While reverse osmosis is a relatively energy-efficient form of seawater desalination, it is nonetheless made less efficient by a problem known as biofouling. A new membrane coating, however, could address biofouling like never before.
Putting it very basically, reverse osmosis involves using high pressure to force seawater through a semipermeable membrane. That membrane filters the salt out of the water, but bacteria and other microbes also gradually accumulate on its surface. Over time, those microorganisms form into a slimy biofilm, which makes the membrane less permeable. As a result, more energy is required to push the water through.
And although there already are membrane coatings that help reduce biofouling, many of them incorporate toxic chemicals that bind them to the membrane. These chemicals ultimately end up being passed into the sea.
Additionally, according to scientists at Saudi Arabia's King Abdullah University of Science and Technology (KAUST), such coatings don't completely eliminate the biofilms. This limitation prompted the researchers to develop a non-toxic "polyelectrolyte" coating.
It's introduced as a liquid into the stream of untreated seawater, and binds to the surface of the membrane via electrostatic interactions – so no chemicals are required.
Once an obstructive biofilm has formed on the resulting coating, a high-saline solution is flushed through the desalination system at an increased velocity. This causes the coating to detach from the membrane, breaking apart and carrying the entire biofilm away with it. A new coating is subsequently applied.
In lab tests, after the biofilm-removal process had been performed, a polyelectrolyte-coated membrane exhibited a two-fold higher liquid flow rate than an untreated control membrane.
The research is being led by PhD student Maria Fernanda Nava-Ocampo, under the supervision of Prof. Johannes Vrouwenvelder. It is described in a paper that was recently published in the journal Desalination.
Source: KAUST
