The following equation shows the stoichiometry for recovery of phosphorus and ni

Question

The following equation shows the stoichiometry for recovery of phosphorus and nitrogen from wastewater through precipitation of struvite.

If a wastewater sample has an NH4-N concentration of 25 mg/L and Mg2+ concentration of 40 mg/L, what is the equilibrium concentration of PO43- in the water? Express your answer in units of mg P/L. The solubility product for struvite (MgNH4PO4) is roughly 10-14. Assume all phosphorus exists as PO43- and that struvite controls the solubility of P in solution.

The following equation shows the stoichiometry for recovery of phosphorus and nitrogen from wastewater through precipitation of struvite. If a wastewater sample has an NH4-N concentration of 25 mg/L and Mg2+ concentration of 40 mg/L, what is the equilibrium concentration of PO43- in the water? Express your answer in units of mg P/L. The solubility product for struvite (MgNH4PO4) is roughly 10-14. Assume all phosphorus exists as PO43- and that struvite controls the solubility of P in solution.

Explanation / Answer

Solubility product of struvite = Ksp = 1E-14 = [Mg^2+][NH4^+][PO4^3-]

1E-14 = 40mg/L x 25mg/L x [PO4^3-]

[PO4^3-] = 1x10^-17mg/L = 1x10^-17mg P/L

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