Ion-exchange softening of human urine to control precipitation

A major barrier to widespread implementation of urine source-separation is the uncontrolled precipitation of magnesium (Mg) and calcium (Ca) minerals in waterless urinal plumbing, which results in clogging that increases maintenance requirements. The goal of this research was to investigate the use of ion-exchange softening of undiluted, fresh human urine to control precipitation reactions in hydrolyzed urine. The working hypothesis was that selective removal of Ca and Mg from undiluted, fresh urine by cation exchange resin would decrease the precipitation of Ca and Mg minerals in hydrolyzed urine. Ion-exchange experiments and precipitation experiments were used to investigate ion-exchange reactions in synthetic urine. Cation exchange resin (Amberlite A200C) removed both Ca and Mg from undiluted, fresh urine with a greater affinity for Ca over Mg. Precipitation experiments showed that struvite, MgNH₄PO₄·6H₂O, and hydroxyapatite, Ca₅(PO₄)₃(OH), were the dominant minerals in untreated and ion-exchange treated hydrolyzed urine. Overall, ion-exchange softening decreased the mass of solids that would be deposited in the urinal plumbing and increased the phosphate available for nutrient recovery.