ASSESSMENT OF BIOLOGICAL PHOSPHORUS REMOVAL IN AN OXIDATION DITCH PROCESS

Though commonly associated with stable and robust BOD removal and nitrification, oxidation ditch systems are now being designed and operated to achieve nitrogen removal through separate anoxic zones and simultaneous nitrification-denitrification. In some cases, operating data suggest that some degree of unintentional biological phosphorus removal (Bio-P) could be occurring in these processes, but it is typically difficult to assess. The operating sequence of the proprietary Phases Isolation Ditch (PID) process, which can be designed for efficient total nitrogen removal and eliminates the need for secondary clarifiers, was modified at a 0.6 MGD treatment plant to include an anaerobic fill phase in hopes of achieving some degree of Bio-P. The objective of this investigation was to evaluate polyphosphate accumulating organism (PAO) activity in the PID process. This work was done to determine whether PAO activity could be responsible for the relatively efficient and otherwise unexplainable P removal currently being observed at this plant. Using phosphorus release rate measurements and comparing these values to that previously published in a survey of Bio-P plants, it is clear that this facility is successfully achieving some degree of biological phosphorus. However, influent Al and Fe loading is significant and could be responsible for some degree of the observed P removal.