Abstract:　Biofilms　inhabiting　pipeline　walls　are　critical　to　drinking　water　quality　and　safety.　With　massive　pipeline　replacement　underway,　however,　biofilm　formation　process　in　newly　built　pipes　and　its　effects　on　water　quality　are　unclear.　Moreover,　differences　and　connections　between　biofilms　in　newly　built　and　old　pipes　are　unknown.　In　this　study,　early　succession　(≤　120 days)　of　biofilm　bacterial　communities　(abundance　and　diversity)　in　upper,　middle　and　bottom　areas　of　a　newly　built　cement-lined　ductile　iron　pipeline　were　evaluated　using　improved　Propella™　biofilm　reactor　and　multi-area　analysis.　A　comparison　with　old　pipelines　(grey　cast　iron,　10 years)　was　performed.　In　the　newly　built　pipeline,　the　abundance　of　biofilm　bacteria　did　not　change　significantly　between　40　and　80 days,　but　increased　significantly　between　80　and　120 days.　The　biofilm　bacterial　abundance　(per　unit　area)　in　the　bottom　area　was　always　higher　than　that　in　the　upper　and　middle　areas.　Based　on　alpha　diversity　index　and　PCoA　results,　biofilm　bacterial　community　richness,　diversity　and　composition　did　not　change　significantly　during　the　120-day　operation.　Besides,　biofilm　shedding　from　the　walls　of　newly　built　pipeline　significantly　increased　bacterial　abundance　in　the　outlet　water.　Opportunistic　pathogen-containing　genera,　such　as　Burkholderia,　Acinetobacter　and　Legionella,　were　identified　in　both　water　and　biofilm　samples　from　newly　built　pipelines.　The　comparison　between　new　and　old　pipelines　suggested　a　higher　bacterial　abundance　per　unit　area　at　the　middle　and　bottom　areas　in　old　pipelines.　Moreover,　the　bacterial　community　composition　of　biofilms　in　old　pipelines　was　similar　to　that　of　newly　built　pipelines.　These　results　contribute　to　accurate　prediction　and　management　of　biofilm　microbial　communities　in　drinking　water　pipelines,　ensuring　the　biosafety　of　drinking　water.　Key　Points:　•　Biofilm　bacterial　communities　in　different　areas　of　pipe　wall　were　revealed.　•　The　abundance　of　biofilm　bacteria　increased　significantly　between　80　and　120 days.　•　Biofilm　bacterial　community　compositions　of　newly　built　and　old　pipes　were　similar.　Graphical　Abstract:　[Figure　not　available:　see　fulltext.]　©　2023,　The　Author(s),　under　exclusive　licence　to　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature.