The　Up-flow　biological　aerated　filter　(UBAF)　based　on　a　one-stage　autotrophic　nitrogen　removal　process　has　been　widely　investigated　nowadays.　In　this　work,　the　biochemical　characteristic　along　the　volcanic-filled　UBAF　reactor　had　been　studied.　The　results　indicate　that　short-rod,　spherical　and　elliptical　(averaged　0.2-1.0　μm)　microorganisms　with　a　specific　irregular　cauliflower　profile　existed　in　the　system.　Species　identification　showed　Nitrosococcus-　and　Nitrosomonas-related　aerobic　ammonium-oxidizing　bacteria　(AerAOB)　and　Candidatus　Kuenenia　stuttgartiensis-like　anaerobic　ammonium-oxidizing　bacteria　(AnAOB)　were　the　predominant　functional　bacteria　that　mixed　with　each　other　and　showed　no　distinct　niche　in　the　system.　However,　the　bioactivity　of　functional　microorganisms　displayed　differently　at　different　filter　layers,　with　a　better　pollutant-removal　activity　in　the　lower　parts　than　in　the　upper　parts　of　the　UBAF.　In　the　lower　parts,　compact　and　small　zooglea　formed,　whereas　it　trended　to　be　larger　and　looser　along　the　filter.　Moreover,　there　was　better　biodiversity　of　AerAOB　in　the　lower　part,　while　AnAOB　showed　stable　and　low　biodiversity　along　the　filter.