U.S. patent application number 09/813469 was filed with the patent office on 2002-12-05 for process for selective coagulant recovery from water treatment plant sludge.
Invention is credited to Prakash, Prakhar, SenGupta, Arup K..
Application Number | 20020179531 09/813469 |
Document ID | / |
Family ID | 25212453 |
Filed Date | 2002-12-05 |
United States Patent
Application |
20020179531 |
Kind Code |
A1 |
SenGupta, Arup K. ; et
al. |
December 5, 2002 |
Process for selective coagulant recovery from water treatment plant
sludge
Abstract
Alum, used as a coagulant in water treatment, is recovered from
clarifier sludge by adjusting the pH of the sludge downward to
produce an aqueous clarifier sludge solution, and contacting the
aqueous clarifier sludge solution with one side of a semi-permeable
cation exchange membrane while contacting the other side of the
membrane with an acidic sweep solution. By virtue of the Donnan
co-ion exclusion phenomenon, aluminum ions, which are trivalent,
pass readily through the membrane, in preference to divalent and
monovalent cations, and consequently heavy metal carryover is
relatively low. Organic matter carryover is substantially excluded,
and consequently, the recovered alum can be reused without the
potential for trihalomethane formation. The reactor is preferably
in the form of a stack of spaced membranes, with the aqueous
clarifier sludge solution and the acid sweep solution flowing
through alternate spaces. The same process can be used for recovery
of ferric iron coagulants.
Inventors: |
SenGupta, Arup K.;
(Bethlehem, PA) ; Prakash, Prakhar; (Bethlehem,
PA) |
Correspondence
Address: |
HOWSON AND HOWSON
ONE SPRING HOUSE CORPORATION CENTER
BOX 457
321 NORRISTOWN ROAD
SPRING HOUSE
PA
19477
US
|
Family ID: |
25212453 |
Appl. No.: |
09/813469 |
Filed: |
March 21, 2001 |
Current U.S.
Class: |
210/649 ;
210/650; 210/651 |
Current CPC
Class: |
C02F 1/44 20130101; C02F
2101/20 20130101; C02F 1/5245 20130101; C02F 1/66 20130101; B01D
61/243 20130101; C02F 2303/16 20130101 |
Class at
Publication: |
210/649 ;
210/650; 210/651 |
International
Class: |
B01D 061/00 |
Claims
What is claimed is:
1. A process for the selective recovery of a trivalent metal
coagulant compound from clarifier sludge solution comprising the
steps of: contacting the aqueous clarifier sludge solution with a
first side of a semi-permeable cation exchange membrane having
first and second opposite sides, while simultaneously contacting an
acidic sweep solution to the second side of said cation exchange
membrane, thereby causing trivalent metal ions from the coagulant
compound to pass through the membrane from the first side to the
second side, and hydrogen ions to pass through the membrane from
the second side to the first side.
2. The process according to claim 1, in which the cation exchange
membrane comprises a plurality of sheets of cation exchange
membrane disposed in a stack, the sheets being separated from one
another in the stack by spaces, and in which the solutions are
directed to and from the stack so that the clarifier sludge
solution and the acidic sweep solution flow through alternate
spaces.
3. The process according to claim, in which the cation exchange
membrane comprises a plurality of sheets of cation exchange
membrane disposed in a stack, the sheets being separated from one
another in the stack by spaces, in which the solutions are directed
to and from the stack so that the clarifier sludge solution and the
acidic sweep solution flow through alternate spaces, and in which
the spaces through which the clarifier sludge solution flows are
connected in series, and the spaces through which the acidic sweep
solution flows are also connected in series.
4. The process according to claim 1, in which the pH of the
clarifier sludge solution is approximately 3.
5. The process according to claim 1, in which the pressure
difference across said membrane is maintained substantially at
zero.
6. The process according to claim 1, in which the coagulant
compound comprises alum.
7. The process according to claim 1, in which the coagulant
compound comprises a ferric compound.
8. A process for the selective recovery of alum from clarifier
sludge containing aluminum hydroxide comprising the steps of:
adjusting the pH of the clarifier sludge to a level such that at
least the majority of the aluminum content of the clarifier sludge
is in solution, thereby producing an aqueous clarifier sludge
solution; contacting the aqueous clarifier sludge solution with a
first side of a semi-permeable cation exchange membrane having
first and second opposite sides, while simultaneously contacting an
acidic sweep solution to the second side of said cation exchange
membrane, thereby causing aluminum ions to pass through the
membrane from the first side to the second side, and hydrogen ions
to pass through the membrane from the second side to the first
side.
9. The process according to claim 8, in which the cation exchange
membrane comprises a plurality of sheets of cation exchange
membrane disposed in a stack, the sheets being separated from one
another in the stack by spaces, and in which the solutions are
directed to and from the stack so that the clarifier sludge
solution and the acidic sweep solution flow through alternate
spaces.
10. The process according to claim 8, in which the cation exchange
membrane comprises a plurality of sheets of cation exchange
membrane disposed in a stack, the sheets being separated from one
another in the stack by spaces, in which the solutions are directed
to and from the stack so that the clarifier sludge solution and the
acidic sweep solution flow through alternate spaces, and in which
the spaces through which the clarifier sludge solution flows are
connected in series, and the spaces through which the acidic sweep
solution flows are also connected in series.
11. The process according to claim 8, in which, in the pH adjusting
step, the clarifier sludge is adjusted to a pH of approximately
3.
12. The process according to claim 8, in which the pressure
difference across said membrane is maintained substantially at
zero.
13. A process for the selective recovery of alum from clarifier
sludge containing aluminum hydroxide comprising the steps of:
adjusting the pH of the clarifier sludge to a level such that at
least the majority of the aluminum content of the clarifier sludge
is in solution, thereby producing an aqueous clarifier sludge
solution; circulating the aqueous clarifier sludge solution through
a first circulatory path in which the circulating clarifier sludge
solution contacts a first side of a semi-permeable cation exchange
membrane having first and second opposite sides, while
simultaneously circulating an acidic sweep solution through a
second circulatory path in which the acidic sweep solution contacts
the second side of said cation exchange membrane, thereby causing
aluminum ions to pass through the membrane from the first side to
the second side and into the second circulatory flow path, and
hydrogen ions to pass through the membrane from the second side to
the first side and into the first circulatory flow path while
substantially preventing passage of anions, organic molecules and
suspended solids through the membrane and selectively favoring the
passage through the membrane of aluminum ions over monovalent and
divalent cations; whereby the concentration aluminum in the second
circulatory path increases while the solution in the second
circulatory path remains substantially free of anions and organic
molecules, the concentration of toxic metals in the second
circulatory flow path is maintained at a low level, and the
hydrogen ions regenerate the membrane and acidify the solution in
the first circulatory path, thereby causing further quantities of
aluminum in the first circulatory path to go into solution.
14. The process according to claim 13, in which the cation exchange
membrane comprises a plurality of sheets of cation exchange
membrane disposed in a stack, the sheets being separated from one
another in the stack by spaces through which said solutions flow,
and in which the solutions are directed to and from the stack so
that the circulating flow path for clarifier sludge solution and
the circulating flow path for acidic sweep solution occupy
alternate spaces.
15. The process according to claim 13, in which the cation exchange
membrane comprises a plurality of sheets of cation exchange
membrane disposed in a stack, the sheets being separated from one
another in the stack by spaces through which said solutions flow,
in which the solutions are directed to and from the stack so that
the clarifier sludge solution and the acidic sweep solution flow
through alternate spaces, and in which the spaces through which the
clarifier sludge solution flows are connected in series, and the
spaces through which the acidic sweep solution flows are also
connected in series.
16. The process according to claim 13, in which, in the pH
adjusting step, the clarifier sludge is adjusted to a pH of
approximately 3.
17. The process according to claim 13, in which the pressure
difference across said membrane is maintained substantially at
zero.
Description
BACKGROUND OF THE INVENTION
[0001] In the U.S., there are over one thousand drinking water
treatment plants which use alum,
Al.sub.2(SO.sub.4).sub.3.multidot.14H.sub.2O, as a coagulant for
efficient removal of particulate solids and colloids. In the
treatment process, alum is finally converted into insoluble
aluminum hydroxide, Al(OH).sub.3, which constitutes a major
component, e.g., from about 25% to 50%, of the solids in water
treatment residuals (WTR), i.e., clarifier sludge. The water
treatment sludge is essentially a bulky, gelatinous slurry composed
of suspended inorganic particles, natural organic matter (NOM),
trace amounts of heavy metal precipitates and aluminum hydroxide.
Clarifier sludges are biologically inert and retain near-neutral
pH. The total solids content of the sludge normally ranges from 2
to 10 percent in mass per unit volume. Water treatment plants in
the U.S. produce over 2 million tons of aluminum-laden disposable
solids every day. Due to recent regulatory changes, disposal of
sludges must now be carried out by way of landfills or land
application. Because of the magnitude and pervasiveness of the
disposal problem, alum recovery from clarifier sludge has received
considerable attention. The toxicity of free and complexed aluminum
species to aquatic life, including benthic organisms, is also a
matter of concern, and has been the focus of several studies.
[0002] An ideal solution to the problems of sludge disposal and
toxicity would be a simple-to-operate process which selectively
recovers alum from the sludge to reduce the volume of the
disposable solids, and which delivers the recovered alum in a form
sufficiently pure to be recycled for use as a coagulant at the
front end of the water treatment plant. Such a process will truly
combine pollution prevention with resource recovery, thereby
significantly reducing the stress on the environment.
[0003] When clarifier sludge is sufficiently acidified with
sulfuric acid, insoluble aluminum hydroxide is dissolved in the
form of dilute liquid alum. The stoichiometry of this reaction is
as follows: 1
[0004] This reaction illustrates the underlying concept of the acid
digestion process, which has been tried both at laboratory and
pilot-scale levels.
[0005] The basic concept of the process of alum recovery is simple.
However, the process is subject to shortcomings which have ruled
out the possibility of reusing the recovered liquid alum as a
coagulant. The process is non-selective. Along with alum it also
recovers all other substances that are soluble under highly acidic
conditions or that exist as colloids. If this occurs and the
recovered alum is recycled for water treatment, the potability of
the water will be degraded. Consequently, the acid digestion
process may not be used in areas where such impurities present
problems.
[0006] Potential impurities which can be converted to soluble form
by acidification include iron, manganese, chromium and other
metals, including those metals which are inherently present as
impurities in the sulfuric acid used in the process. For example,
significant concentrations of manganese exist in raw water at some
locations, and the concentration of manganese in the delivered
water may be increased to unacceptable levels when alum is
recovered for reuse as a coagulant. Heavy metals, such as copper,
lead, cadmium, etc. are normally present in relatively low
concentrations in clarifier sludge. However, the concentrations of
these heavy metals may increase to significant levels if the sludge
is recycled in order to recover alum. The problem of increased
concentration of an undesirable substance in the recovered alum can
occur with many different substances present in ionic or colloidal
form in the raw water, especially substances which have a low
solubility and a high settling rate in the clarifier.
[0007] Naturally occurring organic material (humates and fulvates),
which are generally removed quite well by alum coagulation, will be
present in the recovered alum. Should this recovered alum be reused
as a coagulant, the concentration of organic matter in the treated
water will tend to increase, thereby significantly increasing the
potential for formation of trihalomethanes (THMs), which are
suspected carcinogens.
[0008] Since aluminum oxide is amphoteric, theoretically alum could
be recovered from clarifier sludge under alkaline conditions as
well as under acidic conditions. However, dissolved organic carbon
(DOC) tends to increase with dissolved Al(III) under both acidic
and alkaline conditions. Since a high concentration of dissolved
organic matter is very undesirable in recovered alum due to its
potential for formation of trihalomethanes, neither acid nor alkali
digestion processes have been able to achieve satisfactory
selective alum recovery in practice.
[0009] Acid digestion, that is, acidic extraction using sulfuric
acid, is the most widely used method for alum recovery. When
clarifier sludge is sufficiently acidified with sulfuric acid,
insoluble aluminum hydroxide is dissolved in the form of dilute
liquid alum as shown in the equation:
2Al(OH).sub.3.multidot.3H.sub.2O+3H.sub.2SO.sub.4+2H.sub.2O=Al.sub.2(SO.su-
b.4).sub.3.multidot.14H.sub.2O
[0010] This equation illustrates the underlying concept of the acid
digestion process, which has been tried on a laboratory scale, in
pilot scale studies, and in full-scale at one of the water
treatment plants of Durham, N.C. Studies on this process have shown
that the aluminum concentration in recovered supernatant liquid
ranged from 360 to 3700 mg/l. In addition to aluminum, the
recovered alum was also found to contain other metals such as
manganese, zinc and lead. The concentration of heavy metals such as
As, Cr, Cu, Ni, Pb and Zn ranged from 0.002 to 8.5 mg/l. The total
DOC ranged from 326 to 1800 mg/l, which was of the same order of
magnitude as the recovered aluminum concentration. Similarly, the
concentration of humic substances ranged from 160 mg/l to 1140
mg/l. Thus, the acid digestion process is non-selective; it cannot
prevent DOC and heavy metals from being carried over into the
recovered alum. The process is not capable of yielding high
concentrations of aluminum ions, and the recovered alum is subject
to reduced coagulation efficiency due to the presence of dissolved
organic carbon.
[0011] In the case of Alkali digestion, at increased pH, the total
DOC increases markedly. At a pH of 12, the DOC of water treatment
residuals increases rapidly to over 1000 mg/liter. Furthermore at
high pH levels, undissolved organics tend not to settle adequately
and lead to poor quality in the recovered solution.
[0012] Liquid Ion Exchange (LIE) has also been tried. According to
one study, this process can concentrate aluminum to a level as high
as 4000 mg/l from an initial level of 1000 mg/l. However, this
takes place in a second stage of stripping. In the first stage of
extraction, the concentration ratio is 1:1. Entrainment issues are
always a concern in the stripping process, because it involves
separating aluminum ions from an organic phase into which the
aluminum ions are dissolved in the first stage. In the stripping
process, aluminum is recovered from the organic phase and the
latter recycled. Since ideal 100% separation cannot be achieved,
organics are carried over with dissolved aluminum. The liquid ion
exchange process is operationally complex and expensive, and
capable of delivering only a low concentration of recovered alum.
Solvent carryover in the recovered alum is also a problem inherent
in liquid ion exchange, and requires additional treatment
steps.
[0013] Ultrafiltration is another technique that can be employed
following acid treatment. However, ultrafiltration suffers from
various shortcomings including fouling, decreased membrane life due
to pressure differential, a decrease in flux with continued
deposition, and the relatively high cost of pumping.
[0014] Still another approach is the cyclic composite membrane
process described in U.S. Pat. No. 5,304,309, dated Apr. 19, 1994.
In this process, aluminum ions are selectively sorbed from an
aqueous phase (containing dissolved aluminum in acidified WTR) onto
a composite membrane and thereafter desorbed, with the release of
aluminum ions, as the composite membrane is regenerated in a
sulfuric acid solution. The process is carried out in a continuous
cycle characterized by the following equations:
Al(OH).sub.3(s)+{fraction (3/2 )}R--N(CH.sub.2COOH).sub.2={fraction
(3/2 )}R--N(CH.sub.2COO.sup.-).sub.2Al.sup.3++{fraction (3/2
)}H.sub.2O
3R--N(CH.sub.2COO.sup.-).sub.2Al.sup.3++3H.sub.2SO.sub.4={fraction
(3/2 )}R--N(CH.sub.2COOH).sub.2+Al.sub.2(SO.sub.4).sub.3
[0015] The cyclic composite membrane process overcomes many of the
shortcomings of the previous processes in that it selectively
recovers aluminum ions and prevents passage of natural organic
materials, heavy metals and manganese into the recovered alum.
However, composite ion exchange materials are not available in
sizes appropriate for large-scale applications, and the process is
not capable of concentrating alum to high levels. Furthermore, the
process is a two-stage process with inherent complexity, and
subject to various operational problems such as difficulties in
rinsing the membrane.
[0016] An object of this invention is to overcome at least some,
and preferably all, of the above-mentioned deficiencies in
previously proposed alum recovery techniques.
[0017] In accordance with the invention, the pH of clarifier sludge
is adjusted to a level, preferably about 3, such that at least the
majority of its aluminum content is in solution. The pH adjustment
results in an aqueous clarifier sludge solution, which is then
brought into contact with a first side of a semi-permeable cation
exchange membrane, preferably by circulating the solution through a
first circulatory flow path. Simultaneously, an acidic sweep
solution is brought into contact with the opposite side of the
membrane, preferably by circulating it through a second circulatory
flow path. By Donnan dialysis, aluminum ions are caused to pass
through the membrane in one direction while hydrogen ions pass
through the membrane in the opposite direction. Anions, organic
molecules and suspended solids are substantially prevented from
passing through the membrane, and aluminum ions are selectively
favored over monovalent and bivalent cations for passage through
the membrane. Thus, the concentration aluminum in the second
circulatory path increases while the solution in the second
circulatory path remains substantially free of anions and organic
molecules, the concentration of toxic metals in the second
circulatory flow path is maintained at a low level, and the
hydrogen ions regenerate the membrane and acidify the aqueous
clarifier sludge solution thereby causing further quantities of
aluminum in the first circulatory path to go into solution.
[0018] In a preferred mode, the cation exchange membrane comprises
a plurality of sheets of cation exchange membrane disposed in a
stack, the sheets being separated from one another in the stack by
spaces through which the solutions flow, and the solutions are
directed to and from the stack so that clarifier sludge solution
and acidic sweep solution flow through alternate spaces. Preferably
the spaces through which the clarifier sludge solution flows are
connected in series, and the spaces through which the acidic sweep
solution flows are also connected in series.
[0019] The process is preferably carried out with the pressure
difference across the membrane maintained substantially at
zero.
[0020] The process provides for a simple and effective recovery of
alum, substantially free of solids, natural organic matter and
dissolved organic carbon, and with relatively low carryover of
toxic metals. The recovered alum is therefore suitable for reuse as
a coagulant in the same water treatment plant from which it was
recovered, and the formation of trihalomethanes upon chlorination
is substantially avoided.
[0021] Further objects, advantages and details of the invention
will be apparent from the following detailed description when read
in conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a graph illustrating the influence of pH on
dissolved organic carbon (DOC) and Al(III) concentrations in a
typical water treatment plant sludge;
[0023] FIG. 2 is a schematic diagram illustrating an apparatus for
carrying out the process in accordance with the invention;
[0024] FIG. 3 is a graph comparing concentration of recovered
aluminum with the concentration of DOC recovery over time, using
the process in accordance with the invention;
[0025] FIG. 4 is a graph comparing the percentage of aluminum
recovered with DOC permeation over time;
[0026] FIG. 5 is a graph illustrating the sulphate ion
concentration on both sides of a semi-permeable cation exchange
membrane;
[0027] FIG. 6 is a bar chart showing the relative carryover of
aluminum and other metal ions through the semi-permeable cation
exchange membrane;
[0028] FIG. 7 is a pie chart illustrating the distribution of
various dissolved species in alum recovered in accordance with the
invention.
[0029] FIG. 8 is a graph illustrating the selective recovery of
Fe(III) ions over time, using the process of the invention; and
[0030] FIG. 9 is a graph illustrating the amount of Fe(III)
recovered over time.
DETAILED DESCRIPTION
[0031] The invention takes advantage of a process known as the
Donnan Membrane Process (also referred to as Donnan Dialysis) for
removal of aluminum ions from clarifier sludge and aluminum
enrichment. The Donnan membrane process is based on the Donnan
co-ion exclusion phenomenon, according to which negatively charged
cation exchange membranes will reject anions, while positively
charged anion exchange membranes will reject cations. Unlike other
membrane processes, the Donnan membrane process does not require a
pressure gradient or an electric current supply, and operates by
virtue of the electrochemical potential difference between
electrolytes on two sides of an ion exchange membrane.
[0032] The principle underlying the Donnan membrane process may be
summarized briefly as follows. If solutions of electrolyte are
placed on opposite sides of a cation exchange membrane, the anion
compositions will remain unchanged, but the cations will
redistribute themselves between the two sides to satisfy the
following condition at equilibrium: 1 [ C iR z + C iL z + ] 1 z = [
C jR y + C jL y + ] 1 y = K
[0033] Where C is the molar concentration of cations "i" and "j"
with charges "z" and "y" respectively, subscripts L and R refer to
the Left and Right hand sides of the membrane and K is a constant.
The equation is equally valid for an anion exchange membrane, which
allows only anions to permeate and redistribute but is impermeable
to cations. In the case of multivalent cations having a charge z
and a monovalent cation, having a charge y, used as a sweep
solution, it will be observed from the above equation that the
multivalent cations can be concentrated from a lean feed solution
by counter-transport of the monovalent cation used as a sweep
solution.
[0034] FIG. 1 shows both Dissolved Organic Carbon (DOC) and Al(III)
concentrations in a typical water treatment plant clarifier sludge
(in this case the plant at Allentown, Pa.) at different pH levels.
The Allentown Water Treatment Plant (AWTP) clarifier sludge, which
ordinarily has a pH of 7.1, had a total suspended solids content of
112 g/L, a total aluminum content of 5.6 g/L, a total iron content
of 1.6 g/L, and other constituents as follows: 860 mg/L total
organic carbon, 30 mg/L dissolved calcium, 15 mg/L dissolved
sodium, 85 mg/L total manganese, 23 mg/L total zinc, 3.2 mg/L total
copper, and a negligible amount of cadmium. The components given as
total amounts were determined by analysis after digestion of sludge
for 24 hours at ambient temperature (24.2.degree. C.) and at a pH
less than 1.0 by addition of concentrated sulfuric acid.
[0035] As shown by FIG. 1, DOC tends to increase with dissolved
Al(III) under both acidic and alkaline conditions. Since a high
concentration of dissolved organic matter is very undesirable in
recovered alum due to its potential for the formation of
trihalomethanes, neither acid nor alkali digestion is able to
achieve selective alum recovery.
[0036] As illustrated in FIG. 2, clarifier sludge 10 is collected
in a vessel 12 . The clarifier sludge is rich in aluminum
hydroxide, Al(OH).sub.3, natural organic matter and suspended
solids. The pH of the sludge, which is normally about 7.1, is
adjusted to about 3.0, by the addition of an acid such as sulfuric
acid, to cause at least the majority of the aluminum content of the
clarifier sludge to go into solution. Later on, H.sup.+ ions from
the acid sweep solution cause the pH to fall further, and the
remaining aluminum still locked up in the solid phase will also
dissolve. Transfer of aluminum ions to the sweep side will also
promote further solution of aluminum hydroxide precipitates.
[0037] After pH adjustment, the aqueous clarifier sludge solution
is pumped by a low pressure pump 14, in a first circulatory flow
path, through an exchanger 16 comprising cation exchange membranes.
The membranes can be Nafion membranes available from E. I. DuPont
de Nemours, Wilmington, Del. A preferred membrane is DuPont Nafion
350, which is a robust membrane capable of withstanding extremes of
pH. Other suitable cation exchange membranes are available from
Sybron Chemicals, Birmingham, N.J., U.S.A. and Mitsubishi Chemical
Armerica Inc., in White Plains, N.Y., U.S.A. At the same time,
sulfuric acid solution 18, (H.sub.2SO.sub.4), is pumped as a sweep
solution through the exchanger in a second circulatory flow path,
from a supply vessel 20, by a low pressure pump 22. Preferably, the
sulfuric acid in the sweep solution in vessel 20 is initially at a
concentration in the range from 5-10% by volume.
[0038] The membranes are arranged in a stack, with spaces for the
flow of liquid provided at 24, 26, 28 and 30 between adjacent
membranes, and at 32 and 34 on the exterior sides of end membranes
36 and 38, respectively. As illustrated in FIG. 2, the sludge is
directed through an elongated path including spaces 32, 26 and 30
in series, and returned to vessel 12. The sweep solution of
sulfuric acid is pumped through a path including spaces 34, 28 and
24 in series, and returned to vessel 20. The spaces 32, 26 and 30
of the sludge solution path and the spaces 34, 28 and 24 of the
sweep solution path are in alternating arrangement.
[0039] In the operation of the apparatus illustrated schematically
in FIG. 2, Aluminum hydroxide is converted to soluble aluminum
sulfate as a result of the addition of sulfuric acid for pH
adjustment. As the solution is pumped though the clarifier sludge
solution path (which includes spaces 32, 26 and 30 of the
exchanger), anions (e.g., chloride, sulfate, etc,) dissolved
organic matter, and large neutral organic molecules are prevented
from passing through the membranes and therefore, their
concentrations on opposite sides of the exchanger remain
practically unchanged. On the other hand, aluminum (III) ions,
because they are multivalent, pass readily through the membranes
from the clarifier sludge solution path to the sweep solution path,
while hydrogen ions pass through the membranes in the opposite
direction, from the sweep solution path to the clarifier sludge
solution path. Monovalent and divalent metals pass through the
membranes to a limited extent along with aluminum, but the
carryover of monovalent and divalent metals is inherently limited
by the Donnan membrane process as described above. The passage of
ferric ions, Fe(III), through the membranes along with aluminum is
desirable since Fe(III) enhances the effectiveness of alum as a
coagulant and has no other adverse effects. It is unnecessary to
attempt to prevent carryover of Fe(III). Ferric salts are also used
as coagulants in many water treatment plants. The process described
here can also selectively recover Fe(III) from clarifier sludge, as
discussed subsequently.
[0040] Ultimately, after operation of the exchanger for a time,
vessel 12, which initially contained significant quantities of
Al(OH).sub.3 along with natural organic matter and suspended
solids, will contain primarily insoluble suspended solids, and some
monovalent and divalent metal compounds, but will be substantially
aluminum-free. The vessel 20, on the other hand, which initially
contained a solution of H.sub.2SO.sub.4, will ultimately contain an
acidic solution of Alum, in a concentration in the range of 4-10
g/L, virtually free of dissolved organic matter, suspended solids
and toxic metals.
[0041] Experiments were carried out using clarifier sludge obtained
from the Allentown, Pa. Water Treatment Plant (AWTP). The AWTP has
a production capacity of 8 million gallons (30,000 m.sup.3) per
day. The plant utilizes alum addition, rapid mixing, flocculation,
settling, chlorination and filtration to treat surface water from
the Little Lehigh River. Alum is used to remove turbidity from the
surface water, which ranges from 2 NTU to 680 NTU, and the alum
dosage ranges from 10 to 50 mg/l as alum. Settled sludge is drained
about two times per week from the storage zone of the clarifier.
The clarifier sludge was used in Donnan membrane process
experiments carried out in a laboratory using a single membrane,
Donnan Membrane exchanger. The exchanger was divided into a feed
chamber and a sweep chamber, each having a length of 30 cm, a width
of 7 cm and a height of 40 cm. The feed chamber contained the
clarifier sludge and the other chamber held an acid solution. The
two chambers were separated by an acrylic resin frame holding a
Nafion 350 cation exchange membrane 20 cm in length and 30 cm high.
Mild agitation was maintained by bubbling air into the solutions
through a distributor with small nozzles. The air supply was
adjusted to a pressure of 1 psig to ensure uniform agitation.
[0042] The clarifier sludge in the feed chamber was laden with alum
and other constituents including NOM, suspended solids etc. The
sweep chamber contained 5-10% sulfuric acid.
[0043] Samples, collected at regular intervals, were analyzed for
ions. Aluminum was analyzed using a UV-VIS Spectrophotometer. Its
concentration in the clarifier sludge fell with transport of
hydrogen ions from the acid sweep solution. The concentrations
obtained at various times were used to calculate the aluminum
transfer flux. Other cations were analyzed using a Perkin Elmer
Model 2380 Atomic Absorption Spectrophotometer. The DOC was
measured using a Shimadzu Model 5050A TOC Analyzer. Anions were
analyzed for Donnan exclusion, using Dionex Model DX-120C Ion
Chromatograph.
[0044] FIGS. 3 and 4 compare aluminum recovery versus DOC recovery
as a function of time. While aluminum concentration (FIG. 3)
increased to over 5000 mg/L as Al in the recovered alum, DOC was
practically negligible, i.e., less than 3.0 mg/L. FIG. 4 shows that
more than 65% of the Al in the sludge was recovered and that the
recovered Al was virtually free of NOM and suspended solids. In a
separate set of experiments, about 85% of the Al originally present
in the sludge was recovered by increasing the membrane surface
area.
[0045] As shown in FIG. 5, the sulfate concentrations on the sweep
side and sludge side remained practically unchanged, confirming the
impermeability of the cation exchange membranes to anions.
[0046] The following table provides a comparison of the composition
of the recovered alum between an acid digestion process and the
Donnan membrane process in accordance with the invention.
1 Donnan Membrane Acid Digestion Constituents in mg/l Process
Process Al(III) 5650 2900 Fe(III) 187 159 Total Suspended solids
negligible significant DOC or NOM 0-3 200-225
[0047] It is apparent from the table that the Donnan membrane
process achieves much higher purity in the recovered alum. In a
visual comparison of recovered alums from these two processes, the
alum recovered by acid digestion was dark and cloudy, whereas the
alum recovered by the Donnan membrane process was clear and
transparent, similar to fresh liquid alum. Due to its negligible
DOC content, the alum recovered by the Donnan membrane process in
accordance with the invention may be safely reused as a coagulant
with no possibility of trichloromethane formation upon
chlorination. In the recovered alum, aluminum and iron are the only
major species, and, as mentioned previously, the presence of iron
or Fe(III) in the recovered alum does not have any adverse effect.
On the contrary, Fe(III) enhances the effectiveness of alum as a
coagulant.
[0048] The bar chart in FIG. 6 shows that the relative recovery of
aluminum is also selective with respect to other metal cations,
including heavy metal cations. The carryover of all other cations
into the recovered alum is lower than that for aluminum. As shown
in FIG. 7, aluminum and iron are practically the only major species
in the alum ultimately recovered. Of the cations, typically 98% are
Al(III) and 2% are Fe(III), the remainder being trace amounts of
zinc, calcium, cupper, arsenic and other elements along with trace
amounts of dissolved organic carbon. Thus, in the recovered alum,
the concentration of aluminum is over two orders of magnitude
greater than heavy metals and dissolved organic carbon. In essence
the recovered solution is a pure, concentrated solution of aluminum
sulfate, with minor trace compounds.
[0049] The advantages of the Donnan membrane process for selective
alum recovery may be summarized as follows.
[0050] First, the process is operationally simple, requiring only
two low-pressure pumps, a stack of cation exchange membranes, and
sulfuric acid.
[0051] Preliminary cost calculations indicate that savings
resulting from the reuse of recovered alum and the reduction in
sludge disposal costs make the proposed process economically
viable.
[0052] The acid sweep solution allows both solution of
Al(OH).sub.3(s) and selective Al.sup.3+ recovery through the
semi-permeable cation exchange membrane. No pressure differential
is required across the membrane, and, since pressure is not the
driving force in the process, the high concentration of solids in
clarifier sludge does not foul the membrane or adversely affect its
performance. Changes in sludge composition have only a minor impact
on the operation of the process. Except for the pumps, the process
does not require any moving part and is, therefore, operationally
simple. The cation exchange membranes are chemically stable over
the entire range of pH and mechanically strong. The only expendable
chemical used in the Donnan membrane process is the acid used for
preliminary pH adjustment and for the acid sweep solution. The
aluminum recovered by the process is concentrated and does not
contain NOM, suspended solids or other objectionable
constituents.
[0053] The Donnan membrane process has particular advantages in
regard to its ability to: concentrate aluminum in the recovered
solution; achieve near-complete rejection of natural organic matter
(NOM) or dissolved organic carbon (DOC); reduce carryover of heavy
metals such as copper, zinc, etc., into the recovered alum; provide
for the use recovered alum as a coagulant in the same plant without
the possibility of trichloromethane formation upon chlorination;
and reduce the volume of the sludge and the cost of its
disposal.
[0054] Various modifications can be made to the apparatus and
process described. For example, instead of sulfuric acid, other
acids, such as hydrochloric acid (HCl) can be used for pH
adjustment and as the acid in the acid sweep solution. Sulfuric
acid and hydrochloric acid sweep solutions allow both solution of
Al(OH).sub.3(s) and selective Al.sup.3+ recovery through the
semi-permeable cation exchange membrane."
[0055] Fe(III) salts are also used as coagulants in water treatment
plants, and the Donnan membrane technique is equally effective for
selective Fe(III) recovery and reuse. FIG. 8 shows how a simulated
sludge containing 1800 mg/l Fe(III) can be concentrated with this
process and FIG. 9 confirms that recovery as high as 80% can be
obtained.
[0056] The exchanger can take various alternative forms. For
example, the exchanger can consist of a single membrane separating
two chambers, one for the sludge solution, and one for the acid
sweep solution. Alternatively, the exchanger can be constructed as
a stack of membranes, similar to the stack shown in FIG. 2, but
provided with manifolds directing each liquid in plural paths, so
that for a given liquid, e.g., the sludge solution, the liquid in
each sludge solution path flows in the same direction as the liquid
in every other parallel sludge solution path.
[0057] Various other modifications can be made to the invention
described without departing from the scope of the invention as
defined in the following claims.
* * * * *