UBC Theses and Dissertations
Nanofiltration and tight ultrafiltration membranes for drinking water treatment – system design and operation Winter, Joerg
Nanofiltration (NF) and tight Ultrafiltration (UF) membranes represent a technology that is well suited for high quality drinking water production from source waters with high concentrations of natural organic matter (NOM) or other contaminants. However, the application of these membranes is limited, mainly because of the challenges related to fouling, concentration polarization (CP), system complexity and cost. The aim of the present study was to address these issues and to enable a more widespread application of this technology. Opportunities for simplifying NF and tight UF systems by, for instance, operation in dead-end mode, were explored. Experiments were conducted to evaluate the contribution of CP and fouling to the increase in resistance to permeate flow when filtering raw waters containing NOM. Continuous and periodic hydraulic measures to control CP and fouling were assessed. When filtering model raw waters containing humic substances, the increase in resistance to permeate flow was dominated by CP. When humic substances are effectively rejected and are present at high concentrations, CP becomes extensive, leading to a significant increase in the resistance to permeate flow by the formation of a cake/gel layer at the membrane surface. In the presence of calcium and particulate matter, the increase in resistance to permeate flow was dominated by fouling rather than CP. Filtration tests using periodic hydraulic measures to control fouling indicated that it was difficult to hydraulically remove the accumulated material once a foulant layer had been formed. Therefore, cross-flow operation, which reduces CP and prevents the formation of a foulant layer, is recommended. To optimize cross-flow operation, a framework was developed to compare the performance of NF membranes of different configurations (i.e. spiral wound and hollow fiber configurations) and geometries in terms of the permeate flux that can be sustained, and to optimize the operating set point of NF membranes with respect to system configuration, cross-flow velocity and operating flux with respect to cost. The results suggest that, despite higher manufacturing costs for hollow fiber NF, hollow fiber NF can be cost competitive to spiral wound NF. Because of operational advantages, the application of hollow fiber configurations is recommended.
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