Activated sludge phosphorus removal mechanisms

To assist in explaining the mechanism of phosphorus removal in the activated sludge process, three completely mixed model activated sludge units were operated over a range of solids residence times found at most treatment plants. Each model unit was fed a wastewater of different composition in order to isolate the particular phosphorus removal mechanism involved. In one unit a wastewater containing organic matter but little ${\rm Ca}^{2+}$ was used. Another unit contained the same organic concentration but a high ${\rm Ca}^{2+}$. In the last unit organic matter was eliminated and the ${\rm Ca}^{2+}$ was allowed to remain high. Based on the results obtained the level of ${\rm Ca}^{2+}$ was shown to influence phosphorus removal as was the value of mean cell residence used. "Luxury uptake" was shown to be accounted for by the mechanism of chemical precipitation.