Rethinking membrane biofouling: A functional and mechanistic perspective

Fouling has long been recognized as a critical constraint in membrane-based water treatment and reuse. Yet, the field remains conceptually fragmented, hindered by maligned definitions and reductionist experimental models that fail to reflect operational realities. Despite decades of research, biofouling is still frequently conflated with biofilm formation, with performance decline attributed to microbial activity rather than the full complexity of fouling layer structural and mechanical development. This review identifies a central flaw in prevailing approaches: the interchangeable use of "biofilm" and "biofouling" obscures the dynamic and multiphase interactions between biological, organic, and inorganic constituents that shape real-world fouling layers. Traditional models such as cake filtration and resistance-in-series frameworks describe hydraulic resistance based on inert particulate accumulation, but do not account for the mechanical and interfacial behavior of hydrated, viscoelastic foulant matrices. In membrane systems exposed to continuous flow, initial deposition of natural organic matter, inorganics, and colloids combined with mass transfer of soluble organics can facilitate microbial growth and production of extracellular polymeric substances, resulting in structurally complex and adhesive fouling layers. Laboratory studies relying on synthetic feedwaters and isolated foulants fail to replicate this synergistic evolution, limiting the translational value of mitigation strategies tested under idealized conditions. This perspective calls for a systematic shift: functional membrane autopsies and field-derived fouling layers are proposed as necessary tools to capture real-system behaviors and guide cleaning protocols based on material resilience rather than bulk biomass.