Modulation of struvite composition in full-scale nutrient recovery system using source-separated urine

Using urine collected from a public restroom at a highway rest stop in Northern California, a full-scale nutrient recovery system involving a two-part system consisting of urine distillation followed by the precipitation of struvite crystals was characterized. The study examined the effects of different operational parameters of the coupled ammonium distillation and struvite process (CADSP) on the composition of struvite crystals and the overall nutrient recovery rates. System inputs that were investigated included the feed:steam (F:S), alkalinity source, and urinary pH. Overall, the findings demonstrate that the composition of the struvite produced can be varied by adjusting the preceding distillation unit process. Low F:S distillation operations result in high ammonium distillation rates which coincided with increased formation of magnesium potassium phosphate (MPP) while higher F:S operations led to conditions favorable to magnesium ammonium phosphate (MAP) precipitation. Therefore, low energy distillation operations (high F:S) are more conducive to MAP formation and high phosphorus recovery rates while high energy operations (Iow F:S) are more conducive to MPP formation. Sodium-based alkalinity sources should also be avoided in instances of low ammonium concentrations in the struvite precipitation tank to minimize the co-precipitation of magnesium sodium phosphate (MSP) with MPP.