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Pilot testing of outside-in MF and UF modules used for cooling tower blowdown pretreatment of power plants
Desalination 214 (2007) 287–298
Pilot testing of outside-in MF and UF modules used for cooling tower blowdown pretreatment of power plants Jingdong Zhang*, Lia Chen, Huiming Zeng, Xiaoxuan Yan, Xiaoning Song, Hong Yang, Chunsong Ye Department of Environmental Engineering, School of Resources and Environmental Science, Wuhan University, Wuhan 430079, PR China Tel. +86 (27) 6877-5699; Fax +86 (27) 6877-3516; email: [email protected] Received 5 December 2005; Accepted 18 December 2006
Abstract This paper describes pilot test results obtained on cooling tower blowdown of a power plant in HeBei province, using both 0.1 µm MF and 0.03 µm UF, which are outside-in hollow fiber membrane modules. In this pilot, the MF and UF modules were installed at the same system, although at different testing periods. Operation parameters such as flux, backwash (BW) time, BW frequency, frequency of air backwash (ABW) were changed; the MF and UF modules were operated at constant operating conditions during the same testing period. The study showed that both the MF and the UF membrane systems can produce permeate with high and consistent quality. The filtrate SDI15 of the permeate produced by both systems was below 3 for most of the time (except the period of flocculation addition). This water quality meets the requirements of the reverse osmosis (RO) system. In-line rapid pre-coagulation using polyaluminium chloride (PAC) at a concentration of 2.5–5 ppm did not decrease the filtrate silt density index (SDI15) and may have even led to a sharp increase in filtrate SDI15, especially when combined with polyacrylamide (PAM). In addition, the injection of polymerized ferric sulfate (PFS) and sodium hypochlorite (NaClO) to the pretreatment system feed water had little influence on the filtrate SDI15. Other water quality parameters such as total Fe, Cu, COD and colloidal Si were also measured. The UF membrane module reached a higher level of FPI than the MF membrane module did, while the latter had more stable performance than the former. The MF/UF test has achieved stable membrane permeability, and both MF and UF modules were found suitable to provide adequate pretreatment prior to RO desalination. Keywords: Outside-in; MF; UF; SDI; Cooling tower blowdown
*Corresponding author. 0011-9164/07/$– See front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.12.004
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1. Introduction Power plants are among the biggest waterconsuming industries. Of all systems in a power plant, most water is consumed in the cooling tower system, of which the feed water of the cooling tower accounts for the largest portion of water demand. Because a significant amount of water is lost due to evaporation, wind action, leakage and drainage, a large amount of feed water is needed to keep the water balance and cooling water operation at a steady-state. The loss caused by evaporation, wind action and leakage is related to the climate conditions and the configuration of the cooling system equipment, while drainage loss which constitutes the biggest portion of the feed water losses, varies greatly with changes in source water quality and cooling water treatment. For a long time, the cooling tower blowdown of power plants was not reused in China, and instead the blowdown was discharged directly to surface water bodies. This not only caused environmental contamination but also wasted water resources. Until recently, in most domestic power plants, cooling tower blowdown has been used for ash-flushing. However, with the development of high concentration ash hydraulic transport and dry dust removing technology, the amount of cooling tower blowdown has increased significantly as compared to the demand of the ash-slag disposal system. Meanwhile, the quality of cooling tower blowdown has improved as compared to the regenerating wastewater of ion-exchange systems, so reusing cooling tower blowdown of power plants has become viable and necessary [1]. Presently, cooling tower blowdown in most of domestic power plants is treated with a combination of conventional techniques such as biochemical treatment prior to desalination by ion exchange. However, ion-exchange technology has a number of disadvantages such as significant space requirements, long treatment time, frequent regeneration of resin, high running cost, etc.
UF can effectively remove particulates, colloids, bacteria and some viruses, although it cannot provide significant removal of inorganic ions [2–4]. MF has a similar removal capacity although it cannot remove viruses effectively. The main differences between MF and UF are the nominal pore diameter and molecular weight cutoff of the membrane fibers. As the pretreatment step for RO desalting, three processes were compared by Gabelich et al. in 2002. The pretreatment processes tested were conventional treatment (coagulation, flocculation, sedimentation and dual-media filtration), conventional treatment with pre-ozonation and biologically active dual-media filtration (biofiltration) and microfiltration. The main results are shown in Figs. 1 and 2 [5], which show that no matter what the change is in terms of turbidity, particle-count and SDI of raw feed water, MF always has superior performance and was therefore the best choice for RO pretreatment for the site-specific feed water tested. In recent years, the use of MF or UF pretreatment in RO plants has gained significant popularity [6] and its application for treatment of power plant cooling tower blowdown is increasing. The power plant where this pilot test has been completed is located in the northwest of HeBei province of China, where the water resources are in serious shortage and groundwater is mainly used for potable purposes. This coal-fueled power plant is constructed with eight power generators (8×300 MW), and the amount of circulating cooling water used is about 20,000 m3/h per generator. So reusing the cooling tower blowdown, the total amount of which can reach 785 m3/h, by the treatment of an integrated membrane system would be beneficial. However, the safe, consistent, and cost-effective operation of the MF or UF system is critical for the performance of the whole system. In order to select the proper membrane type and obtain actual running data, this pilot test was completed between March and
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J. Zhang et al. / Desalination 214 (2007) 287–298
June 2005. One of the purposes of this pilot test was to compare the reliability and performance of MF and UF systems as pretreatment for RO, and to identify which type of membrane system (MF or UF) is more suitable for the quality of this
cooling tower blowdown. The other purpose was to test the effect of various water conditioning chemicals (such as PAC, PAM, PFS and NaClO) on membrane performance and permeate water quality. The filtrate quality, the operational parameters and the performance of the tested MF/UF systems were also evaluated. During continuous operation of nearly three months, flux, TMP and filtrate quality were monitored.
2. Characteristics of cooling tower blowdown
Fig. 1. Raw water turbidity (top) and particle-count (bottom) data (Bos-and-whisker plots show minimum, 25th percentile, median, 75 percentile, and maximum values).
The most distinctive characteristics of the circulating cooling water quality at this test power plant site were the significant fluctuation of turbidity and the wide range of temperature variation. The turbidity varied from 2.0 NTU to 32.8 NTU during testing, and the lowest temperature in March was less than 4EC while in May the temperature reached 25EC, as shown in Figs. 3 and 4. These fluctuations are due to the geographical location and weather conditions. In the northwest of HeBei province where there is a significant amount of sand blown by wind, and for the opening pattern of the storage tank under the cooling tower, when the wind blows, the turbidity of the raw water increased sharply. In addition, rainy weather could also lead to an increase in raw water turbidity. Due to the siteTable 1 Cooling tower blowdown quality during test
Fig. 2. Treated water turbidity (top), particle-count (middle) and SDI (bottom) data (Bos-and-whisker plots show minimum, 25th percentile, median, 75th percentile, and maximum values).
Parameters
Range
Mean value
Turbidity, NTU Temperature, EC pH CODMn, mg/L Total Fe, μg/L Cu, μg/L Silica as SiO2 (colloidal), mg/L PO43!, mg/L
2–33 4–28.5 8.2–8.8 2.9–4.8 47–197 35.5–103.5 28–109
9.87 16.4 8.5 3.5 93.3 70.7 56.3
0.72
1.1
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Fig. 3. Cooling tower blowdown turbidity during testing.
Fig. 4. Cooling tower blowdown temperature during testing.
specific geographic location, the temperature between spring and summer varied greatly and the diurnal temperature variation exceeded 10EC. Large amounts of scale inhibitor, corrosion inhibitor and biocide, which contain cupreous chelate complex and phosphorous compound, were typically added to the circulating cooling water every day. Therefore, Cu, PO43! and other water quality parameters were also measured; the means and the variation ranges of these water quality parameters are shown in Table 1.
3. Pilot unit and membrane description The pilot unit is a fully automated system with data recording features. It operates at a constant filtrate flow and performs time-based backwash (BW) and air backwash (ABW) sequences, and all operational processes of the entire system are run by a programmable logic controller (PLCs). The pilot system was equipped with four membrane modules: two of these four modules were outside-in hollow fiber membrane systems;
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291
Fig. 5. Schematic diagram of the MF/UF systems.
one was equipped with MF membranes (Module A) and the other was with UF membranes (Module B). They used the same feed water and backwash water. The cooling tower blowdown was pumped from the pool of cooling tower, and after pre-filtration by a disk filter (nominal pore diameter of 50 µm), the prescreened blowdown was conveyed to a feed tank. The permeate water produced by the MF and UF systems was combined. A portion of the permeate was used for the BW and ABW processes, and the rest was drained back into the pool of the cooling tower. The pool was approximately 2 m deep and 10 m in diameter and the volume of tower blowdown was about 20,000 m3/h. For comparison, the volume of the pilot plant permeate was significantly smaller and did not have a measurable effect on the feed water quality. Both Modules A and B operated in dead-end mode. Fig. 5 shows a schematic diagram of the pilot testing system. The key treatment steps incorporated in Modules A and B were:
C The filtration process: feed 6 feed pump A 6 valve 1 6 Module A 6 valve 2 6 backwash tank. C The backwash process: permeate 6 BW pump 6 valve 3 6 Module A 6 valve 4 6 BW effluent A (step 1). C Permeate 6 BW pump 6 valve 3 6 Module A 6 valve 5 6 BW effluent A (step 2). C The air scouring process: air 6 valve 6 6 Module A 6 valve 5 6 BW effluent A. C The ABW process: permeate 6 BW pump 6 valve 3 6 Module A 6 valve 5 6 BW effluent A. C At the same time, air 6 valve 6 6 Module A 6 valve 5 6 BW effluent A. C The forward flush process: feed 6 feed pump A 6 valve 1 6 Module A 6 valve 5 6 BW effluent A. Both the MF and UF membranes have a hollow fiber outside-in configuration made of polyvinylidene fluoride (PVDF). Compared to
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Table 2 MF and UF membrane characteristics Item Membrane material Membrane area, m2 Number of hollow fiber per module Nominal pore diameter, µm Fiber inner/out diameter, mm Cartridge size, mm Pure water permeability, L/(m2·h·bar)
Table 3 Key operational parameters of the pilot system during test period
Module A (MF)
Module B (UF)
PVDF 36 4680
PVDF 33 6000
0.1 0.7/1.3
0.03 0.7/1.25
139.6 φ × 2338 L —
— $ 100 (@25EC)
inside-out flow configuration, outside-in flow configuration has the following benefits: less plugging, higher solids loading, higher flow area and easy cleaning. The flexible PVDF fibers show very good elasticity performance, allowing strong repetitive mechanical shocks such as frequent backwashing or air scouring. In addition, the PVDF fiber is chemically stable and very resistant to oxidants [7]. Table 2 shows the key characteristics of MF and UF membranes. 4. Pilot operation During initial tests not presented in this paper, preliminary operating parameters such as flux and backwash frequency were adjusted. The initial runs were followed by optimization tests described below. The key operational parameters of the pilot system during the test period are presented in Table 3. Four test runs were completed. The first test run continued from March 14, 2005 to April 6, 2005: a period when the filtrate flux was set at 60 L·m!2h!1, the filtration time was set at 30 min and the ABW interval was set at 8 times. The period of air scrubbing and air backwash
Parameter
Module A (MF)
Module B (UF)
Inlet flux rate, L·m!2h!1 Flux of filtrate, L·m!2h!1 Permeate water flux, m3·h!1 BW flux (step 1/step 2), m3·h!1 Filtrating time, min BW time (step 1), s BW time (step 2), s Air scrubbing time, s ABW time, s Forward flushing time, s ABW interval Air flux, m3·h!1 Recovery of permeate water, %
60–110 60–110 2.2–4.0 3.3/3.1
60–110 60–110 2.0–3.6 3.8/4.1
30–45 45 45 30 20 60 8–12 3.3–3.5 88.46– 94.76
30–45 45 45 30 20 60 8–12 5–6 86.41– 94.81
frequency was 4 h (4 h = 30 min×8). During the second period (April 6, 2005–May 2, 2005), the filtrate fluxes of both Module A and Module B were increased to 90 L·m!2h!1, and the period of air scrubbing and air backwash frequency was increased to 9 h (9 h = 45 min×12). At the end of this period, a chemical cleaning using HCl was completed of both Modules A and B. Then the filtrate flux was adjusted to 75 L·m!2h!1 to assess the effect of flux on membrane recovery ratio (the third period encompassed between May 2, 2005– May 20, 2005). The production rate was further increased during the fourth period (May 21, 2005 –May 31, 2005), when the filtrate fluxes of both Modules A and B reached about 110 L·m!2h!1. During this period, another chemical cleaning was completed on May 23, after two days of operation at flux of 110 L·m!2h!1. In addition, during the second and third periods, PAC, PAM, PFS, NaClO and sulfuric acid (H2SO4) were injected to the feed water, to assess their effect to the permeate SDI15.
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What has to be pointed out is that during each period, both of the two membrane modules were operated continuously 24 h per day, with the exception of brief periods when there was a malfunction of the pilot system. After the first and fourth test periods, membrane integrity of both MF and UF was measured, and the results showed that there is no evident change. 5. Results and discussion 5.1. Permeate quality of MF and UF Determining whether the permeate quality of these two modules could meet the pretreatment requirements of the RO system is the most important purpose of the trial. One of the important RO feed water quality parameters is SDI15. In addition, the RO system has requirements regarding the concentration of Fe, colloidal Si and COD in feed water. Therefore, during testing, not only SDI15 but also the concentrations of Fe , colloidal Si and COD of permeate were measured. The results of the turbidity and SDI15 measurements of the two filtrate modules are presented in Figs. 6 and 7, respectively. The concentrations of total Fe, colloidal Si, COD and Cu etc. are shown in Table 4. According to Fig. 6, although the turbidity of the raw water fluctuated in wide range (2.0– 32.8 NTU), the turbidities of MF module and UF module permeate were much more stable, and were in a range between 0.1 NTU and 0.3 NTU during the entire trial. In the third period, there was an increase in the turbidity of the permeate of both membranes modules. This permeate water quality deterioration is most likely due to the injection of PAC, PAM, PFS, NaClO and H2SO4. Fig. 7 shows the SDI15 trend of both module permeates. For both the MF and UF module, the permeate SDI15 was below 3 for most of the time, although there was a slight increase and fluctuation during the last two testing periods. On April 29, the permeate SDI15 of the MF module was
Table 4 Results of chemical analyses of feed water and two MF/UF module permeates Parameters
Feed water (permeate of disc filter)
MF module
UF module
CODMn, mg/L Total Fe, μg/L
2.9–4.8 47–197
1.5–3.5 19–48
1.4–4 15–44
Cu, μg/L
35.5–98.5
20–86
20–83
14–109 Silica as SiO2 (colloidal), mg/L PO43!, mg/L 0.7–2
9.6–70.4 1.2–32 0.1–0.8
0.2–0.8
more than 3. High levels of organic compounds and associated biofauling, micro-air bubbles and colloidal substances are the three most likely reasons for the unexpected increase in SDI15. The higher SDI levels may also be related to the increase in colloidal matter due to the injection of PAC and PAM. A layer of colloidal matter was observed over the surface of SDI test pads, which was considered to be Al(OH)3. Sulfuric acid was also added to raw water on May 15 to adjust the pH of feed to 6.5. This chemical addition also caused an increase in permeate SDI15. The addition of PAC at 2.5 ppm or 5 ppm did not result in a decrease in permeate SDI15, either. When raw water was conditioned with 5 ppm of PFS and 3 ppm of NaClO, a small SDI15 decrease was observed. Fig. 7 also shows that the permeate SDI15 of the MF module was lower than that of UF module in the first two testing periods. However, the permeate water quality fluctuated greatly after the injection of PAC, PAM, PFS, NaClO and H2SO4. This allows the conclusion that MF membrane performance was more easily affected by the change of source water feed. Compared to MF, the permeate SDI15 of UF module was more stable during the entire test period. Key feed and permeate water quality parameters for the two MF and UF modules are
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Fig. 6. Turbidity of MF and UF permeate during the trial.
Fig. 7. SDI15 trends of MF and UF permeate.
shown in Table 4. According to Table 4, both the MF and the UF modules had a high removal efficiency of total Fe, with average removal of 69.8% and 70%, respectively. As to removal of colloidal silica, UF module reached average efficiency of 65.3%, while MF module could
only achieve 45.3% removal, a little lower than that of UF module. In addition, the MF module achieved average removal of CODMn and PO43! of 37.5% and 56.5%, respectively. For the UF module, the average removal of CODMn and PO43! was 40.8% and 55.4%. In addition, both the MF
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Fig. 8. Flux and TMP over time of the MF module.
Fig. 9. Flux and TMP over time of the UF module.
and the UF modules did not remove significant amounts of copper. The average removal efficiencies were less than 17%.
5.2. Flux and TMP trends of the MF/UF system As shown in Figs. 8 and.9, the TMP of both the MF module and the UF module increases with
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flux increase. From the first testing period to the second testing period, flux was increased from 60 L·m!2h!1 to 90 L·m!2h!1, and then was adjusted to 75 L·m!2h!1 during the third testing period. Correspondingly, the TMP of the MF module increased from about 0.7 bars to about 1.1 bars, then dropped to 0.8–0.9 bars, while the TMP of the UF module increased from 0.5 bars to 0.8– 0.9 bars, then dropped to about 0.7 bars. Then during the fourth testing period, neither the MF nor the UF module could perform adequately at a higher flux of 110 L·m!2h!1. Both TMPs increased sharply and flux decreased rapidly. This illustrates that at this level of very high flux, membrane fouling of the MF module and UF module vas very rapid and significant. In addition, comparing Fig. 8 with Fig. 9, the TMP of the MF module fluctuated at greater rate, and was higher than that of the UF module by about 0.2 bars at the same flux. This result indicates that the MF membranes fouled at a higher rate than UF membranes. Because the pore
Fig. 10. FPI comparison of MF and UF modules.
diameter of MF membranes is larger than that of UF membranes, particulate substances are more likely to enter into the membrane pores of MF membranes, which likely contributes to the faster fouling rate. 5.3. Comparison of FPI of MF and UF modules FPI is an important economic indicator of MF/UF filtrate. The FPI is defined as flux divided by transmembrane pressure. Fig. 10 shows the FPI of the MF and UF modules for the entire test. In the first two testing periods, the FPI of the UF module was much higher than that of the MF module. However, the performance of the MF module was more stable and consistent: the FPI was in a range of 70–80 L·m!-2h!1bar!1, while the FPI of the UF module fluctuated in a wider range, from 110 to 135 L·m!2h!1bar!1. After chemical cleaning, there was a little increase of the FPI of the MF module in the third testing period. This illustrates that membrane perfor-
J. Zhang et al. / Desalination 214 (2007) 287–298
mance of both modules could be recovered successfully, although the membrane fouling of the MF module was more serious than that of the UF module. In addition, during the high-flux run (fourth period), with the continuous increase of TMP, a sharp decline of FPI could be observed for both the MF and the UF modules.
297
5.4. Comparison of chemical cleaning effect on performance of the MF and UF modules During each chemical cleaning, feed water was conditioned with HCl to adjust pH to about 2.9, and the MF and UF membranes were submerged in the acid solution for about 2 h before forward flushing and backwashing. A high
Fig. 11. Chemical cleaning effect after the second testing period.
Fig. 12. Chemical cleaning effect after two-day running at a flux of 110 L·m!2h!1.
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recovery ratio of both MF and UF membranes after chemical cleaning was observed (see Figs. 11 and 12).
6. Conclusions Both the MF and the UF membrane modules performed well in terms of permeate consistency and quality. Permeate SDI15 of the UF module remained below 3 at all times and permeate SDI15 of MF module was below 3 for most of the time of the test. The UF membrane module reached a higher FPI than the MF membrane module, while the performance of the latter was more stable. Both the MF and the UF modules operated at adequately high recovery, low TMP and simple ABW to reduce the plant’s overall treatment costs. The addition of PAC, PAM, PFS or NaClO to the feed water did not result in a significant improvement of the permeate quality. In summary, this pilot study demonstrates that both MF and UF modules are suitable for RO pretreatment for this industrial application.
References [1] J.P. Su, L.Q. Yi, L.P. Mo and S.L. Tan, Using superfilter film to treat circulating cooling water drainage, Hebei Electric Power Technol., 3 (2004) 35–37. [2] L. Viker, C.K. Colton, K.A. Smith and D.L. Green, The osmotic pressure of concentrated protein and lipoprotein solutions and its significance to ultrafiltration IV, J. Membr. Sci., 70(1) (1984) 63–77. [3] S. Nakao, J.G. Wijmans and C.A. Smolders, Resistence to the permeate flux in unstirred ultrafiltration of dissolved macromolecular solutions. J. Membr. Sci., 26(2) (1986) 185–187. [4] J.G. Wijmans, S. Nakao and C.A. Smolders, Flux limiting in ultrafiltration: osmotic pressure model and gel layer model. J. Sci., 20(1) (1984) 115–129. [5] C.J. Gabelich, T.I. Yun, B.M. Coffey and I.H. Suffet, Pilot-scale testing of reverse osmosis using conventional treatment and microfiltration. Desalination, 154 (2003) 207–223. [6] A. Massaki, I. Satoru, I. Hiroshi and T. Naoki, Peculiar or unexpected behavior of silt density index of pretreated seawater for RO desalination. IDA, BAH03-071, 2003. [7] D. Vial and G. Doussau, The use of microfiltration membranes for seawater pre-treatment prior to reverse osmosis membranes, Desalination, 153 (2002) 141–147.
Pilot testing of outside-in MF and UF modules used for cooling tower blowdown pretreatment of power plants Jingdong Zhang*, Lia Chen, Huiming Zeng, Xiaoxuan Yan, Xiaoning Song, Hong Yang, Chunsong Ye Department of Environmental Engineering, School of Resources and Environmental Science, Wuhan University, Wuhan 430079, PR China Tel. +86 (27) 6877-5699; Fax +86 (27) 6877-3516; email: [email protected] Received 5 December 2005; Accepted 18 December 2006
Abstract This paper describes pilot test results obtained on cooling tower blowdown of a power plant in HeBei province, using both 0.1 µm MF and 0.03 µm UF, which are outside-in hollow fiber membrane modules. In this pilot, the MF and UF modules were installed at the same system, although at different testing periods. Operation parameters such as flux, backwash (BW) time, BW frequency, frequency of air backwash (ABW) were changed; the MF and UF modules were operated at constant operating conditions during the same testing period. The study showed that both the MF and the UF membrane systems can produce permeate with high and consistent quality. The filtrate SDI15 of the permeate produced by both systems was below 3 for most of the time (except the period of flocculation addition). This water quality meets the requirements of the reverse osmosis (RO) system. In-line rapid pre-coagulation using polyaluminium chloride (PAC) at a concentration of 2.5–5 ppm did not decrease the filtrate silt density index (SDI15) and may have even led to a sharp increase in filtrate SDI15, especially when combined with polyacrylamide (PAM). In addition, the injection of polymerized ferric sulfate (PFS) and sodium hypochlorite (NaClO) to the pretreatment system feed water had little influence on the filtrate SDI15. Other water quality parameters such as total Fe, Cu, COD and colloidal Si were also measured. The UF membrane module reached a higher level of FPI than the MF membrane module did, while the latter had more stable performance than the former. The MF/UF test has achieved stable membrane permeability, and both MF and UF modules were found suitable to provide adequate pretreatment prior to RO desalination. Keywords: Outside-in; MF; UF; SDI; Cooling tower blowdown
*Corresponding author. 0011-9164/07/$– See front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.12.004
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1. Introduction Power plants are among the biggest waterconsuming industries. Of all systems in a power plant, most water is consumed in the cooling tower system, of which the feed water of the cooling tower accounts for the largest portion of water demand. Because a significant amount of water is lost due to evaporation, wind action, leakage and drainage, a large amount of feed water is needed to keep the water balance and cooling water operation at a steady-state. The loss caused by evaporation, wind action and leakage is related to the climate conditions and the configuration of the cooling system equipment, while drainage loss which constitutes the biggest portion of the feed water losses, varies greatly with changes in source water quality and cooling water treatment. For a long time, the cooling tower blowdown of power plants was not reused in China, and instead the blowdown was discharged directly to surface water bodies. This not only caused environmental contamination but also wasted water resources. Until recently, in most domestic power plants, cooling tower blowdown has been used for ash-flushing. However, with the development of high concentration ash hydraulic transport and dry dust removing technology, the amount of cooling tower blowdown has increased significantly as compared to the demand of the ash-slag disposal system. Meanwhile, the quality of cooling tower blowdown has improved as compared to the regenerating wastewater of ion-exchange systems, so reusing cooling tower blowdown of power plants has become viable and necessary [1]. Presently, cooling tower blowdown in most of domestic power plants is treated with a combination of conventional techniques such as biochemical treatment prior to desalination by ion exchange. However, ion-exchange technology has a number of disadvantages such as significant space requirements, long treatment time, frequent regeneration of resin, high running cost, etc.
UF can effectively remove particulates, colloids, bacteria and some viruses, although it cannot provide significant removal of inorganic ions [2–4]. MF has a similar removal capacity although it cannot remove viruses effectively. The main differences between MF and UF are the nominal pore diameter and molecular weight cutoff of the membrane fibers. As the pretreatment step for RO desalting, three processes were compared by Gabelich et al. in 2002. The pretreatment processes tested were conventional treatment (coagulation, flocculation, sedimentation and dual-media filtration), conventional treatment with pre-ozonation and biologically active dual-media filtration (biofiltration) and microfiltration. The main results are shown in Figs. 1 and 2 [5], which show that no matter what the change is in terms of turbidity, particle-count and SDI of raw feed water, MF always has superior performance and was therefore the best choice for RO pretreatment for the site-specific feed water tested. In recent years, the use of MF or UF pretreatment in RO plants has gained significant popularity [6] and its application for treatment of power plant cooling tower blowdown is increasing. The power plant where this pilot test has been completed is located in the northwest of HeBei province of China, where the water resources are in serious shortage and groundwater is mainly used for potable purposes. This coal-fueled power plant is constructed with eight power generators (8×300 MW), and the amount of circulating cooling water used is about 20,000 m3/h per generator. So reusing the cooling tower blowdown, the total amount of which can reach 785 m3/h, by the treatment of an integrated membrane system would be beneficial. However, the safe, consistent, and cost-effective operation of the MF or UF system is critical for the performance of the whole system. In order to select the proper membrane type and obtain actual running data, this pilot test was completed between March and
289
J. Zhang et al. / Desalination 214 (2007) 287–298
June 2005. One of the purposes of this pilot test was to compare the reliability and performance of MF and UF systems as pretreatment for RO, and to identify which type of membrane system (MF or UF) is more suitable for the quality of this
cooling tower blowdown. The other purpose was to test the effect of various water conditioning chemicals (such as PAC, PAM, PFS and NaClO) on membrane performance and permeate water quality. The filtrate quality, the operational parameters and the performance of the tested MF/UF systems were also evaluated. During continuous operation of nearly three months, flux, TMP and filtrate quality were monitored.
2. Characteristics of cooling tower blowdown
Fig. 1. Raw water turbidity (top) and particle-count (bottom) data (Bos-and-whisker plots show minimum, 25th percentile, median, 75 percentile, and maximum values).
The most distinctive characteristics of the circulating cooling water quality at this test power plant site were the significant fluctuation of turbidity and the wide range of temperature variation. The turbidity varied from 2.0 NTU to 32.8 NTU during testing, and the lowest temperature in March was less than 4EC while in May the temperature reached 25EC, as shown in Figs. 3 and 4. These fluctuations are due to the geographical location and weather conditions. In the northwest of HeBei province where there is a significant amount of sand blown by wind, and for the opening pattern of the storage tank under the cooling tower, when the wind blows, the turbidity of the raw water increased sharply. In addition, rainy weather could also lead to an increase in raw water turbidity. Due to the siteTable 1 Cooling tower blowdown quality during test
Fig. 2. Treated water turbidity (top), particle-count (middle) and SDI (bottom) data (Bos-and-whisker plots show minimum, 25th percentile, median, 75th percentile, and maximum values).
Parameters
Range
Mean value
Turbidity, NTU Temperature, EC pH CODMn, mg/L Total Fe, μg/L Cu, μg/L Silica as SiO2 (colloidal), mg/L PO43!, mg/L
2–33 4–28.5 8.2–8.8 2.9–4.8 47–197 35.5–103.5 28–109
9.87 16.4 8.5 3.5 93.3 70.7 56.3
0.72
1.1
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Fig. 3. Cooling tower blowdown turbidity during testing.
Fig. 4. Cooling tower blowdown temperature during testing.
specific geographic location, the temperature between spring and summer varied greatly and the diurnal temperature variation exceeded 10EC. Large amounts of scale inhibitor, corrosion inhibitor and biocide, which contain cupreous chelate complex and phosphorous compound, were typically added to the circulating cooling water every day. Therefore, Cu, PO43! and other water quality parameters were also measured; the means and the variation ranges of these water quality parameters are shown in Table 1.
3. Pilot unit and membrane description The pilot unit is a fully automated system with data recording features. It operates at a constant filtrate flow and performs time-based backwash (BW) and air backwash (ABW) sequences, and all operational processes of the entire system are run by a programmable logic controller (PLCs). The pilot system was equipped with four membrane modules: two of these four modules were outside-in hollow fiber membrane systems;
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291
Fig. 5. Schematic diagram of the MF/UF systems.
one was equipped with MF membranes (Module A) and the other was with UF membranes (Module B). They used the same feed water and backwash water. The cooling tower blowdown was pumped from the pool of cooling tower, and after pre-filtration by a disk filter (nominal pore diameter of 50 µm), the prescreened blowdown was conveyed to a feed tank. The permeate water produced by the MF and UF systems was combined. A portion of the permeate was used for the BW and ABW processes, and the rest was drained back into the pool of the cooling tower. The pool was approximately 2 m deep and 10 m in diameter and the volume of tower blowdown was about 20,000 m3/h. For comparison, the volume of the pilot plant permeate was significantly smaller and did not have a measurable effect on the feed water quality. Both Modules A and B operated in dead-end mode. Fig. 5 shows a schematic diagram of the pilot testing system. The key treatment steps incorporated in Modules A and B were:
C The filtration process: feed 6 feed pump A 6 valve 1 6 Module A 6 valve 2 6 backwash tank. C The backwash process: permeate 6 BW pump 6 valve 3 6 Module A 6 valve 4 6 BW effluent A (step 1). C Permeate 6 BW pump 6 valve 3 6 Module A 6 valve 5 6 BW effluent A (step 2). C The air scouring process: air 6 valve 6 6 Module A 6 valve 5 6 BW effluent A. C The ABW process: permeate 6 BW pump 6 valve 3 6 Module A 6 valve 5 6 BW effluent A. C At the same time, air 6 valve 6 6 Module A 6 valve 5 6 BW effluent A. C The forward flush process: feed 6 feed pump A 6 valve 1 6 Module A 6 valve 5 6 BW effluent A. Both the MF and UF membranes have a hollow fiber outside-in configuration made of polyvinylidene fluoride (PVDF). Compared to
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Table 2 MF and UF membrane characteristics Item Membrane material Membrane area, m2 Number of hollow fiber per module Nominal pore diameter, µm Fiber inner/out diameter, mm Cartridge size, mm Pure water permeability, L/(m2·h·bar)
Table 3 Key operational parameters of the pilot system during test period
Module A (MF)
Module B (UF)
PVDF 36 4680
PVDF 33 6000
0.1 0.7/1.3
0.03 0.7/1.25
139.6 φ × 2338 L —
— $ 100 (@25EC)
inside-out flow configuration, outside-in flow configuration has the following benefits: less plugging, higher solids loading, higher flow area and easy cleaning. The flexible PVDF fibers show very good elasticity performance, allowing strong repetitive mechanical shocks such as frequent backwashing or air scouring. In addition, the PVDF fiber is chemically stable and very resistant to oxidants [7]. Table 2 shows the key characteristics of MF and UF membranes. 4. Pilot operation During initial tests not presented in this paper, preliminary operating parameters such as flux and backwash frequency were adjusted. The initial runs were followed by optimization tests described below. The key operational parameters of the pilot system during the test period are presented in Table 3. Four test runs were completed. The first test run continued from March 14, 2005 to April 6, 2005: a period when the filtrate flux was set at 60 L·m!2h!1, the filtration time was set at 30 min and the ABW interval was set at 8 times. The period of air scrubbing and air backwash
Parameter
Module A (MF)
Module B (UF)
Inlet flux rate, L·m!2h!1 Flux of filtrate, L·m!2h!1 Permeate water flux, m3·h!1 BW flux (step 1/step 2), m3·h!1 Filtrating time, min BW time (step 1), s BW time (step 2), s Air scrubbing time, s ABW time, s Forward flushing time, s ABW interval Air flux, m3·h!1 Recovery of permeate water, %
60–110 60–110 2.2–4.0 3.3/3.1
60–110 60–110 2.0–3.6 3.8/4.1
30–45 45 45 30 20 60 8–12 3.3–3.5 88.46– 94.76
30–45 45 45 30 20 60 8–12 5–6 86.41– 94.81
frequency was 4 h (4 h = 30 min×8). During the second period (April 6, 2005–May 2, 2005), the filtrate fluxes of both Module A and Module B were increased to 90 L·m!2h!1, and the period of air scrubbing and air backwash frequency was increased to 9 h (9 h = 45 min×12). At the end of this period, a chemical cleaning using HCl was completed of both Modules A and B. Then the filtrate flux was adjusted to 75 L·m!2h!1 to assess the effect of flux on membrane recovery ratio (the third period encompassed between May 2, 2005– May 20, 2005). The production rate was further increased during the fourth period (May 21, 2005 –May 31, 2005), when the filtrate fluxes of both Modules A and B reached about 110 L·m!2h!1. During this period, another chemical cleaning was completed on May 23, after two days of operation at flux of 110 L·m!2h!1. In addition, during the second and third periods, PAC, PAM, PFS, NaClO and sulfuric acid (H2SO4) were injected to the feed water, to assess their effect to the permeate SDI15.
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What has to be pointed out is that during each period, both of the two membrane modules were operated continuously 24 h per day, with the exception of brief periods when there was a malfunction of the pilot system. After the first and fourth test periods, membrane integrity of both MF and UF was measured, and the results showed that there is no evident change. 5. Results and discussion 5.1. Permeate quality of MF and UF Determining whether the permeate quality of these two modules could meet the pretreatment requirements of the RO system is the most important purpose of the trial. One of the important RO feed water quality parameters is SDI15. In addition, the RO system has requirements regarding the concentration of Fe, colloidal Si and COD in feed water. Therefore, during testing, not only SDI15 but also the concentrations of Fe , colloidal Si and COD of permeate were measured. The results of the turbidity and SDI15 measurements of the two filtrate modules are presented in Figs. 6 and 7, respectively. The concentrations of total Fe, colloidal Si, COD and Cu etc. are shown in Table 4. According to Fig. 6, although the turbidity of the raw water fluctuated in wide range (2.0– 32.8 NTU), the turbidities of MF module and UF module permeate were much more stable, and were in a range between 0.1 NTU and 0.3 NTU during the entire trial. In the third period, there was an increase in the turbidity of the permeate of both membranes modules. This permeate water quality deterioration is most likely due to the injection of PAC, PAM, PFS, NaClO and H2SO4. Fig. 7 shows the SDI15 trend of both module permeates. For both the MF and UF module, the permeate SDI15 was below 3 for most of the time, although there was a slight increase and fluctuation during the last two testing periods. On April 29, the permeate SDI15 of the MF module was
Table 4 Results of chemical analyses of feed water and two MF/UF module permeates Parameters
Feed water (permeate of disc filter)
MF module
UF module
CODMn, mg/L Total Fe, μg/L
2.9–4.8 47–197
1.5–3.5 19–48
1.4–4 15–44
Cu, μg/L
35.5–98.5
20–86
20–83
14–109 Silica as SiO2 (colloidal), mg/L PO43!, mg/L 0.7–2
9.6–70.4 1.2–32 0.1–0.8
0.2–0.8
more than 3. High levels of organic compounds and associated biofauling, micro-air bubbles and colloidal substances are the three most likely reasons for the unexpected increase in SDI15. The higher SDI levels may also be related to the increase in colloidal matter due to the injection of PAC and PAM. A layer of colloidal matter was observed over the surface of SDI test pads, which was considered to be Al(OH)3. Sulfuric acid was also added to raw water on May 15 to adjust the pH of feed to 6.5. This chemical addition also caused an increase in permeate SDI15. The addition of PAC at 2.5 ppm or 5 ppm did not result in a decrease in permeate SDI15, either. When raw water was conditioned with 5 ppm of PFS and 3 ppm of NaClO, a small SDI15 decrease was observed. Fig. 7 also shows that the permeate SDI15 of the MF module was lower than that of UF module in the first two testing periods. However, the permeate water quality fluctuated greatly after the injection of PAC, PAM, PFS, NaClO and H2SO4. This allows the conclusion that MF membrane performance was more easily affected by the change of source water feed. Compared to MF, the permeate SDI15 of UF module was more stable during the entire test period. Key feed and permeate water quality parameters for the two MF and UF modules are
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Fig. 6. Turbidity of MF and UF permeate during the trial.
Fig. 7. SDI15 trends of MF and UF permeate.
shown in Table 4. According to Table 4, both the MF and the UF modules had a high removal efficiency of total Fe, with average removal of 69.8% and 70%, respectively. As to removal of colloidal silica, UF module reached average efficiency of 65.3%, while MF module could
only achieve 45.3% removal, a little lower than that of UF module. In addition, the MF module achieved average removal of CODMn and PO43! of 37.5% and 56.5%, respectively. For the UF module, the average removal of CODMn and PO43! was 40.8% and 55.4%. In addition, both the MF
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Fig. 8. Flux and TMP over time of the MF module.
Fig. 9. Flux and TMP over time of the UF module.
and the UF modules did not remove significant amounts of copper. The average removal efficiencies were less than 17%.
5.2. Flux and TMP trends of the MF/UF system As shown in Figs. 8 and.9, the TMP of both the MF module and the UF module increases with
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flux increase. From the first testing period to the second testing period, flux was increased from 60 L·m!2h!1 to 90 L·m!2h!1, and then was adjusted to 75 L·m!2h!1 during the third testing period. Correspondingly, the TMP of the MF module increased from about 0.7 bars to about 1.1 bars, then dropped to 0.8–0.9 bars, while the TMP of the UF module increased from 0.5 bars to 0.8– 0.9 bars, then dropped to about 0.7 bars. Then during the fourth testing period, neither the MF nor the UF module could perform adequately at a higher flux of 110 L·m!2h!1. Both TMPs increased sharply and flux decreased rapidly. This illustrates that at this level of very high flux, membrane fouling of the MF module and UF module vas very rapid and significant. In addition, comparing Fig. 8 with Fig. 9, the TMP of the MF module fluctuated at greater rate, and was higher than that of the UF module by about 0.2 bars at the same flux. This result indicates that the MF membranes fouled at a higher rate than UF membranes. Because the pore
Fig. 10. FPI comparison of MF and UF modules.
diameter of MF membranes is larger than that of UF membranes, particulate substances are more likely to enter into the membrane pores of MF membranes, which likely contributes to the faster fouling rate. 5.3. Comparison of FPI of MF and UF modules FPI is an important economic indicator of MF/UF filtrate. The FPI is defined as flux divided by transmembrane pressure. Fig. 10 shows the FPI of the MF and UF modules for the entire test. In the first two testing periods, the FPI of the UF module was much higher than that of the MF module. However, the performance of the MF module was more stable and consistent: the FPI was in a range of 70–80 L·m!-2h!1bar!1, while the FPI of the UF module fluctuated in a wider range, from 110 to 135 L·m!2h!1bar!1. After chemical cleaning, there was a little increase of the FPI of the MF module in the third testing period. This illustrates that membrane perfor-
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mance of both modules could be recovered successfully, although the membrane fouling of the MF module was more serious than that of the UF module. In addition, during the high-flux run (fourth period), with the continuous increase of TMP, a sharp decline of FPI could be observed for both the MF and the UF modules.
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5.4. Comparison of chemical cleaning effect on performance of the MF and UF modules During each chemical cleaning, feed water was conditioned with HCl to adjust pH to about 2.9, and the MF and UF membranes were submerged in the acid solution for about 2 h before forward flushing and backwashing. A high
Fig. 11. Chemical cleaning effect after the second testing period.
Fig. 12. Chemical cleaning effect after two-day running at a flux of 110 L·m!2h!1.
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recovery ratio of both MF and UF membranes after chemical cleaning was observed (see Figs. 11 and 12).
6. Conclusions Both the MF and the UF membrane modules performed well in terms of permeate consistency and quality. Permeate SDI15 of the UF module remained below 3 at all times and permeate SDI15 of MF module was below 3 for most of the time of the test. The UF membrane module reached a higher FPI than the MF membrane module, while the performance of the latter was more stable. Both the MF and the UF modules operated at adequately high recovery, low TMP and simple ABW to reduce the plant’s overall treatment costs. The addition of PAC, PAM, PFS or NaClO to the feed water did not result in a significant improvement of the permeate quality. In summary, this pilot study demonstrates that both MF and UF modules are suitable for RO pretreatment for this industrial application.
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