Simultaneous removal of fluorine and dissolved silica from semiconductor wastewater by an in-situ crystal nucleation method towards near-zero liquid discharge

Abstract

Near-zero liquid discharge (Near-ZLD) treatment process for semiconductor (SC) wastewater often encounters membrane fouling caused by calcium fluoride (CaF2) and silicates, which affect the process stability and efficiency. However, achieving low chemical dosing while simultaneously removing fluoride ions (F−) and dissolved silica (D-SiO2) remains a significant challenge in SC wastewater treatment. This study developed an in-situ crystal nucleation (ICN) method for Near-ZLD pretreatment, ensuring the simultaneous removal of F− (<8.0 mg/L) and D-SiO2 (<6.0 mg/L) to meet Near-ZLD requirements while mitigating membrane fouling. The method induced the in-situ generation of CaF2 seeds through a simple process adjustment without external additions, promoting the growth of CaF2 particles size and enhancing the subsequent coagulation effect. Consequently, membrane fouling was significantly reduced, and its reversibility was increased by more than ten-fold. In addition, the coagulant dosing was further optimized (Fe/Al mixed coagulants), which enhanced the acid-base interaction between the membrane surface and flocs, reducing irreversible membrane fouling by 70 %. In practical applications, the ICN method consistently achieved a 104.87 % increase in membrane flux compared to the conventional method, significantly extending the membrane's service life. This method provides a new perspective with practical guidance for the stable and efficient Near-ZLD operation in SC wastewater.