Direct solid-phase nitrogenous fertilizer recovery from wastewater: The hybrid system of membrane contactor and solvent-driven fractional crystallization

Abstract

We propose a novel configuration that integrates a membrane contactor with solvent-driven fractional crystallization (SDFC) to recover ammonia from wastewater and produce it as solid-phase nitrogenous fertilizers. A liquid-gas membrane contactor strips ammonia from wastewater in a gaseous form, which enters a strip tank containing a binary mixture of an aqueous anion solution and an organic solvent. There, the ammonia reacts with anions, instantly protonating and forming solid-phase fertilizers. Batch SDFC experiments identified phosphate and sulfate as viable options for producing solid-phase fertilizers from the ammonia gas entering the strip tank. The hybrid system utilizing these acids produced high-grade fertilizers free from soil acidification concerns: a mixture of monoammonium phosphate and diammonium phosphate, and pure ammonium sulfate. Ammonium sulfate crystals in the strip tank grew epitaxially, representing a unique ammonium sulfate crystallization pattern when ammonium concentration gradually increased to supersaturation. A single system run produced solid fertilizers that amounted to 81.54 and 83.84% of the initially added phosphoric and sulfuric acid, respectively. Organic solvents in the strip tank could be recycled for at least five cycles while maintaining crystallization efficiencies of $\ge$82.63%. These results highlight the potential for semi-permanent operation of the system without the need for solvent replenishment.