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Development of a two-stage electrodialysis set-up for economical desalination of sea-type artesian and surface waters Presented at the conference on Desalination Strategies in South Mediterranean Countries, Cooperation between
Mediterranean Countries of Europe and the Southern Rim of the Mediterranean, sponsored by the European Desalination
Society and Ecole Nationale dIngenieurs de Tunis, September 1113, 2000, Jerba, Tunisia.
0011-9164/01/$ See front matter © 2001 Elsevier Science B.V. All rights reserved
Desalination 137 (2001) 207214
Development of a two-stage electrodialysis set-up for economical
desalination of sea-type artesian and surface waters
A.D. Ryabtsev
a
, N.P. Kotsupalo
a
, V.I. Titarenko
a
, I.K. Igumenov
b*
, N.V. Gelfond
b
,
N.E. Fedotova
b
, N.B. Morozova
b
, V.A. Shipachev
b
, A.S. Tibilov
c
a
Ekostar-Nautekh, Inc., Bogdana Khmelnitskogo Str. 2, 630075 Novosibirsk, Russia
b
Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences,
Lavrentiev Ave. 3, 630090 Novosibirsk, Russia
Tel. +7 (3832) 344556; Fax +7 (3832) 344489; email: igumen@che.nsk.su
c
Novosibirsk Plant of Chemical Concentrates Joint-Stock Company,
Bogdana Khmelnitskogo Str. 78, 630110 Novosibirsk, Russia
Received 26 July 2000; accepted 14 August 2000
Abstract
A two-stage desalination process is proposed for mineralized water with a salt content of >8 kg/m
3
in order to
improve economical efficiency of the electrodialysis (ED) technique. The first stage involves ED desalination of the
initial water in the galvanostatic regime at increased current density; the liquor is obtained with the salt content which
is limiting for this method, and the dialyzate with the residual salt content allowing to avoid membrane polarization. The
second stage involves profound desalination of the dialyzate in the potentiostatic regime resulting in desalinated water
and the concentrate that is mixed with the initial water. For the mineralized water from the Tyumen region, two-stage
ED desalination and water preparation were investigated. A two-stage desalinating installation was developed and tested
under industrial conditions; a model technological scheme of the industrial installation was developed for the complex
processing of mineralized water from the oil and gas industry of western Siberia.
Keywords: Mineralized water; Two-stage electrodialysis desalination; Galvanostatic and potentiostatic regimes;
Liquor; Dialyzate; Concentrate; Desalinated water
The lack of drinking water all over the world
makes it urgent to use natural waters of various
composition in the production of drinking water.
*Corresponding author.
The most promising solution for this problem is
the use of ocean water and underground brackish
water. Oceans account for about 97% of the total
water resources of the earth. Therefore, the
preparation of drinking water from sea-type A.D. Ryabtsev et al. / Desalination 137 (2001) 207214
208
waters is one of the best ways to replenish
drinking water resources. An urgent problem is to
obtain desalinated water from natural salty
waters.
The electro-osmotic technique is a rather
widespread method of seawater desalination.
However, the installations based on this tech-
nique are very expensive and require special high
headwater pumps which puts substantial
limitations on their application. Desalinating
installations based on electrodialysis (ED) are
much cheaper, but till recently they have been
believed [1] to be economical (taking into
account maintenance costs) only for desalination
of water with a salt content up to 8 kg/m
3
.
However, in our opinion, this limitation is
caused not by the features of the ED desalination
technique itself but by non-optimal conditions of
its performance when processing water with high
salt content. Under such conditions, ED
desalinating units operate in the regime of mem-
brane underloading with respect to the transport
of salts, i.e., at low current densities, which
causes the decrease of the productivity and, as a
consequence, to an unreasonable increase in
investment and maintenance cost. The reason is
the impossibility to obtain, simultaneously in one
and the same apparatus, highly purified water
(with a residual salt content of 0.5 kg/m
3
) and the
concentrate (liquor) with salt content which
would be maximum for this technique.
In order to improve the economical efficiency
of the ED technique, we propose to perform ED
desalination in two stages [2]. The first stage
involves ED desalination of the initial
mineralized water in the galvanostatic regime at
increased current density in order to obtain the
flow of liquor with a salt content that would be
maximum for this technique, and the flow of
dialyzate with residual salt content avoiding the
polarization of membranes. The second stage is
profound ED desalination of the dialyzate in a
potentiostatic regime (classical desalinating ED),
resulting in desalinated water (with a residual salt
content of 0.5 kg/m
3
and below) and the flow of
concentrate to be mixed with the initial mineral-
ized water. A classical desalination scheme (a)
and the scheme proposed by us (b) are shown in
Fig. 1.
For the water from the Tyumen district
(underground water of the Senoman stage, and
water accompanying oil mining, mineralized up
to 20 kg/m
3
), we studied the process of their two-
stage ED desalination. Quantitative ion composi-
tion of the mineralized waters under investigation
is shown in Table 1.
Definite limitations on ED desalination are
made by the presence of impurities in the
mineralized water, including insoluble disperse
phase, iron(II), alkaline earths, carbonate and
bicarbonate ions, as well as an organic phase in
the water accompanying oil production. In order
to cope with this problem, we studied and
mastered water preparation procedures that
would allow the preparation of fresh water and
liquors by means of ED desalination of sea-type
mineralized natural waters. In the case when
Senoman water, containing no oil or other
organic phases, is used as the initial raw, in order
to provide profound purification from iron and
disperse phase, it is reasonable to remove gases
(methane, hydrogen sulfide, and other reducing
gases) before ED concentrating by dispersing the
initial water followed by aeration with air,
settling and infusive filtration with specific load
of 2.22.5 m
3
/m
2
h through four layers of needle-
punching Dornite-type Dacron material.
The following processes take place:
2HCO
3 + 1/2O
2

CO
32 + CO
2
+ 2OH (1)
2Fe
2+
+ O
2
+ 2OH
-

Fe
2
O
3
+ H
2
O
(2)
Ca
2+
+ CO
32
CaCO
3
(3)
Mg
2+
+ 2OH-
Mg(OH)
2
(4) A.D. Ryabtsev et al. / Desalination 137 (2001) 207214
209
Fig. 1. Desalination of mineralized water. a: traditional scheme of the process; b: scheme proposed in the present study.
F, membrane surface area; J, current density; S, salt content, Q, flow rate; , pumps. Subscripts: i, initial water; l, liquor;
d, dialyzate; c, concentrate; p, purified water.
It follows from Table 2 that there is an
optimal time of aerated water settling (40
80 min), as well as an optimal gas-to-liquid ratio
for aeration (56 volume units of gas per 1
volume unit of aerated water).
In order to diminish the risk of CaCO
3
deposition on the membranes of the concentrator,
the residual carbonate and bicarbonate ions are
decomposed by acidifying the filtered water to
pH 5.56.0 before ED concentrating (Table 3).
The decomposition is accompanied by the
processes: A.D. Ryabtsev et al. / Desalination 137 (2001) 207214
210
Table 1
Quantitative ion composition of mineralized waters
Content of
components, kg/m
3
Water type
Senoman
Accompanying
a
Organic phase
Insoluble disperse phase
Na
+
K
+
Ca
2+
Mg
2+
S
22+
Li
+
Cl HCO
3 J Br Fe
2+
SO
42 Total Up to 0.05
6.55
0.02
0.80
0.18
0.06
0.4×10
3
12.43
0.20
0.02
0.04
1.55
<0.01
21.86
0.02
Up to 0.10
6.00
0.07
0.74
0.03
0.22
4.0×10 3
11.37
0.35
0.10
0.02
0.10
<0.01
18.92
a
Contains oxidized iron.
HCO
3 + H
+

CO
2

+ H
2
O
(5)
CO
32 + 2H
+

CO
2

+ H
2
O
(6)
When accompanying water is used as the
mineralized sample, in order to purify it from
admixtures, it is reasonable to apply ascending
floatation using atmospheric air (Table 4) and
taking an air to water volume ratio of 2530:1
and a settling time of 4050 min.
Dynamic tests of ED desalination in the
galvanostatic regime (current density, 200 A/m
2
)
using mineralized waters preliminarily purified
from admixtures and acidified to 6.0 (for results,
see Fig. 2) showed that for the galvanostatic ED
of pre-treated Senoman water (a), salt transfer
remains constant for a fixed concentrating-
desalinating level. As far as the accompanying
Table 2
The effect of the settling and aeration regimes on the degree of water purification from iron and disperse phase
Experiment no.
Process parameters
Impurities in the filtrate
Settling time, min
Air volume per water
volume, m
3
/m
3
Iron, mg/l
Disperse phase,
mg/l
1
20
7
0.15
0.02
2
30