Desalination 278 (2011) 443–446
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Effect of powdered activated carbon on immersed hollow fiber ultrafiltration membrane fouling caused by particles and natural organic matter Yonghong Li a, b, Xiaojian Zhang a, Wei Zhang a, Jun Wang a, Chao Chen a,⁎ a b
School of Environment, Tsinghua University, 100084, Beijing, China Department of Engineering, The Second Artillery Command College, 430012, Wuhan, China
a r t i c l e
i n f o
Article history: Received 30 November 2010 Received in revised form 19 May 2011 Accepted 20 May 2011 Available online 12 June 2011 Keywords: Ultraﬁltration Powdered activated carbon Membrane fouling Particles Natural organic matter
a b s t r a c t Membrane fouling is one of the primary concerns on ultraﬁltration (UF) application in drinking water treatment. There is a dispute about whether or not powdered activated carbon (PAC) addition is able to alleviate the membrane fouling. This investigation was conducted to further understand the effect of PAC addition on UF membrane fouling. Immersed polyvinylidene ﬂuoride (PVDF) hollow ﬁber membrane was utilized in the experiment. Kaolinite and humic acid (HA) were added in tap water to simulate the particles and natural organic matters (NOM) in raw water. The results veriﬁed that PAC addition could mitigate the total membrane fouling effectively by HA, while has little effectiveness on that by kaolinite and HA-kaolinite. This effect was attributed to the enhancement of organic matter removal and the reduction in irreversible fouling by HA in PAC-UF process. Results of molecular weight (MW) distribution and XAD fractionation indicated that PAC addition was mainly responsible for the removal of HPI fraction and organic matters with MW lower than 1 kDa. SEM images illustrated that the PAC cake layer on the membrane surface partially protects NOM from adsorption into the membrane pores, decreasing irreversible membrane fouling and resulting in signiﬁcantly higher membrane speciﬁc ﬂux recovery by water backwashing. But the PACcontained cake layer increases the reversible resistance. Batch dosing was recommended for its simplicity and higher effectiveness over continuous dosing. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Membrane fouling mechanisms are not only a function of membrane type, material, pore size and structure, but they are also dependent on source water quality . The interactions between the membrane surface and dissolved substances in source water play an important role in UF membrane fouling. Additionally, recent studies have concluded that particles also played an important role in membrane fouling [2,3]. As far as the inﬂuence of water quality concerned, turbidity-causing particles and natural organic matter (NOM) are two major components of surface water, which can lead to membrane fouling during UF. Many researchers have shown that NOM is generally recognized as the main foulant during the natural water ultraﬁltration [4–6]. It has been suggested that the organics and their various fractions demonstrated differences both in rejection and ﬂux decline. It was reported that the larger and more UV-absorbing fraction of humic acid (HA) was primarily responsible for irreversible pore adsorption and plugging . Although membrane fouling in UF is inevitable, the development of fouling can be postponed by some pretreatment processes. Most reports supported the positive effectiveness of PAC pre-treatment and
its improvement of membrane performance [8–10], while other researchers contended that PAC aggravated membrane performance . However, most of these studies were performed with pressured membrane module. Moreover, there are few reports concerning the membrane fouling of PAC-UF process which utilized source water containing NOM and (or) particles. Thus, further investigations on the effect of PAC on UF membrane fouling are needed. In this study, kaolinite and HA were used to simulate the particles and NOM in source water, respectively. The goal of this experiment was to better understand the effectiveness of PAC addition on immersed UF membrane fouling when different components contained source water was ﬁltrated. The mechanism of PAC addition in alleviating the immersed UF membrane fouling was analysed through scanning electron microscope (SEM) images, molecular weight (MW) distribution, and organic matter fractionation. The effect of PAC and different PAC dosing procedures on the performance of UF membrane was investigated as well. 2. Materials and methods 2.1. Membrane and PAC
⁎ Corresponding author. Tel./fax: + 86 10 6278 1779. E-mail address: [email protected]
(C. Chen). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.05.050
The immersed hollow-ﬁber ultraﬁltration membrane made of Polyvinylidene Fluoride (PVDF,Tianjin Motimo Membrane Technology
Y. Li et al. / Desalination 278 (2011) 443–446
LTD, China) were used in this experiment. The outside-in UF membrane possesses a nominal pore size of 0.03 μm. The total ﬁltration area is 4 m2. Commercial PAC (FJ074, Shanxi Xinhua Protective Equipment Ltd. Co., China) with particles size of 200 mesh was used in this study. 2.2. Ultraﬁltration test equipment and operation The UF experimental equipments consist of UF membrane modules, a raw water preparation system, a PAC feeding system, and an air-water backwashing system. The hollow ﬁber UF membrane modules were directly submerged in the membrane tank and driven by a diaphragm pump to provide negative pressure suction. Four perforated pipes were placed at the bottom of the membrane tank to dislodge particles accumulating on the membrane surface by air bubbles rinsing. The ﬁltration procedure was set to ﬁltration (1.75 min) and ﬁltration + aeration (0.25 min) were alternately repeated until backwashing. The membrane reactor was operated with a constant membrane ﬂux of about 1.4 m 3/(m 2.d). The air compressor ﬂow rate was 12 m 3/m 2.h. The strength of the air-water backwashing system was about 120 L/(m 2.h) of water ﬂushing and 6.25 L/(m 2.s) of air ﬂushing. The backwash pressure was about 50 kPa. The backwashing interval is 2 h and the duration was 2 min. About 1.7 g/L HA (Jinke ﬁne chemical industry research institute, Tianjin, China) solution or (and) about 5 g/L kaolinite (Fuchen chemical reagent factory, Tianjin, China) solution were prepared in the stock solution tank (100 L). Then it was diluted in the raw water tank (15 L) with tap water. The concentration of raw water was controlled by adjusting the ﬂow rates of the tap water and the stock solution. The prepared raw water ﬂowed into the PAC tank (20 L) and the membrane tank (80 L) sequentially by gravity. 2 g/L PAC slurry was prepared in advance and was then added either continuously or by batch mode. For the former, PAC slurry was added in the pipeline before the PAC tank by a peristaltic pump. For the latter mode, total demanded PAC slurry was added to the membrane tank at the beginning and discharged once the set amount of raw water was treated. 2.3. Analytical methods The turbidity was measured by a turbidity meter (HACH-2100P, USA). UV absorbance was measured by an ultraviolet grating spectrophotometer (Lengguang 752 N, China). DOC and TOC were analyzed by a total organic carbon analyzer (Shimadzu TOC-VCPH, Japan). The 0.45 μm HA MF membrane ﬁlters (HAWP04700, MILLIPORE) were used to take ﬁltrate containing DOC fraction before measuring DOC. MW distributions of HA in prepared raw water were determined using the high performance size exclusion chromatography (HPSEC). The instrument consisted of a size exclusion macroporous silica-based column (TSK-GEL G3000PW, Japan). XAD-8 resins were utilized for the isolation and fractionation of NOM into hydrophobic (HPO) and hydrophilic (HPI) following that developed by Leenheer . Morphological analysis of the membranes was performed by a scanning electronic microscope (SEM, JSM-6460LV, Japan).
PAC addition forms an incompact cake layer upon the membrane surface. The coverage and thickness of PAC cake was in relationship with PAC dosage. The SEM images of the membrane outer surface illustrates that 50 mg/L dosage of PAC formed a denser and thicker cake layer. The PAC cake layer could be removed almost completely by backwashing regardless of the PAC dosage.
3.1.2. Test with particles contained raw water Kaolinite solution (30 mg/L) was used to simulate the particlescontained raw water, which had a turbidity of about 20 NTU. PAC addition did not inﬂuence the efﬂuent turbidity, which was always below 0.1 NTU. Fig.1 shows that membrane speciﬁc ﬂux declination by Kaolinite and PAC particles was almost completely recovered by air-water backwashing. PAC addition brought about 1–3 percentage more declination of JSF/JSF0 at the end of every cycle when compared with only UF process. However, JSF recovery is about 95.1% JSF0 and 94.1% JSF0 in UF and PAC-UF processes respectively.
3.1.3. Test with NOM contained raw water A 10 mg/L HA solution was used to simulate the NOM-contained raw water, which had a CODMn of about 8 mg/L. PAC addition increased not only the JSF when HA-contained source water was treated, but also the JSF recovery after backwashing, as shown in Fig. 2. For sole UF process, the JSF at the end of three periods was 80.9%, 80.9% and 80.5% of JSF0, the JSF after backwashing was 86.0%, 85.3% and 84.1% of JSF0, and the JSF recovery was 5.1%, 4.4% and 3.6% of JSF0, respectively. For PAC-UF process, the JSF/JSF0 at the end of three periods was 81.7%, 82.3% and 80.9%, the JSF/JSF0 after backwashing was 90.6%, 89.4% and 87.5%, and the JSF recovery was 8.9%, 7.1% and 6.6% of JSF0 in every run period, respectively. The results above indicate that PAC addition can effectively alleviate the irreversible fouling caused by HA. SEM images (Fig. 3) demonstrated that the looser cake layer containing PAC particles prevented HA from sorption into the membrane pores. The PACcontained fouling layer is easily removed by backwashing, allowing for an easy recovery of the membrane ﬂux.
3.1.4. Test with NOM-particles contained source water HA-kaolinite mixture solution (10 mg/L HA and 30 mg/L kaolinite) was used to simulate the NOM-particles contained raw water. The most important contribution of PAC addition was to improve JSF recovery after air-water backwashing in this case. Fig. 4 shows the comparison of JSF/JSF0 decline for UF and PAC-UF process. As far as sole UF process concerned, JSF/JSF0 recovery in every run period was 10.6%, 10.6% and 11.3%, respectively. For PAC-UF process, the JSF/JSF0 recovery achieved 14.9%, 15.1% and 13.9% in every run period, respectively.
3. Results and discussions 3.1. Effect of PAC addition on membrane speciﬁc ﬂux 3.1.1. Test with tap water Tap water was ultra-ﬁltrated with different PAC dosages, i.e. 0, 20, 50, 100 mg/L for 5 h without backwashing. JSF declined sharply during the ﬁrst 30 min and then remained at nearly 95.3%, 94.5%, 93.0% of initial membrane speciﬁc ﬂux (JSF0) with 20, 50, 100 mg/L PAC dosage, respectively. The results reveal that the addition of PAC to tap water leads to a slight JSF decline. The higher the PAC dose added, the more the JSF declined.
Fig. 1. Comparison of JSF/JSF0 decline in Kaolinite contained raw water ultra-ﬁltrated by UF process with the PAC dosage of 0 mg/L and 20 mg/L respectively.
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Fig. 2. Comparison of JSF/JSF0 decline in HA-contained raw water ultra-ﬁltrated with the PAC dosages of 0 mg/L and 20 mg/L respectively.
Fig. 4. Comparison of JSF/JSF0 decline in HA-Kaolinite contained raw water ultra-ﬁltrated with PAC dosages of 0 mg/L and 20 mg/L respectively.
3.2. Comparison of NOM removal by UF and PAC-UF processes
was selected as 0, 20, and 50 mg/L. The results reveal that PAC addition has a positive effect on the membrane fouling decrease regardless of PAC dosage and procedure. For identical PAC dosages, JSF decreases slightly faster when continuous dosing is adopted. Compared to sole UF process, the average JSF/JSF0 increased 5% and 2% when 20 mg/L PAC was added in batch and continuous dose procedures respectively. The JSF/JSF0 increase for a 50 mg/L dose of PAC added via batch and continuous dosing methods were 8% and 5% respectively. The improvement of recovery of 50 mg/L and 20 mg/L was 3% and 1% respectively. It is concluded that batch dosing of PAC and batch discharge after adsorption saturation is a simple, effective operation method in the PAC-UF process according to the study.
Compared with sole UF process, PAC-UF process can signiﬁcantly improve the removal rate of organic matters with MW lower than 1 kDa from 15% to 48%. The results reveal that PAC addition can improve NOM removal rate by adsorption, especially improve the removal rate of NOM in the range of MW lower than 1 kDa. Fraction analysis (measured by UV254) revealed that HA in source water contained dominant HPO fraction, whose percentage was 89.9%. PAC pre-treatment improved the UV254 removal efﬁciency of UF process by 3.8%. Specially, PAC addition led to 3.4% higher HPI removal than HPO removal. 3.3. Effect of PAC addition mode on membrane speciﬁc ﬂux Batch and continuous dosage modes of PAC addition were compared for their inﬂuence on membrane speciﬁc ﬂux. PAC dosage
4. Conclusion PAC addition into tap water leads to an incompact cake layer on the membrane surface, which can increase the TMP slightly. The higher the PAC dose added, the more the JSF declined. PAC contributed to mitigate the irreversible fouling caused by HA. Meanwhile, PAC itself also contributed the reversible fouling. The irreversible fouling was prominent during ultraﬁltration treatment of HA ultraﬁltration, while reversible fouling was prominent during ultraﬁltration treatment of HA-kaolinite. The PAC and kaolinite interceptted by membrane increased the thickness of cake layer on membrane surface, which led to higher reversible fouling and higher total membrane fouling. PAC addition can improve the NOM removal rate, especially for the lower-MW less than 1 kDa. SEM images indicated that PAC-contained cake layer would contribute to signiﬁcantly lower NOM adsorption into membrane pore. Batch dosing of PAC is recommended due to its simplicity and effectiveness. Acknowledgments This work was supported by the National Eleventh Five-year Scientiﬁc and Technical Support Plans Project (No. 2006BAD01B03). References
Fig. 3. SEM images of backwashed membrane's section fouled by HA. (A) UF process, (B) PAC-UF process.
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