Natural organic matter (NOM) — the mixture of humic substances, fulvic acids, and other organic molecules derived from decomposing plant and animal material — is present in virtually all natural water sources. NOM significantly affects PFAS removal by activated carbon and must be considered in filter design for many water supplies.
How NOM Competes With PFAS
Activated carbon has a finite number of adsorption sites. NOM molecules also adsorb onto carbon surfaces, occupying sites that would otherwise be available for PFAS. In waters with high dissolved organic carbon (DOC) content — typical of surface waters drawing from forested watersheds — NOM competition reduces PFAS adsorption capacity and can decrease the volume of water treatable to a target PFAS removal before breakthrough.
Pre-Treatment for NOM Removal
Municipal treatment plants commonly use coagulation, flocculation, and sedimentation — or enhanced coagulation — to remove NOM before carbon filtration. This pre-treatment extends carbon system life and improves PFAS removal reliability. For residential systems treating surface water or water with elevated DOC, a pre-treatment stage targeting NOM (such as oxidation and sedimentation at large scale, or additional carbon volume at small scale) improves PFAS treatment economics.
Membrane Systems Are Less Affected
RO and nanofiltration membranes reject both NOM and PFAS through size exclusion, with NOM rejection typically high because most NOM molecules are larger than PFAS. However, NOM can contribute to membrane fouling — building up on membrane surfaces and reducing water flux. Pre-filtration to remove particulate NOM protects membranes and maintains PFAS rejection performance over time.