Editor-in-Chief Elisabeth Lisican showcases a handful of features to read in the April 2017 issue of Water & Wastes Digest.
A tubular membrane filter system separates solids & liquids at Korean semiconductor plant
One of the largest semiconductor manufacturing companies in Korea recently received a 3,000-cu-meter-per-day tubular membrane filter (TMF) system for wafer backgrinding water reclamation.
During the deposition, etching and intermediate chemical mechanical polishing stages, the thickness of a semiconductor integrated circuit (IC) wafer is typically kept at three or more times greater than required for the final device. The extra thickness ensures handling strength and geometric stability during those manufacturing processes. Prior to IC packaging, the wafer is ground to final thickness in a backgrinding process. Large amounts of ultrapure water are used for rinsing off the fine silicon particles and cooling the wafer during the grinding operation; this is discharged from the wafer packaging facility. This wastewater primarily contains high-value ultrapure water, silicon and colloidal silica particles.
In some cases, a small amount of grinding additive fluid may be present. If the fine particles can be removed, the treated water is exceptionally pure due to the resulting low total dissolved solids (TDS) (low conductivity) and can be recycled. The packaging facility’s ultrapure watermaking costs are reduced by introducing this recycled water to intermediate or tertiary purification stages. Wastewater discharge costs also are reduced.
Challenges With Conventional Treatment
Filtration with hollow-fiber ultrafiltration (UF) presents the following challenges:
Using Porex 0.05-µ TMF modules, Veolia built a system for the solid/liquid separation process without the need for any chemical dosing. This filtered water then is sent to a reverse osmosis (RO) system for ionic silica removal only—not for desalination—because the original influent water conductivity is less than 10 µs/cm. The RO product water then is sent back to the workshop for reuse as deionized (DI) water. The slurry from the TMF system is sent to the facility’s central wastewater system for further treatment.
The influent suspended solids concentration is 500 to 1,000 mg/L. A disc centrifuge analysis shows that most of the particles are larger than 50 nm (0.05 µ). The mixed raw wastewater has a brown appearance with numerous fine particles in the liquid.
The system capacity is designed at 3,000 cu meters per day. Currently the plant utilizes 2,000 cu meters per day, and as its processing volume grows, it will utilize the entire 3,000-cu-meter-per-day capacity.
TMF Characteristics & Advantages
When used to treat backgrinding wastewater, this system provides the following benefits:
Additional System Information
The module configuration of 61-tube TMF modules were applied in the wafer backgrinding application system. Specifications of the modules include:
The backgrinding wastewater in this system is collected in the equalization tank, then pumped to the TMF concentration tank (also called the circulation tank). The water then is fed into the TMF unit for solid/liquid separation. Most of the water stream will recycle between the TMF modules and concentration tank in a cross-flow filtration process. Filtered water is sent to a separate product water tank and fed into an RO system for ionic-type silica removal. It then is reused as DI water. Silicon particles that are rejected by the TMF membranes accumulate in the concentration tank during operation. Typically, the partially concentrated liquid is continuously bled off at a set rate. Currently, this stream, and approximately 95 cu meters per day of RO reject water, are sent to the existing wastewater station for further treatment. No chemicals are fed into the system during normal service.
Construction was completed and system commissioning began in July 2013. The system performance has met or exceeded the original design specifications.
As a unique total solution for backgrinding wastewater reclamation, TMF was selected and installed as a key component in this system. The membranes’ structure and properties make it possible to achieve outstanding filtrate quality together with 95% recovery rates in a simplified treatment process.
This system not only reduces wastewater discharge volume, but also recovers a large amount of high-quality DI water. By saving in these two areas, the end user estimates that this plant will ultimately save approximately $1 million per year in reduction of ultrapure water usage and sewer discharge fees.
The Porex TMF membranes were supplied by SI Membranes of Seoul, Korea, and the system was constructed by JM Tech Co. Ltd., also based in Korea.