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Membrane Ultrafiltration: Industrial Applications for the Removal of Environmental Pollutants
Michael J. Pardus Chief Chemist, Cerro Metal Products
Bellefonte, Pennsylvania
Introduction
One factor limiting conventional lime 6nd settle technology in meeting
more stringent discharge limitations is the effectiveness of the solid/
liquid separation. Gravity separation alone can rarely meet the discharge
limits currently imposed on industrial dischargers; filtration is frequently
required. A viable alternative to the clarifier/filter combination is
membrane ultrafiltration (1) . Ultrafiltration systems typically require less
floor space than conventional systems and are adaptable to a wide variety of
chemical treatments. Reviews of ultrafiltration technology and its relation
to other membrane process are found in references (2) through (6).
Ultrafiltration is demonstra-ted to be an effective treatment in both pilot
and full scale operations, for the removal of oil, suspended solids, heavy
metals and some organics from pretreated...industrial waste streams.
presentation will review the general principles of membrane ultrafiltration
This
and describe a variety of industrial applications for removal of inorganic
and organic pollutants.
The Ultrafiltration Process
Ultrafiltration is a membrane separation process utilizing a hydraulic
pressure gradient to effect solid/liquid separation.
mechanism is due to selective sieving through the membrane pores.
salts and small particles ((0.1 micron) pass through the semi-permeable
The primary separation
Dissolved
. .
membrane i n t h e l i q u i d phase w h i l e l a r g e r s o l i d s a r e r e j e c t e d and c o n c e n t r a t e d
( F i g u r e I ) . As a f i r s t approx imat ion t h e volume o f t r e a t e d wa te r p e r u n i t
membrane area ( o r f l u x ) i s p r o p o r t i o n a l t o t h e d r i v i n g f o r c e and i n v e r s e l y
1
p r o p o r t i o n a l t o membrane r e s i s t a n c e ;
F = P/R = [e(a-Z)Pp] / [8 (TA2)ud ]
where; F = l i q u i d f l u x r a t e
P = h y d r a u l i c p ressu re
R = membrane r e s i s t a n c e
e = membrane p o r o s i t y
a = pore r a d i u s
p = l i q u i d d e n s i t y
T = pore t o r t u o s i t y f a c t o r
u = l i q u i d v i s c o s i t y
d = membrane t h i c k n e s s
P r i m a r y f a c t o r s i n t h e des ign o f an u l t r a f i l t r a t i o n system a r e t h e h y d r a u l i c
g r a d i e n t , membrane p o r o s i t y , membrane p o r e r a d i u s , membrane t h i c k n e s s and
membrane d e n s i t y .
r e f e r e n c e (2).
D e r i v a t i o n ' o f t h e b a s i c f l u x equa t ions can be found i n
Pore s i z e s range f r o m 0.05 t o 0.2 m ic rons i n d iamete r depending on t h e
s p e c i f i c a p p l i c a t i o n . Pore s i z e c o n t r o l s t h e s e l e c t i v i t y and degree o f
separa t i on as w e l l as t h e permeate f l u x r a t e . From t h e f l u x equa t ion , i t can
be shown t h a t d o u b l i n g t h e po re d iamete r shou ld r e s u l t i n a f o u r - f o l d i n c r e a s e . .
i n t h e f l u x r a t e . The mo lecu la r d iamete r o f t h e p a r t i c l e s , o i l s o r greases,
t o be removed f rom t h e wastewater i s t h e l i m i t i n g f a c t o r i n e s t a b l i s h i n g t h e
r e q u i r e d po re d iameter .
s e p a r a t i o n i s by conven t iona l f i l t r a t i o n processes.
When t h e p o r e d iamete rs exceed 50 t o 100 microns,
FLUID FLOW
8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8
8 8 8 8 8 8 8 8 8.8 8 8 8
FILTER BED
FILTRATE flLTRATE
A. BARRIER FILTRATION B. CROSS-FLOW MEMBRANE FILTRATION
CONVENTIONAL BARRIER FILTRATION VS.
CROSS-FLOW MEMBRANE FILTRATION
FIGURE 1
. .
. ... .. .
T y p i c a l h y d r a u l i c p ressures i n u l t r a f i l t r a t i o n systems a r e f rom 40 t o
100 p s i g . T h i s i s a f u n c t i o n o f t h e membrane pore s t r u c t u r e and membrane
geometry. Systems a r e u s u a l l y des igned t o use t h e l owes t d r i v i n g f o r c e
p o s s i b l e t o min imize energy c o s t s and t o reduce t h e s i z e o f t h e pumping
equipment.
o f 600 t o 1000 p s i g .
I n comparison reve rse osmosis systems r e q u i r e h y d r a u l i c p ressures
Because o f t h e r e l a t i v e l y low pressures r e q u i r e d t o
e f f e c t separa t i on , u l t r a f i l t r a t i o n i s n o t a f f e c t e d by t h e osmot ic p ressure
e x e r t e d by t h e wastewater ( 2 ) . d
U l t r a f i l t r a t i o n systems a re opera ted i n a c r o s s f l o w f i l t r a t i o n mode. I n
c r o s s f l o w f i l t r a t i o n , f l u i d f l o w i s p a r a l l e l t o r a t h e r than p e r p e n d i c u l a r t o
t h e f i l t e r media. S o l i d s separa t i on takes p l a c e i n one s u r f a c e l a y e r o r p lane .
I n conven t iona l b a r r i e r f i l t r a t i o n , s e p a r a t i o n takes p l a c e th rough seve ra l
l a y e r s o f t h e f i l t e r media.
used i n c l u d i n g f l a t p l a t e , s p i r a l wound, h o l l o w f i b e r , and l a r g e tube
geometr ies ( 3 , 4 , 5 ) . Due t o a lower f o u l i n g p o t e n t i a l t h e l a r g e tube geometry
has become t h e p r e f e r r e d c o n f i g u r a t i o n f o r i n d u s t r i a l wastewater a p p l i c a t i o n s .
A v a r i e t y o f membrane c o n f i g u r a t i o n s have been
Wastewater i s pumped th rough t h e l a r g e membranes a t h i g h v e l o c i t i e s which
r e s u l t i n a t u r b u l e n t f l o w regime. The r e j e c t e d s o l i d s a r e f l u i d i z e d by t h e
t u r b u l e n t f l o w and scour t h e membrane su r face .
membrane s u r f a c e c l e a n and m a i n t a i n s acceptab le f l u x r a t e s . Optimum f l u x
r a t e s a r e observed when t h e s o l i d s . l e v e l s a r e ma in ta ined a t a c o n c e n t r a t i o n
o f 10,000 t o 40,000 mg/L ( 7 ) .
a re 200 - 400 gpd/ sq f t o f membrane s u r f a c e (6 ) .
The s c o u r i n g a c t i o n keeps t h e
F l u x r a t e s u s i n g advanced membrane des igns
Key components o f an u l t r a f i l t r a t i o n system a r e a p r e t r e a t m e n t u n i t
( t y p i c a l l y hyd rox ide neutralization/precipitation), a c o n c e n t r a t i o n tank,
process pump, membrane modules and f l o w and p ressu re c o n t r o l s ( F i g u r e 11).
The p r e t r e a t e d wastewaters e n t e r t h e c o n c e n t r a t i o n tank and a r e c i r c u l a t e d
th rough t h e porous membranes a t a c o n s t a n t p r e s s u r e b y t h e process pump.
CHEMICAL ABOlTlON (ACID / BASE / PAC)
SLU D E RECl RCU U T I ON I I
WASTE WATER __c
- I I - - ‘ I REACTION
TANK
I 1
1
MEMBRANE MODULES
CONCENTRATION TANK
R
. I i i
FILTRATE SLUDGE - CLEANING - DEWATERING
SOLIDS I LDISPOSAL 1
TYPICAL ULTRAFILTRATION UNIT FIGURE 2
t PERMEATE
COUECTlON
. .- . . . . .- . . -. . . -
. .
The a p p l i e d h y d r a u l i c p ressu re causes t h e l i q u i d t o permeate th rough t h e
pores w h i l e r e j e c t i n g t h e s o l i d s .
ad jus ted , i f necessary, p r i o r t o d ischarge. The r e j e c t e d s o l i d s a r e
accumulated i n t h e c o n c e n t r a t i o n tank. The s o l i d s c o n c e n t r a t i o n i s c o n t r o l l e d
b y blowdown t o t h e s ludge dewater ing system.
The permeate i s c o l l e c t e d and t h e pH
P e r i o d i c c l e a n i n g and backwashing ensure s a t i s f a c t o r y l ong - te rm o p e r a t i o n .
The membrane c l e a n i n g process r e q u i r e s r e c i r c u l a t i o n o f a s u i t a b l e c l e a n i n g
s o l u t i o n t o remove t h e i n o r g a n i c / o r g a n i c b8 i ld -up . T y p i c a l c l e a n i n g c y c l e s
a re o f 30 t o 60 minutes d u r a t i o n . D i l u t e h y d r o c h l o r i c a c i d i s used f o r
removal o f meta l hyd rox ide s ludges f rom t h e membrane pores . H y p o c h l o r i t e
s o l u t i o n i s e f f e c t i v e i n removing o i l s and grease f r o m t h e membrane system.
The use o f an ac id /pe rox ide s o l u t i o n has been r e p o r t e d t o be e f f e c t i v e i n
removing bo th i n o r g a n i c and o r g a n i c f o u l a n t s (8 ) . Systems t h a t i n c o r p o r a t e
an au tomat ic backwash c a p a b i l i t y have been r e p o r t e d t o be e f f e c t i v e i n
reduc ing t h e f o u l i n g problems assoc ia ted w i t h h o l l o w f i b e r membrane
geometr ies ( 2 7 ) .
U l t r a f i l t r a t i o n techno logy i s n o t a new technique. UF s t u d i e s were
r e p o r t e d d u r i n g t h e l a t e 1800's. Recovery o f tramp o i l s by UF techno logy
has been w i d e l y used s i n c e ca. 1960-1970.
t r e a t i n g i n d u s t r i a l wastewaters was impeded by two f a c t o r s . F i r s t , e a r l y
membranes had r e l a t i v e l y smal l pores r e s u l t i n g i n h i g h o p e r a t i n g pressures
and low f l u x r a t e s . Secondly, t h e ' e a r l y membranes were f a b r i c a t e d f r o m
c e l l u l o s e d e r i v a t i v e s and c o u l d n o t s t a n d t h e extreme pH, c o r r o s i v e and
The use o f UF techno logy i n
. .
o x i d a t i v e c o n d i t i o n s .
p o l y s u l f o n e s , t h a t p e r m i t s use under r i g o r o u s i n d u s t r i a l a p p l i c a t i o n s .
Advanced membrane des igns use an i n e r t polymer, e.g.
The advantages o f UF t r e a t m e n t techno logy a r e r e l a t e d t o i t s i n h e r e n t l y
h i g h e f f i c i e n c y i n removing o i l s and suspended s o l i d s . S ince IJF i s adaptab le
t o a wide v a r i e t y o f p r e t r e a t m e n t techn iques , t h e process can be op t im ized
. . . . . . . . . . ... -. -
. .
for the removal of specific inorganic and, in some circumstances, organic
pollutants. The membrane establishes a positive barrier between the waste
and the external environment. The UF system is more resistant to upsets due
to sudden variations in the waste stream than conventional treatment systems.
The equipment is compact and requires little floor space. Since the membrane
modules are normally skid mounted, the units can easily be adapted to existing
systems minimizing the cost of. system up-grade. Maintenance and man power
requirements are less than or equal to conventional treatments. Coagulant 5
aids or other sludge conditioning chemicals are not required.
The temperature, pH and oxidation limitations of the early membranes has
been largely eliminated. Certain solvents at high concentrations may result
in damage to the membranes. Membrane fouling is the largest potential
problem.
the large tube geometry and high velocities through the membranes. Pre-
filtration is sometimes required to remove objects that may clog or puncture
the membranes. Control of pH is crucial as a rapid change in pH can greatly
Solids build up on the membrane surface is minimized by utilizing
intensify the plugging problem. This is most likely the result of
concentration polarization o f the membrane surface.
interfere with the operation of the ultrafiltration system; the surfactants
should be non-ionic or have the same charge as the membrane surface.
Surfactants can greatly
The
affect of surfactants is most likely due to changes in the charge distributions
on the membrane surface. Sufactant'mediated removal of heavy metals is
described i n the next section.
Removal of Heavy Metals
The use of ultrafiltration for removal of precipitated heavy metals has
Applications include removal of heavy grown steadily since the late 1970's.
metals from wastewaters from the following: industrial laundries ( 6 , 9 ) , brass
- . . - . . . . . . . .. . - . . .
wire production (7,25), adhesives and sealant production ( 9 ) , chlor-alakl i
production ( l o ) , printed circuit bo.ard manufacture (ll?lZ), gallium-arsenide
chip manufacture (13) , electroplating (14 ) , and shipbuilding (15). The
treated effluent is typically of high quality suitable for re-use. Effluent
TSS of less than 0.5 mg/L are common. Metal removal efficiency is typically
greater than 90%. Table I summarizes some of the available data. With proper
pretreatment many heavy metals can be removed below 0.1 mg/L.
chromium, mercury, zinc, aluminum, lead, and arsenic have been demonstrated
Copper, i
to be removed below drinking water standards. Metal removal efficiency is a
function of the pretreatment of the wastewater. IJF has been used in conjunction
with hydroxide precipitation, sodium borohydride reduction, hydrazine
reduction and dithiocarbamate precipitation. Other chemical pretreatment
technologies, such as, sulfide precipitation, may be adaptable to the [IF
sys tern.
Metal-ion extraction using UF technology and water soluble starches has
been reported (26). Amination of naturally occurring polysaccharides produces
chelating polymers with the required properties for use in IJF systems. High
molecular weight (MW>10,000) is required to ensure adequate rejection of the
metal complex. Excellent water solubility is required to prevent membrane
fouling.
at concentrations of 10 mg/L.
Removal efficiencies of 98-99% were reported for copper and nickel
This modification of [IF technology may offer
advantages for recovering precious metals f r o m dilute wastewaters.
The use of charged surfactants in conjunction with (IF treatment has been
shown to remove a variety of ionic inorganics and organics and non-ionic
organics (28).
to be removed, is added to the waste stream.
or binds the counterion to the surface of the micelle.
A surfactant, whose charge is the opposite of the pollutant
A micelle i s formed that absorbs
The metal-bearing
T A B L E I SUMMARY OF I J L T R A F , I L T R A T I O N REMOVAL E F F I C I E N C Y
METAL I NF*- E F F * X REMOVAL P R E T R E A T M E N T REF.
6,7,9,14,22,25 c u 200-900 . 2 99-1- OH cu 100-150 c.2 99-1. OH -1- SBH 11 c u 200 < 99-1- DTC 12 Ga 1500 4 2 99-1- S B H 13 As 3000 4.01 ' 9 9-1- S B H 1 3 Hg 10-50 4 . 0 0 5 99-1- OH 10,22 Cr +3 100-1000 ( - 1 - 1 . 5 99-1- OH 7 Zn 900 :. .1 - ,3 99-1- OH 7 A1 100 < .1 99-1- OH 22
Pb 1-10 < 0 1 90-1- OH w, 7 Cd N.S. c $01 -- N.S. 22
Ag 100 4 9 94- OH -I- HYD 1 5
N i 194 a 3 5 99.1- N . S . 24
* influent/effluent concentrations i n mg/L OH = hydroxide precipitation SBH = sodium borohydride DTC = diethyldithiocarbamate HYD = hydrazine N.S. = not specified
micelles are rejected by the UF membrane and concentrated. The permeate is
collected for recycle or discharge. Removal of non-ionic organics, such as
o-cresol, is believed to be due to dissolution of the organic solute within
the interior of the micelle. Removal of calcium, cadmium, copper and zinc
ranged from 98.9 to 99.5% at concentrations of 400-500 mg/L.
removal of 99.4% at 900-1000 mg/L was also reported. Sodium dodecyl sulfate
was found to be suitable for removal of cations while cetylpyridinium
chloride was used for anionic species.
approach is its effectiveness in treating dilute wastewaters.
efficiency reportedly increases as pollutant concentration decreases.
Chromate
A 9ignificant advantage of this
Removal
Many of the early industrial applications o f ultrafiltration technology
were metals removal in the printed circuit industry. Ultrafiltration is now
utilized, however, to remove heavy metals from more traditional industrial
wastewaters. Treatment of rinsewaters from the manufacture of brass wire has
been demonstrated to be cost competive with conventional treatment technology.
The standard process flow for brass wire fabrication included heat treatment
o f the extruded coil stock, -chemical removal o f the surface oxides, and a
drawing step. The cycle was repeated until the desired size and hardness were
achieved. The heat treatment step, requ-ired 'to relieve internal stresses
generated during the drawing operation, resulted in the formation of surface
oxides on the wire surface. Removal of the surface oxides through the use of
acidic pickling produced wastewaters containing chromium, copper, lead, nickel,
zinc, suspended solids, acidic pH, and oil and grease. Evaluation of a variety
of treatment alternatives demonstrated that a combination of hydroxide
precipitation and ultrafiltration was cost effective in producing an effluent
that satisfied the regulatory requirements ( 7 3 5 ) .
installed in May 1986 and has demonstrated removal efficiencies of greater
than 99% for chromium, copper and zinc at influent concentrations of
The treatment system was
.. 200-1500 mg/L.
90% at influent concentrations of 1-10 mg/L. Average effluent metal
concentrations were less than 0.1 mg/L except zinc which averaged less than
0.3 mg/L. Cost of the treatment system for flows up to 50 gpm was $173,800.
The system costs as well as operating and maintenance costs were competitive
with conventional treatment alternatives, e.g. lime, settle and filter, while
producing a higher quality effluent.
Removal efficiencies for lead and nickel were greater than
Treatment of acid mine drainage usingdcharged ultrafiltration membrane
has been reported to offer a cost effective alternative to conventional lime/
settle technology (21). Negatively charged, non-cellulosic membranes were
used to separate the ionic solutes, in this case heavy metals. The rejection
mechanism is reportedly dub to the repulsion of the co-ions by the fixed
charged groups on the membrane surface. The acid mine drainage was treated
with lime to a pH of 4.0 to 4.5 prior to IJF treatment. Removal o f iron and
aluminurn was greater than 95% at 30-100 mg/L, manganese removal was greater
than 89% at 2-10 mg/L and calcium removal was greater than 83% at 50-200 mg/L.
The advantages of the charged membrane system are due to reduced chemical
costs (lime), a reduction in the volume of solid wastes generated, and
improved permeate quality due to lower concentrations of calcium sulfate.
Ultrafiltration has been used in remedial action work by the Department of
Energy (DOE) in the clean-up of radioactive mine tailing sites in Pennsylvania
and the mid-west. (6). In this application the iJF system was used to treat
contaminated groundwater and run-off from the contaminated site while disposal
cells were constructed. The significance of ultrafiltration in treating
groundwater i s certain t o increase as the technology becomes more widely
accepted and recognized as a cost effective alternative for removing inorganics
pol 1 utants.
.. Removal of Organic Pollutants
Removal of organics using 1JF technology is reported with the primary
applications being treatment of industrial laundries (6,9,20), adhesive and
seal ant wastewaters ( 9 ) , synthetic rubber wastewaters (9), removal of color
bodies (lignins) from Kraft mill effluents (16), treatment of latex paint
wastes (17) and removal of oil and grease from wastewaters and emulsified
oils (9,18,19). Treatments combining powdered activated carbon have proven
to be effective in treating some industrialswastes; however, pretreatment is
usually required to prevent membrane fouling. In these instances the
pretreatment typically is lime addition followed by clarification. The use
of silica as a filter aid has also been demonstrated to be effective in
reducing membrane fouling due to the presence o f organics (16).
efficiency for organics is variable depending upon the specific pollutant.
Organic solids removal i s usually greater than 99%. When [IF is used to
The removal
recover oils from water/wastewaters oleophilic pollutants may be removed
along with the oil. Reductions in total toxic organics would tend to
support this hypothesis (24)‘.- Other treatment technologies are probably more
cost effective in removing organics. When the IJF system is utilized for
removal of inorganics, modification o f the system to accomodate organic
removal by PAC may be less costly. ‘?able I1 summarizes some of the data for
UF treatment of organic wastes.
priority organic pollutants is limited. The combined UF/PAC treatment may
not remove organic pollutants to drinking water standards.
The available data for the removal of
The combined
IJF/PAC system may be suitable as a pretreatment to remove organic particulates
and oils that may foul GAC columns.
The effectiveness o f ultrafiltration technology in removing toxic
organics has been demonstrated in the treatment of washwaters from industrial
laundries (6,20,23). Existing technologies for treating the washwaters
T A B L E I 1 SIJMMGRY O F REMOVAL OF O R G A N I C S BY U L T R A F I L T R A T I O N
P O L L U T A N T I N F * EFF* T R E A T M E N T R E F
oil & grease COD
T O C
TS S TTO oil & grease COD
TOC
T S S B O D 5 chloroform benzene perchlorethylene toluene ethyl benzene naphthalene to1 uene trichloroehtylene
2000-80,000
11,000
5780-34,500
, 200-9000 12-1425
200-1100
'3230-5000
1000-1400
852-1000 1300
3.3 2 . 5 9 . 1 5 . 2 3 . 1 1 . 5 1 . 0
0.036
L35-224
1400
380-918
< 2 - t 3 5 <. 5-233
L 1 0 - ~ 1 0 0
4 100
59-98
13-20 L 30 c.1 N . D .
N . D .
N . D .
c. 001
< ,001 < * 001
< ,001
none none none none none P A C
P A C
P A C
P A C P A C P A C
P A C PAC
PAC
PAC
P A C
PAC
PAC
8,18,19,24
19 8,17
8,17,18,19 24
6,20,23
6,20
20,23
20,23 6
6
6 6
6 23
23 23
23
he,
.
* influent/effluent concentrations i n m g / L .
N.D. = not detected, limits of detection not specified
. .. .. . . -.
'.
consisted of coagulation and flocculation followed by dissolved air flotation
(DAF). High dosages of costly inorganic coagulants resulted in large volumes
of sludge. Incomplete removal o f suspended solids and toxic organics was also
reported for the DAF treatment of these waters.
recyclable effluent at competitive costs resulted in the development of a
system utilizing powdered activated carbon (PAC) adsorption and ultrafiltration.
A full scale system was constructed to process 180 gpm of washwater at an
industrial laundry in Florida (23). The washwaters are pretreated with lime
and settled prior to processing by ultrafiltration.
washwaters in an agitated tank with a detention time of 30 minutes. The PAC
mixture was processed through the ultrafiltration membranes where the PAC
and toxic organics were removed. Chemical and sludge disposal costs were
reportedly reduced so that the ultrafiltration system was cost competitive
The desire to produce a
g
PAC was mixed with the
with DAF. Volatile organics, such as toluene and
reported to be reduced to less than 1 part-per-bi
Conclusions
The ultrafiltration process is a demonstrate
trichloroethylene were
lion.
technique for the control
o f heavy metals, oils and greases,-and organic substances found in industrial
wastewaters.
system performance.
hazardous and toxic wastes (HTW), but does not inherently provide any
elimination of the toxic or hazardous characteristics. The residual wastes
Proper pretreatment o f the wastes is a key factor in optimizing
The IJF process serves to greatly reduce the volume o f
may require further processing for ultimate disposal.
the use of [IF in treating industrial wastes is expected particularly in the
areas of coil coating,
textile manufacture, and ground water clean-up.
Continued growth in
porcelain enameling, battery manufacture, dye and
References
(1) Peterson, K., "Replaces a Clarifier," Prod. Finish. (February, 1985).
(2) Weber, W., Physico-chemical Treatment Processes, John Wiley 81 Sons: New York (1986).
( 3 ) Khatib, Z., "An Overview of Membrane Filtration," Chem. Process; 49(4) :87 (1986).
(4) Josephson, J., "Crossflow Filtration," Environ. Sci. Technol.; 18(12):375A (1984).
(5) Halverson, D., "Ultrafiltration Casebdok," Waterworld News 1(1):12 (1985).
(6) Tran, T . , Chem. Engr. Prog.; 81(3):29 (1985).
(7) Pardus, M., Material Substitution: The Effects of Waste Minimization on Effluent Metal Concentrations, M.S;.Thesis, Penn. State Univ. (Aug.1987).
(8) Personal communication with K. Sargavakian, Memtek Corp.
(9)
(14)
Treatability Manual, Vol. 111, USEPA,EPA-600/8-80-042~, U.S. Government Printing Office (July,1980).
Anon., "Mercury Recovery System Utilizes Ultrafiltration Technology to Minimize Metal Discharge," Chem. Process .(May, 1982).
Lopez, N. and J. Regan, "Chelated Copper Extricated by Membrane Filtration System," Chem. Process (Oct., 1984).
Personal communications- with G. Valentine, Triangle Circuits.
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