During storage of urine, urea (the main form of nitrogen in urine) is hydrolysed by an enzyme (urease) to ammonia and carbamate. Carbamate decomposes to carbonic acid and ammonia.
Although this hydrolysis increases the pH (due to NH3 release) of stored urine up to pH 9.3 and triggers the precipitation of hydroxyapatite (Ca10(PO4)6(OH)2), calcium carbonate (CaCO3) and struvite (MgNH4PO4.6H2O), there is a lack of polyvalent cations (i.e., Mg2+, Ca2+) for a complete removal of phosphorus.
In the figure below the basic principal of Phosphate recovery via Struvite precipitation is demonstrated. Suitable Magnesium salts (Mg-salt) are Mg(OH)2, MgCl2 and MgO (with limitation due to its low solubility).
Struvite exist in two main forms potassium struvite (MKP or KMP) - MgKPO4.6H2O - and ammonium struvite (MAP) - MgNH4PO4.6H2O - , due to the composition and the conditions found in hydrolyzed urine the mostly MAP is formed during phosphorus recovery.
An interesting article (Gell et al., 2011) for further reading on effectiveness on struvite as a fertilizer can be obtained from www.ccsenet.org. The article is available under a Creative Commons Attribution 3.0 License (CC BY 3.0).