Feed spacers are important for the impact of biofouling on the performance of spiral-wound reverseosmosis (RO) and nanofiltration (NF) membrane systems.The objective of this study was to propose a strategy for developing, characterizing, and testing of feedspacers by numerical modeling, three-dimensional (3D) printing of feed spacers and experimentalmembrane fouling simulator (MFS) studies.The results of numerical modeling on the hydrodynamic behavior of various feed spacer geometriessuggested that the impact of spacers on hydrodynamics and biofouling can be improved. A goodagreement was found for the modeled and measured relationship between linearflow velocity andpressure drop for feed spacers with the same geometry, indicating that modeling can serve as thefirststep in spacer characterization.An experimental comparison study of a feed spacer currently applied in practice and a3D printed feedspacer with the same geometryshowed (i) similar hydrodynamic behavior, (ii) similar pressure dropdevelopment with time and (iii) similar biomass accumulation during MFS biofouling studies, indicatingthat 3D printing technology is an alternative strategy for development of thin feed spacers with acomplex geometry. Based on the numerical modeling results, a modified feed spacer with low pressuredrop was selected for 3D printing. The comparison study of the feed spacer from practice and themodified geometry 3D printed feed spacerestablished that the 3D printed spacer had (i) a lower pressuredrop during hydrodynamic testing, (ii) a lower pressure drop increase in time with the same accumu-lated biomass amount, indicating that modifying feed spacer geometries can reduce the impact ofaccumulated biomass on membrane performance.The combination of numerical modeling of feed spacers and experimental testing of 3D printed feedspacers is a promising strategy (rapid, low cost and representative) to develop advanced feed spacersaiming to reduce the impact of biofilm formation on membrane performance and to improve thecleanability of spiral-wound NF and RO membrane systems. The proposed strategy may also be suitableto develop spacers in e.g. forward osmosis (FO), reverse electrodialysis (RED), membrane distillation(MD), and electrodeionisation (EDI) membrane systems.

Membrane systems are commonly used in the water industry to produce potable water and for advanced wastewater treatment. One of the major drawbacks of membrane systems is biofilm formation (biofouling), which results in an unacceptable decline in membrane performance. Three novel in situ biofouling characterization techniques were assessed: (i) optical coherence tomography (OCT), (ii) planar optodes, and (iii) nuclear magnetic resonance (NMR). The first two techniques were assessed using a biofilm grown on the surface of nanofiltration (NF) membranes using a transparent membrane fouling simulator that accurately simulates spiral wound modules, modified for in situ biofilm imaging. For the NMR study, a spiral wound reverse osmosis membrane module was used. Results show that these techniques can provide information to reconstruct the biofilm accurately, either with 2-D (OCT, planar optodes and NMR), or 3-D (OCT and NMR) scans. These non-destructive tools can elucidate the interaction of hydrodynamics and mass transport on biofilm accumulation in membrane systems. Oxygen distribution in the biofilm can be mapped and linked to water flow and substrate characteristics; insights on the effect of crossflow velocity, flow stagnation, and feed spacer presence can be obtained, and in situ information on biofilm structure, thickness, and spatial distribution can be quantitatively assessed. The combination of these novel non-destructive in situ biofilm characterization techniques can provide real-time observation of biofilm formation at the mesoscale. The information obtained with these tools could potentially be used for further improvement in the design of membrane systems and operational parameters to reduce impact of biofouling on membrane performance.