Note: Descriptions are shown in the official language in which they were submitted.
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METHODS FOR TREATING WASTEWATER SLUDGE
Background
1. Field of the Invention
This invention relates to methods for the treatment and stabilization of
municipal wastewater sludge, and, in particular, to methods for treating
wastewater
sludge cake with acid and calcium carbonate to produce a finished product,
useful as
a fertilizer, a soil conditioner or a synthetic soil component.
2. Description of the Background
Methods for the treatment of wastewater sludge to economically and
safely produce a useful, soil-like end product, such as fertilizer, are needed
in order to
deal with the large amounts of wastewater sludge being produced by
municipalities and
industries. Alkaline stabilization and advanced alkaline stabilization are two
conventional municipal wastewater sludge processes that have been used to
produce an
alkaline a soil-like end product used as a soil conditioner to control soil
pH. Although
the products of these processes may qualify as either Class A or Class B with
regard to
pathogen standards per 40 C.F.R. 503 Rule of the USEPA, the end products and
processes have the negative characteristic of emitting significant amounts of
ammonia
and amines, such as trimethyl amine or diethyl amine both during processing as
well as
from the final product. As a result, the end products and processes for making
them are
clearly nuisances. For example, the laws of the State of New Jersey with
regard to odor
emissions make it very difficult to distribute such products for agricultural
usage within
the state.
Summary of the Invention
The present invention overcomes the problems and disadvantages
associated with current strategies and designs and provides methods for
treating
wastewater sludge cake with acid and calcium carbonate or calcium carbonate-
containing materials under acidic, low heat conditions to produce a granular
or soil-like,
finished product containing calcium carbonate.
The present invention modifies or improves conventional sludge
stabilization processes in that the undesirable emission of ammonia and amines
does
not occur during processing or upon product completion or storage. The present
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invention also introduces a group of oxidant and non-oxidant odor control
agents both
prior to the addition of the acid and after the addition of the acid. These
odor control
agents include, for example, activated copper sulfate, ferrous sulfate, and
other oxidants
such as chlorine dioxide, stabilized liquid chlorine dioxide, hydrogen
peroxide, sodium
hypochlorite, ozone or combinations thereof. In addition, the invention
produces a safe,
odorless or nearly odorless product useful as a fertilizer, synthetic soil or
soil
conditioner for pH control as an ag-lime. This property is due at least in
part to the
calcium carbonate content of the final product. The invention also provides
the
advantage over traditional alkaline stabilization in that nitrogen contained
within the
ammonium and amines within the sludge is preserved in the product so that the
nitrogen
is available for agricultural use.
Accordingly, one embodiment of the invention is directed to a method
for treating wastewater sludge to produce a stabilized product comprising the
steps of
mixing wastewater sludge comprising preferably between 10% and 40% solids with
an
acid to form an acid-sludge mixture, optionally adding an oxidant to assist
with odor
control of the mix, mixing the acid-sludge mixture with an amount of a
material
containing calcium carbonate, wherein the amount is such that the pH of the
mixture
does not exceed a safe handling pH of between 6 and 9, preferably no more than
8, and
drying the mixture to produce the stabilized product. Generally, when the
mixture is
at least pH 7.0, the carbonate is in a carbonic acid form and will not control
pH as
equivalent to an agricultural limestone or a~ lime.
Another embodiment of the invention is directed to a method for
treating wastewater sludge to produce a stabilized product comprising the
steps of
mixing wastewater sludge comprising preferably between 10% and 40% solids with
an
amount of a material containing calcium carbonate, wherein the amount is such
that the
pH of the mixture does not exceed about 7.0, mixing the calcium carbonate-
containing
sludge with an acid to form an acid-sludge mixture, and drying the mixture to
produce
the stabilized product.
Another embodiment of the invention is directed to a stabilized product
made by the method of the invention.
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Other embodiments and advantages of the invention are set forth in part
in the description which follows, and in part, will be obvious from this
description, or
may be learned from the practice of the invention.
Description of the Drawings
Figure 1 Schematic of a preferred embodiment of the process of the invention.
Figure 2 Graph showing an absence of odorants emitted by products made by the
process of Figure 1.
Figure 3 Schematic showing another embodiment of the process of the invention.
Figure 4 Schematic showing another embodiment of the process of the invention
that introduces heat from the reaction of anhydrous ammonia with acid
during the formation of the product but immediately prior to the
introduction of the calcium carbonate containing materials.
Description of the Invention
As embodied and broadly described herein, the present invention is
directed to methods for treating wastewater sludge cake with acid and calcium
carbonate under acidic, low heat conditions to produce a soil-like finished
product
containing calcium carbonate, useful as a fertilizer, synthetic soil component
or soil
conditioner. The invention is also directed to products produced by these
methods.
It has been surprisingly discovered that wastewater sludge can be treated
so that the material is stabilized and safe wherein the treatment process
produces little
or no emissions of ammonia or amines from the material during mixing or after
product
completion. The invention includes a recipe that processes a sludge cake of
preferably
between 10% and 40% solids and mixes the cake with acid (e.g. hydrochloric,
sulfuric,
nitric and/or phosphoric). After this initial mix, the cake is further mixed
with material
containing calcium carbonate, being careful to ensure that the pH of the
mixture is
between 6 and 9, preferably less than 8. The odor emissions that occur during
mixing
may be controlled by the optional addition of odor control agents, oxidizing
agents such
as, for example, chlorine dioxide, ozone, or hydrogen peroxide, prior to or
after,
preferably immediately after, addition of the acid in the sludge processing.
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By treating and stabilizing municipal wastewater sludge according to the
invention, a desired product is produced that:
is safe with regard to pathogens, i.e. achieves either the USEPA's Class
A (preferably) or Class B standard;
S is treated under acidic conditions to minimize and prevent the emission
of amines and ammonia during and after sludge processing;
omits no significant ammonia (less than 10 ppm) into the inter-soil space
or head space above the product;
omits no significant amines (trimethyl amine, diethyl amine) (less than
2 ppm) into the inter-soil space or head space above the product;
omits no significant dimethyl sulfide (less than 2 ppm) into the inter-soil
space or head space above the product;
is greater than 50% solids, preferably greater than 80% solids, and, more
preferably, greater than 90% solids (i.e. 10% or less water);
can be marketed as an ag-lime (calcium carbonate equivalent product)
and/or as a fertilizer for the N, P, K, S (nitrogen, phosphorus, potassium and
sulfur)
value; and
provides a finished product in the form of a granule or a soil-like
mixture.
The present invention is the first stabilization process that uses product
heating below 93°C (200°F) to make a product which has
substantially no significant
sludge odors of ammonia, amines or sulfides. The process of the invention
creates a
stable, microbial pathogen-safe, odorless product that can be beneficially
used in homes
or gardens, commercial agriculture or in synthetic soil production. The end
product
preferably contains greater than about 5% calcium carbonate, more preferably,
greater
than about 8% calcium carbonate, and most preferably, greater than about 10%
calcium
carbonate. Pilot production according to the invention, using about 1,000
pounds of wet
weight sludge cake per batch, produced an odorless or nearly odorless finished
product
with the desirable characteristics listed above.
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Conventional municipal sludge cake processing involves either raising
the pH of the mixed material to a strongly alkaline pH such as, for example, a
level
above pH 11.0 as an alkaline processing step, and/or raising the temperature
of the
sludge cake to over 93°C (200°F). In contrast, the present
invention is the first sludge
stabilization process to be operated in acidic or nearly neutral conditions
under low heat
conditions that still produces a stable product which has a significant
calcium carbonate
(e.g. greater than 5 to 10%).
A schematic of a preferred embodiment of the invention is depicted in
Figures 1 and 3. As shown in Figure 1, sludge cake is placed in a mixer where
odor
control agents may optionally be added. Acid is added to the sludge and
further mixing
occurs. Calcium carbonate-containing material is then added, and optionally,
binder
agents. The calcium carbonate-containing material may be selected from the
group
comprised of calcium carbonate, commercial ag-lime, Class C fly ash, Class F
fly ash,
cement kiln dust, lime kiln dust, wood ash, fluidized bed ash, lime-injected
multistage
burner ash, soil or dredgings. Although the actual amount of calcium carbonate-
containing material in the finished product is determined by the level of acid
so that the
pH is between 6 and 9, preferably less than 8, it is preferably desirable that
this percent
is greater than S%. The mixture may be granulated as desired. To assist with
formation
of hard granules a binder agent such as, for example, lignin or molasses may
be added
to the mix. Alternatively, wastewater sludge comprising preferably 10% to 40%
solids
is mixed with an amount of a material containing calcium carbonate, wherein
the
amount is such that the pH of the mixture is between 6 and 9, preferably less
than 8
(Figure 3). The calcium carbonate-containing sludge is mixed with an acid to
form an
acid-sludge mixture.
The resulting mixture from either alternative is then dried to produce the
finished product. Drying may be accomplished by exposing the mixture to
mechanical
drying by fluidized bed, rotary drum, infrared or microwave processing. The
initial
mixer permits the addition of one or more odor control agents which minimize
the
generation of odors, especially sulfides, that are created as when sulfuric
acid is added
in the second mixing step. The odor control agents are comprised of a group
which may
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include stabilized liquid chlorine dioxide, activated copper sulfate, ferrous
sulfate, and
other oxidants such as hydrogen peroxide, sodium hypochlorite and ozone. These
agents have the additional advantage of disinfecting the sludge cake material
in the
mixing process. The acid addition step may utilize one or more acids such as,
for
example, sulfuric acid, sulfamic acid, phosphoric acid, hydrochloric acid,
nitric acid
and/or acetic acid, propionic acid, butyric acid or a combination thereof.
Figure 2 is a graph that depicts the odorant production of a typical
unprocessed sludge (sludge), an alkaline stabilized product (N-Viro Soil) and
the
product of this invention (STABLE Stabilized Thermoprocessed Acidified
Biosolid
with Lime and Essential Nutrients). The data in Figure 2 show the lack of
specific
odorants (as detected by instrumentation) emitted by the product (STABLE) made
by
the process of the invention. In addition, the human nose is unable to detect
sludge
odors (amines or sulfides) emitted from the finished product. Very little
ammonia is
detected (as determined by smell) in the finished product. This is significant
in view
of regulations making the use of odorous products difficult. The material
produced by
the invention (which may be odor-free or nearly so) should prove acceptable to
the
wastewater industry, the consumer, the public and the regulatory community.
Processes performed according to preferred embodiments of the
invention meet USEPA 40 C.F.R. Part 503 Class A pathogen standards. For
example,
the following two embodiments have achieved these standards in operations at
the pilot
scale level.
1. One such preferred embodiment for achieving Class A product
designation is based upon the exothermic reaction of concentrated acids that
have been
mixed with the sludge with various compounds in the calcium carbonate-
containing
admixture (e.g. calcium oxide and calcium hydroxide) as well as reactions with
compounds present in the sludge cake (i.e. ammonia or ammonium ion). The
calcium-carbonate containing material may be supplemented with one or more
oxidants
such as, for example, calcium oxide, calcium hydroxide, sodium hydroxide,
potassium
hydroxide, ammonium hydroxide or other similar oxide or hydroxide compounds to
assist in achieving the necessary exothermic reaction. The exothermic
reactions may
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be achieved using one or more acids, e.g. sulfuric, sulfamic, phosphoric,
hydrochloric
or nitric acids. These exothermic reactions are coupled with the production of
heat of
hydration from the reaction of the acid with the water present in the sludge
cake. The
amount of acid and admixture is adjusted so that temperatures of over
75°C are
achieved. This allows the mixture to achieve the time/temperature requirements
published by the USEPA in 40 C.F.R. Part 503 so that the pathogen kill
component of
the 503 rule is met. This time/temperature requirement is met by application
of the
following formula: T (minutes) = 1.896 x 10"/10°v4c. The value C is the
temperature
in degrees Celsius.
2. Another preferred embodiment of achieving Product Class A
standards, described more fully in Example 1 and illustrated in Figure 4,
involves
heating the sludge-acid mixture by adding sufficient anhydrous ammonia and any
additional acid (e.g. concentrated sulfuric acid) such that the temperature of
the mix is
raised to 80°C or greater, according to the same time/temperature
requirement as
described above. Such additions could occur in a mixer at atmospheric
pressures or
could take place under pressure as in a cross-pipe reactor as described in
U.S. Patent
No.5,984,992. The pH of the final product after addition of the calcium
carbonate-
containing admixture step would be preferably less than pH 8.0 so as to
prevent
significant ammonia emissions.
3. Another preferred embodiment for achieving Product Class A
standards, described more fully in Example l, involves heating the soil-like
or granular
product in a mechanical heater such that the product is maintained at a
temperature of
80°C or greater, according to the same time/temperature requirement
described above.
The mechanical heater may be a fluidized bed dryer, a rotary drum dryer, a
infrared
dryer or a microwave dryer.
When the product is dried to over 90% solids it also achieves the Option
#8, which requires wastewater sludge products to be at 90% solids or greater
as
published in the Vector Attraction Rule of 40 C.F.R. Part 503.
The following example is offered to illustrate embodiments of the
invention, and should not be viewed as limiting the scope of the invention.
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Examples
Example 1
Vv astewater sludge cake was received by the Hydropress biosolids
treatment facility located in Phillipsburg, N.J. at 28% solids. The source of
the sludge
was a wastewater treatment facility processing sewage from New York City.
This sludge cake was placed into a mechanical mixer which allowed the
blending of additional materials into the sludge cake. The first addition was
a solution
of an acidified stabilized liquid chlorine dioxide (0.5% by weight of a 10%
solution).
After this was blended, the second addition of an acidified activated copper
sulfate
(0.5% by weight of a 6% solution) was blended. The third addition was 5% by
weight
of concentrated sulfuric acid (93%). As soon as this was blended, an amount of
calcium
carbonate-containing admixture (in this example, a Class C fly ash containing
less than
5% calcium oxide) was added and blended such that the pH of the mixture did
not
exceed pH 8Ø In this example, the amount of Class C fly ash added was 30
lbs. A final
addition of 0.5% ferrous sulfate was added and blended to assist with odor
control of
the finished product.
The final step in the processing of this mixture was to feed the mixture
into a fluidized bed dryer such that the product was heated to 85°C
(185°F). This
temperature was achieved for a period sufficient to achieve the time
temperature
requirement of 503 Rule Class A pathogen standard of the USEPA. In addition,
because the drying achieved over 96% solids, the mixture also achieved Option
8 of the
503 Rule Vector Attraction requirement. By satisfying both provisions of the
503 Rule,
the end product achieves the Class A designation. As such, it can be
beneficially used
in gardening, farming or in synthetic soil production. The product produced in
this
example caused no emission of ammonia or amine or sulfide into either the
inter-soil
space or into the air head space above the product.
Example 2
Wastewater sludge cake was received by the Cypress Chemical
Company in Helena, AR at 28% solids. This sludge cake was vigorously mixed
with
sulfuric acid and then the oxidant, 50% hydrogen peroxide was added until a
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concentration of 500 ppm was reached in the mix. After thorough mixing this
material
was pumped into a Cross-pipe reactor into which was injected an amount of
concentrated sulfuric acid and anhydrous ammonia sufficient to raise the
temperature
of the mix to 85°C (185°F) within the reactor. The mix was
retained in the reactor at
this temperature for over 16 seconds before it was discharged into a
granulator. The pH
of the mix was adjusted to pH 7.8 within the granulator by adding 25% by
weight
pulverized calcium carbonate and a small amount of additional anhydrous
ammonia.
The granular discharge from the granulator was dried in a rotary dryer to 98%
solids.
This process yielded a valuable granular product made up of 8-1-0 (N-P-K)
fertilizer
with 10% sulfate, about 28% ag-lime (calcium carbonate) and about 20% organic
material.
Example 3
In this example, the order of addition of the alkaline material is changed
such that it precedes the addition of the acid. Specifically, the sludge cake
is first
1 S optionally treated with odor conditioning agents as in the Example 1.
Following the
blending in of this material the mixture is next treated with a calcium
carbonate
containing material such as a Class C fly ash. The blending in of this
alkaline material
will temporarily raise the pH of the mixture to above pH 7Ø The pH should be
reduced
as soon as possible by next adding an acid, such as hydrochloric, nitric,
sulfuric or
phosphoric acid, to the alkaline material such that the pH of the mixture is
reduced.
This pH reduction should be at least to pH 10.0 but preferably to below pH 8.0
and
more preferably to between pH 6.0 and pH 7Ø The reduction of pH to these
levels
stops the loss of nitrogen that occurs at alkaline pHs through volatizing
ammonia and
amines while keeping the carbonate species for pH control. This mixture can
then be
dried to over 90% solids by mechanical means during which said mixture is
heated to
above 82°C (180°F) for more than 40 seconds. A diagram
illustrating this method is
presented in Figure 3. The mixture when treated in this manner will achieve
the
USEPA's Class A pathogen standard so that the product of this Example can be
used
in a beneficial manner without restriction and without any public health
concern.
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When allcaline admixtures such as Class C or Class F fly ashes are used
which contain significant amounts of calcium oxide or calcium hydroxide the
amount
of acid that must be added so that the final pH is reduced to below pH 8.0 and
preferably below pH 7.0 and above pH 6.0 is much greater. Although this will
significantly increase the cost of the STABLE process it will still result in
a satisfactory
product that will not emit significant amounts of ammonia or amines. The
calcium
carbonate containing material used in this invention may be selected from the
group
comprising all or some or one of the following: Class F fly ash; Class C fly
ash; lime
injected multistage burner ash; bed ash; wood ash; cement kiln dust; lime kiln
dust;
mined calcium carbonate; pure calcium carbonate; calcium carbonate containing
soils
or dredgings; and other calcium containing materials.
Example 4
In this example, sludge cake was treated with about 33% by weight with
a Class C fly ash containing over 30% calcium hydroxide. This mixture
increased in
1 S temperature to over 52°C but not over $0°C. This temperature
was maintained for over
12 hours. The pH of the mixture was increased to over pH 12.0 and this pH did
not
become reduced to below pH 12.0 for over 72 hours. At this point in the
process
significant amount of ammonia and amines emit from said mixture. This mixture
was
next treated by blending in a solid sulfamic acid such that the pH was
reduced. The pH
reduction which could be accomplished using acids, accomplished a reduction in
the
emission of ammonia and amines. The pH reduction improved the product odor
with
increasing reduction of pH to an optimum of below pH 8.0 and above pH 7Ø
This
soil-like mixture has a percent solids greater than 50% and may be optionally
dried to
a higher percent solids. Said processed mixture maintains a pH of below pH
10.0,
optimally below pH 8.5 and more optimally below pH 7.5. This product can be
used
beneficially as a weak fertilizer, a synthetic soil component or substitute,
or, because
of its calcium carbonate concentration, as an agricultural liming agent (ag-
lime).
Other embodiments and uses of the invention will be apparent to those
skilled in the art from consideration of the specification and practice of the
invention
disclosed herein. All references cited or otherwise identified herein are
specifically and
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entirely incorporated by reference. It is intended that the specification and
examples be
considered exemplary only, with the true scope and spirit of the invention
indicated by
the following claims.
