Novel filamentous spore‐forming iron bacteria causes bulking in activated sludge

Several Gram‐positive iron bacteria were isolated from bulking sludge. They were filamentous and had false branching. They had sheaths with iron deposits and formed spores on modified sucrose casitone yeast extract agar. They did not grow on influsion agar for longer than 1 month but could withstand 80°C for 1 h on the same medium. Adding them to sewage before aeration increased the biochemical oxygen demand of waste water and caused poor settling properties of activated sludges. They were the predominant filamentous micro‐organisms in bulking activated sludges. At present, these strains cannot be assigned to recognized taxa of Bacillus spp. or sheathed iron bacteria.