1995-JMS-Fouling and retention of nanofiltration membranes
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* Corresponding author. 0376-7388/95/$9.50 © 1995 Elsevier Science B.V. All fights reserved
masses higher than ~ 300 g/mol. Of course, the retention depends on the nature of the NF membrane. NF membranes have been on the market for about 10 years, but have gained in popularity during the last five years. Today most membrane manufacturers also produce NF membranes. The membranes are made of many different materials, mostly from polymers such as aromatic polyamides [ 1--6], polysulfones [ 1 ], polyethersulfones [ 1 ] and substituted poly(vinyl alcohols) [1], poly(acrylonitrile) [7], poly(phenylene oxide) [8] as well as from different modifications of them. They can be made with varying permeabilities, having water fluxes from 10 to 100 l / m 2 h at a pressure of 10 bar. The better the flux, the more open and less retentive is the membrane. NF membranes today can also be made of inorganic materials such as 3,-alumina
Received 28 March 1994; accepted in revised form 19 August 1994
Abstract Nanofiltmtion membranes retain substances with molar masses higher than ~ 300 g/mol and multivaient ions. The retention characteristics depend much on how much free volume there is in the membranes, which can for some membranes be related to the flux. In this study, fouling and retention of four different nanofiltration membranes (NF40, NTR-7450, NTR-7410 and NTR7250) were followed using different model substances. It could be seen that multivalent salts were retained better than small organic substances, but retention was much depending on the pH. As nanofiltration membranes have characteristics of both ultrafiltration as well as reverse osmosis membranes, their fouling characteristics are also rather individual. The tighter membranes foul less. Multivalent salts foul mostly only when together with organic compounds, e.g.. lignosulfonate containing calcium fouls more than if it contains sodium as counterion. Maize starch containing small amounts of protein fouls more than potato starch. The NTR-7250 membranes could be modified by chloride ions at high pH to give better flux without loss of retention. Anionic model substances were retained better and fouled less due to charge repulsion effects especially at high pH
[9], titania, hematite, and/or silica on alumina [ 10] or of mixtures of organic and inorganic materials such as zirconia and polyphosphazene [ 11 ]. The inorganic membranes are not yet used industrially [ 12]. NF membranes have been used for textile effluents, dye salts and pigments [ 13-16], water softening or purification [ 17-20], lactic acid removal from fermentation broths [21] or whey [22,23] as well as in the pulp and paper industry [24-26]. More fundamental studies of NF membranes have been made looking at their mode of retention for ions, complexes and other model substances [ 5,15,25,27,29-36 ]. Fouling of NF membranes has not been studied very extensively up to date. Because fouling can decrease the flux very much it is important to investigate what types of foulants should be avoided in NF. In this study NF membranes with different water fluxes were tested with salts, small and large organic molecules, as well as with combinations of these. Most of the substances tested were model substances. Retention of the substances was measured as well as fouling, defined as the % change in pure water flux before and after filtration of the test substance. The reasons for fouling are explained from case to case taking surface effects into account. Some of the model substances were chosen to resemble molecules contained in effluent and circulation waters from the pulp and paper industry. The investigation aimed to scientifically test the possibilities for NF to be useful in cleaning these waters. The experiments were made on a laboratory scale.
aLaboratory of Technical Polymer Chemistry, Departmentof Chemical Technology, Lappeenranta Universityof Technology, P.O. Box 20, 53851 Lappeenranta, Finland b Departmentof Chemical Engineering, Universityof Oviedo, 33071 Oviedo, Spain
Keywords: Nanofiltration membranes; Fouling; Retention; Charge repulsion; Pulp and paper effluent
1. Introduction
Many effluents contain macromolecules as well as small molecules that should be removed if the waters are to be cleaned properly. For this purpose ultrafiltration membranes are too open for the smaller size molecules to be retained. Reverse osmosis could be used, but in many cases the salts contained in the waters do not have to be removed. The best alternative for purification is then nanofiltration (NF). In NF a lower pressure can be used, and the flux is higher than for reverse osmosis ( R O ) . Multivalent ions can often be retained and most substances with molar
SSD103rOm et al. / Journal of Membrane Science 98 (1995) 249-262
Table 1 Rejection of various solutes by NTR-7410, NTR-7450, NF40 and NTR-7250 [ 1,3,5,6,29,37] Solute MW NTR-7410 ~ NTR-7450 ~ NF40 b NTR-7250 15 55 4 9 51 92 13 32 45 95 c 70 97 24 43 90 98 50 e
journalo! MEMBRANE
SCIENCE ELSEVIER
Journal of Membrane Science 98 (1995) 249-262
Fouling and retention of nanofiltration membranes
Marianne Nystr0m a.., Lena Kaipia ", Susana Luque b
masses higher than ~ 300 g/mol. Of course, the retention depends on the nature of the NF membrane. NF membranes have been on the market for about 10 years, but have gained in popularity during the last five years. Today most membrane manufacturers also produce NF membranes. The membranes are made of many different materials, mostly from polymers such as aromatic polyamides [ 1--6], polysulfones [ 1 ], polyethersulfones [ 1 ] and substituted poly(vinyl alcohols) [1], poly(acrylonitrile) [7], poly(phenylene oxide) [8] as well as from different modifications of them. They can be made with varying permeabilities, having water fluxes from 10 to 100 l / m 2 h at a pressure of 10 bar. The better the flux, the more open and less retentive is the membrane. NF membranes today can also be made of inorganic materials such as 3,-alumina
Received 28 March 1994; accepted in revised form 19 August 1994
Abstract Nanofiltmtion membranes retain substances with molar masses higher than ~ 300 g/mol and multivaient ions. The retention characteristics depend much on how much free volume there is in the membranes, which can for some membranes be related to the flux. In this study, fouling and retention of four different nanofiltration membranes (NF40, NTR-7450, NTR-7410 and NTR7250) were followed using different model substances. It could be seen that multivalent salts were retained better than small organic substances, but retention was much depending on the pH. As nanofiltration membranes have characteristics of both ultrafiltration as well as reverse osmosis membranes, their fouling characteristics are also rather individual. The tighter membranes foul less. Multivalent salts foul mostly only when together with organic compounds, e.g.. lignosulfonate containing calcium fouls more than if it contains sodium as counterion. Maize starch containing small amounts of protein fouls more than potato starch. The NTR-7250 membranes could be modified by chloride ions at high pH to give better flux without loss of retention. Anionic model substances were retained better and fouled less due to charge repulsion effects especially at high pH
[9], titania, hematite, and/or silica on alumina [ 10] or of mixtures of organic and inorganic materials such as zirconia and polyphosphazene [ 11 ]. The inorganic membranes are not yet used industrially [ 12]. NF membranes have been used for textile effluents, dye salts and pigments [ 13-16], water softening or purification [ 17-20], lactic acid removal from fermentation broths [21] or whey [22,23] as well as in the pulp and paper industry [24-26]. More fundamental studies of NF membranes have been made looking at their mode of retention for ions, complexes and other model substances [ 5,15,25,27,29-36 ]. Fouling of NF membranes has not been studied very extensively up to date. Because fouling can decrease the flux very much it is important to investigate what types of foulants should be avoided in NF. In this study NF membranes with different water fluxes were tested with salts, small and large organic molecules, as well as with combinations of these. Most of the substances tested were model substances. Retention of the substances was measured as well as fouling, defined as the % change in pure water flux before and after filtration of the test substance. The reasons for fouling are explained from case to case taking surface effects into account. Some of the model substances were chosen to resemble molecules contained in effluent and circulation waters from the pulp and paper industry. The investigation aimed to scientifically test the possibilities for NF to be useful in cleaning these waters. The experiments were made on a laboratory scale.
aLaboratory of Technical Polymer Chemistry, Departmentof Chemical Technology, Lappeenranta Universityof Technology, P.O. Box 20, 53851 Lappeenranta, Finland b Departmentof Chemical Engineering, Universityof Oviedo, 33071 Oviedo, Spain
Keywords: Nanofiltration membranes; Fouling; Retention; Charge repulsion; Pulp and paper effluent
1. Introduction
Many effluents contain macromolecules as well as small molecules that should be removed if the waters are to be cleaned properly. For this purpose ultrafiltration membranes are too open for the smaller size molecules to be retained. Reverse osmosis could be used, but in many cases the salts contained in the waters do not have to be removed. The best alternative for purification is then nanofiltration (NF). In NF a lower pressure can be used, and the flux is higher than for reverse osmosis ( R O ) . Multivalent ions can often be retained and most substances with molar
SSD103rOm et al. / Journal of Membrane Science 98 (1995) 249-262
Table 1 Rejection of various solutes by NTR-7410, NTR-7450, NF40 and NTR-7250 [ 1,3,5,6,29,37] Solute MW NTR-7410 ~ NTR-7450 ~ NF40 b NTR-7250 15 55 4 9 51 92 13 32 45 95 c 70 97 24 43 90 98 50 e
journalo! MEMBRANE
SCIENCE ELSEVIER
Journal of Membrane Science 98 (1995) 249-262
Fouling and retention of nanofiltration membranes
Marianne Nystr0m a.., Lena Kaipia ", Susana Luque b