Sustainable nutrient recovery from synthetic urine by Donnan dialysis with tubular ion-exchange membranes

Abstract

To address interconnected issues related to deteriorated water quality and unsustainable fertilizer production, Donnan dialysis was employed for nutrient recovery from model waste solutions and synthetic urine. Conventional Donnan dialysis reactors with flat-sheet, ion-exchange membranes and 500-mL solutions were deployed to determine the impact of the synthetic urine matrix on nutrient recovery relative to model wastes that only contained sodium phosphate (NaH₂PO₄) or ammonium chloride (NH₄Cl). Compared to the model wastes, the competing ions in the synthetic urine caused orthophosphate (P(V)) and ammonium (NH₄ ⁺) removal to decrease from 90.4 % to 55.2 % and from 87.8 % to 84.8 %, respectively. To improve reactor design for future scale up, an innovative Donnan dialysis system was established by placing tubular anion- and cation-exchange membranes into a 30-L waste solution and continuously recirculating 5 L of an NaCl-based draw solution through the inside of the membranes. P(V), NH₄⁺, and other nutrients were simultaneously recovered, and the concentrations in the draw solution exceeded those in the initial synthetic urine in accordance with Donnan equilibrium for unequal solution volumes. Over 75 % P(V) and 74 % NH₄⁺ were removed from the synthetic urine. MgCl₂ and NaOH were added to the nutrient-enriched draw solution to precipitate solids and reset the electrochemical potential gradient, enabling enhanced nutrient recovery. Chemical equilibrium modeling and solids characterization confirmed that the recovered precipitates were ∼73–90 % struvite. The proof-of-concept tubular reactor represents a promising strategy for scaling up Donnan dialysis systems for selective nutrient recovery from urine and other wastes.