(84ac) Modified Donnan Dialysis Process for Selective Nutrient Removal from Agricultural Liquid Waste

Significant amounts of important macronutrients such as phosphorus, nitrogen, and potassium in high-strength wastewaters such as animal manures, makes nutrient recovery from such waste streams a compelling opportunity to achieve a circular economy and address increasing global fertilizer demand. However, the proposed technologies should be able to deal with high suspended solids, organic matter content, and ion concentrations of such waste streams. Donnan dialysis with ion exchange membranes (IEMs), are one technology option with low energy consumption that has the potential to deal with these challenges and could be used for nutrient recovery from different wastewater sources.

In this talk, I will discuss a new modified cation exchange membrane (CEM)-separated Donnan dialysis process in which a sacrificial magnesium plate is placed in the anolyte to recover phosphate precipitates through electrochemically induced reactions. In addition, the cell produces a side-stream of liquid fertilizer for fertigation applications through the simultaneous separation of ammonium and potassium. A simulated agricultural liquid waste, based on water chemistry of real animal manure samples, has been used as the anolyte. Two different cation exchange membranes have been tested to selectively separate ammonium and potassium from wastewater, while the pure sacrificial magnesium plate in the anolyte was the only source of magnesium in the system. Also, different draw solutions have been tested as the catholyte to understand the role of draw ions on overall system performance.

Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to track ion concentrations in each chamber over time for kinetic purposes. A set of characterization techniques including scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transform infrared spectrometry (FT-IR) were used to identify the morphology and nature of the precipitates. The results indicated complete inorganic phosphorus removal in form of solid precipitates, mainly struvite. Also, selective separation between ammonium and potassium was achieved with decreased ammonium ion cross over the CEM compared to potassium. These results show that the proposed process can successfully recover valuable, diverse products from wastewater while avoiding the disadvantages of other commonly used chemical precipitation methods of struvite recovery, such as magnesium salt dosing and adding base to the system for pH control.