It is reported the impact of biofilms on the quality of tap water inside of water networks. The analysis covers the factors determining the occurrence and growth of biofilms on the inner surface of water pipes. The composition of biofilms upon the age and their influence on the degree of bio-contamination of tap water at the consumption points are considered. The threats for water consumers, associated with the appearance and multiplication of pathogenic microbes in the biofilms matter have described. The special attention has devoted the action, which along with the native processes of water pollution inside of distribution systems, could lead the strong artificial contamination of tap water by pathogens when somebody makes it with a malicious aim. Because of this, the additional purification and sterilisation of tap water are necessary before using. Current and prospective water purification devices designed for solving this task has described in comparison of each – other.