- Email: [email protected]
water cleaning (AWC) in spiral wound elements
Desalination 236 (2009) 266–272
Optimization of air/water cleaning (AWC) in spiral wound elements E.R. Cornelissena*, L. Rebourb, D. van der Kooija, L.P. Wesselsa a
Kiwa Water Research, Groningenhaven 7, 3430 BB Nieuwegein, The Netherlands email: [email protected] b Ecole Nationale Supe´rieure de Chimie de Mulhouse, 3 rue Alfred Werner 68200 Mulhouse, France Received 30 June 2007; revised accepted 7 October 2007
Abstract Air/water cleaning (AWC) is effective in reducing problems with biofouling and particulate fouling of spiral wound membrane elements. One hour daily AWC was found to be useful, however, leading to a given downtime of NF/RO installations. This work deals with optimization of the AWC process to reduce the downtime of the installations using AWC. Reference, daily AWC and weekly AWC measurements were compared in two pilot runs using spiral wound membrane elements. AWC on spacer-filled channels with flat sheet membranes were carried out using a membrane fouling simulator (MFS) for visual observations, determining the amount of air during AWC and to determine the biomass removal on biofouled flat sheet membranes. Daily AWC was found to be more effective than weekly AWC, expressed as the increase in normalized pressure drop (NPD) of respectively 5% and 144% after 21 days. Weekly AWC was effective in reducing the high increase in NPD down to 20% at day 21. From visual observations of the MFS it was found that air bubbles were broken up to the size of the squares of the feed spacer. The amount of air in a vertically positioned MFS correlated to the set air/water ratios higher than four. Biomass from biofouled membrane sheets in the MFS by AWC was removed to 83%. A further optimization of AWC in spiral wound elements will be carried out using the MFS set-up. Keywords: Air/water cleaning; Spiral wound elements; Biofouling; Nanofiltration; Reverse osmosis
1. Introduction The application of membrane filtration in drinking water, process water and waste water treatment, is increasing worldwide. Membrane *Corresponding author.
filtration, specifically spiral wound nanofiltration (NF) and reverse osmosis (RO), is a robust barrier for emerging substances, inorganic compounds, viruses and bacteria. The use of membrane filtration is, however, compromised by membrane fouling. As a result of particulate fouling and
Presented at the International Membrane Science and Technology Conference, IMSTEC 07, 5–9 November 2007, Sydney, Australia 0011-9164/09/$– See front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2007.10.076
E.R. Cornelissen et al. / Desalination 236 (2009) 266–272
biofouling, operational problems occur, which lead to the increase in pressure drop over spiral wound membrane elements [1]. To control the negative effects of membrane fouling, a regular membrane cleaning is applied in practice, commonly known as cleaning in place (CIP). Chemical action during membrane cleaning is usually not enough to reduce problems with biofouling and particulate fouling effectively. To control biofouling for example, it is known that chemical action can be effective in inactivating biomass, however being ineffective in removing the inactivated biomass [2]. In order to remove particulate fouling and biofouling from membrane elements, air/water cleaning (AWC) of spiral wound membrane elements has been successfully applied to prevent and control particulate fouling and biofouling problems [3,4]. In this work, the AWC procedure was applied daily during 1 h, leading to a certain downtime of the installation. The efficiency of AWC depends strongly on the frequency and duration of AWC and the air and water flow during AWC. This work deals with optimization of the AWC process to reduce the downtime of the installations using AWC.
2. Materials and methods 2.1. AWC of spiral wound membrane elements Pilot studies were carried out with two parallel vertically positioned 2.5-inch Hydranautics ESPA2 spiral wound membrane elements without permeation, which were fed with tap water enriched with 60 mg C/L sodium acetate to enhance microbial growth to obtain biofouling. More details of the pilot installation, feed water quality and the investigated membrane types were presented in previous papers [3,4]. Before entering the pilot installation, tapwater was prefiltered using a series of 10 and 1.0 mm cartridge filters (both Amafilter), which were replaced daily during the pilot run to prevent high loads
267
of suspended solids entering the pilot installation. One element was used as reference (REF) and was compared to a periodically AWC element. In a first period pilot run (August to December 2005) the AWC membrane element was daily cleaned with air/water cleaning for 1 h. In a second period pilot run (January to March 2007) the AWC membrane element was weekly cleaned with air/water cleaning for 1 h. The water and air flow during AWC was respectively 350 L/h and 700 NL/h per element. The normalized pressure drop over the membrane elements was measured in time during the pilot trials. Optimization of the AWC process with this pilot trial approach is a time consuming process, due to the relatively slow development of biofouling in virgin membrane elements (approximately 3 weeks). 2.2. AWC of flat sheet membranes (MFS) A further optimization of the AWC process was carried out using a MFS (Membrane Fouling Simulator), i.e. a flow cell for flat sheet membranes including a feed spacer with a transparent window for in situ observations (see Fig. 3). The MFS has similar hydraulic conditions compared to spiral wound membrane elements. More details about the MFS are given elsewhere [5]. Air was introduced into the feed line towards the MFS and was fed together with water to the feed flow channel of the MFS. First, the effect of the introduction of air to the MFS was studied by visual observation during AWC. After this, the amount of air in the MFS was measured at different air/water ratios. Finally, the effect of AWC in the MFS was tested on a flat membrane sheet from a spiral wound membrane element which was prefouled with biofouling. The feed spacer used in the MFS was a diamond pattern type spacer also used in the Hydranautics ESPA2 membrane elements. Demineralized water containing 0.1 g/L Eriochrome Black (Backer Analyzed) was
268
E.R. Cornelissen et al. / Desalination 236 (2009) 266–272
MFS
Mixing point
Air Water
FI FI
100 L TANK
Fig. 1. Schematic drawing of the MFS set-up for air/ water cleaning.
recirculated over the MFS (see Fig. 1) at a water flow of 25 L/h (0.2 m/s) and 50 L/h (0.4 m/s). Eriochrome Black was added in the water to enhance the contrast between air and water in the flow channel of the MFS. Pressurized air (at 6 bar) was added before the MFS at a mixing point at different velocities, ranging from 0.4–2.4 m/s, to obtain different air/water ratios. During the AWC experiments at different air/ water ratios, digital photographs were made of the flow channel of the MFS. The amount of air in the flow channel was determined graphically using Mesurim software [6]. The MFS was positioned both vertically and horizontally. Removal of biomass using a horizontally positioned MFS set-up was investigated using prefouled membrane sheets. Biofouled flat
membrane sheets were obtained from a 2514type RO element (Filmtec TW30) which was fed with tap water enriched with 60 mg C/L sodium acetate for 3 weeks. The amount of accumulated biomass was collected from the flat sheet membranes before and after AWC treatment, subsequently transferred to autoclaved tapwater and sonicated. The obtained suspension was analyzed to assess the total amount of biomass with adenosinetriphosphate (ATP) measurements. This procedure is described by Vrouwenvelder et al. [7]. The MFS was positioned vertically using 12 L/h water and 50 NL/h air (air/water ratio ¼ 4) for 30 min.
3. Results and discussion 3.1. AWC of spiral wound membrane elements The relative normalized pressure drop (NPD) increase in time is given in Fig. 2 for daily AWC (left) and weekly AWC (right) in different pilot runs. During both pilot runs, the NPD increased within 30 days for all membrane elements as a result of biofouling. For the first pilot run the NPD increase after 21 days was 120% and 5% for the reference element and the daily AWC element respectively. For the second pilot run the NPD increase after 21 days was 370% and 144% for the reference element and the weekly 400
Reference Daily AWC
100
NPD increase [%]
NPD increase [%]
120
80 60 40 20 0
Reference Weekly AWC
300 200 100 0
0
10
Time [days]
20
30
0
10
20
30
Time [days]
Fig. 2. The relative normalized pressure drop (NPD) increase in time for period one (left; reference versus daily AWC) and period two (right; reference versus weekly AWC). The arrows mark air/water cleaning for the weekly AWC membrane element.
E.R. Cornelissen et al. / Desalination 236 (2009) 266–272
269
Fig. 3. Air and water flow in MFS with an air flow of 100 NL/h and a water flow of 50 L/h (Air to water ratio is 2:1).
AWC element respectively. During both pilot runs the NPD increase of the reference element was larger than for the AWC membrane element. This indicated the effectiveness of air/water cleaning. The increase in NPD was more pronounced during the second pilot run, which followed from the NPD increase of the reference elements (compare 120% during the first pilot run and 370% during the second pilot run after 21 days). This is attributed to either a difference in feed water quality or to variations in the membrane elements during the second pilot run. Daily AWC was more effective in controlling biofouling and particulate fouling than weekly AWC. The NPD increase for daily AWC was 5% after 21 days, while the increase for weekly AWC was 144% after 21 days. The large difference in NPD increase was, however, partly due to the difference between the two pilot runs. A disadvantage of using daily AWC is a higher membrane installation downtime compared to a downtime for weekly AWC. Despite poorer results, periodical weekly air/water cleaning was effective in decreasing the NPD from 144% down to 20% (see right-hand Fig. 2) at day 21 and day 28. 3.2. AWC on flat sheet membranes (MFS) The MFS was used to investigate different air/ water ratios in spacer-filled channels containing flat sheet membranes. Air bubbles (appeared as
white) were observed in contrast to water (appeared as black) in the feed spacer on top of the flat membrane sheet in the MFS (see Fig. 3). Air bubbles were finely distributed over the whole length and width of the flow channel of the MFS (see Fig. 3) and occurred already at the lowest air flow rates applied in this study (not shown). The size of the air bubbles in the spacer-filled channel appeared to fit approximately within the size of one or more squares of the diamond patterned feed spacer. Larger air bubbles entering the feed chamber were apparently broken up by the feed spacer. During air/water flow through the MFS, it was observed that the amount of air in the flow channel seemed not to be constant, but rather to be pulsating through the system. Moments with low air amounts in the flow channel were followed up by temporarily bursts of large air amounts. This contributed to high turbulence within the feed channel of the MFS during AWC. In the MFS, air and water was introduced at two different water flow rates with an increasing amount of air resulting in different air/water ratios. Furthermore, the MFS was positioned both vertically and horizontally. The amount of air during these AWC experiment was measured by using digital photography and software techniques in order to verify the amount of air introduced to the MFS. It is expected that the amount of air introduced to the MFS will translate directly to the measured amount of air, resulting
270
E.R. Cornelissen et al. / Desalination 236 (2009) 266–272 16 Horizontal Vertical
20
Measured air [%]
Measured air [%]
25
15 10 5 0
Horizontal Vertical 12
8
4
0 0
2
4
6
8
10
Air/water ratio [%]
0
1
2
3
4
5
6
Air/water ratio [%]
Fig. 4. Correlation between the applied air/water ratio and the measured amount of air in the MFS with a constant air flow of 25 L/h (left) and 50 L/h (right). The straight line is the best fit for a vertically positioned MFS when the intercept crosses zero.
in a straight line. The result of this experiment is given in Fig. 4 for a constant water flow of 25 L/h (left-hand side) and 50 L/h (right-hand side). These water flows correspond to velocities of respectively 0.2 and 0.4 m/s within the flow channel of the MFS (without air). This was higher than velocities applied normally in spiral wound elements, in which average water velocity range from 0.1–0.2 m/s. A correlation was found between the set air/ water ratio and the measured amount of air for a vertically positioned MFS at both water velocities (see Fig. 4). This correlation intercepted zero only for set air/water ratios higher than four. For lower air/water ratios than four the measured amount of air was higher than the correlation. This is attributed to the equipment in the MFS set-up, which only seemed to work properly at high air/water ratios. Also higher pulsations were a possible cause at lower air/ water ratios. No correlation was found for the horizontally positioned MFS at both velocities. The higher air amounts found were probably due to entrapment of air bubbles in the flow channel when positioned horizontally. AWC does not seem to operate ideally on horizontally positioned spacer-filled feed channels. Removal of biofouling (and particulate) fouling in a horizontally positioned MFS set-up was studied using biofouled membrane sheets obtained
from a 2514-type RO element. From visual observation it was found that severe biofouling occurred on both membrane surface and spacer material obtained from the 2514-type element (see Fig. 5A). Brown colored biomass material adhered to the membrane surface and was stuck onto the feed spacer. After AWC a substantial part of this biomass was removed as can be seen from Fig. 5B. After 30 min of AWC both the membrane surface and the feed spacer were considerably cleaned. The major part of this cleaning was already accomplished within the first minute of the AWC process, as was observed visually. This corresponds to earlier findings [4]. Complete removal of biomass could not be reached after 30 min of AWC, as can be seen from remaining biomass material in on the membrane surface and the feed spacer (see Fig. 5B). Quantification of biomass removal was carried out using ATP measurements. Biomass expressed as ATP could be successfully removed from biofouled flat membrane sheets in the MFS using AWC. Biomass concentrations decreased from 14,000 pg ATP/cm2 before AWC to 2500 pg ATP/cm2 after AWC, resulting in a biomass removal of 83% (Fig. 6). These results correspond with the visual observation that biomass is substantially, however, not fully removed by AWC. The air/water ratio of 4 was higher than the air/water ratio used in the spiral
E.R. Cornelissen et al. / Desalination 236 (2009) 266–272
271
Fig. 5. Biofouled membrane surface and feed spacer in a horizontally positioned MFS before (A) and after (B) 30 min of AWC with an air flow of 100 NL/h and a water flow of 25 L/h.
wound membrane elements, which makes a good comparison not possible. The MFS was successfully applied as a tool to investigate the removal of biomass with AWC. Further optimization studies can be performed with the MFS using biofouled membrane sheets at different AWC settings, using different air/water cleaning duration and air/ water ratios. Evaluation of biomass removal can be carried out both visually and by using ATP measurements. 16,000
ATP (pg/cm2)
14,000 12,000 10,000 8000 6000 4000 2000 0 Before AWC
After AWC
Fig. 6. Removal of biomass in ATP from biofouled membrane sheets in a MFS with an air flow of 100 NL/ h and a water flow of 25 L/h (Air to water ratio is 4:1) (n ¼ 2).
4. Conclusions Air/water cleaning is an efficient way to control both particulate fouling and biofouling of spiral wound membrane elements [3]. Daily AWC proved to be more efficient than a weekly AWC, which is more efficient than no AWC. Using pilot runs with vertically positioned spiral wound elements is a time consuming process which is not ideal for optimization studies of AWC in spiral wound elements. Adding air to water in spacer-filled channels containing membrane sheets increased the turbulence substantially. This was observed visually in the MFS where air bubbles were broken up to the size of the squares of the feed spacer and covered both the length and the width of the flow channel. A linear increase in the amount of air was found with the air/water ratio, at set air/water ratios higher than four in a vertically positioned MFS. The turbulence increased visually in the MFS when the air/water ratio increased. Removal of biomass of biofouled membrane sheets in the MFS by AWC reduced the biomass concentration with 83%. Biomass could not be
272
E.R. Cornelissen et al. / Desalination 236 (2009) 266–272
completely removed from biofouled membrane surfaces and feed spacers using only AWC. A further optimization of AWC in spiral wound elements will be carried out using the MFS set-up.
[2]
[3]
Acknowledgements This study was conducted in the frame work of the Joint Research Programme (BTO) of the water supply companies in the Netherlands. Ton van Dam (Kiwa Water Research, The Netherlands) and Sidney Meijering (Kiwa Water Research, The Netherlands) are acknowledged for their help during the construction of the pilot and MFS set-ups. David Biraud (ENSC Mulhouse, France) and Patricia Fiadeiro (Universidade Lusofona de Humanidades e Te´cnologias, Portugal) are acknowledged for their assistance during the experimental work.
[4]
[5]
[6]
[7]
References [1] C.E. Hickman, Membrane cleaning techniques, Awwa Seminar Proceedings, Membrane Technologies in
the Water Industry, March 10–13, Orlando (Fl) (1991) pp. 329–344. J.S. Vrouwenvelder and D. Van der Kooij, Diagnosis, prediction and prevention of biofouling of NF and RO membranes, Desalination, 139 (2001) 65–71. E.R. Cornelissen, J.S. Vrouwenvelder, S.G.J. Heijman, X.D. Viallefont, D. Van Der Kooij and L.P. Wessels, Periodic air/water cleaning for control of biofouling in spiral wound membrane elements, J. Membr. Sci., 287 (2007) 94–101. E.R. Cornelissen, J.S. Vrouwenvelder, S.G.J. Heijman, X.D. Viallefont, D. Van Der Kooij and L.P. Wessels, Air/water cleaning for biofouling control in spiral wound membrane elements, Desalination, 204 (2007) 145–147. J.S. Vrouwenvelder, S.M. Bakker, L.P. Wessels and J.A.M. Van Paassen, The membrane fouling simulator as a new tool for biofouling control of spiral-wound membranes, Desalination, 204 (2007) 170–174. Website of the Academie d’Amiens, http://www.acamiens.fr/pedagogie/svt/info/logiciels/Mesurim2/ Index.htm (2006). J.S. Vrouwenvelder, J.A.M. van Paassen, H.C. Folmer, J.A.M.H. Hofman, M.N. Nederlof and D. Van der Kooij, Biofouling of membranes for drinking water production, Desalination, 118 (1989) 157–166.
Optimization of air/water cleaning (AWC) in spiral wound elements E.R. Cornelissena*, L. Rebourb, D. van der Kooija, L.P. Wesselsa a
Kiwa Water Research, Groningenhaven 7, 3430 BB Nieuwegein, The Netherlands email: [email protected] b Ecole Nationale Supe´rieure de Chimie de Mulhouse, 3 rue Alfred Werner 68200 Mulhouse, France Received 30 June 2007; revised accepted 7 October 2007
Abstract Air/water cleaning (AWC) is effective in reducing problems with biofouling and particulate fouling of spiral wound membrane elements. One hour daily AWC was found to be useful, however, leading to a given downtime of NF/RO installations. This work deals with optimization of the AWC process to reduce the downtime of the installations using AWC. Reference, daily AWC and weekly AWC measurements were compared in two pilot runs using spiral wound membrane elements. AWC on spacer-filled channels with flat sheet membranes were carried out using a membrane fouling simulator (MFS) for visual observations, determining the amount of air during AWC and to determine the biomass removal on biofouled flat sheet membranes. Daily AWC was found to be more effective than weekly AWC, expressed as the increase in normalized pressure drop (NPD) of respectively 5% and 144% after 21 days. Weekly AWC was effective in reducing the high increase in NPD down to 20% at day 21. From visual observations of the MFS it was found that air bubbles were broken up to the size of the squares of the feed spacer. The amount of air in a vertically positioned MFS correlated to the set air/water ratios higher than four. Biomass from biofouled membrane sheets in the MFS by AWC was removed to 83%. A further optimization of AWC in spiral wound elements will be carried out using the MFS set-up. Keywords: Air/water cleaning; Spiral wound elements; Biofouling; Nanofiltration; Reverse osmosis
1. Introduction The application of membrane filtration in drinking water, process water and waste water treatment, is increasing worldwide. Membrane *Corresponding author.
filtration, specifically spiral wound nanofiltration (NF) and reverse osmosis (RO), is a robust barrier for emerging substances, inorganic compounds, viruses and bacteria. The use of membrane filtration is, however, compromised by membrane fouling. As a result of particulate fouling and
Presented at the International Membrane Science and Technology Conference, IMSTEC 07, 5–9 November 2007, Sydney, Australia 0011-9164/09/$– See front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2007.10.076
E.R. Cornelissen et al. / Desalination 236 (2009) 266–272
biofouling, operational problems occur, which lead to the increase in pressure drop over spiral wound membrane elements [1]. To control the negative effects of membrane fouling, a regular membrane cleaning is applied in practice, commonly known as cleaning in place (CIP). Chemical action during membrane cleaning is usually not enough to reduce problems with biofouling and particulate fouling effectively. To control biofouling for example, it is known that chemical action can be effective in inactivating biomass, however being ineffective in removing the inactivated biomass [2]. In order to remove particulate fouling and biofouling from membrane elements, air/water cleaning (AWC) of spiral wound membrane elements has been successfully applied to prevent and control particulate fouling and biofouling problems [3,4]. In this work, the AWC procedure was applied daily during 1 h, leading to a certain downtime of the installation. The efficiency of AWC depends strongly on the frequency and duration of AWC and the air and water flow during AWC. This work deals with optimization of the AWC process to reduce the downtime of the installations using AWC.
2. Materials and methods 2.1. AWC of spiral wound membrane elements Pilot studies were carried out with two parallel vertically positioned 2.5-inch Hydranautics ESPA2 spiral wound membrane elements without permeation, which were fed with tap water enriched with 60 mg C/L sodium acetate to enhance microbial growth to obtain biofouling. More details of the pilot installation, feed water quality and the investigated membrane types were presented in previous papers [3,4]. Before entering the pilot installation, tapwater was prefiltered using a series of 10 and 1.0 mm cartridge filters (both Amafilter), which were replaced daily during the pilot run to prevent high loads
267
of suspended solids entering the pilot installation. One element was used as reference (REF) and was compared to a periodically AWC element. In a first period pilot run (August to December 2005) the AWC membrane element was daily cleaned with air/water cleaning for 1 h. In a second period pilot run (January to March 2007) the AWC membrane element was weekly cleaned with air/water cleaning for 1 h. The water and air flow during AWC was respectively 350 L/h and 700 NL/h per element. The normalized pressure drop over the membrane elements was measured in time during the pilot trials. Optimization of the AWC process with this pilot trial approach is a time consuming process, due to the relatively slow development of biofouling in virgin membrane elements (approximately 3 weeks). 2.2. AWC of flat sheet membranes (MFS) A further optimization of the AWC process was carried out using a MFS (Membrane Fouling Simulator), i.e. a flow cell for flat sheet membranes including a feed spacer with a transparent window for in situ observations (see Fig. 3). The MFS has similar hydraulic conditions compared to spiral wound membrane elements. More details about the MFS are given elsewhere [5]. Air was introduced into the feed line towards the MFS and was fed together with water to the feed flow channel of the MFS. First, the effect of the introduction of air to the MFS was studied by visual observation during AWC. After this, the amount of air in the MFS was measured at different air/water ratios. Finally, the effect of AWC in the MFS was tested on a flat membrane sheet from a spiral wound membrane element which was prefouled with biofouling. The feed spacer used in the MFS was a diamond pattern type spacer also used in the Hydranautics ESPA2 membrane elements. Demineralized water containing 0.1 g/L Eriochrome Black (Backer Analyzed) was
268
E.R. Cornelissen et al. / Desalination 236 (2009) 266–272
MFS
Mixing point
Air Water
FI FI
100 L TANK
Fig. 1. Schematic drawing of the MFS set-up for air/ water cleaning.
recirculated over the MFS (see Fig. 1) at a water flow of 25 L/h (0.2 m/s) and 50 L/h (0.4 m/s). Eriochrome Black was added in the water to enhance the contrast between air and water in the flow channel of the MFS. Pressurized air (at 6 bar) was added before the MFS at a mixing point at different velocities, ranging from 0.4–2.4 m/s, to obtain different air/water ratios. During the AWC experiments at different air/ water ratios, digital photographs were made of the flow channel of the MFS. The amount of air in the flow channel was determined graphically using Mesurim software [6]. The MFS was positioned both vertically and horizontally. Removal of biomass using a horizontally positioned MFS set-up was investigated using prefouled membrane sheets. Biofouled flat
membrane sheets were obtained from a 2514type RO element (Filmtec TW30) which was fed with tap water enriched with 60 mg C/L sodium acetate for 3 weeks. The amount of accumulated biomass was collected from the flat sheet membranes before and after AWC treatment, subsequently transferred to autoclaved tapwater and sonicated. The obtained suspension was analyzed to assess the total amount of biomass with adenosinetriphosphate (ATP) measurements. This procedure is described by Vrouwenvelder et al. [7]. The MFS was positioned vertically using 12 L/h water and 50 NL/h air (air/water ratio ¼ 4) for 30 min.
3. Results and discussion 3.1. AWC of spiral wound membrane elements The relative normalized pressure drop (NPD) increase in time is given in Fig. 2 for daily AWC (left) and weekly AWC (right) in different pilot runs. During both pilot runs, the NPD increased within 30 days for all membrane elements as a result of biofouling. For the first pilot run the NPD increase after 21 days was 120% and 5% for the reference element and the daily AWC element respectively. For the second pilot run the NPD increase after 21 days was 370% and 144% for the reference element and the weekly 400
Reference Daily AWC
100
NPD increase [%]
NPD increase [%]
120
80 60 40 20 0
Reference Weekly AWC
300 200 100 0
0
10
Time [days]
20
30
0
10
20
30
Time [days]
Fig. 2. The relative normalized pressure drop (NPD) increase in time for period one (left; reference versus daily AWC) and period two (right; reference versus weekly AWC). The arrows mark air/water cleaning for the weekly AWC membrane element.
E.R. Cornelissen et al. / Desalination 236 (2009) 266–272
269
Fig. 3. Air and water flow in MFS with an air flow of 100 NL/h and a water flow of 50 L/h (Air to water ratio is 2:1).
AWC element respectively. During both pilot runs the NPD increase of the reference element was larger than for the AWC membrane element. This indicated the effectiveness of air/water cleaning. The increase in NPD was more pronounced during the second pilot run, which followed from the NPD increase of the reference elements (compare 120% during the first pilot run and 370% during the second pilot run after 21 days). This is attributed to either a difference in feed water quality or to variations in the membrane elements during the second pilot run. Daily AWC was more effective in controlling biofouling and particulate fouling than weekly AWC. The NPD increase for daily AWC was 5% after 21 days, while the increase for weekly AWC was 144% after 21 days. The large difference in NPD increase was, however, partly due to the difference between the two pilot runs. A disadvantage of using daily AWC is a higher membrane installation downtime compared to a downtime for weekly AWC. Despite poorer results, periodical weekly air/water cleaning was effective in decreasing the NPD from 144% down to 20% (see right-hand Fig. 2) at day 21 and day 28. 3.2. AWC on flat sheet membranes (MFS) The MFS was used to investigate different air/ water ratios in spacer-filled channels containing flat sheet membranes. Air bubbles (appeared as
white) were observed in contrast to water (appeared as black) in the feed spacer on top of the flat membrane sheet in the MFS (see Fig. 3). Air bubbles were finely distributed over the whole length and width of the flow channel of the MFS (see Fig. 3) and occurred already at the lowest air flow rates applied in this study (not shown). The size of the air bubbles in the spacer-filled channel appeared to fit approximately within the size of one or more squares of the diamond patterned feed spacer. Larger air bubbles entering the feed chamber were apparently broken up by the feed spacer. During air/water flow through the MFS, it was observed that the amount of air in the flow channel seemed not to be constant, but rather to be pulsating through the system. Moments with low air amounts in the flow channel were followed up by temporarily bursts of large air amounts. This contributed to high turbulence within the feed channel of the MFS during AWC. In the MFS, air and water was introduced at two different water flow rates with an increasing amount of air resulting in different air/water ratios. Furthermore, the MFS was positioned both vertically and horizontally. The amount of air during these AWC experiment was measured by using digital photography and software techniques in order to verify the amount of air introduced to the MFS. It is expected that the amount of air introduced to the MFS will translate directly to the measured amount of air, resulting
270
E.R. Cornelissen et al. / Desalination 236 (2009) 266–272 16 Horizontal Vertical
20
Measured air [%]
Measured air [%]
25
15 10 5 0
Horizontal Vertical 12
8
4
0 0
2
4
6
8
10
Air/water ratio [%]
0
1
2
3
4
5
6
Air/water ratio [%]
Fig. 4. Correlation between the applied air/water ratio and the measured amount of air in the MFS with a constant air flow of 25 L/h (left) and 50 L/h (right). The straight line is the best fit for a vertically positioned MFS when the intercept crosses zero.
in a straight line. The result of this experiment is given in Fig. 4 for a constant water flow of 25 L/h (left-hand side) and 50 L/h (right-hand side). These water flows correspond to velocities of respectively 0.2 and 0.4 m/s within the flow channel of the MFS (without air). This was higher than velocities applied normally in spiral wound elements, in which average water velocity range from 0.1–0.2 m/s. A correlation was found between the set air/ water ratio and the measured amount of air for a vertically positioned MFS at both water velocities (see Fig. 4). This correlation intercepted zero only for set air/water ratios higher than four. For lower air/water ratios than four the measured amount of air was higher than the correlation. This is attributed to the equipment in the MFS set-up, which only seemed to work properly at high air/water ratios. Also higher pulsations were a possible cause at lower air/ water ratios. No correlation was found for the horizontally positioned MFS at both velocities. The higher air amounts found were probably due to entrapment of air bubbles in the flow channel when positioned horizontally. AWC does not seem to operate ideally on horizontally positioned spacer-filled feed channels. Removal of biofouling (and particulate) fouling in a horizontally positioned MFS set-up was studied using biofouled membrane sheets obtained
from a 2514-type RO element. From visual observation it was found that severe biofouling occurred on both membrane surface and spacer material obtained from the 2514-type element (see Fig. 5A). Brown colored biomass material adhered to the membrane surface and was stuck onto the feed spacer. After AWC a substantial part of this biomass was removed as can be seen from Fig. 5B. After 30 min of AWC both the membrane surface and the feed spacer were considerably cleaned. The major part of this cleaning was already accomplished within the first minute of the AWC process, as was observed visually. This corresponds to earlier findings [4]. Complete removal of biomass could not be reached after 30 min of AWC, as can be seen from remaining biomass material in on the membrane surface and the feed spacer (see Fig. 5B). Quantification of biomass removal was carried out using ATP measurements. Biomass expressed as ATP could be successfully removed from biofouled flat membrane sheets in the MFS using AWC. Biomass concentrations decreased from 14,000 pg ATP/cm2 before AWC to 2500 pg ATP/cm2 after AWC, resulting in a biomass removal of 83% (Fig. 6). These results correspond with the visual observation that biomass is substantially, however, not fully removed by AWC. The air/water ratio of 4 was higher than the air/water ratio used in the spiral
E.R. Cornelissen et al. / Desalination 236 (2009) 266–272
271
Fig. 5. Biofouled membrane surface and feed spacer in a horizontally positioned MFS before (A) and after (B) 30 min of AWC with an air flow of 100 NL/h and a water flow of 25 L/h.
wound membrane elements, which makes a good comparison not possible. The MFS was successfully applied as a tool to investigate the removal of biomass with AWC. Further optimization studies can be performed with the MFS using biofouled membrane sheets at different AWC settings, using different air/water cleaning duration and air/ water ratios. Evaluation of biomass removal can be carried out both visually and by using ATP measurements. 16,000
ATP (pg/cm2)
14,000 12,000 10,000 8000 6000 4000 2000 0 Before AWC
After AWC
Fig. 6. Removal of biomass in ATP from biofouled membrane sheets in a MFS with an air flow of 100 NL/ h and a water flow of 25 L/h (Air to water ratio is 4:1) (n ¼ 2).
4. Conclusions Air/water cleaning is an efficient way to control both particulate fouling and biofouling of spiral wound membrane elements [3]. Daily AWC proved to be more efficient than a weekly AWC, which is more efficient than no AWC. Using pilot runs with vertically positioned spiral wound elements is a time consuming process which is not ideal for optimization studies of AWC in spiral wound elements. Adding air to water in spacer-filled channels containing membrane sheets increased the turbulence substantially. This was observed visually in the MFS where air bubbles were broken up to the size of the squares of the feed spacer and covered both the length and the width of the flow channel. A linear increase in the amount of air was found with the air/water ratio, at set air/water ratios higher than four in a vertically positioned MFS. The turbulence increased visually in the MFS when the air/water ratio increased. Removal of biomass of biofouled membrane sheets in the MFS by AWC reduced the biomass concentration with 83%. Biomass could not be
272
E.R. Cornelissen et al. / Desalination 236 (2009) 266–272
completely removed from biofouled membrane surfaces and feed spacers using only AWC. A further optimization of AWC in spiral wound elements will be carried out using the MFS set-up.
[2]
[3]
Acknowledgements This study was conducted in the frame work of the Joint Research Programme (BTO) of the water supply companies in the Netherlands. Ton van Dam (Kiwa Water Research, The Netherlands) and Sidney Meijering (Kiwa Water Research, The Netherlands) are acknowledged for their help during the construction of the pilot and MFS set-ups. David Biraud (ENSC Mulhouse, France) and Patricia Fiadeiro (Universidade Lusofona de Humanidades e Te´cnologias, Portugal) are acknowledged for their assistance during the experimental work.
[4]
[5]
[6]
[7]
References [1] C.E. Hickman, Membrane cleaning techniques, Awwa Seminar Proceedings, Membrane Technologies in
the Water Industry, March 10–13, Orlando (Fl) (1991) pp. 329–344. J.S. Vrouwenvelder and D. Van der Kooij, Diagnosis, prediction and prevention of biofouling of NF and RO membranes, Desalination, 139 (2001) 65–71. E.R. Cornelissen, J.S. Vrouwenvelder, S.G.J. Heijman, X.D. Viallefont, D. Van Der Kooij and L.P. Wessels, Periodic air/water cleaning for control of biofouling in spiral wound membrane elements, J. Membr. Sci., 287 (2007) 94–101. E.R. Cornelissen, J.S. Vrouwenvelder, S.G.J. Heijman, X.D. Viallefont, D. Van Der Kooij and L.P. Wessels, Air/water cleaning for biofouling control in spiral wound membrane elements, Desalination, 204 (2007) 145–147. J.S. Vrouwenvelder, S.M. Bakker, L.P. Wessels and J.A.M. Van Paassen, The membrane fouling simulator as a new tool for biofouling control of spiral-wound membranes, Desalination, 204 (2007) 170–174. Website of the Academie d’Amiens, http://www.acamiens.fr/pedagogie/svt/info/logiciels/Mesurim2/ Index.htm (2006). J.S. Vrouwenvelder, J.A.M. van Paassen, H.C. Folmer, J.A.M.H. Hofman, M.N. Nederlof and D. Van der Kooij, Biofouling of membranes for drinking water production, Desalination, 118 (1989) 157–166.