Coagulation and ultrafiltration in seawater reverse osmosis pretreatment

Seawater desalination is a globally expanding coastal industry with an installed capacity of over 80 million m3/day. Algal blooms pose a challenge to the operation of seawater reverse osmosis (SWRO) membranes and pre-treatment systems due to high concentrations of algal cells and algal organic matter (AOM) present in the seawater. During such periods, operators must ensure continuous production of high quality feed water for SWRO membranes while maintaining stable hydraulic operation of the pre-treatment system. Coagulation is commonly applied to stabilize hydraulic performance and enhance permeate quality of ultrafiltration (UF) systems in SWRO pre-treatment. From an operational point of view it is desirable to completely eliminate coagulation from the process in order to reduce associated costs and complexities including treatment, handling and discharge of coagulant-rich sludge. This study investigates the role of coagulation in enhancing hydraulic performance and permeate quality of UF membranes and provides insight into options for minimizing or ideally eliminating coagulation from UF pre-treatment to SWRO. Results show that coagulation improves UF hydraulic performance mainly by reducing non-backwashable fouling of the membranes. This can be achieved at very low coagulant dose (~ 0.5 mg Fe/L) by coating the membranes with sub-micron particles. Further reducing particle size of the coating suspensions to the lower nanometre range is expected to be more effective in reducing the required equivalent dose and is recommended for future research. The work highlights the applicability of UF membranes with low molecular weight cut-off as the coagulant free future of SWRO pre-treatment. Major benefits in terms of reduced environmental impact is expected when applying membranes with low molecular weight cut-off, as the need for coagulation (for AOM removal) is eliminated while ensuring longevity of downstream SWRO membranes. Further improvements in material properties of these membranes should be directed at increasing the surface porosity of the membranes to enhance permeability recovery and ensure stable hydraulic operation. In general terms, the research indicates that coagulant consumption can be significantly reduced in UF pre-treatment of SWRO by optimizing operational parameters and applying alternative solutions.Water ManagementCivil Engineering and Geoscience