electronic

1. Saline waste disposal reuse for desalination plants for the chlor-alkali industry
The particular case of pozo izquierdo SWRO desalination plant
N. Melián-Martel ⁎, J.J. Sadhwani, S. Ovidio Pérez Báez
Department of Process Engineering, University of Las Palmas de Gran Canaria, Campus de Tafira Baja, 35017, Las Palmas de Gran Canaria, Spain
a b s t r a c t
a r t i c l e i n f o
Article history:
Received 23 July 2010
Received in revised form 15 July 2011
Accepted 16 July 2011
Available online 15 August 2011
Keywords:
Brine
Desalination
Reverse osmosis
Environmental impact
Chlor-alkali
Membrane cells
Seawater desalination has become an important and ever-increasing industry which faces up the
environmental situation of water scarcity present in some Mediterranean countries and in the Canary
Islands (Spain). This activity presents several environmental drawbacks and negative impacts on marine
ecosystems, originated mainly by the discharge into the sea of the generated brine. This emphasizes the need
of introducing, in the short-term, new management proposals for this particular case which should be both
economically viable and effective, not only for new setting up plants, but also for those already installed. As an
alternative to brine disposal, an adequate system has been proposed and developed for the reuse of this saline
waste coming from reverse osmosis desalination plants in the chlor-alkali industry by NaCl electrolysis in
membrane cells. In this paper, the various treatment phases, necessary for the adaptation of this residue as an
alternative raw material resource in the chlor-alkali manufacturing industry, are described. This study has
been adapted to Pozo Izquierdo Reverse Osmosis Desalination Plant, in Gran Canaria.
This new and different residue reuse as raw material supposes the production and exploitation of new
chemical resources, as for example: chlorine, hydrogen gas, and caustic soda.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
At present, seawater desalination seems to be the main and most
feasible technological bet in order to satisfy the ever-increasing fresh
water demand at the Spanish Mediterranean basin. At the same time,
it has become the most important non-natural water resource in the
Canary Islands.
Among the various several existing desalination techniques,
reverse osmosis seawater desalination is the most common method
used in fresh water production. This is due to both new advances in
the technology and to its minor energy consumption and space
requirement [1]. While a reverse osmosis desalination plant works, an
important volume of reject water is generated, this being a
concentrated seawater by-product, its concentration factor depending
upon membrane efficiency (55–60%, the highest concentrations
amounting to about 90%) [2], and several other additives used during
the desalination process (anti-scaling, anti- fouling, biocides and
cleaning chemicals) and heavy metals from corrosion [3–5]. In nearby
coastal desalination plants, this reject water is mainly discharged into
the marine environment which can very often severely damage the
receiving environment. Anoxic condition on the seabed, changing
light conditions and impact on marine species and seagrass could be
generated.
An overview on the composition and effects of these saline
residues can be found in a WHO recent document [6], where they are
discussed in detail by Lattemann and Höpner [5] and MEDRC [7].
Moreover, in latest publications, special attention is drawn to some
regional cases which present a desalination activity increase, such as
the Red Sea [8], the Mediterranean Sea [9–11], the Arabian Gulf [12],
and Gran Canaria Island coastline [13,14].
The environmental impact of this residue has been minimized
both through adequate recommendations [15,5] and good strategy
planning prior to desalination plants building. There are many
emerging alternative technologies that can be combined to achieve
a minimization and valorisation of brine and thus an appropriate
management such as hydrotherapy uses, heat carrier fluid (in solar
ponds or as a cooling fluid), wetlands regeneration, aquaculture,
growth of halophilic species, capacitive deionization, membrane
distillation, nanofiltration, osmotic power, evaporation ponds (by
natural o induced means), selective precipitation, freezing–melting
process and rapid-spray evaporation. However in desalination plants
already installed, minimization corrective measures, being considered
at present, are inviable in most cases. This fact emphasizes the need of
developing new effective and economically viable proposals for this
residue management, not only for new setting-up plants, but also for
those already installed.
In this framework, the sodium chloride high content (doubling
approximately that of seawater) in saline waste products coming
from desalination plants, could be used through previous treatment,
in the chlor-alkali manufacturing industry for chlorine production,
Desalination 281 (2011) 35–41
⁎ Corresponding author. Tel.: +34 6601631 30; fax: +34 928 458975.
E-mail address: nmelian@proyinves.ulpgc.es (N. Melián-Martel).
0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.desal.2011.07.040
Contents lists available at ScienceDirect
Desalination
journal homepage: www.elsevier.com/locate/desal

2. caustic soda, and hydrogen, by means of electrolytic processes. In
addition, this residue has an added value as a byproduct and, what is
more, it avoids the marine environmental impact its disposal should
imply.
The importance of this study has, lies not only in the scientific field,
but also in current practical application, both in the Canary Islands and
in the rest of Spain.
1.1. Chlor-alkali manufacturing technologies
Chlor-alkali manufacturing industry produces chlorine and sodi-
um hydroxide or potassium hydroxide through saline solution
electrolysis. The main technology applied in the chlor-alkali industry
is electrolysis, either in membrane, diaphragm or mercury cells, using
mainly sodium chloride as raw material or potassium chloride, in a
lesser degree, when it is used to produce potassium hydroxide.
The process global electrochemical reaction is as follows:
2 NaCl + 2 H2O →
energy
Cl2 + H2 + 2 NaOH ð1Þ
Energy, as direct current electricity, is supplied to drive the
reaction. The amount of electric energy required will depend upon the
electrolytic cell design, the voltage used and the brine concentration
used. For each tonne of chlorine produced, 1.1 sodium hydroxide
tonnes and 28 hydrogen kilograms are manufactured.
Broadly speaking, the process carried out in order to obtain
chlorine, sodium, and hydrogen is the same one for the three different
technologies. This process can be divided into three large phases, as it
can be observed in Fig. 1.
However, there are important technical differences among these
three technologies, both in terms of product quality and in relation to
each technology operation [16]. One of the main differences among
these technologies lies in those phases necessary for brine purification
treatment. Diaphragm and mercury cell technology do not require
high purity brine. Yet, membrane processes do require high purity
brine. On the other hand, for diaphragm and mercury technologies,
precipitation and filtration are the adequate measures for brine
purification. However, for membrane cell processes, an additional
brine treatment containing ion exchange resins is necessary.
In relation to operation systems, each process represents a
different method of keeping aside the chlorine produced in the
anode from both the caustic soda and the hydrogen produced in the
cathode, either directly or indirectly. As a consequence, each process
produces a different chlorine gas purity and a different caustic soda
concentration [17]. In Table 1, some industrial characteristics of these
processes are briefly shown.
The advantage mercury cell technology has, is that of producing a
high purity caustic soda by a merely simple brine purification.
However, this is the highest energy-consuming process (up to
3400 kWh of electric energy per chlorine tonne produced). Dia-
phragm processes produce low-quality caustic soda and moreover,
they require a higher energy consumption than that required by
membrane cells. Consequently, the installed capacity of these
processes is being diminished [18]. All new chlor-alkali plants use
membrane cell technology. This is due to the fact that the expenditure
concerning capital investment, operating costs and energy consump-
tion, are lower than that of diaphragm and membrane technology
[16–18]. What is more, this is the most environmental friendly
technology, yet this technology requires a high purity brine (see
Table 2).
Among the electrochemical technologies studied in this work,
membrane cell technology is a clean, economically viable technique
for the reuse of these residue, but, at the same time, it is dependent on
many factors, such as brine purity, flow density, and pH factor.
The treatment stages definition for chlorine production using
membrane technologies is most clear, since its specifications and
characteristics are well-known. The adaptation to a particular
alternative resource for chlorine production, as it is that of saline
residues coming from waters desalination, however, is a great
challenge. Due to its composition and origin-dependence, the use of
brine often implies several various treatments which have been, up to
now, very little studied.
The total volume of impurities, present in these alternative
resources, form a most diverse elements group, taken as a whole,
and having different behaviors. The treatment of this saline residue
must be carried out through both identification and quantization of all
influential parameters, i.e., by a precise and exact brine analysis.
A practical case of about 8400 m3
/day brine reuse coming from
Pozo Izquierdo desalination plant has been developed in the present
study. It also shows one adequate method for brine purification and
concentration for further use in chlor-alkali manufacturing produc-
tion electrolytic cells. This study is focused on a particular case located
in Gran Canaria Island.
2. Materials and methods
We shall analyze each treatment phase necessary for the reuse of
saline residue coming from one reverse osmosis desalination plant
sited in Gran Canaria Island, i.e., the particular case of Pozo Izquierdo
SWRO Plant, which has been thoroughly studied.
2.1. Pozo Izquierdo desalination plant
This desalination plant is located in the south-eastern part of the
Island, i.e., in Pozo Izquierdo (Tenefé Point) within Santa Lucía de
Tirajana municipal area. Both the technical specifications and the
residue chemical composition generated after desalination are shown
in Tables 3 and 4 respectively.
Fig. 1. Chlor-alkali process simplified scheme.
Table 1
Comparative study among chlor-alkali electrolysis processes [17,18].
Mercury cell process Diaphragm cell process Membrane cell process
Electric energy demand (kWh/t Cl2) 3100–3400 2300–2900 2100–2600
Total energy demand (kWh/t Cl2) for 50%
of NaOH
3100–3400 3200–3800 2400–2900
NaCl purification Simple Simple Expensive (Ca2+
+Mg2+
b20 ppb)
NaOH quality 50 wt.% from cells, low chloride
content
12 wt.% from cells, up to 1% chlorides in 50 wt.%
NaOH
32 wt.% from cells, low chloride
content
Cl2 quality b1% O2 in Cl2, no further cleaning 2–3% O2, further cleaning required 1–3% O2,further cleaning required a
Environmental issues Hg used as cathode material Asbestos used for diaphragms None
a
0.5% O2 with HCl addition to anolyte.
36 N. Melián-Martel et al. / Desalination 281 (2011) 35–41

3. 2.2. Initial data
The reuse of 8400 m3
/day of brine has been chosen as the
computation baseline to follow because of its NaCl content, this
being 61.48 g/L. The treatment stages relating chlorine, caustic soda,
and hydrogen production through membrane cell technology is quite
clear, taking into account that both its specifications and character-
istics are well known [16,17]. Due to this fact, and, in order to reuse
these residue, the removal of several impurities becomes necessary,
mainly Ca2+
+Mg2+
(b20 ppb), Sr2+
(b40 ppb) and SO4
2−
(b6 g/L),
in addition to 290–310 g/L of NaCl concentration. The brine reassess-
ment process block diagram for the chlor-alkali manufacturing
industry is shown in Fig. 2.
2.3. First purification: chemical precipitation, clarification and filtration
In this process stage, the addition of precipitants which carry out
the impurities removal, takes place. Most of the calcium appearing as
CaCl2 and those sulphates which appear as Na2SO4, having a rate of
2.66 kg/m3
and 8.95 kg/m3
respectively, are removed by chemical
precipitation using sodium carbonate (Na2CO3) and barium chloride
(BaCl2) solutions. The reactions that take place are the following:
Na2CO3•10H2OðacÞ þ CaCl2ðacÞ→CaCO3ðsÞ þ 2NaClðacÞ
þ 10H2OðacÞ ð2Þ
BaCl2•2H2OðacÞ þ Na2SO4ðacÞ→BaSO4ðsÞ þ 2NaClðacÞ
þ 2H2OðacÞ ð3Þ
In order to remove magnesium found as MgCl2 and rating
10.99 kg/m3
, a sodium hydroxide (NaOH) solution is used. Conse-
quently, the following reaction occurs:
2NaOHðacÞ þ MgCl2ðacÞ→MgðOHÞ2ðsÞ þ 2NaClðacÞ ð4Þ
The volumes of precipitants (Na2CO3•10H2O, BaCl2•2H2O and
NaOH) are determined by operation mass balance. Concentrations
of each solution have been determinate taking into account the
solubility of substances. The NaOH solution comes from the
membrane cell at 32% in weight, once the NaCl electrolysis has
taken place.
Throughout this stage, various different metals, such as iron,
molybdenum, nickel, and chrome, may also precipitate as hydroxides
during operation.
All salts that precipitate together form a sludge that silts out and,
consequently, is removed. For this, some potato starch will be used as
flocculent. The sludge produced in the clarificator tank is basically
formed by the following precipitates: calcium, sulphates and
magnesium (CaCO3, BaSO4, and Mg(OH)2). Yet, these precipitates
are never recovered. These sludges will be treated through filtration.
Pre-coat filtration of the decanted waters is used to eliminate small
diameter suspended solids. This, generally, is a complementary
operation to flocculation and sedimentation stages. Sand, anthracite,
and diatomaceous earth may be used as filter medium contributory
materials.
2.4. Multieffect evaporation saturation
Feed brine containing a sodium chloride concentration of about
61.48 g/L, needs a pre-concentration phase.
The volume of water that evaporates during operation for
obtaining a saturated NaCl at the desired concentration, i.e. 36%
approximately is determined by operation material balance, taking
into account, at the same time, that the input NaCl amount equals the
output NaCl quantity.
In order to determine the NaCl concentration reached in each one
effect, it becomes necessary to know the volume of water evaporated
in each one effect. For this, it is initially supposed that the evaporated
water total volume in each particular effect is the same for both.
For heat transfer integral coefficient calculus, the following
correlation recovered by Mandani, F. et al., is used [20]:
Ue¼ 1; 9394 þ 1; 4053 ·10
3
·Tb2; 0752 ·10
4
·T
2
bþ2; 3186 ·10
6
·T
3
b
ð5Þ
where Ue is the heat transfer global coefficient, expressed in kW/m2
°C,
and Tb the brine boiling temperature in each effect, expressed in °C.
The specific heat of solutions varies according to concentration.
Therefore, the following correlation recovered by Mandani, F. et al.
[20], will be used for its own determination.
Cp¼ A þ BT þ CT
2
þDT
3
· 10
3
ð6Þ
Variables A, B, C, and D, are obtained by means of the following
expression which emerges according to water salinity function (S):
A = 4206; 8−6; 6197 · S + 1; 2288 · 10
−2
·S
2
ð7Þ
B ¼ 1; 1262 þ 5; 4178 ·10
2
· S 2; 2719 · 10
4
· S
2
ð8Þ
Table 3
Pozo Izquierdo desalination plant technical specifications [19].
Parameter Value
Production capacity 33,000 m3
/day
Conversion 50%
TSD 400 mg/L
Application Domestic consumption
Table 4
Chemical composition of reject brine from Pozo Izquierdo desalination plant [19].
Parameter Value Units
Calcium 960.00 mg/L Ca2+
Magnesium 2867.00 mg/L Mg2+
Strontium 14.55 mg/L Sr2+
Barium 0.018 mg/L Ba2+
Silicon 20.50 mg/L SiO2
Nickel 0.01 mg/L Ni
Sulphates 6050.00 mg/L SO4
2−
Carbonates 0.00 mg/L CO3
2−
Bicarbonates 1829.00 mg/L HCO3
−
Chlorides 41,890.00 mg/L Cl−
Sodium 25,237.28 mg/L Na+
Fluorides 1.82 mg/L F−
Potassium 781.82 mg/L K+
Boron 8.00 mg/L B
pH 7.50 –
Conductivity 85,200.00 μS/cm
TDS 79,660.00 mg/L
Table 2
Typical specifications for feed brine in membrane cells [16].
Component Concentration
Calcium+magnesium (mg/L) b 0.02
Strontium (mg/L) b 0.04
Soluble SiO2 (mg/L) b 10.00
Aluminium (mg/L) b 0.10
Heavy metals (mg/L) b 0.10
Iron (mg/L) b 1.00
Barium (mg/L) b 0.50
Sulphates (g/L) b 6.00
37
N. Melián-Martel et al. / Desalination 281 (2011) 35–41

4. C ¼ 1; 2026 · 10
2
5; 4178 · 10
4
·S þ 1; 8906 ·10
6
· S
2
ð9Þ
D ¼ 6; 8777 · 10
7
þ1; 517 ·10
6
· S 4; 4268 · 10
9
· S
2
ð10Þ
where Cp is expressed in
kJ
kg∘
C
, T in °C and S in g/L.
The energy balance in each evaporator is given by:
L0 · ΔHL0 + Q1 = L1 ·ΔHL1 + V1 ·ΔHV1 ð11Þ
where L0 is the brine flow entering the evaporator, L1 is the brine flow
leaving the evaporator, and V1 is the amount of water evaporated in
the evaporator. Q1 is the exchanged heat. ΔHL0 and ΔHL1 are the liquid
enthalpies both at the exchanger input and output, and ΔHV1 is the
vapour enthalpy.
In order to optimize the exchanged energy in each evaporator and,
therefore, diminishing the feed vapour expenditure coming from the
boiler, a series of shell-and-tube preheaters, heated by each effect
vapour, is used. Besides, five heat exchangers will be used for
preheating the feeding brine. The determination of the obtained
temperatures in each exchanger has been calculated using the
effectiveness-NTU method [21].
2.5. Second purification: ion exchange
The precipitation phase alone is not enough, by itself, to reduce
calcium, magnesium, and strontium amounts. A brine depuration
phase, as a second purification, must be planned in advance.
The resin selection will depend upon both the input brine
composition and on the loading cycle limit criterium. In both cases,
a higher amount of calcium, magnesium, and strontium should be
removed. This is why the resin character used should belong to
aminomethylphosphonic functional group (AMP), since this is in
closer relation to Ca2+
, and Mg2+
.
Once the brine is purified, both its heating (up to about 90 °C [16])
and acidifying (at pH 4), become necessary in order to avoid 12 and 13
secondary reactions to form.
Cl2 þ H2O→Cl
−
þ H
þ
↔ClO
−
þ Cl
−
þ 2H
þ
ð12Þ
HClO + ClO
−
→ClO
−
3 + 2Cl
−
+ 2H
þ
ð13Þ
2.6. Depleted brine dechlorination
When the membrane cell electrochemical output amounts to
about 60%, a 210–250 g/L concentration reject brine is generated. This
depleted brine is then recirculated as it comes out of the electrolysers
so that it can be mixed with that other brine coming from the
multieffect evaporators. The depleted brine, however, must be
dechlorinated before its being recirculated. For this, some chloridric
acid is already added in a previous step (a 2–2.5 pH is needed), so that
a better and more viable chlorine extraction is obtained. A pH 2–2.5
chloridric acid acidification not only reduces chlorine solubility
through the hydrolysis equilibrium point change, but it also inhibits
the chlorate and hypochlorite formation. In order to remove the brine
remaining chlorine, this is made to go through a dechlorinator, so that
the in-brine chlorine concentration is reduced.
3. Results
The brine reassessment process flow diagram for the chlor-alkali
industry is shown in Fig. 3.
The initial stage of purification uses BaCl2•2H2O, Na2CO3•10H2O
and NaOH to precipitate sulphate anions as BaSO4 and calcium and
magnesium cations as CaCO3 and Mg(OH)2 respectively. This process
is largely controlled by the solubility product of the barium sulphate,
magnesium hydroxide and calcium carbonate. The concentrations of
the precipitants are essentially fixed at levels which can be
determined from their respective solubility. Barium chloride and
sodium carbonate solubility at 20 °C are 35.7 g and 21.5 g of
Chemical Precipitation
Clarification
Filtration
Saturation
Ion exchange
Electrolysis
Dechloration
Chlorine
Brine from desalination plant
Depleted Brine
Precipitants
HCl
HCl
Sludge
Potato Starch
Purified Brine
Caustic Soda
Hydrogen
Fig. 2. Block diagram for the brine reassessment in the chlor-alkali industry.
38 N. Melián-Martel et al. / Desalination 281 (2011) 35–41

5. anhydrous salt in 100 g of water respectively [21]. Therefore a
solution of 30 wt.% of BaCl2•2H2O and 20 wt.% of Na2CO3 can be used
to carry out the impurities removal.
All salts that precipitate together form a sludge that silts out and,
consequently, is removed from the clarifier tank by means of
flocculation using potato starch. The generated residues which are
mainly solids coming from brine purification are moisturized twice its
own weight and removed as brine sludge. From the environmental
point of view, these industrial sludges do not pollute. Nevertheless,
they should be adequately managed. Table 5 overviews the final
composition of the sludge removed from the clarifier unit.
The clarified brine is filtered in a pre-coat filtration unit with
diatomaceous earth before entering the multi-effect evaporator unit
where the residual calcium and magnesium and remaining metal ion
impurities are removed. After filtration, brine contains the following
impurities: Mg+2
b1 ppm, Ca+2
b5 ppm, Sr+2
b5 ppm and Ba+2
b
0.5 ppm.
Next after a six-step multieffect evaporation, that brine containing
initially 5.54% NaCl in weight, reaches a degree of saturation of 36%
NaCl in weight. This saturation is most adequate in order to carry out
the membrane cell electrolysis. During this last effect, a 0.0105 MPa
pressure is reached. Boiler vapour (T=192 °C) at a saturation
pressure of 1.31 MPa, is used as a heating medium in the first effect
jacket. In order to optimize the exchanged energy in each evaporator
and, therefore, diminishing the feed vapour expenditure coming from
the boiler, a series of shell-and-tube preheaters, heated by each effect
vapour, is used. Besides, five heat exchangers will be used so that the
feeding brine pre-heating gets up to about 60.94 °C. The main
parameters of the multieffect evaporation unit are shown in Table 6.
Once brine is saturated, a second purification becomes necessary.
This time using an ion-chelating exchange resin, type AMP (amino-
methylphosphonic acid), and chosen by its greater effectiveness when
eliminating Ca2+
and Mg2+
ions (b20 ppb) and remaining metal ion
impurities.
From the ion exchanger unit, the brine solution is passed to the
electrolyser where Cl2 is produced at the anode and H2 at the cathode.
Sodium cations migrate through a semi-permeable membrane in the
electrolyser from the brine anolyte to the catholyte (NaOH). When the
membrane cell electrochemical output amounts to about 60%, 25 wt.%
NaCl concentration reject brine is generated. This depleted brine is
then recirculated as it comes out of the electrolyser that it can be
mixed with that other brine coming from the multieffect evaporators
after a dechlorination. Some chloridric acid is added in a previous step
Fig. 3. Flow diagram for the brine reassessment in the chlor-alkali industry.
Table 5
Composition of the sludge removed from the clarifier unit.
Compounds Flow (kg/h)
CaCO3 839.5
BaSO4 5146.6
Mg(OH)2 2355.6
H2O 16,683.4
NaCl 6473.09
Na2CO3•10H2O 116.69
BaCl2•2H2O 261.90
Table 6
Results of the main parameters of six-step multieffect evaporation unit.
Effect NaCl
concentration (%)
Boiling
point (°C)
Specific
heat (kJ/kg °C)
Steam generated
(kg/h)
Pressure
(MPa)
Feed 5.54 60.94 3.91 – –
1 6.45 147.18 3.97 31,213.06 0.438
2 7.72 125.20 3.87 44,403.19 0.231
3 9.60 105.50 3.77 53,932.53 0.0112
4 12.71 87.30 3.63 60,159.63 0.0156
5 18.79 70.10 3.40 63,770.48 0.0124
6 36.00 53.55 2.99 64,832.12 0.0105
39
N. Melián-Martel et al. / Desalination 281 (2011) 35–41

6. (2–2.5 pH) so that the in-brine chlorine concentration is reduced to
10–30 mg/L.
Membrane cell electrolysis works at 90 °C, with a current intensity
of 5 kA/m2
, which requires brine pre-heating.
The chemical products obtained after electrolysis is carried out
are: chlorine-gas (Cl2) caustic soda (NaOH), and hydrogen (H2) can be
viewed in Table 7. Concerning chlorine-gas, a 2% moisture flow is
obtained. This is why purification through refrigeration and stepping
through a demister, are two required measures. On the other hand,
concerning sodium hydroxide and hydrogen, no purification becomes
necessary.
Specifically and in relation to the case studied, 101.16 kt/year Cl2;
253.71 kt/year NaOH and 2.82 kt/year H2 are produced, revaluating
8400 m3
/day of saline residues.
From the energy point of view, a consumption of about 2150 kWh/
t NaOH is estimated. The obtained hydrogen will be used in situ for
gaining electric energy through proton exchange by means of fuel
cells (PEMFC), so that some of this consumption will be self-supplied.
4. Conclusions
When facing the need for developing new proposals for saline
residue management, being not only economically-viable but also
effective for both new setting-up plants and those already in
operation, the information recovered and given in this paper may
be regarded as most useful for implementation in future projects.
The conclusions drawn out of this work refer to knowledge and
control improvements concerning desalination processes, to impact
reduction generated by brine disposal, and to saline residue reassess-
ment as raw material in the chlor-alkali manufacturing industry. It
may be considered, on the one hand, as a step forward in relation to
the study and development of the various different existing
alternatives in respect to the reassessment of saline residue from
reverse osmosis desalination plants and, on the other, regarding
desalination process control.
In view to previous assessments, the creation of a chlor-alkali
industry annexed to desalination facilities appears to be a technical
and economically viable option facing residue disposal. What is more,
the creation of new plants having such characteristics will make these
residues earn an added value as raw material for the production of Cl2,
H2, and NaOH, revaluating a one type residue having no previous
value ever before.
Concerning the implementation possibilities, requirements do not
include a sophisticated technology, so that plant maintenance will be
easy to follow, since membrane cell technology for chlor-alkali
production is technical and economically viable. Furthermore, this
type of technology is environmentally friendly, and does not require
big lots for it to be set up. This is of special significance in the Canary
Islands, since the production of desalinated water in this area reaches
2.6% of worldwide production. The setting-up of a chlor-alkali
manufacturing industry in the Canary Islands would not only reduce
the environmental impact coming from desalination plants, but also,
would be an economical precursor for the islands, and would be
regarded as a new and stable economical model, based upon
manufacturing (or secondary) sector as an alternative to the present
model, service-dependent, as it is tertiary sector, supported by
external economies and factors. Consequently, the Canary Islands
would innovate, one more time, regarding the desalination industry,
and, moreover, this time contributing to a new sustainable develop-
ment. However and in spite of this, the execution of an economical
study as a new researching line to follow would have to be assessed.
After NaCl electrolysis, the obtained products have various distinct
ultimate uses and very different market dynamics. Concerning
hydrogen, this will be used in situ so as to obtain electric energy
through proton exchange by means of fuel cells.
On the one hand, chlorine may be used for public supply water
chlorination, while meeting the maximum concentration allowed and
established by the WHO demands, i.e., (0.5 ppm). This would cover
the total demand in Gran Canaria Island (147,500 kg Cl2/year). On the
other hand, chlorine might be used in the various different existing
industries in the islands, requiring of this product, as for example,
waste management industries (glass and paper), and food and
agriculture industries (containers disinfection, tanks, and processing
lines).
Sodium hydroxide may be used, in turn, in Canarian industries
producing detergents, food industry (bottle cleansing and pH
control), and for future-coming biodiesel plants, and, in lesser
amount, for industrial activity in common general uses (paint
stripping, in-dried removing agent, enamel work, and even in metal
degreasing and cleansing).
Acknowledgements
This paper has been made possible with the cooperation of the
University of Las Palmas de Gran Canaria and the Industrial
Engineering Technical College of Las Palmas de Gran Canaria
University, which are gratefully acknowledged. The authors would
also like to thank Mr José Manuel Díaz García, Chief-in-Plant from
Pozo Izquierdo Desalination Facility, for the information given.
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Table 7
Final products' composition (before later processing).
Cl2 NaOH H2
Cl2 N98% (vol.) NaOH 32 wt.% N 99.9% (vol.)
H2 b2% (vol.) NaClb20 ppm
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