Researchers have concluded that pressure-retarded osmosis (PRO) was commercially “dead” primarily due to biofouling.
PRO has, since the 1970s, been considered to hold great potential as a sustainable energy source and as a possible contributor to desalination systems. But a study by scientists at the Zuckerberg Institute for Water Research at Israel’s Ben-Gurion University of the Negev (BGU) and Yale University in the US has found that biofouling was, as yet, too great a challenge to the technology.
“Power generation by PRO produces little and next to nothing due to biofouling caused by bacteria that clog the membrane structure and the feed channel,” said the researchers. Their findings echo those of earlier studies at Yale University.
“Our research shows that commercial PRO is currently dead in the water,” said Zuckerberg’s Dr Edo Bar-Zeev. “Biofouling is detrimental to the process and can’t be mitigated since there are no membranes today that are specifically designed for PRO.”
Bar-Zeev said PRO required either sterile streams on both sides or a new membrane design “dedicated for PRO technology instead of using the current forward osmosis (FO) membranes.”
His team operated a PRO system under biofouling conditions using thin-film composite FO membrane and fabric feed spacers. It deployed synthetic wastewater secondary effluents and seawater reverse osmosis desalination brine.
“The study showed that organic matter and bacteria in the feed wastewater stream resulted in extreme biofouling development across the feed spacer as well as the membrane support layer, thereby crippling PRO performance,” said Bar-Zeev. He d predicted that his results would be repeated with natural waters where dissolved bacteria are usually prevalent.
PRO uses a hydro turbine to exploit the osmotic pressure created by salinity gradients - for example at estuaries where sea and river water mix.