New process makes sea water drinkable in minutes

A team from the Korea Institute of Civil Engineering and Building Technology (KICT) has developed a membrane distillation process to turn sea water into drinking water, using a stable-performance electrospun nanofibre membrane.

Membrane wetting is the most challenging issue in membrane distillation. If a membrane exhibits wetting during membrane distillation operation, it must be replaced. If a membrane gets fully wetted, the membrane leads to inefficient membrane distillation performance, leading to low-quality permeate.

Led by Dr Yunchul Woo, the KICT research team developed coaxial electrospun nanofibre membranes fabricated by an alternative nanotechnology called electrospinning. The technology can prevent wetting issues and improve the long-term stability of the membrane distillation process. Improved hydrophobicity is achieved by the formation of a three-dimensional hierarchical structure, which is created by the membranes’ nanofibres for higher surface roughness.

Dr Woo’s team used poly(vinylidene fluoride-co-hexafluoropropylene) as the core, and silica aerogel mixed with a low concentration of the polymer as the sheath, to produce a coaxial composite membrane and obtain a superhydrophobic membrane surface. The membrane performed a 99.99% salt rejection for one month. The results are published in the Journal of Membrane Science.

The membrane operated well, without wetting or fouling issues, due to its low sliding angle and thermal conductivity properties. Temperature polarisation is one of the significant drawbacks in membrane distillation. It can decrease water vapour flux performance during membrane distillation operation due to conductive heat losses. The new membrane is suitable for long-term membrane distillation applications as it possesses several important characteristics, such as a low sliding angle and low thermal conductivity for avoiding temperature polarisation, and reduced wetting and fouling problems while maintaining supersaturated high water vapour flux performance.

The research team noted that it is more important to have a stable process than a high water vapour flux performance in a commercially available membrane distillation process. Dr Woo commented that the coaxial electrospun nanofibre membrane has strong potential for the treatment of seawater solutions without suffering from wetting issues, and may be appropriate for pilot-scale and real-scale membrane distillation applications.

Image caption: Schematic of the coaxial electrospinning device. Image credit: Elsevier.