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The structure parameter of the forward osmosis membrane has been defined as the characteristics of the support membrane, not related to the separation layer. Therefore, studies for improving the performance of the forward osmosis (FO) membranes have been focused on the optimization of the support structure. This study demonstrates that the separation thin-film polyamide (PA) layer has a strong impact on the performance of the FO membrane thus the structure parameters (S). We intentionally utilized a polyethersulfone hollow fiber substrate and two types of active separation skin were prepared at the inner surface: the first MPD/TMC and the second by adding triethylamine (TEA) into the aqueous phase during the interfacial polymerization (IP). By adding TEA, a rough PA layer with a leaf-like structure and a high crosslinking degree was obtained followed with high FO flux and low reverse salt flux attributed to the increased effective filtration area and crosslinking degree. The structure parameter of the FO membrane was reduced from 0.78 mm (for MPD/TMC system) to 0.45 mm (for MPD-TEA/TMC system). The result indicated that for the same substrate, lower structure parameter corresponded to the skin layer with higher permeance and rejection. This study confirmed that the structure parameter is more an inherent property of the entire TFC FO membrane than the character of the porous substrate. Future design for FO membranes needs to regulate the substrate, but more importantly to optimize the separation layer. • An approaching ratio (α) was designed as a simple guideline for control the structure parameter of FO membranes. • Diethylene glycol was used as the nonsolvent to control the approaching ratio. • Lowest structure parameter and highest FO performance were obtained at α = 50 %. • The optimized TFC membrane is of practical interest for lithium concentration. [ABSTRACT FROM AUTHOR]

Hemodialysis consumes significant amounts of pure water and produces large amounts of spent dialysate. Recovering water from spent dialysate is a new attempt in medical industries to save water and costs. The recent proposal of using forward osmosis (FO) to concentrate the spent dialysate showed economic viability. However, there is still one key concern of potentially extended hemodialysis duration because commercial FO membranes have limited urea rejection, which would reduce the urea concentration gradient during hemodialysis. In this paper, a model was developed to quantify the effect of incomplete urea rejection in FO on subsequent hemodialysis duration. The impacts of average urea rejection, water recovery, and the number of hemodialysis sessions were systematically analyzed. Counter-intuitively, the results indicated that relatively low urea rejection of FO membranes actually has little impact on hemodialysis duration. Low urea concentration in the spent dialysate, partial urea rejection by the FO membrane, and further dilution of pre-diluted dialysis concentrate, largely reduces this impact. The result further supports the viability of FO in spent dialysate reuse. Although FO membrane with 100% rejection is expected, the present work indicates that commercial FO membranes with a low rejection of neutral matters are of practical utility in certain applications. • Osmotic dilution was used for recycling spent dialysate of hemodialysis. • A model was developed to evaluate the effect of several factors in the osmotic dilution on the hemodialysis duration. • A relatively low urea rejection of an FO membrane has a minor impact on the subsequent hemodialysis duration. • The reasons for the counter-intuitive result in recycling the spent dialysate by FO were revealed. [ABSTRACT FROM AUTHOR]