Desalination 262 (2010) 115–120

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Effect of chlorine and acid injection on hollow fiber RO for SWRO

Jia Xu a, Guoling Ruan b, Linda Zou c, Congjie Gao a,⁎a Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education; College of Chemistry and Chemical Engineering, Ocean University of China, Songling road 238,Qingdao, Shandong, 266100, Chinab Institute of seawater desalination and multipurpose utilization, Tianjin 300192, Chinac SA Water Centre for Water Management and Reuse, University of South Australia, Adelaide SA5095, Australia

⁎ Corresponding author. Tel./fax: +86 532 66781872E-mail addresses: qdxujia@sina.com.cn, qdxujia2007

0011-9164/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.desal.2010.06.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 February 2010Received in revised form 12 May 2010Accepted 1 June 2010Available online 2 July 2010

Keywords:Hollow fiberCellulose triacetateSeawater reverse osmosisChlorine injectionHCl injection

Bio-fouling results in deterioration of permeate quality and flux in reverse osmosis (RO) process. Cellulosetriacetate hollow fiber membrane used for RO in SWRO has anti-chlorine property that makes technicalsense compared to polyamide spiral-wound RO for overcoming bio-fouling. Mode of chlorine injection is thecontrolling factor of sterilization in SWRO system. This work evaluated the effects of five different modes ofchlorine injecting system as well as HCl injection on the performance of HF RO system. In addition, effects ofHCl and anti-scalants on anti-scale performance of HF RO system were also investigated, respectively. A brieflist of chemical costs under different abovementioned operating conditions was covered in a cost analysis.The results indicated that the injecting mode that employed continuous chlorine with a relatively lowconcentration of 0.3–0.4 mg/L at pH 5.8–6.2 was preferable in the aspects of total bacterial count (TBC)removal, permeate flow rate, differential pressure, permeability (Pp), performance factor (η) and chemicalcosts. HCl injection could improve the sterilization and prevent scaling on the membrane surface efficiently.The expense of anti-scalant (MDC220) was higher than HCl.

.@gmail.com (C. Gao).

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, serious bio-fouling has become an emergingproblem that impedes the performance of SWRO and has attractedgreat attention. Many approaches [1,2] have been conducted toidentify the robust membranes and appropriate operating conditionsto overcome bio-fouling, but none of them has fundamentallyaddressed the problem. At the same time, with the improvementsof membranematerials andmodules [3], the cellulose triacetate (CTA)hollow fiber (HF) RO membranes manufactured by TOYOBO [4]exhibited the superiority on chlorine tolerance over the polyamide ROmembranes. Because these polyamide membranes have a lowchlorine tolerance (less than 0.1 mg/L), the no residual chlorinerequirement in the feed prior to or in the SWROmembranes results inthe breeding of bacteria and other microorganisms compromising theproperties and performance of the RO membrane.

Sodium hypochlorite is well known as one of the most efficientsterilizers for seawater to control the bio-fouling and is used widely inwater treatment process. Chlorine injecting systems are mainlyclassified into intermittent chlorine injection (ICI) and continuouschlorine injection (CCI), which are indispensable to HF SWRO system.Some literatures reported that ICI could eliminate bio-fouling moreefficiently, while CCI was not as effective as ICI [4,5]. For example, the

required minimum chlorine concentration and injecting time of ICIbased on the experiments of the growth and sterilization rates ofmicroorganisms in seawater of Middle East were simulated, and theoperational results proved that ICI was the most effective chlorineinjection mode to achieve best RO performance of desalination [6].However, there are few researches conducted on a systematicinvestigation on the performance of different modes of ICI and lowchlorine CCI.

The objective of this work is to investigate the effects of differentmodes of ICI and low chlorine CCI on the overall performance of HF ROsystem. Hydrochloric acid injection which cooperated with chlorinewas used to improve the efficiency of anti-bio-fouling and preventscaling on the membrane surface without anti-scalants. In addition,effects of HCl and anti-scalants on reducing the scaling of HF ROsystem were also investigated, respectively. A brief list of chemicalcosts under the abovementioned operating conditions was covered ina cost analysis.

2. Experimental

2.1. Description of HF SWRO system

Raw seawater was taken from the surface seawater (open intake)in Jiaozhou Bay, Yellow Sea, China, and it was pumped through anultrafiltration (UF) system which was the main pretreatment prior toHF SWRO system. The pretreated seawater was sent to the cartridgefilter (for removing 5 μmparticles in order to protect ROmembrane in

Fig. 1. Picture of HF SWRO system (excluding UF pretreatment system).

Table 1Characteristics of the HF modules.

Items HF module

Module and membrane Module HR 8355Membrane material Cellulose Triacetate

(CTA)Length/diameter of module, mm 1330/305

Feed water qualities SDI15 b4pH 3–8Residual chlorine b1.0

Operating conditionsa Operation pressure, MPa b6.05–40

Temperature, °C N10Permeate flow rate, m3/d 15–150Brine flow rate, m3/d N99.2Salt rejection,%

a The benchmark values are based on the following conditions: 35,000 mg/L NaCl,5.5 MPa, 30%, and 25 °C.

116 J. Xu et al. / Desalination 262 (2010) 115–120

emergent operation) and pumped to the test plant by high-pressurepump with a frequency converter. The reagent used in this studyinvolved sodium hypochlorite (NaClO), sodium bisulfite (SBS),hydrochloric acid (HCl) and a commercial anti-scalant (HypersperseMDC220, purchased by US General Electric). System operations suchas the on-off, reagent injecting and in-situ cleaning procedures werefully automated and controlled by a programmable logic controller(PLC). The picture and main characteristics of the HF SWRO system ispresented in Fig. 1 and Table 1, respectively.

Fig. 2. Different modes of ch

2.2. Experimental procedure

Chlorine injection is an indispensable step to HF SWRO system.The reagents of ICI system included NaClO, SBS and HCl. NaClO as anefficient sterilizer and SBS as a reducing agent can be assembled indifferent modes of ICI system. HCl instead of other commercial anti-scalants was used to improve the efficiency of bio-fouling reductionand to adjust pH to prevent scaling on the membrane surface. Thechemical injecting points (IP) and monitoring points (MP) of fivemodes of ICI and low chlorine CCI are shown in Fig. 2, and theoperating conditions and chemical dosages are summarized inTable 2. Operation of the HF RO system in each injecting mode lasted140–150 h. In order to evaluate the efficiency of different injectingmodes, the residual chlorine, pH and total bacterial counts (TBC) weremeasured at different MPs, and the RO performance in terms ofpermeate flow rate, differential pressure and water quality wereinvestigated, as well as a brief comparison of chemical costs. Inaddition, effects of different chemical reagents such as HCl and acommercial anti-scalant on the anti-scaling performance were alsoinvestigated, where HCl works by obtaining a negative Stiff-Davisindex value.

2.3. Water quality analysis

Water quality analysis was conducted according to the standardmethods recommended by Chinese regulation.

3. Results and discussion

3.1. Efficiency of different chlorine injecting modes

In order to find out the variation of chlorine concentration throughthe overall system and determine the optimum reagent dosage,chlorine at 1.0 mg/L concentration was injected into raw seawaterand this test was conducted as a reference data. Fig. 3 illustrates thevariance of residual chlorine, pH and TBC. With fluid from MP2 toMP5, chlorine was consumed from 1.0 to 0.4 mg/L mainly due tobacteria and other microorganisms. TBC was around 2600 cfu/ml inraw seawater and decreased by 80.8% at MP2 when chlorine at1.0 mg/L concentration was injected at IP1, which indicated thatchlorine could eliminate bacteria efficiently. Residual chlorine at0.4 mg/L, however, did not result in the total elimination of bacteria inthe system, which means that the efficiency of sterilization not onlydepends on the residual chlorine concentration, but also depends onother factors such as contact time and pH. The contribution of UF onTBC removal was around 92–94%. If RO system applied the polyamidespiral-wound membranes, SBS had to be injected at MP4, where thereduction state might cause the serious breeding of anaerobes oranaerobic organisms and even the bio-film formation on themembrane surface. Based on the results from Fig. 3, different injecting

lorine and HCl injection.

Table 2Description of injection system and reagent dosage.

mode Description of injection and reagent dosage (mg/L)

ICI 1 NaClO (IP1), continuously injected all day, 1.0 mg/LSBS (IP2), injected twice a day and 11 h each time, 0.5–0.6 mg/L

2 NaClO (IP1), continuously injected all day, 0.5–0.6 mg/LHCl (IP1), continuously injected all day, adjusting pH to 5.8–6.2SBS (IP2), injected twice a day and 11 h each time, 0.1–0.2 mg/L

3 NaClO (IP2), injected twice a day and 1 h each time, 0.8–1.0 mg/L4 HCl (IP1), continuously injected all day, adjusting pH to 5.8–6.3

NaClO (IP2), injected twice a day and 1 h each time, 0.5–0.6 mg/LCCI 5 NaClO (IP1), continuously injected all day, 0.3–0.4 mg/L

HCl (IP1), continuously injected all day, adjusting pH to 5.8–6.2

117J. Xu et al. / Desalination 262 (2010) 115–120

modes with different chemical reagent dosages were applied forimproving the RO performance (as shown in Table 2).

The efficiency of the different modes of chlorine injecting systemwascharacterized by the TBC removal at MP5 and the overall performance ofHF RO system, as shown in Figs. 4–7. Fig. 4 illustrates the removalefficiencies of TBC versus time. It can be seen that the fluctuations in theTBC removalwere in the rage of 62–83%, 75–100%, 5–78%, 5–99% and 88–100% with mode 1, 2, 3, 4 and 5, respectively, which indicated that thechlorine injecting modes with different injecting time and dosages wereresponsible for the differences in the TBC removal. For instance, inmode 1and 2 (ICI), continuous chlorine fluid was injected at IP1 and SBS wasinjected intermittently to decompose chlorine at IP2, which means thatthe feed before IP2 was sterilized by chlorine all day, while the feed afterIP2 as well as in the RO module was in an oxidating state only twice perday for 1 h each time. The relatively low values (62–75%) of TBC removalwere caused by the bacterial reproduction due to the neutral andreducting state of the feed after IP2. In modes 3 and 4 (ICI), chlorine wasinjected at IP2 twice per day for 1 h each time and the TBC removal waslowered significantly to 0–5% due to the absence of chlorine through theoverall system. It indicated that chlorine played an important role in thefeed quality so as to influence the TBC removal. Mode 5 which employedCCI with a relatively low NaClO concentration of 0.3–0.4 mg/L exhibitedthehigher TBC removal of 88–100%, indicating that chlorine could achievethe improved sterilizing efficiency by making it easier for the overallsystem to be in aweak oxidizing state even at a lowNaClO concentration.

Moreover, when comparing with the TBC removals in mode 1 and2 or in mode 3 and 4, it also can be found from Fig. 4 that the modewith HCl injection for adjusting the feed pH at 5.8–6.2 yielded a muchhigher TBC removal, which showed the significant synergy of HCl on

Fig. 3. Variance of residual chlorine, pH and total bacterial counts with MP (reference te

the chlorine sterilization. For example, in mode 2 with HCl injection,an increase of 12–20% in the TBC removal was achieved compared tomode 1 under the same conditions. This can be explained in considerthat the sterilization effect is influenced by the ratio of HClO to ClO−

obtained from NaClO hydrolysis, as shown in Eqs. (1) and (2), andNaClO hydrolysis depends on water pH. Main form (above 90%) ofchlorine in water at pH 5–6 is HClO, while that at pH more than 9 isClO−.

NaClO + H2O⇔HClO + NaOH ð1Þ

HClO⇔Hþ + ClO−: ð2Þ

Although both HClO and ClO− are oxidants, the oxidability of HClOis much stronger than ClO− based on the standard electrode potential.It is well known that HClO is the dominatingly active sterilizing agentin water and possesses the sterilizing efficiency roughly 50–100 timesstronger than ClO−. Therefore, according to Eqs. (1) and (2), lower pHis beneficial to the production of more HClO in water so as to enhancethe sterilization efficiency.

In order to investigate the performance of HF RO system, basicparameters such as feed pressure, brine pressure, permeate flow rateand product quality were recorded during the testing period. Fig. 5displays the trends of the differential pressure (the differencebetween feed pressure and brine pressure) and permeate flow ratewith the period of RO operation in different chlorine injecting modes.The following identical operating conditions of each mode wereapplied: feed flow rate 25–26.5 m3/d/module, feed pressure of 5.1–5.2 MPa, system recovery of 39–41%, the initial differential pressurearound 0.01 MPa and initial permeate flow rate 10.5–10.8 m3/d/module. These operating conditions such as operating pressure andsystem recovery were a little lower than the previous work [4] whichapplied operating pressure of 6.5 MPa and system recovery of 45–50%.During testing period of about 150 h, the differential pressureincreased differently for each mode, which indicates that modes ofchlorine injection can influence not only the feed quality but also thedifferential pressure, and the increase in differential pressure could beprevented efficiently by the better chlorine injecting mode. Forexample, the slightest and sharpest increases in differential pressurewere observedwith the operation inmode 5 andmode 3, respectively,which showed the superiority of mode 5. Overall, the increase ofdifferential pressure was still less obvious compared to other reportedresearch [6]. As for the permeate flow rate, the operation in mode 5

st). (Scaling will occur at a pH of more than 6.7 according to the S&DSI calculation).

Fig. 4. Variance of total bacterial counts at MP5 with different modes of chlorineinjection system.

Fig. 6. Variance of permeability (Pp) and performance factor (η) during RO operation.

118 J. Xu et al. / Desalination 262 (2010) 115–120

exhibited more stable permeate flow rate compared to the operationsin other modes, which confirmed that low chlorine CCI was the moreefficient injecting mode for the stable RO performance. Additionally,comparing with modes 1–4, ICI with HCl injection can declinedifferential pressure obviously.

In order to further evaluate the effectiveness of injecting mode onthe RO performance, the permeability (Pp) and performance factor (η)were analyzed using Eqs. (3) and (4), and the results are plotted inFig. 6.

Pp =Qt

Pdiftð3Þ

η = ΔQ⋅ΔPdif = Qt−Q0ð Þ⋅ Pdift−Pdif0� �

ð4Þ

Fig. 5. Variance of differential pressure and permeate flow rate during RO operation.(Feed flow rate of 25–26.5 m3/d/module, feed pressure of 5.1–5.2 MPa, and systemrecovery of 39–41%).

where Q is the permeate flow rate, Pdif is the differential pressure,subscript 0 and t stands for the initial time and t time during ROoperation, respectively.

Pp here different from the permeability used for UF reported inprevious work [7] could clearly assess the performance of RO membraneby relating the product flow and the differential pressure. As shown inFig. 6, Pp decreased with the operating time for each mode and yieldeddifferent declining rates due to the membrane fouling which could bepartly controlled by chlorine injectingmode. On thewhole, Ppwithmode5 remainednearly constant integrally compared to that of othermodes. Asfar as η is concerned, η is associated additionallywith the initial permeateflow rate and initial differential pressure compared to Pp, and hence canassessdirectly the comprehensiveeffect of theproductflowreduction anddifferential pressure elevation. The higher η obtained, the more seriousmembrane fouling is and thus the worse performance is. It was clearlyobserved that η increased dramatically with the operation in mode 3(from0 to 0.0043)while it remainedmore constantwith the operation inmode 5 (from 0 to 0.0003), which means that chlorine injecting mode 3caused the most serious membrane fouling while low chlorine CCI wasproven to be efficient to maintain a stable RO performance. Note that thetrend of Pp is not as obvious as the trend ofη, indicating thatη can be usedas amore hypersensitive indicator to investigate a slight change of the ROperformance. In addition, comparing the injecting modes with andwithout HCl injection under the same operating conditions, more than50% reduction of ηwas achieved when HCl injection was applied, whichconfirmed the effectiveness of HCl injection to probably hinder the bio-fouling even scale on the ROmembrane surface. The removal efficiency ofscaling by HCl will be discussed later in Section 3.2.

Conductivity as the main parameter of water quality here wasmonitored with the operating time. The conductivity in the feed (rawseawater) seemed stable in the rage of 44,000–47,000 μS/cm. Fig. 7 showsthe variance in the product conductivity with the operation in differentchlorine injecting modes. The conductivity values of RO product werebelow 155 μS/cm and 88% of all measurements less than 138 μS/cm, with

Fig. 7. Variance of product conductivity during RO operation.

Table 3Cost of chemicals with different modes of chlorine injection system.

Dosage (kg/year/module)

Cost of each reagent($/year/module)

Total cost($/year/module)

NaClO SBS HCl NaClO SBS HCl

Mode 1 94.90 4.79 0 17.520 1.628 – 19.148Mode 2 52.20 1.31 227.765 9.636 0.449 26.28 36.365Mode 3 17.08 – – 3.145 – – 3.145Mode 4 10.44 – 227.765 1.926 – 26.28 28.206Mode 5 33.22 – 227.765 6.132 – 26.28 32.412

Fig. 8. Variance of S&DSI of feed and brine with pH. (system recovery of 40%, feed flowrate of 26 m3/d/module, feed pressure of 5.1 MPa and temperature at 24 °C).

119J. Xu et al. / Desalination 262 (2010) 115–120

themeanvalueof130 μS/cm.According to the results above, themeansaltrejectionwasabout99.71%, indicating that theexcellentqualityofproductwater with relatively low TDS of about 65 mg/L can be obtained by HFSWROsystem. It isworth noticing that, however, it seems to be difficult toevaluate the effect of different chlorine injecting modes on the productconductivity because very little differences were observed.

Estimation of chemical costs for each RO module operated for oneyear is displayed in Table 3. It turns out that the costs of modes 2 and 3were the highest and lowest from all the tested modes, respectively.In consideration of the RO performance, modes 1, 2 and 3 are notrecommended although the cost of mode 3 is relatively low. Mode 5 isacceptable due to a litter high cost of chemicals and the evidently highperformance of RO. As for mode 4, it is recommended to be run for alonger period to further investigate the impact of TBC removal on theRO performance due to the drastic fluctuation of the TBC removal inthe feed prior to HF RO membrane, as shown in Fig. 4.

3.2. Anti-scaling efficiency

Based on the results from Section 3.1, it was found that HCl playedan important role in hindering the membrane bio-fouling andsimultaneously decreasing the required chlorine dosage to achievethe same sterilizing efficiency. In addition, it should be noticed thatHCl could also lower water pH to achieve a negative Stiff-DavisStability index (S&DSI) value and thus prevent scaling on themembrane surface in the feed and brine profile without any givenanti-scalant. To investigate the scaling potential across the ROmembrane under different operating conditions, S&DSI was calculat-ed according to the following empirical equations shown in Eqs. (5)–(7):

S & DSI = pHb−pHs ð5Þ

pHs = − lg Ca2 +h i

− lg Alk½ � + K ð6Þ

pHb = 6:35 + lg HCO−3½ �− lg CO2½ � + 2 lg f ð7Þ

where K is the ionic strength constant, depending on watertemperature and ionic strength; f is activity coefficient of monovalention in water.

Main ion concentrations in the feed and brine were measured forcalculating S&DSI, shown in Table 4, and the results of S&DSI in the

Table 4Main ions concentration of feed and brine.

Items Feed (mg/L) Brine (mg/L)

K+ 346 578Na+ 9820 16,387Ca2+ 355 595Mg2+ 1220 2033SO4

2− 2000 3349Cl− 17,900 29,910HCO3

− 118 191TDS 33,330 55,665

feed and brine under different operating conditions are plotted versuswater pH in Fig. 8. It can be seen that the critical pHs for scaling on themembrane surface in the feed and brine profile were 6.7 and 6.25,respectively, which indicates that scales are inclined to deposit on themembrane surface in the brine profile compared to the feed profile . Inthis test, HCl was used to adjust the water pH to 5.8–6.2, which seemssuitable for this RO system to prevent scaling on the membranesurface so as to achieve a good and stable performance of HF ROsystem.

Generally, anti-scalant is the dominating chemical reagent tohinder scaling in a practical RO system. Thus, it is necessary toinvestigate the anti-scaling efficiencies and the chemicals costs of theanti-scalant and HCl for comparison. In this study, the anti-scalant(Hypersperse MDC220, purchased by US General Electric) wasinjected alone (2 mg/L in the feed) in a separate test. The performanceof this anti-scalant and HCl was shown in Fig. 9 and the costcomparison was given in Table 5. Viewed from the technique, HClinjection is more likely to achieve the higher Pp and lower η, both ofwhich indicate for the better performance of HF RO system, resultingfrom the suitable pH in the feed. Moreover, the feed pHwasmore than7.5 when MDC220 was injected alone, causing that NaClO worksincompletely. It might result in that bacteria and other microorgan-isms in the feed might be accumulated to form a biofilm on themembrane surface to degrade the RO performance. Viewed from theeconomy, although MDC220 dosage is much less than HCl, the cost ofMDC220 used was 1.4 times higher than HCl due to the expensive unitprice of MDC220.

Fig. 9. Effect of HCl and anti-scalant (MDC220) on performance of HF RO.

Table 5Cost of chemicals for preventing scale on membrane surface.

Dosage (kg/year/module) Total cost ($/year/module)

HCl 227.765 26.28MDC220 18.981 36.37

120 J. Xu et al. / Desalination 262 (2010) 115–120

4. Conclusion

From the conducted experimental study, two conclusions can bedrawn:

Firstly, comparing all the testingmodes of chlorine injection, mode5 that employed continuous chlorine with a relatively lowconcentration of 0.3–0.4 mg/L at pH 5.8–6.2 is preferable in theaspects of TBC removal, permeate flow rate, differential pressure,permeability (Pp) and performance factor (η). Note that the trendof Pp is not as obvious as the trend of η so that η can be used as amore accurate indicator to investigate the slight change of the ROperformance. HCl injection can improve sterilizing efficiency. Goodproduct quality with TDS of about 65 mg/L can be obtained by thisHF SWRO system, yet it cannot be used to evaluate the efficiency ofdifferent modes because very little differences in the productquality were observed. Viewed from the economy, chemical costsof modes 2 and 3 are the highest and lowest among all the testingmodes, respectively. Mode 5 is acceptable due to a slightly highercost of chemicals and its high RO performance.

Secondly, comparing with HCl and MDC220, it is found that HClinjection is the efficient alternative to other commercial anti-scalants for preventing scaling on the membrane surface, and ismore propitious to achieve the high Pp and low η. In addition, theannual cost of HCl is lower than MDC220.

Acknowledgements

The authors would like to acknowledge gratefully the Institute ofseawater desalination and multipurpose utilization, SOA (Tianjin),and Huangdao Power Plant in China for their contributions to thesuccess of this pilot test.

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