Towards effective phosphorus recycling from wastewater: quantity and quality.

Precipitation in the Mg-Ca-NH3-PO4 system has been explored to improve understanding of likely phases recoverable from complex wastewaters. Over a range of Mg/Ca combinations (0-100%) and pH 5-11, at least seven identifiable crystalline phases could be precipitated from artificial wastewater including: struvite, hydroxylapatite, newberyite, brushite, merrilite/whitlockite, octocalcium phosphate, and monetite. This experimental study has outlined the physicochemical conditions required to produce various phosphate products from synthetic wastewater, and found that large differences exist between experimentally formed phases and thermodynamical predictions. Struvite formation is the most desirable precipitate for the recovery of phosphate based upon purity, growth characteristics, dewatering properties, phosphate removal efficiency, and its ability to simultaneously remove ammonia. This study has also demonstrated that in specific cases the preliminary precipitation of brushite is a possible means of decreasing calcium content such that subsequent struvite formation could achieve higher-purity. Utilising experimental results and information on current commodity prices, discussion on the choice of Mg and Ca sources for phosphorus recovery provides guidance on appropriate means to optimise the formation and yield of high quality cost-optimised products.