Struvite, MgNH4PO4, can be recovered from wastes, such as sludge supernatants. Recovery processes rely on the equilibrium between magnesium, ammonia, and ortho-phosphate. The predictability of Struvite Recovery depends on factors with significant uncertainty: 1. Waste constituents vary and one will be limiting; 2. pH is variable and must be adjusted to maintain favorable conditions; and, 3. Equilibrium constants are not exactly known, and, temperature and ionic strength vary. A model was developed to adjust operating conditions to maximize Struvite recovery. This technique uses a chemical equilibrium term, Ω, using median equilibrium constants to develop a Conditional Solubility Product, Kspcond, or Ω = [Mg][N][P]/Kspcond (1), where Ω &gt; 1.0 implies precipitation; Ω &lt; 1 implies no precipitation; Ω = 0 implies equilibrium; and [Mg] = magnesium, [N] = aammonia and [P] = ortho-phosphate concentrations. The median Kspcond was approximated by a polynomial: pKspcond = - 0.2116(pH)2 + 4.45(pH) -14.04 (2) From field data, ortho-phosphate was most often limiting (86.4% of time), and, maximum struvite recovery (stoichiometric) was limited by ortho-phosphate. However, actual recovery was equilibrium limited by pH with a maximum at Ω = 1.0. Practically, recovery was limited by the need to maintain Ω at 2.6 or above (the practical minimum). This could be achieved by elevating pH to 9 or above; however, solids precipitate too rapidly as Ω↑ and struvite separation is not achieved. For this case, projected recoveries are shown in Table 1. These projections show that the struvite recovery is most influenced by pH with the recovery of struvite approaching the stoichiometric limit.

Equilibrium modeling of struvite recovery in wastewater processes

2019 Fall National Meeting

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