This study investigated stoichiometric relationships between metal cations (Ca2+, Mg 2+, K+, Na+, Al3+ and Fe (II and III)) and P uptake and release in full-scale EBPR activated sludge treatment plants with
a range of configurations and influent wastewater characteristics. The plants had specific P uptake and release rates ranging from 1.4 to 24 mg P/g VSS-h and ratio of anaerobic volatile fatty acids (VFA) uptake to P release ranging from 0.2 to 0.5 (gP/gHAc), indicating a significant
difference in populations of the poly-phosphateaccumulating organisms (PAO) in the mixed liquors tested. The K+/P and Mg2+/P mole ratios observed during the P release and uptake tests varied from plant to plant and ranged from 0.15 to 0.48 for K+/P
and from 0.27 to 0.48 for Mg2+/P. We found that cation/P mole ratios measured during anaerobic P release can differ significantly from those measured during aerobic P uptake. In general the cation/Prelease ratio was lower than the cation/Puptake
ratio for Ca and Mg. While no consistent difference between K+ and Prelease and K+/Puptake ratios was observed. Three out of the six plants clearly showed the association of Ca with phosphate uptake and release. The Ca2+/Prelease
ratio ranged from 0.07 to 0.28. The other three did not show a direct correlation of soluble Ca2+ and phosphate uptake and release based on the soluble Ca2+ and P measured. However, further evaluation of Ca2+ and PO43−
concentrations and pH indicated chemical precipitation could occur under the study conditions. The correlation of Ca2+ to P uptake and release clearly shown in this study demonstrated that Ca2+ actually plays just as important a role as Mg2+
in the biological P-removal process. The Ca2+ availability in the influent should thus be evaluated together with Mg2+ when investigating whether poor performance of an EBPR process may be related to possible metal limiting conditions.
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