Struvite precipitation from urine with electrochemical magnesium dosage.

When magnesium is added to source-separated urine, struvite (MgNH(4)PO(4)·6H(2)O) precipitates and phosphorus can be recovered. Up to now, magnesium salts have been used as the main source of magnesium. Struvite precipitation with these salts works well but is challenging in decentralized reactors, where high automation of the dosage and small reactor sizes are required. In this study, we investigated a novel approach for magnesium dosage: magnesium was electrochemically dissolved from a sacrificial magnesium electrode. We demonstrated that this process is technically simple and economically feasible and thus interesting for decentralized reactors. Linear voltammetry and batch experiments at different anode potentials revealed that the anode potential must be higher than -0.9 V vs. NHE (normal hydrogen electrode) to overcome the strong passivation of the anode. An anode potential of -0.6 V vs. NHE seemed to be suitable for active magnesium dissolution. For 13 subsequent cycles at this potential, we achieved an average phosphate removal rate of 3.7 mg P cm(-2) h(-1), a current density of 5.5 mA cm(-2) and a current efficiency of 118%. Some magnesium carbonate (nesquehonite) accumulated on the anode surface; as a consequence, the current density decreased slightly, but the current efficiency was not affected. The energy consumption for these experiments was 1.7 W h g P(-1). A cost comparison showed that sacrificial magnesium electrodes are competitive with easily soluble magnesium salts such as MgCl(2) and MgSO(4), but are more expensive than dosing with MgO. Energy costs for the electrochemical process were insignificant. Dosing magnesium electrochemically could thus be a worthwhile alternative to dosing magnesium salts. Due to the simple reactor and handling of magnesium, this may well be a particularly interesting approach for decentralized urine treatment.
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