Experimental evaluation on concentrating cooling towerblowdown water by direct contact membrane distillation

Xianguo Yu a, Hai Yang a, Hui Lei b, Andrew Shapiro c,⁎a Separation & System Lab, GE Global Research, China Technology Center, Shanghai 201203, Chinab Coating & Membrane Technology Lab, GE Global Research, China Technology Center, Shanghai 201203, Chinac Thermal Systems, GE Global Research, One Research Circle, Niskayuna, NY 12309-1027, USA

H I G H L I G H T S

► MD process is feasible for desalting simulated cooling tower blowdown water.► Conductivity limit during the concentration process was about 17000 μS/cm.► Above this concentration, large amount of CaCO3 scale formed on the membrane.► Antiscalant was effective at preventing silica, carbonate and sulfate scale.► A cleaning procedure was developed to recover membrane performance after scaling.

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

Article history:Received 6 June 2012Received in revised form 25 January 2013Accepted 28 January 2013Available online 28 February 2013

Keywords:Membrane distillationDesalinationCooling tower blowdownConcentration

Concentration of simulated cooling tower blowdown water (CTBD) by the desalination process of direct con-tact membrane distillation (DCMD) has been evaluated. A bench-scale DCMD setup was used to test the de-salination performance. Both silica-free and silica-containing simulated CTBD water were prepared andconcentrated. The concentration process using DCMD produced a permeate flux of about 30 L/m2·h and asalt rejection of more than 99.95% under feed side temperature of about 60 °C. Membrane scaling was exam-ined for the simulated feeds. It was exhibited that insoluble calcium carbonate scale formed on membraneunder concentration factor of about 3.7–4.0 for silica-free simulated CTBD water. Silica, calcium carbonateand sulfate scaling precipitated together under concentration factor of about 3.2–5.0 for silica-containingsimulated CTBD water. The scales resulted in the drop of both permeate flux and salt rejection, while the per-formance was recovered after membrane cleaning. The addition of antiscalant enhanced the concentrationfactor to about 8.0 and the corresponding water recovery to about 87%. The experimental results indicatedthat MD is a potential feasible technology for the water recovery of CTBD water.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Water plays critical functions in modern thermoelectric powerindustry [1]. It works as the physical source of energy conversionand the medium of heat exchange. In a typical steam-driven andwet cooling tower power plant, two significant water loops areemployed. One closed loop of high-quality boiler water is convertedto steam and drives a steam turbine which powers an electrical gen-erator. Another recirculating loop of water extracts heat from thecondenser on the low pressure side of the steam turbine, and dissi-pates heat in the wet cooling tower through evaporation. A largeamount of water is consumed to support the power generation andthe majority comes from the recirculating cooling loop [2]. Coolingtower water is typically withdrawn from a freshwater source. Due

to the water loss through tower evaporation, leakage and wind ac-tion, the concentrations of ions in the cooling water loop such asCa2+, Mg2+, CO3

2− and HCO3−, silica, microorganisms and chemicals

increase, which may lead to scale formation or corrosion. To avoidscaling and corrosion, concentrated water is discharged as blowdownand freshwater is supplied as make-up to the tower [3]. For example,one 300 MW power generator required about 20,000 m3/h circulat-ing cooling water and produced about 98 m3/h [4] of blowdown. Typ-ically the blowdown accounts for 10–20% of the consumed water; therest is lost to evaporation.

Freshwater scarcity, environment protection and cost reduction aredrivers to treat and reuse the large volume of blowdown water. Chemi-cal coagulation such as lime-soda softening [5] and brine thermal evap-oration [6] are the typical traditional treatment approaches for coolingtower blowdown (CTBD)water. Lime-soda softening removes hardness,but it also consumes chemicals and suffers from the sludge disposal.Thermal evaporation by mechanical vapor recompression is able to

Desalination 323 (2013) 134–141

⁎ Corresponding author. Tel.: +1 518 387 4753; fax: +1 518 387 7258.E-mail address: shapiro@research.ge.com (A. Shapiro).

0011-9164/$ – see front matter © 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.desal.2013.01.029

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Desalination

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produce high quality distillate and reaches high water recovery whileconsuming large amounts of electricity (~20–25 kwh/m3). In recentyears, membrane technologies as alternative treatment processes haveattracted considerable attention. Microfiltration (MF), ultrafiltration(UF) and nanofiltration (NF) were attempted as pretreatment processes[4,7,8] to remove particles, bacteria, colloids, and silica. In practical appli-cations the membrane fouling problems still need to be further solvedand the cost needs to be reduced. At the same time, the desalination re-lied on reverse osmosis (RO) [9,10]. RO is so far the most reliable andcost effective membrane desalting technology. It produces relativelyhigh quality water with less energy consumption than thermal distilla-tion systems. However, the complicated compositions contained in thecooling tower blowdown, such as chemicals, residual chlorine, silicaand other scaling minerals negatively affect RO performance. The resid-ual chlorine or chlorine oxide can decompose RO membrane [11]. AndRO membranes have low resistance for colloidal or reactive silica. Also,fouling or scaling is apt to take place [12]. Finally, RO is not appropriatefor high concentration water because of the high osmotic pressure andthe rejection of all dissolved species is not always sufficient [13].

The above mentioned shortcomings of RO are possibly overcomeby an emerging desalination technology: membrane distillation(MD). MD combines the use of both thermal distillation and mem-brane separation. Its fundamental separation mechanism is basedon vapor–liquid equilibrium theory [14,15]. A hydrophobic micropo-rous membrane supports the MD desalination process. The drivingforce for mass transport in MD derives from the temperature differ-ence across the membrane, which results in vapor pressure differ-ence. The hot feed side needs to contact with membrane, but thevapor penetrating through micropores can be transported by differ-ent mediums, such as water, vacuum, air and gas. Those correspondto direct contact MD (DCMD), vacuum MD (VMD), air gap MD(AGMD) and sweeping gas MD (SGMD) respectively. Compared toother desalination processes such as RO and thermal evaporation,the advantages of MD are [16]: (i) lower operating temperature andvapor space required than conventional distillation; (ii) lower operat-ing pressure than RO; (iii) nearly 100% salt rejection of non-volatilesolutes; (iv) unlimited by high osmotic pressure; and (v) lower ener-gy costs in cases of using waste or low-grade heat. Finally, the mem-brane material for MD is hydrophobic and typically chemically inert.Materials such as polytetrafluoroethylene (PTFE) and polypropylene(PP) are commonly used for MD, and therefore MD is not highly sen-sitive to oxidants and some polymers and chemicals.

Thermoelectric power plants have residual heat such as the steamdischarged by a backpressure turbine [17] and the low-grade heatfrom steam/water separation. Membrane distillation leveraging wasteheat in coal fired power plants for the saline water recovery has alsobeen reported [18]. The energy streams for driving membrane distilla-tion include boiler blowdown, steam diverted from bleed streams andthe cooling water system. But it is not viable to use steam divertedfrombleed streams because thatwill affect the stable power generation.The cooling recirculating water (i.e. blowdown) from a condenser gen-erally kept a temperature range at about 8–40 °Cwith the seasonal var-iation, which is not hot enough for MD usage directly. Boiler blowdowncan have a high temperature of about 260 °C. It is possible to make useof the boiler blowdown for heating the CTBD to a temperature above50–60 °C through MD with special configuration such as multi-effective membrane distillation (MEMD).

As a thermally-driven membrane separation process, the imple-mentation of MD encounters the obstacles of membrane scaling andfouling as applied to the purification of saline water [19]. Generally,the recirculating cooling water is kept at a steady-state with relativelyhigh concentration factor to minimize water usage and with moderatealkalinity to avoid corrosion. Under such operation conditions, blow-downwater has a tendency of scaling. The scalingmainly includes inor-ganic hardness precipitations and silica scale. In this study, simulatedCTBD water was prepared by a reference of water chemistry from one

customer's cooling tower recirculating water in a power plant. The ob-jective is to investigate the feasibility of desalination by MD processand to examine the scaling tolerance. Different configurations of MDsuch as VMD, AGMD can be used to treat CTBD water and are expectedto be more thermally efficient than DCMD. However, DCMD is the sim-plest configuration to test membrane scaling/fouling on the laboratoryscale and the results will be applicable to other MD configurations.

2. Experimental

2.1. Membrane and membrane module

A flat-sheet hydrophobic microporous polypropylene (PP) mem-brane provided by GE Osmonics was used for the desalination of sim-ulated CTBD water and for the evaluation of membrane scaling. Thiswas a symmetrical membrane and there were no support layers oneither side of the membrane. The contact angle of water on the sur-face of membrane was about 128°–132°. The average pore size was0.1 μm and the thickness was 100 μm. The porosity of membranewas between 65 and 70%.

A polyoxymethylene (POM) plastic module wasmachined to fix thePP membrane and to facilitate flow channels. It consisted of two sym-metrical flat-sheet POM plates. The thickness of each POM plate was4 cm, which served to reduce the heat loss from module to ambient.Three small flow channels were engraved in one side of each plate,and a small support bar was between the neighboring channels toavoid the crinkle of membrane caused by water flow. The length,width and height were 6.0 cm, 1.0 cm and 0.25 cm respectively ineach small channel. In order to prevent the salt water from penetratingthrough edges to distillate side, awide overlap and Parafilm sealingwaxwas employed during the assembly of module. The effective contactarea of water withmembranewas about 16 cm2 because of the overlapof hermetic gasket and membrane. After the completion of assembly,the module was purged with compressed air with pressure of 12–14 psig for 5 min to thoroughly dry the pores and to ensure the hydro-phobicity of the membrane.

2.2. DCMD setup and test procedure

The present studies were carried out in a laboratory bench-scaleDCMD setup. The schematic diagram of this setup was shown inFig. 1. The installation included feed hot concentrate loop and perme-ate cool distillate loop as well as DCMD module. Both heating andcooling loops were connected to the membrane module. The modulewas assembled in a vertical position in terms of the flow direction.And the feed and distillate streams were oriented in a counter-flowconfiguration. A diaphragm pump (DP130, max 1.7 L/min, 130 psig)pumped the feed water into a titanium coil immersed in a heater(Julabo, ED-27) to heat salty solution and then circled through mem-brane module. The distillate stream heated by the condensing perme-ate was cooled by a chiller (Julabo F12). The K-type thermocouplesensors and the high resolution pressure gauges (0–6 psig) were setto monitor the temperature and pressure respectively at inlet andoutlet. The conductivity and pH sensors were also put in feed and per-meate tank. A balance (Mettler-Toledo Instruments Co. Ltd, PS 4001)was used to weigh the permeate water for determination of fluxthrough the membrane.

When the setup was built, a test procedure was followed to per-form the DCMD evaluation. Both the hot and cold loop were rinsedby deionized water (0.8–2.0 μS/cm) until the conductivity of thecold water was in the range of 1.0–5.0 μS/cm. About 400 ml deionizedwater remained in permeate vessel to maintain circulation of the coldside. The vessels in both sides were sealed to avoid water evapora-tion. The feed side water was heated in advance to the desired tem-perature by a bypass loop using a diaphragm pump. The inlettemperature for brine and distillate was set at 60.3 °C and 18.9 °C

135X. Yu et al. / Desalination 323 (2013) 134–141

respectively. In each DCMD experiment, the inlet temperature of bothbrine and distillate were kept at the same range. The hot water flowrate was 600 ml/min and cold distillate 550 ml/min.

2.3. Simulated blowdown water and membrane rinse

The compositions of the simulated CTBD water prepared in pres-ent study derived from the water analysis of the field site raw CTBDwater in one customer's power plant. The raw CTBD water qualitywith main compositions is listed in Table 1.

For the purpose of examination of the different scaling behaviorsthrough DCMD process, four types of simulated feed water samplesnamed A, B, C and D were prepared respectively. The chemicals andcorresponding concentrations of different types of simulated feedwater samples were shown in Table 2. Na2SO4, NaHCO3 and CaCl2were added and dissolved into the distilled water in turn during prep-aration to form feed A, which was used for the study of hardness scal-ing. Na2SiO3·9H2O water solution with pH adjustment from 10.6 to8.5 was prepared to obtain feed B for the baseline simulation of silicascaling. Na2SiO3·9H2O solution with pH=7.0 and certain concentra-tion was added into the feed A to form feed C. The feed D was pre-pared by addition of 20 ppm antiscalant (MDP 150), which wasobtained from GE Water and Process Technologies. The simulatedCTBD water was usually prepared in 5 L batches. The membrane

cleaning recovery process for feed C and feed D was conducted with2.5 L batches.

An acid solution with 1 wt.% HCl was prepared for the inorganicscaling membrane washing. One liter of acid solution was circulatedthrough membrane module in feed side to wash membrane for0.5 h at room temperature and then the module was flushed with dis-tilled water. The membrane for the concentration of feed A wascleaned by acid solution. The membrane containing silica scale pro-duced by the concentration of feed C was cleaned first by acid solu-tion washing and then the alkaline solution (2 wt.% NaOH, 1 L).Finally the module was dried with compressed air with pressure of5 psig for 20 min.

2.4. Material characterization and performance evaluation

Themorphological images of freshmembrane and scaledmembranewere attained by scanning electron microscopy (SEM, Quanta, FEG250,OXFORD). Energy dispersion spectrometry (EDS) provided the detec-tion of inorganic elements on the surface of the scaled membrane. Themembrane samples for the SEM and EDX characterizationwere sprayedwith chromium coating for electron conduction. The Fourier transforminfrared (FTIR) spectra of samples were measured by the dispersive in-frared spectrophotometer (IRPrestige-21, SHIMADZU company). Thetotal dissolved solids (TDS) in the water were achieved by weighingthe dried solids after the complete evaporation of water using aMoisture Analyzer (Excellence HX204, Mettler-Toledo Instruments Co.Ltd,). Dissolved silica (SiO2) was measured through the Hach DR5000colorimeter using the silicomolybdate method (Hach Method 656).

The ionic conductivity and pH value of brine and distillate wererecorded by computer through a monitor (S40 SevenMulti, Mettler-Toledo Instruments Co. Ltd). The DCMD specific flux J was calculatedfrom the equation: J=(W2−W1)/(A×t), where (W2−W1) (kg) is theweight increase during the interval of record time. A (m2) is the

Fig. 1. Schematic diagram of bench scale DCMD experimental setup.

Table 1Average feed water quality with main constitutes of the referred original cooling towerblowdown at power plant of customer.

Analytes Units Values

pH pH Units 8.5Conductivity μS/cm 7132P-alkalinity mg CaCO3/L 5M-alkalinity mg CaCO3/L 254Sulfate mg/L 2341Chloride mg/L 399Phosphate mg/L 8.2Nitride mg/L 17Silica mg/L 96Calcium mg CaCO3/L 578Magnesium mg CaCO3/L 116Sodium mg/L 1158Potassium mg/L 52TSS mg/L 32TDS mg/L 4749

Table 2Simulated cooling tower blowdown feed water samples with the corresponding addedchemicals and concentrations for the DCMD evaluation.

Simulated feed Concentration of chemicals in water (unit: mg/L)

NaHCO3 Na2SO4 CaCl2 Na2SiO3·9H2O Antiscalant

A 213.4 3462.7 623.8 – –

B – – – 454.4 –

C 213.4 3462.7 623.8 454.4 –

D 213.4 3462.7 623.8 454.4 20

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membrane area and t (hr) is the interval time of the pure water collec-tion. The salt rejection was calculated from the equation

rejection %ð Þ ¼ 100 � 1−σp

σ f

!

σp ¼mi þmp

� �σ c−miσ i

mp:

σi The initial circulating distilled water conductivityσc The water conductivity of the cold sideσp The permeate water conductivityσf The feed side water conductivitymi The initial distilled water weightmp The permeate water weight

For each experiment, mi=400 g and σib5 μS/cm. When the valueof σi and mi was low and that of mp was high, the salt rejection couldbe simplified as the following equation.

rejection %ð Þ ¼ 100 � 1 − σ c

σ f

!

σc and σf were measured on-line by the conductivity monitor.The feed water was cycled through DCMD till a certain concentra-

tion factor (CF) was reached. The value of CF was calculated by thefollowing equation:

CF ¼ mf ;i= mf ;i − mp

� �;

wheremf,i was the initial feed water weight andmp was the permeatewater weight.

The Langelier Saturation Index (LSI) and the theoretical ionic con-ductivity were calculated using Watsys software (GE Osmonics, Ver-sion 2.2). The calculated conductivity of feed water was obtained byinputting the calculated ions concentration to Watsys software. Thecalculated ion concentration is equal to the initial ion concentrationmultiplied by concentration factor. Through this software, the LSIcould be determined.

3. Results and discussion

3.1. Performance of DCMD

Permeate flux, permeate conductivity and salt rejection are impor-tant performance parameters for the desalination process of membranedistillation. Different compositions of CTBD feed, especially contents ofhardness and silica, were evaluated using DCMD. Fig. 2 shows the per-meate flux and the permeate conductivity of DCMD for the concentra-tion and desalination of simulated CTBD feed A and feed C. Theperformance of DCMD on the concentration of a single silica solution(feed B) is also shown to highlight the influence of silica. The results re-vealed that DCMD performed an average membrane specific flux ofabout 30 L/m2·h for both feed A and feed C under a temperature differ-ence of 40 °C across membrane. But their flux decrease and permeateconductivity increase occurred at different concentration factors: CF ap-proximately 3.7–4.0 for feed A and 3.2–5.0 for feed C. These results indi-cate the distinct concentration limitations for these simulated CTBDsolutions. The flux remained relatively constant up to a certain rangeof concentration factor due to the moderate feed salinities (initialTDS=0.34 wt.% and final TDS=1.5–2.6 wt.%). Essentially, membranedistillation is a thermally-driven process. The vapor pressure difference,which comes from the temperature difference between hot side andcold side of membrane, has the predominant effect on permeate flux,

and is weakly affected by salt concentration at these TDS levels. Theseresults verify that the MD process was not significantly sensitive tofeed water concentration despite the concentration polarization duringfeed cycling process. No changes of flux and permeate conductivity forfeed B were observed even with the CF rose to more than 10 times.Fig. 3 shows the salt rejection of DCMD process. Salt rejection of morethan 99.95% was obtained for each type of feed. The drop of rejectioncorresponded to the drop in flux at a critical CF. Thus, hardness and sil-ica in CTBD affected DCMD performance and limited the concentrationfactor and feed water recovery.

3.2. Membrane scaling

Membrane scaling was the cause for the drop of flux and decreaseof salt rejection. The original CTBD water contained relatively high al-kalinity and high hardness as well as high content of silica. For thepurpose of verification of different types of scaling and its correlationto the limitation of concentration factor, the simulated CTBD feedswere prepared to be clarified into silica-free water (feed A) andsilica-containing water (feed C). The feed A was apt to form eitherCaCO3 scale or CaSO4 (anhydrite)/CaSO4·2H2O (gypsum) scale,while the feed C had the potential to precipitate silica besides hard-ness scaling.

The simulated CTBD exhibited tendency to form scale in terms of LSI.Feed A had a positive LSI (LSI>1) shown in Fig. 4a, which indicated itwas inclined to form hardness scale. Also, the theoretically calculatedionic conductivity (Fig. 4b) and the experimentally measured

Feed A: simulated silica-free CTBDFeed B: silica baseline solutionFeed C: simulated silica-containing CTBD

Feed B

Feed B

Feed A

Feed A Feed C

Feed C

Fig. 2. DCMD flux and permeate conductivity for different types of simulated CTBD feed asa function of feed concentration factor. Feed A: simulated silica-free CTBD; feed B: simu-lated silica water solution as baseline and feed C: simulated silica-containing CTBD.

Fig. 3. Salt rejection of DCMD for different types of simulated CTBD feed as a function offeed concentration factor.

137X. Yu et al. / Desalination 323 (2013) 134–141

conductivity (Fig. 4c) of feed A increased almost linearly with the con-centration process. But the calculated feed conductivity was higherthan the measured ones. This was due to the decrease of conductiveions in feed A, which resulted from the formation of precipitates. In ad-dition, this discrepancy became significant fat CF>1.5 (Fig. 4).

Concentrating feed A generated carbonate scale, but not sulfatescales at the range of CF=1.0–4.0. SEM morphology of the scale onthe hot side surface of membrane was shown in Fig. 5. Associatedwith the concentration process, the following equilibrium reactionswere possible.

H2O ⇌ Hþ þ OH− ð1Þ

HCO−3 ⇌ Hþ þ CO2−

3 ð2Þ

2 HCO−3 → H2O þ CO2 ↑ þ CO2�

3 ð3Þ

Ca2þ þ CO2−3 ⇌ CaCO3↓ ð4Þ

Ca2þ þ SO2−4 ⇌ CaSO4↓ ð5Þ

It was the ionization or decomposition of bicarbonate ions intocarbonate ions that resulted in the scaling of CaCO3. The slightly solu-ble (from about 2400 mg/L to 3000 mg/L, Ksp=9.1×10−6) calciumsulfate scale for feed A (supposed Ca2+ totally combined with SO4

2−,CaSO4 concentration were about 750 mg/L initially) could not formunder low concentration factors. For this moderately alkaline feed A(pH=8.5), at the initial conditions of the DCMD process after heatingfeed to 60 °C, it is possible to form microcrystals of CaCO3 scale. Butthe observed onset of scaling with feed A occurred at a CF about3.7–4.0. The FTIR spectrum of the scaling particles on the surface ofmembrane verified that the scale was carbonate, not sulfate. Com-pared with the standard spectrum of CaCO3, the curve of scaling par-ticles exhibited the similar strong characteristic adsorptions at thewave numbers of 1440 cm−1, 850 cm−1 and 710 cm−1. While thecharacteristic adsorption peaks of anhydrite at the wave numbers of980 cm−1 and 652 cm−1 and that of gypsum at 1146 cm−1 [20]were not observed in the scaling particles spectrum. The changes ofpH value for the feed water during the concentration process furtherconfirmed the scaling species. The formation of CaSO4 did not changethe pH value of feed. But once a large amount of CaCO3 scalingformed, from equilibrium (Eqs. (2) and (3)) it is deduced that eitherCO2 or H+ was generated, which would lead to the decrease of pH

value. The pH value decreased from about 8.4 to about 7.6 at the con-centration factor range of 3.7–4.0.

Silica is expected to exist in raw CTBD water and it is inevitable forsilica to precipitate onMDmembrane at certain cycles of concentration.But itwas demonstrated thatMDhad a relatively high tolerance to silicascale in terms of MD flux and salt rejection from concentration a singlesilica baseline solution (feed B). Although the flux and permeate con-ductivity kept stable (see Fig. 1) under CF>8.0, the membrane surficialmorphology actually was changed due to the scaling of silica. The for-mation of silica scale on membrane was verified by SEM-EDS analysisas shown in Fig. 6a. Si and O elements reflected the formation of onlySiO2 and the C element came from PPmembrane. The appearance of sil-ica scale presented an amorphous status, as also observed in other re-search [21,22]. The amorphous and scattered scale didn't affect theflux and was not capable of tearing the membrane, so the MD perfor-mance remained constant. But it is believed that in the cases of elevatedof hardness and prolonged accumulation of silica scale, MD perfor-mance would be affected significantly.

When DCMD concentrated silica-contained simulated CTBD feed Cto CF=6, besides silica scaling, sulfate scale also formed together withcarbonate. The soluble silica concentration of initial feed C (CF=1)was 72 mg/L, yet the concentrated (CF=6) was 156 mg/L which im-plies significant precipitation of silica. Fig. 6b shows the morphologyand elements of scaling from the characterization of SEM-EDS. The

Fig. 4. Variation of feed conductivity and LSI versus feed A concentration factor. (a) thecalculated LSI corresponding to the theoretical brine conductivity; (b) the calculatedconductivity of feed brine under the same concentration factor and (c) the measuredconductivity of feed brine.

Fig. 5. SEM morphology for the comparison of (a) fresh PP membrane and (b) scaledmembrane by the simulated silica-free CTBD.

138 X. Yu et al. / Desalination 323 (2013) 134–141

identification of elements of Si, C S and O from EDS indicates differenttypes of scaling deposited on membrane. The overlap of C element incarbonate and in PPmembranemakes it difficult to determine the exis-tence of carbonate from the EDS spectrum. FTIR spectra in Fig. 7 furtherconfirmed the presence of silica (1072 cm−1, 790 cm−1) [23] and gyp-sum (3401 cm−1, 1614 cm−1, 1116 cm−1, 670 cm−1) [20], but thecharacteristic peaks of carbonate were not evident. The characteristicspectrum of CaCO3 was not observed by EDS and FTIR. It cannot beconcluded that the carbonate was absent from the scale, because thescaling thickness was more than 50 μm while the EDS and FTIR couldonly reflect the sample signal less than 3 μm. However, it is reasonable

(a) Feed B

(b) Feed C

Fig. 6. SEM-EDS analysis results of scales on PP membranes obtained by concentrating (a) feed B: silica baseline solution and (b) feed C: simulated silica-containing CTBD.

Fig. 7. FTIR spectra for the membranes of (a) blank PP membrane; (b) silica scaledmembrane and (c) silica-containing simulated CTBD scaled membrane.

(Right) cleaned membrane(Left) fresh membrane

(b)

(a)

(Left) fresh membrane (Right) cleaned membrane

Fig. 8. Flux recovery curves and concentration factor variations with evolution ofDCMD elapsed time. (a) membrane for feed A and (b) membrane for feed C.

139X. Yu et al. / Desalination 323 (2013) 134–141

to speculate that carbonate scale formed; it was merely buried in thedeep layers of the scales as it might precipitate first. CaCO3 scalebegan to deposit from CF=1.5 (see Fig. 3). pH value of feed C decreasedfrom pH=8.5 (CF=1) to pH=7.7 (CF=6) and the alkalinity de-creased to 28 mg/L (as CaCO3, CF=6). Therefore, the calcium carbonatescale also precipitated when concentrating the feed C together with sil-ica as well as sulfate.

3.3. Recovery of membrane

The performance of the scaled membrane was able to be recov-ered when the membrane was not damaged by crystal tearing andmechanical pressure. This recovery required the removal of scalesand the restoration of membrane hydrophobicity. The scales deposit-ed on the surface prevented the feed water from contacting the mem-brane directly and thus the effective membrane area was reduced.The scales located in the pores of membrane blocked the vapor trans-ferring channels. So scaling of membrane resulted in permeate fluxdecrease. Scales could even tear the membrane in case of sharp crys-tal formation. In addition, with blocked pores by scales the membranemay have become hydrophilic, which explains the increase of perme-ate conductivity and the decrease of salt rejection.

Acid and alkaline solution rinse was the generally employed proce-dures for scale cleaning. Insoluble carbonate reacted with acid. Slightlysoluble sulfatewas re-dissolved into solution gradually under a relativelyhigher flow rate of acid. Silica scale also increased solubility at higher pH(e.g, pH>9) and temperature (>50 °C). Thus, acid and alkaline washingwas employed for the clean-in-place (CIP) procedure of module.

The scaled membrane performance was fully recovered by follow-ing the described CIP processes. The membrane performance beforeand after cleaning through DCMD for concentration of feed A isshown in Fig. 8a. The results confirmed that after CIP, flux returnedto the initial level of about 30 L/m2·h and similar curves of flux andconcentration factor via elapsed time were obtained. Fig. 8b presentsthe results of the performance recovery for the membrane producedscale by cycling of feed C. Its flux also was recovered. Fig. 9 furthershows the permeate conductivity and salt rejection of concentratingfeed A for both before and after CIP. Again, the similar behavior tookplace when the feed water was concentrated to about 4 times. Thesame phenomenon for concentrating feed C occurred in terms of thepermeate conductivity and salt rejection. The experiments were re-peated for several times and the similar results were obtained.

3.4. Influence of antiscalant

Membrane scaling limited the concentration of CTBD water.Antiscalant contributed to enhance the concentration factor and in-crease water recovery. Fig. 10 compares the DCMD performance onthe concentration of feed C and feed D. For feed D, the concentrationfactor was enhanced to about 8.0 and then the permeate flux began todecrease. The water recovery was enhanced to about 87% by additionof antiscalant into simulated CTBD feed C.

The typical increase of permeate conductivity that occurs simulta-neously with flux decline was not observed at CF=8. This suggeststhat scaling formed on membrane, while it was enough not to wet the

Fig. 9. Comparison of DCMD performance in permeate conductivity and salt rejectionbefore and after membrane cleaning respectively for the feed A feed.

Fig. 10. MDperformance enhancement for the simulated CTBD by addition of antiscalant.

Fig. 11. SEM-EDS analysis for the scaled membrane by addition of antiscalant into feed C.

140 X. Yu et al. / Desalination 323 (2013) 134–141

membrane at CF=8.0. SEM-EDS in Fig. 11 confirmed the precipitationof scaling, and the scales were silica and calcium phosphate. This calci-um phosphate derived from the decomposition of antiscalant.

4. Conclusions and remarks

Direct contactmembrane distillationwas evaluated for water recov-ery of simulated cooling tower blowdown. DCMD demonstrated a rela-tively stable flux of about 30 L/m2·h and a salt rejection of about99.95% under feed water temperature at 60 °C and permeate tempera-ture of 20 °C. Scaling occurred on membrane during the concentrationof simulated CTBD water. For silica-free simulated CTBD feed water,only calcium carbonate scale formed leading to flux decline. Forsilica-containing simulated CTBD feed water, silica and sulfate as wellas carbonate scale deposited at a relatively high concentration factor.DCMD exhibited a distinct tolerance to different scales when concen-trating simulated CTBD water. For carbonate scale, the maximumconcentration factor was about 3.7–4.0, which corresponded to a limitof 72% water recovery. For composite scale of silica, carbonate and sul-fate, the maximum concentration factor was about 3.2–5.0, whichcorresponded to a limit of 68%water recovery. The use of antiscalant en-hanced the maximum concentration factor to about 8.0 and thus thewater recovery was capable to achieve about 87%. Technically, it is fea-sible for MD to concentrate and reuse cooling tower blowdown water.

Although membrane distillation technology has better salinity tol-erance to feed water than that of reverse osmosis, the scaling andfouling still are obstacles for its application in industrial wastewaterreuse such as CTBD. Therefore, the understanding of scaling formationmechanism and the development of scale inhibition as well aspretreatment are important for MD's real application. Raw coolingtower blowdown water may have higher TDS and alkalinity thanthe present simulated water. Some other scaling and fouling compo-sitions, e.g., phosphates, polymers and biological fouling, have notbeen included in this study. Thus it is necessary to evaluate rawCTBD water using MD in future work.

Thermoelectric power plants have some waste heat of the may beuseful for MD, unfortunately overall it is of low quality and it is unlikelyto meet the full needs of treating the huge amount of blowdown water.Additional heat may be supplemented and high efficiency MD systemconfigurations need to be developed.

References

[1] J.R. Wolfe, R.A. Goldstein, J.S. Maulbetsch, C.R. McGowin, J. Contemp. Water Res.Educ. 143 (2009) 30–34.

[2] T.J. Feeley, T.J. Skone, G.J. Stlegel, A. McNemar, M. Nemeth, B. Schimmoller, J.T.Murph, L. Manfredo, Water: a critical resource in the thermoelectric power indus-try, Energy 33 (2008) 1–11.

[3] S.J. Altman, R.P. Jensen, M.A. Cappelle, A.L. Sanchez, R.L. Everett, H.L. Anderson Jr.,L.K. McGrath, Membrane treatment of side-stream cooling tower water for reduc-tion of water usage, Desalination 285 (2012) 177–183.

[4] J.D. Zhang, L. Chen, H.M. Zeng, X.X. Yan, X.N. Song, H. Yang, C.S. Ye, Pilot testing ofoutside-in MF and UF modules used for cooling tower blowdown pretreatment ofpower plants, Desalination 214 (2007) 287–298.

[5] J.V. Mattson, T.G.I. Harris, Zero discharge of cooling water by side stream soften-ing, Water Pollut. Control Fed. 51 (1979) 2602–2614.

[6] M.N. DiFilippo, Use of produced water in recirculating cooling systems at powergenerating facilities, National Energy Technology Laboratory, Palo Alto, CA,2004, p. 68.

[7] J.D. Zhang, H.M. Zeng, C.S. Ye, L. Chen, X.X. Yan, Pilot test of UF pretreatment priorto RO for cooling tower blowdown reuse of power plant, Desalination 222 (2008)9–16.

[8] R.J. Jones, Waste Water Purification System, Wastewater Resources Inc., USA,1991.

[9] Z.Wang, Z.F. Fan, L.X. Xie, S.C. Wang, Study of integratedmembrane systems for thetreatment of wastewater from cooling towers, Desalination 191 (2006) 117–124.

[10] S. Kallappan, C. Sathish, T. Niramlkumar, Recovery and reuse of water from efflu-ents of cooling tower, J. Indian Inst. Sci. 85 (July–Aug. 2005) 215–221.

[11] Takuji Shintani, Hideto, et al., Effect of heat treatment on performance ofchlorine-resistant polyamide reverse osmosis membranes, Desalination 247(2009) 370–377.

[12] J.M. Kavanagh, S. Hussain, T.C. Chilcott, H.G.L. Coster, Fouling of reverse osmosismembranes using electrical impedance spectroscopy: measurements and simula-tions, Desalination 236 (2009) 187–193.

[13] Lauren F. Greenlee, Desmond F. Lawler, et al., Reverse Osmosis desalination: watersources technology, and today's challenges, Water Res. 43 (2009) 2317–2348.

[14] M.S. El-Bourawi, Z. Ding, R. Ma, et al., A framework for better understandingmembrane distillation separation process, J. Membr. Sci. 285 (2006) 4–29.

[15] KevinW. Lawson, Douglas R. Lloyd,Membrane distillation, J.Membr. Sci. 124 (1997)1–25.

[16] L. Mariah, C.A. Buckley, C.J. Brouckaert, E. Curcio, E. Drioli, D. Jaganyi, D.Ramjugernath, Membrane distillation of concentrated brines — role of water ac-tivities in the evaluation of driving force, J. Membr. Sci. 280 (2006) 937–947.

[17] M. Cohen, I. Ianovici, D. Breschib, Power plant residual heat for seawater desalina-tion, Desalination 152 (2002) 155–165.

[18] C.W. Morrow, B.P. Dwyer, S.W. Webb, S.J. Altman, Water Recovery Using WasteHeat from Coal Fired Power Plants, SAND2011–0258, Sandia National Laborato-ries, Albuquerque, NM, 2011.

[19] E. Curcioa, X.S. Jia, G.D. Profioa, et al., Membrane distillation operated at high sea-water concentration factors: role of the membrane on CaCO3 scaling in presenceof humic acid, J. Membr. Sci. 346 (2010) 263–269.

[20] Hasan Boke, Sedat Akkurt, Serhan Özdemir, et al., Quantification of CaCO3–CaSO3_0.5H2O–CaSO4_2H2O mixtures by FTIR analysis and its ANN model,Mater. Lett. 58 (2004) 723–726.

[21] M. Badruzzaman, A. Subramani, J. DeCarolis, et al., Impacts of silica on the sustain-able productivity of reverse osmosis membranes treating low-salinity brackishgroundwater, Desalination 279 (2011) 210–218.

[22] S. SalvadorCob, C. Beaupin, B. Hofs, et al., Silica and silicate precipitation as limitingfactors in high-recovery reverse osmosis operations, J. Membr. Sci. 423–424 (2012)1–10.

[23] A. Fidalgo, L.M. Ilharco, The defect structure of sol-gel-derived silica / polytetra-hydrofuran hydrid films by FTIR, J. Non-Cryst. Solids 283 (2001) 144–154.

141X. Yu et al. / Desalination 323 (2013) 134–141

ID Title Pages

623785Experimentalevaluationonconcentratingcoolingtowerblowdownwaterbydirectcontactmembrane

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