Abstract
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.
Highlights
Hydraulics are controlled by the linear flow velocity of the system, which in turn is controlled by factors like thickness and porosity of spacer-filled channel, feed spacer orientation and geometry
The feed spacers were first compared with respect to geometry (Section 3.1)
The results of the various techniques for porosity measurement of spacer-filled feed channels were compared for the six feed spacers (Section 3.2)
Summary
Some strategies have focussed on reducing the impact of biomass accumulation on the performance of spiral-wound membrane systems by modifying the hydraulics of the membrane systems (Sablani et al, 2001; Valladares Linares et al, 2014). Research into the modification of feed spacer geometry, either experimentally or by numerical modelling has provided insight into (i) the impact of hydraulics on the performance of spiral-wound membrane systems (FimbresWeihs and Wiley, 2010; Koutsou et al, 2007; Madireddi, 1999; Picioreanu et al, 2009; Saeed et al, 2012; Siddiqui et al, 2016; Wiley and Fletcher, 2002), (ii) possibilities to reduce the impact of biofouling on membrane performance (Siddiqui et al, 2017), and (iii) possibilities to enable effective cleaning strategies. Modifying feed spacer properties such as thickness, orientation or filament angle affects the porosity of spacer-filled feed channels
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