Maximizing liquid fertilizer concentration during ammoniacal nitrogen recovery using hollow fiber membrane contactors

Abstract

Hollow fiber membrane contactors (HFMC) can recover high-purity ammonium (liquid) fertilizer from wastewater with low energy and area footprint. Previous studies examined factors such as pH, initial ammonia concentration, flow rate, stream configuration, and acid-stripping solution. However, water flux through the membrane, impacting %N in acid stripping, remains a key barrier to producing commercial-grade liquid fertilizer. This study tested %N enhancement by increasing feed-side salinity to reduce vapor pressure, thereby reducing undesired water flux to the acid side. Two salinity levels (2 M and 5 M NaCl) representing zeolite regeneration solutions were tested. With 5 M NaCl, water flux was nullified, yielding 12 %N (NH₄⁺) over two cycles, considered very high for this technology. In contrast, 2 M NaCl allowed water flux, achieving only 9 %N. In a third cycle, 5 M NaCl further increased %N to ∼14 %, an unprecedented result for HFMC. Water flux was negative (-0.031 L/m²·h) with 5 M and positive (0.015 L/m²·h) with 2 M NaCl, indicating reverse or forward flow. Ammonia removal efficiency and transfer coefficient (K) remained stable. Furthermore, the membrane prevented ion cross-contamination, producing high-purity liquid fertilizer. Operating at higher salinity, as in ammonia-laden regeneration solutions, may be economically feasible due to solution reusability. This approach optimizes feed properties for highly concentrated liquid fertilizer production.