Wave-powered water treatment

Oceans could be cleaned using entirely self-powered systems, if research from a group of Chinese researchers proves to be successful.

As the saying goes, “Water, water, everywhere, but not a drop to drink”. The high-salinity of seawater may be what makes it undrinkable, but increasing water pollution levels give this ancient rhyme a very modern relevance. Untreated wastewater contains harmful pathogens, but it can also cause algal blooms to thrive, depleting oxygen levels in our waterways. Current water treatment approaches include membrane separation and ultraviolet sterilisation, but while both are highly effective, they can treat only small volumes of water at a time, and can be very energy-intensive.

Researchers from three Beijing universities recently published [Nano Energy – DOI: 10.1016/j.nanoen.2015.09.017] a system to sterilise water that doesn’t require an external power supply – instead, it is powered using the motion of the water itself. To harness the kinetic energy in the waves, a triboelectric nanogenerator (TENG) was developed, consisting of two layers of indium tin oxide (ITO)-coated polyethylene terephthalate (PET) separated by a nanostructured polytetrafluoroethylene (PTFE) film. The top layer formed an arched shape, to allow it to ‘flex’ in response to any moving water. As these layers came into contact, the surface charges generated could flow through the TENG, giving rise to a small electrical current. 50 of these TENGs were set up in a large bath and together, they provided enough power (peak 210 V and 4.8 mA AC) for the attached electrochemical water treatment system.

This was, in effect, a seawater electrolysis set-up, with two mixed-metal oxide and titanium electrodes, coated in reduced graphene oxide. The graphene sheets were shown to reduce the growth of microbes on the surface of the electrodes, and the chlorine present in the seawater actively removed bacteria and algae, and it was very effective. After 60 s, 99.9999% of bacteria (including Escherichia coli and Streptococcus faecalis) and mixed marine algae present in the seawater were found to be deactivated.

The team believe that if scaled up, this TENG-powered system could be a practical solution for treating polluted ocean water. On the shorter-term, their low-cost system would be suitable for use as a pre-treatment or post-treatment step in existing water treatment cycles.

Jiang, Y. Jie, Y. Han, C. Gao, H. Zhu, M.Willander, X. Zhang, X. Cao – Nano Energy (2015) Vol 18, 81-88, “Self-powered electrochemical water treatment system for sterilization and algae removal using water wave energy.” DOI: 10.1016/j.nanoen.2015.09.017

This story first appeared here on 11 December 2015: http://www.materialstoday.com/materials-chemistry/news/wavepowered-water-treatment/