The Evaluation of Ammonium Sulphate as a Potential Draw Solute in a Hybrid FO-RO Process to Concentrate Nutrients (NPK) from a Simulated Liquid Digestate-Part I: Deionized Water as a Feed Solution.

The ultimate objective of this research is to concentrate nutrients-nitrogen (N), phosphorus (P), and potassium (K)-and produce process water from a chemically pretreated liquid digestate using an FO-RO hybrid process. However, in this manuscript, we assessed the suitability of (NH4)2SO4 and NaCl as draw solutes in a series of FO experiments employing a commercial CTA membrane and DI water as the feed solution. We also examined the regeneration of (NH4)2SO4 in a series of RO experiments at various feed concentrations and pressures using a commercial polyamide (PA) thin-film composite (TFC) membrane, ACM4. Additionally, the RO experiments enabled the experimental determination of the osmotic pressure of (NH4)2SO4 at various feed concentrations, which is crucial for designing the FO part of the hybrid process. The CTA membrane exhibited a significantly greater selectivity for (NH4)2SO4 than for NaCl at any osmotic pressure. The RO experiments demonstrated the possibility of reconcentrating (NH4)2SO4 to 0.5 mol/L, with a corresponding water flux of 60 L h-1 m-2 at 40 bars. The experimentally determined osmotic pressures were lower than those predicted by van't Hoff's equation but were consistent with those reported in the literature using an indirect hygrometric method.