This work deals with an experimental investigation on the effect of different pretreatments on the critical flux of both an ultrafiltration and a nanofiltration membrane process, applied at pilot scale, on the purification of an olive washing wastewater. Two pretreatments were considered: flocculation and biofiltration. First, the flocculation process was optimized, taking into account two different coagulants and varying the dosage and the mixing procedure. The same wastewater was treated by a biological filter. The obtained pretreated solutions were then processed by using wound spiral membrane modules of ultrafiltration and nanofiltration to determine the critical flux for each membrane process. For both the examined membrane processes the critical flux value was around 40% higher when the flocculation pretreatment was applied.