Assessing calcium nitrate and nitrogen-phosphorus-potassium (NPK) as draw solutions in fertilizer-drawn forward...

Forward osmosis (FO), particularly fertilizer-drawn forward osmosis (FDFO), offers a promising approach to tackling water scarcity challenges efficiently. This research project aims to investigate the performance of calcium nitrate (Ca(NO₃)₂) and commercial nitrogen, phosphorus, and potassium (NPK) as draw solutions (DS) for desalination using the FDFO process on both bench and pilot scales. The feed solution (FS) consisted of synthetic brackish water (BW) with moderate salinity and trace levels of strontium (Sr²⁺) and barium (Ba²⁺), reflecting concentrations commonly found in natural brackish groundwater. This investigation was carried out using different concentrations of Ca(NO₃)₂ and NPK (0.5 M, 1 M, and 2 M) on the bench-scale trials. The performance of each draw solution was evaluated by measuring water flux, water recovery, specific reverse solute flux, and ion rejection efficiency. The results showed that water flux increased with DS concentration. Surface characterization via energy-dispersive X-ray spectroscopy (EDX) revealed distinct elemental deposition patterns on the polyamide (PA) membrane. Greater elemental intrusion and fouling were observed in the NPK-treated membrane compared to Ca(NO₃)₂. The membrane demonstrated consistently high rejection rates for both Sr²⁺ and Ba²⁺ ions, with Ca(NO₃)₂ DS achieving up to 99% rejection. The evaluation of the pilot-scale FO system was based on using the highest-performing DS concentration as optimized from the bench-scale trials. Using 2 M Ca(NO₃)₂ resulted in the best performance of Pilot FO, achieving an average water flux of 2.23 ± 2.7 LMH and 115 ± 40 L of recovered water. Na⁺ rejection was 90%, while Sr²⁺ and Ba²⁺ rejections were 90% and 87%, respectively, with final concentrations within Egyptian agricultural reuse limits. The low specific reverse solute flux (SRSF) values (0-0.18 g/L) indicated high membrane selectivity.