Laboratory studies on recovery of N and P from human urine through struvite crystallisation and zeolite adsorption

Struvite [Mg (K, NH4)(PO4) . 6H(2)O] crystallisation and adsorption to zeolite have been proposed as a method for nutrient recovery from human urine collected with urine separating toilets. The aim of the present study was to optimise the use of MgO (to precipitate struvite) and zeolite (to adsorb ammonia) in this process. The experiments were performed with fresh urine, to which various amounts of MgO and zeolite were added. After repeated stirring and settling for 3 days the supernatant was analysed for pH, total-N, total-P and acute toxicity for Daphnia magna (24- and 48-h EC50). The results show that addition of MgO reduced P and addition of zeolite reduced N in the supernatant, as expected. The required concentration of MgO added was less than expected from the stoichiometric Mg-P-ratio for struvite. In combination with zeolite the requirement for MgO was reduced even further. Zeolite was effective in reducing total-N, but because of its interaction with MgO, the effect of zeolite on N as well as P reduction was rather complicated. The optimal combination of MgO and zeolite for combined N and P recovery was found to be around 0.5 MgO per litre urine and 15 g zeolite per litre urine. These additions reduced supernatant P from about 1300 mg l(-1) to 10 mg l(-1) and N from 8000 mg l(-1) to 1000 mg l(-1). The 24-h EC50 for D. magna was not significantly affected by these additions. The expected recovery potentials for P and N by addition of MgO and zeolite are, thus, about 99% for P and 90% for N. However, these figures need to be verified, and pilot plant experiments at a science centre with urine separation wastewater treatment (www.universeum.se) are in progress.