Toward zero liquid discharge treatment of semiconductor wastewaters with a hybrid system integrating forward osmosis and multi-stage nanofiltration

The semiconductor industry produces large quantities of acidic wastewaters containing high levels of hydrofluoric acid (HF) and sulfuric acid (H2SO4), which poses severe environmental concern. Zero liquid discharge (ZLD) treatment of these wastewaters is a pressing need for sustainable growth of the semiconductor industry. Herein, we propose an innovative membrane-based hybrid system that combines forward osmosis (FO) with multi-stage NF process for simultaneous treatments of H2SO4 and HF wastewaters. The preceding FO process was designed to operate with HF wastewater (i.e., 200 ppm F- and 189 ppm Cu2+ at different pH of 3 and 5) as a feed stream and neutralized H2SO4 wastewater (i.e., 1.0 M Na2SO4) as a draw stream. We demonstrate that the FO process allows >55 % dilution of the Na2SO4 waste stream while providing >50 % rejection of all ionic contaminants in the HF wastewater, especially over 90 % rejection of copper ions. As a result, the concentration of Cu-2(+) increased more than three-fold in the feed stream, highlighting the potential for valuable metal recovery from the HF wastewaters. The following two-stage NF process was employed to produce fresh water from the diluted Na2SO4 waste stream at a maximum water recovery rate which satisfies the ZLD requirement. Our results show that two-stage low-salt-rejection NF (LSRNF) configuration can concentrate the Na2SO4 stream to 1.3 M, which is the critical concentration required for ZLD treatment, at a relatively moderate hydraulic pressure of 40 bar, while simultaneously producing high-quality water that meets discharge standards. We further modeled the multi-stage NF process to highlight the feasibility of the promising FO and two-stage LSRNF hybrid design for ZLD treatment of semiconductor wastewaters.