Reusing wastewater from oil-related industries is becoming increasingly important, especially in water-stressed oil-producing countries. Before oily wastewater can be discharged or reused, it must be properly treated, e.g., by membrane-based processes like ultrafiltration. A major issue of the applied membranes is their high fouling propensity. This paper reports on mitigating fouling inside ready-to-use ultrafiltration hollow-fiber modules used in a polishing step in oil/water separation. For this purpose, in-situ polyzwitterionic hydrogel coating was applied. The membrane performance was tested with oil nano-emulsions using a mini-plant system. The main factors influencing fouling were systematically investigated using statistical design of experiments.