Algal organic matter (AOM) is one of the main causes of organic/particulate fouling in seawater reverse osmosis (RO) membranes and pretreatment systems, but studies on the role of AOM on performance of conventional pretreatment options are limited. In this study, we investigate the removal of AOM derived from marine diatom species Chaetoceros Affinis (CA), with three pretreatment scenarios consisting of dual media-filter (DMF) without coagulation, DMF with coagulation and sedimentation, and DMF with coagulation and dissolved air flotation (DAF), implemented for different AOM concentrations (0–0.4 mg C/L). Results revealed that AOM released by CA is mainly composed of biopolymers, and AOM can induce organic fouling after DMF during simulated algal bloom conditions at biopolymer concentrations as low as 0.2 mg C/L. Biopolymers could be removed at different extent by tested pretreatment scenarios. The most significant improvement in performance was observed for the DMF preceded by coagulation and DAF, which exhibited 72 % removal of biopolymers. This result is comparable to previously-reported membrane pretreatment with 50 K Da ultrafiltration (UF) membranes, which substantially reduced downstream SWRO fouling.