U.S. patent application number 17/739850 was filed with the patent office on 2022-08-18 for biological solids processing.
The applicant listed for this patent is Schwing Bioset, Inc.. Invention is credited to Thomas M. Anderson, Lakshminarasimha Krishnapura, Charles M. Wanstrom.
Application Number | 20220259113 17/739850 |
Document ID | / |
Family ID | |
Filed Date | 2022-08-18 |
United States Patent
Application |
20220259113 |
Kind Code |
A1 |
Anderson; Thomas M. ; et
al. |
August 18, 2022 |
BIOLOGICAL SOLIDS PROCESSING
Abstract
A method of processing biological solids includes blending a
sludge with calcium oxide and delivering the blended sludge and
calcium oxide to a pressurized container; injecting, into the
blended sludge and calcium oxide in the pressurized container, an
additive capable of exothermic reactions with the calcium oxide;
regulating pH in the pressurized container to produce class A
biological solids from the sludge; and pumping the blended sludge,
calcium oxide, and additive from the pressurized container to a
reactor. A system used for this process include sources of calcium
oxide and biological solids, an additive injector, and a
pressurized reactor.
Inventors: |
Anderson; Thomas M.;
(Naples, FL) ; Wanstrom; Charles M.; (Maplewood,
MN) ; Krishnapura; Lakshminarasimha; (Boca Raton,
FL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Schwing Bioset, Inc. |
Somerset |
WI |
US |
|
|
Appl. No.: |
17/739850 |
Filed: |
May 9, 2022 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16319689 |
Jan 22, 2019 |
11358907 |
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PCT/US2017/043911 |
Jul 26, 2017 |
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17739850 |
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62366797 |
Jul 26, 2016 |
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International
Class: |
C05F 7/00 20060101
C05F007/00; C02F 11/18 20060101 C02F011/18; C05F 17/943 20060101
C05F017/943; C05F 17/10 20060101 C05F017/10; C02F 1/20 20060101
C02F001/20; C02F 1/66 20060101 C02F001/66; C05D 3/02 20060101
C05D003/02 |
Claims
1. A process for treating pathogen-containing sludge of biological
solids, the process comprising: blending a sludge with an alkaline
product; delivering the sludge and the alkaline product to a
pressurized container; injecting, into the pressurized container, a
gaseous or liquid additive; mixing the additive with the sludge and
the alkaline product; delivering the sludge, the alkaline product,
and the additive into a temperature and pressure controlled
reactor; regulating pH and temperature in the reactor from an
exothermic reaction produced by the sludge, the alkaline product,
and the additive to reduce pathogens in the sludge, creating a
reacted sludge; removing ammonia from the reacted sludge with a
vapor and odor recovery mechanism and a water scrubber; and
delivering the sludge to a recovery station to recover end products
from the system.
2. The process of claim 1, and further comprising: blending an acid
with the sludge and the alkaline product before delivering the
sludge and the alkaline product to the pressurized container,
wherein the acid is sulfamic acid.
3. The process of claim 1, wherein ammonia is scrubbed from the
sludge with an air blower and is dissolved into a spent additive
selected from the group consisting of spent sulfuric acid, spent
nitric acid, spent phosphoric acid, and combinations thereof.
4. The process of claim 1, wherein ammonia is scrubbed from the
sludge with an air blower and is dissolved into water.
5. The process of claim 1, wherein the alkaline product is selected
from the group consisting of calcium oxide, fly ash, bed ash,
calcium carbonate, low grade lime, kiln dust, gypsum, and
combinations thereof.
6. The process of claim 1, wherein the sludge is between 50% and
95% water by weight.
7. The process of claim 1, wherein the additive is selected from
the group consisting of carbon dioxide, phosphoric acid, sulfuric
acid, nitric acid, sulfamic acid, carbonic acid, hydronium ions,
and combinations thereof.
8. The process of claim 1, wherein the blending and delivering
occur via a delivery mechanism selected from the group consisting
of a belt conveyor, a conveyer, a drag chain, a feed screw, a
hopper, a twin-screw auger, a positive displacement pressure pump
and combinations thereof.
9. A system for treating pathogen-containing sludge of biological
solids, the system comprising: a first delivery mechanism that
mixes the sludge and an alkaline product; a pressurized container
that receives and temporarily holds the sludge and the alkaline
product; an injector that injects an additive into the pressurized
container with the sludge and the alkaline product; a temperature
and pressure controlled reactor in which the alkaline product and
the additive produce an exothermic reaction that reduces pathogens
within the sludge; and a vapor and odor recovery mechanism that
removes ammonia containing vapor from the sludge.
10. The system of claim 9, and further comprising: a recovery
station that receives the sludge after processing; and a second
delivery mechanism that transports the sludge, the alkaline
product, and the additive from the pressurized container to the
reactor, from the reactor to the vapor and odor recovery mechanism,
and from the vapor and odor recovery mechanism to the recovery
station.
11. The system of claim 10, wherein the recovery station recovers
fertilizer products from the exothermic reaction between the
alkaline product and the additive.
12. The system of claim 9, wherein ammonia is scrubbed from the
sludge by the vapor and odor recovery mechanism with an air blower
and is dissolved into water.
13. The system of claim 9, wherein ammonia is scrubbed from the
sludge by the vapor and odor recovery mechanism with an air blower
and is dissolved into a spent additive selected from the group
consisting of spent sulfuric acid, spent nitric acid, spent
phosphoric acid, and combinations thereof.
14. The system of claim 9, wherein the alkaline product is selected
from the group consisting of calcium oxide, fly ash, bed ash,
calcium carbonate, low grade lime, kiln dust, gypsum, and
combinations thereof.
15. The system of claim 9, wherein the sludge is between 50% and
95% water by weight.
16. The system of claim 9, wherein the additive is selected from
the group consisting of carbon dioxide, phosphoric acid, sulfuric
acid, nitric acid, sulfamic acid, carbonic acid, hydronium ions,
and combinations thereof.
17. The system of claim 9, wherein the additive is injected into
the pressurized container as a liquid or a gas.
18. The system of claim 9, wherein the first delivery mechanism and
the second delivery mechanism are selected from the group
consisting of a belt conveyor, a conveyer, a drag chain, a feed
screw, a hopper, a twin-screw auger, a positive displacement piston
pump, and combinations thereof.
19. A system for treating pathogen-containing sludge of biological
solids, the system comprising: a first delivery mechanism that
receives and mixes the sludge, an alkaline product, and an acid,
wherein the first delivery mechanism is enclosed; a pressurized
container that receives and temporarily holds the sludge, the
alkaline product, and the acid; an injector that injects a liquid
or gaseous additive into the pressurized container; a mixing
mechanism in the pressurized container to combine the additive with
the sludge, the alkaline product, and the acid; a second delivery
method to move the sludge, the alkaline product, the acid, and the
additive through the system; a temperature and pressure controlled
reactor in which the alkaline product, the additive, and the sludge
produce an exothermic and pH altering reaction that reduces
pathogens within the sludge; a vapor and odor recovery mechanism to
remove ammonia containing vapors from the pathogen-reduced sludge;
a water scrubber that scrubs and collects ammonia-containing
products from the pathogen reduced sludge and the ammonia
containing vapors; and a recovery station to recover end products
from the system.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 16/319,689, filed Jan. 22, 2019, which is a
National Stage Application of International Patent Application No.
PCT/US2017/043911, filed Jul. 26, 2017, which claims benefit of the
U.S. Provisional Patent Application No. 62/366,797, filed Jul. 26,
2016, incorporated by reference herein.
BACKGROUND
[0002] This application relates generally to processes for the
treatment of biological sludge, and specifically to processing
biological sludge such that bacteria, viruses, parasites, and other
pathogens are reduced, and to produce biological solids as
characterized by the U.S. Environmental Protection Agency
("EPA").
[0003] Traditionally, biological solids are processed in a method
mixing lime with sludge to sterilize the sludge. Mixing lime into
biological solids in a pressurized container allows for increased
temperature due to exothermic reactions of lime and biological
solids, and an increased pH of the sludge. This results in
processed biological solids that meet class A requirements for
heating, pH, and water content under the EPA, which can be legally
used as fertilizers. This method, referred to as the "Bioset
Method," is described in detail in U.S. Pat. Nos. 5,635,069,
5,868,942, 6,056,880, 6,214,064, and 6,221,261.
[0004] However, as the additives must be purchased, there is an
operating cost associated with these technologies to create the
necessary high temperature environment inside a reactor pipeline.
Therefore, it is advantageous to find a combination of additives
that can attain the same elevated temperature and pH conditions to
reduce the operating expenses of these technologies.
SUMMARY
[0005] A process for treating pathogen-containing sludge of
biological solids includes blending a sludge with calcium oxide and
delivering the blended sludge and calcium oxide to a pressurized
container; injecting, into the blended sludge and calcium oxide in
the pressurized container, an additive capable of exothermic
reactions with the calcium oxide; regulating pH in the pressurized
container to produce class A biological solids from the sludge; and
pumping the blended sludge, calcium oxide, and additive from the
pressurized container to a reactor.
[0006] A system for treating pathogen-containing sludge of
biological solids includes a storage container configured to hold
calcium oxide, a biological solids conveyer configured to move
biological solids, an acid container configured to hold acid, a
twin-screw mixer configured to mix the calcium oxide, the
biological solids, and the acid into a sludge, a piston pump
fluidly connected to the twin-screw mixer, the piston pump
configured to move the sludge, a reactor connected to the piston
pump and configured to the sludge, an injector configured to inject
an additive capable of exothermic reactions with the calcium oxide
into the system upstream of the reactor, and a recovery system
configured to receive and process the sludge.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 depicts a schematic drawing of a biological solids
processing system.
[0008] FIG. 2 is a flowchart depicting a method of processing
biological solids.
DETAILED DESCRIPTION
[0009] This application proposes a method of processing biological
solids using calcium oxide (CaO) in conjunction with an additive
such as carbon dioxide (CO.sub.2). The use of an additive allows
exothermic reactions to occur inside a pressurized reactor
containing watered down biological solids ("sludge"), and increases
the temperature and pH of the sludge such that the resulting
product meets "class A" biosolid requirements as dictated by the
EPA. The resulting product can then be used as fertilizer.
[0010] FIG. 1 depicts a schematic drawing of biological solids
processing system 10, which includes lime storage 12, biological
solids feed screw 14, acid storage 16, hopper 18, auger 19, piston
pump 20, additive injector 22, reactor 24, water scrubber 26, vapor
and odor recovery mechanism 28, and recovery station 30. In system
10, lime storage 12, biological solids feed screw 14, and acid
storage 16 feed into hopper 18, which in turn sends resulting
sludge into piston pump 20. Additive injector inserts an additive
into system 10 in an enclosed, pressurized space downstream of
auger 19, wherein the additive is mixed with the sludge. Sludge is
then fed into piston pump 20 and pumped to reactor 24 where sludge
is treated. Reactor 24 sends treated biological solids into vapor
and odor recovery 28, where recovery mechanism 28 and water
scrubber 26 further treat biological solids. Recovered items end in
recovery station 30.
[0011] Lime storage 12 holds calcium oxide (CaO), often referred to
as "quick lime." Calcium oxide can be mixed with biological solids
to induce exothermic and pH altering reactions. Alternatively,
calcium oxide can be derived from fly ash, bed ash, gypsum, low
grade lime, kiln dust, or other appropriate sources of lime. Lime
storage 12 is connected to hopper 18 such that calcium oxide can be
delivered into hopper 18, typically by a quick lime screw feed.
[0012] Biological solids feed screw 14 feeds biological solids into
system 10. Feed screw 14 is a conveyer, and can alternatively be a
belt conveyor, drag chain, or other suitable conveyance. Biological
solids include organic matter from sewage. Biological solids feed
screw 14 delivers biological solids needing processing to hopper 18
in system 10, where the biological solids are mixed with calcium
oxide. Acid storage 16 contains sulfamic acid or other appropriate
acid that will be mixed in hopper 18 with biological solids and
calcium oxide. Acids are delivered to hopper 18 by a feeder.
[0013] Hopper 18 is a totally enclosed hopper that contains all
dust and odors, and receives calcium oxide, biological solids, and
acid. A counter-rotating, intermeshing, twin-screw auger 19
downstream of hopper 18 provides efficient homogenization of the
biological solids and chemicals, producing sludge. Preferably,
auger 19 is a twin-screw design for more efficient mixing, though
other configurations can be leveraged to mix the sludge with
additives.
[0014] The sludge is delivered to pump 20, can be, for example, a
positive displacement piston pump capable of pumping organic
materials up to 50% solids content at pressures over 1,500 psi.
Piston pump 20 can also include a poppet valve discharge assembly
that allows the use of a sludge flow measuring system that can
determine within 5% of the amount of biological solids pumped
through piston pump 20.
[0015] Additive injector 22 is located downstream of twin screw
auger 19 under hopper 18, but upstream to piston pump 20. Additive
injector 22 is attached to a pressured, sealed compartment in
system 10, through which sludge flows between twin screw auger 19
and piston pump 20. This is the preferred location as it the lowest
pressure area to inject a gaseous additive. Other locations in the
system could be contemplated. Additive injector 22 inserts an
additive (either gaseous or liquid, although a liquid additive
could instead be inserted into hopper 18) into the sludge prior to
sludge entering reactor 24. The additive is a chemical that will
exothermically react with calcium oxide and other compounds (for
example, byproducts of a calcium oxide reaction), driving up the
temperature of the sludge when it is in reactor 24.
[0016] Reactor 24 is an enclosed pipeline through which sludge is
pumped. Reactor 24 is enclosed to container odor and dust, and
thermally insulated. Temperature sensors in reactor 24 monitor
temperature ranges of the sludge. In reactor 24 of system 10, the
reaction of lime with water in the sludge elevates the temperature
of the sludge. Specific methods include using a mixture of lime
(calcium oxide) and the additive (such as carbon dioxide) leveraged
to increase the temperature of biological sludge between 70 and
55.degree. C. under pressurized conditions. Specific required
temperatures are a function of a formula provided by the EPA. The
higher the temperature, the shorter the retention time required in
the reactor. Under the EPA, 55.degree. C. is the minimum
temperature allowed for biological solid processing, while
temperatures above 70.degree. C. are not common. Such reactions
increase pH of the sludge and result in the release of ammonia
naturally present in the sludge at elevated pH and temperature. The
temperature and ammonia act as the stressors to reduce pathogens in
the sludge. Thus, no external heat is required. These reactions are
discussed in more detail with reference to FIG. 2.
[0017] Water scrubber 26 is downstream of reactor 24 and serves as
an ammonia scrubber and collection line with typically 1 to 10 gpm
of water flow. Vapor and odor recovery 28 is an odor control hood
to regulate outgoing sludge, which has been temperature and pH
regulated in reactor 24. Recovery 30 collects end products from
system 10. Recovery 30 leads to the scrubber 26. A blower
associated with scrubber 26 pulls air through vapor and odor
recovery 28 on the discharge of sludge, through recovery 30 and
into water scrubber 26. This allows for capture of ammonia evolved
in system 10. The ammonia is dissolved in water recycled for other
use. In the case of making ammonia sulfate or the other fertilizers
downstream of reactor 24, water scrubber 26 is replaced with
alternate technologies. This type of device would utilize sulfuric
acid rather than water to react with the ammonia to product
ammonium sulfate fertilizer.
[0018] FIG. 2 is a flowchart depicting method 40 of processing
biological sludge. Method 40 includes blending calcium oxide and
biological sludge in a pressurized container (step 42), injecting
one or more industrial byproducts capable of exothermic reactions
with the calcium oxide into the pressurized container (step 44),
and regulating the pH and temperature in the pressurized container
to produce class A biological solids (step 46). Method 30
optionally includes sequestering carbon dioxide generated in the
pressurized container (step 48), recovering fertilizer products
from an exothermic reaction between the calcium oxide and the
industrial byproduct (step 50), and/or scrubbing ammonia with spent
sulfuric acid to precipitate ammonium sulfate in the pressurized
container (step 52).
[0019] Process 40 begins with mixing calcium oxide into biological
solids as described in reference to FIG. 1 (step 42). Added calcium
oxide reacts exothermically in biological solid sludge as it reacts
with water present in the sludge. The reaction for the exothermic
hydrolysis of anhydrous calcium oxide is shown below (Where
.DELTA.H.sub.f.sup.c represents standard enthalpy of formation in
kJ/mol):
CaO+H.sub.2O.fwdarw.Ca(OH).sub.2
.DELTA.H.sub.f.sup.c=-65.05 KJ/gmol
[0020] The moles of calcium oxide to raise the sludge to 55.degree.
C. is 1.92 moles (56 grams/mole) or 107.7 grams per liter of
calcium oxide. The dry weight of calcium oxide to sludge is 107.7
grams of lime to 200 grams of sludge or about 0.53 pounds of
anhydrous lime to pound of sludge. Thus, addition of calcium oxide
to sludge results in heat production and increased temperature
inside reactor 24 of FIG. 1, but a large quantity of calcium oxide
is required. Chemical Calcium Oxide, or "quick lime," used in this
alkaline stabilization process is a major cost component and
alternatives to decrease the quantity of calcium oxide consumed are
preferred.
[0021] Thus, in step 44, an additive is injected into system 10 to
reduce consumption of calcium oxide but maintain high levels of
heat inside reactor 24 to create class A biosolids. One additive
that may be used is carbon dioxide (CO.sub.2). Reaction between
calcium oxide and carbon dioxide, along with other similar
reactions, produce more heat than simply adding Calcium Oxide and
allowing it to react with water. Specifically, addition of an
additive such as carbon dioxide reduces the amount of calcium oxide
required by about 60% (1.92 moles versus 0.80 moles).
[0022] A multitude of exothermic reactions occur when carbon
dioxide is added to the sludge and calcium oxide mixture. First,
the reaction between carbon dioxide and calcium oxide to form
calcium carbonate (CaCO.sub.3) occurs:
CaO+CO.sub.2.fwdarw.CaCO.sub.3
[0023] This reaction creates an enthalpy change of
.DELTA.H.sub.f.sup.c=-158 KJ/gmol.
[0024] Meanwhile, carbon dioxide also reacts with water to form
carbonic acid and calcium oxide reacts with water to form calcium
hydroxide, with the following enthalpy changes:
CO.sub.2+H.sub.2O.fwdarw.H.sub.2CO.sub.3
.DELTA.H.sub.f.sup.c=0.0 KJ/gmole
CaO+H.sub.2O.fwdarw.Ca(OH).sub.2
.DELTA.H.sub.f.sup.c=-65 KJ/gmole
[0025] Subsequently, calcium oxide reacts with the carbonic acid to
produce calcium carbonate and water, and carbon dioxide reacts with
the calcium hydroxide to form calcium carbonate and water, with the
following enthalpy changes:
CaO+H.sub.2CO.sub.3.fwdarw.CaCO.sub.3+H.sub.2O
.DELTA.H.sub.f.sup.c=-158 KJ/gmole
Ca(OH).sub.2+CO.sub.2.fwdarw.CaCO.sub.3+H.sub.2O
.DELTA.H.sub.f.sup.c=-93 KJ/gmole
[0026] Finally, calcium hydroxide additionally reacts with carbonic
acid to produce calcium carbonate and water, while calcium oxide
reacts with hydronium ions to form calcium cation and water with
the following enthalpy changes:
Ca(OH).sub.2+H.sub.2CO.sub.3.fwdarw.CaCO.sub.3+2H.sub.2O
.DELTA.H.sub.f.sup.c=-93 KJ/gmole
Ca(OH).sub.2+2H.sup.+.fwdarw.Ca.sup.+2+2H.sub.2O
.DELTA.H.sub.f.sup.c=-134 KJ/gmole
[0027] These reactions all occur within reactor 24 of system 10
where calcium oxide and carbon dioxide are mixed with the sludge.
The sum of these reactions produces a sufficient amount of enthalpy
to heat reactor 24 up to desired temperatures for processing
biological solid sludge.
[0028] However, using carbon dioxide to react with calcium oxide to
generate more heat requires proper mixing of the gaseous carbon
dioxide and calcium oxide. This can be challenging, as the gas
quickly dissipates into the available air space above the
biological solids through conveyors, mixers, and other
non-pressurized processes. In addition, heat generated is lost
quickly without an insulated closed system, furthermore, ammonia
released a byproduct of high temperature and elevated pH and an
essential component of pathogen reduction will be lost in the open
system.
[0029] Thus, in step 44, carbon dioxide (or other appropriate
additives) are added either in liquid or gas form into the sludge
and calcium oxide matrix by injecting into an area of a pressurized
system that is already pressurized and the carbon dioxide and other
suitable chemical additives remain intimately in contact with the
biological solids and calcium oxide. In system 10, the additive is
added into the feed zone of a pump as described in reference to
FIG. 1. However, carbon dioxide and any other suitable chemical
additives could be injected anywhere within the pressurized system.
In system 10, the location of injector 22 shown in FIG. 1 is a
preferred location. This is because at that point in system 10, the
pressures needed to inject a gaseous additives are at their lowest
point requiring the least amount of energy to accomplish the task.
Additionally, the combined sludge mixture is still being mixed as
it passes through piston pump 22, allowing a more efficient
homogeneous mixing between the sludge and the additive, enabling a
more complete, efficient process.
[0030] Alternatively, in step 44, a different additive can be used
instead of carbon dioxide. If carbon dioxide is still preferred but
unavailable, carbonic acid can be used. Otherwise, inorganic acids
such as sulfuric, nitric, and phosphoric acids will react with
hydrated calcium oxide and reduce the amount of calcium oxide
necessary to maintain high temperatures in reactor 24. These
additives have slightly different results in enthalpy changes,
require different amounts of additives, and additionally allow for
recovery of different fertilizers from the sludge.
[0031] The following reaction shows use of hydronium as an
additive, along with the change in enthalpy:
Ca(OH).sub.2+2H.sup.+----Ca.sup.+2+2H.sub.2O
.DELTA.H.sub.f.sup.c=-133.6 KJ/gmole
[0032] This reaction is intended to show the general chemical
reactions and enthalpy result for use of an inorganic acid with
calcium oxide in system 10.
[0033] The following reaction shows use of sulfuric acid as an
additive (instead of carbon dioxide), along with the change in
enthalpy:
Ca(OH).sub.2+H.sub.2SO.sub.4---CaSO.sub.4.2H.sub.2O
.DELTA.H.sub.f.sup.c=-226.6 KJ/gmole
[0034] Sulfuric acid requires a dry weight of 0.29 pounds sulfuric
acid to every 1 pound of biological solids to heat one liter of
sludge to 55.degree. C. Use of sulfuric acid allows recovery of
ammonium sulfate fertilizer at the end of the process. Spent
sulfuric acid (that is, sulfuric acid post exothermic reaction) can
be recovered with naturally released ammonia to produce this
fertilizer.
[0035] The following reaction shows use of nitric acid as an
additive, along with the change in enthalpy:
Ca(OH).sub.2+2HNO.sub.3---Ca(NO.sub.3)2+2H.sub.2O
.DELTA.H.sub.f.sup.c=-192.47 KJ/gmole
[0036] Nitric acid requires a dry weight of 0.20 pounds nitric acid
to every 1 pound of biological solids to heat one liter of sludge
to 55.degree. C. Use of nitric acid allows recovery of ammonium
nitrate fertilizer at the end of the process. Thus, nitric acid
both provides heat for the process and fertilizer.
[0037] The following reaction shows use of phosphoric acid as an
additive, along with the change in enthalpy:
Ca(OH).sub.2+2/3H.sub.3PO.sub.4---1/3Ca.sub.3(PO.sub.4).sub.2+2H.sub.2O
.DELTA.H.sub.f.sup.c=-175.72 KJ/gmole
[0038] Phosphoric acid requires a dry weight of 0.60 pounds
phosphoric acid to every 1 pound of biological solids to heat one
liter of sludge to 55.degree. C. Use of phosphoric acid allows
recovery of ammonium phosphate fertilizer at the end of the
process. Any of these acids (or carbon dioxide) used as additives
can regulate the temperature in reactor 24 as necessary.
[0039] Additionally, to make biological solids categorized as Class
A by the EPA, an additional level of treatment by pH is necessary.
Thus, step 46 includes pH regulation of the sludge. The calcium
oxide, in addition to providing the heat of reaction, also provides
the increase in pH. The limiting reaction is the temperature, which
requires more calcium oxide than necessary to achieve the proper
pH.
[0040] If more heat is available with the carbon dioxide (or other
additive) and calcium oxide reactions, less calcium oxide can be
used which will lower the overall operating expenses for the
treatment process. This additional heat also allows the opportunity
for lower cost calcium oxide sources that can be used to generate
the necessary heat and also achieve the required pH. Sources such
as waste products from power plants such as fly ash, bottom ash,
and calcium carbonate (aka synthetic gypsum) as well as low grade
lime and kiln dust from the lime kilns which can be obtained for
significantly lower cost than calcium oxide can now be considered.
Other alkaline products (aside from calcium oxide) can also be used
to provide the heat and pH adjustment required for Class A
biological solids. Other alkaline products can also be blended with
calcium oxide or the above waste products in the biological solids
treatment process.
[0041] If carbon dioxide is used, then in step 44, carbon dioxide
can ideally be extracted from the wastewater plant at a low cost to
further reduce expenses. Wastewater plants utilizing Biological
Nutrient Removal processes are a multi-stage process wherein one of
the stages naturally produces carbon dioxide from the respiration
of the organisms in that stage. Wastewater Plants utilizing
anaerobic digestion and producing combustible gases for power
production and other uses, the by-products of this combustion also
produce carbon dioxide that can also be recovered. Other sources
within the plant are potentially available and all carbon dioxide
could be harvested and injected into the process to lower
costs.
[0042] Additionally, the wastewater treatment plant can now reduce
their greenhouse gas output by sequestering the recovered carbon
dioxide within the plant primarily as Calcium Carbonate
(CaCO.sub.3), which is the primary byproduct of the carbon dioxide
reactions within the residual Class A biological solids product.
Alternatively, if carbon dioxide is injected earlier in the process
it will automatically be sequestered as calcium carbonate--this
would not be a separate fertilizer that is recovered. Separate
fertilizer recovery only occurs if the water scrubber of FIG. 1 is
replaced with an ammonia sulfate system where sulfuric acid is the
additive.
[0043] Optionally, in step 50, fertilizer can be recovered from
system 10. As described in reference to step 46, depending on the
additive used, ammonium sulfate fertilizer, nitrate fertilizer, or
phosphate fertilizer can be recovered. These compounds are created
by reactions between the inorganic acid additive and the ammonium
molecules evolved during pH regulation of the sludge by calcium
oxide. For example, ammonia may be scrubbed with spent sulfuric
acid (or other inorganic acid) to precipitate ammonium sulfate (or
other fertilizers).
[0044] Method 40 allows for efficient, cost-effective processing of
biological solids leveraging quick lime (calcium oxide) and
additives that exothermically react with the quick lime and
biological solid sludge. Additionally, carbon dioxide can be
sequestered and recycled if carbon dioxide is used as the additive.
Alternatively, fertilizers can be recovered from inorganic acids
used as additives.
[0045] Although the present invention has been described with
reference to preferred embodiments, workers skilled in the art will
recognize that changes may be made in form and detail without
departing from the spirit and scope of the invention.
* * * * *