A fresh look at mega-scale SWRO: using brine-staged reverse osmosis to optimize energy efficiency and membrane performance

Seawater reverse osmosis (SWRO) desalination has been in wide use for over 25 y. This paper exam-ines limitations of the prevalent membrane configuration, based on a single stage consisting of one or more membrane housings operating in parallel with 6-7 elements per membrane housing. A brine -stage configuration using interstage pressure boosting is proposed to address limitations in single stage systems including high biofouling potential, excessive permeate salinity and high energy con-sumption. Membrane array optimization requires careful review of concentration polarization (Beta or CP) and the feed flow rate through each membrane element (Q(ele)), (a proxy for cross-flow velocity). CP is the formation of a salinity gradient normal to the membrane surface generated by a combina-tion of boundary layer formation and permeation. CP is often characterized by the term Beta, which is defined as the ratio of salinity at the membrane surface divided by the bulk feed salinity. A high Beta means that the membrane surface is exposed to elevated salinity (hence higher osmotic pres-sure and increased salt passage) and a greater concentration of foulants. These conditions increase the required pressure, increases permeate total dissolved solids (TDS) and promotes fouling of membrane spacers and surfaces. A high Q(ele) is preferred to minimize boundary layer thickness and to better scour feed channel spacers and the membrane surface. This paper explores a brine staged SWRO membrane array that achieves preferred levels of Beta and Q(ele) to achieve improved biofouling resistance and lower permeate TDS while providing a recovery of 60% in typical SWRO applications using standard membranes, pressure vessels and energy recovery devices (ERDs). Major membrane suppliers have endorsed high recovery SWRO using brine staging as described in this paper. The focus also includes optimal ways to implement brine staging over very large SWRO systems using turbochargers in a unique configuration that maximizes brine stage performance as well as energy recovery efficiency. The paper also addresses related ERD technologies and variable frequency devices that together can reduce energy consumption as well as provide a substantial reduction in capital costs (CAPEX) and operating costs (OPEX) with realistic reductions of 16% in mega-scale SWRO facilities.