Desalination 279 (2011) 445–450

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Desalination

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Solar energy storage by salinity gradient solar pond: Pilot plant construction andgradient control

César Valderrama a,⁎, Oriol Gibert a,b, Jordina Arcal a, Pau Solano a, Aliakbar Akbarzadeh d,Enric Larrotcha b,c, José Luis Cortina a,b

a Departament d'Enginyeria Química, Universitat Politècnica de Catalunya, Spain.b Water Technology Center CETaqua, Barcelona, Spainc Aigües de Barcelona Agbar, Barcelona, Spaind School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Australia

⁎ Corresponding author at: Departament d'Enginyerinica de Catalunya, Carrer Colom 1, Terrassa 08222, B4011818; fax: +34 93 401 58 14.

E-mail address: cesar.alberto.valderrama@upc.edu (

0011-9164/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.desal.2011.06.035

a b s t r a c t

a r t i c l e i n f o

Article history:Received 4 April 2011Received in revised form 16 June 2011Accepted 20 June 2011Available online 20 July 2011

Keywords:Salinity gradient solar pondSolar energySalinityTemperatureBrinesRenewable energy

An experimental solar pond pilot plant was constructed in Solvay-Martorell, facilities, Catalonia (NE part ofthe Iberian Peninsula) to capture and store solar energy. The body of the pond is a cylindrical reinforcedconcrete tank, 3 m height, 8 m diameter and total area of 50 m2. Salinity and thermal gradient were properlyestablished by using the salinity distribution methodology. The gradient in the pond was maintained byfeeding salt (NaCl) through a cylindrical salt charger to the bottom at a height of 80 cm from the pond floor.Continuous surface washing using tap water supply maintained the salinity of the top convective layer at alow level and compensate loses by evaporation. An acidificationmethod by addition of HCl at different heightswas used to control the clarity of the pond. The salinity gradient was fully established on 30 September 2009and has been maintained until the date. After winter time (February 2010), the pond warms up and thetemperature increased continuously until it reached its maximum (55 °C) in August 2010. The salinitygradient observed great stability after one year of continuous control and maintenance and under differentweather conditions.

a Química, Universitat Politèc-arcelona Spain. Tel.: +34 93

C. Valderrama).

l rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Nowadays, with rising oil prices and increasing supply instability,together with international requirements for greenhouse gas emis-sion reduction, lowering traditional energy requirements for desali-nation by making use of renewable sources such as solar energy andwaste heat is essential [1,2]. Thermal energy storage is required inorder to save fuel andmake the systemmore effective by reducing thewastage of energy further; energy storage is convenient where theenergy source is intermittent such as solar energy [3–5]. An attractivesolution, especially for areas with abundant solar radiation, is tocouple a low-temperature multi-effect evaporation (MEE) processwith a solar–thermal source. Salinity Gradient Solar Ponds (SGSP), asintegral devices for collecting and storing solar energy, have beenrecognized as potential heat source technology and attractive solutionfor thermal desalination [2,6–8]. Compared with other solar desali-nation technologies, solar ponds provide the most convenient andleast expensive option for heat storage, which is very important, forboth operational and economical aspects, if steady and constant water

production is required [9]. In a SGSP, a temperature gradient (withhighest temperatures at the bottom and lowest temperatures at thetop) is established and a salt concentration gradient (denser at thebottom and lighter at the top) is therefore created and supposed toprevent convective motions that would otherwise promote the returnof the stored energy to the outside ambient and thus destroy thepond's purpose. A double diffusion process occurs where thetemperature and salinity fields make opposite contributions to thefluid density [10]. A typical salinity-gradient solar pond generallyconsists of three regions namely the upper convective zone (UCZ), themiddle non-convective zone (NCZ), and the lower convective zone(LCZ). When solar radiation is incident on the solar pond, a part of theradiation is reflected back from the top surface while most of theincident sunlight is transmitted inside through the top surface of theUCZ. The fraction of the transmitted radiation is first rapidly absorbedin the surface layer. However, this absorbed heat is lost to theatmosphere by convection and radiation heat transfer. The remainingradiation is then subsequently absorbed in the middle NCZ andbottom LCZ before the rest of the radiation reaches the bottom of thepond. In the LCZ, the absorbed solar energy is converted to heat andstored as sensible heat in high concentration brine. Since there are noheat losses by convection from the bottom layer, the temperature ofthis layer can rise substantially. The temperature difference betweenthe top and the bottom of the solar ponds can be as high as 50–60 °C

Fig. 1. a) Schematic viewof thepond showing thedistributionof the threezones andb)viewof the 50 m2 experimental solar pond at Solvay-Martorell facilities, Catalonia (Spain).

446 C. Valderrama et al. / Desalination 279 (2011) 445–450

[11]. Thermal energy stored in the solar pond can be used for heatingof buildings, power production and industrial processing [8,12].

The efficiency of a solar pond in collecting energy depends on thestability of the gradient zone and water clarity. Maintaining the stateof the salt gradient zone (boundaries, level and salt gradient of NCZ)as its initial design is essential to the successful operation of a salinitygradient solar pond. Both of the upper and lower zones cause erosionof the boundaries of the salt gradient zone. The progress of erosionleads to the reduction of thickness of the NCZ; thus the salinitygradient gets destroyed, whether care is not taken [13].

Numerous experimental and theoretical studies have been under-taken [14–20]. Most of the experimental work [14,15] concentrates ondesign, application, thermal measurements, efficiency and investiga-tions of the thermal performance of various types of solar ponds atlaboratory scale. Many experimental studies [16–20] focus on deter-mining the efficiency of inner zones in solar ponds, and determining thezone performance that yields the best solar pond system.

In this work, a 50 m2 circular pond has been constructed in orderto evaluate the feasibility of collecting and storing solar energy bymeans of a salinity gradient in order to storage heat to be delivered forspecific applications. The first part of the project is reported in thisarticle, which reports the construction and evaluation of the salinitygradient after one year of continuous maintenance, the effect of theweather conditions and the lessons learned on the control of the threezones inside the pond are reported.

2. Materials and methods

2.1. Description of the solar pond

An experimental salinity-gradient solar pondwas constructed in theSolvay-Martorell facilities, Catalonia (41.475556N, 1.931667E). Thebody of the pond was a cylindrical reinforced concrete with a thicknessof 0.2 m, 3 m height and 8 m diameter (total area of 50 m2). The pondwas insulated by 60 mm of rock wool and then covered with 0.8 mm ofsmooth aluminum plates. The inner surface of the concrete was coatedwith Remmers Aida® Kiesol (supplied by BASF) to waterproof andprotect the concrete against corrosion. The level of water in the pondwas fixed at 2.8 m above the bed by using an overflow system. Freshwater continuously flowed from a mains tap to flush the surface andcompensate losses by evaporation. Typically, the rate of flushing wasabout twice the evaporation rate. Floating rings were distributed overthe surface of the pond to reduce the surface mixing caused by wind-driven currents [21]. Thefloating rings suppresswave action bydividingup the surface area of the pond into small isolated cells to reduce thefetch length, which in turn limits the wave lengths and henceamplitudes of the waves. A salt charger was employed to replenishthe salt (NaCl) in the pond. The charger was made from a polyvinylchloride (PVC) cylinder with a diameter of 0.8 m fixed to thewall of thepond. The cylinderwas filledwith salt up to the surface of the pond. Saltcoming out of the bottom of the cylinder produced a salt pile in theshape of a semi-cone round the charger. A sketchof a cross section of thepondwith three zones and salt charger is shown in Fig. 1a and a view ofthe experimental solar pond is observed in Fig. 1b. The pondwas abovethe ground, thus, it was necessary to build a stairway in order to getaccess to the pond surface. Amobile PVC tub (6 mmof diameter and3 mheight) supported in a fixed tub and a peristaltic pump (3 L/h)was usedas sampler mechanism to measure density, pH and turbidity inside thepond at different heights [22]. A portable densitymeter DMA 35 (AntonPar, accuracy: ±0.001 g/cm³) was used for measurement of density.Further, pH and turbidity were measured by portable pHmeter (CrisonpH25, accuracy: ±0.01 pH), and portable turbidity meter (HannaHI93703C, accuracy: ±0.5NTU), respectively. The temperature mea-surement at different heights was performed by means of 21 sensors(thermo-resistances Pt-100-K type, Abco, Spain) uniformly distributedeach 14 cm (starting at 0.2 cm from the bottom) installed in a plastic

support fixed to the pond wall. Heat losses by the wall and the bottomwere also considered, thus, other 6 sensors were installed inside andoutside of the wall concrete, and inside of the slab of the pond. Thetemperature was measured every 2 s and the averages after 10 min aswell as the hourly and daily average were recorded. The monthlyaverage temperature of each zone was determined by averaging thedaily recorded values.

The weather parameters weremeasured bymeans of an automaticweather station CR1000 Measurement and Control System (CampbellScientific, Barcelona, Spain) which was programmed to measure andstore data (Datalogger CR1000) of different meteorological sensorswith high accuracy as follows: rain gages (52202/52203, 2% up to25 mm/h); solar radiation (CS300, ±5% for daily total radiation);wind speed (03002 ±0.5 m/s); relative humidity (CS215, ±2% 10 to90% RH); barometric pressure (CS106, ±0.6 mb 0° to 40 °C) and airtemperature (CS215, ±0.4 °C over +5 to +40 °C). The sensors takemeasures every 10 s, the hourly average was recorded as well as thedaily average (24 h). The monthly average ambient temperature wasdetermined by averaging the daily recorded values.

2.2. Uncertainty analysis

The thermo-resistances (K type) were used to measure thetemperature at different heights inside the pond. The operating rangeand accuracy were −200 °C to +1250 °C and ±2.2 °C or 0.75%,respectively. The thermo-resistance sensors were connected to a datalogger, which had a temperature measurement accuracy of ±(0.06% ofthe reading+offset). The total uncertainty (δtotal) in every measure-ment can be obtained by combining the sensor accuracy (δsensor)andmeasuring instrument (δinstrument) accuracy using the root-sum-of-squares method as follows [6].

δtotal =ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiδsensorð Þ2+ δinstrumentð Þ2

qð1Þ

Fig. 2. Picture and scheme of the diffuser used to setup salinity gradient.

447C. Valderrama et al. / Desalination 279 (2011) 445–450

2.3. Settling the salinity gradient

The establishment of salinity gradient profile is a critical task of solarpond technology. Previous works [23–25] demonstrate that waterinjection is the most common and efficient method for settling thesalinity profile. The first step in establishing the salinity gradient is to fillthe pond up to half the depth of the planned gradient zone with highconcentration brine (In this study, 83 m3of brinewith a concentration of25% byweight). Then, fresh or low-salinitywater is injected horizontallyinto the brine through a diffuser [23,26,27]. Fig. 2 shows the semi-circular diffuser designed and used in this work (overall diameter:500 mm; overall thickness: 27 mm; gap vertical dimension: 3 mm).

The injection starts from the desired level of the boundarybetween the gradient zone and the storage zone. As the injectionproceeds and the level of the water in the pond rises, the diffuser israised in increments from its position within the brine solutiontowards the surface of the pond. It is recommended that the speed ofthe upward lifting of the diffuser is twice the speed of the rise of thepond water. At the end of this process, the pond is full and the desiredsalinity gradient created. A complete description of the process isreported by Leblanc et al., [2].

The key parameter of this technique is the Froude number (Fr),which is a dimensionless number representing the ratio of the kineticenergy to the gravitational potential energy of the injected fluid.

Fr =ρV2

gΔρBð Þ

" #1=2

ð2Þ

where ρ is the density of the surrounding saline fluid; V is theinjection velocity at the diffuser outlet; g is the acceleration due togravity;Δρ is the density difference between the injected fluid and thesurrounding fluid; and B is the vertical gap dimension of the diffuser.Finally flow rate should be determined by fixing a Fr number andsolving Eq. (2) for the injection velocity and considering the surfacearea of the pond (50 m2).

0

0,5

1

1,5

2

2,5

3

1,00 1,04 1,08 1,12 1,16 1,20

Hei

gh

t (m

)

Density (g/cm3)

Fig. 3. Density profile during gradient setup procedure.

A value of Fr=15 should be maintained during the filling process[28]. However, because of the diffuser design was possible to obtain ahigh flow rate (horizontal gap dimension) and the Fr number wasfinally recalculated to 11. During the injection process the resultingsalinity gradient profile was monitored after each movement of thediffuser, obtaining the final profile shown in Fig. 3.

3. Results and discussion

The solar pond at Solvay facilities has been continuously main-tained since 30th of September 2009 until the date. The maximumtemperature of 54 °C was reached after 10 months of operation(summer) at the border between NCZ and LCZ; and the minimumtemperature was observed in winter time reaching 18 °C at samepoint; temperature was kept over 45 °C for about 5 months.

Fig. 4 shows the evolutions of the UCZ, NCZ (measured at 0.92 mfrom the bottom) and LCZ since the pond started operating until awhole year of operation, it is observed that temperature values for theNCZ were quite higher than those of LCZ. It can be explained by theshape of the temperature profile as can be seen in Fig. 5. Themaximum temperature is reached in the NCZ and then temperaturedecreases in the LCZ. This phenomenon can be related to the fact thatbottom of the pond was not insulated, and thus, temperaturedifference between ground and LCZ reduced the effectiveness of thesolar pond. In Figs. 4 and 5, the error bars in the y-axis and x-axisdirection, respectively, were calculated for temperaturemeasurementusing the uncertainty analysis procedures described in Section 2.2

The salinity was measured every week as well as the turbidity andpH. Themonthly average density, turbidity and pHwere determined byaveraging the recorded values of each month. The density evolution ofthe three zones was fairly constant during one year operation as can beseen in Fig. 6. This behavior confirmed the great stability of the salinitygradient under the influence of the weather parameters. The evolutionof the minimum, average and maximum temperatures, solar radiation;rain and wind average speed measured in-situ by means of the

05

10152025303540455055

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Nov-10

T (

°C)

Ambient

UCZ

NCZ (0.92 m from the bottom)

LCZ

Fig. 4. Monthly average temperature evolution of ambient and three zones LCZ, NCZand UCZ from October 2009 to October 2010 (with error bars shown).

0

0,3

0,6

0,9

1,2

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1,8

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Hei

gh

t (m

)

T (°C)

Oct-2009Dec-2009Feb-2010May-2010Jun-2010Aug-2010Oct-2010

UCZ

LCZ

NCZ

Fig. 5. Temperature profile along the pond height (based on monthly averagecalculation) from October 2009 to October 2010 (with error bars shown).

1,00

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1,20

1,25

Aug-09

Oct-09

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Dec-10

Den

sity

(g

/cm

3 )

bottom 0,7m 0,8m 0,9m 1m1,1m 1,5m 1,9m 2,3m surface

Fig. 6. Salinity evolution atdifferentheightof thepond fromOctober2009 toOctober2010.

0%

5%

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25%

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-10

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Heat losses to the bottom

Heat losses to the wall

Hea

t lo

sses

(%

of

sola

r ra

dia

tio

n)

So

lar

rad

iati

on

(M

J/m

on

th)

Fig. 7. Montly solar radiation and heat losses to the bottom and to the wall aspercentage of solar radiation from November 2009 to October 2010.

448 C. Valderrama et al. / Desalination 279 (2011) 445–450

automaticweather station is summarized in Table 1. The values indicatethat July reported themaximum temperature and solar radiation, whileJanuary the lowest temperature. On the other hand thewind speedwasfairly constant along thewhole year,which represented lowsignificancein the variability of the UCZ.

It is observed how after the establishing of the salinity gradient,density in the bottom of the pond (0.3 m) was above 1.2 g/cm3 andfew weeks later after the stabilization in the salt addition the densityincreased over the 1.2 g/cm3 and maintained constant a long wholeyear. On the other hand, density at surface observed higher variabilitywhich is due to the evaporation, the salt diffused to the surface andthe flushing system used to compensate the evaporated water and toreduce salinity at surface.

3.1. Heat losses

The heat losses to the bottom and to the side wall of the pondwere monitored by means of temperature sensor outside and inside

Table 1Weather parameters measured at the experimental solar pond from November 2009 to Oc

Nov-09 Dec-09 Jan-10 Feb-10 Ma

Average monthly temperature (°C) 11.97 7.90 7.08 7.35Average minimum temperature (°C) 6.10 3.71 3.26 1.75Average maximum temperature (°C) 20.36 13.89 12.42 13.66 1Solar radiation (MJ/m2 month) 208.40 163.55 184.95 246.27 43Rain (L/m2) 2.60 53.00 44.80 62.30 5Wind average speed (m/s) 3.74 4.34 4.12 5.05

the pond (concrete wall) and can be estimated by the followingequations [14]:

Qw = ARw Tin−Toutð Þ ð3Þ

Qb = ARb Tin−Toutð Þ ð4Þ

where Qw and Qb are the total heat loss to the wall and to the bottom,respectively; A is the area; R is thermal resistance of the side orbottom wall and Tin and Tout are the inside and outside temperature,respectively. The heat losses to the side wall were monitored at twoheights (0.8 and 2.5 m from the bottom of the pond). Since the solarpond wall was insulated, then the heat losses were fairly notsignificant along the experiment as can be seen in Fig. 7. On theother hand the heat losses to the bottom of the pond were rangingbetween 5 and 15% of solar monthly radiation (also in Fig. 7). Heatlosses were expected, since the bottom of the pond was not insulated,with summer as the season that reaches the highest heat losses by thebottom. This fact confirms the necessity of a complete insulation of thepond in order to avoid heat losses that can reduce the pond capacityand the heat available for a given application. A typical solar pondwith a depth of 3 m would receive around 20 to 25% of the radiationincident upon the pond [29], which means that accounting losses tothe walls and bottom (0.3 and 9.7% of total solar radiation along oneyear), between 10 and 15% of the incoming radiation is available forextraction to an application. In this study, 5 GJ/m2/year was the totalsolar radiation, which indicates that between 25 and 40 GJ/year wereavailable for extraction.

3.2. Maintenance of the salinity gradient

A mass balance of the solar pond indicates that sodium chlorideand low-salinity water are required to maintain the salinity gradient[29]. In this sense a salt charger was constructed and installedfollowing the design procedure proposed by Jaefarzadeh andAkbarzadeh [22]. The bottom of the cylinder was designed to release

tober 2010.

r-10 Apr-10 May-10 Jun-10 Jul-10 Aug-10 Sep-10 Oct-10

9.86 14.29 16.67 21.50 26.77 25.27 21.42 15.883.96 10.98 10.44 14.95 19.92 19.33 15.49 10.296.63 22.16 24.10 29.32 35.22 33.02 29.12 23.746.33 529.43 552.03 549.72 610.73 535.14 396.94 302.818.80 14.00 101.70 46.60 2.80 1.10 13.20 31.405.11 5.08 5.09 5.33 6.10 5.33 4.65 4.72

Fig. 8. a) Salt charger b) flushing system and c) the acidification system installed in the solar pond in order to maintain the clarity.

449C. Valderrama et al. / Desalination 279 (2011) 445–450

salt by means of windows located at 0.80 m above the bottom, thus,the border between the LCZ and NCZ was determined by the saltcharger design. The salt charger installed (before to set-up the salinitygradient) empty and plenty of salt is shown in Fig. 8a. In this study, theamount of salt consumed to maintain the salinity gradient after oneyear was 2600 kg.

Additionally, a flushing system was also implemented in order tocompensate the losses caused by evaporation and to renovate thesurface water [2]. The average consumption of low-salinity water was3000 L/month and almost twice during summer season. The flushingsystem installed is shown in Fig. 8b, it can be observed that board onthe surface avoids to disturb the UCZ during the flushing procedure.

The pH and turbidity parameters were measured in order to controlthe clarity maintenance of the solar pond. An increase in turbidity iscaused bymany factors including dust, leaves and debris. Some of theseparticles settle to the bottom of the pond while others simply remainsuspended at some level in the gradient layer [2]. This is one of themost

0

2

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bottom 0,7m 1,5m1,9m 2,3m surface

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Nov-10

Tu

rbid

ity

(NT

U)

bottom 0,7m 1,5m1,9m 2,3m surface

Fig. 9. Evolution of pH and turbidity from October 2009 to October 2010 in the solarpond.

important elements in achieving high efficiency in collecting anddelivering heat at desired temperatures [29]. The acidification systemwas selected as a simple and effective strategy to control the growth ofalgae inside the pond. For this purpose a system delivering acid atdifferent heights was used in this work, thus, five tubswere installed onthe pond in order to deliver the acid (25 L of HCl (9%v/v)) from thesurface to different heights of the pond (0.2, 0.6, 0.95, 1.5 and 2.1 m) ascan be seen in Fig. 8c. The total amount of HCl added after one year ofcontinuous maintenance of the pond was 130 L. As a result, the pH waskept between 2 and 4 with exception of the bottom (due to acid notreached this depth) and surface (due to the surface renovation) wherepH reported values around6. The pH at different heights and as functionof time is shown in Fig. 9. It is important to point out that nometalswereput inside thepond in order to avoid corrosionof thismaterial at lowpH.On the other hand, the turbidity was also reduced to values around 2NTU. It should be pointed out that at the beginning of the spring theturbidity increased and the frequency of the acidification was con-sequently increased. Thus, the turbidity values were reduced above 2NTUatmiddle of summer. The evolution of turbidity on the solar pond isalso shown in Fig. 9.

The future prospects of the project are the heat extraction processby means of heat exchangers installed inside the pond (NCZ and LCZ).These experiments will take place during the next spring season.

4. Conclusions

The diffusor design used to establish the salinity gradient allowed toincrease the flow-rate and complete the injection process with arecalculated Froude number of 11. The salinity gradient demonstratedhigher stability after one year of continuous operation and underdifferentweather conditions. Only the surface zone reported variability,which is expected since the renovation or compensation of low-salinitywater is continuously required in this zone. Furthermore, the gradientstability demonstrates the efficient maintenance and control measures(i.e. salt charger and flushing system). The pH and turbidity control bymeans acidification at different heights of theponddemonstrates to be asimple and effective method to control these two parameters. Howevera deep study in this area is required since even after acidificationprocedure some algae still remain inside the pond.

The maximum temperature was observed at the NCZ and thentemperature decreased at the LCZ. This fact can be related to the lackof a suitable insulation of the slab which penalizes the heat storage inthe bottom. Further, around 10% of total solar radiation along one yearis quantified as heat losses to the bottom of the pond, while heatlosses to side wall were not significant with less of 0.3% of total solarradiation.

450 C. Valderrama et al. / Desalination 279 (2011) 445–450

NomenclatureA Area (m)B vertical gap dimension of the diffuser (m)Fr Froude numberG acceleration due to gravity (m/s2)Q total heat loss (MJ)R thermal resistance of the side or bottom wall (°C/MJ)T temperature (°C)V injection velocity at the diffuser outlet (m/s)Δρ density difference between the injected fluid and the

surrounding fluid (kg/m3)δsensor error due to the sensorδinstrument error due to the measuring instrumentδtotal total error estimationρ density of the surrounding saline fluid (kg/m3)

Subscriptsb bottomin insideout outsidew wall

Acknowledgments

The authors gratefully acknowledge M. Degouys, R. Onandia,F. Franco and personnel from Solvay-Martorell facilities for practicalassistance in the construction and maintenance of the solar pond;N. Gasulla for helping settling the salinity gradient and J. Flores, andC. Aladjem for their helpful discussions and valuable cooperation. Thisresearch was financially supported by the Spanish Centre for theDevelopment of Industrial Technology (CDTI) and Aigües de Barce-lona (Agbar) through the SOSTAQUA project (CEN-20071039). Theauthors are also grateful to the three anonymous reviewers for theirconstructive criticism of the original paper.

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