A preliminary study on electrically enhanced crossflow microfiltration of CMP (chemical-mechanical polishing) wastewater

Chemical-mechanical polishing (CMP) is currently one of the main technologies used by wafer plants in the semiconductor industry. Normally, a large amount of de-ionized water should be used to wash out the abrasives adhered to the surface of wafers during the grinding process. Therefore, CMP wastewater not only has a great quantity but also contains very small size of suspended solids. Generally, these suspended solids would not settle. This phenomenon results in a low visibility of CMP wastewater. To solve this problem, a study on electrically enhanced crossflow microfiltration of CMP wastewater was conducted. Normally, the membrane of traditional dead-end filtration is easily blocked by the filter cake resulting in a small flux of filtrate. Therefore, the form of crossflow filtration (CFF) is used to reduce the blocking of the membrane. Furthermore, if CFF is accompanied by an external electric field, the negatively charged suspended solids in wastewater would move toward the positive electrode. Meanwhile, the flux of filtrate would increase. In this investigation, CMP wastewater was obtained from a wafer plant and characterized by various standard methods. Before testing, the CMP wastewater was pre-filtered using a 1.2 μm pore size filter. Then it was operated by a crossflow microfiltration (0.1 μm pore size) system under an appropriate crossflow velocity, filtration pressure, and electric potential. The filtrate was also characterized by various standard methods. Experimental results show that the filtrate has a turbidity of zero value. Thus, the filtrate could be reused for other purposes. However, the suspended solids of the CMP wastewater would be concentrated by this method. Therefore, the recovery of the suspended solids would be worth considering.