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

Yong-Qiang Liu,Simone Cinquepalmi

doi:10.3390/pr9101710

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.

Highlights

Aerobic granular sludge is a promising technology to replace suspended sludge for wastewater treatment
It can be seen that SVI5 increased sharply in both reactors in the first 3 days and and decreased quickly to less than 100 mL g−1 after day 7
This phenomenon is in agreement agreement with those reported by Liu et al [23] and Liu et al [24], indicating a typical with those reported by Liu et al [23] and Liu et al [24], indicating mL

Summary

Introduction

Aerobic granular sludge is a promising technology to replace suspended sludge for wastewater treatment. Granular sludge is a kind of self-immobilized biofilm without carriers, which was developed in the 1980s in Upflow Anaerobic Sludge Blanket (UASB) systems for anaerobic wastewater treatment, and in the 2000s in Sequential Batch. Reactor (SBR) systems for aerobic wastewater treatment. Granular sludge possesses a compact structure and large size, which is believed to be favorable to inorganic precipitation and accumulation in granules. Reactor operation mode might affect inorganic mineral accumulation in granules. After the operation mode was changed from sequential batch to continuous, the ash content of aerobic granules increased from 20 to 84% with inorganic minerals identified by XRD analysis as CaCO3 , Ca2+ concentration in wastewater as low as 19 mg L−1 [4]

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