Nanofiltration (NF) is an attractive option for the treatment of wastewater e.g. municipal wastewater and landfill leachate. However, membrane fouling can be a major obstacle in the implementation of this technology. Fouling of nanofiltration membranes by hu-mic acids (HA) was investigated using bisphenol A (BPA) as an indicator chemical to dif-ferentiate between various mechanisms that may lead to a change in solute rejection. Three commercially available NF membranes were investigated and an accelerated foul-ing condition was achieved with a foulant mixture containing humic acids in an electro-lyte matrix. The effects of membrane fouling on the rejection of BPA were interpreted with respect to the membrane pore sizes and the fouling characteristics. Results report-ed here indicate that calcium concentration in the feed solution could be a major factor governing the humic acid fouling process. Moreover, a critical concentration of calcium in the feed solution was observed, at which membrane fouling was most severe. Mem-brane fouling characteristics were observed by their influence on BPA rejection. Such influence could result in either an increase or decrease in rejection of BPA by the three different membranes depending on the rejection mechanisms involved. It is hypothe-sised that these mechanisms could occur simultaneously and that the effects of each might not be easily distinguished. However, it was observed that their relative contribu-tion was largely dependent upon membrane pore size. Pore blocking, which resulted in a considerable improvement in rejection, was prominent for the more open pore size TFC-SR2 membrane. In contrast, the cake-enhanced concentration polarisation (CECP) effect was more severe for the tighter NF270 and NF90 membranes. For hydrophobic solutes such as BPA, the formation of the fouling layer could also interfere with the so-lute-membrane interaction, and therefore, exert considerable influence on the separa-tion process.
The combined impact of humic acid fouling and CaCO3 scaling on the rejection of trace organic contaminants by a commercially available nanofiltration membrane was inves-tigated in this study. Due to the presence of humic acid in the feed solution, CaCO3 scal-ing behaviour differed substantially from that of a pure CaCO3 solution. A prolonged induction period was consistently observed prior to the onset of membrane scaling. In addition, membrane scaling following humic acid fouling did not result in a complete loss of permeate flux. This is consistent with the absence of any large CaCO3 crystals. In fact, the CaCO3 crystals on the membrane surface were quite small and similar in size, which would result in a relatively porous cake layer. At the onset of CaCO3 scaling the rejection of all three trace organic contaminants started to decrease dramatically. The observed decrease in rejection of the trace organic contaminants was much more se-vere than that reported previously with a single layer of either organic or colloidal foul-ing. Such severe decrease in rejection can be attributed to the extended cake-enhanced concentration polarisation effect occurring as a result of the combination of membrane fouling and scaling. The porous CaCO3 scaling layer could lead to a substantial cake-enhanced concentration polarisation effect. In addition, the top CaCO3 scaling layer could reduce the wall shear rate within the underlying humic acid fouling layer, causing an additional concentration polarisation (CP) effect.:1 INTRODUCTION 1
1.1 Fundamentals of NF/RO 1
1.1.1 Solute transport through NF/RO membranes 2
1.1.2 Separation mechanisms 3
1.1.2.1 Steric size exclusion 3
1.1.2.2 Donnan effect 3
1.1.2.3 Electrostatic repulsion 4
1.1.2.4 Adsorption 4
1.1.3 Environmental applications of NF/RO 5
1.1.4 Drinking water treatment from groundwater and surface water sources 5
1.1.5 Water/Wastewater reclamation 7
1.2 Classification and materials of NF/RO membranes 7
1.2.1 Membrane classes 7
1.2.2 Membrane materials 8
1.2.3 Organic membrane materials 9
1.2.3.1 Polyamide membranes 9
1.2.3.2 Cellulose acetate membranes 9
1.2.4 Inorganic membrane materials 10
1.3 Removal of trace organic contaminants 11
1.3.1 Impact of membrane characteristics 14
1.3.1.1 Molecular weight cut-off/pore size 14
1.3.1.2 Surface charge 14
1.3.1.3 Hydrophobicity/hydrophilicity 15
1.3.1.4 Surface morphology 15
1.3.2 Impact of feed characteristics 17
1.3.2.1 pH value 17
1.3.2.2 Ionic strength 18
1.3.2.3 Organic matter 19
1.3.2.4 Presence of divalent ions 20
1.3.2.5 Presence of foulants 20
1.3.2.6 Temperature 20
1.3.3 Impact of solute characteristics 22
1.3.3.1 Molecular weight 22
1.3.3.2 Molecular size (length and width)/molecular volume 22
1.3.3.3 Minimum projection area/Equivalent width 23
1.3.3.4 Charge 23
1.3.3.5 Hydrophobicity/hydrophilicity 24
1.3.4 Impact of operational characteristics 25
1.3.4.1 Transmembrane pressure/permeate or transmembrane flux 25
1.3.4.2 Cross-flow velocity/recovery/concentration polarisation 25
1.3.5 Impact of fouling on rejection 26
1.3.5.1 Organic fouling 28
1.3.5.2 Colloidal fouling 30
1.3.5.3 Inorganic fouling (scaling) 31
1.3.5.4 Biological fouling 32
1.3.6 Impact of membrane cleaning on rejection 32
1.3.6.1 Changes of membrane morphology due to cleaning 32
1.3.6.2 Impact on rejection of TrOCs due to cleaning 33
1.3.7 Validation at pilot and full scale systems 35
2 MEMBRANE FOULING IN THE NANOFILTRATION OF LANDFILL LEACHATE AND ITS IMPACT ON TRACE CONTAMINANT REMOVAL 37
2.1 Introduction 37
2.2 Materials and Methods 40
2.2.1 Analytical reagents and chemicals 40
2.2.2 Nanofiltration membrane 40
2.2.3 Membrane filtration set-up and protocol 41
2.2.4 Analytical technique 42
2.3 Results and discussion 42
2.3.1 Landfill leachate characterisation 42
2.3.2 Physico-chemical properties of bisphenol A 43
2.3.3 Influence of the calcium concentration on the flux 44
2.3.4 Influence of fouling on the rejection of organic contaminants 46
2.4 Conclusions 48
3 CHARACTERISING HUMIC ACID FOULING OF NANOFILTRATION MEMBRANES USING BISPHENOL A AS A MOLECULAR INDICATOR 50
3.1 Introduction 50
3.2 Materials and methods 52
3.2.1 Model NF membranes and membrane characterisation 52
3.2.2 Model trace organic contaminant 52
3.2.3 Organic foulant 53
3.2.4 Membrane filtration set-up 54
3.2.5 Filtration protocol 55
3.2.6 Analytical technique 56
3.3 Results and discussions 56
3.3.1 Membrane characteristics 56
3.3.2 Membrane fouling behaviour 58
3.3.3 Change of membrane hydrophobicity 61
3.3.4 Effects of organic fouling on the nanofiltration of BPA 63
3.3.5 Effects of organic fouling on rejection: the mechanisms 65
3.4 Conclusions 67
4 EFFECTS OF FOULING AND SCALING ON THE REJECTION OF TRACE ORGANIC CONTAMINANTS BY A NANOFILTRATION MEMBRANE: THE ROLE OF CAKE-ENHANCED CONCENTRATION POLARISATION 69
4.1 Introduction 69
4.2 Materials and methods 71
4.2.1 Nanofiltration membrane 71
4.2.2 Chemicals and reagents 71
4.2.3 Crossflow membrane filtration system 72
4.2.4 Experimental protocol 73
4.2.5 SEM-EDS analysis 74
4.2.6 Analytical methods 75
4.3 Results and discussion 75
4.3.1 Membrane characteristics 75
4.3.2 Membrane fouling and scaling development 76
4.3.3 Effects of fouling/scaling on the membrane rejection behaviour 79
4.3.4 Cake-enhanced concentration polarisation 85
4.4 Conclusions 87
5 SUMMARY AND CONCLUSIONS 88
6 REFERENCES 94
7 ACKNOWLEDGEMENTS 112
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:34037 |
| Date | 20 May 2019 |
| Creators | Vogel, Dirk |
| Contributors | Dornack, Christina, Lerch, André, Nghiem, Long D., Technische Universität Dresden |
| Publisher | Eigenverlag des Forums für Abfallwirtschaft und Altlasten e. V. |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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