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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]