A thin, slimy film of bacteria that adheres to surfaces, which are regularly in contact with water. The structure of biofilms is quite complex consisting of colonies of bacteria and other microorganisms such as yeasts, fungi, and protozoa. Biofilms can be a problem for drinking water supply (pipe fouling). But they are also widely used water purification (e.g. biosand filters, slow sand filtration) or in biological wastewater treatment (e.g. rotating biological contactor, trickling filter).