Abstract:
Biological phosphorus removal in activated sludge is not fully understood. Acinetobacter has been implicated as the principal phosphorus removing agent. A fluorescent antibody and Acridine Orange staining technique combined with mem brane filtration and epifluorescence microscopy was developed and applied for the in situ identification and enumeration of Acinetobacter and the total number of bacteria in activated sludge after chemical dispersion or sonication of activated sludge in a 0,5 % tripolyphosphate solution. Whereas it has been genera1ly assumed that all Acinetobacter cells were relatively large. the use of different membrane pore sizes combined with fluorescent antibody staining has shown that these cells can range from relatively very small to very large cells. This confirms the erroneous conclusions that can be made by considering morphological characteristics only for identification of bacteria using light microscopy. The Acinetobacter numbers in the different activated sludge zones differed significantly in time. The primary aerobic and secondary anoxic Acinctnhacter numbers differed significantly from the other zones when using membrane filters with a pore size of 0.45 μm suggesting that the size of these bacteria increased with higher metabolic activity in the primary aerobic zone resulting in carryover into the secondary anoxic zone. Although statistical analysis showed no correlation between Acinetobacter numbers and phosphorus removal in activated sludge, density gradient centrifugation of activated sludge revealed that Acinetobacter was associated with phosphorus in activated sludge. Using transmission electron microscopy, the cell volume of volutin containing cells and the volutin volume were determined. This confirmed that only large Acinetobacter cells contained phosphorus. Electron dispersive micro-analysis of X-rays (EDAX) confirmed that the volutin contained phosphorus. Using Acinetobacter numbers, volutin volumes, densities and phosphorus content it was determined that a maximum of 34% of the observed phosphorus removal in activated sludge could be removed by Acinetobacter as polyphosphate. Other organisms or mechanisms therefore, also had to be involved in the observed phosphorus removal.
