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The porosity of spacer-filled feed channels influences the hydrodynamics of spiral-wound membrane systems and impacts the overall performance of the system. Therefore, an exact measurement and a detailed understanding of the impact of the feed channel porosity is required to understand and improve the hydrodynamics of spiral-wound membrane systems applied for desalination and wastewater reuse. The objectives of this study were to assess the accuracy of porosity measurement techniques for feed spacers differing in geometry and thickness and the consequences of using an inaccurate method on hydrodynamic predictions, which may affect permeate production. Six techniques were applied to measure the porosity namely, three volumetric techniques based on spacer strand count together with a cuboidal (SC), cylindrical (VCC) and ellipsoidal volume calculation (VCE) and three independent techniques based on volume displacement (VD), weight and density (WD) and computed tomography (CT) scanning. The CT method was introduced as an alternative for the other five already existing and applied methods in practice. Six feed spacers used for the porosity measurement differed in filament thickness, angle between the filaments and mesh-size. The results of the studies showed differences between the porosities, measured by the six methods. The results of the microscopic techniques SC, VCC and VCE deviated significantly from measurements by VD, WD and CT, which showed similar porosity values for all spacer types. Depending on the maximum deviation of the porosity measurement techniques from -6% to +6%, (i) the linear velocity deviations were -5.6% and +6.4% respectively and (ii) the pressure drop deviations were -31% and +43% respectively, illustrating the importance of an accurate porosity measurement. Because of the accuracy and standard deviation, the VD and WD method should be applied for the porosity determination of spacer-filled channels, while the CT method is recommended for numerical modelling purposes. The porosity has a linear relationship with the flow velocity and a superlinear effect on the pressure drop. Accurate porosity data are essential to evaluate feed spacer performance in spiral-wound membrane systems. Porosity of spacer-filled feed channels has a strong impact on membrane performance and biofouling impact.

The application of nanofiltration is growing rapidly in drinking water and wastewater treatment. The main problem during the operation of nanofiltration membranes is membrane fouling, part of which is due to the presence of Natural Organic Matter (NOM) in sources for drinking water. In this work the effect of calcium was investigated on the nanofiltration fouling behavior in relation to NOM. From Li and Elimelech (2004) it is known that calcium enhanced membrane fouling significantly due to the formation of calcium-NOM complexes. Two techniques were used in our research to determine the part of calcium which is complexated by NOM and the free calcium ion in solution. Results showed that a minimum calcium concentration and a minimum NOM concentration were required for calcium-NOM complex formation. Furthermore, the influence of the calcium concentration on the flux decline during nanofiltration experiments was investigated for different types of feed water. The observed flux decline was proven to be only caused by NOM fouling rather than other membrane fouling types, such as biofouling, scaling or particulate fouling. Fouling of nanofiltration membranes was related to the calcium content in the feed water, and more specifically to the calcium-NOM complex concentration. Membrane cleaning with SDS was found to be very effective.