Nutrient recovery from human urine by struvite crystallization with ammonia adsorption on zeolite and wollastonite

Urine-separation toilets are a possible route for achieving maximum recovery and recycling of urine nutrients not contaminated by hazardous compounds such as heavy metals. However, the direct use of human urine as agricultural fertiliser is problematic and controversial with regard to hygiene, storage, transport and spreading. In this paper, simple methods for capturing the nutrients in urine by transformation into solid mineral form are presented. On the addition of small amounts of MgO to synthetic or natural human urine most of the phosphorous and significant amounts of the potassium and nitrogen were precipitated, with crystalline struvite [Mg(K,NH4)(PO4)•6H2O] as a major component together with montgomeryite, newberyite, brucite and epsonite. Nitrogen recovery could be improved by adsorption. Clinoptilolite, wollastonite and a natural zeolite all showed excellent adsorbent properties in contact with ammoniacal solutions. In combination with struvite crystallisation 65–80% of the nitrogen was recovered as crystalline or adsorbed ammonium.