Trace metal fouling and cleaning of seawater R.O. membranes

271

Desalination, 66 (1987) 271-284 Elsevier Science Publishers B.V., Amsterdam

-

Printed in The Netherlands

TRACE METAL FOULING AND CLEANING OF SEAWATER R.O. G.

PEPLOW (1)

and F.

1

Department

of

2

Department of Chemistry and Applied Salford M5 4WT (Great Britain).

MEMBRANES

VERNON (2)

Chemistry,

University

of

Malta,

Msida

Chemistry,

(Malta).

University

of

Salford,

SUMulARY The low concentrations of heavy metals in seawater are very often ignored potential foulants for RD membranes in seawater desalination. Cumulative deposition of such metals may result, which is difficult to remove by conventional cleaning techniques.

as

followed by a mass balance Destructive tests on hollow fibre membranes, exercise during routine operation of the Ghar Lapsi seawater RC plant in Malta, revealed the existance of trace metal depositions. lead and vanadium were detected found to be the only effective

The metals iron, copper, zinc, chromium, these membranes. Dilute mineral acids were cleaning agents in ‘in vitro’ experiments. A more efficient pre-treatment and cleaning investigated to control these deposits.

technique

must

therefore

on

be

INTRODUCTION The projected refer

to

foulant

the for

of

to

carried

Fe(II)

to

dissolved

of

seawater

be oxidised Studies

feedwater

control

iron

out

(Fe

on pilot is

metal

species feed

in

water

sea

the

presence

of

major

by concentration

the

ions

-log,D.

anions or

may exist

0011.9X4/87/$03.50

in

have

in

metal

solution as the

(ref.

2)

dissolved

(ref.11, potential

(FeII)

can

hydroxide.

suggest

oxygen

The lowering some of

of

these

etc.

found

that

oxidation

concentration

and the

+ + 8 CO,

existence the

of

pH to

species

in

seawater. on the

These fibre

several

7 or

into

Such formations

?

less

more are

metal

trace in

metal

RO plant

soluble

possible

oxides, due to

complexes

membrane

surface,

some metal

ions

may especially

effects. of the

1 indicates

equilibrium

= 4Fe(OH),

shown the

precipitated

polarization

Figure

systems

on pH,

sulphates

At the working pH of 7-7.5 Z+ z+ solubility, where M is PM is

iron

guidelines

trace

precipitates

+ O2 + 2H20

form.

be adsorbed

RO plant

concentration.

may convert

carbonates,

The ferrous

membrane

analyses

particulate systems

hydroxides,

eventually

water

in

as a primary

III),which

dependent

4Fe(HCOs ), Surface

cited

concentration

RO membranes.

ferric

Fe(II1)

trace

specifications iron

with

the

PC feedwater,

metal the their

concentration limits oxide

at or

0 1987 Elsevier Science Publishers

of

which

state

precipitation

of

hydroxide

B.V.

have

oxidation

form.

low z+, such

The Fe2+

p

272

o

o

~ ~

+

N o

?;

~-~

0 ~

f

J

J

J

J

(NI ~r-

.r.t

, i

i

i

I

o I

1,,

o

213

ions

have

low solubilities

The ions

pH )3.

For

optimum to

particles

have

ions

have

progressively

solubilities

RC performance,

remove

thus

gross

also

three

in

months,

solution

prevail

recycled

1s then

in

the

followed

colloidal

require

at

removal

low solubilities

higher

for

the

ions

at

solubilities

like

permeators and to

by the

at

and Agi

Mg’*

product

a pressure

of

cartridge

water

at

and other

500

and

smaller

usually

every

Cleaning cleaning

kPa for

progranme

pretreated

filters.

flow.

standard

about

cleaning

is

coagulate

programnes,

recommended

by a surfactant silica

the

cleaning

improve

is

to

material

periodic

to

pH 4)

plant

material,

fed of

their

order

(1% at

water

particles

facilitating

The membranes

acid is

higher

Fe ‘+

whereas

pH.

filtered

or

pH >9,

Cu2+ and Al ‘+

low pH and conversely high

at

Zn’+,

(0.5%

with

two citric

procedure

which This

120 minutes. at

pH 10)

to

dislodge

deposits.

TRACE METAL CONCENTRATION IN THE MEDITERRANEAN SEA. The Mediterranean its

enclosed

distinction

must

open sea the

of

Sea

metals shore

to

areas, wind

particulate

in

Some of

quite blowing

from

matter,

range

in that

appropriate particular Table

sea

waters

in

sea water the

condition

to

of

the

in

applicable

RO plants.

the

is

or in

terms

of

practically most

Fe”. their at

Table

different

liberty

stable

to

form

areas

near

particles

in

metals

the

off-

found

monitoring total

shore.

brought

The speciation

such

about as of

trace

trace

metals

iron

concentrations

are

1 shows how minor species. form

It

complexes

corresponding

to

can be with

the

the

seawater.

values

of

Mediterranean

several Sea.

trace

metals

in

of

the

can contribute

waters

example,

unless

A

the

that

concentration

trace 5). for

For

meaningless

of

of

(ref.

particulate

the

surface

it.

in

enclosed

indicate

of

levels

mg/l

in

water

the

into

found

semi

matter

The amounts

found

or

recently

12-15 quite

assume

2 shows recent areas

shore.

areas

due to

flowing

concentration

same levels

increased

concentration

rivers

Particulate

is

are

of

the

4).

water

exist

metals

ion(s)

enclosed

have

Fe(II1)

metal

conducted

have

metal

large

and for

between

raw feed

as Fe(II),

elenlents noticed

in

trace

especially

the

of

surveys

(ref.

levels

trace

number

the

and cobalt

naturally,

and brine

qualified

the

samples

increased

composition

feed

different

Mediterranean,

nickel

concentration

limited

be made between

31.

copper,

has a wide-ranging

and the

and near-shore

by the

metal

the

(ref.

markedly These

Sea

nature

surface

waters

of

274

TABLE 1

Trace

metal

speciation

in

marine

system

Metal

Major

Al’ +

Al(Cti),

Cr3 +

Cr(OH)2+

Cr042 -

cro4,

Mnz+

MI? + 58% MnCl+ 30%,

Fe’ +

FeOH+ 84%,

(ref.

6)

Forms

o

-

85%, Cr 0; 94%,

(a)

Fe’+

Fe(OH),

(b)

Fe’+

Fe(Cti),+

HCr04-,

FeCl+

14% KCr04-

2%, Cr20

7

‘-

MnS040 7%

7%, Fe2 + 7%

o 95%,

Fe(OH),+

5%

60%,

Fe(Ctl14-

40%

co’ +

co* + 54%,

coc1+

31%,

coca, 0 7%

Ni2 +

Ni’ ’

NiCl+

31%,

NiCOJO 9%, NiSQ40

53%,

(a)

Cu2+

Cu(OH)z O 83%,

(b)

Cu’+

CuOHCl”

65%,

(a)

Zn2+

Zn(OH),

50%, Zn’+

(b)

Zn2+

ZnCl+

44%,

6%

C&O, o 11% CuCQ”

Zn*+

22%, 22%,

16%,

‘&Cl

ZnCl+

16%, C”CH+ 4% 8%, ZnCO, o 6%

ZnCl, O 15%, ZnOHClO 13%

-

MO(W)

Mo04,

Ag+

AgC143 -

54%, AgCl,‘-

24%,

AgClz

17%

(a)

Cd’+

CdCl, o 50%, CdCl+

40%,

(b)

Cd2+

CdClz o 38%,

CdCl+

29%, CdC13 - 28%

AU+

AuCl, - 91%,

AuClBr-

Hg’ +

HgC14’ -

66%,

Pb’+

PtXX,O

76%,

(b)

Pb’+

PbCl, O 42% PbCl+

(b):

PbCl+

Different

CdClp - 6%

9%

HgCIJBr-

(a)

(a);

2%

12%, HgCl,-

11%, 19%,

PbClr”

5%

PbOH+ lo%,

Soruces

12%

of

PbHClO 9%

Reference

215 TABLE 2 Trace metals

in the surface

Location

waters

of the Mediterranean

Concentration ug/l

Analytical Methods

Sea Authors

Chromium Adriatic

Sea

0.68

X-Ray Emiss

Marijanovic, 1982 (Ref. 10)

Sea

0.39

X-Ray Emiss

Marijanovic, 1982 (Ref. IO)

0.21

AAS

Kremling,

4,.5

X-Ray Emiss

Marijanovic, 1982 (Ref. IO)

Ligurian

2.9

AAS

Baffi,

Various

0.25

AAS

Kremling & Peterson, 1981 (Ref. 11)

0.15

AAS

Frache,

Manganese Adriatic Various

1981 (Ref.11)

Iron Adriatic

Sea

1982

(Ref. 12)

Cobalt N Ligurian

Sea

1976

(Ref. 13)

Nickel West Mid Medit Ligurian

Coast

N Ligurian Adriatic

Sea

Sea

Spivack,

9.4 0.3-9.0

Baffi,

c.73

AAS

Frache,

1.3

X-Ray Emiss

MariJanOviC,

1982,

1981

1982

(Ref. 14)

(Ref. 12)

1980

(Ref. 15)

(Ref. IO)

Copper West-Mid

Medit

Gulf of Gera

(Gr)

4.45

Spivack,

2.4-3.3

1981

Scoullos,

N Ligurian

1.43

AAS

Frache,

(Ref 14)

1982

1980

(Ref 16)

(Ref. 15)

French Coast

0.2

AAS/ASV

Fukai,

NW Medit

0.165

AAS/ASV

Whitehead, 1983 (Ref. 18)

Various

0.21

AAS

Kremling & Peterson, 1981 (Ref. II)

1982

(Ref. 17)

Zinc Various

1.4-3.7

French Coast

0.4-1.0

Fukai, 1970-1976 Ref. 17) ASV/SP

Fukai,

I\lwMedit

0.316

ASV/AAS

Whitehead, (Ref. 18)

Various

0.4

ASV

Kremling,

1980

(Ref. 19) 1983

1981

(Ref.11)

216

Cadmium West-Mid

Medit

0.49

Spivack,

1981

(Ref.

141

Various

Areas

0.15-0.19

Fukai,

1970-1975

(Ref.

Ligurian

Sea

0.2-0.33

Baffi,

1976-1980

(Ref.

0.016-0.08

Nurnberg,

La Spezia N Ligurian

0.5

Sea

AAS

1976

Baffi,

1976 1980

0.015

AAS/ASV

Fukai,

NW Medit

0.007

AAS/ASV

Whitehead,

Various

0.017

AAS

Kremling,

French

Coast

(Ref.

(Ref.

12) 20)

12)

(Ref.

19)

1983 1981

17)

(Ref.

18)

(Ref.

111

TRACE METAL CONCENTRATIONS IN UNTREATED, TREAlED AND BRINE WATER AT GHAR LAPS1 REVERSE OSMOSIS PLANT, MALTA. The plant about

is

situated

25,00Om”/day

77,00Om’/day conducted

of

product treated

during

operation:

producing permeators

seawater

is

the

sample

complexing the

untreated

water

the

about

study

first

with

10 independent since

this

was

phase

of

900

E-10

about

trains

(Fig

porous

limestone

21.

The raw feed rock

strata

sea.

the

concetrations

of

the

seawater

at

different

-

treated

_

brine

-

(product

feed feed

metals

water). low levels

for

of Table

sampling

3).

The sample August

1984

determining

on 5 litres

were

3.

then

metal results

up to

and analysed

are about

programne

for

the

1985. metals

This

with

curve. in

average

the

out of

of

along at

by

digested

with

was incorporated

in

to the

samples the

three

25 samples

manganese,

The pooled

residue

near-shore

values

one kilometre

was carried

was carried

chloroform.

calibration

chromium,

of

was performed

and the additions

A total

analysis

diethyldithiocarbamate

standard

the

the

concentration

8).

dryness

concentrations

situated

November

metal (Ref

complex

of

water,

test.

and sodium

to

a standard

trace

collection

the

evaporated

These

product

trace

samples

The method

to draw

sights

the

8-hydroxyquinoline

acids.

the

in

a preliminary

pH and extracting

method

from

after

with

and nitric

shown in

seawater,

seawater,

effluent,

analysis

extracts

The results

five

open sea,

extraction

analytical

collected

the

-

appropriate

sulphuric

from

-

of

the

chloroform

are

the

was discontinued

by solvent

the

product

the

producing

handles

for

in

currently

plant

performed

was still

boreholes

examine

insignificantly

The method

at

for

to

the

is

namely:

Due to

out

filter

It

Malta.

means that

plant

over

inshore

of

The work

20,000m3/day

distributed

was necessary

points,

this

about

coast

This

water.

when the

pumped from

as a natural It

south

sea water.

1984-86,

Permasep

acts

on the

shore

monthly were

iron,

plant

collected (Figure intervals,

therefore cobalt,

nickel,

RO

TURBO

Fihre

:

LAYOUT

OF

RO

PUMP

PLAN,,

BUILOING

HALL

GHAR

LAPSI

(RCC

T)

278

,

‘X ‘?... .:.

,\ i

0

.,

TABLE 3 Trace metal

Metal

concentrations

in the sea and R.O. plant waters

Near Shore Sea Water

Untreated

(ug/l)

Feedwater

Treated

Feedwater

Chromium

0.44

0.48

0.55

Manganese

0.49

0.62

0.68

Iron

6.97

11.71

13.53

Cobalt

0.08

0.09

0.11

Nickel

0.48

0.55

0.69

Copper

0.58

0.41

0.51

Zinc

0.47

0.68

0.76

:admium

0.003

0.00

0.00

.ead

0.05

0.07

0.06

copper,

zinc,

Samples open- sea Table

This

for

Most

could

have

dissolved

due to

high

feed

slow metal

of

the

were

collected

the

from

species

indicate

the

lower

were

presence

pH of

the

rock

carried

the

higher to

species

simultaneously

and treated values

the

of

of

particulate

treated

trace

Mn Fe co Ni cu Zn

in

treated

feed

0.01 6.00 (0.32) 0.33 0.32 0.72

TABLE 5 -Analysis

after

detergent

cleaning

Fe

2.820

mg/l

Cr

0.075

mg/l

co

0.031

mg/l

Zn

0.233

mg/l

Mn

0.741

mg/l

CU

0.038

mg/l

Ca

0.831

mg/l

Mg

0.406

mg/l

Analysis

after

citric

acid

cleaning

Fe

12.364

mg/l

Cr

0.173

mg/l

co

0.441

mg/l

Zn

11 .346

mg/l

Mn

3.897

mg/l

cu

0.843

mg/l

Ca

1.213

mg/l

Mg

0.409

mg/l

water

metal

shown in to

- January

Table

the shown in

in

the water.

species reason

The concentration

may be

of particulate 4 reveal

toal

concentration,

1986

ug/l

Cu and Zn.

matter

metals

Another

TABLE 4 Particulate

are

untreated

trace

sea water.

The results compared

with

sea water

corresponding

and pipework.

out.

of particulate

and analysed

untreated

when compared

been due to at

for

results

samples

leaching

proportion Fe,

water

The results

sea water

which

trace

of

samples.

3.

treated

cadmium and lead.

the

notably

281 CONCENTRATIONS Samples

OF TRACE METALS

of washing

IN PERMASEP

solution

effluent

July and August

1984 and analysed

method

the digestion

involved

solution, 5.

followed

by dilute

The values obtained

mostly

by the citric

Further

indicate

This was undertaken

The results

that some trace metals

the interim

STRANDS

concentrations.

of the residue after evaporation acid dissolution.

from the previous fibre

strands

during

The analytical

of 500cms:

are shown

were being

in turn that cumulative

tests were made on hollow

permeator.

MEMBRANE

from the plant were collected

for trace metal

acid, proving

had taken place during

B-10 PERMEATOR

removed

deposition

cleaning

obtained

of metals

programne.

from an open B-IO

by the use of an XRF spectrophotometer,

the metals

determine

qualitatively

confirmed

the pressrce of V, Cr, Fe, Sr, Si, Ca, Cu and Ti. Destructive

tests were then conducted

deposited

on the strands

on the strand

to confirm

of

in Table

surface.

and quantify

to

The test chemical

some of these

metals. Determination

for vanadium

content

of strands

in nickel crucibles.

parallel.

The fused mixture

and dissolved carried

in deionised

was carried

A set of spiked

was treated

water.

with NaOH

A calorimetric

out using N-benzoylphenylhydroxylemine

vanadium

complex.

by leaching resultant

The determination

the metals

solution

absorption

with 6M HN&

to remove brown

were fused

in

fused again, cooled

determination

solution

known weights

strands

pellets,

for the other

from the strands

was boiled

out by fusing standard

was then

to give the coloured

trace metals

was carried

at 60°C for 3 days.

fumes and analysed

out

The

on an atomic

spectrophotometer.

The results obtainedare shown

in Table 6.

TABLE 6 Trace elements Fe

0.54

Zn

0.16

CU

0.15

in membrane

Sr

0.08

Cr

0.013

Mn

0.11

V

0.76

These

results

the presence

support

of metals

the XRF method. the same strands.

strands

mg/kg

the values from

in the membrane

Its presence

the citric acid cleaning

strands.

is however

solution

for

Zinc could not be observed

conspicuous

in the acid

leaching

of

by

282

TRACE METAL MASS BALANCE ON THE GHAR LAPS1 RC PLANT Quantitative applied

to

in the

feed

analyses

draw

and brine

is

It

operation.

water

was comparatively

The treated under

obtained,

in

feed

losses boost

the

various

retained the

total

could

flow

and for

elements. membranes

quantity

be drawn

for

of

(Table

brine

metal

were concentraticns

metals

by the

train

during

routine

in

product

concentration

analysed

flow

Zn,

Cr,

for

Fe,

degrees.

Zn,

depositing

Cu,

the

Table

the

three

nine

trace

The results by multiplying

energy

recovery

deposit 8 give

basis

the

membrane

mass balance

found these

on the

for of

on the the

of

months

with

be adjusted

Cr and Pb were

On the in

the

had to

and Pb to

shown in

in

hour

rates

used

Cu,

for

technique.

flow

The metals

metal

RC plant

of

the

to mgs per

These

The results

each

of

were

converted

varying

the

on one single metal

samples

Fe,

of

be ignored.

rates. the

uptake

preconcentration

were

litre,

confirmed.

by the

water

solvent

The tendency

was thus

the

trace

to

water

The variance

out

that

low enough

water

leaks

pumps.

to

was carried

and brine

ugs per

and brine

exercise.

equivalent

by the

and brine

strands of

feed study

due to

feed

was assumed

plant

the

the

The experiment

permeators.

metals

of

up a mass balance

to

be

results, membranes

7).

TABLE 7 -Total

quantities

of

metals

retained

Membrane Retention in gms/quarter

Membrane Retention in mgs/hr December Test

Test

by membranes

Fe

61.73

134.82

cu

8.19

17.89

Zn

5.38

11.75

co

1 .OO

Cr

10.00

Pb

2.40

2.184 21.84 5.2416

REMDVAL OF TRACE METAL DEPOSITS Three

100

weight) were

of

added

was then cycle rinsed

cm’

separating

membrane to

each

removed

was repeated thoroughly

strands, funnel

slowly

were

packed and left

deionised

At

set

above to

and replaced

20 times. with

FROM RC MEMBRANE STRANDS

funnels

the

soak with

each

stopcock

the

strands

another

end of

water.

up,

the

this

containing stem. for

20 ems’ treatment,

IO gms (dry 20 ems’

12 hours. of

the the

of

2M HNO,

The acid

acid.

This

strands

were

283 TABLE 8 Corrected

mass contrations

Test

Concentrations

of

ions

in

calculated

RO plant

in mgs per

train

no.1

hour

Feed (a)

Product (b)

Brine (c)

Fe

3667

7.27

3598

61.73

CU

173

0.89

164

8.19

5

Zn

154

1.62

147

5.38

4

Ni

200

7.27

193

(-1

0.27

/

Mn

118

4.04

114

(-)

0.04

/

co

34

0.00

33

1 .oo

3

Cr

95

0.00

85

Cd

0.25

0.00

22

Pb

Mass Balance (a-(b+c) 1

loss % 2

11

10.00 (-1

0.33

0.00

Feed

19.6

0.08

/

2.40

11

TABLE 9 Results

on acid

cleaned

membrane

strands

Sample A (Control)

Wt.

of

Membrane

Wt.

of

Ash gms

B

10.11

gms

D

I (Blank)

10.10

8.93

10.12

0.021

0.02

0.02

0.02

0.01

Fe mg/kg

0.23

0.12

0.10

0.11

0.00

Zn mg/kg

0.04

0.01

0.05

0.01

0.01

Cu mg/kg

0.00

0.02

0.00

0.03

0.00

Sr mg/kg

0.12

0.08

0.10

0.02

0.00

Si

0.44

0.11

0.08

0.04

0.00

0.68

0.00

0.00

0.00

0.00

mg/kg

V mg/kg

9.65

C

Cu mg/kg

0.04

0.00

0.00

0.01

0.01

Cr mg/kg

0.00

0.00

0.00

0.00

0.00

Mn mg/kg

0.00

0.00

0.00

0.00

0.00

The appearance yellow. sample

The

results

(Control),

of

the

strands

shown in unused

improved

Table

strands

9,

considerably:

present

(Blank)

values

and the

from for

three

dark

untreated treated

brown

to

light

strand samples,

B,C,D.

284

CQNCLUSICNS The amount citric

acid

depatable

Zn,

complete

amy not

(Table

9)

technique

is

investigate

than

Another permeators

field

of

with

trace

performance feed

for

find

all

metals

for,

way to practical

of

trace

method

investigation metal

on permeators

is

in

is

to

(namely

Fe,

acid

cleaning

acid

solutions This

necessary

to

be more

to

control

deposition

deposition.

monitor

the

performance

may be conducted

variable

the

completely.

would

of

that

strong

method

onset

by the

still

metals

It

removal.

the

citric

therefore

cleaning

is

metals

that

It

This

handling

trace

use.

required

deposits.

of proves

away all

remove

reduced it

The fact

occurs.

cleaning

metal

grossly

However

strip

routine

to

is

5.

some quantities

a better

a ‘vigorous’

Table

Even Rigorous

method

to

of

unaccounted

effective

unacceptable

perhaps

rather

in

being

only

a milder

practible

stripping

be adequate.

was the

membranes

as shown in

resulted

Cr and Pb)

on the

metals,

technique

exercise

Co,

alone,

the

deposited

whether

Mass Balance Cu,

of

cleaning

concentrations

of

by comparing of

trace

such

metals

in

water.

REFERENCES Du Pont J.M.

Company,

Jackson

M. Bernhard,

VI

R. Frache, 365-375. 5

Technical

and D.

F.

Journ.

Baffi,

P. Bust-Menard, 1005-1009.

Information

Ladott, Etud. A.

Goldberg,

Poll.,

Dadone

M. Arnold

Section (19731,

C.I.E.S.M.,

Ves.

Journ.

(19821,

237-243. 4,

Polut.

E.D.

W.T. Andrews, J.H. Sleigh and R.A. Bergman, The Malta osmosis facility, Desalination, 46, (1983) 111.

8

M.E.

9

W. Stumn and J.J.

IO

P. Marijanovic, J. Cannes, C.I.E.S.M.,

Makjanic (1981).

and V.

If

K.

Petersen,

Meteor

12

F. Baffi, C.I.E.S.M.

13

R.

Kremling

and H.

Frache,

F.

A.J.

15

R.

16

M. Scoullos

17

R.

Spivack, Frache,

18

N.E.

19

R.

20

H.W.

Eaffi

L.

R. Frache A.

Workshop: etal,

et,

Trace

Nurnberg,

Huynh-Ngoc

Huynh-Ngoc P. Valenta

VI

Vies

etal,

and R.

Etud.

Chem, 4 365-375 in

1975. reverse

Etud. 7,

Jugos., Chem.

Poll. 9-12

16 2-4, 282

(1970).

Etud.

Pollut.,

(1981). Pollut.,

(Eric)

1079-1082,

Cannes,

(1981). (1980).

Cannes,

CIESM (1982)

(1976).

Fukai,Oceanologica

Anal.

(19811,

(1976).

Sea Water, 27A,

Bull.,

Thalas.

etal,Z.

171,

Kiel,

Journees

Journees Poll.

Berlin,

Journees

Forchergebniesse,

Metals

Med.,

seawater

N.Y.,

Vies

Deep Sea Research,

Mar.

Conference,

Wiley

Valkovic,

Mar.

Mar.

(19761,

(1984).

Chemistry,

etal,

Dadone

Huynh-Ngoc, L.

University,

Aquatic

and M. Dassenakis,

and L.

Whitehead, Fukai,

Baffi, Nato

F.

Salford

Morgan,

M. Fabiano, (1982).

14

Fukai

Thesis,

1-3.

Chemistry,

Etud,

Dahlem

(1983)

Marine

7

M.Sc.

seawater,

XIII, 361-378.

Cannes,

6

Cmar,

of

12,

and G. Zanicchi,

etal,

The nature

Manual,

Desalination,

Acta 171-176

357-367

(1985). (1980).

(1976).