Regardless of the plant capacity, the selection of the right pre-treatment is critical to the performance and longevity of every sea water reverse osmosis (SWRO) plant.

Overview A successful pre-treatment package for sea water reverse osmosis (SWRO) desalination is designed to mitigate the risks of membrane fouling, and deliver capacity designed production and long-term reliability in the operation of the SWRO plant. Both the levels of dissolved and insoluble matter in seawater are a concern in the design process for a SWRO plant. The feed water quality data needs to include an analysis of: the amount of suspended solids and silt in the seawater feed, the concentrations of mineral scaling species and the presence of insoluble metal hydroxide forming species, natural organic species from the marine environment that foul the membranes, microbial species including marine organisms and associated organic compounds that can cause bio-film formation on the membranes, and colloidal materials that concentrate and form barriers on the membrane surface. Designing the Right Pre-Treatment System Designing the requisite pre-treatment system that eliminates any risks from (1) to (5) and at the same time delivers a solution that is reliable in performance and cost-competitive in operation is crucial for the entire SWRO plant design. The identification of an optimum location for the seawater intake (and type of seawater intake system) will minimise the load of silt and fine matter that enters the desalination plant. All seawater desalination systems will however, include a pre-filtration stage to remove fine matter, silt and microbial matter. High turbidity solutions may require the use of a sedimentation or coagulation plant together with fine media filtration and single-micron cartridge filtration, or a sedimentation or coagulation plant in combination with a membrane pre-treatment plant, whereas lower turbidity feeds may use multiple stage filters or a membrane filtration pre-treatment process only. The chemistry of the seawater matrix, including the high salinity of the seawater assists in the prevention of scale formation on the membrane surface. However, the measurement of the concentrations of metals and colloidal forming species always needs to be assessed for scale-formation concentration levels, at the operating pH and temperature of the plant process. Microbial matter typically found in seawater contains a diverse array of organisms, and the pre-treatment tools available include biodegradation of the organic load that feeds organisms or the biological disinfection of the organism themselves. Septech utilises an extensive history in process design, installation and operation of domestic, commercial and mega project plants in the delivery of site and water quality optimised pre-treatment packages, in every SWRO desalination project.