Thermodynamic-driven evaluation of nutrient recovery as Struvite from Istanbul’s wastewater treatment plant

Pollution by nutrients from anthropogenic sources, an important component of wastewater treatment plants (WWTPs), has become a critical problem for aquatic ecosystems worldwide. In this paper, struvite precipitation and nutrient recovery from the supernatant of sludge thickening supernatant (TS) and the supernatant of sludge dewatering (DS) of WWTPs are investigated and the thermodynamic properties of the formed struvite are studied. A thermodynamic approach was investigated relating to the ions of magnesium, ammonium and phosphate by calculating the various parameters (ionic strength, activity coefficient, Ksp, saturation index). The most economic recoveries of NH₄⁺-N, PO₄³⁻-P and Mg²⁺ were 79%, 82% and 91%, respectively, at a stoichiometric N: P:Mg molar ratio of 1:0.5:0.5 and a Ksp value of 1.5 × 10-5 for the DS supernatant. Analysis of struvite precipitate showed that it was composed of mostly of non-uniform, fine particles with orthorhombic crystal structure of more than 96%. The total reagent cost was calculated to be $4.43/m³ for TS and $5.30/m³ for DS. The recovery potential of DS was higher than that of TS. The change in solution characteristics, particularly the increase of initial amounts of ions of Mg²⁺, NH₄⁺, and PO₄^3–, resulted in a shifted saturation.