Alginate, a sustainable product, can be synthesized by organic substances in wastewater by aerobic granules. Although chemical precipitation is effective for extraction, this typically consumes considerable amounts of chemicals, causing chemical pollution. Alternatively, membrane separation can be used as a potential separation method. Resistance reduction and fouling mechanisms were investigated by constant-pressure dead-end filtration for the membrane recovery of alginate from a suspension formed in an aqueous solution of sodium alginate (SA) by the addition of Ca2+. Flux decline was analyzed by the Ruth's cake filtration theory and the Ruth's filtration coefficient, which indicated that feed filterability exponentially increased as Ca2+ was added. The increased Ruth's filtration coefficient corresponded to decreased concentrations and average specific cake resistances of the components present in the suspension, e.g., colloids and polymers. The optimal amount of Ca2+ should be economically feasible while also affording filtration resistance. Residual Ca2+ in the filtrate can be reused. Residual salt ions increased filtration resistance. The filter cake formed on the membrane was compressible. The effect of protein on the ultrafiltration of alginate was considered. Microfiltration or filtration with a filter paper, exhibiting a high recovery rate of alginate, was suggested.