The spatial arrangement of functionally important ammonia oxidizing bacteria (AOB) in activated sludge flocs is largely unknown and activated sludge flocs are regarded as heterogeneous agglomerations of bacterial communities. This study aimed to determine both the location of AOB microcolonies within flocs and the size variation of these microcolonies based on their location. Fluorescent in situ hybridization (FISH) was used in combination with a confocal scanning laser microscope (CSLM) to achieve these goals. Grab samples taken from a full-scale plug flow wastewater treatment plant were examined with probes targeted to AOB b-Proteobacteria (Nso1225) and a genus specific probe targeted to the genus Nitrosospira (Nsv443). Frequencies of AOB microcolonies were observed to increase towards the center of activated sludge flocs (ASFs). In addition, the size of AOB microcolonies was found to rise from the edges to the centers of flocs. This study indicates that ASFs might have a non-random agglomeration of AOB microcolonies. However, FISH is a relatively preliminary method for the aims targeted in this study and the results are not sufficient to justify definite conclusions.

The spatial arrangement of functionally important ammonia oxidizing bacteria (AOB) in activated sludge flocs is largely unknown and activated sludge flocs are regarded as heterogeneous agglomerations of bacterial communities. This study aimed to determine both the location of AOB microcolonies within flocs and the size variation of these microcolonies based on their location. Fluorescent in situ hybridization (FISH) was used in combination with a confocal scanning laser microscope (CSLM) to achieve these goals. Grab samples taken from a full-scale plug flow wastewater treatment plant were examined with probes targeted to AOB b-Proteobacteria (Nso1225) and a genus specific probe targeted to the genus Nitrosospira (Nsv443). Frequencies of AOB microcolonies were observed to increase towards the center of activated sludge flocs (ASFs). In addition, the size of AOB microcolonies was found to rise from the edges to the centers of flocs. This study indicates that ASFs might have a non-random agglomeration of AOB microcolonies. However, FISH is a relatively preliminary method for the aims targeted in this study and the results are not sufficient to justify definite conclusions.