Reportedly the properties of sludge have been changed by installing iron ion releasers (also known as Σ Balls) in the bottom layer in the estuary of a river to supply ferrous iron ion into seawater. This study compared the time course between a tank with Σ Balls in sludge and a tank with sludge only. The emergent eubacteria were analyzed. As a result, in Tank A (sludge only), bacterial clones closely related to Holophaga were detected at a high frequency (number of clones = 5, homology = 82.3-86.3%), and in Tank B (with Σ Balls in sludge), bacterial clones closely related to Chthoniobacter (number of clones = 9, homology = 83.6-88.0%), and Candidatus Magnetobacterium (number of clones = 6, homology = 88.0-89.6%) were detected at high frequency.