Monitoring of polymeric membrane fouling in hollow fiber module using ultrasonic nondestructive testing

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Transactions of Nonferrous Metals Society of China Trans. Nonferrous Met. SOC.China 16(2006) s845-s848 www.csu.edu.cn/ysxb/

Monitoring of polymeric membrane fouling in hollow fiber module using ultrasonic nondestructive testing LIU Jing-xia($g'&$), LI Jian-xin(q@$%), CHEN Xue-mei(%%@), ZHANG Yu-zhong(%E,%!) School of Material and Chemical Engineering, Key Lab of Hollow Fiber Membrane Material and Processes, Ministry of Education, Tianjin Polytechnic University, Tianjin 300 160, China Received 10 April 2006; accepted 25 April 2006 Abstract: This study describes the development of novel protocols extending the real-time ultrasonic reflectometry(UTDR) for the detection of membrane fouling in hollow fiber module during ultrafiltration(UF) of oily water treatment. A specially designed acoustic sensor with a frequency of 2.5 MHz was used. The hollow fiber membranes used were polysulphone(PSf) UF membranes with MWCO 40 m a . The wastewaters with three different oily concentrations of 100, 500 and 1 000 mg/L were investigated. Diesel oil was utilized as the primary foulant. The results show that the permeate flux declines with operation time and its value becomes lower with the increase of the oily concentration in wastewater. It is found that ultrasonic measurement can detect the fouling and cleaning processes. A new signal analysis protocol-ultrasonic reflected energy was developed. Ultrasonic reflected energy obtained indicates the deposition of oily layer as a function of operation time and its removal after cleaning. The overall flux decline is reasonably correlated with the changes in ultrasonic reflected energy. This research provides the evidence that the ultrasonic reflectometry technique is capable of monitoring membrane fouling and cleaning in hollow fiber modules.

Key words: nondestructive testing; membrane fouling; hollow fiber; ultrasonic time-domain reflectometry; ultrafiltration

1 Introduction Recently oily water in inland waterways and coastal zone has become one of the most serious issues of water pollution which needs to be resolved urgently[ 1,2]. There are several sources of oily waters such as crude oil spill from wrecked tankers, oily bilge water, leak of oil caused faulty handling of crude cargo at the port and along the coast, and oily wastewater discharged from several industries such as steel industry, machine industry, petroleum refinery, petrochemical industry, textile industry, metal industry[3-51. For the time being, regulations imposed on oily water pollution have become tighter and thus the small quantity of remaining oil in the water must be reduced to an acceptable limit before the water can be discharged into sea or rivers or re-injected for water flooding[4,6]. The membrane separation processes applied to oily water have been paid extensive attention to due to its unique advantages, such as lower capital cost, the absence of chemical addition and the

subsequent generation of oily sludge[7-10]. Membrane fouling is, however, a critical problem limiting the wider application of membrane filtration separations. A significant amount of research has been conducted to better understand the dynamics of the membrane fouling [ 11-131. Although the progress has been made, the mechanism of membrane fouling, especially the oily fouling, remains ambiguous due to the complexity of oily wastewater and invisibility of fouling processes. Better detection and control of fouling in membrane-based oily wastewater treatment are essential if membranes are to find increased use in a variety of industrial applications. Recently ultrasonic time-domain reflecto-metry(UTDR) as a nondestructive and real-time testing technique has been successfully utilized to investigate and quantify membrane fouling and cleaning in flat-sheet, tubular and spiral membrane systems[ 14-18]. LI et al[ 161 described an ultrasonic time-domain reflectometry (UTDR) technique applied to detect fouling-layer initiation and growth on the flat-sheet microfiltration(MF) membrane modules in real-time. The experimental results show a

Foundation item: Project supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry of China;

Projects(043612611, 05YFGDGX10000) supported by the Natural Science Foundation and Development Program of Science and Technology of Tianjin, China Corresponding author: LI Jian-xin; Tel: +86-22-2452 875 1; Fax: +86-22-2451 9341; E-mail: [email protected]

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LIU Jing-xia, et al/Trans. Nonferrous Met. SOC.China 16(2006)

good correspondence between the UTDR signal response from the membranes and the development of a fouling layer on the membrane surface. ZHANG et a1[17] described the use of UTDR for the in situ, non-invasive observation of fouling deposition and growth on spiral reverse osmosis(R0) membrane modules. They introduced drift factor concept due to the complexity of ultrasonic reflected signal in spiral RO unit. The results show that drift factor and amplitude have a good correspondence with the membrane flux-decline. LI et a1[18] recently reported results describing the use of a focused ultrasonic sensor for the investigation of membrane fouling in tubular UF membrane modules. The results show that the method can distinguish signal from support and bend surface of tubular membrane. However there is little literature describing the use of UTDR for the investigation of fouling on the hollow fiber membranes due to the much more complex situation occurring in the hollow fiber membrane module. The aim of this study is to employ the UTDR technique for monitoring membrane fouling in a hollow fiber module during ultrafiltration (UF) of oily wastewater treatment. A novel signal analysis protocol was developed in this study.

was utilized as the primary foulant in three different concentrations of 100, 500 and 1 000 mg/L by adding diesel oil in tap water. A multi-cycle protocol was adopted whereby each cycle consisted of three stages: pure water equilibration, fouling and cleaning. All the fouling experiments were carried out at a flow rate of (0.8h0.05) m / s for 240 min, the operation pressure of (50*5) kPa and the temperature of (20h1) "C. A daily detergent was used to clean the fouled membranes. The same membrane module was used to treat the wastewaters with oily concentrations of 100, 500 and 1 000 mg/L, in turn. The ultrasonic measurement system included a specially designed ultrasonic transducer of 2.5 MHz, a pulser-receiver, a 350 MHz digital oscilloscope with sensitivity of lns/div and a computer for storing the ultrasonic data. The focused transducer was externally and perpendicularly mounted in contact with the cylindrical test cell. Permeation rate as well as acoustic amplitude and arrival time measurements were made at regular intervals during three complete cycles. A morphological study of the fouled membrane samples was also carried out to establish a correlation between the measurement of UTDR and the fouling layer.

3 Results and discussion 2 Experimental Fig. 1 shows a schematic representation of a hollow fiber UF experimental system and UTDR measurement system. The assembly consists of a 51 feed tank for storage and supply of the oily wastewater, a peristaltic pump for pressurization of the feed solution, a hollow fiber module, and a stirring device. The hollow fiber membranes used were polysulphone(PSf) UF membranes with MWCO 40 kDa from Tianjin Motian Membrane Engineering and Technology Co, China. The dimension of the membranes was 0.8 mm in inside diameter and 380 mm in length. A cylindrical test module containing 20 fibers was designed and used in this study. Diesel oil

The changes in permeate flux of the hollow fiber membranes with operation time during the fouling experiments carried out with the oily wastewaters with three different oily concentrations of 100, 500 and 1 000 mgL are illustrated in Fig.2. The membrane flux declined rapidly at commencement and then reached a near steady-state in each fouling experiment. The initial rapid drop in flux is attributed to the adhesion of oily foulant on membrane surface or plugging in the membrane pores. The gradual decline is a result of the slow growth of the oily layer.

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LIU Jing-xia, et aVTrans. Nonferrous Met. SOC.China 16(2006)

It is also shown in Fig.2 that the values of the membrane flux becomes lower with the increase of the oily concentration in waste-water. The value of the membrane flux was 14% of initial pure water flux after UF of 1 000 mg/L oily wastewater. This suggests thzt the membranes suffer from severe oily fouling. The membrane samples were taken for scanning electronic microscopy(SEM) analysis after 900 rnin of operation (UF of 1 000 mg/L oily wastewater). Fig.3 shows the representative SEM micrographs of the new and fouled membrane surfaces. Morphological analysis of the new membrane surface showed no fouling layer on the membrane surface (Fig.3(a)). SEM analysis of the membrane surface after 900 rnin of operation indicates membrane coverage with the oily foulants (Fig.3(b)).

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layer on the membranes. The similar results were obtained during the fouling experiments carried out with 100 mg/L and 1 000 mg/L oily wastewater. The cleaning processes after the fouling experiments carried out with 500 mg/L oily wastewater were also measured by UTDR. The results are shown in Fig.5. It can be seen in Fig.5 that the acoustic waveforms moved backward it-, arrival time and the amplitude increased after cleaning. Furthermore, Fig.5 illustrates that the cleaning by a detergent is more effective than by pure water. 0.8 0.4

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UTDR signals obtained during the fouling experiments carried out with 500 mg/L oily wastewater are shown in Fig.4. Systemic changes in time-domain as a function of operation time during the fouling experiments were observed in Fig.4. Peak A generated from the membranes moved ahead for 70 ns in time-domain after 240 min of fouling operation carried out with 500 mg/L oily wastewater. The movement in time-domain is related to the build-up of oily fouling layer on membrane surface. The amplitude of ultrasonic signals correspondingly dropped with the operation time due to the deposition of the lower density of oily fouling

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To better understand and interpret the relationship between ultrasonic signals obtained and the membrane fouling and cleaning processes in hollow fiber membrane modules, a novel analysis protocol-ultrasonic reflected energy was developed and used. The ultrasonic frequency spectra and ultrasonic reflected energy from the hollow fiber membranes during cross-flow UF of 500 m g L oily wastewater are shown in Fig.6. The decline in the amplitude and ultra-sonic reflected energy with the fouling operation time was

LIU Jing-xia, et aVTrans. Nonferrous Met. SOC.China 16(2006)

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membranes. Traditional flux data and SEM analysis corroborated the ultrasonic measurements. This research provides the evidence that the ultrasonic reflectometry technique is capable of monitoring membrane fouling in hollow fiber modules. Overall, it suggests a strong basis for the continued development of the acoustic reflectometry technique with an ultimate goal of successfilly monitoring membrane fouling and cleaning in the membrane industry.

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Fig.6 Acoustic spectrum(a) and ultrasonic reflected energy(b) vs operating time during UF of 500 m g L oily wastewater

treatment observed in Fig.6. The decreases in ultrasonic reflected energy are related to the deposition of the oily fouling on the membranes. To further make sure the results observed, the ultrasonic reflected energy in overall operation processes is summarized in Fig.2. It is shown in Fig.2 that the changes in ultrasonic reflected energy through the overall experiments are consistent with the flux-decline behavior. The fouling layer on the membrane resulted in the loss of the ultrasonic reflected energy from the membranes. These results indicate that the ultrasonic reflected energy is reasonably correlated with membrane fouling.

4 Conclusions The results of this study show that the UTDR technique is successhlly used to detect the adhesion and growth of oily fouling on hollow fiber membranes and its removal non-destructively, in situ, under actual operating conditions. The data obtained demonstrate a good correspondence between the ultrasonic response signal and the development of oily foulant on hollow fiber

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