The Fate of Phosphate in Full-Scale Aerobic Granular Sludge Systems
S.M.L. Stubbé
M.K. de Kreuk – Mentor
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Abstract
In previous works, biologically induced phosphate precipitation was observed in lab-scale aerobic granular sludge (AGS) reactors. In this study, the contribution of biologically induced phosphate precipitation to the total removal of phosphate was investigated in full-scale AGS installations in Utrecht and Garmerwolde, the Netherlands. A longitudinal phosphate balance showed that under current operating conditions, phosphate does not accumulate noticeably in mature full-scale AGS systems. The total phosphate content of granule fractions, the scanning electron microscope (SEM) images coupled with energy dispersive X-rays (EDX) and the X-ray fluorescence (XRF) analyses on reactor granules showed no substantial phosphate crystals in the granules. The X-ray diffractions (XRD) revealed the presence of quartz crystals (SiO4) and, at some moments in time, different types of other minerals (e.g., potassium struvite, dolomite, magnesium calcite, brushite). Other XRD measurements however, showed no crystals at all besides quartz. It is suggested that a dynamic precipitation - dissolution process occurs in full-scale AGS systems within each process cycles or over multiple cycles. Overall, no indications were found phosphate precipitation contributed significantly to the total phosphate removal. In addition, the potential of phosphate to contribute to the process control of full-scale AGS installations is discussed.