Phosphorus removal from wastewaters: Experimental and theoretical support for alternative mechanisms

Through batch sorption tests with synthetic solutions, it was found that aluminum hydroxide has a significant sorptive capacity for orthophosphate, condensed phosphate and organic phosphate. In all tests, except that with organic phosphate at pH 3.6, the measured soluble aluminum was consistent with solutions saturated with solid-phase Al(OH) 3. With organic phosphate at pH 3.6, an aluminum-organic phosphate precipitate appeared to form. A theoretical analysis of Al(OH) 3 and Al(PO 4) precipitation showed that orthophosphate removal is not achieved through Al(PO 4) precipitation when dosing with conventional alum, but rather through a mixed aluminum hydroxide phosphate precipitate. Jar tests performed with alum sludge and wastewater showed that phosphate is removed almost exclusively by the particulate fraction of the sludge. The removal of reactive phosphate (orthophosphate) decreased with an increase in the storage time of the alum sludge. This decrease was not seen with the non-reactive phosphate.