U.S. patent application number 11/616247 was filed with the patent office on 2007-05-10 for nitrogen recovery system and method using heated air as stripping gas.
This patent application is currently assigned to ENVIRONMENTAL ENERGY & ENGINEERING CO.. Invention is credited to Dennis A. Burke.
Application Number | 20070102352 11/616247 |
Document ID | / |
Family ID | 35150537 |
Filed Date | 2007-05-10 |
United States Patent
Application |
20070102352 |
Kind Code |
A1 |
Burke; Dennis A. |
May 10, 2007 |
NITROGEN RECOVERY SYSTEM AND METHOD USING HEATED AIR AS STRIPPING
GAS
Abstract
A process for the recovery of nitrogen from anaerobically
digested liquid waste and for the collection of the nitrogen as
nitrate compounds that can be used to produce fertilizer and
compost, includes stripping ammonia from anaerobically digested
liquid waste, and converting the ammonia into nitrates via
nitrification. The stripping gas is heated above ambient
atmospheric temperature to improve nitrogen recovery. The heat can
be reclaimed by burning gases generated during the anaerobic
digestion process.
Inventors: |
Burke; Dennis A.; (Olympia,
WA) |
Correspondence
Address: |
CHRISTENSEN, O'CONNOR, JOHNSON, KINDNESS, PLLC
1420 FIFTH AVENUE
SUITE 2800
SEATTLE
WA
98101-2347
US
|
Assignee: |
ENVIRONMENTAL ENERGY &
ENGINEERING CO.
6007 Hill Street NE
Olympia
WA
98516
|
Family ID: |
35150537 |
Appl. No.: |
11/616247 |
Filed: |
December 26, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10826131 |
Apr 16, 2004 |
7153427 |
|
|
11616247 |
Dec 26, 2006 |
|
|
|
10625198 |
Jul 22, 2003 |
6866779 |
|
|
10826131 |
Apr 16, 2004 |
|
|
|
60398296 |
Jul 22, 2002 |
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Current U.S.
Class: |
210/603 |
Current CPC
Class: |
C02F 3/302 20130101;
Y02P 20/145 20151101; C02F 2103/20 20130101; C02F 1/001 20130101;
C02F 1/66 20130101; Y10S 210/903 20130101; C02F 1/24 20130101; C02F
2101/16 20130101; Y02E 50/30 20130101; C02F 3/06 20130101; C02F
3/30 20130101; Y02W 30/40 20150501; C02F 1/20 20130101; C05F 17/50
20200101; Y02A 40/20 20180101; C02F 1/02 20130101; C05F 3/00
20130101; C05F 17/40 20200101; C02F 3/28 20130101; C05F 17/15
20200101; Y02W 10/10 20150501 |
Class at
Publication: |
210/603 |
International
Class: |
C02F 3/28 20060101
C02F003/28 |
Claims
1. A method for recovering nitrogen from liquid waste, comprising:
(a) anaerobically digesting liquid waste into digested liquid waste
containing ammonia; (b) stripping said ammonia from said digested
liquid waste to produce gas containing ammonia; and (c) converting
stripped ammonia into nitrogen-containing compounds in a
biofilter.
2. The method of claim 1, further comprising separating solids from
the digested liquid waste prior to stripping.
3. The method of claim 1, further comprising raising the pH of the
digested liquid waste.
4. The method of claim 1, further comprising raising the
temperature of the digested liquid waste.
5. The method of claim 1, wherein ammonia is stripped by contacting
the digested liquid waste with air.
6. The method of claim 1, further comprising collecting organic
biomass containing nitrogen.
7. The method of claim 1, wherein the pH of the digested liquid
waste is about 7 to about 8.
8. The method of claim 1, wherein the pH of the digested liquid
waste is about 8 to about 9.
9. The method of claim 2, wherein the separated solids are recycled
to anaerobic digestion.
10. A method for recovering nitrogen from anaerobically digested
liquid waste, comprising: (a) stripping ammonia from anaerobically
digested liquid waste to produce gas containing ammonia; and (b)
converting stripped ammonia into nitrogen-containing compounds in a
biofilter.
11. The method of claim 10, further comprising raising the pH of
the digested liquid waste.
12. The method of claim 10, further comprising raising the
temperature of the digested liquid waste.
13. The method of claim 10, wherein ammonia is stripped by
contacting the digested liquid waste with air.
14. The method of claim 10, further comprising collecting organic
biomass containing nitrogen.
15. The method of claim 10, wherein the pH of the digested liquid
waste is about 7 to about 8.
16. The method of claim 10, wherein the pH of the digested liquid
waste is about 8 to about 9.
17. A method for making fertilizer from liquid waste containing
manure, comprising: (a) anaerobically digesting liquid waste
containing manure into digested liquid waste containing ammonia;
(b) stripping said ammonia from said digested liquid waste to
produce gas containing ammonia; and (c) converting stripped ammonia
into nitrogen-containing compounds in a biofilter.
18. The method of claim 17, further comprising separating solids
from the digested liquid waste prior to stripping.
19. The method of claim 17, further comprising raising the pH of
the digested liquid waste.
20. The method of claim 17, further comprising raising the
temperature of the digested liquid waste.
21. The method of claim 17, wherein ammonia is stripped by
contacting the digested liquid waste with air.
22. The method of claim 17, further comprising collecting organic
biomass containing nitrogen.
23. The method of claim 17, wherein the pH of the digested liquid
waste is about 7 to about 8.
24. The method of claim 17, wherein the pH of the digested liquid
waste is about 8 to about 9.
25. The method of claim 17, wherein the separated solids are
recycled to anaerobic digestion.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
10/826,131, filed Apr. 16, 2004, now U.S. Pat. No. 7,153,427, which
is a continuation-in-part of application Ser. No. 10/625,198, filed
Jul. 22, 2003, now U.S. Pat. No. 6,866,779, which claims the
benefit of Provisional Application No. 60/398,296, filed Jul. 22,
2002. All the above are incorporated herein expressly by reference
in their entirety.
FIELD OF THE INVENTION
[0002] The field of the invention is related to the removal and
recovery of nitrogen from anaerobically digested liquid waste and
the collection of the nitrogen as nitrates and organic nitrogen
compounds that can be used to produce fertilizer and compost.
BACKGROUND OF THE INVENTION
[0003] In U.S. application Ser. No. 10/625,198, fully incorporated
herein by reference in its entirety, a method is described that is
related to the recovery of nitrogen from ammonia that is produced
from anaerobic digestion of waste. In the prior process,
atmospheric air at ambient temperature is used as the stripping
gas. However, if ambient air is used, especially during winter, the
temperature of the digested liquid waste can be significantly
reduced to a temperature, such as 40.degree. F. to 50.degree. F.,
that will lower the amount of ammonia and hydrogen sulfide that is
stripped from the digested liquid waste. The lower temperature will
also negatively affect the efficiency of the biofilter that fixes
the nitrogen into a solid form.
[0004] Accordingly, there is a need to improve on the prior method
to more efficiently recover ammonia, and subsequently nitrogen. The
present invention overcomes the drawbacks of the prior method and
has further related advantages.
SUMMARY OF THE INVENTION
[0005] The present invention is related to a process for the
recovery of nitrogen from wastewater liquids, and collection and
sequestration of the nitrogen in organic matter taking the form of
inorganic nitrogen and organic nitrogen compounds. Recovered
nitrate, ammonia, and organic nitrogen compounds can be used to
produce nitrogen-rich organic compost fertilizer. One embodiment of
the method according to the invention can recover nitrogen from
liquid waste to produce nitrogen-rich fertilizer. The process
produces less greenhouse gases as compared with the typical
nitrification and denitrification processes. Such reduction in
ammonia levels and greenhouse gases is thought to produce
beneficial health benefits for people and lessen the environmental
impact.
[0006] In one embodiment of the present invention, a method for
recovering nitrogen from liquid waste is provided. The method
includes anaerobically digesting liquid waste in an anaerobic
digester. Through this process, digested liquid waste is produced
containing amounts of dissolved carbon dioxide, hydrogen sulfide,
and ammonia. Dissolved ammonia can exist in liquid as ammonium
ions. The digested liquid waste can be stripped of carbon dioxide
and ammonia with a heated gas, such as heated air that is heated
above the ambient atmospheric temperature, or combustion gas. Upon
stripping the carbon dioxide, the ammonium ions form ammonia gas
and the ammonia gas is likewise stripped from the digested liquid
waste. The gases leaving the stripping unit can be fed to a
biofilter containing bacteria that convert the stripped ammonia
into nitrate compounds and bacterial biomass, for example.
[0007] In one embodiment of the invention, the heated stripping gas
is produced by collecting the combustible gases, such as methane,
which are generated during the digestion process, and burned. The
burning of the methane can be used in power generation equipment.
Alternatively, the methane and other combustible gases can be
burned in a flare stack. Heat to heat stripping air can be
reclaimed during the burning process via several methods. For
example, a heat exchanger can be provided to reclaim heat from
various hot media, such as hot water, condensate, steam, and
combustion gas. Alternatively, the combustion gas produced during
burning can be used as the stripping gas. However, combustion gas
will contain an excess amount of carbon dioxide that will lower the
pH of the digester effluent and therefore favor the removal of
hydrogen sulfide rather than ammonia nitrogen. Stripped hydrogen
sulfide can be converted into sulfur-containing compounds in a
biofilter. Heated stripping gas can be collected in the form of
heated ambient atmospheric air from a building housing the power
generation equipment.
[0008] Using heated air as the stripping gas will increase the
removal of carbon dioxide and ammonia. Using heated air will also
increase the temperature of the biofilter, thus increasing moisture
content (90+% saturation from stripping), biological activity, and
nitrogen and/or hydrogen sulfide fixation into solid compounds.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The foregoing aspects and many of the attendant advantages
of this invention will become more readily appreciated as the same
become better understood by reference to the following detailed
description, when taken in conjunction with the accompanying
drawings, wherein:
[0010] FIG. 1 is a schematic illustration of one embodiment of a
method according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0011] FIG. 1 schematically illustrates one embodiment of a method
according to the present invention. Anaerobically digesting liquid
waste 10, such as flush water containing cow manure from dairy
farms, is followed by gas stripping or desorption 28 of dissolved
carbon dioxide and ammonia from the digested liquid. The stripped
gases are treated in a bacteria-activated "biofilter" 38, that
converts ammonia into nitrate compounds and other nitrogenous
compounds that can then be used as fertilizer. The method according
to the invention combines the processes of anaerobic digestion,
desorption, and nitrification to advantageously recover nitrogen,
which would otherwise be discharged into the atmosphere.
[0012] The system to carry out the method according to the present
invention, includes an anaerobic digester 10, a gas desorption or
stripping unit 28, a nitrification absorption unit or biofilter 38,
and optionally a solids separator 18. It is to be appreciated that
the figure represents a portion of what may be a larger integrated
system. For example, the influent liquid waste 12 may have
undergone pretreatment before arriving at the anaerobic digester
10, such as by being processed through several screens and
sedimentation, or holding ponds. However, pretreatment of the waste
in this manner may also potentially reduce the amount of solids
that can be converted into valuable products. These predigester
unit operations are described in Dairy Waste Anaerobic Digestion
Handbook, by Dennis A. Burke (2001), incorporated herein by
reference in its entirety. The Handbook is available on the
Internet at www.makingenergy.com. The system according to the
invention may be integrated into existing dairy farms to recover
the nitrogen that is otherwise lost into the atmosphere through the
conventional process of treating water generated by dairy
operations.
[0013] According to the invention, the anaerobic digester 10
digests the influent liquid waste stream 12. The influent liquid
waste 12 can include, but is not limited to, water, collected
during the typical operation of a dairy farm. Anaerobic digestion
10 produces digested liquid waste 14 through the breakdown of
organic material via a microbial population that lives in the
oxygen-free environment in the digester 10. When organic matter
decomposes in an anaerobic environment, the bacteria produce, at a
minimum, methane and carbon dioxide gas. Nitrogen-containing
compounds are converted to ammonia, and sulfur-containing compounds
are converted into hydrogen sulfide. In the system according to the
invention, these gases can be vented from the digester 10 via vent
line 22. Some gases are soluble and can remain dissolved within the
liquid. Vented methane gas can be used as an energy source.
Methane, and other combustible gases, produced during digestion,
and drawn from vent line 22 can be routed to power generation,
block 44. For example, boilers can burn the methane gas to produce
steam that can be used to drive turbines. Heat can be recovered
from the boiler via numerous methods. Forms of power generation
include steam turbines, methane engines, and fuel cells. Each power
generation method produces some amount of heat that can be
reclaimed as heated stripping gas. Alternatively to power
generation, the methane and other combustible gases can be disposed
of by flaring to a stack. Heat can be reclaimed by this later
method as well.
[0014] The digested liquid waste 14, leaving the digester 10,
includes water, soluble organic and inorganic compounds, such as
soluble gases, and insoluble organic and inorganic compounds.
Reference is again made to the Dairy Waste Anaerobic Digestion
Handbook for a more detailed description of anaerobic digestion. In
particular, representative examples of suitable anaerobic digesters
for use in one embodiment of the method according to the present
invention are described therein, including, but not limited to,
covered anaerobic lagoons, plug flow digesters, mesophilic
completely mixed digesters, thermophilic completely mixed
digesters, anaerobic contact digesters, and hybrid contact/fixed
film reactors, and the like. Such digesters are at least suitable
for processing dairy waste. Other anaerobic digesters include
packed fixed film reactors, upflow anaerobic sludge blanket
reactors (UASB), and horizontal baffled reactors, described also in
the Handbook.
[0015] After anaerobic digestion 10, the liquid waste 12 contains
carbon dioxide, ammonia, hydrogen sulfide, and other dissolved
inorganic components, such as alkaline compounds. Typically, the
majority of ammonia remains dissolved in digested waste liquid 14,
as the ammonium ion. The pH of digested liquid waste 14 can be
between about 7 and about 8. The anaerobic digestion process also
produces carbon dioxide that dissolves in the digested liquid waste
14. The amount of dissolved carbon dioxide is a function of at
least the partial pressure of carbon dioxide, which is typically
between about 25% to about 40% of the total pressure within the
anaerobic digester 10. In accordance with the present invention,
the anaerobic digestion process can be used to produce a digested
liquid product, which has a substantial amount of carbon dioxide
and ammonia dissolved therein. The anaerobic digestion process also
uses heat through the thermophilic or mesophilic digestion of the
liquid waste influent 12. Since the subsequent process of ammonia
stripping is both temperature and pH dependent, higher digestion
temperatures and pH values in the anaerobic digester 10 are
advantageous in the stripping process to recover ammonia. Lower pH
values are advantageous in the stripping process to recover
hydrogen sulfide. Higher pH values are achieved by stripping carbon
dioxide, lower pH values are achieved by carbon dioxide through the
stripping process by using a high carbon dioxide content gas. Both
low and high pH values to remove hydrogen sulfide and ammonia can
be achieved by first stripping with the combustion gas followed by
stripping with air.
[0016] In one embodiment of the present invention, a liquid/solid
separator 18 can be provided after anaerobic digestion 10. The
separator 18 is an optional piece of equipment. Separation is
optional, according to one embodiment of the invention. Digested
liquid waste 14 includes a solid phase and a liquid phase. The
separator 18 is provided to separate the solid phase from the
liquid phase. The separated solid phase 20 can be recycled to the
anaerobic digestion process 10. Additionally, or alternatively, the
separated solid phase 20 can be diverted to other operations, which
process the solid phase into compost or other beneficial products
24, or alternatively used in biofilter 38. The solid phase will
contain some nitrogen compounds and the majority of the phosphorus.
Digested liquid waste having reduced solids is represented by
reference numeral 16. Representative separators 18 include filters,
screens, screw presses, flotation, and gravity separators. A
suitable flotation separator is described in U.S. application Ser.
No. 10/194,451, filed Jul. 11, 2002, incorporated herein by
reference in its entirety. Solid-phase separation methods and units
are explained in many engineering textbooks, such as the Chemical
Engineers' Handbook, 5th ed., by Perry and Chilton, incorporated
herein in its entirety by reference. A stripping unit 28 to recover
ammonia follows the separator 18 or, if no separator 18 is
provided, the anaerobic digester 10.
[0017] According to the present invention, digested liquid waste,
containing solids 14 or reduced solids 16, is stripped of at least
some ammonia, carbon dioxide, or hydrogen sulfide in a stripping
unit 28. As used herein, digested liquid waste can refer to
digested liquid waste containing solids 14, or digested liquid
waste with reduced solids 16 if the optional separator 18 is used
prior to stripping 28. The stripping unit 28 receives digested
liquid waste 14 or 16, containing at a minimum dissolved carbon
dioxide, dissolved hydrogen sulfide and dissolved ammonia. Carbon
dioxide, hydrogen sulfide and ammonia (acting as the sorbate) can
be stripped from the liquid by contact with a stripping gas 30
(acting as the sorbent) through a process known as desorption. The
stripping gas 30 is preferably heated above the prevailing ambient
atmospheric temperature. In one embodiment of the present
invention, the heated stripping gas 30 is produced in the power
generation step, block 44. Heat produced during power generation
can be reclaimed by employing various methods. For example, the
combustion gas that is produced during burning of the methane, or
other combustible gas produced via the digestion process, can be
used as the stripping gas 30. Combustion gas used in stripping can
have a carbon dioxide content of at least 30% by volume and at
least 5% oxygen by volume. The temperature of the heated stripping
gas 30 in this instance can be about 350.degree. F. or greater.
Combustion gas contains higher amounts of carbon dioxide as
compared with atmospheric air that will tend to lower the pH. A
lowering of the pH favors the removal of hydrogen sulfide gas as
compared with ammonia gas. This is due, it is believed, to the
hydrogen sulfide ions in solution (HS.sup.-) gaining a hydrogen ion
(H.sup.+) under reduced pH conditions, and thus increasing the
amount of hydrogen sulfide (H.sub.2S) in solution. Heat can also be
reclaimed using heat exchangers wherein ambient atmospheric air can
be directed to pass on one side of the heat exchanger and hot
water, condensate, steam, or combustion gas is passed on the
opposite side of the heat exchanger to thus transfer heat from the
hot medium to the relatively colder ambient air. The temperature of
air is thus heated above the ambient temperature. The temperatures
achieved by heat exchangers can be as much as about 200.degree. F.
or greater. Alternatively, a power generator can be housed in a
building whereby the ambient air surrounding the power generation
equipment is heated via radiant and convection means. The air from
the building can be collected and used as the heated stripping gas
30. Air heated via this later method can be about 80.degree. F. to
about 110.degree. F. Heated stripping gas 30 produced from radiant,
convection, and conduction sources, such as heated atmospheric air,
will be lower in carbon dioxide as compared with combustion gas,
and thus will be more efficient at removing carbon dioxide. The
reduction in carbon dioxide will thus increase the pH of the
digested liquid waste and improve ammonia recovery. In one
embodiment of the present invention, therefore, the pH of the
digested liquid waste may first be lowered by stripping with heated
air, effectively removing carbon dioxide and ammonia, followed by a
second stripping process using combustion gas, effectively lowering
the pH, and stripping hydrogen sulfide from the digested liquid
waste. However, in other embodiments, stripping hydrogen sulfide
with combustion gas can be followed by stripping ammonia with
heated air. In yet other embodiments, heated air or combustion gas
can be employed as the sole stripping gas 30. Stripping with heated
air raises pH, removes acidic gases, such as carbon dioxide, from
the digested liquid waste (with or without solids) for ammonia
removal, while stripping with combustion gas lowers pH, adds acidic
gases, such as carbon dioxide, to the digested liquid waste (with
or without solids).
[0018] The stripping gas 30 can contain comparatively lower amounts
of carbon dioxide and ammonia as compared with the digested liquid
waste 14 or 16, so as to provide a concentration gradient that will
cause the diffusion of the carbon dioxide and ammonia from the
digested liquid waste 14 or 16 into the stripping gas 30. It is to
be appreciated that molecular diffusion is but one process that can
be occurring to cause the carbon dioxide and ammonia to transfer
into the stripping gas 30. As desorption of carbon dioxide takes
place, the pH of the digested liquid waste 14 or 16 may increase
due to the removal of the carbon dioxide. As the pH of the digested
liquid waste increases, some of the ionized ammonia (ammonium ions)
will be converted into gaseous ammonia. A portion of this ammonia
may be stripped in conjunction with the carbon dioxide. Eventually,
the carbon dioxide concentration of the digested liquid waste 14 or
16 will be reduced, such that the concentration of carbon dioxide
approaches equilibrium with the concentration of the carbon dioxide
of the stripping gas 30. At this point, the pH of the digested
liquid waste 14 or 16 can be between about 8 to about 9. At this
pH, the ammonium ions tend to form into ammonia and are removed by
the stripping gas. As the gaseous ammonia is removed, ammonium ions
continue to be converted to gaseous ammonia. The fundamentals of
desorption or stripping is explained in many engineering books,
such as the Chemical Engineers' Handbook, 5th ed., by Perry and
Chilton, pp. 14-2 to 14-16, incorporated herein by reference, in
its entirety. Typically, packed towers or plate towers are used to
carry out gas desorption from liquids, each type of tower having
its advantages and disadvantages, depending on the ultimate
application. The engineering literature has fuller descriptions of
suitable stripping towers. For example, reference is made to the
United States Environmental Protection Agency Paper, EPA
832-F-00-019 (September 2000), incorporated herein by reference in
its entirety.
[0019] As an alternative embodiment, caustic, or alkaline chemicals
26, may be added to the digested liquid waste 14 or 16, to raise or
lower the pH to assist in the stripping of ammonia or hydrogen
sulfide from the digested liquid waste. As shown in the figure,
alkalinity can be added to the digested liquid waste 14 or 16
either before the stripping unit 28 or to the stripping unit 28.
The amount of alkalinity can be varied. A suitable amount of
alkalinity to add can be obtained by balancing the consumption of
energy required in the stripping process and the cost of alkaline
chemical addition. A suitable alkaline compound can be sodium or
calcium hydroxide, and the like. Magnesium hydroxide or magnesium
oxide can also be added, in this case, before or after anaerobic
digestion.
[0020] As another alternative embodiment, the temperature of the
digested liquid waste 14 or 16 can be increased. Increasing the
temperature of the digested liquid waste 14 or 16 will result in an
increased rate of stripping the ammonia from the liquid. A heat
exchanger can be provided in the line to the stripping unit 28 or,
additionally or alternatively, the stripping unit can be provided
with a jacket surrounding the stripping vessel and so provide for
heat exchange between a comparatively hot fluid and the digested
liquid waste 14 or 16. Heat-providing media 32, such as steam, or
other condensable fluids, or hot liquids, can be introduced
directly, or alternatively, on the shell or tube side of a heat
exchanger or the jacket of the stripping vessel 28. The temperature
of the digested waste liquid 14 or 16 may be varied according to
the desired amount of stripping performance. The system can readily
be provided with heat transfer equipment to provide heat, in
addition to the heat that is produced during anaerobic
digestion.
[0021] Increasing the pH of the digested liquid waste 14 or 16 by
introducing alkalinity to the digested liquid waste 14 or 16,
either before the stripping unit 28 or in the stripping unit 28,
and raising the temperature of the digested liquid waste 14 or 16,
is believed to increase the rate at which ammonia can be stripped
from the digested liquid waste 14 or 16. The stripping unit 28 can
produce a stripped liquid-phase component 34 having reduced
quantities of soluble compounds and a gas-phase component 36
containing the soluble compounds.
[0022] Decreasing the pH of the digested liquid waste 14 or 16 can
be achieved by adding acidity. Such pH reduction can come about
through the introduction of acidic compounds, such as but not
limited to hydrochloric, sulfuric, and phosphoric acids. The
stripped liquid waste 34 is discharged from the stripping unit 28
and may be processed further. Stripped gas 36 from the stripping
unit 28, contains, at a minimum, ammonia hydrogen, sulfide, or
both. A nitrification biofilter 38, located downstream of the
stripping unit 28, can be used to convert the ammonia that is
stripped from the digested liquid waste 14 or 16 into nitrogenous
compounds, other than ammonia, by bacterial activity. Nitrate
compounds are the result of a process referred to as nitrification.
The literature is replete with descriptions of the nitrification
process that oxidizes ammonia into nitrite by Nitrosomonas
bacteria, and from nitrite to nitrate by Nitrobacter bacteria. The
nitrate compounds 40 can be collected, and further processed,
and/or refined into desirable products, such as nitrogen-rich
compost, or fertilizer. The gas-phase component 42 can be
discharged from the biofilter 38.
[0023] A nitrification biofilter 38 according to the invention may
include a fibrous material, such as compost or a synthetic porous
media capable of supporting a bacterial consortium for the
conversion of ammonia to nitrate compounds and organic biomass
containing nitrogen. The biofilter 38 can have sufficient nutrient
value to support the bacterial consortia. Additionally or
alternatively, nutrients and micronutrients may be added to ensure
adequate bacterial performance. Moisture may also be added, if
sufficient moisture is not already present in the gas 36 from the
stripping unit 28. The quantity of moisture present in the gas 36
may be a function of the temperature at which the digestion and
stripping phases are carried out. According to the present
invention, the biofilter can support a bacterial consortia that
will absorb and precipitate the ammonia gas into nitrate compounds
and/or organic biomass containing nitrogen. Depending on the type
of biofilter used, the biofilter media 40 can be replaced from time
to time with new media. During the replacement of the biofilter
media, the newer media may be seeded with bacterial consortia of
the previous filter media. The filter media that are removed will
be substantially higher in nitrogen compounds than the original
filter media.
[0024] Biofilters are presently being used to eliminate odors from
buildings, such as barns, by venting the barn air through a bed of
organic material. As the air passes through the organic medium,
microorganisms convert the organic gases into carbon dioxide and
water. Literature on biofiltration regarding the recovery of
nitrogen from ammonia is also available. For example, reference is
made to the articles, "Biofilters for Odor Control," D. Schmidt, et
al., University of Minnesota, and "Gaseous Ammonia Removal in
Biofilters: Effect of Biofilter Media on Products of
Nitrification," J. A. Joshi et al., Rutgers, The State University
of New Jersey, both incorporated herein by reference in their
entirety.
[0025] Alternatively, aerobic biofiltration can be utilized to
convert hydrogen sulfide into sulfuric acid or elemental sulfur.
Hydrogen sulfide gas is readily soluble in water. Thus, any
hydrogen sulfide in the stripped gas 36 can be removed with the use
of an aerobic biofilter.
[0026] While the preferred embodiment of the invention has been
illustrated and described, it will be appreciated that various
changes can be made therein without departing from the spirit and
scope of the invention.
* * * * *
References