Biofilms consist of a layer of mucus (film) in which microorganisms (e.g., bacteria, algae, fungi, protozoa) are embedded. Biofilms are formed when microorganisms settle at interfaces. They form predominantly in aqueous systems, either on the water surface or at an interface to a solid phase.
In principle, however, all interfaces of biofilms may be overgrown: between gas and liquid phases (e.g., free water level), liquid and solid phases (e.g., gravel at the bottom of the water), or between different liquid phases (e.g., oil droplets in the water). The interface on which the biofilm is formed is called substratum.
Due to the existing in the pipeline network biofilm may cause damage to health due to the concentrated accumulation of bacteria and microorganisms. Biofilms are often perceived in everyday life as a “mucus layer” or “coating”. Other, colloquial designations are growth, residue film or sewer films.