2025 AIChE Annual Meeting

Direct Crystallization of Struvite Using Selective Bipolar Membrane Electrodialysis

Over the last century, human agricultural output has relied increasingly on synthetic fertilizers to meet the needs of the world’s growing population. However, upwards of 40-80% of the nutrients applied to farmland are lost to run-off, creating an extremely wasteful nutrient cycle. As global food demand continues to rise, this marks a major opportunity for scientific advancement, as synthetic fertilizer production methods such as the Haber-Bosch process are unsustainable in the long term. With these concerns, improved methods for nutrient recovery and recycling must be developed to minimize nutrient loss.

Struvite (magnesium ammonium phosphate) precipitation is a well-established method for nitrogen and phosphorus recovery from wastewater. Nutrients are simultaneously removed from wastewater and upcycled into a valuable slow-release fertilizer containing critical plant nutrients. Struvite crystallization systems typically introduce external magnesium and apply alkaline chemicals to induce precipitation; however, these additional inputs make these systems cost-ineffective. We propose a struvite crystallization system that will remove the need for these additional inputs, concentrating existing magnesium from wastewater and generating internal alkaline conditions using selective bipolar membrane electrodialysis (SBED).

Electrodialysis (ED) as a means of nutrient recovery is of particular interest due to its low energy consumption and absence of harsh or expensive chemical additives. ED employs an electric field through wastewater, where separation occurs when target ions migrate across exchange membranes from dilute to concentrate channels. Additionally, selective electrodialysis (SED) utilizes monovalent ion-exchange membranes that allow monovalent ions to pass, but reject multivalent ions. Furthermore, bipolar membrane electrodialysis (BMED) utilizes bipolar membranes, which combine an anion- and a cation-exchange layer with a water-splitting catalyst, directly producing protons (H+) and hydroxide ions (OH-). In SBED, we incorporate SED to concentrate phosphate in one channel and BMED to generate alkaline conditions in another, optimizing struvite crystallization conditions when these channels are combined in the crystallizer.

One barrier to membrane-based processes such as electrodialysis is membrane scaling or fouling. If precipitates form within the system, cell resistance increases, effective membrane surface area decreases, and in severe cases, membranes puncture. Our system works to mitigate traditional causes of scaling. Hydroxide ions are concentrated in a channel that does not contain an excess of phosphate, reducing the chance of premature struvite precipitation. Additionally, crystallization occurs externally, and the recycle stream is filtered before reentering the cell, reducing the chance that scalants precipitate inside the cell or enter through the feed. Finally, the bipolar membrane generates protons that may be used as an acid source for washing the membranes of any scalants.

In trials, our system was tested with simulated concentrated animal feeding operation (CAFO) wastewater and was run for three hours without scaling. Precipitates were collected from the crystallizer, and after analysis by scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS), were determined to be struvite.