In trials with synthetic wastewater at 5 bar, the flux decline was 58.3% for GR60PP and 64.1% for OT050. Flux decline can be attributed mainly to fouling phenomenon, due to tannin adsorption, cake formation as well as pore blocking. The rejected particles accumulated near the membrane surface forming a cake layer that assisted in tannin removal and increased permeate flux resistance. This is because vegetable tannins are miscible with water and form poly disperse solutions, partly of colloidal type. In the effluent, labile aggregates form non-associated molecules with approximately spherical shape with 14–40˚A diameter. Besides colloidal particles,