A biofilm is a complex structure of bacteria living as a community within a sticky slime layer of their own making. Biofilms can attach to any surface that stays wet. Biofilms are a developmental process, and as the sticky polymers that produce the slime attach to a surface and build up, their appearance under magnification resemble microscopic mushrooms and streamers.
Biofilms are a water issue because such films support the accumulation of bacterial colonies and can result in a mat capable of causing pipe corrosion or clogging problems. Bacteria biofilms within water distribution systems are somewhat protected from low-level disinfection methods, so they can cause a number of water quality problems, depending on the type of bacteria cultures present in the biofilm. Biofilms pose a variety of health risks in a water distribution system. Pseudomonas aeruginosa is one opportunistic pathogen known to produce biofilms and it can cause animal disease.
The dangers of not prohibiting biofilm growth are if chunks of this type of biofilm break loose and move into water supply pipelines, health officials will see the effect. Nitrification within a water distribution system is caused by biofilms. The nitrifying bacteria within such biofilms can oxidize residual ammonia leftover from chloramination to produce nitrate and nitrite in the treated water as it moves through a distribution system. A higher nitrate level at the tap than that in the raw water source, especially in a system using chloramines for disinfection, is a good indicator of biofilm induced nitrification within the distribution system.
In some cases antibiotics and biocides can to some degree help but they also produce problems. The huge doses of antimicrobials required to rid water systems of established biofilm bacteria are environmentally undesirable and medically impractical in hospitals since they would kill the patient. The primary problem with the general use of antibiotics to prevent biofilm formation is that this practice tends to produce bacterial resistance to antibiotics. For example, Pseudomonas aeruginosa biofilms are a chief cause of hospital infections and some strains pose a special problem because they have become resistant to most antibiotics. In water distribution systems the biocide level required to control biofilm formation leaves residual levels that make the water too toxic for consumption.