Biofilms are ubiquitous in drinking water systems, either in the form of thin and patchy colonies or as surface-covering multiple layers. In biofilters they are used for the elimination of biologically degradable substances. However, they occur in other sites, e.g., on the walls of containers and pipes, on sediment and on suspended particles. They can rise problems by contamination of the water phase by detaching biofilm organisms. Biofilms provide a possible habitat for hygienically relevant microbes in which they can persist and even multiply. Here they are protected against disinfectants, in particular if located in corrosion products, sediments or ingested by protozoa which feed on biofilm cells. Biofilms are related to the occurrence of “black water” and malodours. They are involved in the corrosion of metals, mineral materials and synthetic polymers. The limiting factor for biofilm growth is usually the availability of nutrients, mainly provided either by biodegradable substances leaching from materials or from by the water phase. The extent of biofilm growth and of the occurrence of hygienically relevant organisms is still unknown and to be investigated. However, latest research indicates that such organisms do not multiply in large numbers in drinking water biofilms; it is possible that drinking water biofilms can inhibit the propagation of invading pathogens.

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