Biofouling is still a major challenge in membrane science and accounts for up to 70% of the total costs of membrane-operated filtration systems. Thus, in this work, a thin layer of diverse synthetic hydrogels has been applied via electron beam-initiated polymerization on a polyethylene terephthalate (PET) track-etched membrane (fundamental study) as a simple model membrane and on a conventional used polyethersulfone (PES) ultrafiltration membrane (applied study) to study its anti-biofouling properties. The effect of hydrophilicity, roughness, surface charge, and degree of swelling of the attached hydrogel on algae (Chlorella Vulgaris) fouling was studied in a fundamental way, and long-time tests in a membrane bioreactor (MBR, filtration area 1.5 m²) over 11 months were performed on an applied scale. The hydrophilicity, roughness, and surface charge had only a minor to no effect on algae fouling. Only the degree of swelling had an impact on biofouling. The membranes with hydrogels with a low degree of swelling (6%) showed a lower loss of permeability (only 5%) after fouling, compared to a 25% loss with the highest degree of swelling (25%). During the operation over 11 months in a membrane bioreactor (MBR), the permeability is higher (ca. 10%) in a module equipped with hydrogel modified membranes, compared to a module with reference membranes. Thus, modification of membranes with a thin layer of a hydrogel results in an anti-biofouling surface. The degree of swelling is a decisive parameter to control biofouling.