A Modified Donnan Dialysis Process Using Sacrificial Magnesium Plates to Improve Phosphorus Recovery and Capture Ammonium and Potassium for Use as a Liquid Fertilizer

Donnan dialysis (DD) processes can be used to leverage the electrochemical potential gradient across ion exchange membranes to recover targeted nutrients from liquid waste streams. However, the slow separation rate of diffusion-based systems limits their practical applications. To accelerate phosphorus recovery rates, we used a modified Donnan dialysis (MDD) system that incorporated a sacrificial magnesium (Mg(s)) plate in the feed chamber. Using a second adjoining chamber, we simultaneously recovered ammonium (NH₄⁺) and potassium (K⁺) transported across the cation exchange membrane (CEM), producing a solution that could be used as a liquid fertilizer. Comparisons between MDD and DD across 1×, 5×, and 10× feed concentrations demonstrated that the MDD system captured P efficiently, achieving removal efficiencies of up to 99.6% within 60 min, primarily as struvite (plate surface area to reactor volume ratio of 8.9 m²/m³). Despite a slight reduction in K⁺ and NH₄⁺ diffusion through the CEM due to struvite reactions in the feed, the simultaneous capture of K⁺ and NH₄⁺ in the solid and liquid phases improved their overall recovery by up to 33.6%. These results show the feasibility of the MDD process, which offers both solid and liquid fertilizers from a single operation with improved P recovery rates.