Quantifying the potential of ultra-permeable membranes for water desalination

Abstract

In the face of growing water scarcity, it is critical to understand the potential of saltwater desalination as a long-term water supply option. Recent studies have highlighted the promise of new membrane materials that could desalinate water while exhibiting far greater permeability than conventional reverse osmosis (RO) membranes, but the question remains whether higher permeability can translate into significant reductions in the cost of desalinating water. Here, we address a critical question by evaluating the potential of such ultra-permeable membranes (UPMs) to improve the performance and cost of RO. By modeling the mass transport inside RO pressure vessels, we quantify how much a tripling in the water permeability of a membrane would reduce the energy consumption or the number of required pressure vessels for a given RO plant. We find that a tripling in permeability would allow for 44% fewer pressure vessels or 15% less energy for a seawater RO plant with a given capacity and recovery ratio. Moreover, a tripling in permeability would result in 63% fewer pressure vessels or 46% less energy for brackish water RO. However, we also find that the energy savings of UPMs exhibit a law of diminishing returns due to thermodynamics and concentration polarization at the membrane surface. Broader contextThe development of affordable, reliable and energy-efficient technologies for converting saltwater into fresh water is one of the most important research goals of this century. Yet the best technology available today, reverse osmosis (RO), remains costly. Recent advances in materials research suggest that new membranes could reject salt while permeating water much faster than nonporous RO membranes. However, considerable confusion exists regarding the likelihood that future RO systems will continue to become smaller, more productive or more energy-efficient. Given the critical importance of water technology research for human development goals, it is essential to carefully evaluate what future RO systems can and cannot achieve on the basis of more permeable membranes. Beginning with fundamental transport equations and extending to applied engineering scenarios, we demonstrate that membranes with 3x higher permeability could reduce the energy consumption of RO by 15-46% for seawater and brackish water respectively, or alternatively reduce the number of pressure vessels by 44-63%. Given many recent advances in the design of RO membranes, this work highlights the likely development of a new generation of desalination plants with higher throughput and a smaller spatial footprint than what is achievable today. Motivation and research questionThe orders-of-magnitude increase in permeability that UPMs could potentially enable require an in-depth assessment of the physical mechanisms that occur at the membrane's surface.

Multilayer Nanoporous Graphene Membranes for Water Desalination

“…Indeed, a membrane with 3x higher permeability than current commercial membranes would exhibit most of the energy efficiency and cost reduction improvements to be gained from more permeable membranes. 1 Our results also provide guidelines for the design of multilayer membranes. We have found a critical region for membrane design at a layer separation of H ≤ 8 Å, in which both layer separations and pore alignment both play a crucial role in determining desalination performance.…”

“…1,2,3 Most studies to date have considered an idealized, single-layer system with variations in several key parameters (i.e., nanopore size, separation, chemistry, or substrate morphology), with the hope of guiding the experimental synthesis of NPG membranes. 2,3,4 Although multiple efforts have targeted the synthesis of large-scale graphene films, 5 producing perfect monolayer graphene over large areas remains highly challenging experimentally.…”

Review on carbon nanotubes and carbon nanotube bundles for gas/ion separation and water purification studied by molecular dynamics simulation

“…A combination of water desalination technologies and renewable energy sources will most likely be needed to win such a challenge. Desalination is one of the most promising approaches to freshwater recycle (Amokrane et al 2015;Cohen-Tanugi et al 2014;Ghaffour et al 2013;Porada et al 2013;Wang and Karnik 2012). As nanotubes are innately insoluble in water (Fatemi and Foroutan 2015b), aqueous dispersion requires modification of the nanotube surface.…”

Shielding membrane surface carboxyl groups by covalent-binding graphene oxide to improve anti-fouling property and the simultaneous promotion of flux

“…However, higher water permeability means a higher concentration polarization effect for the same operating conditions (Cohen-Tanugi et al, 2014), implying more serious membrane fouling (Bacchin et al, 2006). Our fouling tests (Fig.…”