On the occurrence of anoxic microniches, denitrification, and sulfate reduction in aerated activated sludge

A combination of different methods was applied to investigate the occurrenc e of anaerobic processes in aerated activated sludge. Microsensor measureme nts (O-2, NO2-, NO3-, and H2S) were performed on single sludge flocs to det ect anoxic niches, nitrate reduction, or sulfate reduction on a microscale. Incubations of activated sludge with (NO3-)-N-15 and (SO42-)-S-35 were use d to determine denitrification and sulfate reduction rates on a batch scale . In four of six investigated sludges, no anoxic zones developed during aer ation, and consequently denitrification rates were very low. However, in tw o sludges anoxia in flocs coincided with significant denitrification rates. Sulfate reduction could not be detected in any sludge in either the micros ensor or the batch investigation, not even under short-term anoxic conditio ns. In contrast, the presence of sulfate-reducing bacteria was shown by flu orescence in situ hybridization with 16S rRNA-targeted oligonucleotide prob es and by PCR-based detection of genes coding for the dissimilatory sulfite reductase. A possible explanation for the absence of anoxia even in most o f the larger flees might be that oxygen transport is not only diffusional b ut enhanced by advection, i.e., facilitated by flow through pores and chann els. This possibility is suggested by the irregularity of some oxygen profi les and by confocal laser scanning microscopy of the three-dimensional flee structures, which showed that flocs from the two sludges in which anoxic z ones were found were apparently denser than flocs from the other sludges.