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New thin-film composite low pressure reverse osmosis membranes and spiral wound modules
387
Desalination,64 (1987) 387-401 Elsevier SciencePublishers B.V.,Amsterdam-PrintedinTheNetherlanh
NEW THIN-FILM COMPOSITE LOW PRESSURE REVERSE OSMOSIS MEMBRANES AND SPIRAL WOUND MODULES
I. KAWADA, KAMIYAMA
K.
INOUE,
Y.
KAZUSE,
H.
ITO, T. SHINTANI AND Y.
NITTO ELECTRIC INDUSTRIAL CO., LTD. KUSATSU, SHIGA (JAPAN)
SUMMARY
This paper describes improved thin-film composite low pressure reverse osmosis membranes developed by Nitto Electric Industrial Company. These membranes exhibit high salt rejection and high flux at low operating pressures, combined with good permeate chlorine resistance and thermal stability. The characteristics of the two new membrane tvoes. the NTR-739HF and the NTR-729HF. both available as spiral-wound modules, are discussed in detail.. The NTR-739HF exhibits up to 95% salt rejection at a flux over 0.8 ma/mad (20 gfd), when tested at 1 MPH (143 psi) and 25'C (77'F) on 1500 ppm NaCl solution. The NTR-729HF has a water flux greater than 1.3 m3/mad (32 gfd) and a salt rejection above 92% In actual module trials with when tested in the same conditions. brackish water, salt rejections of 96% (product water = 60 ppm TDS) have been obtained. The membranes are stable to chlorine in feed water, as shown by long-term reverse osmosis tests with tap water containing about 1 ppm residual chlorine.
INTRODUCTION The phase
asymmetric
consisting layer)
cellulose acetate
inversion process.
(CA) membranes are made
They have an
asymmetrical
by
structure
of a dense surface layer (a reverse osmosis (RO) skin
and a porous substrate.
use in 1960s Ill.
They had been put to
practical
Since then the RO membranes have been improved
to enhance RO performance and durability. The main aim of development of RO membranes realization as
in 1970 to 1980 was
of membranes which ensure such a pure product
drinking
water
after
single
stage
seawater
water
desalination
process. Enhancement of salt rejection by CA membranes results in reduction
of product water fluxes and reduction of usability
the CA membranes. a
hollow
This discrepancy
of
could be removed by applying
fiber membrane module with remarkably
large
membrane
area per module volume, and single stage seawater desalination
B.V. OOll-9164/87/$03.50@1987ElsevierSciencePublishen,
by
388 CA
hollow fiber membrane module was proved to be effective
To
exceed
the limit of CA membranes,
membranes
synthetic polymers were studied.
various
asymmetric membrane
polyamide
B-10
Among them,
These
of
aromatic
has been used practically
hollow fiber membrane module for single stage seawater tion.
r21.
consisting
as
desalina-
asymmetric membranes are put into practical use as
hollow fiber membranes.
For other module type,
such as
spiral
wound module, the RO membranes with high product water fluxes are The phase inversion process is restricted in reduction
required.
thickness of skin layer of the RO membrane.
of
composite
thin-film
ultrafiltration
an
in-situ polymerization result
of
these
desalination NTR-7199
developed
(UF)
membrane
the The
[31. the
is
following used
as
a
a dense skin layer which exhibits RO capacity
and
substrate
were
thin-film composite membranes has
representative structure;
membranes
Therefore,
reaction is formed on the substrate. the
researches
such as PA-300 [41,
RO
membranes
for
by As a
seawater
PEC-1000 161,
FT-30 [51,
and
171 were put into use in the form of spiral wound module
and plate and frame module. In 198Os, the RO membrane, which had been improved to increase salt rejection capacity,
took various turns.
If an RO membrane
the same RO capacity as before at lower pressure,
keeps
can be applied to low-pressure process.
membrane running
cost.
processing, rejections
On the other hand,
the so
have
for industrial use
long
as
it
such
as
has
sufficient
rejections
for
a
To diversify the membrane processes,
been demands for function to osmose
inorqanics
condense organics and function to osmose inorqanics and and
RO
reduces
RO membranes is allowed to be satisfied in salt
substance to be separated. there
the
This
condense colored components.
and
orqanics
The low-pressure RO membranes
which can meet these demands are playing an important role
among
all RO membranes. Low-pressure RO Membrane Loose RO membrane. --
1.
(1)
reverse
90% (for example 10 to 50%), because
There had been available the
osmosis CA membranes with salt rejections of of
narrow
range
loose
less
than
however, they were not worth notice of
pH
of
CA
membranes
and
use
temperature. The thin-film composite membrane NTR-7250 we developed in 1982 [7]
had
higher
orqanics rejections than the
former
loose
RO
389
membranes
and
pressure
maintained high product water fluxes even at
tions were low; disadvantage chlorine
50 to 60%.
which
membrane
is
Moreover the membrane was freed from
contained
in
with
pressure
was
tap water
performance
without
thin-film
as
composite
California.
50%.
However,
reducing its
Since
the
psi)),
the
electrical
it
indicated for
capacity
12,000
Thus its unique separating capacity and excellent energy
saving have been appreciated, in
in
low (0.4 MPa (57
conductance rejection was approx. hours.
usual
Fig. 1 shows the result of continuous running of the
NTR-7250
running
low
rejec-
(lacking in resistance to oxidizing agents such
membranes).
stable
salt
of less than 2 MPa (286 psi) although its
Japan
for
fermentation, thin-film
ultrapure
and it finds extensive applications
water
pharmaceutical
productions,
manufacturing
food
fields.
industries, Recently, the
membranes SC-200 [81 and NF-40
composite
having
the
same performance as our product have been developed. these
However, pressure
loose
RO
membranes could be used
operation from the viewpoint of product
for
water
low-
fluxes,
but its salt rejections were too low to apply to desalination. When
the membranes are used for the purpose of brackish water
desalination,
they are required to have salt rejections of 90% at
least or, if possible, 95%.
In
(2) Low-pressure RO membrane with high salt rejection capacity brackish water desalination of low salt content of less than
1500 ppm the osmotic pressure is lower (for example, pressure
of
brackish water of 1000 ppm
is
the osmotic
approx.
0.1
MPa).
Therefore, even RO membranes do not require running pressure of 2 to 3 MPa (286 to 429 psi) in principle. were
The former RO membranes
not applied at such a low pressure because at low pressures
of less than 1 MPa (143 psi) the salt rejections decrease and the product use
of the membranes.
afford the
water fluxes reduce too excessively
to ensure
If there were any membrane
practical
which
could
the RO ability at low pressure which we obtained by using
conventional
RO membrane at 3 MPa (429 psi),
cost of
and piping of equipment with the membranes would be reduced, the initial cost would be lowered. as power consumption
Moreover,
pumps and
running cost such
would be remarkably reduced.
These are the
key reasons why the low-pressure RO membranes are expected. Thus, rejections
we
began
development of membranes
having
without impairing the features of NTR-7250;
high
salt
1)
high
390
product
water
During
fluxes
and
2)
excellent
chlorine
resistance.
this development we obtained some variations of
shown
in Fig.
729HF
are
brackish
2.
membrane
Among them the membranes NTR-739HF and
low-pressure
RO membranes which can be
water desalination
NTR-
applied
at low pressure of 1 MPa (143
for psi).
Below are discussed these two kinds of RO membranes. These RO membranes are thin-film composite membranes the
polysulfone
UF
substrates
are covered
with
in
which
skin
layers
(ultrathin layer) featuring high salt rejections at low pressure. The skin layer of NTR-729 consists of the same polyvinyl material
as that of the above-mentioned
NTR-7250,
alcohol
and this skin
layer material is partially modified and applied to the
membrane
NTR-739HF. 3 shows the effects of feedwater pressure, one of the RO
Fig.
characteristics.
As expected,
the flux is increased in propor-
tion to increase of pressure in its measurement
range.
rejections
particularly
curve (286
resembles a hyperbola,
curve
psi)
nearly reaches equilibrium.
is 98%,
The
the
2
MPa
NTR-739HF
the rejection performance of 95% at 1 MPa (143 psi) and
90% at 0.5 MPa (71 psi). usable
The salt
whose rejection performance at
for the NTR-739HF,
maintains
and
flux
membranes
Another membrane NTR-729HF possesses an
at 0.5 MPa.
This suggests that the two
is useful for brackish water desalination
kinds
even at
of low
pressure of 1 MPa (143 psi). The
conventional
advantage, however,
extremely
thin-film composite membranes have low chlorine resistance as
these new RO membranes,
like NTR-7250,
stated
a
disabove,
ensure the same
or higher chlorine resistance than the CA membranes. All
the
RO
membranes are used practically as
spiral
wound
module. 2.
Solute Rejection Performance -of RO Membranes 4 show the rejection performance when treat-
Table 1 and Fig.
ing various solutes. ions
of
the
The rejection performance against inorganic
RO membranes
is equal to
that
of
CA
membranes,
however, the the rejection performance against organic solutes of RO
membranes
is remarkably higher.
membrane having rejection performance tion
performance
against i-Propanol
though a
For example, to NaCl is 97%, is
lower,
CA
its rejec-
45%.
On
the
contrary,
the rejection performance against NaCl of NTR-739HF is
equal
that of CA membranes,
to
but its
rejection
performance
391
0
e-
xm0.4
_......
~~0.2 8H t-
. . .
-
. . . . . ._*. . . . . . . .-a...
._. . . .
-6a. . . . . . .4O’Z
* . . . ...*
-202
0 0
2030
4am
am
Elapse?%ne
Test Condition
IZaxl
lam
(h)
: Element:NTR--7250~S2B (2.5-inch spiral wound element) :Tap Water, Feed Chlorine Concentration : 0.5-0.9
ppm
Brine Flow Rate :2 Vmin frntecmittentlY) Temperature
Fig. 1
:
20-28’c
LONG-TERM OPERATION WITH TAP WATER AT SANTA CLARA, CA., USA
NEW
COMPOSITE
RO MEMBRANES
NTR-723HF
GB NTR-7250
@B
1 0
I I
2
4
3 Flux
5
(m’/m’.d)
Operating Condition NaCl Concentration Pressue
:
:
0.15 wt%
1MPa (143 psi)
Feed Temperature : 25’c PH
Fig.2
PERFORMANCE
:
6-7
OF LOW-PRESSURE
TYPE RO
392
103
20
3x3
4cu
NTR-729HF
01 0
Fig.3
(psi)
200
c
’ I
0
’
1
0
2 Feed Water Pressure (MPa)
WATER
3
FLUX AND SALT REJECTION
Brme flow vate : 20 Vmin ; Temperature Element : 4-inch spiral wound element Feed concentration : 0. 15 wt%
TABLE
1
:
25C
SOLUTE REJECTION OF RO MEMBRANE ( %1
NaCl
95
92
mg so4
) 99
> 99
Ethanol
30
25
I - Propanol
75
70
N-Methyl
Pyrmlidone
Saooharorc
Prcssur.
) IMPa
1
a4
86
99
) 99
I l143priJ
97 )
99 9 45 60
)
99
3 (429LSi
I
i-Propanol
against This
is
polymer
4).
composite
thin-film
Such performance enables to apply the RO membranes to
membranes. various
of synthetic
feature
a
75% (based on data in Figure
is higher,
conventional
than
other
fields
brackish
water
desalination. 2 shows salt passage of RO membranes.
Table
affecting
factors
salt passage.
There
are
two
molecular
One of them is the
size of a solute,
and another is the charge repulsion of ion
fixed
functional
groups of the membrane.
kinds
of
Since both
the
by two
membranes contain fixed carboxyl groups in their
skin
layers, their considerably high rejection against divalent anions can be explained by above-mentioned
two factors.
The difference between salt passages of NaCl and MgClz depends on
the
molecular
however,
size,
the difference
between
of NaCl and NazSOr is determined by not only
passages
salt
molecular
size but also charge repulsion. Chlorine Resistance
3.
Besides
the
water fluxes, as
chlorine
RO performance an
important
wider
solutions
with
moreover,
cleaning
the
condition for
use range
properties
various agent
As
the RO membianes,
for
and
resistance.
of
can
real
product
use
membrane,
of
the
RO more
processed,
be
can be selected among many
agents when the membrane performance chlorine
rejections
the use range of pH, temperature and oxidizer such is The
membranes.
such as
and,
kinds
of
lowers due to fouling.
much importance is
given
to
the
The CA membranes has a certain degree
of
chlorine resistance, although their use ranges of pH and temperature
smaller
are
of
CA
as compared to those of
membranes
that in the
membrane deterioration
well
as
composite suffer poured slime prevent
membranes,
the disadvantage natural
polymer,
is caused remarkably rapidly by attack
Namely, a feed water is turned to bacteria-free
bacteria.
by pouring chlorine, as
CA
composite
thin-film
This chlorine resistance compensates
membranes.
deterioration
deterioration.
consisting by bacteria,
to prevent deterioration.
Since
of synthetic
the
fouling,
it
the
thin-film
polymers
they do not require On the contrary,
takes place easily owing to propagation of
intermittently.
water
and this prevents fouling by bacteria slime
CA membrane
membranes
of
is necessary
to
add
do
not
chlorine
fouling
bacteria. chlorine
by To even
Besides, the cleaning agent used most frequently
in food industry processes is made from chlorine.
394
loo -
Test
(
Feed
Pressure
Cone
wt x 9
,
MPa
NTR739HF
80 -
I1 .0.151 NTR-729HF, II , 0.151
si ,$60
s
I 1
Z t %
NTR7250 12 0.21 ,o
14.2 30.5)
/
*
“; 40 ._
1; !’
C ,‘ll.5.0.11 /
20 -
,/ .’
1
I
** -(2 *o-21 ,
20
40
60
_-_,, 0 0
NaCl
Fig.4
TABLE
2
Rej
NaCl REJ
**
CA
membrane
,
,
I
60
100
(X) vs
i-Propanol
REJ
SALT REJECTON AND PASSAGE BY NEW MEMBRANES FOR DIFFERENT SOLUTES
NTR-739HF
NTR-729HF
NaCI
4
6
MQCIZ
3
7
NaCI
4
a
Na.SO.
0.5
0.5
395
Fig. 5 shows the data of chlorine resistance obtained when the solution
containing
100
ppm of active
chlorine
processed
is
continuously at running pressure of 1 MPa at various pH. NaClO is dissociated at various pH as shown in Fig. 6, and the Therefore, it is
oxidizing ability of HClO is higher than ClO-.
solu-
said that if solution has the same chlorine concentration, tion with lower pH have higher oxidizing ability. all the membranes, the
However, with
the time until the membranes fail to maintain
retention of rejection of 100% (the time before decrease
of
salt rejection) is decreased remarkably near the pH7. Fig. 7 shows the data in continuous operation of membranes with a solution
(pH = 7) containing 10 ppm of active chlorine at running
pressure of 1 MPa (143 psi). membranes
composite however
The rejections of these
increases
at
the
beginning
thin-film
of
running, fouling
after this it decreases gradually owing to iron
but it is restored by acid cleaning. The
order of the time until decrease of rejection is CA membrane
< NTR-739HF the
< NTR-729HF.
critical
approx.
1
chlorine
area,
if the chlorine
ppm, resistance of approx.
20,000 ppm-Cl,-hr.
performed
containing
is
practical
and
approx.
continuous running test with tap water
usually
respectively.
Lonq-term Continuous Running with = We
concentration
the NTR-729HF and NTR-739HF maintain
10,000 ppm-C12-hr, 4.
As is evident from this figure, even in
pH = 7,
Water
1 ppm or less of active chlorine by using the 4"
dia.
elements of NTR-739HF. Fig.
8
shows
the RO performance
in long-term
running,
table 3 indicates the analysis results of quality of feed
and
water,
product water and brine after running for 3,500 hours. The
product water exhibits the stable conductivity of 6 to
MS/cm while the feed water conductivity is 120 to 130 After 5,000 hours of running, the performance is stable,
7
US/cm. and at
present this test is continued. 5.
Desalination We
conducted
Performance -in Synthetic Brackish Water the desalination performance test by using
kinds of synthetic brackish water with the 2.5" dia. NTR-739HF.
Table 4 shows the test results.
element
two of
4000, i?
Y a
A NTR-739 HF 0 NTR729 HF
f G I 3000.
1 NTR-
15951CAI
E a ,a
n
e ._ 2 2000 ” 2 % 8 0lOOOlz
01 4
5
6
7
8
Feed Water
:
Tap
Water
PreSSUV2
1 MPa
(143
:
Temperature
Fig.5
:
5
IO
6
7
psi)
20-257~
CHLORINE RESISTANCE (d-l6 TO IO. AND CHLORINE
4
9
PH
Test Condition
8
AS A FUNCTION CONCENTRATION
9
to
PH
Fig. 6
EOUlLlBRlUM DIAGRAM OF HOC1 AND OCI-
pH 100PPMj
397
4-
Cleaning
B
3B&&;: 2 1c
Ev
Time Exposure to Chlorine (ppm-c/,-hour) Test Condition :Feed : 0.15 wt% NaCI. Pressure : 1 MPa (143 psi) Temperature : 18-28C, pH : 7 Fig.
7
LONG -TERM OPERATION CHLORINE OXIDATION (CHLORINE
CONCENTRATION
ON 10 PPM)
398
Elapsed time (h) Test Condition
:
:
Element
:
Feed Chlaine
NTR-739HF-S4 (4-inch spiral wound element) Tap Water
Concentration : 0.3-0.8
Pressure
:
: 12 Vmin : ZO-3OC
Brine Flow Rate Temperature
Fig. 8
LONG-TERM AT KUSATU
ppm
1 MPa (143 psi) (3.2 gpm)
OPERATION WITH TAP WATER CITY, SHIGA, JAPAN
TABLE 3 FEED, PRODUCT AND BRINE ANALYSIS AFTER 3500 HOURS OF OPERATION
Unit PH
Concbctivity
#S/cm
TDS
9/t
Nil’
T/t
K’
T/d
CS”
=?/L
Msl” Total hardness
ss/l ~..caC0~/6
Feed
Prodnet
6.6
5-a
120
5.6
73 8.8
(
10 0.7
Brine
6.8 151 91 10.2
I .9
(0.1
2.0
12.6
(0.1
14..8
2.6
(0.1
3.1
42.6
1.5
49.6
Cl-
V/d
18.1
1.2
20.6
HCOa-
s/t
24.6
1.8
30.5
so,--
4/6
SiO2
=?/1
1.1
TOC
PPb
1700
15
(0.5 0.2 112
18.7 1.4
1900
399
At
first,
the test was performed by using synthetic brackish
water A (TDS 601.5 ppm, feedwater. TDS
of
total hardness 330.8 ppm and pH 8.2)
In this test,
at pressure of 1 MPa (143 psi),
product water was 26.3 ppm,
lower than 6.6 ppm;
but
At pressure of 0.2 MPa
the TDS of product water was 69.2 ppm, was lower than 11.9 ppm;
hardness more
than
was
and the total hardness
the rejection of TDS was 96%, and the rejec-
tion of total hardness was more than 90%. (29 psi),
as the
96%
of the
total
and its
the rejection of TDS was 88%,
rejection
of
total
hardness
was
maintained. total
When another synthetic brackish water B (TDS 327.6 ppm, feedwater,
the
rejection of TDS was 94% and the rejection of total hardness
was
hardness more of
191.7
ppm
and pH 7.9) was tested
as
than 97% at pressure of 1 MPa (143 psi),
while at pressure
0.2 MPa (29 psi) the rejection of TDS was 88% and the
rejec-
tion of total hardness was more than 97%. The and
table shows also the calculated values of
flux
water
quality
which were obtained with the aid of personal
computer
program developed by us. Table
5 indicates the desalination
brackish
test results of
water (TDS 1474 ppm) by using the 4"
dia.
synthetic element
of
NTR-739HF. As is evident from this table, the rejection of TDS is 96% and the rejection of total hardness is more than 99% at pressure of 1 MPa (143 psi) even when the TDS of feedwater is approx. 1500 ppm. When
the pressure is increased to 1.5 MPa (214 psi),
tion
of
TDS reaches 97%,
exceeds 99%.
and the rejection of
the rejec-
total
hardness
On the contrary, even when the pressure is lowered
to 0.5 MPa (72 psi),
95% of rejection of TDS and 99% of
rejec-
tion of total hardness were maintained. This brackish In
suggests
that
water desalination at low pressure
addition,
if
the
function satisfactorily (72 psi).
the new RO membranes are membranes are used
effective
(1 MPa for
(143
for
psi)).
softening,
even at low pressures lower than 0.5
they MPa
400
TABLE
4
BRACKISH
WATER DESALINATION
Elamant:NTR-739HF-S2 T
-
K
spiral wound element)
Model soutlhe-n California Feed
NP
(2.5-inch
Permeats~ll OBS.
CAL.
A
OBS.
Californls M&l
Scuthern
Permeate IZI CAL.
Permeatell)
Feed
OBS.
B
Psrmwts 12)
CAL.
OES.
83.9
5.1
4.8
15.0
17.4
48.1
3.5
2.8
7.9
8.1
5.6
( 0.1
0.3
< 1.0
1.2
1.2
( 0.1
0.1
( 0.1
0.2
I
4.2
45.2
(
1.0
0.6
<
1.o
1 .I?
1.5
19.2
(
I-O
0.2
(
I.0
0.7
2.1
2.7
6.9
7.2
Gil
84.6
(
1.0
I .o
MQ
29.1
(
I.0
0.4
( I.0
Cl
38.2
4.3
4.2
17.2
13-I
25.0
3.
*unit
CAL.
(a. knl=v//a
HCOl
161 .7
8.4
7.6
18.5
28.0
131.8
6.9
6.2
13.0
18.3
SO,
186.4
4.4
4.5
8.4
17.7
43.1
2.0
1.o
3.0
3.2
SiO*
12
2
I.5
5
4.6
14
2
I.8
5
4.6
bsSiQh/~
Total tirdmaa
330.8
( 6.6
4.1
(11 .9
16.7
I91 .7 ( 6.6
2.3
( 6.6
7.4
(a W&/L
M-Alkalinity
132.6
6.9
6.2
15.2
23.0
108.1
5.7
5-l
10.7
15.0
TDS
601.5
26.3
24.3
69.2
87.7
327.6
18.6
15.3
37.9
44.1
Cm&xivity
648
27.8
72.3
384
14.7
35.7
6.7
6.9
6.8
6.8
PH
8.2
I .o 15.4
I .o 10-l
7.9
0.2
0.2
7.2
5.
I .o
I .o
I
15.1
(lonductivity
0.2
MPa
7.6
6.3
%
: Element: 4einch
spiral wound, Temperature : 25’c
“S/an
Na’
=?/L
K’
q/p/L
Ca’*
Y/L
Mg **
v/p/L
Cl-
=?/L
HCO,--
1370
76
417
21
62 16.5
o-
46 11.8
-
7.8
(0.3
(0.3
(0.3
20.5
(0.2
(0.2
(0.2
224
17.2
11.9
8.4
Y/P/r
661
33.1
30.5
20.6
SO.--
W/l
I43
3.5
3.0
2.0
SiOz
v/j
Tool
hmdmass
TDS
ws~cwlo4 w/L
Robct Flow Fbte
5% mvd
<
1474 8.8
PH
RkOV~Y
oI 76 7.4 6 2.2
(
1 63 7.3 7 4.7
(
I 44 7.
I
19 7.4
rsb
0.2
12.0
TABLE 5 BRACKISH WATER DESALINATION BY NTR-739HF MEMBRANES Test Condition
e/l
401 CONCLUSION The tion
two kinds of new RO membranes performance
and
have excellent salt
high flux at low pressure of 1
Moreover, their chlorine resistance
psi). than
that of the CA membranes.
over
a
long
period
with
MPa
(143
is equal to or higher
They can be
stable
rejec-
used
performance
continuously
for feed
water
containing active chlorine. In
addition, since
they have a
high
rejection
performance
against organic solutes, they can be applied not only to brackish water
desalination
(T.O.C.) and
but
also to removal
in ultrapure water production,
concentration
various
process
of
organic
separation,
of valuable products in food applications,
as
well
as
substance refinement
industries for
waste
and water
treatment. The
low-pressure reverse osmosis was realized for
time
by the thin-film composite loose RO membranes.
that
the
development
investigation
in
this
field
will
be
the We
first expect
advanced
by
of low-pressure high rejection RO membranes.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
S. Loeb and S. Sourirajan, Advan, Chem. Ser., 38, 117 (1962) S. Kimura et al., Desalination, 54, 43 (1985) J.E. Cadotte and L.T.,Rozelle, OSW PB-Rep., No.927 (1972) R.L. Riley, R.A. Case, A.L. Loyd, C.E. Milstead and M. Tagami, Desalination, 36, 207 (19811 R.J. Petersen, R.E. Larson and E.E. Erickson, J.E. Cadotte, Desalination, 32, 25 (1980) M. Kurihara et al., Desalination, 32, 13 (19801 Y. Kamiyama et al., Desalination, 51, 79 (19841 M. Kurihara et al., Desalination, 54, 75 (1985)
Desalination,64 (1987) 387-401 Elsevier SciencePublishers B.V.,Amsterdam-PrintedinTheNetherlanh
NEW THIN-FILM COMPOSITE LOW PRESSURE REVERSE OSMOSIS MEMBRANES AND SPIRAL WOUND MODULES
I. KAWADA, KAMIYAMA
K.
INOUE,
Y.
KAZUSE,
H.
ITO, T. SHINTANI AND Y.
NITTO ELECTRIC INDUSTRIAL CO., LTD. KUSATSU, SHIGA (JAPAN)
SUMMARY
This paper describes improved thin-film composite low pressure reverse osmosis membranes developed by Nitto Electric Industrial Company. These membranes exhibit high salt rejection and high flux at low operating pressures, combined with good permeate chlorine resistance and thermal stability. The characteristics of the two new membrane tvoes. the NTR-739HF and the NTR-729HF. both available as spiral-wound modules, are discussed in detail.. The NTR-739HF exhibits up to 95% salt rejection at a flux over 0.8 ma/mad (20 gfd), when tested at 1 MPH (143 psi) and 25'C (77'F) on 1500 ppm NaCl solution. The NTR-729HF has a water flux greater than 1.3 m3/mad (32 gfd) and a salt rejection above 92% In actual module trials with when tested in the same conditions. brackish water, salt rejections of 96% (product water = 60 ppm TDS) have been obtained. The membranes are stable to chlorine in feed water, as shown by long-term reverse osmosis tests with tap water containing about 1 ppm residual chlorine.
INTRODUCTION The phase
asymmetric
consisting layer)
cellulose acetate
inversion process.
(CA) membranes are made
They have an
asymmetrical
by
structure
of a dense surface layer (a reverse osmosis (RO) skin
and a porous substrate.
use in 1960s Ill.
They had been put to
practical
Since then the RO membranes have been improved
to enhance RO performance and durability. The main aim of development of RO membranes realization as
in 1970 to 1980 was
of membranes which ensure such a pure product
drinking
water
after
single
stage
seawater
water
desalination
process. Enhancement of salt rejection by CA membranes results in reduction
of product water fluxes and reduction of usability
the CA membranes. a
hollow
This discrepancy
of
could be removed by applying
fiber membrane module with remarkably
large
membrane
area per module volume, and single stage seawater desalination
B.V. OOll-9164/87/$03.50@1987ElsevierSciencePublishen,
by
388 CA
hollow fiber membrane module was proved to be effective
To
exceed
the limit of CA membranes,
membranes
synthetic polymers were studied.
various
asymmetric membrane
polyamide
B-10
Among them,
These
of
aromatic
has been used practically
hollow fiber membrane module for single stage seawater tion.
r21.
consisting
as
desalina-
asymmetric membranes are put into practical use as
hollow fiber membranes.
For other module type,
such as
spiral
wound module, the RO membranes with high product water fluxes are The phase inversion process is restricted in reduction
required.
thickness of skin layer of the RO membrane.
of
composite
thin-film
ultrafiltration
an
in-situ polymerization result
of
these
desalination NTR-7199
developed
(UF)
membrane
the The
[31. the
is
following used
as
a
a dense skin layer which exhibits RO capacity
and
substrate
were
thin-film composite membranes has
representative structure;
membranes
Therefore,
reaction is formed on the substrate. the
researches
such as PA-300 [41,
RO
membranes
for
by As a
seawater
PEC-1000 161,
FT-30 [51,
and
171 were put into use in the form of spiral wound module
and plate and frame module. In 198Os, the RO membrane, which had been improved to increase salt rejection capacity,
took various turns.
If an RO membrane
the same RO capacity as before at lower pressure,
keeps
can be applied to low-pressure process.
membrane running
cost.
processing, rejections
On the other hand,
the so
have
for industrial use
long
as
it
such
as
has
sufficient
rejections
for
a
To diversify the membrane processes,
been demands for function to osmose
inorqanics
condense organics and function to osmose inorqanics and and
RO
reduces
RO membranes is allowed to be satisfied in salt
substance to be separated. there
the
This
condense colored components.
and
orqanics
The low-pressure RO membranes
which can meet these demands are playing an important role
among
all RO membranes. Low-pressure RO Membrane Loose RO membrane. --
1.
(1)
reverse
90% (for example 10 to 50%), because
There had been available the
osmosis CA membranes with salt rejections of of
narrow
range
loose
less
than
however, they were not worth notice of
pH
of
CA
membranes
and
use
temperature. The thin-film composite membrane NTR-7250 we developed in 1982 [7]
had
higher
orqanics rejections than the
former
loose
RO
389
membranes
and
pressure
maintained high product water fluxes even at
tions were low; disadvantage chlorine
50 to 60%.
which
membrane
is
Moreover the membrane was freed from
contained
in
with
pressure
was
tap water
performance
without
thin-film
as
composite
California.
50%.
However,
reducing its
Since
the
psi)),
the
electrical
it
indicated for
capacity
12,000
Thus its unique separating capacity and excellent energy
saving have been appreciated, in
in
low (0.4 MPa (57
conductance rejection was approx. hours.
usual
Fig. 1 shows the result of continuous running of the
NTR-7250
running
low
rejec-
(lacking in resistance to oxidizing agents such
membranes).
stable
salt
of less than 2 MPa (286 psi) although its
Japan
for
fermentation, thin-film
ultrapure
and it finds extensive applications
water
pharmaceutical
productions,
manufacturing
food
fields.
industries, Recently, the
membranes SC-200 [81 and NF-40
composite
having
the
same performance as our product have been developed. these
However, pressure
loose
RO
membranes could be used
operation from the viewpoint of product
for
water
low-
fluxes,
but its salt rejections were too low to apply to desalination. When
the membranes are used for the purpose of brackish water
desalination,
they are required to have salt rejections of 90% at
least or, if possible, 95%.
In
(2) Low-pressure RO membrane with high salt rejection capacity brackish water desalination of low salt content of less than
1500 ppm the osmotic pressure is lower (for example, pressure
of
brackish water of 1000 ppm
is
the osmotic
approx.
0.1
MPa).
Therefore, even RO membranes do not require running pressure of 2 to 3 MPa (286 to 429 psi) in principle. were
The former RO membranes
not applied at such a low pressure because at low pressures
of less than 1 MPa (143 psi) the salt rejections decrease and the product use
of the membranes.
afford the
water fluxes reduce too excessively
to ensure
If there were any membrane
practical
which
could
the RO ability at low pressure which we obtained by using
conventional
RO membrane at 3 MPa (429 psi),
cost of
and piping of equipment with the membranes would be reduced, the initial cost would be lowered. as power consumption
Moreover,
pumps and
running cost such
would be remarkably reduced.
These are the
key reasons why the low-pressure RO membranes are expected. Thus, rejections
we
began
development of membranes
having
without impairing the features of NTR-7250;
high
salt
1)
high
390
product
water
During
fluxes
and
2)
excellent
chlorine
resistance.
this development we obtained some variations of
shown
in Fig.
729HF
are
brackish
2.
membrane
Among them the membranes NTR-739HF and
low-pressure
RO membranes which can be
water desalination
NTR-
applied
at low pressure of 1 MPa (143
for psi).
Below are discussed these two kinds of RO membranes. These RO membranes are thin-film composite membranes the
polysulfone
UF
substrates
are covered
with
in
which
skin
layers
(ultrathin layer) featuring high salt rejections at low pressure. The skin layer of NTR-729 consists of the same polyvinyl material
as that of the above-mentioned
NTR-7250,
alcohol
and this skin
layer material is partially modified and applied to the
membrane
NTR-739HF. 3 shows the effects of feedwater pressure, one of the RO
Fig.
characteristics.
As expected,
the flux is increased in propor-
tion to increase of pressure in its measurement
range.
rejections
particularly
curve (286
resembles a hyperbola,
curve
psi)
nearly reaches equilibrium.
is 98%,
The
the
2
MPa
NTR-739HF
the rejection performance of 95% at 1 MPa (143 psi) and
90% at 0.5 MPa (71 psi). usable
The salt
whose rejection performance at
for the NTR-739HF,
maintains
and
flux
membranes
Another membrane NTR-729HF possesses an
at 0.5 MPa.
This suggests that the two
is useful for brackish water desalination
kinds
even at
of low
pressure of 1 MPa (143 psi). The
conventional
advantage, however,
extremely
thin-film composite membranes have low chlorine resistance as
these new RO membranes,
like NTR-7250,
stated
a
disabove,
ensure the same
or higher chlorine resistance than the CA membranes. All
the
RO
membranes are used practically as
spiral
wound
module. 2.
Solute Rejection Performance -of RO Membranes 4 show the rejection performance when treat-
Table 1 and Fig.
ing various solutes. ions
of
the
The rejection performance against inorganic
RO membranes
is equal to
that
of
CA
membranes,
however, the the rejection performance against organic solutes of RO
membranes
is remarkably higher.
membrane having rejection performance tion
performance
against i-Propanol
though a
For example, to NaCl is 97%, is
lower,
CA
its rejec-
45%.
On
the
contrary,
the rejection performance against NaCl of NTR-739HF is
equal
that of CA membranes,
to
but its
rejection
performance
391
0
e-
xm0.4
_......
~~0.2 8H t-
. . .
-
. . . . . ._*. . . . . . . .-a...
._. . . .
-6a. . . . . . .4O’Z
* . . . ...*
-202
0 0
2030
4am
am
Elapse?%ne
Test Condition
IZaxl
lam
(h)
: Element:NTR--7250~S2B (2.5-inch spiral wound element) :Tap Water, Feed Chlorine Concentration : 0.5-0.9
ppm
Brine Flow Rate :2 Vmin frntecmittentlY) Temperature
Fig. 1
:
20-28’c
LONG-TERM OPERATION WITH TAP WATER AT SANTA CLARA, CA., USA
NEW
COMPOSITE
RO MEMBRANES
NTR-723HF
GB NTR-7250
@B
1 0
I I
2
4
3 Flux
5
(m’/m’.d)
Operating Condition NaCl Concentration Pressue
:
:
0.15 wt%
1MPa (143 psi)
Feed Temperature : 25’c PH
Fig.2
PERFORMANCE
:
6-7
OF LOW-PRESSURE
TYPE RO
392
103
20
3x3
4cu
NTR-729HF
01 0
Fig.3
(psi)
200
c
’ I
0
’
1
0
2 Feed Water Pressure (MPa)
WATER
3
FLUX AND SALT REJECTION
Brme flow vate : 20 Vmin ; Temperature Element : 4-inch spiral wound element Feed concentration : 0. 15 wt%
TABLE
1
:
25C
SOLUTE REJECTION OF RO MEMBRANE ( %1
NaCl
95
92
mg so4
) 99
> 99
Ethanol
30
25
I - Propanol
75
70
N-Methyl
Pyrmlidone
Saooharorc
Prcssur.
) IMPa
1
a4
86
99
) 99
I l143priJ
97 )
99 9 45 60
)
99
3 (429LSi
I
i-Propanol
against This
is
polymer
4).
composite
thin-film
Such performance enables to apply the RO membranes to
membranes. various
of synthetic
feature
a
75% (based on data in Figure
is higher,
conventional
than
other
fields
brackish
water
desalination. 2 shows salt passage of RO membranes.
Table
affecting
factors
salt passage.
There
are
two
molecular
One of them is the
size of a solute,
and another is the charge repulsion of ion
fixed
functional
groups of the membrane.
kinds
of
Since both
the
by two
membranes contain fixed carboxyl groups in their
skin
layers, their considerably high rejection against divalent anions can be explained by above-mentioned
two factors.
The difference between salt passages of NaCl and MgClz depends on
the
molecular
however,
size,
the difference
between
of NaCl and NazSOr is determined by not only
passages
salt
molecular
size but also charge repulsion. Chlorine Resistance
3.
Besides
the
water fluxes, as
chlorine
RO performance an
important
wider
solutions
with
moreover,
cleaning
the
condition for
use range
properties
various agent
As
the RO membianes,
for
and
resistance.
of
can
real
product
use
membrane,
of
the
RO more
processed,
be
can be selected among many
agents when the membrane performance chlorine
rejections
the use range of pH, temperature and oxidizer such is The
membranes.
such as
and,
kinds
of
lowers due to fouling.
much importance is
given
to
the
The CA membranes has a certain degree
of
chlorine resistance, although their use ranges of pH and temperature
smaller
are
of
CA
as compared to those of
membranes
that in the
membrane deterioration
well
as
composite suffer poured slime prevent
membranes,
the disadvantage natural
polymer,
is caused remarkably rapidly by attack
Namely, a feed water is turned to bacteria-free
bacteria.
by pouring chlorine, as
CA
composite
thin-film
This chlorine resistance compensates
membranes.
deterioration
deterioration.
consisting by bacteria,
to prevent deterioration.
Since
of synthetic
the
fouling,
it
the
thin-film
polymers
they do not require On the contrary,
takes place easily owing to propagation of
intermittently.
water
and this prevents fouling by bacteria slime
CA membrane
membranes
of
is necessary
to
add
do
not
chlorine
fouling
bacteria. chlorine
by To even
Besides, the cleaning agent used most frequently
in food industry processes is made from chlorine.
394
loo -
Test
(
Feed
Pressure
Cone
wt x 9
,
MPa
NTR739HF
80 -
I1 .0.151 NTR-729HF, II , 0.151
si ,$60
s
I 1
Z t %
NTR7250 12 0.21 ,o
14.2 30.5)
/
*
“; 40 ._
1; !’
C ,‘ll.5.0.11 /
20 -
,/ .’
1
I
** -(2 *o-21 ,
20
40
60
_-_,, 0 0
NaCl
Fig.4
TABLE
2
Rej
NaCl REJ
**
CA
membrane
,
,
I
60
100
(X) vs
i-Propanol
REJ
SALT REJECTON AND PASSAGE BY NEW MEMBRANES FOR DIFFERENT SOLUTES
NTR-739HF
NTR-729HF
NaCI
4
6
MQCIZ
3
7
NaCI
4
a
Na.SO.
0.5
0.5
395
Fig. 5 shows the data of chlorine resistance obtained when the solution
containing
100
ppm of active
chlorine
processed
is
continuously at running pressure of 1 MPa at various pH. NaClO is dissociated at various pH as shown in Fig. 6, and the Therefore, it is
oxidizing ability of HClO is higher than ClO-.
solu-
said that if solution has the same chlorine concentration, tion with lower pH have higher oxidizing ability. all the membranes, the
However, with
the time until the membranes fail to maintain
retention of rejection of 100% (the time before decrease
of
salt rejection) is decreased remarkably near the pH7. Fig. 7 shows the data in continuous operation of membranes with a solution
(pH = 7) containing 10 ppm of active chlorine at running
pressure of 1 MPa (143 psi). membranes
composite however
The rejections of these
increases
at
the
beginning
thin-film
of
running, fouling
after this it decreases gradually owing to iron
but it is restored by acid cleaning. The
order of the time until decrease of rejection is CA membrane
< NTR-739HF the
< NTR-729HF.
critical
approx.
1
chlorine
area,
if the chlorine
ppm, resistance of approx.
20,000 ppm-Cl,-hr.
performed
containing
is
practical
and
approx.
continuous running test with tap water
usually
respectively.
Lonq-term Continuous Running with = We
concentration
the NTR-729HF and NTR-739HF maintain
10,000 ppm-C12-hr, 4.
As is evident from this figure, even in
pH = 7,
Water
1 ppm or less of active chlorine by using the 4"
dia.
elements of NTR-739HF. Fig.
8
shows
the RO performance
in long-term
running,
table 3 indicates the analysis results of quality of feed
and
water,
product water and brine after running for 3,500 hours. The
product water exhibits the stable conductivity of 6 to
MS/cm while the feed water conductivity is 120 to 130 After 5,000 hours of running, the performance is stable,
7
US/cm. and at
present this test is continued. 5.
Desalination We
conducted
Performance -in Synthetic Brackish Water the desalination performance test by using
kinds of synthetic brackish water with the 2.5" dia. NTR-739HF.
Table 4 shows the test results.
element
two of
4000, i?
Y a
A NTR-739 HF 0 NTR729 HF
f G I 3000.
1 NTR-
15951CAI
E a ,a
n
e ._ 2 2000 ” 2 % 8 0lOOOlz
01 4
5
6
7
8
Feed Water
:
Tap
Water
PreSSUV2
1 MPa
(143
:
Temperature
Fig.5
:
5
IO
6
7
psi)
20-257~
CHLORINE RESISTANCE (d-l6 TO IO. AND CHLORINE
4
9
PH
Test Condition
8
AS A FUNCTION CONCENTRATION
9
to
PH
Fig. 6
EOUlLlBRlUM DIAGRAM OF HOC1 AND OCI-
pH 100PPMj
397
4-
Cleaning
B
3B&&;: 2 1c
Ev
Time Exposure to Chlorine (ppm-c/,-hour) Test Condition :Feed : 0.15 wt% NaCI. Pressure : 1 MPa (143 psi) Temperature : 18-28C, pH : 7 Fig.
7
LONG -TERM OPERATION CHLORINE OXIDATION (CHLORINE
CONCENTRATION
ON 10 PPM)
398
Elapsed time (h) Test Condition
:
:
Element
:
Feed Chlaine
NTR-739HF-S4 (4-inch spiral wound element) Tap Water
Concentration : 0.3-0.8
Pressure
:
: 12 Vmin : ZO-3OC
Brine Flow Rate Temperature
Fig. 8
LONG-TERM AT KUSATU
ppm
1 MPa (143 psi) (3.2 gpm)
OPERATION WITH TAP WATER CITY, SHIGA, JAPAN
TABLE 3 FEED, PRODUCT AND BRINE ANALYSIS AFTER 3500 HOURS OF OPERATION
Unit PH
Concbctivity
#S/cm
TDS
9/t
Nil’
T/t
K’
T/d
CS”
=?/L
Msl” Total hardness
ss/l ~..caC0~/6
Feed
Prodnet
6.6
5-a
120
5.6
73 8.8
(
10 0.7
Brine
6.8 151 91 10.2
I .9
(0.1
2.0
12.6
(0.1
14..8
2.6
(0.1
3.1
42.6
1.5
49.6
Cl-
V/d
18.1
1.2
20.6
HCOa-
s/t
24.6
1.8
30.5
so,--
4/6
SiO2
=?/1
1.1
TOC
PPb
1700
15
(0.5 0.2 112
18.7 1.4
1900
399
At
first,
the test was performed by using synthetic brackish
water A (TDS 601.5 ppm, feedwater. TDS
of
total hardness 330.8 ppm and pH 8.2)
In this test,
at pressure of 1 MPa (143 psi),
product water was 26.3 ppm,
lower than 6.6 ppm;
but
At pressure of 0.2 MPa
the TDS of product water was 69.2 ppm, was lower than 11.9 ppm;
hardness more
than
was
and the total hardness
the rejection of TDS was 96%, and the rejec-
tion of total hardness was more than 90%. (29 psi),
as the
96%
of the
total
and its
the rejection of TDS was 88%,
rejection
of
total
hardness
was
maintained. total
When another synthetic brackish water B (TDS 327.6 ppm, feedwater,
the
rejection of TDS was 94% and the rejection of total hardness
was
hardness more of
191.7
ppm
and pH 7.9) was tested
as
than 97% at pressure of 1 MPa (143 psi),
while at pressure
0.2 MPa (29 psi) the rejection of TDS was 88% and the
rejec-
tion of total hardness was more than 97%. The and
table shows also the calculated values of
flux
water
quality
which were obtained with the aid of personal
computer
program developed by us. Table
5 indicates the desalination
brackish
test results of
water (TDS 1474 ppm) by using the 4"
dia.
synthetic element
of
NTR-739HF. As is evident from this table, the rejection of TDS is 96% and the rejection of total hardness is more than 99% at pressure of 1 MPa (143 psi) even when the TDS of feedwater is approx. 1500 ppm. When
the pressure is increased to 1.5 MPa (214 psi),
tion
of
TDS reaches 97%,
exceeds 99%.
and the rejection of
the rejec-
total
hardness
On the contrary, even when the pressure is lowered
to 0.5 MPa (72 psi),
95% of rejection of TDS and 99% of
rejec-
tion of total hardness were maintained. This brackish In
suggests
that
water desalination at low pressure
addition,
if
the
function satisfactorily (72 psi).
the new RO membranes are membranes are used
effective
(1 MPa for
(143
for
psi)).
softening,
even at low pressures lower than 0.5
they MPa
400
TABLE
4
BRACKISH
WATER DESALINATION
Elamant:NTR-739HF-S2 T
-
K
spiral wound element)
Model soutlhe-n California Feed
NP
(2.5-inch
Permeats~ll OBS.
CAL.
A
OBS.
Californls M&l
Scuthern
Permeate IZI CAL.
Permeatell)
Feed
OBS.
B
Psrmwts 12)
CAL.
OES.
83.9
5.1
4.8
15.0
17.4
48.1
3.5
2.8
7.9
8.1
5.6
( 0.1
0.3
< 1.0
1.2
1.2
( 0.1
0.1
( 0.1
0.2
I
4.2
45.2
(
1.0
0.6
<
1.o
1 .I?
1.5
19.2
(
I-O
0.2
(
I.0
0.7
2.1
2.7
6.9
7.2
Gil
84.6
(
1.0
I .o
MQ
29.1
(
I.0
0.4
( I.0
Cl
38.2
4.3
4.2
17.2
13-I
25.0
3.
*unit
CAL.
(a. knl=v//a
HCOl
161 .7
8.4
7.6
18.5
28.0
131.8
6.9
6.2
13.0
18.3
SO,
186.4
4.4
4.5
8.4
17.7
43.1
2.0
1.o
3.0
3.2
SiO*
12
2
I.5
5
4.6
14
2
I.8
5
4.6
bsSiQh/~
Total tirdmaa
330.8
( 6.6
4.1
(11 .9
16.7
I91 .7 ( 6.6
2.3
( 6.6
7.4
(a W&/L
M-Alkalinity
132.6
6.9
6.2
15.2
23.0
108.1
5.7
5-l
10.7
15.0
TDS
601.5
26.3
24.3
69.2
87.7
327.6
18.6
15.3
37.9
44.1
Cm&xivity
648
27.8
72.3
384
14.7
35.7
6.7
6.9
6.8
6.8
PH
8.2
I .o 15.4
I .o 10-l
7.9
0.2
0.2
7.2
5.
I .o
I .o
I
15.1
(lonductivity
0.2
MPa
7.6
6.3
%
: Element: 4einch
spiral wound, Temperature : 25’c
“S/an
Na’
=?/L
K’
q/p/L
Ca’*
Y/L
Mg **
v/p/L
Cl-
=?/L
HCO,--
1370
76
417
21
62 16.5
o-
46 11.8
-
7.8
(0.3
(0.3
(0.3
20.5
(0.2
(0.2
(0.2
224
17.2
11.9
8.4
Y/P/r
661
33.1
30.5
20.6
SO.--
W/l
I43
3.5
3.0
2.0
SiOz
v/j
Tool
hmdmass
TDS
ws~cwlo4 w/L
Robct Flow Fbte
5% mvd
<
1474 8.8
PH
RkOV~Y
oI 76 7.4 6 2.2
(
1 63 7.3 7 4.7
(
I 44 7.
I
19 7.4
rsb
0.2
12.0
TABLE 5 BRACKISH WATER DESALINATION BY NTR-739HF MEMBRANES Test Condition
e/l
401 CONCLUSION The tion
two kinds of new RO membranes performance
and
have excellent salt
high flux at low pressure of 1
Moreover, their chlorine resistance
psi). than
that of the CA membranes.
over
a
long
period
with
MPa
(143
is equal to or higher
They can be
stable
rejec-
used
performance
continuously
for feed
water
containing active chlorine. In
addition, since
they have a
high
rejection
performance
against organic solutes, they can be applied not only to brackish water
desalination
(T.O.C.) and
but
also to removal
in ultrapure water production,
concentration
various
process
of
organic
separation,
of valuable products in food applications,
as
well
as
substance refinement
industries for
waste
and water
treatment. The
low-pressure reverse osmosis was realized for
time
by the thin-film composite loose RO membranes.
that
the
development
investigation
in
this
field
will
be
the We
first expect
advanced
by
of low-pressure high rejection RO membranes.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
S. Loeb and S. Sourirajan, Advan, Chem. Ser., 38, 117 (1962) S. Kimura et al., Desalination, 54, 43 (1985) J.E. Cadotte and L.T.,Rozelle, OSW PB-Rep., No.927 (1972) R.L. Riley, R.A. Case, A.L. Loyd, C.E. Milstead and M. Tagami, Desalination, 36, 207 (19811 R.J. Petersen, R.E. Larson and E.E. Erickson, J.E. Cadotte, Desalination, 32, 25 (1980) M. Kurihara et al., Desalination, 32, 13 (19801 Y. Kamiyama et al., Desalination, 51, 79 (19841 M. Kurihara et al., Desalination, 54, 75 (1985)