Hydroxyapatite Precipitation and Accumulation in Granules and Its Effects on Activity and Stability of Partial Nitrifying Granules at Moderate and High Temperatures

Abstract

Precipitation and accumulation of calcium phosphate in granular sludge has attracted research attention recently for phosphate removal and recovery from wastewater. This study investigated calcium phosphate accumulation from granulation stage to steady state by forming heterotrophic granules at different COD/N ratios at 21 and 32 °C, respectively, followed by the transformation of heterotrophic granules to partial nitrifying granules. It was found that mature granules accumulated around 60–80% minerals in granules, much higher than young granules with only around 30% ash contents. In addition, high temperature promoted co-precipitation of hydroxyapatite and calcite in granules with more calcite than hydroxyapatite and only 4.1% P content, while mainly hydroxyapatite was accumulated at the moderate temperature with 7.7% P content. The accumulation of minerals in granules at the high temperature with 75–80% ash content also led to the disintegration and instability of granules. Specific ammonium oxidation rates were reduced, as well, from day 58 to day 121 at both temperatures due to increased mineral contents. These results are meaningful to control or manipulate granular sludge for phosphorus removal and recovery by forming and accumulating hydroxyapatite in granules, as well as for the maintenance of microbial activities of granules.