Using supercritical CO₂ for alginate-fouled seawater reverse osmosis cleaning and for carbonate mineral production from concentrated brine

Organic fouling is recognized as one of the main challenges in seawater reverse osmosis (SWRO). To overcome this challenge, membrane cleaning is routinely conducted. In this study, membrane cleaning with supercritical carbon dioxide (sCO₂) was implemented. To understand the mechanism of sCO₂ cleaning, the effects of pH, shear force, phase of CO₂ (supercritical/gaseous), and type of organic foulant (alginate only, alginate with calcium (Ca²⁺) ions, or transparent exopolymer particles (TEP)) were investigated. sCO₂ cleaning had 82–87 % of cleaning efficiency, regardless of foulant types. With alginate-fouled membranes, sCO₂ presented 89.7 % cleaning efficiency, while de-ionized water flushing, pH 5 solution, liquid CO₂ showed respective cleaning efficiencies of 12.2 %, 6.2 %, 9.3 % and 7.2 %. The superior cleaning efficiency of sCO₂ for organic fouling is mainly attributed to the supercritical phase that promotes shear force and dissolution (local acidic pH). Moreover, the mineralization of brine using the spent sCO₂ cleaning agent was evaluated. sCO₂ was able to convert divalent ions (Ca²⁺ and magnesium(Mg²⁺)) into nano-sized carbonate precipitates (∼46 nm) that can be potentially traded to reduce water production cost and environmental burdens when brine is released into seawater.