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Acoustic sensor: a novel technique for low pressure membrane integrity monitoring
DESALINATION ELSEVIER
Desalination 119 (1998) 73-77
Acoustic sensor: a novel technique for low pressure membrane integrity monitoring J.M. La~ne a*, K. Glucina a, M. Chamant b, P. Simonie b aLyonnaise des Eaux-CIRSEE, 38 rue du President Wilson, 78230 Le Pecq, France Tel. +33 (1) 34 80 22 59; Fax +33 (1) 30 53 62 07; email: [email protected] bMETRA VIB R.D.S., France, 200 chemin des Ormeaux, 69760 Limonest, France
Received 15 June 1998
Abstract
Control of membrane integrity is critical to assuring process efficacy in terms of microbial removal. Various monitoring techniques such as turbidity, particle counting, and air testing are commonly used in membrane operation. However, membrane integrity is not necessarily reflected by a changing of the permeate quality, i.e., turbidity as well as particle count. Therefore, acoustic integrity monitoring (AIM) was developed to monitor the integrity of the membrane, especially when other techniques such as particle counting are not applicable. Based on hydrophonic sensor technology, the acoustic monitoring technique consists of measuring the noise due to a compromised fiber. The advantage of a hydrophonic sensor is that integrity of the membranes is monitored continuously during filtration. Moreover, AIM enables detection of a non-cut compromised fiber with a hole of 0.5 mm which therefore guarantees more than 6 log removal of viruses. Keywords." Ultrafiltration; Integrity; Acoustic
1. I n t r o d u c t i o n
Microfiltration (MF) and ultrafiltration (UF), the primary low pressure filtration processes, have received a great deal o f attention in recent years. *Corresponding author.
Both MF and UF ensure a high level o f removal for bacteria and cysts without the use of disinfectant. It should be noted that UF technology, due to its tighter cut-off as compared to MF, also removes viruses. However, complete microbial removal is provided only under the conditions of intact membrane operation.
Presented at the Conference on Membranes in Drinking and Industrial Water Production, Amsterdam, September 21-24, 1998, International Water Services Association, European Desalination Society and American Water Works Association 0011-9164/98/$ - See front matter © 1998 Elsevier Science B.V. All fights reserved. PII S0011-9164(98)00111-8
74
J.M. Lafne et al. / Desalination 119 (1998) 73-77
Damaged fibers may have an impact to some extent on the treated water quality; thus membrane integrity control will insure complete disinfection. Various monitoring techniques such as turbidity, particle counting, and air testing are commonly used in membrane operation. However, particle counting, which is a more sensitive technique than turbidity for membrane integrity monitoring, may not be suitable in some cases. This has been demonstrated especially in the case of membrane operation in dead-end on low turbidity water (high dilution effect). Therefore, alternative techniques such as the acoustic monitoring technique enable control of membrane integrity independently of water quality. The objectives of this study were to design an acoustic integrity monitoring (AIM) system to monitor the integrity of the membrane, especially when other techniques such as particle counting are not applicable.
Fig. 1. Hydrophonic sensor.
2. Materials and methods 2.1. Principle
Based on hydrophonic sensor technology, the AIM technique consists of measuring the noise (i.e., pressure fluctuation) due to a compromised fiber. The advantage of a hydrophonic sensor is that integrity of the membranes is monitored continuously during filtration, as compared to other methods such as air pressure tests which are non-continuous. This on-line sensor, shown in Fig. 1, is mounted on each individual membrane module (Fig. 2). The operation of the hydrophonic sensor is based on piezo-electric ceramic bending. This ceramic is cast in a silicon resin which is in contact with the fluid (Fig. 3). The electric signal given out is amplified and analyzed in frequency modulation. 2.2. Experiments
Preliminary experiments were conducted at pilot scale in order to evaluate the signal for a
Fig. 2. Hydrophonic sensor mounted on the UF module.
compromised fiber depending on the filtration mode, the location of the sensor(s) on the module, and the size of the hole in a fiber. Further experiments were performed on two full-scale plants toe valuate the acoustic signal for different modules on one membrane rack unit and its evolution with time. The two UF plants selected were La Filli~re (0.5 mgd) and Avoriaz (0.9 mgd). On the second site, the permeate flow rate was controlled by a variable frequency drive, whereas for the other site, the permeate flow rate was controlled by an actuated valve located on the feed water side. Based on the results of the previous studies, a prototype was designed and installed on
75
J.M. Latne et al. / Desalination 119 (1998) 73-77 Module air bleed port
Table 1 Impact of the system operation on background noise
Test 1 Test 2 Test 3
signal
Fig. 3. Schematic diagram of the hydrophonic sensor.
one rack of the UF plant of Vigneux sur Seine (15 m gd).
3. Results and discussion
Preliminary results from pilot testing showed that a comprised fiber (a cut fiber) provides an increase in acoustic level up to 20dB in the frequency band range from 280 to 650Hz. This acoustic signature was obtained when the UF system operated in dead-end filtration. In recirculation mode, the noise generated by the recirculation pump drowned the noise due to the compromised fiber. It should be noted that only one hydrophone was enough, and it could be placed on the lower air bleed port, which was the opposite location of the cut fiber. A non-cut compromised fiber with a hole of 0.5mm also provided a response of 20 dB. The results from the full-scale plants showed that the acoustic technique performances greatly depend on the background noise. It has been found that the lower background noise was obtained at the plant which had its flow regulated with a variable frequency drive (Table 1). When the
La Filli6re actuated valve, dB
Avoriaz Variable frequency drive, dB
6.7 4.9 5.9
-24.8 -23.5 - 24.8
permeate flow is regulated by an actuated valve, high background noise is generated, and the sensor has a much lower response. The acoustic detection also depends on the flow rate. The higher the flow rate, the higher the noise generated by a compromised fiber (Figs. 4a, 4b). Also, it was shown that the background noise was still stable regardless of the module location and time. Based on these results, an AIM prototype was built to check 28 modules. This prototype presented in Fig. 5 includes 28 acoustic sensors, two collectors and one processor. The signals from the sensors are transmitted to the collectors. The collectors treat the signal from the sensors. Each collector is able to treat 12 sensors one by one. The measures performed by the collector are collected by the processor which compares the measures to a threshold. A preliminary economic study showed that to monitor a 28-module unit (developing 8300ft 2 of membrane surface area), the cost for a full system would be approximately $7000, i.e., $250 per module. Compared to a particle counter, this AIM technology is competitive. Indeed, the cost for a particle counter is around $6500.
J.M. Latne et aL / Desalination 119 (1998) 73-77
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J.M. Lafne et al. / Desalination 119 (1998) 73-77
4. Conclusions
This study showed that the AIM system is an efficient method to control the membrane integrity. This technology is able to ensure more than 6 log
77
removal of viruses 100% of the time, whatever the water quality. AIM is able to identify the compromised module and it is a competitive system to the current technologies.
Desalination 119 (1998) 73-77
Acoustic sensor: a novel technique for low pressure membrane integrity monitoring J.M. La~ne a*, K. Glucina a, M. Chamant b, P. Simonie b aLyonnaise des Eaux-CIRSEE, 38 rue du President Wilson, 78230 Le Pecq, France Tel. +33 (1) 34 80 22 59; Fax +33 (1) 30 53 62 07; email: [email protected] bMETRA VIB R.D.S., France, 200 chemin des Ormeaux, 69760 Limonest, France
Received 15 June 1998
Abstract
Control of membrane integrity is critical to assuring process efficacy in terms of microbial removal. Various monitoring techniques such as turbidity, particle counting, and air testing are commonly used in membrane operation. However, membrane integrity is not necessarily reflected by a changing of the permeate quality, i.e., turbidity as well as particle count. Therefore, acoustic integrity monitoring (AIM) was developed to monitor the integrity of the membrane, especially when other techniques such as particle counting are not applicable. Based on hydrophonic sensor technology, the acoustic monitoring technique consists of measuring the noise due to a compromised fiber. The advantage of a hydrophonic sensor is that integrity of the membranes is monitored continuously during filtration. Moreover, AIM enables detection of a non-cut compromised fiber with a hole of 0.5 mm which therefore guarantees more than 6 log removal of viruses. Keywords." Ultrafiltration; Integrity; Acoustic
1. I n t r o d u c t i o n
Microfiltration (MF) and ultrafiltration (UF), the primary low pressure filtration processes, have received a great deal o f attention in recent years. *Corresponding author.
Both MF and UF ensure a high level o f removal for bacteria and cysts without the use of disinfectant. It should be noted that UF technology, due to its tighter cut-off as compared to MF, also removes viruses. However, complete microbial removal is provided only under the conditions of intact membrane operation.
Presented at the Conference on Membranes in Drinking and Industrial Water Production, Amsterdam, September 21-24, 1998, International Water Services Association, European Desalination Society and American Water Works Association 0011-9164/98/$ - See front matter © 1998 Elsevier Science B.V. All fights reserved. PII S0011-9164(98)00111-8
74
J.M. Lafne et al. / Desalination 119 (1998) 73-77
Damaged fibers may have an impact to some extent on the treated water quality; thus membrane integrity control will insure complete disinfection. Various monitoring techniques such as turbidity, particle counting, and air testing are commonly used in membrane operation. However, particle counting, which is a more sensitive technique than turbidity for membrane integrity monitoring, may not be suitable in some cases. This has been demonstrated especially in the case of membrane operation in dead-end on low turbidity water (high dilution effect). Therefore, alternative techniques such as the acoustic monitoring technique enable control of membrane integrity independently of water quality. The objectives of this study were to design an acoustic integrity monitoring (AIM) system to monitor the integrity of the membrane, especially when other techniques such as particle counting are not applicable.
Fig. 1. Hydrophonic sensor.
2. Materials and methods 2.1. Principle
Based on hydrophonic sensor technology, the AIM technique consists of measuring the noise (i.e., pressure fluctuation) due to a compromised fiber. The advantage of a hydrophonic sensor is that integrity of the membranes is monitored continuously during filtration, as compared to other methods such as air pressure tests which are non-continuous. This on-line sensor, shown in Fig. 1, is mounted on each individual membrane module (Fig. 2). The operation of the hydrophonic sensor is based on piezo-electric ceramic bending. This ceramic is cast in a silicon resin which is in contact with the fluid (Fig. 3). The electric signal given out is amplified and analyzed in frequency modulation. 2.2. Experiments
Preliminary experiments were conducted at pilot scale in order to evaluate the signal for a
Fig. 2. Hydrophonic sensor mounted on the UF module.
compromised fiber depending on the filtration mode, the location of the sensor(s) on the module, and the size of the hole in a fiber. Further experiments were performed on two full-scale plants toe valuate the acoustic signal for different modules on one membrane rack unit and its evolution with time. The two UF plants selected were La Filli~re (0.5 mgd) and Avoriaz (0.9 mgd). On the second site, the permeate flow rate was controlled by a variable frequency drive, whereas for the other site, the permeate flow rate was controlled by an actuated valve located on the feed water side. Based on the results of the previous studies, a prototype was designed and installed on
75
J.M. Latne et al. / Desalination 119 (1998) 73-77 Module air bleed port
Table 1 Impact of the system operation on background noise
Test 1 Test 2 Test 3
signal
Fig. 3. Schematic diagram of the hydrophonic sensor.
one rack of the UF plant of Vigneux sur Seine (15 m gd).
3. Results and discussion
Preliminary results from pilot testing showed that a comprised fiber (a cut fiber) provides an increase in acoustic level up to 20dB in the frequency band range from 280 to 650Hz. This acoustic signature was obtained when the UF system operated in dead-end filtration. In recirculation mode, the noise generated by the recirculation pump drowned the noise due to the compromised fiber. It should be noted that only one hydrophone was enough, and it could be placed on the lower air bleed port, which was the opposite location of the cut fiber. A non-cut compromised fiber with a hole of 0.5mm also provided a response of 20 dB. The results from the full-scale plants showed that the acoustic technique performances greatly depend on the background noise. It has been found that the lower background noise was obtained at the plant which had its flow regulated with a variable frequency drive (Table 1). When the
La Filli6re actuated valve, dB
Avoriaz Variable frequency drive, dB
6.7 4.9 5.9
-24.8 -23.5 - 24.8
permeate flow is regulated by an actuated valve, high background noise is generated, and the sensor has a much lower response. The acoustic detection also depends on the flow rate. The higher the flow rate, the higher the noise generated by a compromised fiber (Figs. 4a, 4b). Also, it was shown that the background noise was still stable regardless of the module location and time. Based on these results, an AIM prototype was built to check 28 modules. This prototype presented in Fig. 5 includes 28 acoustic sensors, two collectors and one processor. The signals from the sensors are transmitted to the collectors. The collectors treat the signal from the sensors. Each collector is able to treat 12 sensors one by one. The measures performed by the collector are collected by the processor which compares the measures to a threshold. A preliminary economic study showed that to monitor a 28-module unit (developing 8300ft 2 of membrane surface area), the cost for a full system would be approximately $7000, i.e., $250 per module. Compared to a particle counter, this AIM technology is competitive. Indeed, the cost for a particle counter is around $6500.
J.M. Latne et aL / Desalination 119 (1998) 73-77
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J.M. Lafne et al. / Desalination 119 (1998) 73-77
4. Conclusions
This study showed that the AIM system is an efficient method to control the membrane integrity. This technology is able to ensure more than 6 log
77
removal of viruses 100% of the time, whatever the water quality. AIM is able to identify the compromised module and it is a competitive system to the current technologies.