U.S. patent application number 13/530948 was filed with the patent office on 2012-12-27 for treating acidic water.
Invention is credited to John F. Bossler, Hari Bhushan Gupta, Jospeh C. Jimerson, Kenneth R. Workman.
Application Number | 20120325745 13/530948 |
Document ID | / |
Family ID | 38172223 |
Filed Date | 2012-12-27 |
United States Patent
Application |
20120325745 |
Kind Code |
A1 |
Bossler; John F. ; et
al. |
December 27, 2012 |
TREATING ACIDIC WATER
Abstract
The invention relates to systems and methods of treating water
by directing the water to a first reverse osmosis (RO) membrane;
separating the water using the first RO membrane; adding a
chelating agent to first permeate and/or raising the pH of the
first permeate to between about 5.5 and 7.5 before a second RO
membrane; and separating the first permeate into a second permeate
and a second concentrate using the second RO membrane, thereby
separating constituents from the water.
Inventors: |
Bossler; John F.;
(Edwardsburg, MI) ; Gupta; Hari Bhushan;
(Schaumburg, IL) ; Workman; Kenneth R.; (Hoffman
Estates, IL) ; Jimerson; Jospeh C.; (Conroe,
TX) |
Family ID: |
38172223 |
Appl. No.: |
13/530948 |
Filed: |
June 22, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11303160 |
Dec 15, 2005 |
8206592 |
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13530948 |
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Current U.S.
Class: |
210/639 ;
210/202 |
Current CPC
Class: |
B01D 61/022 20130101;
C02F 1/441 20130101; C02F 1/42 20130101; C02F 1/66 20130101; C02F
1/001 20130101; C02F 1/683 20130101; B01D 61/025 20130101; C02F
1/56 20130101; B01D 2317/025 20130101 |
Class at
Publication: |
210/639 ;
210/202 |
International
Class: |
C02F 9/08 20060101
C02F009/08; C02F 9/04 20060101 C02F009/04; C02F 1/42 20060101
C02F001/42; C02F 1/52 20060101 C02F001/52; C02F 1/44 20060101
C02F001/44; C02F 1/66 20060101 C02F001/66 |
Claims
1-35. (canceled)
36. A method for treating wastewater having a pH level of about 0.5
to about 4.5 and including an undesirable level of one or more of
ammonia, fluoride and phosphate species, the method comprising:
dosing the wastewater having a pH level of about 0.5 to about 4.5
with a coagulant; introducing the dosed water to at least one media
filter to produce pre-treated water; introducing the pre-treated
water to a first reverse osmosis unit to produce a first permeate;
adjusting a pH level of the first permeate to within a range of
about 5.5 to about 7.5; introducing the first permeate at the
adjusted pH level to a second reverse osmosis unit; and discharging
treated water having a pH level of about 6.5 to about 8.5
downstream of the second reverse osmosis unit, the treated water
having a predetermined discharge level of one of more of the
ammonia, fluoride and phosphate species.
37. The method of claim 36, wherein the water to be treated has a
pH level of below about 3.0.
38. The method of claim 36, wherein producing pre-treated water
further comprises introducing the dosed water to a cartridge filter
downstream of the at least one media filter.
39. The method of claim 36, further comprising introducing an
antiscalant to the pre-treated water upstream of the first reverse
osmosis unit.
40. The method of claim 36, wherein the pre-treated water has a
turbidity of less than about 2 NTU or a salt density index of less
than about 4.
41. The method of claim 40, further comprising introducing a
concentrate stream from the second reverse osmosis unit to a feed
side of the first reverse osmosis unit.
42. The method of claim 36, wherein the coagulant is an organic
coagulant.
43. The method of claim 36, wherein the predetermined discharge
level is less than about 8 ppm for fluoride, less than about 1 ppm
for ammonia, and less than about 0.5 ppm for phosphorous.
44. The method of claim 36, wherein the coagulant is dosed to
produce a coagulant concentration of about 8 ppm to about 10 ppm in
the dosed water.
45. The method of claim 36, further comprising adding a chelating
agent to the first permeate.
46. The method of claim 45, wherein the chelating agent is added to
produce a chelating agent concentration of from about 5 ppm to
about 50 ppm.
47. The method of claim 45, further comprising monitoring a pH
level of the first permeate.
48. The method of claim 36, further comprising introducing the
treated water to an ion exchange system.
49. A system for treating wastewater, the system comprising: a
source of wastewater having a pH level of about 0.5 to about 4.5
and including an undesirable level of one or more of ammonia,
fluoride and phosphate species; a pre-treatment subsystem
comprising a coagulant injection system and at least one media
filter, the pre-treatment subsystem fluidly connected to the source
of wastewater; and a reverse osmosis separation subsystem fluidly
connected downstream of the pre-treatment subsystem, the reverse
osmosis separation subsystem comprising: first and second reverse
osmosis units, and a pH adjustor system positioned between the
first and second reverse osmosis units, the reverse osmosis
separation subsystem configured to produce treated water having a
pH level of about 6.5 to about 8.5 and a predetermined discharge
level of at least one of the ammonia, fluoride and phosphate
species.
50. The system of claim 49, wherein the predetermined discharge
level is less than about 8 ppm for fluoride, less than about 1 ppm
for ammonia, and less than about 0.5 ppm for phosphorous.
51. The system of claim 49, wherein the pre-treatment subsystem
further comprises a cartridge filter.
52. The system of claim 49, wherein the coagulant injection system
is configured to produce a coagulant concentration of about 8 ppm
to about 10 ppm.
53. The system of claim 49, wherein the pH adjustor system is
configured to adjust a pH level of permeate from the first reverse
osmosis unit to within a range of about 5.5 to about 7.5.
54. The system of claim 49, wherein a concentrate stream of the
second reverse osmosis unit is fluidly connected to a feed side of
the first reverse osmosis unit.
55. The system of claim 49, further comprising an ion exchange
system downstream of the reverse osmosis subsystem.
Description
TECHNICAL FIELD
[0001] The present invention relates to treatment of acidic water
and, more particularly, to treatment of acidic water using reverse
osmosis systems.
BACKGROUND
[0002] Process water associated with and produced by phosphate
manufacturing operations is typically acidic and typically contains
various dissolved constituents such as fluoride, ammonia, silica,
sulfate, calcium, heavy metals, phosphate, magnesium, colloidal
matter, organic carbon, and, in some instances, radium (a
radioactive element). Ponds associated with past phosphate
processing contain billions of gallons of this waste water. There
is an urgent environmental need to treat this wastewater,
particularly in environmentally sensitive areas, or areas where
population growth has come into closer contact with phosphate
processing sites. Treatment of this waste to reduce its toxicity
and its volume has been a technological challenge of significant
interest. The toxic or harmful contaminants must be either reduced
or eliminated before treated water can be discharged into the
environment.
[0003] Various techniques have been used to reduce the level of
such constituents before water is discharged. For example, double
liming, followed by air stripping, can be used. This process adds
lime in two stages, to promote precipitation of fluoride species
and phosphate species, followed by high pH air stripping to remove
ammonia. In another technique, water is treated by techniques
involving chemical precipitation followed by reverse osmosis. Like
double liming, such techniques raise the pH of influent water to
promote precipitation and solids separation before reverse
osmosis.
[0004] Reverse osmosis involves separating water from a solution of
dissolved solids by forcing water through a semi-permeable
membrane. As pressure is applied to the solution, water and other
molecules with low molecular weight and low ionic charge pass
through small pores in the membrane. Larger molecules and those
with higher ionic charge are rejected by the membrane.
[0005] Some constituents that can be found in water, such as
fluoride and phosphate, tend to form soluble acids under acidic
conditions thus reducing the potential for scaling of reverse
osmosis membranes. Other constituents that can be found in water,
such as ammonia, tend, under acidic conditions, to form salts that
are easily rejected by the membranes. In dual-pass reverse osmosis
systems, the pH of permeate from the first pass reverse osmosis
membranes can be adjusted upwards towards neutral conditions
between the first and second pass membranes to make it easier to
remove constituents that tend to exist in soluble form under highly
acidic conditions.
[0006] Antiscalants can be added before first pass and/or second
pass reverse osmosis membranes. Typically, antiscalants are
materials that interfere with precipitation reactions by mechanisms
such as crystal modification in which negative groups located on
the antiscalant molecule attack the positive charges on scale
nuclei interrupting the electronic balance necessary to propagate
the crystal growth. Similarly, some antiscalants adsorb on crystals
or colloidal particles and impart a high anionic charge, which
tends to keep the crystals separated.
[0007] Some treatment systems include pretreatment before the
reverse osmosis membranes to remove constituents such as suspended
solids that can clog the reverse osmosis membranes. Some treatment
systems include polishing technologies to reduce the residual
concentrations of constituents for which allowable discharge
concentrations are very low. Although these polishing technologies
may be necessary to meet discharge criteria, they can add
significantly to the overall treatment system operating costs.
SUMMARY
[0008] The invention is based, at least in part, on the discovery
that one can effectively and efficiently treat water (e.g., water
associated with the production of ammonium phosphate) having a low
pH to remove contaminants by using multiple reverse osmosis
membranes arranged in series and by controlling the pH between the
reverse osmosis membranes. For example, in a dual-pass reverse
osmosis system, a first pass reverse osmosis membrane operating
under highly acidic conditions can be used to separate influent
water into a first permeate and a first concentrate. The first
permeate can then be separated into a second concentrate and a
second permeate using a second pass reverse osmosis membrane. The
pH of the first permeate can be raised to between about 5.5 and
7.5, e.g., from about 5.5 to 6.0, before it contacts the second
pass reverse osmosis membranes. The optimum pH for treatment of
water with multiple constituents can be outside the optimal ranges
for rejection of individual species. The new methods also reduce
the need to use polishing systems through the use of chelating
agents (also called complexing or sequestering agents) in place of
antiscalants and/or the careful control of interpass pH. In
contrast to antiscalants, chelating agents are compounds that form
stable complexes with metal ions. Thus, precipitation of sparingly
soluble salts, e.g., of calcium and magnesium with inorganic
anions, fatty acids, and anionic surfactants can be reduced.
[0009] In one aspect, the invention features methods of removing
constituents from water having a pH below about 4.5 (e.g., below
3.5) by: directing the water to a first pass reverse osmosis
membrane; separating the water into a first permeate and a first
concentrate using the first pass reverse osmosis membrane; adding a
chelating agent (e.g., ethylenediaminetetraacetic acid) to the
first permeate before the second pass reverse osmosis membrane; and
separating the first permeate into a second permeate and a second
concentrate using a second pass reverse osmosis membrane, thereby
separating constituents from the water.
[0010] In another aspect, the invention features methods of
treating acidic water (e.g., water with a pH of below about 4.5,
e.g., having constituents including at least ammonia, fluoride, and
phosphate species) by: directing acidic water to a first pass
reverse osmosis membrane; separating the water into a first
permeate and a first concentrate with the first pass reverse
osmosis membrane; adjusting the pH of the first permeate to between
about 5.5 and 6.0 by adding an alkali to the water; and separating
the first permeate into a second permeate and a second concentrate
with a second pass reverse osmosis membrane. Some embodiments of
this aspect also include adding a chelating agent to the first
permeate.
[0011] In another aspect, the invention features methods of
removing constituents from water having a pH below about 4.5 by:
directing the water to a first pass reverse osmosis membrane;
separating the water into a first permeate and a first concentrate
using the first pass reverse osmosis membrane; adding a chelating
agent to the first permeate before the second pass reverse osmosis
membrane; separating the first permeate into a second permeate and
a second concentrate using a second pass reverse osmosis membrane,
thereby separating constituents from the water; and increasing an
amount of chelating agent being added to the first permeate in
response an accumulation of solid material on the second pass
membrane.
[0012] In some embodiments of these methods, the constituents can
include at least ammonia, fluoride, and phosphate species. The
constituents can also include at least one of calcium, magnesium,
and silica, e.g., at concentrations in the water above 100 parts
per million. At least one of fluoride, calcium, magnesium, and
silica can be present in the first permeate at conditions that can
cause scaling on a reverse osmosis membrane at a pH between about
5.5 and 8.0 and/or can be present at concentrations in the first
permeate above about 10 parts per million.
[0013] In some embodiments, the new methods also include raising a
pH of the first permeate to between about 5.5 and 7.5 (e.g.,
between about 6.5 and 7.5 or between about 5.5 and 6.0). For
example, the pH of the first permeate can be raised by adding an
alkali (e.g., sodium hydroxide) to the first permeate.
[0014] In some embodiments, separating the first permeate includes
concentrating one or more of ammonia, fluoride, and phosphate
species in the second concentrate. In other embodiments, the
methods can also include adding at least a portion of the second
concentrate to the water directed to the first pass reverse osmosis
membrane. In various embodiments, methods can also include
pre-treating the water prior to directing the water to the first
pass reverse osmosis membrane. The water can be pre-treated by
reducing the concentration of suspended solids in the water (e.g.,
by passing the water through one or more media filters). The
concentration of suspended solids can also be reduced by adding a
coagulant to the water before passing the water through the
filters.
[0015] In certain embodiments, the methods can also include passing
the second permeate through an ion exchange system. The methods can
also include adding an antiscalant (e.g., a sodium salt of
phosphonomethylated diamine) to the water before the first pass
reverse osmosis membrane, or can include lowering the pH of the
water by adding an acid to the water before the first pass reverse
osmosis membrane. The new methods can also include passing the
second permeate through an ion exchange system.
[0016] In various embodiments, the methods can also include raising
the pH of the first permeate to form conditions in the first
permeate that favor formation of salts over other ammonia,
fluoride, and phosphate species. In some instances, the pH is
raised (e.g., to between about 5.5 and 6.0 or to between about 6.5
and 7.2) by adding an alkali to the first permeate.
[0017] In another aspect, the invention includes systems for
treating acidic water that include a first pass reverse osmosis
unit that separates incoming water into a first permeate and a
first concentrate; a second pass reverse osmosis unit arranged to
receive at least a portion of the first permeate from the first
pass reverse osmosis unit; and an interpass chemical controller
located upstream of the second pass reverse osmosis unit, wherein
the controller is configured to raise the pH of at least the
portion of the first permeate to between about 5.5 and 7.5 and adds
a chelating agent to at least the portion of the first
permeate.
[0018] In some embodiments, the new systems can include thin film
composite polyamide reverse osmosis membranes, which have been
found to have a particularly beneficial combination of membrane
flux, fouling resistance, and chemical resistance characteristics
well-suited for the conditions of operation on phosphate wastewater
streams.
[0019] In some embodiments, the new systems can also include one or
more of the following components: one or more storage units for
holding acidic water; a fluid routing system arranged to direct at
least a portion of a second concentrate from the second pass
reverse osmosis unit to a location upstream of the first pass
reverse osmosis unit; a pretreatment unit (e.g., pretreatment unit
including a first stage media filter and a second stage media
filter) disposed upstream of the first pass reverse osmosis unit;
an ion exchange unit receiving at least a portion of a second
permeate from the second pass reverse osmosis unit.
[0020] The interpass chemical controller can include a chemical
injector that adds the chelating agent to the first permeate and/or
a chemical injector that adds an alkali to the first permeate.
[0021] The invention provides several advantages. Careful control
of interpass pH and/or the use of chelating agents in place of
antiscalants can reduce the need for polishing systems such as ion
exchangers. This can reduce operating costs, eliminate the need for
storage of hazardous regenerants, reduce the salt discharge from
the ion exchange regenerant waste stream, and increase the overall
system recovery. The combined use of pH control and the metering of
low levels of chelating agents on a continuous or semi-continuous
basis, particularly on dual pass reverse osmosis systems where the
chelating agent is added to the first pass permeate before being
fed to the second pass, can also result in a reduction of cleaning
frequency, longer life for the reverse osmosis membranes, and
operation at higher water recovery rates over longer time periods.
Moreover, if radium is present in the water being treated, reducing
scaling can reduce the precipitation and concentration of radium on
the membranes and limit the issues associated with handling the
resultant radioactive material.
[0022] The new methods can also reduce system downtime for cleaning
of membranes due to scaling, plugging, or fouling of the membranes
caused by precipitation or colloid agglomeration or deposition or
binding of foulants on the membrane surfaces. Continuous feeding of
chelating agents can, to some extent, prevent such problems.
However, it has also been discovered that, when such problems
occur, the problems can be addressed by increasing the chelating
agents in the membrane feed water while continuing to operate the
system. Cleaning the membranes in place while the system is
operating reduces costs including, for example, costs associated
with loss of production due to system downtime and/or costs
associated with maintaining onsite membrane cleaning facilities
and/or backup membranes.
[0023] Unless otherwise defined, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this invention belongs. Although
methods and materials similar or equivalent to those described
herein can be used in the practice or testing of the present
invention, suitable methods and materials are described below. All
publications, patent applications, patents, and other references
mentioned herein are incorporated by reference in their entirety.
In case of conflict, the present specification, including
definitions, will control. In addition, the materials, methods, and
examples are illustrative only and not intended to be limiting.
[0024] Other features and advantages of the invention will be
apparent from the following detailed description, and from the
claims.
DESCRIPTION OF DRAWINGS
[0025] FIG. 1 is a flow diagram of a method for treating acidic
water.
[0026] FIG. 2 is a graph showing the equilibrium relative
composition of hydrofluoric acid and fluoride species as a function
of pH.
[0027] FIG. 3 is a graph showing the equilibrium relative
composition of phosphoric acid and phosphate species as a function
of pH.
[0028] FIG. 4 is a graph showing the equilibrium relative
composition of ammonium and ammonia species as a function of
pH.
[0029] FIG. 5 is a schematic of a system for implementing the
method of FIG. 1.
[0030] FIG. 6 is a specific embodiment of a system for implementing
the method of FIG. 1.
[0031] FIG. 7 is a table listing chemical constituents of water
treated by the system of FIG. 6 at various points in the
system.
[0032] FIG. 8 is a table listing chemical constituents of water
treated by the system of FIG. 6 with varying degrees of interpass
pH adjustment.
[0033] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0034] The new dual-pass reverse osmosis systems and methods
described herein can be used for treating acidic water that
contains ammonia, fluoride, and/or phosphate species, such as the
process water produced during the production of phosphate-based
fertilizers. In some embodiments, the systems can include
pretreatment before the first pass reverse osmosis membranes to
remove suspended solids from the influent water. The pH of permeate
from the first pass reverse osmosis membranes is adjusted upwards
to increase the rejection of fluoride and phosphate species in the
second pass reverse osmosis membranes. Chelating agents can be
added to the permeate from the first pass reverse osmosis
membranes.
[0035] General Methodology
[0036] FIG. 1 illustrates a method 10 for treating acidic water
containing constituents including at least species of ammonia,
fluoride, and phosphate, which is based on the use of two reverse
osmosis systems provided in series. Water having a pH below about
4.5 and containing at least ammonia, fluoride, and phosphate
species is directed to a first pass reverse osmosis membrane (step
12). The first pass reverse osmosis membrane is used to separate
the influent stream into a first permeate and a first concentrate
(step 14). Operating under highly acidic conditions (e.g., pH less
than about 4.0, 3.8, 3.6, 3.5, 3.4, 3.2, or less than about 3.0)
reduces the potential for scaling of the reverse osmosis membrane
as the fluoride and phosphate are typically present in the form of
soluble acids (e.g., hydrofluorosilic acid and phosphoric acid)
that pass through the reverse osmosis membrane (see FIGS. 2 and 3).
Operating under these conditions also provides for good rejection
of ammonia in the form of ammonium salts, which are typically the
predominant ammonia species in solutions whose pH is below 4 (see
FIG. 4).
[0037] In dual-pass reverse osmosis systems, the pH of the first
permeate is sometimes raised to close to neutral or higher before
the second pass reverse osmosis membrane, because in this range
(e.g., above about 6.5) the rejection of fluoride and phosphate
improves as they tend to form salts. However as pH increases,
ammonia is increasingly present in the form of free ammonia, which
passes through reverse osmosis membranes more easily than the
ammonium salts. Consequently, residual ammonia in the first
permeate tends to pass through second pass reverse osmosis filters
in systems where the pH of the first permeate is raised before the
second pass reverse osmosis membrane. As the optimal pH for ammonia
removal using reverse osmosis is below 4 and the optimal pH for
fluoride and phosphate removal using reverse osmosis is above 6.0,
the residual ammonia is often dealt with using polishing
technologies (e.g., ion exchange beds) after the second pass
reverse osmosis membrane. Control of interpass pH to between about
5.5 to 6.0 can reduce the need for such polishing in certain
systems. In the present new methods, the pH of the first permeate
can be raised to between about 5.0 and 7.5 (e.g., a minimum pH of
5.1, 5.2, 5.3, 5.4, 5.5, 5.6, or 5.7; and a maximum pH of 7.5, 7.4,
7.3, 7.2, 7.1, 7.0, 6.9, 6.8, 6.7, 6.6, 6.5, 6.4, 6.3, 6.2, 6.1,
6.0, 5.9, or 5.8) (step 16), e.g., within a range of 5.5 to 6.0,
before the pH-adjusted first permeate is separated into a second
permeate and a second concentrate using a second pass reverse
osmosis membrane (step 18). The use of this specific pH range has
been found to concentrate ammonia, fluoride, and phosphate in the
second concentrate to significantly reduce the need to apply
polishing technologies to the second permeate to meet water
discharge criteria.
[0038] In the present new method, one or more chelating agents can
be added to the first permeate (step 16) to form stable complexes
with metal ions. Chelating agents are generally known and include,
for example, ethylenediaminetetraacetic acid (EDTA),
nitrilotriacetic acid (NTA), diethylenetriaminepentaacetic acid
(DTPA), and (hydroxyethyl)ethylenediaminetriacetic acid (HEDTA)
(see, e.g., "Selecting The Correct DOW Chelating Agent," The Dow
Chemical Company, Midland, Mich., the entire contents of which are
incorporated herein by reference). The chelating agent(s) can be
added as an aqueous solution to produce a concentration of the
chelating agent in the treatment stream in the range between about
5 to about 50 parts per million (ppm). The chelating agent(s) can
form complexes with calcium to prevent the precipitation of calcium
fluoride on the reverse osmosis membrane surface. The formation of
such precipitation can inhibit the passage of water through the
reverse osmosis membranes, thus requiring higher pressures to
maintain constant flow. In addition, if precipitation occurs, the
precipitant can be removed by the addition of higher levels of the
chelating agent.
[0039] Chelating agents, being more ionized at higher pHs, tend to
have higher affinity for constituents including calcium and radium
as pH increases. However, the use of chelating agents at pH levels
below pH 7 (in the acid range) has been unexpectedly effective in
eliminating chemical precipitation and membrane fouling even with
the chelating agents at low ppm concentrations in the water being
treated. Moreover, the continuous feed of chelating agent under
these conditions can restore membrane performance that has been
degraded by prior precipitation and fouling without taking the
treatment system offline.
[0040] In some embodiments, the first pass reverse osmosis
membranes can create a leveling effect on the impurities in the
first permeate. This can be important for treatment of feeds that
vary in their levels of constituents to be removed over time, since
the amount of chelating agent that is used interpass can be
relatively constant in spite of the variations in constituent
concentrations in the feedwater. For example, the first pass
reverse osmosis membranes tend to remove a percentage of the
constituents present in the feed water. Thus, if the first pass
reverse osmosis membranes reject 99% of a specific constituent,
e.g., calcium, a change in calcium feed concentrations from 100 ppm
to 200 ppm in the feedwater would result in a first pass permeate
concentration change from 1 ppm to 2 ppm. Accordingly, an interpass
chemical feed system that adds a chelating agent would need to add
only the chemical equivalent 2 ppm of the chelating agent while use
of this technique with a first pass feedwater would require the
addition of the chemical equivalent 200 ppm of the chelating
agent.
[0041] In some embodiments, method 10 can include pretreating the
water (step 20) before it is directed to a first pass reverse
osmosis membrane (step 12). Pretreatment can include, for example,
removing organic matter by adding a disinfectant, a coagulant,
and/or a flocculating agent to the water and/or reducing the level
of suspended solids in the water by adding a coagulant and passing
the water through media filters. Such pretreatment can be
particularly advantageous in treating water that is exposed to the
environment such as process/product water associated with phosphate
manufacturing operations, which is typically accumulated in open
on-site ponds. Similarly, an antiscalant and/or other chemical
amendment such as, for example, a chelating agent, can be added to
the water before the first pass reverse osmosis membrane and/or the
second pass reverse osmosis membrane.
[0042] In some embodiments, method 10 can include polishing the
second permeate (step 22) after it leaves the second pass reverse
osmosis membrane to additionally reduce the levels of constituents
such as species of ammonia, fluoride, and/or phosphate. Polishing
can include, for example, passing the second permeate through an
ion exchange system to further reduce ammonia concentrations prior
to discharge. Although polishing may be necessary to meet
regulatory standards prior to discharge of treated water, such
polishing typically increases the operational costs of associated
with implementing method 10. However, adjusting the pH of the first
permeate (step 16) as described herein can reduce or eliminate the
need for polishing the second permeate.
[0043] In some embodiments, separating the first permeate can
include concentrating ammonia, fluoride, and phosphate species in
the second concentrate (i.e., these constituents are present in
higher concentrations in the second concentrate than in the second
permeate). However, the second concentrate contains these
constituents at lower concentrations than they are present in the
source water directed to the first pass reverse osmosis system.
Consequently, method 10 can also include adding at least a portion
of the second concentrate to the water directed to the first pass
reverse osmosis membrane. In some instances, at least a portion of
the first concentrate is directed back to a storage system from
which the water is directed to the first pass reverse osmosis
membrane.
[0044] Method 10 can also include lowering a pH of the water by
adding an acid to the water between the storage system and the
first pass reverse osmosis membrane. This step is typically not
included when method 10 is used to treat acidic water whose pH is
naturally below 4.5 (e.g., less than about 3.8, 3.6, 3.5, 3.4, 3.2,
or less than about 3.0) such as process/product water accumulated
in on-site ponds at phosphate manufacturing operations. It can be
necessary when treating water from other sources or to maintain
influent at the desired pH when other processes such a double
liming are also being used to treat phosphate and manufacturing
process/product water.
[0045] Systems to Treat Acidic Water
[0046] FIG. 5 illustrates a system 100 that can be used to treat
acidic water whose constituents include at least ammonia, fluoride,
and phosphate species, for example, process/product water
associated with phosphate manufacturing operations. The system 100
receives influent water from, for example, on-site ponds at a
phosphate manufacturing facility holding acidic water with a pH
below about 4.5. A first pass reverse osmosis system 112 is
disposed downstream of the storage units and a second pass reverse
osmosis system 114 receives a first permeate 115 from the first
pass reverse osmosis system. System 100 also includes an interpass
pH adjustor system 116 for raising a pH of first permeate 115 to
between about 5.0 and 6.0 or higher, e.g., up to 6.5, 7.0, or 7.5
in certain embodiments.
[0047] Interpass pH adjustor system 116 includes a chemical
injector 117 that can add an alkali (e.g., sodium hydroxide,
caustic potash or potassium hydroxide) to the first permeate 115
before it reaches second pass reverse osmosis system 114. Interpass
pH adjustor system 116 can also include sensors 119 that measure
the pH of first permeate 115 and controllers 121 that regulate the
injection of the alkali based on readings from the sensors. In some
embodiments, interpass pH adjustor system 116 also includes a
chemical injector 117 adding a chelating agent to first permeate
115 before it reaches second pass reverse osmosis system 114. In
some embodiments, interpass pH adjustor system 116 also includes
additional chemical injectors 117 adding other chemicals such as,
for example, a chelating agent (e.g., EDTA, NTA, or DTPA) to first
permeate 115 before it reaches second pass reverse osmosis system
114. The chelating agents can be injected directly into the feed
stream of the second pass reverse osmosis membranes as an aqueous
solution using a chemical injection pump. The caustic used to
adjust the pH upward is also injected at approximately the same
point also using a chemical feed pump. A pH probe downstream of the
injection point and a static mixer sends a signal to the pH meter.
This meter in turn sends a signal to the chemical feed pump that
can automatically adjust the caustic dosage to maintain a constant,
predetermined pH value.
[0048] In some embodiments, system 100 includes a pretreatment unit
118 disposed upstream of first pass reverse osmosis system 112.
Water stored in storage ponds can contain constituents such as, for
example, organics compounds, algae, and various suspended solids
that may clog the pores of reverse osmosis membranes 112, 114. In
some embodiments, pretreatment unit 118 includes first stage media
filters 120 and second stage media filters 122. Such filters
preferably reduce the turbidity of water to less than about 2
Nephelometric Turbidity Units (NTU) and reduce the Silt Density
Index (SDI) to less than about 4 to reduce the likelihood of
downstream fouling. A chemical injection system 124 can add an
organic coagulant such as, for example, diallyl dimethyl ammonium
chloride (DADMAC) commercially available from Nalco Chemical as
Cat-Floc 8103 Plus, between the storage ponds and first stage media
filters. Such organic coagulants, added for example in amounts
calculated to result in coagulant concentrations between about 8
and about 10 ppm, can increase the agglomeration of suspended
solids to increase their size and facilitate their removal in the
filters.
[0049] Under operating conditions, a bank of cartridge filters 126
receives flow from the second stage media filters. Under cleaning
conditions, backwash water from both first and second stage media
filters is returned to storage ponds 110.
[0050] In other embodiments, pretreatment systems designed to
remove algae from the feedwater can be used in place of or in
addition to filters. Clarification systems including, for example,
ballasted floc reaction technologies (described in more detail in
U.S. Patent App No. 2005/0103719 incorporated herein by reference
in its entirety), can be used to remove algae and other suspended
solids before the first pass reverse osmosis membranes. Examples
include the ACTIFLO.RTM. Ballasted Clarification systems,
commercially available from Kruger Inc. of Cary, N.C. Such
clarifiers can use a disinfectant, such as sodium hypochlorite, to
deactivate any microorganisms or organic matter in the wastewater
stream; a coagulating agent, such as, but not limited to,
bentonite, aluminum sulfate, and ferric chloride, to promote
coagulation of deactivated matter; and a flocculating agent such
as, but not limited to, nonionic, cationic, anionic polymers or
combinations thereof, to promote flocculation of the deactivated,
coagulated matter. Such clarifiers can also use microsand enhanced
settling and hydrocyclone techniques to separate sludge or solids
from the liquid-rich stream.
[0051] Another chemical injection system 128 can be used to add an
antiscalant such as, for example, such as the sodium salt of
phosphonomethylated diamine, commercially available from Nalco
Chemical as EL-5300, before the water reaches first pass reverse
osmosis system 112. Although any suitable antiscalant that inhibits
the formation of scale on the reverse osmosis membranes can be
used, most antiscalants used for reverse osmosis applications do
not function well under highly acidic conditions. In certain
embodiments, the antiscalant is added between the second stage
media filters 122 and cartridge filters 126. The antiscalant
reduces the tendency of some chemical species to precipitate out of
solution onto the reverse osmosis membrane and clog pores of
reverse osmosis membranes 112.
[0052] First pass reverse osmosis system 112 separates influent
water into first permeate 115 and first concentrate 130. As
discussed above, first permeate 115 is the influent to second pass
reverse osmosis system 114. In certain embodiments, first
concentrate 130, containing increased concentrations of ammonia,
fluoride, and phosphate species, is returned to storage ponds 110.
In other embodiments, at least a portion of first concentrate 130
can be mixed with water being treated between storage units and
first pass reverse osmosis membranes 112. Similarly, second pass
reverse osmosis system 114 separates first permeate 115 into second
permeate 132 and second concentrate 134. In some embodiments,
piping 135 returns second concentrate 134 to the inlet side of
first pass reverse osmosis system 112, because the second
concentrate typically has lower levels of ammonia, fluoride, and
phosphate species than water from storage ponds 110. Thus, the
second concentrate dilutes the source water.
[0053] First pass reverse osmosis system 112 should be suitable for
treatment of water having a pH of less than about 4, and flux rates
of about 6 to about 12 gallons per square foot of membrane per day
(GFD) because, it is believed, high flux rate greater than about 12
GFD can lead to fouling and flux rates less than about 6 GFD can
lead to low permeate quality. Similarly, second pass reverse
osmosis membranes 114 should be suitable for treatment of water
having a pH of about 5 to about 7.5 and flux rates of about 12 to
about 20 GFD. Suitable membranes include the FILMTEC.TM. BW30-365
membrane available from FILMTEC.TM., a subsidiary of The Dow
Chemical Corporation, Midland, Mich. It can be particularly
advantageous to use polyamide-based thin film composite reverse
osmosis membranes. Such membranes have a high resistance to pH
condition-induced degradation over a broad range of pH values;
provide high flux and rejection; and have a high resistance to
fouling (see, e.g., FILMTEC.TM. Brackish Water Performance Data,
Form No. 609-00485-704, the entire contents of which are
incorporated herein by reference, or other product literature). The
membranes can be installed in separation apparatus such as, for
example, those commercially available from USFilter Corporation,
Warrendale, Pa.
[0054] In some embodiments, an ion exchange unit 136 receives
second permeate 132 from second pass reverse osmosis system 114.
Ion exchange unit 136 can include anionic and cationic ion exchange
resins that attract and bind residual charged species in the
treated water. The ion exchange resin can be present in mixed- or
separate-bed configurations in any suitable arrangement to further
purify the treated water. For example, the cation and anion resins
can be arranged in a separate bed configuration with a cation bed
followed by a anion bed in series. Examples of suitable ion
exchange resins include the DOWEX.TM. MARATHON.TM. resin family,
available from The Dow Chemical Corporation, Midland, Mich., as
well as the AMBERLITE.TM. resin family available from Rohm and Haas
Company, Philadelphia, Pa.
[0055] The pumps, piping, sensors, and control systems associated
with moving fluids through system 100 are well known to those of
ordinary skill in the art and, consequently, are not specifically
discussed.
Example
[0056] The following example is intended to illustrate the benefits
of the present invention, but does not exemplify the full scope of
the invention. Thus, this example does not limit the claimed
invention.
[0057] Referring to FIG. 6, this example further details one
specific implementation of method 10 using an illustrative
embodiment of system 100. In this embodiment, system 100 was used
to treat water stored in ponds that accumulated process/product
water at a phosphate manufacturing facility. Influent water 110
typically had constituent concentrations as listed in FIG. 7. FIG.
7 also lists constituent concentrations at various points in system
100. System 100 also included a pretreatment system 118, first and
second pass reverse osmosis membranes 112, 114, and an ion exchange
polishing unit 136. Under typical operating conditions,
approximately 135 gallons per minute (gpm) of water was directed
from storage ponds (not shown) to a pretreatment system 118 that
included first and second stage multimedia filters 120, 122 and
cartridge filter unit 126. Four first stage multimedia filters 120
were arranged in parallel. Effluent from first stage multimedia
filters 120 was combined and fed to two second stage multimedia
filters 122, arranged in parallel. Both first and second stage
filters 120, 122 used media comprised of anthracite, sand, and
garnet to reduce the turbidity of the water to less than about 2
NTU and to reduce the SDI to less than about 4. The media is
layered with the anthracite on top followed by sand (in both first
and second stage filters 120, 122) and then garnet (only in the
second stage filters). A chemical feed pump added an antiscalant,
the sodium salt of phosphonomethylated diamine, commercially
available from Nalco Chemical as EL-5300, between the second stage
multimedia filters 122 and the cartridge filter unit 126 in amounts
calculated to result in antiscalant concentrations between about 5
and about 10 ppm. Cartridge filter unit 126 included three 5-micron
12R.times.20 inch cartridge filters 127 arranged in parallel with
each other that discharged to a single 1-micron 12R.times.30 inch
cartridge filter 129. Cartridge filter unit 126 removed additional
suspended solids including media leaking from the second stage
multimedia filters 122. Passage through cartridge filters 126 had
the additional effect of thoroughly mixing the antiscalant into the
water discharged from pretreatment unit 118.
[0058] As is discussed in more detail below, concentrate 134 from
second pass reverse osmosis membranes 114 was added to water
discharged from pretreatment unit 118 before the inlet side of the
first pass reverse osmosis system 112.
[0059] The first pass reverse osmosis system used FILMTEC.TM.
BW30-365 membranes from FilmTec Corporation, a subsidiary of The
Dow Chemical Corporation, Midland, Mich. and was operated at an
average flux rate of about 5 to 7 GFD at about 250-300 psig
operating pressure. Approximately 100 gpm of first pass concentrate
is discharged from the first pass reverse osmosis apparatus back to
the storage ponds. The second pass reverse osmosis system also used
FILMTEC.TM. BW30-365 membranes and was operated at an average flux
rate of about 18 GFD. A chemical feed pump added sodium hydroxide
to the water stream after the first pass reverse osmosis apparatus
and before introduction into the second pass reverse osmosis
apparatus to raise the pH to between about 6.9 and about 7.1
although the system was observed to meet discharge specifications
for ammonia, fluoride and phosphate with interpass pHs between
about 6.5 and 7.2.
[0060] The recovery of the first pass RO is a function of the
dissolved solids in the feed, e.g., the higher the recovery, the
higher the dissolved solids in the reject stream, thus the higher
the scaling potential is. For the influent water treated by the
exemplary plant, the recovery was limited to about 30% with a feed
water that varied between 13,900 and 22,400 .mu.mho/cm
conductivity.
[0061] A chemical feed pump 116 also added chelating agent ethylene
diamine tetra acetic acid (EDTA) to first permeate 115 before it
reaches second pass reverse osmosis system 114 in amounts
calculated to result in concentrations between about 5 and about 10
ppm. Approximately 7 gpm of first concentrate is discharged from
second pass reverse osmosis membrane back to the inlet side of
first pass reverse osmosis system 112 and approximately 28 gpm of
second permeate passes through to ion exchange polishing unit
136.
[0062] Ion exchange polishing unit 136 was configured with the
cation and anion resins arranged in a separate bed configuration
with a cation bed followed in series by a anion bed using DOWEX.TM.
MARATHON.TM. A and DOWEX.TM. MARATHON.TM. C ion exchange resins,
each available from The DOW Chemical Corporation, Midland,
Mich.
[0063] The mixed-bed polisher served to further control the
concentration of ammonia to below about 1 mg/l and to reduce the
concentration of phosphate species to below about 0.5 mg/l.
[0064] Because no other treatment processes were present, it was
not anticipated that the pH of the water in the storage ponds would
significantly change over time. Consequently, no mechanism was
included for maintaining or lowering the pH of influent into the
first pass reverse osmosis membranes.
[0065] Referring to FIG. 7, this system was used to successfully
treat feedwater with a pH of 1.7 containing fluoride, total
ammonia, and phosphorus at concentrations of 5647 ppm, 456 ppm, and
6645 ppm, respectively. As used herein, "successfully treated" is
used to indicate reducing the concentrations of fluoride, total
ammonia, and phosphorus to below 8 ppm, 1 ppm, and 0.5 ppm,
respectively, before discharge with a discharge pH of between 6.5
and 8.5. It should be noted that discharge criteria are
site-specific. Influent feedwater also included substantial levels
of calcium, magnesium, silica, and possibly organic materials,
measured as total organic carbon, which have the potential to
interfere with the reverse osmosis treatment process through
chemical interactions or by causing scaling. The first pass reverse
osmosis membranes significantly reduce the concentrations of most
of the constituents of interest. However, calcium and fluoride are
present in the first pass permeate at concentrations that
potentially could cause scaling on the second pass reverse osmosis
membranes under the higher pH conditions induced by the interpass
caustic addition. Similarly, total hardness, silica, and organic
carbon are present in the first pass permeate and the inorganic and
organic mix of contaminants can cause fouling of the second pass
membranes, particularly as the pH is elevated. Interpass addition
of EDTA effectively prevented scale formation on the second pass
reverse osmosis membranes.
[0066] Referring to FIG. 8, the adjustment of pH caused a large
improvement on overall contaminant rejection in the second pass
membranes. However, as discussed above, the increased pH causes an
increased risk of fouling and scaling of the second stage
membranes, particularly at desired high water recovery rates. It is
believed the continuous addition of the chelating agent in this
example was responsible for increasing the time between required
membrane cleanings from the order of days with no chelating agent
addition to the order of weeks with the addition of 5-50 ppm of
EDTA.
[0067] A system as described above can be used to treat feedwater
with a pH between about 0.5 and about 4.5 (e.g., between about 0.9
and 4.0), and individual constituents such as one or more of the
following: organic materials, measured as total organic carbon,
below about 500 ppm; calcium below about 1,300 ppm; iron below
about 150 ppm; ammonia below about 750 ppm (e.g., below about 500
ppm); fluoride below about 7,500 ppm (e.g., below about 5,000 ppm);
phosphorus below about 7,500 ppm (e.g., below about 7,000 ppm); and
total silica below about 3,000 ppm.
Other Embodiments
[0068] A number of embodiments of the invention have been
described. Nevertheless, it will be understood that various
modifications can be made without departing from the spirit and
scope of the invention. For example, the pretreatment systems
described herein could include a clarifier having ballasted
flocculation subsystems. Accordingly, other embodiments are within
the scope of the following claims.
* * * * *