A critical review on challenges and trend of ultrapure water production process

Ultrapure water (UPW) is of high purity and with almost no other electrolytes except hydrogen and oxygen ions, which is usually used in the electronics industry, photovoltaic industry, and pharmaceutical industry. Calculations based on ionic mobilities yield a specific resistance of ideal UPW is 18.3 MΩ•cm at 25 °C (Singh, 2016). Hence, ordinary distilled water, reverse osmosis water, and high-purity water (resistivity ≥10.0 MΩ•cm) are not considered to be UPW (Singh, 2016). In addition, UPW must be devoid of all contaminants including pathogens, suspended solids, radioactive contaminant, inorganic compounds and organic compounds in ideal condition, so it is also called “highly refined water” (Choi and Chung, 2019; Lee et al., 2016; Singh, 2016). The ultimate goal of UPW production is to completely remove all components except water molecules, but this is virtually impossible. Hence, the specific demand for UPW in different industries determines the purity level of UPW. Among them, the purity grade of UPW used in the semiconductor industry is most stringent. Especially with the booming developments in semiconductor technology, the quality requirements of UPW in the semiconductor industry are being formulated (Lee et al., 2016; Zhao et al., 2019). Thus, strictly controlling the quality of the final effluent is one of the most important challenges in the current UPW production process.

In addition, because most of the manufacturing stages of electronic products involve the use of UPW, UPW represents an important and large market. For example, a semiconductor is a miniature electronic circuit which contains a multitude of transistors. Creating an integrated circuit on a 30 cm wafer requires approximately 2200 gal of water. A typical electronic product manufacturing process needs about 3 to 60 million liters of UPW per day (Lee et al., 2016). Thus, the manufacture of semiconductors is highly water intensive. Along with the development of semiconductor technology, the demand of UPW in the semiconductor industry will greatly increase, which will lead to a huge growth of the raw water demand. Therefore, how to meet a large amount of raw water demanded will be a major problem that has to be solved in UPW production. On the other hand, with rapid urbanization and population growth, water shortage is becoming more and more serious worldwide. Therefore, people have made extensive efforts to promote other water resources, such as rainwater utilization, reclaimed water utilization, seawater desalination, etc. (Charfi et al., 2017; Guo et al., 2014). Among them, reclaimed water and municipal sewage are readily available, in great quantities, and not restricted by geographical environment. These accelerate the change of raw water resource used for UPW production. Recent studies' results have shown the feasibility of using reclaimed water as raw water sources to produce UPW (Lefebvre, 2018; Wang et al., 2019), which is a feasible engineering solution to realize the sustainable utilization of water resources. For instance, Singapore's NEWater, one of that country's four national taps, is high-grade reclaimed water produced from treated effluent of sewage treatment plants utilizing membrane technology and ultraviolet disinfection. Total organic carbon (TOC) and the conductivity of NEWater are about 0.5 mg/L and 100 μm/cm, respectively, which can be employed in industries with lower UPW water quality requirements (Lefebvre, 2018; Wang et al., 2019). In general, making full use of reclaimed water to produce UPW desired in semiconductor industry can save a large quantity of tap water. Moreover, it can improve the recycled utilization rate of water resources. However, to meet the strict UPW quality of the semiconductor industry, the largest problem concerning the raw water change is probably associated with the organic compounds removal, which means the existing UPW preparation process must be improved.

This is the first paper to critically review on challenges of the existing conventional UPW production process using reclaimed water as raw water source. The paper summarizes the technical methods of removing organic matters in the traditional UPW production process and discusses the existing problems inherent in organic matter removal while suggesting the future trend of the UPW production process.