Struvite Recovery from Swine Waste Biogas Digester Effluent through a Stainless Steel Device under Constant pH Conditions.

Objective: To investigate the struvite precipitation under constant and non-constant pH conditions and to test a stainless steel device under different operating regimes to maximize the recovery of struvite. Methods: The molar ratio of NH₄ ⁺: Mg²⁺: PO₄ ³⁻ was adjusted to 1: 1.2: 1.2 and pH was elevated to 9.0. The absorbance measurement was used to trace the process of struvite crystallization. Wastewater and precipitate analysis was done by standard analytical methods. Results: The pH constant experiment reported a significantly higher struvite precipitation (24.6±0.86 g) than the non-constant pH experiment (19.8±1.86 g). The SAR ranged from 5.6 to 8.2 g m⁻² h⁻¹ to 3.6-4.8 g m⁻² h⁻¹ in pH constant and non-constant experiments, respectively. The highest struvite deposit on the device was found in regime 3 followed by in regimes 2 and 4. The highest PO₄ ^3- (97.2%) and NH₄ ⁺ (71%) removal was reported in the R1 regime. None of the influent Cu²⁺ or Zn²⁺ was precipitated on the device. Conclusion: A higher struvite yield is evident in pH constant experiments. Moreover, the stainless steel device facilitates the isolation of heavy metal free pure (around 96%) struvite from swine waste biogas digester effluent contaminated with cu²⁺ and Zn²⁺ and the highest yield is attainable with the device operating at 50 rpm with agitation by a magnetic stirrer. [Copyright &y& Elsevier]