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 Cu²⁺ 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.