- Email: [email protected]
Zero liquid discharge: Heading for 99% recovery in nanofiltration and reverse osmosis
Desalination 236 (2009) 357–362
Zero liquid discharge: Heading for 99% recovery in nanofiltration and reverse osmosis S.G.J. Heijmana,b*, H. Guoa, S. Lia, J.C. van Dijka, L.P. Wesselsb a b
Delft University of Technology, P.O. Box 5048, 2600 GA Delft, The Netherlands Kiwa Water Research, Groningenhaven 7, 3430 BB Nieuwegein, The Netherlands email: [email protected] Received 30 June 2007; revised accepted 7 October 2007
Abstract Concentrate of nanofiltration and reverse osmosis installations is an increasing problem, especially for inland membrane installations. Introduction of membrane filtration in The Netherlands is severely hindered by the concentrate problem. Two approaches are viable for solving or reducing the concentrate problem: (1) low recovery NF/RO without anti-scalant dosing, (2) Zero liquid discharge. This research focuses on the second option: Zero liquid discharge NF/RO. First and main problem to be solved with zero liquid discharge is to increase the recovery of the membrane installation to its limits, without increasing the costs of water produced. A high recovery (>99%) is necessary to reduce energy consumption and costs for evaporation of the remaining waste stream (<1%). The only possibility to achieve a very high recovery in NF/RO, is removing the scaling components from the feed water. A very important advantage of removing the scaling components is that the nanofiltration or RO can be operated at high fluxes. In this paper the results of two pilot experiments are reported. One treatment concept was developed for surface water treatment and one for groundwater treatment. The surface water treatment concept consisted of fluidized ion exchange to remove positive multivalent cations, and then followed by ultrafiltration, nanofiltration and granular activated carbon filtration. With this setup a recovery of 97% was achieved. To achieve an even higher recovery it is also important to remove silica from the feed water because silica can limit the recovery. Silica can be removed at high pH during co-precipitation with magnesium hydroxide. The groundwater concept consisted of: precipitation at high pH, then followed by sedimentation, weak acid cation exchange and nanofiltration. With this setup a recovery of 99% was achieved. Keywords: Concentrate; Membrane filtration; Membrane fouling; Zero liquid discharge
*Corresponding author. 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.2007.10.087
358
S.G.J. Heijman et al. / Desalination 236 (2009) 357–362
1. Introduction 1.1. High recovery surface water treatment A treatment scenario is proposed for the production of drinking water from surface water: • A fluidized weak acid cation exchange resin (FIX) • Ultrafiltration (UF) • Nanofiltration (NF) • Granular activated carbon filtration (GAC) combined with marble filtration. This treatment concept was tested in a small pilot plant (100 L/h) for 3 months. The most important benefit of this concept is the very high log-removal of human pathogens and the excellent removal of organic micro pollutants by the combination NF-GAC. The cation exchange resin removes divalent cations from the raw water. Divalent cations (Ca2þ, Mg2þ, Ba2þ) are an important fouling factor for membrane filtration [1–4,5–7,15]. Removal of divalent ion reduced the NOM-fouling in ultrafiltration membranes [8] as well as in nanofiltration membranes [9]. The resin was used in a fluidized bed operation to avoid clogging of a packed bed, since the feed water was untreated surface water with a high load of suspended matter. A 99% or more removal of the divalent ions is also very important to avoid scaling of sparingly soluble salts on the nanofiltration membrane when operated at high recovery. If all the positive divalent ions are removed the recovery of the system can be increased and, consequently the concentrate production is decreased. A higher flux can be applied in the nanofiltration installation because there is no danger of scaling. With a higher flux the investment costs of the NF will decrease significantly [10]. After the nanofiltration, the majority of the natural organic matter is removed, allowing a very efficient operation of the activated carbon filter: there is no pre-loading, pore blocking or
competition on adsorption sites. Also activated carbon filtration adds additional value because it is good in removing small apolar components. An activated carbon filtration contact time of only a few minutes is sufficient without the effect of pre-loading effect. The principle advantages of the combination of nanofiltration and activated carbon filtration was found in earlier research by Kiwa Water Research [11]. The total treatment scheme includes a double barrier against pathogens (UF and NF) and against micro pollutants (NF and GAC), resulting in a robust and reliable treatment. Furthermore the particle concentration is low and the biological stability is also expected to be excellent. 1.2. High recovery groundwater treatment To achieve an extremely high recovery and to reduce organic fouling in UF membranes to a large extend, a >99% removal of all divalent positive ions is necessary.[5,10,12,16]. Chemical softening is able to remove the majority of Ca2þ ion from a feed water. Main advantage of chemical softening is that it removes calcium with the use of fewer chemicals compared to ion exchange. Main disadvantage is that chemical softening does not remove calcium completely and that it does not remove other scaling components. Also chemical softening reactors always cause carry over of calcium carbonate particles that might clog a spiral wound membrane. Therefore a weak acid cation exchange has additional value: (1) it removes remaining calcium and other divalent positive ions, and (2) it removes the carry over of the chemical softening. The combination of chemical softening and weak acid cation exchange is advantageous compared to sole cation exchange, because the chemical consumption and regenerate problem is reduced significantly. In the experiment we tested a chemical softening at high pH to be able to remove silica as well and used a weak acid
S.G.J. Heijman et al. / Desalination 236 (2009) 357–362
359
Fluidized-IEX Ca-removal
Marble filtration +GAC
Ultrafiltration
Nanofiltration
Fig. 1. Treatment concept for surface water.
cation for removing remaining divalent cations. The water produced was then fed to a nanofiltration unit. The experimental setup of this combination is shown in Fig. 2. For the groundwater concept a sludge softening process was chosen because of the possibility to remove silica at high pH [13]. 2. Materials and methods For the surface water the following setup was used (see Fig. 1). A weak acid cation resin used in this experiment was the Amberlite IRC86. This kind of resin only works properly with sufficient alkalinity. The capacity of this resin is 4.1 eq./L. The membrane used for ultrafiltration NaOH
Precipitation
Sedimentation Cation exchange Weak acid resin
Fig. 2. Experimental setup of the groundwater treatment.
in this experiment was produced by X-Flow Company (UFC M5). The UF is a hydrophilic and negatively charged membrane. A Trisep 2540-TS80-TSF, with a molecular weight cutoff of 200 Da was used for nanofiltration. The cross flow velocity was 0.12 m/s. The carbon used in the column was Norit Row 0.8 supra GAC with an empty bed contact time of 3 min. For the groundwater, the following setup was used (see Fig. 2): In order to force calcium carbonate and magnesium hydroxide precipitation, 3 mmol/L (120 mg/L) NaOH was dosed in the mixed tank and the pH of the tap water was raised to 10. Softening sludge from the sedimentation tank was pumped back to the mixing tank every day to increase the crystallisation surface
360
S.G.J. Heijman et al. / Desalination 236 (2009) 357–362
Table 1 Water quality used for the two experiments
MTC (m/s kpa) 0.6 0.5
Parameter
Kþ Naþ Mg2þ Ca2þ Mn2þ Ba2þ HCO 3 Cl SO2 4 SiO2 Ortho phosphate (P) DOC Fe-total
Surface water concentration raw water (mg/L)
Groundwater concentration tapwater (mg/L)
18 92 26 150 0.99 0.016 nav nav 154 7.5 0.65
1 12 5.8 75 0.003 0.024 255 9.5 0.13 8.5 0.03
15 nav
1.8 0.14
area. The retention time in the mixed vessel was about 3 h. The upward velocity in the sedimentation tank was 0.3 m/h. A weak acidic cation resin (Amberlite IRC86) was used in a packed bed with an empty bed contact time of 15 min. The flux in the nanofiltration was 20 L/m2 h. The crossflow velocity in the Trysep TS80-TSF 2540 membrane element was 0.15 m/s. The quality of the water used in the two experiments is given in Table 1. The surface water used had very high concentrations of organic matter (15 ppm DOC) and sulphate and a high hardness. 3. Results and discussion Fig. 3 shows the result from the experiments with surface water (without silica removal). At 97% recovery, an initial decrease of the Mass transfer Coefficient (MTC in m/(s kpa)) is observed at the start of the experiment. This may be explained either by NOM-fouling (the NOM-
0.4 0.3 MTC tendency at 97% revovery 0.2
MTC tendency at 87% recovery MTC tendency at 80% recovery
0.1 0.0 0
10
20
30
40
50
60
70
80
Time (h)
Fig. 3. Calculated mass transfer coefficient during the experiment with surface water. No scaling was observed during the experiment.
concentration increased to about 500 mg/L at the feed side of the membrane) or adaptation to the higher salt concentration and/or compaction due to the higher applied pressure. After this short decrease the MTC remained constant, indicating that no further scaling occurred [14]. A recovery above 97% were not used during the experiments with surface water. This was not possible at that time because of the difficulties to maintain a very small concentrate flow. The flow was reduced to 6 L/h at 97% recovery. Lower flows were hard to handle in the setup used at that time. Probably the recovery can be further increased without fouling problems. In Fig. 4 the result of an 11 days experiment is shown with the groundwater setup. Again there is a decrease in MTC at the start of the experiment. NOM-fouling is very probable because the concentration at the feed side of the membrane increased to 150 mg/L. Only the clean membrane can foul with NOM-molecules. As all the membrane surface is covered with NOM-molecules, the NOM-fouling stops because NOM-molecules are not associating in the feed solution spontaneously, so this process is also not likely on the membrane surface. The decrease stopped and a very stable MTC was measured over a period of 10 days, indicating the absence of scaling with this feed water. This
S.G.J. Heijman et al. / Desalination 236 (2009) 357–362 0.75
361
14 12 10
MTC
0.5
8
pH 6
0.25
After IEX
4
After sedimentation
2
0 0
48
96
144
192
240
288
Time (h)
0
0
48
96
144
192
240
288
Time (h)
Fig. 4. Mass transfer coefficient calculated for the experiment with groundwater at 99% recovery. No scaling was observed during the eleven days of the experiment.
is the first experiment at 99% recovery and it looks very promising, however a longer experiment (several months) is necessary to draw more specific conclusions about the technical and economical feasibility of the concept. The ion exchange not only removes the remaining multivalent ions after the sludge softener. But it is also dosing acid ions to the water (exchange of divalent ions against Hþ ions). As a result the water becomes rather aggressive (low pH and low calcium concentration). This aggressive water is dissolving all of the CaCO3 carry-over coming from the sedimentation tank. Based on the results of this experiment, we expect no problems with carry-over particles entering a membrane system. This is concluded, based on the results given in Fig. 5: the pressure
Fig. 6. pH trends before and after ion exchange.
drop in the first seven days is constant indicating no fouling of the spacers at the feed side of the membrane. After these 7 days the pressure drop starts to increase as a result of the fouling of the feed spacers. An explanation is that after these seven days the acid ions of the ion exchange are exchanged by monovalent ions (Na, K). The ion exchange resin still removes multivalent ions but now calcium is changed with sodium or potassium. The result is that the pH increases up to the pH of the sedimentation tank. This is indeed concluded from pH measurement as shown in Fig. 6. The water entering the nanofiltration installation will be less aggressive and the carry over will not be removed enough. As a result spacer clogging will be a matter of time. In practice, this problem of spacer clogging caused by carry over, can be solved through pseudo moving bed ion exchange.
ΔP (m bar)
4. Conclusions
280 210 140 70 0 0
48
96
144
192
240
288
Time (h)
Fig. 5. Pressure drop over the feed spacers of the membrane element. The pressure started to increase after 7 days.
From the experiments it appeared to be possible to obtain a very high recovery (99%) if scaling parameters such as calcium, magnesium, barium and silicate are removed in the pre-treatment. The experiments are too short to draw more detailed final conclusions. Future work is necessary to further prove and detail the technological and economical feasibility of the concept. Both groundwater as well as surface water can be treated with softening-ion exchange in
362
S.G.J. Heijman et al. / Desalination 236 (2009) 357–362
order to get the main waste stream as a solid waste that can be reused (calcium carbonate pellets). The choice between sludge softening and pellet softening depends on the necessity to remove silica from the feed water. These preliminary results indicate that the use of a weak acid ion exchange resin can be beneficial for the removal of carry-over. It may be worthwhile to investigate even a higher recovery in future experiments. But the recovery is also limited by the increasing concentration of micro pollutants at the feedside of the membrane. The permeate quality will contain more micro pollutants at a higher recovery. Also the ionic strength increases at higher recovery resulting in higher osmotic pressures and higher energy consumption. So there is probably an optimum between the recovery and water quality and energy consumption. We intend to investigate the retention of micro pollutants high recovery systems in the near future.
References [1]
[2]
[3]
[4]
J. Cho, G. Amy and J. Pellegrino, Membrane filtration of natural organic matter: factors and mechanisms affecting rejection and flux decline with charged ultrafiltration (UF) membrane, J. Membr. Sci., 164 (2000) 89–110. Q. Li and M. Elimelech, Organic fouling and chemical cleaning of nanofiltration membranes: measurements and mechanisms, Environ. Sci. Technol., 38 (2004) 4683–4693. A. Seidel and M. Elimelech, Coupling between chemical and physical interactions in natural organic matter (NOM) fouling of nanofiltration membranes:applications for fouling control, J. Membr. Sci., 203 (2002) 245–255. S. Hong and M. Elimelech, Chemical and physical aspects of natural organic matter (NOM) fouling of nanofiltration membranes, J. Membr. Sci., 132 (1997) 159–181.
[5] W. Wei and A.L. Zydney, Humic acid fouling during ultrafiltration, Environ. Sci. Technol., 34 (2000) 5043–5050. [6] W. Wei and A.L. Zydney, Effects of solution environment on humic acid fouling during microfiltration, Desalination, 122 (1999) 63–76. [7] M. Kabsch-Korbutowicz, K. Majewska-Nowak and T. Winnicki, Analysis of membrane fouling in the treatment of water solutions containing humic acids and mineral salts, Desalination, 126 (1999) 179–185. [8] S.G.J. Heijman, A. Gijsbertsen and M. Kennedy, Water Quality test for dead end ultrafiltration, 6. Aachener Tagung Siedlungswasserwirtschaft und Verfahrenstechnik, Oktober, 2005. [9] E.R. Cornelissen, W.G. Siegers, J. Ogier, and E.F. Beerendonk, I¨nfluence of calcium–NOM complexes on fouling of nanofiltration membranes in drinking water production, Water Sci. Technol.: Water Supply, 6 (4) (2006) 171–178. [10] J. van Paassen, P. Wessels, Meer, W. van der, M. Riemersma and J. Post, 97% recovery nanofiltration: cheaper with less concentrate, Proceedings MDIW Muhlheim an der Ruhr, 2002. [11] S.G.J. Heijman and R. Hopman, Activated carbon filtration in drinking water production, Colloids Surf., Part A: Physicochem. Eng. aspects, 151 (1999) 303–310. [12] B.W. Long, R.G. Bond and S. Veerapaneni, Reducing the Costs for Zero Liquid Discharge Desalination, Proceedings IWA Membrane conference, Harrogate, 2007. [13] S. Chen, T. Chang and C. Lin, Silica pretreatment for RO membrane by softening adsorption or co-precipitation? Proceedings IWA Membrane conference, Harrogate, 2007. [14] S.G.J. Heijman, H. Folmer, F. Domker, B.M. Rietman J.C. Schippers, Application of scale guard at reverse osmosis and nanofiltration installations, Water Supply, 3 (5) (2002) 133–138. [15] J.H. Kweon and D.F. Lawler, Fouling mechanisms in the integrated system with softening and ultrafiltration, Water Res., 38 (2004) 4164–4172. [16] S.G.J. Heijman, S. Li and J.C. van Dijk, Treatment concept for future drinking water production, Proceedings WQTC conference, Denver, 2006.
Zero liquid discharge: Heading for 99% recovery in nanofiltration and reverse osmosis S.G.J. Heijmana,b*, H. Guoa, S. Lia, J.C. van Dijka, L.P. Wesselsb a b
Delft University of Technology, P.O. Box 5048, 2600 GA Delft, The Netherlands Kiwa Water Research, Groningenhaven 7, 3430 BB Nieuwegein, The Netherlands email: [email protected] Received 30 June 2007; revised accepted 7 October 2007
Abstract Concentrate of nanofiltration and reverse osmosis installations is an increasing problem, especially for inland membrane installations. Introduction of membrane filtration in The Netherlands is severely hindered by the concentrate problem. Two approaches are viable for solving or reducing the concentrate problem: (1) low recovery NF/RO without anti-scalant dosing, (2) Zero liquid discharge. This research focuses on the second option: Zero liquid discharge NF/RO. First and main problem to be solved with zero liquid discharge is to increase the recovery of the membrane installation to its limits, without increasing the costs of water produced. A high recovery (>99%) is necessary to reduce energy consumption and costs for evaporation of the remaining waste stream (<1%). The only possibility to achieve a very high recovery in NF/RO, is removing the scaling components from the feed water. A very important advantage of removing the scaling components is that the nanofiltration or RO can be operated at high fluxes. In this paper the results of two pilot experiments are reported. One treatment concept was developed for surface water treatment and one for groundwater treatment. The surface water treatment concept consisted of fluidized ion exchange to remove positive multivalent cations, and then followed by ultrafiltration, nanofiltration and granular activated carbon filtration. With this setup a recovery of 97% was achieved. To achieve an even higher recovery it is also important to remove silica from the feed water because silica can limit the recovery. Silica can be removed at high pH during co-precipitation with magnesium hydroxide. The groundwater concept consisted of: precipitation at high pH, then followed by sedimentation, weak acid cation exchange and nanofiltration. With this setup a recovery of 99% was achieved. Keywords: Concentrate; Membrane filtration; Membrane fouling; Zero liquid discharge
*Corresponding author. 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.2007.10.087
358
S.G.J. Heijman et al. / Desalination 236 (2009) 357–362
1. Introduction 1.1. High recovery surface water treatment A treatment scenario is proposed for the production of drinking water from surface water: • A fluidized weak acid cation exchange resin (FIX) • Ultrafiltration (UF) • Nanofiltration (NF) • Granular activated carbon filtration (GAC) combined with marble filtration. This treatment concept was tested in a small pilot plant (100 L/h) for 3 months. The most important benefit of this concept is the very high log-removal of human pathogens and the excellent removal of organic micro pollutants by the combination NF-GAC. The cation exchange resin removes divalent cations from the raw water. Divalent cations (Ca2þ, Mg2þ, Ba2þ) are an important fouling factor for membrane filtration [1–4,5–7,15]. Removal of divalent ion reduced the NOM-fouling in ultrafiltration membranes [8] as well as in nanofiltration membranes [9]. The resin was used in a fluidized bed operation to avoid clogging of a packed bed, since the feed water was untreated surface water with a high load of suspended matter. A 99% or more removal of the divalent ions is also very important to avoid scaling of sparingly soluble salts on the nanofiltration membrane when operated at high recovery. If all the positive divalent ions are removed the recovery of the system can be increased and, consequently the concentrate production is decreased. A higher flux can be applied in the nanofiltration installation because there is no danger of scaling. With a higher flux the investment costs of the NF will decrease significantly [10]. After the nanofiltration, the majority of the natural organic matter is removed, allowing a very efficient operation of the activated carbon filter: there is no pre-loading, pore blocking or
competition on adsorption sites. Also activated carbon filtration adds additional value because it is good in removing small apolar components. An activated carbon filtration contact time of only a few minutes is sufficient without the effect of pre-loading effect. The principle advantages of the combination of nanofiltration and activated carbon filtration was found in earlier research by Kiwa Water Research [11]. The total treatment scheme includes a double barrier against pathogens (UF and NF) and against micro pollutants (NF and GAC), resulting in a robust and reliable treatment. Furthermore the particle concentration is low and the biological stability is also expected to be excellent. 1.2. High recovery groundwater treatment To achieve an extremely high recovery and to reduce organic fouling in UF membranes to a large extend, a >99% removal of all divalent positive ions is necessary.[5,10,12,16]. Chemical softening is able to remove the majority of Ca2þ ion from a feed water. Main advantage of chemical softening is that it removes calcium with the use of fewer chemicals compared to ion exchange. Main disadvantage is that chemical softening does not remove calcium completely and that it does not remove other scaling components. Also chemical softening reactors always cause carry over of calcium carbonate particles that might clog a spiral wound membrane. Therefore a weak acid cation exchange has additional value: (1) it removes remaining calcium and other divalent positive ions, and (2) it removes the carry over of the chemical softening. The combination of chemical softening and weak acid cation exchange is advantageous compared to sole cation exchange, because the chemical consumption and regenerate problem is reduced significantly. In the experiment we tested a chemical softening at high pH to be able to remove silica as well and used a weak acid
S.G.J. Heijman et al. / Desalination 236 (2009) 357–362
359
Fluidized-IEX Ca-removal
Marble filtration +GAC
Ultrafiltration
Nanofiltration
Fig. 1. Treatment concept for surface water.
cation for removing remaining divalent cations. The water produced was then fed to a nanofiltration unit. The experimental setup of this combination is shown in Fig. 2. For the groundwater concept a sludge softening process was chosen because of the possibility to remove silica at high pH [13]. 2. Materials and methods For the surface water the following setup was used (see Fig. 1). A weak acid cation resin used in this experiment was the Amberlite IRC86. This kind of resin only works properly with sufficient alkalinity. The capacity of this resin is 4.1 eq./L. The membrane used for ultrafiltration NaOH
Precipitation
Sedimentation Cation exchange Weak acid resin
Fig. 2. Experimental setup of the groundwater treatment.
in this experiment was produced by X-Flow Company (UFC M5). The UF is a hydrophilic and negatively charged membrane. A Trisep 2540-TS80-TSF, with a molecular weight cutoff of 200 Da was used for nanofiltration. The cross flow velocity was 0.12 m/s. The carbon used in the column was Norit Row 0.8 supra GAC with an empty bed contact time of 3 min. For the groundwater, the following setup was used (see Fig. 2): In order to force calcium carbonate and magnesium hydroxide precipitation, 3 mmol/L (120 mg/L) NaOH was dosed in the mixed tank and the pH of the tap water was raised to 10. Softening sludge from the sedimentation tank was pumped back to the mixing tank every day to increase the crystallisation surface
360
S.G.J. Heijman et al. / Desalination 236 (2009) 357–362
Table 1 Water quality used for the two experiments
MTC (m/s kpa) 0.6 0.5
Parameter
Kþ Naþ Mg2þ Ca2þ Mn2þ Ba2þ HCO 3 Cl SO2 4 SiO2 Ortho phosphate (P) DOC Fe-total
Surface water concentration raw water (mg/L)
Groundwater concentration tapwater (mg/L)
18 92 26 150 0.99 0.016 nav nav 154 7.5 0.65
1 12 5.8 75 0.003 0.024 255 9.5 0.13 8.5 0.03
15 nav
1.8 0.14
area. The retention time in the mixed vessel was about 3 h. The upward velocity in the sedimentation tank was 0.3 m/h. A weak acidic cation resin (Amberlite IRC86) was used in a packed bed with an empty bed contact time of 15 min. The flux in the nanofiltration was 20 L/m2 h. The crossflow velocity in the Trysep TS80-TSF 2540 membrane element was 0.15 m/s. The quality of the water used in the two experiments is given in Table 1. The surface water used had very high concentrations of organic matter (15 ppm DOC) and sulphate and a high hardness. 3. Results and discussion Fig. 3 shows the result from the experiments with surface water (without silica removal). At 97% recovery, an initial decrease of the Mass transfer Coefficient (MTC in m/(s kpa)) is observed at the start of the experiment. This may be explained either by NOM-fouling (the NOM-
0.4 0.3 MTC tendency at 97% revovery 0.2
MTC tendency at 87% recovery MTC tendency at 80% recovery
0.1 0.0 0
10
20
30
40
50
60
70
80
Time (h)
Fig. 3. Calculated mass transfer coefficient during the experiment with surface water. No scaling was observed during the experiment.
concentration increased to about 500 mg/L at the feed side of the membrane) or adaptation to the higher salt concentration and/or compaction due to the higher applied pressure. After this short decrease the MTC remained constant, indicating that no further scaling occurred [14]. A recovery above 97% were not used during the experiments with surface water. This was not possible at that time because of the difficulties to maintain a very small concentrate flow. The flow was reduced to 6 L/h at 97% recovery. Lower flows were hard to handle in the setup used at that time. Probably the recovery can be further increased without fouling problems. In Fig. 4 the result of an 11 days experiment is shown with the groundwater setup. Again there is a decrease in MTC at the start of the experiment. NOM-fouling is very probable because the concentration at the feed side of the membrane increased to 150 mg/L. Only the clean membrane can foul with NOM-molecules. As all the membrane surface is covered with NOM-molecules, the NOM-fouling stops because NOM-molecules are not associating in the feed solution spontaneously, so this process is also not likely on the membrane surface. The decrease stopped and a very stable MTC was measured over a period of 10 days, indicating the absence of scaling with this feed water. This
S.G.J. Heijman et al. / Desalination 236 (2009) 357–362 0.75
361
14 12 10
MTC
0.5
8
pH 6
0.25
After IEX
4
After sedimentation
2
0 0
48
96
144
192
240
288
Time (h)
0
0
48
96
144
192
240
288
Time (h)
Fig. 4. Mass transfer coefficient calculated for the experiment with groundwater at 99% recovery. No scaling was observed during the eleven days of the experiment.
is the first experiment at 99% recovery and it looks very promising, however a longer experiment (several months) is necessary to draw more specific conclusions about the technical and economical feasibility of the concept. The ion exchange not only removes the remaining multivalent ions after the sludge softener. But it is also dosing acid ions to the water (exchange of divalent ions against Hþ ions). As a result the water becomes rather aggressive (low pH and low calcium concentration). This aggressive water is dissolving all of the CaCO3 carry-over coming from the sedimentation tank. Based on the results of this experiment, we expect no problems with carry-over particles entering a membrane system. This is concluded, based on the results given in Fig. 5: the pressure
Fig. 6. pH trends before and after ion exchange.
drop in the first seven days is constant indicating no fouling of the spacers at the feed side of the membrane. After these 7 days the pressure drop starts to increase as a result of the fouling of the feed spacers. An explanation is that after these seven days the acid ions of the ion exchange are exchanged by monovalent ions (Na, K). The ion exchange resin still removes multivalent ions but now calcium is changed with sodium or potassium. The result is that the pH increases up to the pH of the sedimentation tank. This is indeed concluded from pH measurement as shown in Fig. 6. The water entering the nanofiltration installation will be less aggressive and the carry over will not be removed enough. As a result spacer clogging will be a matter of time. In practice, this problem of spacer clogging caused by carry over, can be solved through pseudo moving bed ion exchange.
ΔP (m bar)
4. Conclusions
280 210 140 70 0 0
48
96
144
192
240
288
Time (h)
Fig. 5. Pressure drop over the feed spacers of the membrane element. The pressure started to increase after 7 days.
From the experiments it appeared to be possible to obtain a very high recovery (99%) if scaling parameters such as calcium, magnesium, barium and silicate are removed in the pre-treatment. The experiments are too short to draw more detailed final conclusions. Future work is necessary to further prove and detail the technological and economical feasibility of the concept. Both groundwater as well as surface water can be treated with softening-ion exchange in
362
S.G.J. Heijman et al. / Desalination 236 (2009) 357–362
order to get the main waste stream as a solid waste that can be reused (calcium carbonate pellets). The choice between sludge softening and pellet softening depends on the necessity to remove silica from the feed water. These preliminary results indicate that the use of a weak acid ion exchange resin can be beneficial for the removal of carry-over. It may be worthwhile to investigate even a higher recovery in future experiments. But the recovery is also limited by the increasing concentration of micro pollutants at the feedside of the membrane. The permeate quality will contain more micro pollutants at a higher recovery. Also the ionic strength increases at higher recovery resulting in higher osmotic pressures and higher energy consumption. So there is probably an optimum between the recovery and water quality and energy consumption. We intend to investigate the retention of micro pollutants high recovery systems in the near future.
References [1]
[2]
[3]
[4]
J. Cho, G. Amy and J. Pellegrino, Membrane filtration of natural organic matter: factors and mechanisms affecting rejection and flux decline with charged ultrafiltration (UF) membrane, J. Membr. Sci., 164 (2000) 89–110. Q. Li and M. Elimelech, Organic fouling and chemical cleaning of nanofiltration membranes: measurements and mechanisms, Environ. Sci. Technol., 38 (2004) 4683–4693. A. Seidel and M. Elimelech, Coupling between chemical and physical interactions in natural organic matter (NOM) fouling of nanofiltration membranes:applications for fouling control, J. Membr. Sci., 203 (2002) 245–255. S. Hong and M. Elimelech, Chemical and physical aspects of natural organic matter (NOM) fouling of nanofiltration membranes, J. Membr. Sci., 132 (1997) 159–181.
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