Organic matter removal mitigates crystallization inhibition to enhance co-recovery of potassium and phosphorus from wastewater.

Magnesium potassium phosphate (MPP) hexahydrate crystallization is a promising approach for the simultaneous recovery of potassium and phosphorus. However, organic matter in the wastewater affects the formation of crystals, which reduces the recovery efficiency. This study constructed the optimal conditions for the formation of MPP crystals, on the basis of which the mechanism of the effect of different types of organic matter on crystallization. The results showed that the using MgCl2 as the magnesium source, potassium and phosphorus recovery efficiencies reached 98.05 % and 99.40 %, respectively, under the following conditions: pH 10, rotation speed of 500 rpm, Mg:K:P molar ratio of 2:1:2, and seed dosage of 5 g/L. Under these conditions, all three organics had an effect on crystallization, with the order of inhibition being sodium alginate > bovine serum albumin > humic acid. Characterization analysis revealed that these organic compounds formed hydrogen bonds with functional groups or complexes with Mg2+ on the crystal surface, hindering crystal growth and modifying crystal morphology. Finally, when liquid digestate was used as the influent for crystallization reaction, removal of organic pollutants led to significant increase in the potassium recovery efficiency.