Review of the Feasibility of Recovering Phosphate from Wastewater for Use as a Raw Material by the Phosphate Industry

Abstract

Phosphate recovered from wastewater may represent an alternative source of raw material for the manufacture of phosphate products. Within wastewater treatment plants phosphate has been found to spontaneously precipitate forming crystalline deposits of a double salt of magnesium ammonium phosphate, known as struvite. Clearly the conditions for its formation can be found naturally within the environment of wastewater treatment works. The implication is that if these conditions can be exploited it may be possible for struvite to be extracted in commercially economic quantities. A literature review was undertaken to examine the precise conditions under which struvite will precipitate together with the morphology of its crystals and their settlement characteristics. For struvite to form its components must be available in at least a molecular ratio of 1(Mg2+):1(NH4 +):1(PO4 3−). Municipal wastewater is typically deficient in magnesium while ammonium is present in excess. Conditions must also be alkaline, above pH 7.5 — 8 for precipitation to occur. Dosing is therefore required to supplement the magnesium supply and raise the ambient pH. Struvite crystal cells are orthorhombic in character ranging from X - shaped to flat trapezoidal depending upon the degree of supersaturation of the solution. A qualitative economic evaluation undertaken demonstrated a number of cost savings and costs that would be incurred if struvite formation was used as a method of nutrient reduction in wastewater streams.