Struvite precipitation thermodynamics in source-separated urine

Struvite (MgNH₄PO₄·6H₂O) precipitation eliminates phosphate efficiently from urine, a small but highly concentrated stream in the total flux of domestic wastewater. Precipitation experiments with hydrolysed urine evaluated the solubility product of struvite. The stored and fully hydrolysed urine had an ionic strength of between 0.33 and 0.56 M and required the estimation of activity coefficients. From our data, we identified the Davies approximation with the two constants A=0.509 and B=0.3 as agreeing best with our laboratory results. The standard solubility product K_s⁰=f₁[NH₄⁺]f ₂[Mg²⁺]f₃[PO₄^3-] ([ ]=concentration of the species; f_x=corresponding activity coefficient) of struvite in urine was found to be 10^{-13.26±0.057} at 25 °C and the enthalpy of struvite formation ΔH was 22.6(±1.1) kJ mol^-1. The equilibrium calculations required the following dissolved complexes: [MgCO₃]_aq, [MgHCO₃]⁺, [MgPO₄]^-, [NH₄HPO₄]^-, and [NaHPO₄]^- and to a lesser extent [MgSO₄]_aq and [NH₄SO₄]^-. Organic complexes do not seem to influence the solubility product substantially. For practical purposes, a conditional solubility product K_s^cond=[Mg_aq]·[NH₄ ⁺+NH₃]·[P_ortho]=10^-7.57 M³ was derived to calculate struvite solubility in urine at 25 °C, pH=9.0 and ionic strength I=0.4 M directly from measured concentrations.