The relationships between bacterial storage products, density, and settling characteristics were determined in a laboratory-scale sequencing batch reactor (SBR) enhanced biological phosphorus removal (EBPR) system. Both long-term and single anaerobic-aerobic cycle variations in these properties were studied. Increased polyphosphate (PP) content of the biomass during long-term operation resulted in improved sludge volume index (SVI) values. End-aerobic phase (after phosphate (P) uptake) values were consistently lower than end-anaerobic phase (after P release) values. Neither filamentous nor slime bulking were evident by microscopic observations. Biomass density increased at a rate of 1.2 mg/L per each 1% increase in biomass P content. End-aerobic phase samples had an average 25% higher buoyant density than end-anaerobic phase samples, which was attributed to aerobic P uptake. Biomass density was negatively correlated with SVI values, and SVI values increased sharply at low biomass density. A mathematical model developed by Mas et al. (1985) was modified to predict total cell density based on literature values of PP, glycogen (GLY), and poly-b-hydroxybutyrate (PHB) densities. Model predictions were in good agreement with experimental results, although improved measurement of PP density is required to improve model predictions.

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