(557d) Nature-Inspired Hydrodynamic Techniques to Enhance Membrane Separation Performance – a CFD-DEM Case Study

According to the United Nations (UN) World Water Development Report from 2023, in 2010 around 32-46% of the world’s population lived under water stress for at least a month per year.(1) It is expected that by 2050 the world’s population will have increased from 8 billion now to approximately 9.8 billion.(2) If global living standards continue to improve, the demand for clean water will be, roughly, the freshwater resources of three planets Earths by 2050.(3) Membrane-based filtration processes are increasingly used in industrial applications such as water treatment, for being energy-saving, cost-effective and space-efficient. However, these processes also have some drawbacks, membrane fouling being one of the most difficult to deal with.

Natural filtration processes such as the kidney’s blood filtration, and the filter-feeding system of certain animals are examples of highly effective membrane filtration techniques. Their efficiency is likely a result of multiple factors, such as the physicochemical and the geometrical characteristics of the membrane. When seeking to develop a nature-inspired membrane, one should understand the underlying mechanisms behind such natural systems, from the nanoscale to the macroscale, in addition to possible dynamic factors.(4, 5)

Since multiple scales are important in natural systems, hydrodynamics might also influence anti-fouling in membrane filtration. The curvature of the glomerulus, and the oral cavities of filter-feeding species, like basking sharks and flamingos, might generate fluid instabilities that decrease fouling. The membrane placement on filter-feeding species, as well as the curvature of glomerular blood vessels are the inspirations for the concept that membrane placement, the formation of Dean vortices, increased near-wall velocity, and particle focusing provided by curved channels can decrease fouling in membrane separation.

Inspired by these natural phenomena, CFD-DEM simulations were performed to investigate how hydrodynamics influence membrane filtration. The results showed that for a membrane-module with patterned membrane positioning and where the membranes are represented by a square wire screen, it was beneficial to have a spiralling tube instead of a straight tube, as this increased the number of particles contained in the retentate by more than 10%. This indicates higher efficiency of the membrane filtration and decreased fouling. Changes in the characteristic parameters of the helical configuration allow to tailor the efficacy of the filtration, where the number of turns is the most influential parameter. A higher filtration performance was obtained for water with a Reynolds number of 250 flowing inside a helical tube with 3 turns, a tube and coil radius of 20.2 and 120 mm, respectively, and a patterned membrane with a membrane-non membrane length ratio of 0.5. An efficiency of 32% was achieved for the lower base footprint helical tube configuration, when 60% of the inlet fluid is filtered. For the same configuration membrane module, where the membranes are represented by a porous material with a thickness of 2.1 mm and a loss coefficient of 10,000, using a sine wave with an amplitude of 0.011 m/s, a period of 403 s (Strouhal number of 0.01), and a 0.01 m/s phase shift to describe the inlet flow to the membrane module, improved the efficiency by 14% with an 11% increase in the filtered flow when compared to the non-pulsed inlet flow.

In conclusion, modulating the membrane module’s geometry and the inlet velocity in a nature-inspired way was established as an efficient anti-fouling strategy, where the membrane positions, addition of curvature and the pulsation of the inlet velocity are highly influential factors.

References

1. UNESCO World Water Assessment, The United Nations World Water Development Report 2023: partnerships and cooperation for water. Handb. Water Purity Qual., 210 (2023).
2. World population projected to reach 9.8 billion in 2050, and 11.2 billion in 2100 | United Nations, (available at https://www.un.org/en/desa/world-population-projected-reach-98-billion-2...).
3. A. Hussam, Potable Water: Nature and Purification. Monit. Water Qual. Pollut. Assessment, Anal. Remediat., 261–283 (2013).
4. M.-O. Coppens, Nature-Inspired Chemical Engineering for Process Intensification. Annu. Rev. Chem. Biomol. Eng. 12, 187–215 (2021).
5. J. Li, Y. Liu, L. C. Campos, M.-O. Coppens, Increased E. coli bio-adsorption resistance of microfiltration membranes, using a bio-inspired approach. Sci. Total Environ. 751, 141777 (2021).