Ultrafiltration and Low Pressure Reverse Osmosis for Point-of-Use Water Treatment

By Gil K. Dhawan | 1997

Introduction

Ultrafiltration and low pressure reverse osmosis are finding increasing use as a final treatment step in the production of ultrapure, particle free and organic free water. Membrane systems are especially attractive for the point-of-use as required in electronic, pharmaceutical, and potable water applications.
Several advancements have been made over the past three years. These include better membranes, better analytical equipment, and better understanding of the effect of impurities on the use of water. This paper reviews some of these developments and gives examples of some of the applications for ultrafiltration and low pressure RO.

Point-of-Use Water for the Electronic Industry

In the electronic industry, water standards are periodically revised to reflect changing needs, changing raw water quality, and the availability of more sophisticated analytical techniques.
The starting point for producing ultrapure water is the Feedwater which is generally obtained from city water supply. The standard for this water is set by the Federal Environmental Protection Agency. Table 1 gives some of the more common contaminants and their maximum concentrations allowed in drinking water.
There has been a great deal of discussion on the quality of water required for final rinsing of wafers. Several water quality specifications have been proposed and projected for the point-of-use water (Table 2.)
The technology to produce the quality of water demanded by the electronic industry has improved over the last five years. The major areas of concern in the ultrapure water are the growth of microorganisms and the presence of colloidal materials. Colloidal contamination can be caused by impurities present in the raw water such as humic acid or colloidal silica, bacterial growth in the system, and bacterial byproducts such as pyrogens. Other sources of contamination are leaching of materials from piping, valves, gauges, pumps, and controls used in the water purification system.
To obtain and maintain the highest quality of water it is essential to use materials for piping and other components that do not leach out in the ultrapure water. Fluoropolymer is replacing PVC as construction material for ultrapure water systems.
At present, the final treatment step for the point-of-use water in the electronic industry is filtration through a 0.2 micron microporous filter to remove microorganisms and colloidal materials that may still be present in the water at that point. Substantial experience now indicates that 0.2 micron filtration is not adequate to maintain the high quality of water.

A recent study (Ref. 2) shows that ultrafiltration provides much superior quality of water at point-of-use than the conventional 0.2 micron filters. Microporous membrane systems are in the 0.2 to 10 micron particle removal range and operate with inlet pressures from 5 to 100 psi. Ultrafiltration, on the other hand, removes particles from 0.001 to 0.05 micron.

One such system designed for point-of-use in micro-electronics industry is a 5 gallon per minute system supplied by Millipore. This system uses spiral wound ultrafiltration membranes with a 0.006 micron rating and a final 0.2 micron membrane. Figure 1 shows the Millipore system with the entire piping constructed of fluoropolymer materials. The system has a sanitization injection post at the inlet to the system. The reject valve for the system is factory set at 0.1 gallon per minute so that a very high recovery (98%) is obtained. Periodically the valve is opened to allow full flow and a flushing action across the membrane.
These systems have been tested (Ref. 2) using colloid retention. These tests were carried out both in the laboratory and in the field. In addition, the wafers were tested to determine the performance of the point-of-use system.

The first comparison of point-of-use microfilters and ultrafilter was done by using a modified Silt Density Index. The D.I. water flows from the point-of-use filter under test into an SDI filter. The rate of plugging of the SDI filter is an indication of the colloidal removal efficiency of the point-of-use filter. A higher rate of plugging would mean a relatively poor efficiency in removing colloids and vice versa.

Figure 2 summarizes the results of this testing. The graph shows that 0.1 and 0.2 micron filters allowed the passage of about the same amount of particles and colloids. A much better performance was shown by the ultrafilter with a 100,000 molecular weight (0.006 micron).

The results of the field test are given in Figure 3. The plot shows plugging of a 0.2 micron filter with DI water filtered through a 0.2 micron filter versus plugging of water treated by ultrafiltration. Clearly, ultrafiltration has produced a much higher quality water.

And, finally, the point-of-use ultrafiltration was also evaluated by measuring wafer contamination. Table 3 gives the results of these tests in four different locations. The table shows a significant reduction of particles on wafers when ultrafiltration was used, as compared with the 0.2 micron filter. Locations 1 and 4 also show the effects of not upgrading the piping downstream from ultrafiltration.

More work is being done by different researches in understanding and improving the quality of water at the point-of-use, but it is clear that point-of-use ultrafiltration provides much better quality rinse water than was possible before with 0.2 micron filters (Table 4).

Conclusions

The applications of ultrafiltration and low pressure reverse osmosis for point-of-use water treatment looks very promising. The development of high flux membranes and “softener” type RO membranes is sure to expedite the use of these membranes at the point-of-use, to meet the very high water quality requirements in a number of industries. We can expect to see an increasing use of these processes in the production of ultrapure water.

References

1. Motomura, H., Microcontamination, March 1984
2. Accomazzo, M. and Gaudet, P.W., Point-of-Use Ultrafiltration of Deionized Water and Effects on Microelectronics Devices Quality, Millipore Corporation.
3. California State Department of Health Services, Sanitary Engineering Branch
4. United States Pharmacopeia, XX Edition, Mack Publishing Company, 1980.
5. Pharmaceutical Technology, October 1983, Part IIb of the report of PMA’s Deionization Water Committee.

Figure 1  Ultrafiltration System  Designed for Point-of-Use in the Micro-electronics Industry	Figure 2   Comparison of Point-of-Use Microfilters and Ultrafilter Using a Modified Silt Density Index	Figure 3 Comparison of UF and 0.2 Micron Filter
UF system using spiral wound ultrafiltration membranes with a 0.006 micron rating and a final 0.2 micron membrane.	Plugging of 0.2 Micron Membrane (293 mm disc) Downstream of Test Filters at Constant Flow Rate (2.0 GPM)	Plugging of 0.2 Micron Disc Filter in DI Water Loop Before and After Ultrafiltration

Table 1 - Primary Drinking Water Standards (Ref. 3)

Table 2 - Specifications for Ultrapure Water for the Semiconductor Industry

a.  Measured with a resistivity meter.

b.  Measured by direct microscopic count.  Particles retained on a poly-carbonate filter are stained and counted with an optical microscope or, when particles are less than 0.1 micron, witha  scanning electron microscope.

c. Measured by ultraviolet oxidation-resistivity detection (Barnstead's Photochem).  Wet oxidation-infrared detection is used for determining total organic carbon levels higher than 0.2 ppm.

d. Measured by the culture method (ASTM F60-68).

e. Measured by the colorimetric-molybdate reactive silica method (ASTM 0689-80).

f. Measured by Winkler's titration.

Table 3 - Point-Of-Use Ultrafiltration Evaluation Using Wafer Contamination

Particle Counts on Wafers

*Plumbing after UF not upgraded

Table 4 - Point-of-Use Ultrafiltration Performance