Biofilm is a gelatinous organic substance that accumulates on the inside of pipework and other surfaces that come in contact with water. This substance is an excretion from certain bacteria particularly (but not limited to) pseudomonas, that they deposit to create a home and protect themselves from things in the environment that can be harmful to them.
Once biofilm has established itself in a water distribution system, problems quickly begin to multiply. Other microorganisms, notably, legionella take up residence within the biofilm.
It is only a matter of time before the biofilm will burst, releasing dangerous bacteria into the water supply. Some of these bacteria will remain in a free floating (planktonic) state and pose a risk of causing infection, while others will latch onto the pipe further downstream and start the process of creating biofilm and forming colonies all over again.
While many water disinfection methods are effective at killing free floating bacteria, there are very few that are able to successfully remove biofilm.
For further information, read our article on Understanding the Relationship between Copper-Silver, Chlorine and Biofilm.