Microbiology of enhanced biological phosphorus removal in aerated-anoxic Orbal processes

The traditional process for enhanced biological phosphorus removal (EBPR) in wastewater treatment involves an anaerobic zone followed by an aerobic zone. Although there is no strict anaerobic zone in aerated-anoxic Orbal processes, phosphorus removal in excess of that required for cell growth does occur. The microbial ecology of polyphosphate-accumulating organisms (PAO) in two full-scale Orbal wastewater treatment plants was investigated using flow cytometry to physically separate PAO from non-PAO and fluorescent in situ hybridization (FISH) to identify organisms. Although Candidatus Accumulibacter phosphatis, an uncultured organism associated with EBPR in acetate-fed laboratory-scale reactors, was detected, it did not seem to be the dominant PAO in these processes. Comparative FISH analyses of the activated sludge and the PAO-rich subpopulation did not reveal the presence of a dominant group of PAO in these full-scale plants. Rather, the analysis suggested that the operational characteristics of aerated-anoxic processes might select for a diverse PAO community that is significantly different from that observed in acetate-fed laboratory reactors or in traditional EBPR configurations.