The Interdisciplinary Nature of Water Purification: Where Materials Science, Chemistry and Thermodynamics Meet

For something as common as water filtration ,the modern technology that enables it, is built upon some serious materials engineering.  Polymer fibers,the active components of any filtration process, are spun using special coaxial extrusion techniques into what are known as hollow fiber membranes. 

These membranes are then submerged inside the semi-treated water tank or wound around a plastic core for a more compact arrangement as water is drawn through these hollow sections the tiny pores on the surface trap impurities —suspended solids,bacteria,viruses,protein molecules depending upon the pore size. 

Achieving this pore size tuning is in itself a quite complex process that relies on phase inversion where polymer solvent is replaced by non solvent on the solution, dissolved polymer coagulates but is diluted by a non solvent that evaporates leaving behind pores in the main structure. 

Ceramic membranes offer a different class of materials that are more robust in harsh chemical environments and at elevated temperatures. But they offer another advantage which is relaxation on materials requirements. Polymer membranes require complex multi step processes for manufacturing of base polymers and equally complex steps of manufacturing solvent systems. Ceramic membranes work with silica and carbon-based systems that are more abundant. Polymer membranes may be manufactured at room temperature but their process chain is far more complex. 

Filtration systems rely on a whole set of support tools to function — tanks,pumps, stirrers to name a few. Plasma based ozone production to breakdown pollutants at pre filtration stage and post filtration stage disinfection are gaining popularity. 

Water filtration is important not just for drinking water but also for industrial processes and of course for H2 production which is both a fuel and an industrial chemical. 

The real prize is large scale desalination  that has the potential to completely get rid of water scarcity and unleash the clean fuel industry and make a significant impact on manufacturing. Currently reverse osmosis technology is the dominant method to desalinate at 3kwh/m3 of fresh water produced. But it suffers from 2 major problems: fouling of membranes requiring constant monitoring and reliance on complex techniques to make thin film composite structures that enable that desalination. 

Nature has demonstrated alternative techniques. Freezing of water pushes out salt ions making the ice fresh. The underlying physics is not much different from sub nanometer pore size membranes that block salt ions in reverse osmosis. As ice freezes crystal lattices become so small that salt crystals can't remain embedded in it and are pushed out. This method consumes about 140kwh/m3 but when combined with waste heat recovery and heat pumps this could make for a very attractive low cost cheap and effective potable water production system. Especially in dry areas where water scarcity occurs along with high heat such a system could provide both cooling and fresh water. The latent heat in ice could also be used to lower the ambient temp of the freezing system ,or to pre-cool the brine water so refrigerator has to do less work on freezing it. Making it possible to reach a simillar 3kwh/m3 as an RO system. 

It is always a pleasure to connect with people working in water filtration technology. Something so essential but deeply complex and immensely satisfying. I look forward to continuing my engagement with the community and learning from the experts.