Feasibility of concentrating textile wastewater using a hybrid forward osmosis-membrane distillation (FO-MD) process:...

The forward osmosis-membrane distillation (FO-MD) hybrid process has shown great promise in achieving zero liquid discharge in the textile industry, recovering valuable dye molecules while producing large amounts of clean water. However, the progress of this technology seems to have stagnated with the direct coupling of commercial asymmetric FO and MD membranes, because water management in the system is found to be rather complicated owing to the processing of the different membranes. Herein, we propose, for the first time, an FO-MD hybrid process using a custom-made self-standing and symmetric membrane and a hydrophobic polytetrafluoroethylene membrane in the FO and MD units, respectively. Three types of operation modes were investigated to systematically study the process performance in the concentration treatment of model textile wastewater; two commercial FO membranes were also tested for comparison. Owing to its low fouling propensity and lack of an internal concentration polarization effect, the water transfer rate of our symmetric FO membrane quickly reaches equilibrium with that in the MD unit, resulting in continuous and stable operation. Consequently, the hybrid process using the symmetric FO membrane was found to consume the least energy, as indicated by its lowest total cost in both lab- and large-scale systems. Overall, our study provides a new strategy for using a symmetric FO membrane in the FO-MD hybrid process and highlights its great potential for use in the treatment of textile wastewater.