Quick Answer: Biofouling — that’s biofilm forming on RO membranes — is the toughest kind of fouling to control. It can slash how much clean water you get by 20-50% and drive up your operating costs. But good news, new anti-biofouling membranes with zwitterionic polymer coatings, silver nanoparticles, and graphene oxide are showing great results in lab tests, really cutting down on biofilm stickiness. We’re starting to see these hit the market too.