Mass Transport and Wetting Resistance in Membranes for Advanced Water Treatment

Lee, Sangsuk

Graduate Thesis Or Dissertation

Mass Transport and Wetting Resistance in Membranes for Advanced Water Treatment Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/rn301327k

Abstract

1 in 4 people on Earth face a lack of clean and safe water sources for drinking, irrigation, sanitation, and economic development. The urgent need for water motivates the use of unconventional water resources, such as seawater and wastewater. Advanced water treatment technologies that allow us to access these unconventional resources are drawing increasing attention. Membrane processes including reverse osmosis (RO) and nanofiltration (NF) have been rapidly growing as advanced water treatment technologies for desalination and water reuse due to high productivity, costeffectiveness, and scalability. The objective of this dissertation research is to further understand mass transport of water and solutes across membranes and increase the effectiveness of innovative distillation-based membrane technologies.

Numerous potentially harmful compounds exist in feed streams entering advanced water treatment facilities, and thus, understanding membrane rejection in reverse osmosis and nanofiltration for hundreds of compounds is critical for securing high quality product water. A large rejection dataset was compiled, and machine learning techniques enhanced by molecular fingerprints were used to predict membrane rejection of organic compounds. These techniques allowed us to interpret the relationship between the molecular structure of the solute and its rejection in membrane processes. The machine learning models showed high prediction accuracy (Spearman and Pearson coefficients of 0.86-0.99) both with training and test sets. Then, the trained models were analyzed using Shapley values to study the effects of sub-structures of organic compounds on membrane rejection.

The second study in this dissertation focused on addressing the low water flux of the osmotic distillation (OD) process. We found that the high membrane thickness (typically 30-100 𝜇𝜇m) of current membranes was the main cause of low water fluxes. An optimal membrane thickness of 0.073 𝜇𝜇m was derived with element-scale simulations, and it was able to achieve water fluxes exceeding those of current commercial forward osmosis (FO) membranes. In addition, comparison of module-scale performance with OD and FO membranes found that optimized OD membranes can outcompete high-performance FO membranes in maximum achievable water flux (25.3 vs. 18.6 kg m^-2h^-1 for OD and FO membranes, respectively) and water recovery (0.28 vs. 0.18).

The third study in this dissertation focused on the transport of volatile compounds in membrane distillation (MD). Rejection of volatile compounds in MD is highly varied and poorly understood. This study analyzed a variety of volatile and semi-volatile organic compounds to yield a comprehensive understanding of transport in MD. The effects of different molecular properties on transport were studied first, and we found the Henry’s constant and diffusion coefficient were important in determining solute flux. Then, the transport resistances across MD membranes were quantified and two distinct transport regimes (membrane resistance regime and boundary layer resistance regime) were defined.

Hydrophobic membranes are susceptible to membrane pore wetting, which results in failure of the system. To overcome this issue in pressure-driven distillation treating low surface tension liquids, the final study in the dissertation focused on fabricating omniphobic, wetting resistant membranes. Nanoporous membranes were modified with re-entrant structures and low surface energy. The results showed the liquid entry pressure values of the modified membranes were much higher than those of conventional membranes with cylindrical pores, allowing for the desalination at 16 bar with a 15% water-ethanol mixture. This low surface tension feed solution wetted the membrane with cylindrical pores.

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