment of gas bubbles in the recirculation system.
48. The method according to claim 37, further comprising venting
biogas from the gas collection area of the digester.
49. The method according to claim 37, further comprising minimizing
foam in the digester.
50. The method according to claim 37, further comprising removing
scum from the digester.
51. A method for improving the efficiency of an anaerobic solids
digestion apparatus comprising: a) feeding a waste stream
comprising anaerobically biodegradable solids to a digester; b)
reacting the anaerobically biodegradable solids in the waste stream
with anaerobic bacteria in the digester to reduce an amount of the
biodegradable solids, thereby producing a mixed liquor and a
biogas; c) imparting a shearing force to the mixed liquor in the
digester; and d) concentrating the mixed liquor, wherein the steps
of reacting the anaerobically biodegradable solids and
concentrating the mixed liquor occur concurrently.
52. A method for minimizing the need for use of a polymer that
inhibits biological activity in a waste stream in a digestion
apparatus, the method comprising: (a) feeding a waste stream to a
digester, wherein a portion of the waste stream is biodegradable;
(b) reacting the biodegradable portion in the waste stream with
bacteria in the digester to produce a mixed liquor and gas; and (c)
concentrating the mixed liquor with a membrane separator, wherein
the steps of reacting the biodegradable portion in the waste stream
and concentrating the mixed liquor occur concurrently.
53. The method according to claim 52, wherein the waste stream
comprises anaerobically biodegradable solids, and the method
further requires reacting the biodegradable solids in the waste
stream with anaerobic bacteria in the digester to produce a mixed
liquor and a biogas, and wherein the steps of reacting the
anaerobically biodegradable solids an concentrating the mixed
liquor occur concurrently.
54. The method according to claim 52, wherein the need for use of a
polymer that inhibits biological activity in a waste stream in an
anaerobic solids digestion apparatus is eliminated.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 60/294,805, entitled "Shear Enhanced
Anaerobic Digestion Apparatus," filed May 31, 2001. The entire
disclosure of U.S. Provisional Patent Application No. 60/294,805 as
filed is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] The invention relates to anaerobic biological treatment of
waste streams with high solids content. Anaerobic biological
treatment has traditionally been applied to the digestion of
primary and secondary sludge at municipal sewage treatment
facilities, but is also applicable to municipal solid waste,
agricultural manures and crop residues, or industrial solid wastes
and slurries where a significant portion of the solids material is
potentially biodegradable.
[0003] Anaerobic digestion of municipal sludge has been performed
for decades to reduce volume, stabilize highly-putrescible material
and destroy pathogens. Conventional digestion is a once-through
process where the sludge resides in the digester for 20 to 40 days
to achieve optimal digestion. This is expressed as solids retention
time (SRT) which in a once-through system is equal to the hydraulic
retention time (HRT). SRT represents the average time that solids
reside in the digester, and HRT represents the average time that
liquids reside in the digester. In order to optimize the digestion
process and to reduce the size of the digester vessel, there is a
need for an improved digestion method that can operate effectively
at a reduced SRT.
[0004] One problem associated with municipal sludge digestion is
the large volume required for the anaerobic digester. Concentrating
the solids in municipal sludge upstream of the digester has been
used for reducing the digester volume. Even though municipal sludge
is relatively high in suspended solids compared to many industrial
wastewaters, typically approximately 99% of the municipal sludge
may be water. To achieve the conventional SRT, the digester must
accommodate the volume of water in the sludge. By concentrating
these sludge solids by a factor of two, the digester volume
required for digestion could be halved. Traditionally, a thickening
process has been applied upstream of the digester to increase the
percentage of solids in the feed to the digester. Traditional
methods involve mechanical thickeners, dissolved air flotation or
similar equipment to concentrate the solids.
[0005] Additives, such as polymers, have been mixed with the sludge
stream to enhance the thickening process. These polymers are known
in the art and include, for example, cationic polyacrylamides in a
water-in-oil emulsion, solution mannich polymers--nonionic
polyacrylamide polymers made cationic by reacting the amide groups
along the polyacrylamide backbone with both a dialkylamine and a
formaldehyde source, and cationic water-soluble polymers in
emulsions, for example polyamine or poly (diallyldialkylammonium
halides). The applicants have discovered that such polymers can
inhibit anaerobic biological digestion. This effect might not be
noticeable in conventional systems with long SRT and relatively low
biological activity. However, as the digestion process is optimized
and the SRT is reduced, this impact becomes more noticeable and
prevents achievement of optimal digestion performance.
[0006] Less conventional methods for thickening the sludge such as
membrane separation have also been used upstream of the digester.
However, the hydrophilic nature of the solids in the waste stream
makes it difficult to extract water efficiently using a membrane
separator and promotes fouling of membranes, a build up of
colloidal hydrophilic compounds which is difficult to penetrate and
disturb. Traditionally, this made membrane separation an
unattractive method for thickening the waste stream. Thus, in order
to optimize anaerobic sludge digestion, there is a need for an
improved method of concentrating the feed stock delivered to the
digester and eliminating the need for the above noted polymers in
optimized digesters.
[0007] Digesters for the anaerobic digestion of municipal
publicly-owned treatment works (POTW) sludge are generally large
tanks of relatively low height providing for 20-40 days of HRT.
Proper treatment of municipal POTW sludge requires a sufficient
inventory of active digesting bacteria and contact of those
bacteria with the biodegradable fraction of the sludge. Contact is
achieved by mixing digester contents. Optimally, the digester
contents are mixed thoroughly. Conventional mixing methods include
mixing by mechanical methods and mixing by using gas. However, the
large and low design of conventional digesters typically results in
"dead zones" which are not mixed and which could reach or exceed
approximately 15% of the digester.
[0008] An "egg-shaped" digester has been developed to address these
problems. This shape has improved the overall performance by
effectively approaching a 100% mixed digester volume. This digester
also requires a smaller widest cross-sectional area because it is
taller relative to the traditionally-shaped tanks noted above.
However, construction of egg-shaped digesters must overcome complex
geometry. Although they are smaller than conventional digesters,
they are still relatively large and expensive to construct. These
structures improve mixing efficiency, but remain limited by the
solids retention time (SRT) dictated by their design parameters and
the typical biological activity of a continuously stirred
system.
[0009] Accordingly, there also remains a need for improved digester
performance by exposing more surface area of the degradable
organics and available digesting bacteria to increase the
opportunity for reactions between them. One way to achieve this is
to fragment the sludge particles so as to expose degradable
organics and digesting bacteria on the interior of the particles.
These components may then be brought into contact in a high-energy
environment. This requires turbulent mixing in the digester.
[0010] One method of mixing in an anaerobic sludge digester is the
loop digester. Loop digesters have a continuous circulating flow
which may be around a draft tube configuration. A mixing method
used in the field of aerobic digestion is the concept of an eductor
nozzle immersed in a liquid filled vessel. The pressure on the
pumped side of the nozzle can be used to accelerate the flow of
liquid at the nozzle outlet thus releasing energy into the liquid
filled vessel and disturbing the vessel contents to effect mixing.
Additionally, this acceleration creates a suction effect (similar
to a Venturi) which can be used to draw a secondary fluid or gas
into the flow stream.
[0011] Eductor nozzles to fragment biological solids have been used
in the treatment of wastewaters using high rate aerobic digesters
that apply a shearing force to the mixed liquor in the digester.
With the supplemental addition of oxygen in the form of air
injection, these digesters rely on contact between wastewater and
biomass particles in an oxygen-rich environment to promote aerobic
bacterial digestion of soluble components contained in the
wastewater.
[0012] Another problem associated with municipal sludge is its
disposal. Regulatory restrictions on the disposal of sludge make it
desirable that the sludge be treated to "Class A" standards prior
to disposal. 40 CFR .sctn.503.32 proscribes EPA standards regarding
the use and disposal of sewage sludge and is incorporated herein by
reference. Accordingly there is a need for an improved digester
design that can provide for operational or process modifications
that achieve sludge which is treated to Class A standards.
BRIEF SUMMARY OF THE INVENTION
[0013] The invention includes an anaerobic solids digestion
apparatus comprising a digester; a mixing device in the digester
capable of directing a flow of a mixed liquor within the digester;
and a shearing device in communication with a mixed liquor inlet to
the digester, the shearing device being capable of imparting shear
to a mixed liquor within the digester.
[0014] The invention also includes an anaerobic solids digestion
apparatus comprising a digester comprising a shear source capable
of imparting shear to a mixed liquor within the digester and a
concentrator in fluid communication with a mixed liquor inlet of
the digester and at least one mixed liquor outlet of the digester,
wherein the concentrator and digester are configured to allow for
concurrent concentration and digestion of a mixed liquor.
[0015] The invention also includes an anaerobic solids digestion
apparatus comprising a digester; at least one draft tube positioned
in the digester and capable of directing a flow of a mixed liquor
and comprising an upper inlet and a lower outlet; at least one
nozzle comprising a gas inlet, a liquid inlet, an outlet and an
interior surface, the nozzle further comprising a gas tube having
an exterior surface, the tube extending from the nozzle gas inlet
to the nozzle outlet, wherein a generally annular space is defined
between the exterior surface of the gas tube and the interior
surface of the nozzle; and a biogas source in communication with
the gas inlet of the nozzle.
[0016] The invention additionally includes a method for digesting a
waste stream in an anaerobic solids digestion apparatus, the method
comprising feeding a waste stream comprising anaerobically
biodegradable solids to a digester; reacting the anaerobically
biodegradable solids in the waste stream with anaerobic bacteria in
the digester to reduce an amount of the biodegradable solids,
thereby producing a mixed liquor and a biogas; introducing a mixed
liquor to the digester through a shearing device; and mixing the
mixed liquor within the digester.
[0017] The invention includes a method for improving the efficiency
of an anaerobic solids digestion apparatus comprising feeding a
waste stream comprising anaerobically biodegradable solids to a
digester; reacting the anaerobically biodegradable solids in the
waste stream with anaerobic bacteria in the digester to reduce an
amount of the biodegradable solids, thereby producing a mixed
liquor and a biogas; imparting a shearing force to the mixed liquor
in the digester; and concentrating the mixed liquor, wherein the
steps of reacting the anaerobically biodegradable solids and
concentrating the mixed liquor occur concurrently.
[0018] The invention additionally includes a method for minimizing
the need for use of a polymer that inhibits biological activity in
a waste stream in a digestion apparatus, the method comprising
feeding a waste stream to a digester, wherein a portion of the
waste stream is biodegradable; reacting the biodegradable portion
in the waste stream with bacteria in the digester to produce a
mixed liquor and gas; and concentrating the mixed liquor with a
membrane separator, wherein the steps of reacting the biodegradable
portion in the waste stream and concentrating the mixed liquor
occur concurrently.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0019] The foregoing summary, as well as the following detailed
description of preferred embodiments of the invention, will be
better understood when read in conjunction with the appended
drawings. For the purpose of illustrating the invention, there is
shown in the drawings embodiments which are presently preferred. It
should be understood, however, that the invention is not limited to
the precise arrangements and instrumentalities shown.
[0020] In the drawings:
[0021] FIG. 1 is a schematic representation of the features of a
shear enhanced anaerobic digestion apparatus according to the
invention;
[0022] FIG. 2 is a schematic representation of a two-phase mixing
and shearing nozzle.
[0023] FIG. 2a is a schematic representation of a single phase
mixing and shearing nozzle.
[0024] FIG. 3 is a schematic representation of a conventional
sludge treatment process;
[0025] FIG. 4 is a schematic representation of a sludge treatment
process according to an embodiment of the invention which includes
a membrane concentrator;
[0026] FIG. 5 is a schematic representation of an anaerobic
digestion apparatus and a membrane concentrator; and
[0027] FIG. 6 is a schematic representation of an alternative
anaerobic digestion apparatus and a concentrator.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The present invention relates to an apparatus, designated
generally in the drawings as 100, and process for the anaerobic
digestion of solids in a waste stream using a shear enhanced
anaerobic digestion apparatus (SEAD). The invention also relates to
an apparatus and method for concurrently concentrating and
digesting the degradable solids fraction of a waste stream. The
invention additionally relates to a method of minimizing the need
for use of a polymer(s) that can inhibit biological activity in a
waste stream. By utilizing a preferred continuous recirculating
flow around a draft tube, mixing of the system can be achieved
without moving parts within the digester. A shearing nozzle may be
used in the apparatus to impart energy to the digester contents so
as to fracture solids particles and expose the maximum reactable
surface area. When energy is released through the nozzle, the fluid
inside the draft tube is accelerated, resulting in about a ten-fold
increase in internal flow rates compared to the recirculation flow,
as described further herein. Shearing of the particles in the waste
stream or waste slurry solids, occurs both within the nozzle, as
well as in a turbulent mixing zone at the outlet of the nozzle,
physically breaking down the solid biodegradable particles of the
waste stream or waste slurry into smaller particles, exposing more
surface area of the biodegradable solids. Biogas produced in the
anaerobic digestion can be recycled in the two-phase nozzle and
mixed with the mixed liquor. Entrained biogas is dispersed
throughout the circulating mixed liquor providing for maximum
turbulence in the mixed liquor to ensure optimal contact of the
biodegradable organics with the anaerobic bacteria. These factors
improve mass transfer rates resulting in significantly improved
reaction efficiency evidenced by reduced SRT.
[0029] A fraction of the digester contents can be concentrated by
using membrane separation or other conventional concentration
process in combination, and preferably concurrently, with the
digestion apparatus. A concentrator is in fluid communication with
an inlet and an outlet of the digester, so that the concentrator
and digester may operate concurrently. As digestion acts on the
degradable waste material in the digester, the hydrophilic
components are being digested, and the digester contents are more
easily concentrated. Because the apparatus can work alongside a
concentrator, they can be operated concurrently and the thickening
of the waste is significantly improved. Thus, the design of the
apparatus including a concentrator as described herein enhances
separation performance over conventional concentration processes
with the result that the feed stock to the digester is effectively
concentrated. Additionally, the performance of the digester is
enhanced because the concentration can be performed without the
need for the use of polymers that can inhibit biological activity
or other additives that can negatively impact an optimized digester
such as the digester described herein. Applicants have additionally
discovered that concurrently concentrating while digesting the feed
stock for other types of digesters, including, for example,
non-shear enhanced anaerobic digesters, egg-shaped anaerobic
digesters, and aerobic digesters, can also minimize the need for
additives, such as thickening polymers that may act to inhibit
biological activity in those digesters when optimized.
[0030] In general, waste streams to be treated according to the
various embodiments and aspects of the present invention may be any
waste streams containing material that is at least partially
biodegradable by anaerobic or aerobic bacteria. However, the
principle aspects of the invention are directed to anaerobic
bacteria unless otherwise specified herein. Preferably, such waste
streams are primary and/or waste-activated sludge from municipal
sewage treatment plants or industrial aerobic wastewater treatment
plants, but may also include wastewater streams from solid waste
agricultural manures, crop residues, industrial solid wastes,
sludge, and slurries or any other high solids waste streams where a
significant portion of the solids material is potentially
biodegradable. The present invention can process streams with high
concentrations of total suspended solids (TSS) and/or fat, oil and
grease (FOG), as well as the slurries or solid waste having
anaerobically digestible material.
[0031] In a preferred embodiment of the invention represented in
the attached Figures, the waste stream is preferably a waste
slurry. More preferably, the waste slurry is a mixture of primary
and secondary sludge in a ratio of about 70:30 to about 30:70, most
preferably about 60:40 to 40:60. In the case of waste sludge, a
total solids concentration of about 2 to about 20 wt % is preferred
for embodiments of the digester of the invention without the
concentrator based on the weight of solids divided by the weight of
the sample. For embodiments of the digester with the concentrator,
the total solids concentration is preferably about 1 to about 10 wt
%, more preferably about 1 to about 5%. About 60 to about 90 wt %
of the solids present in such waste sludge are generally volatile,
potentially biodegradable solids. The waste sludge will also
preferably have an inlet chemical oxygen demand (COD) level of at
least 20,000 mg/l for the embodiments of the digester of the
invention without the concentrator and at least 5,000 mg/l, more
preferably at least 10,000 mg/l, for embodiments of the invention
with concentrator. For waste slurries, COD levels of greater than
2,000 mg/l are preferred. It will be understood, however, based on
the disclosure that the anaerobically digestible stream processed
by the invention can have varied characteristics.
[0032] The following is a detailed description of preferred
embodiments of the invention and should not be considered to be
limiting. The referenced schematics in FIGS. 1-6 are representative
and not drawn to scale. Certain terminology is used in the
following description for convenience only and is not considered to
be limiting. The words "lower" and "upper", "top" and "bottom",
"upward" and "downward" and "left" and "right" designate directions
in the drawings to which reference is made. The terminology
includes the words specifically mentioned, derivatives thereof and
words of similar import.
[0033] A waste stream can be introduced into a digester 30, as
shown in FIG. 1, through one or more inlets 2 located around the
periphery of the digester vessel or through the digester
recirculation conduit 60, 64 as described herein. The digester 30
also preferably includes one or more mixed liquor outlets 50 around
the periphery of the digester vessel, one of which may serve as a
main outlet for discharging digester effluent or the digested
liquor or sludge for disposal or further processing. The digester
may also include one or more sample ports 44 around the periphery
of the digester. The digester 30 additionally may include one or
more drains 40 for emptying the vessel when not in use. The term
"mixed liquor" in this specification includes, but is not limited
to a mixed liquor, a mixture of solids, liquids and gas and the
biodegradable portion therein and bacteria therein, which in the
preferred embodiment are anaerobic bacteria. The mixed liquor may
be within the digester, fed into the digester as a recycle stream
from the digester and any effluent from the digester.
[0034] Preferably, the digester volume is selected such that the
sludge retention time SRT within the digester is about 2 to about
20 days, preferably about 6 to about 12 days. The configuration of
the digester may vary, however, preferably it is a generally
cylindrical vessel with a height as measured along the longitudinal
axis A of the digester of preferably about 25 feet (8 m) to about
75 feet (23 m), and more preferably about 45 feet (14 m) to about
55 feet (17 m), with a height to diameter ratio of about 0.2 to
about 20, and more preferably about 1 to about 4, wherein the
diameter is measured in a transverse direction along the largest
transverse dimension of the digester. The digester can be
constructed of any conventional material that is consistent with
the materials handling and structural requirements of the
particular materials to be digested and digester design chosen.
However, it is preferable that the digester is constructed of
concrete, steel or fiberglass.
[0035] As shown in FIG. 1, the waste stream is preferably fed to
the digester upstream of a recirculation pump 6. The digester inlet
2 is preferably upstream of the digester recirculation pump 6 so
that the feed immediately and intensely contacts with the digesting
bacteria existing in the preferred recirculated stream flowing
through conduit 60 and is mixed proportionately with that stream.
In the preferred embodiment, the waste stream is fed continuously
to the digester 30, although a batch feeding operation may also be
utilized. The rate of feed of waste stream into the digester may
vary, but the maximum feed rate can generally be determined by
dividing the volume of the digester employed by the design
hydraulic retention time (HRT).
[0036] Preferably the digester is operated at a controlled mixed
liquor volume, which is a substantially constant volume, subject to
typical control fluctuations. To maintain the mixed liquor volume
inside the digester at a controlled volume, an amount of mixed
liquor substantially equal to the flow rate of the waste stream
feed is extracted from the digester 30 via a mixed liquor outlet
50, preferably one located to extend generally transversely from a
side 32 of the digester vessel. A control valve 4, such as a
gravity overflow or control valve, or any other appropriate flow
control mechanism can be used to control the mixed liquor discharge
so as to maintain the liquid volume in the digester at a
substantially constant volume.
[0037] In the preferred embodiments illustrated in FIGS. 1 and 5,
the apparatus further includes a recirculation system 46, including
the recirculation pump 6 and recirculation conduit 60, 64. The
recirculation system provides fluid communication for the mixed
liquor L within the digester 30 from a mixed liquor outlet 50,
which in this instance serves as a mixed liquor recirculation
outlet, preferably in the bottom 52 of the digester vessel, to a
liquid inlet 65 of at least one preferred two-phase nozzle 18 as
best shown in FIG. 2. In the preferred embodiment, the rate of
recirculation through the recirculation system is controlled to be
substantially constant such that the digester volume divided by the
digester volume recirculation rate is about 15 to about 150
minutes, and more preferably about 45 to about 75 minutes. Similar
digester recirculation rates would be preferred in the processing
of other waste slurries such as agricultural or industrial
slurries. However, recirculation rates can be altered or optimized
for varying systems. A first conduit 60 conveys liquid from pump 6,
which may be a pump or any liquid pumping apparatus that moves the
liquid through the conduit 60, 64, to the liquid inlet 65 of the
nozzle 18. A second conduit 64 conveys liquid from the mixed liquor
outlet 50 that serves as the mixed liquor recirculation outlet to
the pump 6. It is understood that the term "conduit", as described
in this specification, may be any pipe, conduit, tube, conveyance
mechanism, valve, or indirect or direct connection or the like
which provides fluid communication as described herein.
[0038] If the inlet 2 is connected to the second conduit 64, the
recirculation system 46 provides a continuous blend of waste stream
feed and recirculated mixed liquor from the digester to the liquid
inlet 65 of the preferred two-phase nozzle 18 which would then
discharge fresh waste stream and recirculated mixed liquor and
biogas from a biogas source into the digester 30. The nozzle 18,
best shown in FIG. 2, includes a nozzle gas inlet 13 in fluid
communication with a biogas source, a nozzle liquid inlet 65 and a
nozzle outlet 20. The nozzle further has a gas flow tube 15
extending from the nozzle gas inlet 13 to the gas tube outlet 33 in
proximity to the nozzle outlet 20. A nozzle space 56 which is
generally annular is defined by the exterior surface 35 of the gas
flow tube 15 and the interior surface 54 of the nozzle 18, through
which recirculated mixed liquor is passed.
[0039] As illustrated in FIG. 1, the pump 6 circulates the
recirculation stream and/or feed stream and pressurizes the slurry
upstream of the preferred two-phase nozzle 18. The pump energy is
transferred to the digester contents at the fluid outlet 20 of the
preferred two-phase nozzle 18. The fluid outlet 20 of the preferred
two-phase nozzle 18 is configured such that the nozzle annular
space 56 narrows at the nozzle outlet causing an acceleration of
the mixed liquor at the outlet. The velocity gradient generated via
the nozzle outlet 20, is preferably maintained at a level of about
50 to about 500 sec.sup.-1, as defined by formula (I)
G=(P/(.mu.V)).sup.0.5 (I)
[0040] wherein G is the mean velocity gradient, sec.sup.-1, P is
the power requirement in Watts, V is the digester volume in cubic
meters, and .mu. is the dynamic viscosity of the digester contents
in Ns/m.sup.2.
[0041] The energy transferred to the mixed liquor at nozzle outlet
20 imparts a shearing force on the solids in the stream which
breaks the solids into smaller particles and increases the surface
area. The increase in surface area exposes more unreacted organics
making them accessible to the anaerobic bacteria. The shearing
occurs both inside the nozzle 18 as the fluid is accelerated and in
the mixing zone 66 outside the nozzle outlet 20 where the energy of
the mixed liquor is transferred to the digester contents.
[0042] In the preferred embodiment, the nozzle is a two-phase
nozzle which can provide a shearing force as well as educting a
gas. However, it will be recognized by one skilled in the art that
a single phase nozzle 72, as shown in FIG. 2a, would also be
suitable for the providing shear to the mixed liquor in the
digester. The nozzle in FIG. 2a includes a liquid inlet 73 having a
diameter d.sub.2, which would be in fluid communication with a
mixed liquor outlet 50 of the digester 30 shown in FIG. 1, and a
liquid outlet 74 having a diameter d.sub.3 which is narrower than
the diameter d.sub.2 of liquid inlet 73 for introducing mixed
liquor into the digester, wherein the diameters d.sub.2 and d.sub.3
are measured in the largest dimension and transversely across
openings 73 and 74, respectively. The narrower outlet 74 causes
acceleration of the mixed liquor at the outlet 74. Recirculated
liquor passes through an interior space 75 of the nozzle 72 defined
by interior walls 76 of the nozzle 72. Unlike the two phase nozzle
18 described above, the single-phase nozzle 72 does not include a
gas tube. Additionally, other types of shearing devices, for
example a Venturi valve or nozzle, or an impeller, capable of
fracturing the solids and introducing them into the digester while
not as preferred as the nozzles of the present invention may also
be used within the scope of the invention.
[0043] The number of nozzles used can vary depending upon the
volume of the digester and/or the desired optimized process. For
example, two nozzles are shown in the preferred embodiment of FIG.
5. Preferably, about one nozzle to about 150 to about 1,500 cubic
meters of digester volume is preferred, and more preferably about
one nozzle to about 600 to about 900 cubic meters of digester
volume.
[0044] A "mixing device" which is preferably a draft tube but may
be any mixing device including impellers, injected gas, vacuum
pumping, mixing blades and the like capable of inducing mixing
within the digester are within the scope of the invention. It will
be recognized by one skilled in the art that other forms of mixing
such as mechanical mixers or gas injection or other mixing methods
in the presence of a shear enhanced medium would contribute to an
improvement in mass transfer rates and, accordingly, be suitable
for use in the invention. As best shown in FIG. 1, the nozzle
outlet 20 discharges proximate to the inlet 22 of a draft tube 28
positioned below the level 48 of the mixed liquor in the digester.
The fluid exiting the nozzle flows downwardly through the inner
area 26 defined by the draft tube 28. The diameter d.sub.1 of the
draft tube 28 as measured transversely through the tube is
preferably constant and is preferably about 40 to 200 centimeters.
Preferably the length l.sub.1 of the draft tube 28 as measured
along a longitudinal axis B of the draft tube should be such that
the draft tube inlet 22 is sufficiently far below the surface of
the liquid level to allow circulation of the contents of the
digester into the draft tube inlet 22, as shown in FIG. 1, and the
draft tube outlet 43 is sufficiently far above the bottom of the
digester to minimize flow restriction or excessive pressure drop.
More preferably the length 11 of the draft tube 28 ranges from
about 50% to about 90% of the digester liquid depth Q as measured
longitudinally from the bottom 52 of the digester to the surface 48
of the mixed liquor, with the depth of the liquid above the draft
tube inlet being no greater than the liquid depth below the draft
tube outlet 43. It is understood that more than one draft tube may
be employed according to the considerations discussed above with
respect to the number of nozzles employed. A 1:1 relationship
between the number of nozzles employed and the number of draft
tubes employed is preferred. However, there may be more than one
nozzle per draft tube in the anaerobic digestion apparatus
according to the invention.
[0045] The continuing downward flow of the nozzle effluent into the
draft tube 28 induces a generally downward flow inside the draft
tube. As the mixed liquor exits the draft tube outlet 43, it is
forced upwardly by the digester bottom such that a circulation
pattern is developed within the digester 30 in which liquid flows
back up around the exterior surface 24 of the draft tube 28 and
then is pulled and/or pushed downwardly again into the draft tube
28 through the upper inlet 22. This induced circulation pattern
around the draft tube preferably exceeds the volumetric flow rate
discharged from the nozzle and is beneficial to the mixing of the
mixed liquor, and more preferably the enhanced circulation is about
5 to about 25 times the discharge volumetric flow rate of the
nozzle. The enhanced mixing provided by the preferred circulation
around the draft tube contributes to an increased mass transfer
rate.
[0046] The degree of anaerobic digestion of a particular
biodegradable solid substrate is limited by the organic makeup of
that substrate. However the rate at which this digestion can be
achieved is affected by the mass transfer rate. By improving the
mass transfer rate, a reduction in the time for achieving digestion
can be effected. The induced circulation of the mixed liquor within
the digester 30 provides enhanced mixing of the digester contents
thoroughly dispersing the feed material and exposing the unreacted
organics to the digesting bacteria. Because the shearing effect of
the nozzle 18 has increased the exposed surface area of the
unreacted organics, the mass transfer rates of the anaerobic
digestion process are improved over conventional anaerobic
digesters. Under the influence of the energy imparted to the
digester by the discharge from nozzle 18, these conditions increase
the mass transfer rate.
[0047] The anaerobically biodegradable material contained in the
waste stream is digested through reactions in the digester 30,
where anaerobic bacteria convert the biodegradable material to a
biogas which substantially is made up of methane and carbon
dioxide, with lesser amounts of other gases, such as hydrogen
sulfide. These gaseous components and other similar anaerobic gas
byproducts are generally referred to herein as "biogas". The biogas
may also contain small amounts of water vapor, nitrogen and traces
of other volatile compounds which may be present in the feed or
formed during biodegradation. The composition of the biogas by
volume percent will vary depending on the particular digestible
organics being processed. Preferred methane levels in biogas formed
in the digester of the invention are in the range of about 50 to
about 90 volume percent. Preferred carbon dioxide levels are in the
range of about 5 to about 45 volume percent and hydrogen sulfide
levels can range from about 200 parts per million (volume) to about
6 volume percent. Action of the anaerobic bacteria on the
digestible organics also results in multiplication of the anaerobic
bacteria.
[0048] This apparatus preferably has a biogas source in fluid
communication with the nozzle 18 to provide biogas to the nozzle
18. A preferred biogas source is a biogas recycle system generally
designated 27 which uses a portion of the gas generated in
digestion as gas feed to the nozzle. However biogas or other
anaerobic digestion feed gas can be introduced independently
through inlet 37 and/or used together with a biogas recycle system
as shown in FIG. 1. Below the upper surface 34 of the digester, the
level 48 of the mixed liquor is such that there is an area 3 above
the liquor to allow for collection of the biogas that de-entrains
from the mixed liquor at its upper surface 48. The volume of the
area 3 above the mixed liquor may vary, but there is preferably a
distance of about 4 feet (1 m) to about 7 feet (2.5 m), more
preferably about 5 feet (1.5 m) to about 6 feet (2 m), of space
between the upper surface 48 of the mixed liquor and the upper
surface 34 of the digester 30 to prevent any foam that is generated
from impeding circulation or gas collection. This biogas collection
area 3 is preferably in fluid communication with nozzle 18 in a
biogas recycle system. Preferably, the biogas recycle system is
provided for recycling a portion of the biogas from the collection
area 3 to the nozzle 18. More preferably this biogas recycle system
27 comprises a conduit 5. The conduit provides an outlet at one end
7 for biogas in the collection area 3. The other end 11 of the
conduit is in communication with the gas inlet to the nozzle. The
conduit preferably has a control valve 4 for adjusting the rate of
flow of the biogas in the conduit 5. Biogas is also preferably
vented from the collection area, for example via a defoaming hood
17 through biogas outlet 29, with such venting preferably being
controlled, for example by a further valve 67, to maintain a gas
pressure in the area 3 of about atmospheric pressure to about 50
inches water at 35.degree. C. (12,400 Pa). More preferably, the
pressure range will be about 10 inches water at 35.degree. C.
(2,500 Pa) to about 20 inches water at 35.degree. C. (5,000 Pa).
Any biogas not recycled through the biogas recycle system 27 may be
discharged and may subsequently be burned as fuel or utilized for
other purposes.
[0049] In addition to biogas, additional gases may be introduced to
the nozzle 18 through inlet 37. For example, nitrogen feed gas may
be routed to the nozzle 18, either for control of strippable
toxins, or for altering the carbon dioxide equilibrium between the
biogas and the mixed liquor, thereby affecting the pH of the mixed
liquor in the digester. Alternatively, small amounts of air or
oxygen may be routed to the nozzle 18 through inlet 37 or a
separate gas inlet (not shown) to modulate the oxidation-reduction
potential (ORP) of the mixed liquor. This is desirable since the
tendency of undesirable anaerobic bacterial reactions to produce
hydrogen sulfide is favored by particular ranges of
oxidation-reduction potential. Hydrogen sulfide is malodorous,
corrosive to certain materials, and toxic to humans and the
digesting bacteria. By adjusting the oxidation-reduction potential
of the mixed liquor to a region outside those favoring hydrogen
sulfide production, the level of hydrogen sulfide present in the
mixed liquor may be reduced, thereby mitigating one of the less
desirable features associated with anaerobic digestion. This may be
facilitated by an ORP meter or gauge 14 coupled to the SEAD system
100, preferably somewhere along the recirculation system 46, most
preferably along the conduit 60. This ORP meter or gauge 14 may
signal a motorized control valve such as valve 16 to adjust the
flow of gas from inlet 37.
[0050] As previously described, the mixed liquor is accelerated as
it exits the nozzle 18, which is in close proximity to the outlet
33 of the gas flow tube 15. This creates an eduction effect useful
for the preferred biogas recycle system which draws the biogas and
removes a portion of the biogas from the biogas collection area 3,
through nozzle 18, and introduces the portion of biogas into the
digester 30. As the mixed liquor and the biogas exit the nozzle 18,
further mixing occurs between the portion of biogas and the
recirculating mixed liquor at the nozzle outlet 20 and the outlet
33 of the gas flow tube with the gas creating increased turbulence
at the nozzle discharge. This turbulence exerts an additional
shearing force on the solid particles in the mixed liquor, further
fracturing particles and thereby providing additional surface area
of degradable organics. This additional shearing mechanism further
enhances the performance of the invention by providing for
increased mass transfer rates as described above when more of the
degradable organics are exposed.
[0051] Increase of the mass transfer rate in the anaerobic process
requires an increase in the exposed surface area available to the
digesting bacteria as well as thorough mixing to assure that mass
transfer reactions can occur at an optimum rate. The eduction of
the portion of biogas removed from the gas collection area 3 into
the recirculating mixed liquor stream at the nozzle 18 entrains
fine gas bubbles in the mixed liquor circulating inside the
digester 30. Because the gas velocity differs from the fluid
velocity of the mixed liquor in both the draft tube 28 interior 26,
where the mixed liquor is flowing downward carrying the entrained
gas by overcoming its buoyancy, and in the area outside the draft
tube 28, where the velocities of the mixed liquor and gas are both
upward but different, the entrained gas promotes a high degree of
turbulence on the sheared particles in the mixed liquor. In
conjunction with the induced circulation imparted by the nozzle 18
and draft tube 28, this entrained gas turbulence further promotes
an increase in the mass transfer rate that is beneficial to the
optimum performance of the invention.
[0052] As discussed above, the eduction effect of the nozzle draws
biogas into the mixed liquor from a biogas source, preferably the
biogas recycle system 27. It is preferred to control the amount of
biogas recycled into the mixed liquor, for example by control valve
4 on conduit 5. In the preferred embodiment the volume ratio of
biogas to liquid in the nozzle will be up to about 0.5 of the
volume of biogas per volume of liquid that flows through the
nozzle. However, it is recognized that the characteristics of each
mixed liquor will vary for many reasons including the
characteristics of the feed waste stream and it is further
recognized that these characteristics will impact the rate at which
entrained gas generated within or injected into the mixed liquor is
released. It is also recognized that as the mass transfer rate of
the digester 30 is increased, the rate at which biogas is generated
within the mixed liquor mammoth stream due to the digestion process
also increases. At such point in the operation of the system where
the volume of biogas entrained in the mixed liquor due to digestion
has reached the level sufficient to provide the amount of gas
turbulence preferred for the given application, the biogas recycle
may be shut off by closing control valve 4.
[0053] The concentration of solids in the waste stream feed is
expressed in percent total suspended solids (TSS). In the
illustration case of municipal POTW waste sludge, the TSS of the
feed before thickening is typically less than 1.0% TSS. Typically
this sludge is thickened to about 5% TSS by utilizing polymers in
the thickening process. The applicants have discovered that
performance of an optimized digester, such as the apparatus of the
invention, can be negatively impacted by the presence of such
thickening polymers in the waste stream. Digesting the unthickened
waste stream would effect this, but the digester volume would
become proportionally larger which is not desirable. Thus a method
to thicken the waste feed without the need for polymer addition is
desired.
[0054] In conventional digestion processes, of which FIG. 3 is
representative, concentration or thickening of the feed into the
digester occurs upstream of the digester. However, conventional
methods typically do not digest the hydrophilic compounds in the
waste stream. The hydrophilic compounds typically present in the
solids in the waste stream make it difficult to thicken. The
performance of any concentration system, and in particular a
membrane concentrator, can be improved if the hydrophilic compounds
can be removed from the medium as these compounds reduce the
tendency of the medium to release water. The digester apparatus of
the present invention will digest these hydrophilic compounds and,
when operated concurrently with the concentrator, enhances the
performance of the concentrator by removing these hydrophilic
compounds that make it difficult for the concentrator to
thicken.
[0055] As shown in FIGS. 4 and 5, in a preferred embodiment of the
invention, a concentrator 62 conveys a portion of the mixture of
solids and liquid from a mixed liquor outlet 50 of the digester,
preferably the mixed liquor recirculation outlet, to the
concentrator and back to an inlet 77 of the digester. The
concentrator 62 is in fluid communication with an inlet 77 and with
a mixed liquor outlet 50 of the digester. More preferably, the
concentrator comprises a pump 70, which may be the pump 6 of the
recirculation system, but is preferably one or more separate pumps,
and a separator 58 having an outlet 53 which is in fluid
communication with an inlet 77 of the digester and an inlet 68 in
communication with a mixed liquor outlet 50 of the digester. The
concentrator also preferably includes a conduit 55 with a first end
57 connected to the outlet 53 of the separator 58, and a second end
59 connected to the inlet 77 of the digester. This conduit
preferably conveys the concentrate from a concentrate side 69 of
the separator 58 back to the digester 30.
[0056] Most preferably, the separator is a water-permeable membrane
including and preferably manufactured of a material suitable for
processing a liquid with various concentrations of suspended solids
and suspended solid particles of varying sizes. An example of a
suitable membrane is an ultra-porous, asymmetric, polymeric
ultra-filtration membrane. Commonly used polymers include cellulose
acetates, polyamides, polysulfones, poly
(vinylchloride-co-acrylonitrile)s, and poly (vinylidene fluoride).
Membrane separation is an effective concentrating method, however,
other concentration methods such as a lamella separators, dissolved
air flotation, gravity belt filter, decanter, rotating screen, or
others are suitable separators. The permeate from the concentrator
and any unused concentrate may be discharged from the system or
routed to one or more heat exchangers 8 as discussed below.
[0057] If desired, a flow control meter(s) or gauge(s) 51 and
preferably a motorized valve 16 may be provided to control the flow
of mixed liquor to and from the concentrator. However, any control
mechanism is acceptable for controlling the flow of liquor to and
from the concentrator. Additionally, a pressure meter or gauge 63
coupled with a motorized valve 16 may be employed to control the
pressure of mixed liquor conveyed to the digester 30 from the
separator 58. However, any pressure control mechanism is suitable
for controlling the pressure of the mixed liquor conveyed to the
digester.
[0058] The digester configuration described previously allows for
improved rates of digestion due to increased mass transfer rates
but the volatile solids destruction is limited by the fraction of
biodegradable solids available in said waste stream and is a
function of solids retention time (SRT). The process performance in
the same apparatus can be further improved if the SRT of the
digester can be extended without increasing the digester volume.
This can be achieved by further increasing the solids concentration
in the digester. With the apparatus of the preferred embodiment,
concurrent concentration and digestion allows for adjustment of the
mixed liquor concentration resulting in increased SRT in the
digester at a fixed waste stream feed rate. Thus the design SRT can
be targeted to achieve a particular goal such as, for example, to
increase volatile solids destruction or to achieve specific
effluent solids concentration. Applicants have discovered that
concurrent concentration and digestion in the above manner using a
membrane separator not only minimizes or eliminates the need to use
the potentially inhibitory polymers in optimized digesters,
including the various embodiments of the apparatus of the
invention, but in any digester, including non-shear enhanced
anaerobic digesters, egg-shaped anaerobic digesters, and aerobic
digesters.
[0059] The preferred embodiment is illustrated for the case of a
municipal POTW waste sludge or a waste slurry. Similar digester
designs and use of this method are envisioned for digesting other
waste slurries from agricultural and industrial sources by the
present invention. It is recognized that the percent fraction of
biodegradable material in the slurry will vary based on the source
and also that the concentration of solids in the slurry could be in
the range of about 0.5% to about 12% TSS, but is preferably above
1% TSS. It is further recognized that the treatment objectives or
economics of a given application might make it preferable to
operate the digester 30 or concentrator 62 at parameters outside of
the preferred ranges. Considering these factors it is recognized
that in some applications concentration of the feed may not be
required and, in fact, a dilution stream might instead be preferred
to achieve the desired mixed liquor TSS in the digester. It is also
recognized that this anaerobic digestion process will also digest
the soluble biodegradable organics present in the fluid stream.
These and other variations in the present invention are
contemplated.
[0060] An optional gas de-entrainment zone 45 can be provided
within the digester 30, as shown in FIG. 5. The gas de-entrainment
zone is preferably in the form of a vertical cylinder defined by a
wall(s) 47 with an open top and a closed bottom 61, preferably
contiguous with the bottom 52 of the digester, except for an
opening in the bottom 52 which is in communication with a mixed
liquor outlet 50 of the digester, preferably the mixed liquor
recycle outlet. The shape of the wall(s) 47 of the zone may be
generally cylindrical or otherwise configured so that the
transverse cross sectional area of the zone is sufficient such that
the downward velocity of the mixed liquor in the zone caused by the
suction of the recirculation pump 6 is less than the rate of rise
of gas bubbles of less than about 1 mm in diameter, in order to
allow for such bubbles to de-entrain from the mixed liquor. The
preferred cross sectional area varies, but should be sufficient
such that the downward velocity of the mixed liquor in the
entrainment zone may range from 0.02 to 0.2 m/s, more preferably
0.05 to 0.1 m/s. This gas de-entrainment zone is preferred to avoid
any potential for such gas bubbles to contribute to possible
cavitation at the recirculation pump or the concentrate pump 6,
which could result in mechanical damage.
[0061] Alternatively, an optional gas deflector plate 25, depicted
in FIGS. 1 and 5 is preferably positioned between the lower outlet
43 of the draft tube 28 and the digester bottom 52 to minimize the
entrainment of gas bubbles in the mixed liquor at the point where
it enters the conduit 50 of the recirculation system 46 or the
concentrator 62. Generally there is no limitation on the shape or
materials of construction of the plate. Preferably the gas
deflector plate 25 has a shape that is larger than the outlet of
the draft tube but smaller than a size that would cause the
downward velocity of the mixed liquor flowing around the plate 25
to be increased above the rise rate of gas bubbles less than about
1 mm in diameter, in order to allow for such bubbles to de-entrain
from the mixed liquor. It is understood however that, optional gas
de-entrainment zone and deflector plate may used each alone or in
combination in varying SEAD systems according to the invention.
[0062] Performance of the system can be further enhanced by
operating the system at optimal levels of pH. Any conventional
manual or automated pH control mechanism can be used to control and
optimize these conditions inside the digester. In such cases a
conventional pH control system 10 can be included, preferably in
the recirculation conduit 60, to measure pH and dose appropriate
amounts of adjusting chemicals. The preferred pH level of the
digester for the anaerobic digestion is about 6 to about 8. For
certain waste slurries, such as those with a chemical oxygen demand
(COD) below 30,000 mg/l, adjustment of the pH may be required to
maintain the optimum level in the digester.
[0063] Performance of the system can also be further enhanced by
operating the system at optimal levels of temperature. Any
conventional temperature control mechanism may be used to control
the temperature of the mixed liquor in the digester. One mechanism,
shown in FIG. 1, includes a temperature meter or gauge 12 and a
heat exchanger 8, preferably in the recirculation system 46 of the
invention. A preferred method, shown in FIG. 5, includes the use of
a heat exchanger 8 and a steam injector 9 upstream of recirculation
pump 6. As shown in FIG. 5, the heat exchanger 8 may serve as a
recovery heat exchanger to capture heat from the permeate from
concentrator 62. The waste heat may also be recovered from the
digester effluent or the excess concentrate using a separate heat
exchanger (not shown). The temperature control mechanism preferably
heats the feed into the digester (or into the recirculation
conduit) to a temperature at or slightly above the preferred
reaction temperature prior to the entry of the mixed liquor and/or
feed into an inlet 2 of the digester. The preferred temperature
level of the digester for mesophilic anaerobic digestion is about
80.degree. F. (25.degree. C.) to about 105.degree. F. (40.degree.
C.). The digester may also be operated in the thermophilic range of
about 125.degree. F. (50.degree. C.) to about 145.degree. F.
(60.degree. C.). However, the feed into the digester may be heated
to any temperature that does not damage the anaerobic bacteria to a
degree that negatively impacts the digestion process. In the case
where it is preferred to destroy pathogens in the slurry, such as
for the purpose of producing Class A municipal sludge, operation in
the thermophilic range would allow for achievement of this
objective simultaneously with digestion of the degradable
organics.
[0064] As described herein, the recirculation of mixed liquor
through the nozzle 18 induces a circulation pattern around the
draft tube 28, which provides for mixing of the digester contents.
In addition, the biogas entrained in the mixed liquor enhances the
mixing. In addition to the beneficial effects mixing and turbulence
heretofore mentioned, mixing provides a more uniform pH and
temperature profile across the digester, thereby maintaining stable
reaction conditions within the digester vessel.
[0065] It has been found advantageous for the desired operability
of the shear enhanced anaerobic digestion apparatus and process to
provide elements to allow the successful restart of the digester
operation after an extended shutdown. On shutdown, solid material
tends to settle in the digester 30, potentially building into a
layer with a depth sufficient to block at least the outlet of the
draft tube 28. The solids buildup inhibits restart of the digester
operation since, absent remedial efforts, the solids have a
tendency to remain stationary, thereby blocking the pump suction
and the draft tube 28, as well as potentially blocking the inlet
feed. It has been found that by providing liquid circulation in the
bottom of the digester, it will disturb any layer of built-up
solids sufficiently to improve liquid circulation within the
digester 30 and assist start-up. As shown in FIGS. 1 and 5,
preferably, the liquid circulation would be supplied by pump 6 and
a conduit 36 in fluid communication with the discharge of the pump
6 and a nozzle 38 on the digester 30, and would be directed below
the deflector plate(s) 25 or otherwise in the area above and in
proximity to the digester bottom 52. This liquid circulation may
have any shape that sufficiently disturbs the layer of built up
solids.
[0066] In the event that the liquid circulation is not sufficient
to clear the draft tube 28 of solids build up or in lieu of the
liquid circulation, an additional biogas recycle system 21, may be
utilized. The biogas recycle system 21, preferably includes
internal gas nozzles 23, in communication with a conduit 19, to
force a gas upwardly around and into the draft tube 28 so as to
dislodge the solids, before beginning normal operation of the
nozzle 18. The system may also be further facilitated by an
optional pump 49 to accelerate the flow of gas to nozzles 23.
[0067] It has also been found desirable to use a defoaming spray
system for the optimum operation of the anaerobic digester in the
present invention. Such a system prevents the buildup of foam
inside the digester 30 by spraying liquid preferably continuously
onto the upper surface of the mixed liquor in the digester. Such
spray preferably covers the majority or substantially all of the
liquid surface. The impact of the sprayed liquid on the surface of
the liquid level serves to collapse the foam and inhibit foam
buildup. Preferably, the liquid sprayed would be mixed liquor from
the digester 30. As shown in FIG. 1, at least one spray head 31 is
used, and preferably many such spray heads. This defoaming spray
system can be supplemented if necessary with conventional chemical
defoamer injection, activated through an appropriate foam sensor,
and/or with conventional mechanical methods for foam
destruction.
[0068] The foregoing detailed description refers to the preferred
embodiments of the present invention. However, the apparatus
according to the invention is operable when generally comprising a
digester; any suitable mixing device capable of inducing a
circulation and contributing to an improved mass transfer rate,
including but not limited to the examples discussed above; and any
suitable shearing device capable of fracturing the solids and
introducing them into the digester, including but not limited to
the examples discussed above.
[0069] The area delineated by the dashed rectangle in FIG. 5 may be
replaced by the apparatus shown FIG. 6. FIG. 6 is directed to an
alternative embodiment of the invention comprising a digester,
preferably an anaerobic and shear enhanced digester, coupled with a
concentrator. The apparatus has improved efficiency due to
concurrent operation of the digester and a concentrator. The
apparatus includes a digester 30' preferably including a shear
source 71', such as a shearing nozzle, a Venturi nozzle, an
impeller, or some other device capable of imparting shear to the
mixed liquor within the digester, and a concentrator 62' in fluid
communication with a mixed liquor inlet 77' of the digester and at
least one mixed liquor outlet 50' of the digester 30'. The digester
30' may be the digester 30 above or any digester preferably with a
source for imparting shear to the mixed liquor in the digester. If
used, the shear source 71' may be either within or outside of the
digester 30' or in fluid communication with the digester 30' so
long as it is configured to impart shear to the mixed liquor within
the digester. The concentrator 62' may be any concentrator that
reduces the amount of water in the waste stream, including any of
the concentrators 62 described above with respect to FIG. 5. The
concentrator 62' and the digester 30' can be configured as
described above with respect to FIG. 5 to effect concurrent
concentration and digestion of the mixed liquor. The apparatus may
further comprise any of the devices and adopt any configuration
described above and will optimize the efficiency, particularly of
the anaerobic digestion apparatus. Similar to the concentrator 62
described above, the concentrator 62' may comprise a pump 70' and a
separator 58' having an outlet 53' which is in fluid communication
with an inlet 77' of the digester and an inlet 68' in communication
with a mixed liquor outlet 50' of the digester. The concentrator
may also include a conduit 55' with a first end 57' connected to
the outlet 53' of the separator 58', and a second end 59' connected
to the inlet 77' of the digester 30'. This conduit preferably
conveys the concentrate from a concentrate side 69' of the
separator 58' back to the digester 30'.
[0070] A waste stream may be digested in various embodiments of an
anaerobic digestion apparatus, as described herein, by feeding the
waste stream into the digester, preferably taking into
consideration the parameters discussed above. The biodegradable
material in the waste stream may be reacted with anaerobic bacteria
to produce a mixed liquor and a biogas. This reaction may be
further optimized by any of the methods described above, if
desired, including controlling temperature and pH, concentrating
the mixed liquor, minimizing the entrainment of gas bubbles, etc.
The mixed liquor may be introduced to the digester and any shearing
device or method may be used, preferably in communication with an
inlet of the digester, and the mixed liquor within the digester may
be mixed by any mixing device or method, including those described
herein.
[0071] The invention will now be described in more detail with
respect to the following specific, non-limiting examples.
EXAMPLE I
[0072] The data in Table 1 illustrate the improved mass transfer
rates of the digester apparatus of the present invention. The pilot
apparatus for this study was generally the embodiment of the
digester apparatus as depicted in FIG. 1 of the attached drawings,
hereinafter referred to as a shear-enhanced anaerobic digester
(SEAD digester). The apparatus included one two-phase nozzle, a
draft tube, and a gas impingement plate, but not the concentrator.
The data additionally indicate the impact of typical polymers on
the anaerobic digestion process. The data is taken from a pilot
study at a POTW wherein the feed to the SEAD digester was a mixture
of primary and secondary sludge that had been thickened using a
conventional belt thickener with the addition of polymers. For this
study the digester was operated in a once-through mode without a
concentrator. During the study the POTW elected to change the type
of polymer being used as indicated in the table. At the point
indicated in Table 1, the feed to the digester apparatus was
changed to a point upstream of the polymer addition and the polymer
was purged from the system until it was polymer free.
1TABLE 1 Date Month 3 Month 4 Month 5 Month 6 Parameter Chem. A
Chem. B Purging Purged Feed Conc. (% TS) 5.1 4.1 2.3 2.2 HRT (days)
18.1 36.4 11.4 9.5 SRT (days) 18.1 36.4 11.4 9.5 Bio-activity
(kg/kg/d) 1.3 0.7 1.3 2.0 Rx VFA (meq/L) 5.0 6.9 2.3 0.6
[0073] The above data are monthly average operating data. Chemical
A is a cationic water soluble polymer in emulsion. Chemical B is a
solution mannich polymer. The feed concentration is expressed in
percent total solids (% TS), i.e. lbs. of solids per 100 lbs. of
liquid sludge. The HRT is hydraulic retention time in days, which
in a once-through system equals the solids retention time (SRT).
The bioactivity is a measure of the volatile solids conversion
capacity of the anaerobic biomass and is expressed as kilograms of
volatile solids digested per kilogram of digesting bacteria
(volatile solids) per day.
[0074] Rx VFA is the concentration of volatile fatty acids in the
digester expressed in milliequivalents per liter (meq/L). Rx VFA is
a measure of the process stability in the digester. Rx VFA readings
of less than 1.0 indicate that the digesting environment is very
stable and could likely perform at even higher mass transfer
rates.
[0075] The first column indicates the data for the last month
wherein Polymer A was added. The SRT is roughly equivalent to that
of an aggressively designed conventional digester. The VFA
indicates less than optimal stability. The second column indicates
the data for the subsequent month of operation when the POTW
switched to a more economical polymer. Performance of the SEAD
digester deteriorated as indicated by the much longer SRT as well
as the elevated VFA indicating that Polymer B was significantly
more inhibitory than Polymer A.
[0076] At this point in the study, the location of the feed was
moved to allow the SEAD digester to receive the same sludge mix
before thickening and polymer addition. The next column indicates
the data for the month during which the SEAD digester was gradually
purged of the polymer. The SEAD digester performance increased
significantly achieving an SRT below the conventional 20-40 days
and with a more stable VFA. The final month of the study reflects
the performance of the SEAD digester on the same sludge after the
polymer was completely purged from the SEAD system. An SRT of less
than 10 days was achieved with a very stable VFA indication. The
study was ended before the most optimal SRT achievable for this
application was determined, but the very low VFA indicates that
further reductions in SRT would likely have been possible.
[0077] The effectiveness of the shear enhanced anaerobic digester
apparatus according to the invention is illustrated by this data.
The improved mass transfer rates allow for sludge digestion to be
performed at an SRT that is 50% of the lower design guideline
recommended for conventional systems. The study also identifies
that the benefits of the SEAD apparatus are best obtained if the
feed stock does not contain commonly used polymers in the
thickening process.
[0078] The data in Table 2 are taken from a pilot study on an
industrial waste activated sludge wherein the sludge had been
thickened using a conventional thickener with the addition of a
cationic polyacrylamide in a water-in-oil emulsion as a polymer.
The pilot apparatus for this study was essentially the embodiment
depicted in FIG. 1.
2 TABLE 2 Parameter Industrial High Rate Egg Shaped Feed Conc. (%
TS) 4.1 4.6 5.0 HRT (days) 12.2 26 20 SRT (days) 12.2 26 20
Bio-activity (kg/kg/d) 1.2 0.6 0.8 Rx VFA (meq/L) 4.5 3.7 N/A.
[0079] Data for Conventional Digesters designated as "High Rate" in
Table 2 was taken from WEF (1992): "Manual of Practice No. 8:
Design of Municipal Wastewater Treatment Plants Volume II: Chapters
13-20", pp. 1261-1263. ISBN 0-943244-85-4. Data for "Egg-Shaped"
Digesters in Table 2 was taken from Brinkman, Doug and Voss, Denton
(1999): "Egg Shaped Digesters, are they all they're cracked up to
be?" Water Environment & Technology, November Issue, pp. 28-33.
Each of these sources of data is hereby incorporated by reference.
The SRT of 12.2 days for the SEAD indicates a significant
improvement over the design basis SRT for conventional digesters.
The elevated VFA indicates that the stability of the system was not
optimal. Although no study of the effect of the polymer was
undertaken here, without wishing to be bound by any theory
applicants herein attribute the elevated VFA at least in part to
the inhibitory impact of the polymer on the digestion
efficiency
[0080] The data in Table 3 are taken from a pilot study on a
mixture of primary and secondary sludge taken from a POTW system at
a point before thickening and polymer addition. The pilot apparatus
for this study was essentially as depicted in FIG. 5, with the
exception that only one two-phase nozzle and one draft tube were
used, hereinafter designated as a membrane coupled shear enhanced
digestion apparatus (MCSEAD digester).
3 TABLE 3 SEAD Transition Extrapolated Parameter Only MCSEAD MCSEAD
Feed Conc. (% TS) 0.5 0.5 0.5 HRT (days) 11 5 1 SRT (days) 11 11 11
Bio-activity (kg/kg/d) 1.4 2.0 1.4 Rx VFA (meq/L) 0.6 2.0
<1.0
[0081] The first column indicates the performance obtained on the
unthickened feed without the concentrator in operation. The SRT
again indicates that the improved mass transfer rates of the SEAD
apparatus according to the invention are beneficial to the
digestion process. The low VFA indicates that further reductions in
SRT might be possible. The concentrator was then activated and the
same sludge feed was concurrently concentrated and digested.
Concentration was achieved with a membrane separation system
without the addition of polymers as indicated in the preferred
embodiment. The second column indicates the performance with the
concentrator operating during a transition period as the mixed
liquor TSS is increasing. As the quantity of digesting bacteria
accumulated in the digester, increased demand on the existing
bacteria is indicated by the increase in Bioactivity and slightly
elevated VFA. When the concentration transition is complete,
equilibrium will be restored and the extrapolated results are
indicated in column three. Data for the equilibrium condition were
not obtained due to mechanical limitations at the pilot scale. The
membrane separator performed effectively as evidenced by the
reduction of HRT at a constant feed concentration and SRT.
[0082] Sludge digester systems are typically once through systems
designed based on SRT and HRT. The solids concentration of raw
sludge is typically about 1% TS. In order to keep digester volume
reasonable, sludge is often thickened to about 5% TS before
digestion. Table 2 shows that the embodiment of the digester of the
apparatus of the present invention including a shearing device and
a mixing device can achieve the same process performance with
prethickened sludge as a conventional high rate or egg-shaped
digester, but at a considerably reduced SRT and HRT.
[0083] Further, Table 1 shows that thickening polymers can be
moderately to severely inhibitory to the anaerobic bacteria in the
applicants' digester having a shearing device and a mixing device,
and that elimination of such polymers improves process stability
and bacterial activity. Mere removal of the polymer is not a
practical way in which to improve such process stability and
bacterial activity because it would require treatment of dilute
sludge in such a digester apparatus, which even at reduced SRT and
HRT would require a large digester volume. Further, prethickening
raw sludge from 1% TS to 5% TS with a membrane separator upstream
of the digester is difficult, as discussed above, because the
presence of hydrophilic compounds in sludge can prevent efficient
performance of the upstream membrane. Accordingly, applicants'
embodiment further including concurrent digestion and concentration
of the mixed liquor permits control of SRT independently of HRT,
making it possible to digest dilute sludge without use of
inhibitory polymers and using a digester volume similar to or less
than the digester volumes associated with the prethickened sludge
as used in Table 2. Table 3 illustrates that for a given digester
volume, applicants digester embodiment using the addition of a
membrane concentrator according to the invention permits
maintaining an SRT of 11 days while increasing the digester
throughput by a factor of 11, as shown by the decrease in HRT from
11 days to 1 day.
[0084] It will be appreciated by those skilled in the art that
changes could be made to the embodiments described above without
departing from the broad inventive concept thereof. It is
understood, therefore, that this invention in not limited to the
particular embodiments disclosed, but is intended to cover
modifications within the spirit and scope of the present invention
defined by the appended claims.
* * * * *