Abstract <p>The manuscript considers the features of biofilm formation by groundwater microorganisms on granular activated carbon (GAC), which is used as a sorbent for removal of organic impurities of various genesis and structure from drinking water. Analysis of the development of natural groundwater microbial complexes on easily available (glucose) and difficult-to-mineralize (sodium humate) substrates in vitro revealed that autochthonous organic matter was an important factor provoking the formation of biofilms on GAC. Studies by scanning electron microscopy and spectrophotometry showed that the natural potential of microorganisms was associated with the formation of mycelium and spores by actinobacteria, which retained their stability at different stages of water treatment. Microbial complexes attached to the porous surface of GAC granules contributed actively to the transformation of the aliphatic, aromatic and, chromophore components of humic macromolecules. After the dispersion of spores from biofilms, they spread along the technological line and after 14 days were able to germinate in purified water with the formation of new mycelial pellets and biofilms, significantly worsening the organoleptic properties of water.</p>