Comparison of Computational Fluid Dynamics (CFD) and Experimental Data for Fouling Mitigation with Sinusoidal Reverse Osmosis Spacer

Membrane fouling is a major drawback of reverse osmosis (RO) membrane filtration as it negatively affects the quantity and quality of the permeate and lifespan of the membrane. Spacer plays a pivotal role in hydrodynamics of the membrane channel thus influencing the membrane fouling. In this study, a series of novel sinusoidal spacers that can form unobstructed sinusoidal membrane channels were built and the performance of various novel sinusoidal spacers and the conventional mesh spacer on mitigating foulant deposition onto membrane surface was evaluated both experimentally on a bench-scale RO system and numerically with a three dimensional computational fluid dynamics (CFD) model. The results of the fouling experiments with humic acid and calcium ions showed that sinusoidal spacers were able to mitigate the membrane fouling and increasing the tortuosity of the sinusoidal wave pattern enhanced the efficiency of fouling mitigation. In addition, the CFD simulation revealed the development of fouling process and the CFD results were compared with the scanned fouled membrane images collected at different stages of fouling process. CFD results matched well with the experimental results and showed that the geometry of sinusoidal spacers could make an impact on membrane fouling development. Introduction Membrane fouling, which generally refers to the attachment, accumulation and adsorption on membrane surface or with membrane pores, is a critical issue in membrane filtration [1–6]. Studies on membrane fouling patterns often suggest that spacers play an important role in fouling development. Tran et al. [7] reported that fouling initially started along the feed spacer and then gradually encroached upon the rest of the clean membrane area. Gimmelshitein et al [8] studied the flow in spacerfilled channels and they found spacer exacerbated the particle deposition on places near the mesh spacer filaments. Vrouwenvelder et al. [2] and Paassen et al. [9] independently studied the correlation between spacers and biofouling. They reported that biofouling was largely initiated on feed spacers and the pressure drop caused by biomass accumulation was much higher when the spacer was present. It is well acknowledged that spacer design is critical in creating a favorable fluid dynamics condition to mitigate membrane fouling. However a great effort has been paid on influent pretreatment and membrane surface modification to make anti-fouling or foulant-resistant membranes[10,11], the benefits of those efforts may be diminished if spacer design is not sufficiently addressed. The goal of this study is to test the sinusoidal spacer’s performance on mitigating membrane fouling as a continuum of our previous work of sinusoidal spacer on concentration polarization reduction[12]. There are many studies on improving the current prevailing mesh spacer [13– 16]. For example, Schwinge, et al. [15] built a three-layer mesh spacer that produced higher flux compared to the traditional two-layer spacer, but it incurred higher