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Water and chemical savings in cooling towers by using membrane

capacitive deionization

B. van Limpt a,,1, A. van der Wal a,b,2

a Voltea BV, Wasbeekerlaan 24, 2171 AE Sassenheim, The Netherlandsb Department of Environmental Technology, Wageningen University, Bornse Weilanden 9, 6708 WG Wageningen, The Netherlands

H I G H L I G H T S

Desalination of cooling tower feed stream with MCDI MCDI enabled signicant chemicals, water and waste water savings.

MCDI showed lower energy use and lower fouling susceptibility compared to RO.

Observed preferential uptake of chloride and calcium can further increase savings.

a b s t r a c ta r t i c l e i n f o

Article history:

Received 2 September 2013

Received in revised form 2 December 2013

Accepted 16 December 2013

Available online 21 January 2014

Keywords:

Membrane capacitive deionization

Cooling towers

Preferential ion uptake

Water savings

Chemical savings

Membrane capacitive deionization (MCDI) is a water desalination technology based on applying a voltage

difference between two oppositely placed porous carbon electrodes. In front of each electrode, an ion exchange

membrane is positioned, andbetween them, a spaceris situated, which transportsthe water to be desalinated. In

thisstudywe determined thewater andchemicalsavings that canbe achieved in a coolingtower by desalinating

the feed water stream with a full-scale MCDI system. By monitoring the water use of the cooling tower, and

comparing this to a scenario without MCDI, chemical savings up to 85% could be achieved. Additionally, water

savings up to 28%, and waste water savings up to 48% could be achieved. MCDI energy use for desalination of

cooling tower feed water was between 0.1 and 0.2 kWh per cubic meter of produced desalinated water.

Preferential uptake of chloride and calcium was observed, which lowers the risk of scaling and corrosion in the

cooling tower and allows for further chemical and water savings.

2014 Elsevier B.V. All rights reserved.

1. Introduction

Cooling towers are commonly used to dissipate heat from industrial

processes. This heat is stored in the recirculation water of the cooling

tower, and is released by evaporation. As water evaporates, the solids

that are dissolved in the water are left behind while new water is

drawn into the cooling tower. As a consequence, salt concentrations in

the cooling tower increase over time. This can result in corrosion dueto high concentrations of chloride, and in scaling due to high concentra-

tions of calcium and alkalinity. To reduce scaling and corrosion in the

cooling tower, it is common practice to add chemicals such as

antiscalants and corrosion inhibitors to the recirculation water, and to

discharge the cooling tower water in a blowdown stream when the

conductivity reaches a certain threshold value. This leads to a discharge

of large volumes of waste water containing high levels of chemicals,

which can present a signicant environmental hazard when not

disposed of properly.

Reduction of chemicals use in cooling towers can be achieved by

removing thesalts from therecirculation water,whichresults in a larger

fraction of the recirculation water that can be evaporated before the

conductivity threshold value is reached. Consequently, less frequent

blowdown is needed, and therefore chemicals and water can be saved.Salt removal from the recirculation water can be achieved as

follows; 1) deionizing the recirculation water; 2) deionizing the

blowdown water,or 3) deionizing the feed water that entersthe cooling

tower.

The use of reverse osmosis (RO) [13] has been suggested for deion-

ization of the cooling tower recirculation water and for deionization of

the blowdown water. However, the use of RO is not viable because of

low water recoveries as well as other technical drawbacks, such as silica

scaling on the membranes.Furthermore, the cooling tower recirculation

and blowdown stream contain high levels of foulants, requiring exten-

sive preltration steps such as ultraltration to prevent membrane

fouling[2,3].

Desalination 342 (2014) 148155

Corresponding author.

E-mail addresses:[email protected](B. van Limpt),

[email protected](A. van der Wal).1 Tel.: + 31 252 2001 18.2 Tel.: +31 252 2001 43.

0011-9164/$ see front matter 2014 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.desal.2013.12.022

Contents lists available at ScienceDirect

Desalination

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An alternative method we propose in this work is to reduce the

chemicals and water use of a cooling tower by deionizing the feed

water by membrane capacitive deionization (MCDI). MCDI is an emerg-

ing desalination technology that uses capacitive electrodes and ion

exchange membranes to effectively remove ions such as chloride and

calcium ions from various water sources, such as tap-, well- and even

seawater [410]. Thekey difference of this technology with other capac-

itive deionization (CDI) technologies is the use of ion exchange mem-

branes. With the use of ion exchange membranes in MCDI muchhigher ion removal efciencies and water recoveries can be reached

[5,6,8,11,10]. In addition, the ion exchange membranes increase the

ion storage capacity of the carbon electrodes by up to 40% because of

the selectivity in transport for either anions or cations [5,6,8]. Further-

more, themembranes reducethe sensitivity of the electrodes for scaling

and fouling by forming a physical barrier between the fouling sensitive

electrodes and the spacer channel.

There are four main advantages of MCDI over RO for cooling tower

desalination. The rst advantage is that MCDI is not susceptible to silica

scaling, since at neutral or acidic pH silica is not charged, and therefore

does not interact with MCDI. As a consequence, silica is left in the

cooling tower water, where it can act as a corrosion inhibitor[12,13].

The second advantage is the high water recovery that is obtained with

MCDI, recoveries over 80% are possible, resulting in more efcient use

of the feed water and higher overall water savings in the cooling

tower. The third advantage is that by deionizing the feed water stream

of a cooling tower the preltration and antiscalant requirements are

reduced, since the feed water stream contains much less foulants and

scalants than the recirculation stream. The fourth advantage is the low

energy use of MCDI, allowing for lower operational costs.

In this work we demonstrate the application of MCDI at two cooling

tower sites, one operating on softened water and one operating on

unsoftened water. The relevant water streams of the cooling tower

equipped with MCDI and the MCDI energy use were measured for up

to one year. By comparing the cooling tower operation to a scenario

without MCDI, we demonstrate the amount of water savings that can

be achievedby using MCDI. Furthermore,we demonstrate thelow ener-

gy requirement for MCDI by comparing the measured energy use with

state-of-the-art brackish water RO systems and with literature values

for MCDI. Finally, a critical analysis is made on the preferential uptake

of ions by MCDI with two different feed water types, where we

determine the possible impact on water and chemical savings.

2. Materials and methods

2.1. MCDI design

Fig. 1 shows a schematic overview of a MCDI unit cell, which consists

of two porous carbon electrodes separated from each other by a spacer.

Each of the electrodes is 250 +/ 50 m in thickness and has a specic

weight of 0.5 g/cm3. On top of the electrodes, ion exchange membranes

are placed, whereby the anion exchange membrane is placed on top of

the anode, and the cation exchange membrane on top of the cathode.

The electrodes are connected to thin graphite sheets, called current

collectors. These current collectors serve as electrical conductors to

facilitate the charge transport into and out of the electrodes. The spacer

is porous and acts as a ow channel between both membranes, for

the transport of the feed water. The anion exchange membrane on

the anode only allows for the transport of anions into the anode. At

the same time, the cation exchange membrane on the cathode only

allows for the transport of cations into the cathode.Fig. 2shows the construction of stacks and a module from unit cells

The unit cells are square with a central hole, which allows for an

outside-in ow. The unit cells are combined to form a stack, subse-

quently the stacks are placed on top of each other and placed inside a

housing, to form a module. Each module contains approximately 6 m2

of cell area. A water inlet is positioned on the outer edge of the module

and allows water to reach the outsides of the individual cells. A water

outlet is positioned on the top center of the module, and is connected

to the insides of the unit cells. By applying a pressure difference

between the inlet and the outlet of the module, water will ow via the

spacers through the unit cells, where it becomes deionized. The current

collectors of all separate cells in the module are connected in parallel,

allowing for only two leads to exit the module, one lead connecting to

all anodes, and one lead connecting to all cathodes. These leads are

connected to a power supply which output is limited to 1.2 V to prevent

undesired side reactions such as water splitting.

2.2. MCDI operation

The operation of MCDI consists of two steps. In the rst step, also

called deionization step, ions are removed from the feed water and

stored inside the electrodes. Therst step starts when a potential differ-

ence is applied over the cell. This creates a driving force for ions to

migrate from the water in the spacer compartment through the ion

exchange membranes into the porous electrode surface. The ions are

adsorbed at the carbonwater interface and stored in the electrical

double-layers of the porous electrodes. This results in a decrease of

ions in the spacer compartment, therefore deionizing the salineinuent

water.

Once the electrodes arelled with ions, they need to be regenerated

This is done in a second step, also called regenerationstep. By reversing

the polarity of the applied potential difference over the cell, the ions

stored in the electrodes will migrate back into the spacer compartment

This results in an increase of the concentration in the spacer compart-

ment, which isushed out of the MCDI cell as a concentrate stream.

The operation of MCDI was done under constant current conditions

meaning that during the purication step and the regeneration step a

constant current was applied to the unit cell, and that the absolute

amount of charge transported is the same in both steps. Duringpurica-

tion the constant current operationallows for a constant removal of ions

from the feed stream, which combined with a constant ow of water

through the unit cell gives a constant quality of the deionized water

Spacer

Porous carbon electrode

Porous carbon electrode

Cation

Anion

Saline influent Deionized effluent

Anion exchange membrane

Cation exchange membrane

Current collector

Current collector

Fig. 1.Schematic representation of the different components of a MCDI cell [14].

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During regeneration the direction of the applied current is reversed,

which together with adjustments of the concentrate ow allows for a

precise control of the concentration of calcium and carbonate in the

waste stream. This prevents the precipitation of calcium and carbonate,

and therefore prevents scaling in the MCDI cell.

Since the concentration of salts in the spacer is higher during regen-

eration as compared to purication, the electrical resistance of the

module during regeneration is lower [15]. This allows for higher

currents during regeneration, which results in relatively short regener-

ation times, and consequently in a higher volume of deionized water

produced per unit time.

2.3. Cooling tower design and operation

MCDI modules were installedon the water inlet of a cooling tower at

two sites. The rst site (site 1) is a Unilever site at Pratau, Germany,

whereby a cooling demand of up to 500 kW is delivered by an evapora-

tive cooling tower (KTK Khlturm Karlsruhe GmbH, Type K12-28-3).

Annually, over 2500 m3 of softened city water is used as make up

water for the cooling system to compensate for evaporation and blow-

down. This site was followed for 1 year, consisting of a startup period

of 2 months in which the MCDI operation wasoptimized, and an evalu-

ation period of 10 months. Only the evaluation period was considered

in determining water and chemical savings.

The second site (site 2) is an ofce building in the city of Utrecht, the

Netherlands,whereby a cooling demand of up to 4500 kW is delivered by

an evaporative cooling tower (Gea Polacell, Type CMS 20-DH-90-D/3).

Annually over 10 000 m3 of unsoftened city water is used as make up

water for the cooling system to compensate for evaporation and blow-

down. This site was followed for half a year, consisting of a startup period

of 2 months and an evaluation period of 4 months.

Fig. 3shows a schematic overview of the cooling tower congura-

tion

tted with MCDI technology. At site 1 softened city water wasused as feed steam, whereas for site 2 unsoftened city water was used.

For both sites the feed stream was rst treated in a preltration train

consisting of a nominal 25 micron bag lter (BP-420-25 Pentek), a

nominal 5 micron baglter (BP-420-5 Pentek) and a nominal 1 micron

cartridge lter (ECP1-10BB Pentek) to protect the MCDI modules from

suspended particles in the water.

The MCDI used in this study consists for site 1 of two parallel mod-

ules, and for site 2 of four parallel modules. Valves are used to redirect

the deionized efuent into a buffer tank, and the concentrate stream

to the sewer. A ow controller was used to regulate the ow during

the concentrate and the deionization step. The valves, power supply

andow controllers were regulated by a programmable logic control

box (Siemens SIMATIC S7-1200).

The deionized water from the MCDI was led into a buffer tank,

where chemicals such as biocides, scale inhibitors and corrosion inhibi-

tors were added, to prevent biofouling, scaling and corrosion in the

cooling tower. A sensor was included in the buffer tank to measure

the conductivity of the recirculation water in the cooling tower. Once

this sensor reads a conductivity value above a certain value, the thresh-

old value, a part of the recirculation water was discharged from the

cooling tower into a blowdown stream. This blowdown stream was

discharged into the sewer. Level controllers were included in the buffer

tank to allow the MCDI deionized stream to dilute the recirculation

water and consequently to prevent scaling and corrosion in the cooling

tower. A bypass was included to allow feed water to directly enter the

buffer tank in case the cooling tower water demand exceeded the

MCDI deionization capacity.

Table 1shows a short summary of the differences in operation

between sites 1 and 2.Table 1shows that as the feed conductivity is

lower, less current is needed to achieve 70% removal, as in MCDI the

conductivity removal is directly proportional to the applied current.

Water recoveries were adjusted to controlthe concentrations of calcium

Fig. 2. Schematicof construction of a module, showingunit cellscombinedto stacks,stacks

combined toa module, andpositionof connections towatersupply andpower supply.The

construction of each individual cell is described inFig. 1.

Fig. 3.Schematic of the cooling tower conguration equipped with MCDI. Solid lines () indicate water ows, dotted lines () indicate control lines, and double triangles ( ) indicate valves.

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and alkalinity in the waste stream to such levels that precipitation of

calcium carbonate is prevented. Regeneration current settings were

increased by 1 A compared to the required current for charge balance

to ensure complete discharge of the electrodes during regeneration.

To compare the removal of individual water species versus the total

conductivity removal, a water composition analysis was performed on

the feed stream and the deionized stream. The analysis was performed

by Aqualab Zuid BV (Werkendam, The Netherlands), using inductively

coupled plasma mass spectrometry (ICP-MS) to obtain concentrations

of the relevant cations and metals, ion chromatography (IC) to obtain

concentrations of the relevant anions, titration to obtain the chemical

oxygen demand (COD) and spectrometry to obtain the silica concentra-

tion (SiO2). Furthermore, the concentration of total dissolved solids

(TDS) was calculated from individual ion species concentrations.

The MCDI system was equipped with voltage and current sensors

to determine the energy use per cubic meter of produced water. To

compare the energy use with literature values for CDI and MCDI, the

energy use was normalized on the TDS reduction in the deionized

water to obtain the energy use in kJ/g TDS reduction, as this was

found to be relatively independent of feed composition[1517].

2.4. Scenario calculations

To determine water,blowdown and waste water savings with MCDI,

the actual operation where MCDI was used to treat the ingoing water

was compared to a scenario where the cooling demand is the same,

but where the feed water is not deionized by MCDI.In the situation with MCDI, the total water volume was measured

with a water meter in thefeed stream. The volume of the MCDI concen-

trate stream was dependent on the water recovery (WR) of the MCDI,

which is dened as the ratio between the volume of water of the

deionized stream (VDI) and the feed stream (VF), according to

Eqs.(1a) and (1b).

WRVDIVF

1a

where

VFVDIVC 1b

whereVCis the volume of water in the concentrate stream.

The cycles-of-concentration (CoC) in the cooling tower is dened as

the ratio between the threshold conductivity setpoint for blowdown

(BD) and the conductivity of the deionized stream (DI), according to

Eq.(2).

CoCBD

DI

2

For site 1 BD was set at 1400 S/cm, for site 2 it was set at

1020 S/cm. For both sites DIwas measured by a conductivity

meter located in the deionized stream.

From the CoC, the blowdown volume (VBD) and the evaporation

volumeVEcan be calculated, according to Eqs.(3) and (4).

VBD 1

CoCVDI 3

VE 1 1

CoC VDI 4

Thetotal waste volume (VTW)isdened asthe sum of the blowdown

volume (VBD) and the MCDI concentrate volume (VC) according to

Eq.(5).

VTWVBDVC 5

To compare the situation with MCDI to a scenario without MCDI, an

equal cooling demand, and therefore an equal evaporation volumeVEwas assumed. Using Eqs.(6) and (7), feed volume (VF*) and blowdown

volume (VBD*) in the scenario without MCDI could be determined.

VF VE= 1 1

CoC

6

VBD 1

CoCVF 7

The total waste volume in the situation without MCDI is equal to the

blowdown volume, since without MCDI no concentrate stream is

present. The cycles-of-concentration (CoC*) in the situation without

MCDI is equal to the ratio between the blowdown threshold conductiv-

ity and theconductivity inthe feed stream, which for site1 was 2.15and

for site 2 was 2.9.

Table 1

Comparison differences in operation between the two sites.

Parameter Unit Site 1 Site 2

Feed conductivity mS cm1 0.65 0.35

Conductivity removal % 70% 70%

Water recovery % 84% 80%

Deionization current A/module 60 28

Regeneration current A/module 91 43

Deionizationow L/min/module 7 7

Regenerationow L/min/module 1.5 1.7

a b

Fig. 4. Representative deionizationregeneration cycle of MCDI operation at site 1, showing the applied module voltage andthe resulting current (a) and outlet conductivity and module

ow (b) during a representative deionization and a regeneration cycle. Feed conductivity was approximately 0.6 mS/cm, regeneration conductivity reached approximately 2.5 mScm1

(not shown in Fig. 4b).

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3. Results and discussion

3.1. MCDI operation

Fig. 4a shows a representative voltage and a current prole dur-

ing a deionizationregeneration cycle of the MCDI operation at site

1 after 9 months into the evaluation period; Fig. 4b shows the

outlet conductivity and module ow during the same deioniza-

tionregeneration cycle. The sign of the current inFig. 4a indicates

the direction of charge transport, when the current is positive the

module is charging, resulting in deionization, and when the current

is negative the module is discharging, resulting in regeneration.

Fig. 4a further shows that in the last 10 s of regeneration the current

was no longer constant, as the voltage limit of the power supply was

reached. This is due to the set increased currents during regenera-

tion, causing the electrodes to be completely discharged prior to

reaching the end of the regeneration step.

As can be seen inFig. 4b, during deionization the feed conductivity

was reduced from approximately 0.60 mS cm1 to a conductivity of

approximately 0.18 mS cm1 in the deionized water, corresponding

to a conductivity removal of 70%. During the regeneration step the

water ow was reduced to 1.6 L/min to increase water recovery and

the conductivity reached approximately 2.5 mS cm1 (not shown).

During the last 5 s of regeneration, a positive current was applied and

theow was increased to ensure that the outlet water conductivity is

sufciently low when starting a new deionization step. Water recovery

calculated with Eqs.(1a) and (1b)is equal to 83%.

3.2. Cooling tower operation with MCDI

Fig. 5 shows the puried water conductivity and the achieved water

recovery of the cooling tower operations with MCDI at sites 1 and 2

during the evaluation period. The water recovery and the deionized

conductivity for site 1 remained approximately constant during the

evaluation period. For site 2, water recovery and conductivity removal

were increased by approximately 5% during the evaluation period to

further optimize chemicals and water savings.Fig. 6shows the measured total water use during the evaluation

period when MCDI was used, compared to the total water use in a sce-

nario without MCDI, according to Eq. (6).Fig. 7shows the blowdown

and total waste water during the evaluation period, according to

Eqs. (3) and (4), compared to the total waste (blowdown) in a scenario

without MCDI, according to Eq.(7).

Figs. 6 and 7show a signicant increase in water use and waste

water discharge for site 2 after day 60. This coincides with the start of

a period of high temperature, resulting in a higher cooling demand.

Onthe otherhand, the cooling tower on site 1 wasused for an industrial

process, therefore cooling demand was more constant throughout the

year.

Table 2gives an overview of the total water, chemicals and waste

water savings for the two cooling tower sites over the entire evaluation

period, together with the average energy use for each site.

Water and waste water savings were higher for site 1 compared to

site 2, which is mainly due to the use of softened feed water at site 1,

which allows for higher water recovery. For site 2 a higher chemical

savings were achieved due to the lower conductivity of the deionized

stream at site 2, which results in a higher CoCin the cooling tower and

thus a lower blowdown volume.

Energy use for MCDI was calculated from the applied currents and

resulting voltages during regeneration and purication, whereby

lower applied currents result in lower energy use[15,18]. In this work,

high regeneration currents were used, to minimize regeneration time

and to achieve a high net water production. When only considering

the energy used in the purication cycle, the energy use is signi-

cantly lower, 1.3 kJ g1 TDS reduction for site 1 and 1.1 kJ g1

TDS reduction for site 2. This is comparable to previously reported

values of 0.91.5 kJ g1 TDS reduction[15].

Energyuse forMCDI pervolume of deionizedwaterwas between0.1

and 0.2 kWh m3, which compares favorably compared to an energy

use of 0.861.55 kWh m3 deionized water for RO treatment of brack-

ish water [19]. Energyuse canbe further reduced by reducing theregen-

eration currents and by using energy recovery during the regeneration

step[15].

Fig. 5.MCDI puried water conductivity together with MCDI water recovery during the

evaluation period for both sites.

Fig. 6.MCDI blowdown and total waste water discharge during evaluation period compared to the blowdown discharge in a scenario without MCDI.

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Thenancial benets of using MCDI for cooling towers can be deter-

mined by comparing the achieved water and chemical savings to the

capital and operational expenditures in a nancial model, which for

both sites shows favorable returns of investments for the site owner.

3.3. Preferential ion uptake

Table 3compares the feed compositions, deionized water composi-

tions, and removal of individual species in the deionized stream. At

both sites, no substantial amounts of metals were present (b0.5 mg/L).

Table 3shows that the removal of certain ion species was higher as

compared to the average conductivity removal (highlighted in

Table 3), indicating that these ions show preferential uptake in MCDI.

As a consequence, other ions were removed to a lesser extent. Both

sites showed strong preferential uptake for monovalent anions, andsignicantly reduced uptake of sulfate. Site two showed preferential

uptake of the bivalent hardness ions, and reduced uptake of potassium

and sodium ions.

Silica was not removed from the feed water, since silica is not

charged at the pH values observed during operation. COD was only

removed for 22% for site 1, and for at least 29% for site 2. This is likely

due to the removal of charged, low molecular weight hydrophilic

organic compounds which are able to pass the membrane.

The cause for preferential uptake of ions in MCDI has not been

described before, but it is well understood for electrodeionization

processes without membranes (CDI).

Preferentialuptakeof ions in CDI is determined by thecharge (z)and

the hydrated radius of the ions (rh); small, multivalent ionsare energet-

ically more favorably stored in the double-layer than large-monovalentions [20,16,21], as can be derived from the GouyChapmanStern

model[22]. This agrees with the order of cation uptake observed in

this work, the bivalent cations calcium and magnesium show preferen-

tial uptake over the monovalent cations, and the smaller ion calcium

(rh= 0.412 [23]) shows preferential uptake versus magnesium

(rh= 0.428[23]), which is also observed for potassium (rh= 0.331

[23]) versus sodium (rh= 0.358[23]).

Surprisingly, the order of anion uptake for CDI does not follow the

observations, where at both sites the bivalent anion sulfate was taken

upto a signicantlylower extentthan the monovalent anions. An expla-

nation for this discrepancy is the use of the ion exchange membranes in

MCDI. As described by Sata et. al.[24], anion selectivity in the anion

exchange membrane (AEM) is primarily determined by the degree of

hydration of the transported ion and secondary by the hydrophobicity

of the AEM, whereby the Gibbs free energy of hydration ( Gh0) of

the ion that is transported correlates with the anion selectivity

[24,25]. When ions are transported through the AEM they need to par-

tially lose their hydration shell. Less hydrated anions such as chloride

( Gh0 = 363 kJ mol1 [26]) are therefore more easily transported

through the membrane over strongly hydrated anions such as sulfate

( Gh0 = 1145 kJ mol1 [26]).

Further research with different cation and anion exchange mem-

branes is required to determine the impact of the used ion exchange

membranes on selective ion uptake. For example, use of a monovalent

selective CEM in MCDI is expected to show an increased uptake of

monovalent cations[27]; use of a highly crosslinked CEM in MCDI is

expected to show increased uptake for cations with a small hydrated

radius[28]; and use of a highly hydrophilic AEM is expected to show

increased uptake of highly hydrated anions such as sulfate[24].

Other effects that could inuence preferential uptake include chem-

isorption, whereby in therst few cycles ions may be adsorbed onto thecarbon surface which are not regenerated on reversal of polarity[29,9]

As the measurements were taken in a steady state operation, the effect

of chemisorption in our analysis will be negligible.

The impact of preferential uptake of chloride and calcium is demon-

strated for site number 2, where the conductivity removal was 66%

whereas calcium and chloride removal was at least 73%. This means

that the deionized water stream, and thus the recirculation water

stream, contains 20% less calcium and chloride compared to the reduc-

tion of conductivity of the water, resulting in a lower risk of scaling and

corrosion in the cooling tower. When allowing for an equal scaling and

corrosion risk in the cooling tower, the conductivity threshold of site 2

could be increased from 1020 uS/cm to 1275 uS/cm. This would result

in a further 5% reduction in chemicals use, 13% lower discharge of

waste water and 17% lower use of feed water.

Fig. 7.Measured MCDI water use during evaluation period compared to required water use in a scenario without MCDI.

Table 2

Total savings and energy use for the two cooling tower sites over the entire evaluation

period, per cubic meter of puried water and normalized for the TDS reduction, which

was 0.329 g TDS/L for site 1, and 0.186 g TDS/L for site 2.

Site 1 Site 2

Chemical savings 78% 85%

Water savings 28% 12%

Waste water savings 48% 32%

Energy use

kWh m-3 puried water 0.234 0.105

kJ g1 TDS reduction 2.6 2.2

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4. Conclusion

By using MCDI to deionize the feed water stream of cooling towers

we have demonstrated chemical savings of up to 85%, waste water

savings up to 48% and water savings up to 28%.

Further chemical and water savings are possible by considering that

MCDI shows preferential uptake of calcium and chloride, which results

in lower levels of calcium and chloride in the recirculation water, hence

reducing the risk of scaling and corrosion.

The cause for thepreferential uptake of ions in MCDI is likely to be de-

termined by the use of ion exchange membranes. Further increases in

chemicals and water savings are possible by selecting ionexchange mem-

branes that show an even higher selectivity for calcium and chloride.

Further water and waste water savings are possible by increasing

the water recovery of the MCDI. In the current study it was set at 80%

to prevent calcium carbonate precipitation in the MCDI. A possible

solution is to move the antiscalant dosing from inside the cooling

tower to the inlet of the MCDI. Most antiscalants are organic chemicals,

which, as shown in this work, are only removed at low levels by MCDI.

This means that by dosing antiscalants in front of the MCDI, increased

water recovery for MCDI can be achieved as well as further water

savings in the cooling tower.

As expected, silica was not taken up by the MCDI and therefore no

silica fouling was observed in the MCDI nor in the cooling tower during

the evaluation period. Further research is needed to determine the

relation between the increased silica content in the cooling tower and

inhibition of corrosion, as this can further reduce the need for corrosion

inhibitor dosing.

The average energy use for the site with softened water was0.234 kWh m3 produced water, and 0.105 kWh m3 produced

water for the site with unsoftened water, which compares favorably

with the average energy use of RO for water with low TDS. Further re-

duction in energy use is possible when using lower regeneration cur-

rents and by implementing energy recovery during the MCDI

regeneration step.

Acknowledgments

Thedevelopmentof the MCDI cooling tower application wasexecut-

ed within the AquatForUse EU project, supported by the European

Commission under the Seventh Framework Programme (Contract nr.

211534).

The authors thank Josh Summers for assistance in the experimental

work on preferential ion uptake.

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Table 3

Composition of feed water and deionized water together with calculated removal for both investigated sites. The ions that show preferential uptake (Cl, NO3, HCO3

and Ca2+, Mg2+)

are highlighted.

Feed water Deionized water Removal

Site 1 Site 2 Site 1 Site 2 Site 1 Site 2

Bulk parameters

pH 7.5 7.9 6.7 7.5

Conductivity (S/cm) 633 375 224 127 65% 66%

TDS 503 294 174 108 65% 64%

Anions (mg/L)

SO42 116 20 69 9.0 41% 55%

Cl 25 38 b2 10 N92% 73%

NO3 1.3 2.0 0.4 0.5 69% 75%

HCO3 191 144 35 49 82% 66%

Cations (mg/L)

Ca2+ b0.5 46 b0.5 12 74%

Mg2+ b0.1 5.1 b0.1 1.5 71%

K+ b0.5 2.1 b0.5 0.8 62%

Na+ 150 22 47 10 69% 55%

Others (mg/L)

COD (as O2) 9 7 7 b5 22% N29%

SiO2 19.8 15.2 19 15 3% 1%

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