Metabolic transformations and characterisation of the sludge community in an enhanced biological phosphorus removal system

Enhanced biological phosphorus removal was performed in a continuous laboratory-scale two-reactor system with sludge recirculation over a 75-day period. Influent wastewater was a synthetic medium based on acetate, and the sludge age was kept at 12 days. The adapted sludge stored poly-β-hydroxyalkanoic acids (PHA) in the anaerobic reactor with a conversion ratio of 1.45 PHA/acetic acid (based on chemical O2 demand: COD/COD) and gave ratio of a phosphate-P release to acetic acid uptake of 0.51 P/CH3COOH (w/w). Fractionation of anaerobic and aerobic sludges showed that the main part of phosphorus taken up, was eluted in the trichloroacetic acid fraction indicating that it was polyphosphate. A total of 60 percent of the phosphorus in the aerobic sludge was solubilized in the trichloroacetic acid fraction, whereas this fraction accounted for only 32 percent of the phosphorus in the anaerobic sludge. Only 4 percent of the total phosphorus in the aerobic sludge and 2 percent in the anaerobic sludge was found in the EDTA fraction, indicating low amounts of metal-bound phosphates. Isolation on acetate-based agar medium showed that Acinetobacter strains were present in the sludge. However, a more complete analysis of the bacterial community of the sludge was obtained by creating a clone library based on the 16S rRNA gene. A total of 51 partial clone sequences were phylogenetically evaluated. The predominating group was found in the high-(G + C) (mol percent) gram-positive bacterial subphylum (31 percent of the sequenced clones), while the gamma proteobacteria only constituted 9.8 percent of the clones