Treatment of wastewater containing nano-scale silica particles by dead-end microfiltration: evaluation of pretreatment methods

Chemical mechanical polishing (CMP) is the process to planarize wafer in the IC manufacturing. In the process, a large amount of ultra pure water is used to clean the surface of the wafer, which generates large quantity of wastewater containing high concentration of nano-scale silica particles. The wastewater is generally treated with the traditional coagulation/sedimentation process, producing large quantity of sludge. In this study, a microfiltration (MF) process coupled with chemical pretreatment was investigated to separate the nano-scale particles from the CMP wastewater to reclaim the water. Wastewater was filtered through a polytetrafluoroethylene membrane of pore size 0.5 μm at a low vacuum pressure (0.65 kg/cm2). The permeate flux was monitored and the degree of irreversible fouling of the membrane was estimated. The SEM images of the fouled membrane and the cake layer were used to diagnose the mechanism of membrane fouling. Several coagulants and flocculants were tested for their potential use prior to microfiltration. Polyaluminum chloride (PACl) plus cationic polyacrylamide (PAA) were selected for pretreatment, and their effects on permeate flux and fouling of the membrane were examined. The results indicated that PACl improved silica removal substantially. The addition of trace amount of PAA effectively increased the size of the flocs, and subsequently enhanced the permeate flux. The performance of the dead-end MF operation depended strongly on the turbidity of the influent. The addition of oxidant, i.e. Chlorine, after the flocculation reduced the irreversible fouling and promoted flux recovery of the membrane.