This research describes the start-up phase of Biological Phosphorus Removal (BPR) processes in two reactors. A Submerged Fix Bed Reactor (SFBR) working with a mixture of synthetic wastewater and domestic sewage and seeded with activated sludge coming from a BPR facility. A second reactor, a Biofilm Membrane Reactor (BMR), was fed with synthetic wastewater and seeded with a mixture of a pure culture of Acinetobacter lwoffi and fresh domestic sewage. It was found that Organic Loading Rates (OLR) above 5 gCOD/m2·d do not guarantee the adequate Mean Cellular Retention Time (MCRT) for the development of Poly-P organisms. Anaerobic periods should not be short during start-up phase or facultative bacteria will efficiently compete for food and, eventually, facing long aerobic periods, the Poly-P organisms may not need the phosphate mechanism to survive short anaerobic periods. Even if COD removal is observed during the anaerobic phase, further P accumulation during the aerobic phase will not be significant, unless the COD removal results in PHB formation. A loss in the BPR capacity of the Poly-P microorganisms was observed after seeding a reactor with active Poly-P organisms. The drop of BPR activity after seeding a reactor with active Poly-P organisms can be caused by the loss of genetic material due to the new environmental conditions and the new influent substrate.

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