Desalination 222 (2008) 9–16

Presented at the conference on Desalination and the Environment. Sponsored by the European Desalination Societyand Center for Research and Technology Hellas (CERTH), Sani Resort, Halkidiki, Greece, April 22–25, 2007.

Pilot test of UF pretreatment prior to RO for cooling tower blowdown reuse of power plant

Jingdong Zhanga*, Huiming Zenga,b, Chunsong Yeb, Liang Chena, Xiaoxuan Yana

aDepartment of Environmental Engineering, College of Resources and Environmental Science, Wuhan University, Wuhan 430079, P.R. China

Tel. +86-27 68775699; Fax +86-27 68773516; email: jdzhang@whu.edu.cnbDepartment of Water Quality Engineering, College of Power and Mechanical Engineering,

Wuhan University, Wuhan 430079, P.R. China

Received 12 December 2006; accepted 3 January 2007

Abstract

Membrane processes are more and more used in treating waste water in order to reuse, and then alleviate waterscarcity problems. In this pilot test, ultrafiltration (UF) membrane was chosen as the pretreatment of reverse osmosis(RO) for the reuse of the cooling tower blowdown (CTB) of a power plant in north of China. The permeatequalities, permeability and membrane fouling of the two hollow fiber membrane modules — module A (outside-instyle) and module B (inside-out style) fed with raw CTB only with a disc filter were analyzed and compared whenthe flux changed stage by stage in range from 60 to 110 Lmh/m2. The results suggested that the permeate qualitiesof the two UF modules was similar and which could meet the requirements of the consequent RO for long-termfunction, module A had higher ability to resist to fouling than module B and it could run stable at or less thana flux of 90 Lmh/m2, and module B could only do that at or less than 80 Lmh/m2. The recovery of module A washigher, and the running capital of module A was lower.

Keywords: UF membrane; Cooling tower blowdown; Permeate quality; Recovery

1. Introduction

Water scarcity problems have been more andmore severe around the world and so great atten-tion is being paid into reclamation and reuse ofwastewater from municipalities and industrial

plants [1], especially in regions that shorten offresh water, it is important to recycle the wastewater discharged from industrial plants to meetthe stringent waste water discharge requirementsand save the fresh water sources.

Power plants need much fresh water to functionand a great deal of the waste water is dischargedwithout any recycle process, so a number of new*Corresponding author.

0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.doi:10.1016/j.desal.2007.01.123

10 J. Zhang et al. / Desalination 222 (2008) 9–16

power plants are being required to achieve zeroliquid discharge (ZLD). A zero liquid dischargesystem is required to accept waste water streamssuch as cooling tower blow down (CTBD),scrubber blow down, ash sluicing blow down, etc.,as feed. The CTBD is typically the largest andsometimes the only compound [2]. It is technicalfeasible to clarify and desalt the waste water,and then reuse it as makeup for boiler, coolingtower or other processes needing fresh water.

Membrane processes are thus increasinglypopular for wastewater reuse applications, sincethey could play a key role in removing the complexcomponents of dissolved and particulate mattercontaminants in wastewater [3]. It is reportedthat reverse osmosis (RO) has been used last20–30 years for desalination of water sourcesseawater and brackish water for potable waterproduction. Also, the advanced waste water treat-ment facility, using microfiltration plus RO, willhave a capacity of 38,000 t/d produce water [4].

The critical issue for a successful RO plantis the pretreatment, pretreatment design has toensure that the quality of the water fed to theRO membrane consistently high and avoidvariability in the feed water. In this pilot study,

two ultrafiltration module (one was outside-instyle, the other was inside-out style) were chosenas the pretreatment for consequent RO process,and the permeate quality, the permeability andthe fouling of them were focused and compared.

2. Pilot and membrane description

The pilot test was carried out in the InternationalDatang zhangjiakou Coal-fueled Power Plant(Zhangjiakou, Heibei, China) during March 1,2005 and May 30, 2005 (90days). The control andautomatic analysis system concluded a group oftransducers and some valve connected with apiece of single chip machine (SCM) and a com-puter, which could accomplish to control thefacility, make some of the analysis (pH, velocityof flow and pressure) and data collection.

Fig. 1 shows schematic diagram of the pilottest system: the raw water was pumped up froma cooling tower tank, got through the disc filter,and then went into the feed water tank, the feedpump pumped the feed water into the two UFmodules, and the filtrate of the UF got into theproduce water tank. The backwash pump got thefiltrate from the tank as backwash water, and

Fig. 1. Schematic diagram of the pilot test system.

J. Zhang et al. / Desalination 222 (2008) 9–16 11

then pumped into the two modules to make back-wash; The disposal of backwash was dischargedor reused by other methods. There are three autochemical injection systems in this UF processsystem: one is on the filtrate pipeline of theoutside-in module, the other is on the feed pipe-line of the inside-out module, and another is onthe feed pipeline of the disc filter, which is usedto make on-line coagulation.

The outside-in UF module was definedmodule A, and the inside-out module was definedmodule B. They were both running in dead-endstyle vertically in this pilot test. The characteristicsof the UF membrane were particularly shown inTable 1.

The main operation parameters of the mem-branes were shown in Table 2.

3. Characteristics of cooling water tower discharge in testing place

The site of this pilot test was in north of China,which is part of the typical dry climate regions.The main fresh water source of the power plant is

underground water, whose typical characteristicsare high salinity and hardness, but low turbidityand organic matters. And the concentration ofdissolved solid of the cooling tower blowdownis higher because of chemical injection andbeing concentrated by evaporation. The particularfeed water quality is shown in Table 3.

4. Pilot operation

The main changes of operating modes wereshown in Table 4. From March 7 to May 30, wechanged the flux of the two modules stage bystage, and then analyzed the TMP and the perme-ate qualities in every stages to find out the functionand fouling status of the membranes, in this test,backwash flux and backwash frequency werealso adapted to cost-effective running, at the endof the pilot test, the flux of two membranes wereincreased to 110 Lmh/m2 to analyze the foulingand recovery of chemical cleaning of the twoUF membrane.

Table 1UF membrane characteristics

Items Value

Module A Module B

Membrane material PVDF PES Style Outside-in

hollow fiberInside-out

hollow fiberMembrane area, m2 33 40 Nominal pore

diameter, µm 0.03 0.03

Number of hollowfiber per module

6000 10,992

Fiber inner/out diameter, mm

0.7/1.25 0.8/1.5

Pure water permeability,Lmh/bar

>100 500

Table 2Main pilot process parameters of the two UF modulesduring testing period

Operation parameters Membrane operating range

Module A Module B

Flux, L/m2/h 60–110 60–110 pH 8.5–8.7 8.5–8.7 Temperature, °C 5–30 5–30 BW duration, seconds/

frequency, min90/30–45 90/20–30

BW flux, L/m2/h 112–124 194–240 Feed pressure, bar <1.5 <2.0 Operating TMP, bar 0.3–1.1 0.5–1.1 Max. total suspended

solids, mg/L 80 80

NaOCl, cleaning maximum, mg/L

200 200

Air flush/CEB frequency, h 6–9 6–9 Cleaning chemicals NaOCl HCl Recovery 86.4–94.8% 66.5–81.8%

12 J. Zhang et al. / Desalination 222 (2008) 9–16

5. Results and discussion

5.1. Produced water quality of UF

Table 5 showed the results of chemical analysison filtrate by UF system, the concentration of

CODMn, total Fe, colloidal silica, total Cu, phos-phate and total organic phosphate in the permeatehad decreased in different extent, but the hardnessof the water had changed very little, the removalof total Fe and colloidal was higher than otherdissolved solid in the cooling tower blowdown.In integrity, module A had a bit higher removalefficiency of total Fe and Cu, otherwise, module Bcould remove colloidal silica and phosphate a littlemore. It demonstrated that the two UF membraneshad a similar ability to removal CODMn, total Fe,colloidal silica, etc. Matters in the feed waterremained in solid or colloidal style would havehigher removal, but that remained in ions wouldhave little. Also, in this alkaline condition, the col-loid grew up easily and the removal efficiencyincreased.

It was showed in Fig. 2 that the turbidity of UFfiltrate: the main value of the permeate turbidityis 0.15–0.3 NTU, which declined first, then keptstable, and went up after a long-term function, andfinally kept at a little higher level. The differ-ence between the filtrate of two modules waslittle. It suggested that the turbidity variety ofthe UF membrane was associated with the feedwater temperature and the fouling of the mem-brane. At the beginning of the pilot test, the new

Table 3Water quality of cooling water tower discharge in testsite

Parameter The cooling tower blowdown

Mean value Range

pH 8.55 8.2–8.75 Temperature, °C 18 5.0–21.0 Turbidity, NTU 7 5.0–23 Total dissolved solid, mg/L 1342 1200–1400Suspended solid, mg/L 10 7.0–22 Conductivity, μS/cm 1500 1300–2000Dissolved oxygen, mg/L 9.4 8–10 COD, mg/L 3.5 3–4.8 Total Fe, mg/L 150 50–280 Silica as SiO2 (colloidal),

mg/L 140 50–200

Remaining chlorine, mg/L 0.03 0.02–0.05 Alkalinity (CaCO3), mg/L 356 350–370 Inorganic nitrogen, mg/L 0.424 0.3–0.5

Table 4Main changes of operating modes

Date Flux, Lmh/m2 Backwash flux, Lmh/m2 Backwash frequency, min

Module A Module B Module A Module B Module A Module B

Mar. 7–Mar. 14 60 60 133/112 172 30 20 Mar. 14–Apr. 6 60 60 112/118 170 30 20 Apr. 6–Apr. 11 90 90 124/115 170 30 20 Apr. 6–Apr. 16 90 60 124/115 210 30 20 Apr. 16–Apr. 18 90 60 124/115 210 45 30 Apr. 18–Apr. 26 90 70 124/115 210 45 30 Apr. 26–Apr. 29 90 80 124/115 210 45 30 Apr. 29–May 2 60 80 124/115 210 45 30 May 2–May 21 75 75 124/115 210 45 30 May 21–May 23 110 110 124/115 210 45 30 May 23–May 31 80 80 124/115 210 45 30

J. Zhang et al. / Desalination 222 (2008) 9–16 13

membrane with clean pore could have a littlelower ability to remove the small solid in thefeed water, when the temperature increased, thediameter of the UF membrane pore wouldincrease, and which could decrease the ability ofremoval.

Fig. 3 showed that the trends of UF filtrateSDI over date: the main value of SDI was less

than 2.5, at the beginning of the pilot test, thevalues were the lowest, and then increasedslowly, that lasted to April 6. After that, the valueswere fluctuating around 2.5. The SDI of the fil-trate from module A was smoother though theflux changed stage by stage, while that from mod-ule B fluctuated much harder intense with changesof flux. It demonstrated that the inside-out

Feb. 28 Mar. 13 Mar. 26 Apr. 8 Apr. 21 May 4 May 170.0

0.2

0.4

Tur

bidi

ty (

NT

U)

Date

Turbidity of the out-inside module

Turbidity of the inside-out module

May 30

Fig. 2. Turbidity of UF filtrate.

Table 5Results of chemical analysis on filtrate by UF system

Description Value (mean or range)

Disc filtrate Module A Module B

pH 8.2–8.75 8.2–8.75 8.2–8.75 CODMn, mg/L 3–4.8 1.5–3.0 1.7–2.9 Total Fe, µg/L 50–280 3.1–70.7 9.6–89.2 Silica as SiO2 (colloidal), mg/L 50–200 1–1.5 0.8–1.2 Cu, mg/L 35.5–98.5 20.6–83.2 24–85.7 Hardness, mg/L 8–20 8.8–19.7 8.5–19.4 Athophosphate, mg/L 0.5–2.0 0.2–0.72 0.17–0.71 Total organic phosphate, mg/L 1.2–2.9 1.8–2.2 1.2–1.9

14 J. Zhang et al. / Desalination 222 (2008) 9–16

membrane (module B) was susceptible to changesof the feeding flux or TMP and fouling of themembrane. When the flux increased, the diameterof hollow fibers would increase under the higherinner pressure, and then the solid matter could getthrough the membrane pore more easily. The SDIwould decrease when the flux decreased coulddue to fouling of the membrane pore.

5.2. Flux and TMP trends of the UF

Fig. 4 displayed the trends of flux and TMPover the date: the mainly trends was that TMPincreased along with the flux being changed toanother higher value, but in the same stage of theinitial flux, TMP went up and the flux declinedslowly due to the fouling of the UF membranes.It also shows that module A could run consistentlyat or less than 90 Lmh/m2, while module B couldrun consistently at or less than 80 Lmh/m2 underthis feed water. However, two modules both hadability of recovery by normal backwash after

membrane fouling due to the flux increasingexcept module B could not recover by itself butby thorough chemical cleaning when the fluxgot to 110 Lmh/m2, but module A could recoverwithout chemical cleaning though the mem-brane fouling was also serious.

It indicated that the backwash style played animportant role in long-term function of the mem-brane, in this feeding condition, normal backwashplus air flush and air/water flush was more effi-cient style to resist membrane fouling than CEB,which might due to the matters in the feed waterwere almost inorganic and the main residual in themembrane pore could be particles that could notbe dissolved by HCl or other chemical. Anothercause could be: the inside-out hollow fiber mem-brane pore being expanded and the particlesgetting in under the higher TMP, and then theresidual being difficult to be washed out.

Fig. 5 showed the FPI of the two modulesover the date, the value of which fluctuated inthe range from 75 Lmh/m2/bar to 150 Lmh/m2/bar,

Mar. 1 Mar. 14 Mar. 27 Apr. 9 Apr. 22 May 5 May 180

1

2

3

4

5

SD

I

Date

SDI of outside-in module filtrate

SDI of inside-out module filtrate

May 31

Fig. 3. SDI of UF filtrate.

J. Zhang et al. / Desalination 222 (2008) 9–16 15

Feb. 24 Mar. 9 Mar. 22 Apr. 4 Apr. 17 Apr. 30 May 13 May 26 Jun. 8

–50

0

50

100

150

200

250

300

350

FT

P (

Lmh/

m2 /

bar)

Date

FTP of outside-in module

FTP of inside-out module

Fig. 5. FPI over the date.

Feb

. 24

Mar

. 6

Mar

. 16

Mar

. 26

Apr

. 5

Apr

. 15

Apr

. 25

May

5

May

15

May

25

Jun.

4

–20

0

20

40

60

80

100

120

Flux of outside-in module Flux of inside-out module TMP of outside-in module Tmp of inside-out module

Date

Flu

x (L

mh/

m2 )

0.5

1.0

1.5

2.0

2.5

3.0

TM

P (bar)

Fig. 4. Flux and TMP over the date.

16 J. Zhang et al. / Desalination 222 (2008) 9–16

and was higher at the beginning but lower atthe end, and the trend of module B was moreevident. In integrity, the main value of moduleA was a little higher; the difference was about20 Lmh/m2/bar. It demonstrated that the perme-ability of module A was a little better, whichcould also result from the higher ability of thebackwash style to remove the residual from themembrane pore.

6. Conclusions

The filtrate qualities of the two modules weresimilar to each other, and quite stable, which couldmeet the feed requirement of consequent reverseosmosis process for long-term function. The SDIvalues of module A was a little higher but morestable.

Module A could run stably at or less than90 Lmh/m2, and module B could do that at or lessthan 80 Lmh/m2, they both had ability to recoverafter fouling due to flux changes, the ability ofmodule A to resist to fouling and endure the

variety flux loading was higher by the more effi-cient backwash style — air flush and air/waterflush. The permeability of module A under varietyflux was higher and more stable than module B.

The recovery of module A was higher thanmodule B under the same flux because CEBneeded much more time than air flush.

References

[1] R. Mujeriego and T. Asano, The role of advancedtreatment in wastewater reclamation and reuse,J. Water Sci. Technol., 40 (1999) 1–9.

[2] Allen R. Boyce and Michael Ferrigne, New zeroliquid discharge strategy — boiler makeup fromcooling tower blowdown, Proc. Intl. Water Con-ference, Pittsburgh, PA, October, 1999.

[3] Suck-Ki Kang and Kwang-Ho Choo, Use of MFand UF membranes for reclamation of glass indus-try wastewater containing colloidal clay and glassparticles, J. Membr. Sci., 223 (2003) 89–103.

[4] Manual P. del Pino and Bruce Durham, Waste waterreuse through dual-membrane process: opportuni-ties for sustainable water sources, J. Desalination,124 (1999) 271–277.