Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD OF DEWATERING AQUEOUS WASTE SLUDGE
Background of the Invention
.
This invention relates generally to the treatment
o~ aqueous waste sludge rom food canneries and more specifically
to a novel and useful method of concentrating the solids content
of sludge derived from ~ish waste water from about 10~ to about
~ 40% by wei~ht.
- Previously, liquid waste from fish processing plants
have heen dischar~ed without any treatment. However, this can
be harmful to the environment because the primary waste material
consists o bits and pieces of whole fish which are in a
partially decomposed state. Since the pieces o fish are
organic in nature, they create an oxygen demand on the receivinq
waters, thus depleting the amount or oxygen available to aquatic
life inhabiting the receiving waters. The public concern of
environmental problems relating to the quality of waste water
effl~ent has caused state and federal regulatory agencies to
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impose strict guidelines concerning the disposal of such waste
material. Enforcement of these guidelines in a typical fish
processing plant may require costly disposal methods. For
example, it's a common practice in the fish processing industry
to have waste sludge dewatered by conventional methods to a
concentration of about 6 to 20~ solids and taken to a
l~quid waste disposal site for disposal. This process is highly
uneconomical.
Dewatering processes known in the ~rt for concentrating
aqueous waste sludge from a 6 to 20~ by weight solids content
may include gravity filtration, vacuum filtration, centriuga-
tion, ~lota~ion, and sedimentation. In addition to these
mechanical dewaterlng methods, lt has become standard practice
in the art to chemlcally condltion the waste water prlor to sludge
ormatlon and dewaterlng.
The waste water can b~ treated in any o~e of a
variety of conventional procedures to produae the sludge material
that can be treated in the process of the present invention.
In the broader aspects of the process o~ the present invention,
the process may comprise waste water treatment to aid in the
formation of sludge material as well as treatment of the sludge
material to concentrate the solids content to a~out 40~ by weight.
With no intent to limit the invention, any means that can aid
in the formation of bubbles in the waste water, whether it be
compressed air, aspiratea air or electrolytically produced gas
bubbles may be utili~ed. The bubbles in the waste water suspend
solids as well as fats and oils and move them to the surface
of tne water being treated.
According to the present invention and in conjunction
wlth conventional procedures for treating aqueous waste water,
it is possible to concentrate the waste-water from a fish pro-
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cessing plant having a total solids content of about .02 to 0~5%by weight and a high fat content to a solids concentration of
about 20 to 40% by weight by means of forming a sludge material
and then forming a flocculent material within the sludge and
agglomerating that material and separating insoluble material.
As will be described hereinafter, the concentrated
sludge containing proteinaceous materials can be further
subjected to a dry rendering process and produce blended meat
and bone meal and blended tallow or made into fertilizer.
Alternatively, the dewatered sludge can be incinerated. Thus,
by concentrating the sludge to a relatively high solids content
not only are reduced disposal costs realized, but also efficient
a]ternate uses of the sludge can be obtained.
Summary of the Inv~ntion
Briefly, the present invention comprehends a method of
dewatering aqueous waste water and sludge contalning a high fat
content. The method comprises addin~ a poly~alent metal salt to
the waste water to form a floc. An organic polymer is then
added to agglomerate the floc to finite particles that float to
the surface of the waste water. Bubbles are formed in the waste
water. The particles of floc from the surface of the waste
water are combined with any sediment that has formed. The pH of
this waste water sludge mixture is then adjusted to within the
range of 4.0 - 5.0, sufficient to reach the isoelec~ric point of
the protein, causing the protein to precipitate. An organic
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polymer is added to the sludge mixture to agglomerate sludge
particles. The solid material is then separated from the sludge
to provide a solids content of about 10 to 40 percent by weight,
preferably 25 to 40~ by weight.
These and other aspects and advantages of the
invention will become apparent hereinafter.
Description of the Drawings
In the drawings which illustrate an embodiment of the
present invent:ion,
FIGURE 1 is a flow diagram illustrating the process of
the present invention.
Description of the Preferred Embodiments
... .
The present invention was developed largely for
treating aqueous sludge from the processing of food,
particularly fish having a high fat content :Ln the range of 40
to 70% by weight. Therefore, it will be explained largely with
respect to fish processing and has special application to such
processing, although it can be used for other food processes in
the broader aspects of the invention.
Conventional methods of treating sludge obtained from
waste water commonly involve three general steps: the creation
of flocculent material, agglomerating that material and
separating that insoluble material from the indigenous liquid.
However, it is increasingly more difficult to apply these steps
and dewater sludge containing a high fat or oil content. The
specific gravity differential in the aqueous supernatant and
insoluble solid material is decreased as the fat or oil content
is increased in the aqueous sludge. Notwithstanding this
problem, a critical feature of the present invention is the
adjustment of the pH of the sludge to about the isoelectric
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point of the protein material present in the sludge, and
optionally heating the sludge, to provide proper agglomeration
of flocculent material in the sludge. Further, another critical
feature of the present invention involves the separation and
concentration of solid material by high gravity centrifugation
to unexpectedly provide a dewatered sludge with a concentration
of solid material in the range of about 20 to 40% by weight.
Accordingly, waste water which is obtained from the
processing of canned food products, particularly canned fish
products such as tuna is stored in collection tank 1 after
passing through rotostrainers. Waste water treatment can be
accomplished b~ any conventional procedure and the following
procedure is meant to be illustrative and not intended to limit
the invention. Prior to entering the tank, the waste water is
passed through a screen having openings the size of about .010
to .030 inches to remove large bones and meat particles as well
as impurities~ The waste water typl~ally comprises about 0.02
to 0.5% by weight total solids and about 0.02 to 0.06% by weight
fat on a dry basis. The proteinaceous material content may vary
from about 25 to about 45~ by weight. The solid material
consists mainly of small particles of meat, bone and scales.
Other insoluble material includes native fish oil and soybean
oil from the packing process. Soluble material in the waste
water, besides water from the wash down, thawing, cooling and
draining operations consists primarily of soluble protein from
cooking water, blood and press water from the processing of fish -~
by-products, etc. As an optional feature, compressed air may be
added to tank 1 to help prevent the proliferation of anaerobic
bacteria and to insure the growth of the natural bacteria
present in the waste water. The air can be supplied at
a low~pressure sufficient to overcome the static pressure
of the water. Preferably, about 2 to 10 psi is utilized.
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The waste water is then passed th.ough a line to dissolved air
flotator 2. A metal salt 3 is fed into the line. Generally
speaking, most suspended liquids or solids in waste water possess
a negative charge and polyvalent metal salts are often added to
react with the negative particles, and by coa~u~ation, to break
an~ colloidal emulsion that may have formed. These metal salts,
such as sodium aluminate, aluminum sulfate, alumt fe~ric sulate,
ferric chloride, lime and other calcium salts, etc., ionize to
their h~droxide structure and form flocculen~ particles. About
4 ppm to about 40 ppm active metal ion species are used in the
practice of this invention.
Compressed air at about 40 to 75 psi is added to the
waste water through restricted valve 4. The use of compressed
air is not intended to be unduly limi.tative for any bubble forming
substance may be utilized such as aspirated air or electrolytically
produced gas. As the air pressure iel released, bubbles form
which will eventually float foraign particles and suspended solids
together with ~ats and oils to the surface of the water in
dissolved air flotator 2. There is a bene~icial relationship
between the bubblesr particles and solids which become attached
thereto. The particles and solids act as nuclei for the bubbles
so that the material can be floated to the surface of the water.
Preferably, about 8 to 10 ppm trivalent aluminum ions are pre-
ferred.
To further aid flocculent formation, a polymer 5 may
be added to the aerated w~ste water in the line going to dissolved
air f~otator 2. The addition of the polymer leads to the forma-
tion and agglomeration of floc to finlte particles that ~loat to
the sur~ace of the water and are skimmed off by a sweep arm and
30 collected in holding tank 6. While the lnventlon is not restricted
to a particular organic polymer~ ~t has been found that both
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anionlc and cationic polymers such as acrylamide or other water
soluble salts o~ acrylic acid can be utilized. These polymers
are characterized by having an intrinsic viscosity at 30C in an
aqueous bu~er at pH 7.0, as measured by a G'annon-Fenski
capillary viscometer, o~ at least from about 0.17 to about 24
dec,liter per gram. These polymers are known in the art and are
avallable ~rom several commercial sources. Pre~erably, Percol~
726, available ~rom Allied Collolds, Inc. Ridgewood,.New Jersey,
is utilized. Generally, about .5 ppm to about 15 ppm polymer is
used in the in~tant process.
Arter the waste water enters ~lotation tank 2, the
top M oat sludge is removed and is added to holdlng tank 6, also
sediment ~rom the flotator ls collected by a bottom sweep arm
through an oriflce and optlonally adde~d to holding tank 6. The
sediment con~ists Or coarse grlt material comprising bits of
bones and meat and other impurlties that pass through the screen
ln the line berore the collection tank 1. The partic1es are too
large ~or agglomeratlon and ~lotatlon The solids content o~
the sludge in holding tank 6 1s about 5 to 10% by weight and the
rat content is about 40 to 70% by weight.
The clari~led liquld ~rom ~lotator 2 is substantially
~ree Or impurities and withln regulatory guidelines so it can
be pumped into a sewer wlthout creating environmental problems.
It can be wlthdraun ~rom ~lotator 2 in the middle Or the tank,
equidistant from the top where the top ~loat sludge is located
and the bottom~ where the sediment is collected.
The material present in holding tank 6 comprises a
~lotable scum and sedimented grit material. This putrid material,
~hlch is transported to liquld waste disposal areas in prior
art processes at a great expense is conducive to rapid bacterial
growth and has little commercial value as a fertilizer or feed.
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The rollowing steps are crltical to the process
of the present inventlon.
The sludge ls treated with an acld 7 to precipltate
the tuna proteln by lowering the pH to an acldic value near or -
at the isoelectric point o~ the protein, usually a pH of 4 - 5
wlth the addition o. any protor. donating compound, such as
mlneral acids, preferably sulfuric, hydrochloric, nitric or
phosphoric acids. In ract, any acid will lower the pH of the
~lxture to the preferred pH range to precipitate the protein.
Most preferably, sulfuric acid will be used to lower the pH to
pre~erably 4.5.
The acid treated sludge is then passed through a line
to a high graYity centrifuge 8 such as one available from thè
Sharples Company. that separates a solid phase ~rom a liquid phase
in such a manner to leave a clarlfied centrate as the liquid
phase that can be recycled in the proc:ess and a dewatered sludge
cake as ~he solid phase. Usually, a i~orce o~ at least about
750XG is necessary ror the proper separation. The higher the
gra~itational force the greater the facility for separation. Prior
to this separation however, the acid treated sludge must again be
treated with agglomerating polymer 5 to aid in the flnal floccua-
tlon and separatlon o~ ~aterials. In thls step, the agglomerating
polymer should be added until the clarity of centrate cannot be
~urther improved. Typically~ this should be in amounts ran~ing
~rom about 5 ppm to about 500 ppm and preferably about 100 to
200 ppm.
The acid treated sludge, having a solids content of
about 5~ to 10% by weight and a fat content Or about 40 to 70~
by weight on a dry basis ls concentrated to a solids content Or
about`20 to 40% ~y weight. A typlcal compositlon of sludge con-
~entrated by the process of the present lnvention is as ~ollows:
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TABLE I
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Percent
Total Solids 38
Protein 13
Total Lipids 19
Water 62
The dewatered sludge with a solids concentration of
about 2~to 40% ls formable and can be easily handled. The costs
for disposing this sludge material in a landfill area are minimal
compared to the higher moisture sludge rendered by conventional
dewatering processes which contains more water therefore being
hea~ier and more costly to dispose of.
~ A modified embodiment of the process for dewatering
waste water sludge comprises heating the sludge materlal prior
to lts introduction into the centri~uge. This is not intended
to be limiting, for the acid precipitation at an elevated tempera
ture only promotes coagulation and precipitation of the protein.
The heating must be at a temperature range surriciently hlgh and
for an effective amount of time to coaF,ulate the protein material
present in the sludge. ~he temperature range may preferably be
rrom about 170F to 212F. I~ this procedure is ~ollowed, the
pH o~ the sludge must still be ad~usted to a range of about 4 to
5 prior to entering the high speed centrlfuge.
As previously discussed, due to the relatively high
concentration o~ solids in the dewatered sludge treated by the
process of the instant invention it may be suitable for a number
o~ uses or disposal methods.
~ he dewatered sl~ldge~ contalning about 20 to about 40%
by welght solids may be combined with meat and bone cscraps and
cooked in a dry rendering kettle at about 212 to 250"F for about
120 minutes and then passed through an expeller press to form
blended mea~ and bone meal and blended tallow. Further, an
animal food product may be obtalned by blending the dewatered
sludge concentrated in the process of the present invention with
~re3h fish scrap or meal.
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The composition o~ the sludge shown ln Table I on
a dry basis has a fat content of 50% and a protein level of
33%. These levels are sufriclent for formulating an animal
~ood.
Addltionally, if the dewatered sludge is not sub~ect
to ~urther processing, it may be disposed of in a solid waste
landflll area at a lower cost than sludge containing larger
amounts of water or lncinerated and the residual ash.may be
disposed of ~n a land~ill area.
To assure that one having ordinary skill in the art
understands the invention, the ~ollowing examples are set forth
as illustrated embodiments of the ~nvention and are not to be
taken in any manner as limitlng the scope o~ the invention which
1~ defined by the appended claims.
xample I
Waste water in the amount oi~ 200,000 gallons weighing
834 tons havlng a solids content Or 0"12% by welght, a ~at
content of 0.07~ by weight and a protein content o~ 0.05% by
~eight is passed through a screen having a pore size of 0.030"
to a holding tank where it is subJected to 14 psi o~ compressed
air. The waste water is then treated with 50 ppm o~ 42% by
welght sodium aluminate and sub~ected to 60 psi of compressed
air while in a line going to a dissolved air ~lotator. Just
prior to entering the ~lotator, 4 ppm Percol~ 726 1~ added. The
top M oat scum and sediment is collected ~rom the ~lotator and
treated with about 1.8 liter sulfuric acid to ad~ust the pH of
the material to 4.3. Then, 250 ppm PercoP 726 is added to the
materlal and it is passed through a Sharples Sludgepak ~ high
gravity centrifuge. The centri~uge is operated at 1300XG. The
sludge collected from the centri~uge had a solids content Or 36.3%
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by weight and wa~ disposed of in a solid waste land~ill.
Example II
The process of ~xample I was followed, but after
treating the waste water material with an acid to adjust the
pH to 4~5 prior to entering the high gravity centrifuge, it
; was heated to 200~F or five minutes. It was then subjected to
high gravity centrifugation and resulted in a sludge having a
solids content of 39.5~ by weight.
As various changes could be made in~the above methods
and products without departing from the scope of the invention,
it is intended that all matter contained in the above description
or shown in the accompanying drawing shall be interpreted as
illustr~tive and not in a limiting sense.
