Challenge

A semiconductor plant in Motorola, TX uses reverse osmosis (RO) and ion exchange (IX) to produce ultrapure water for wafer rinsing. In 1990, the plant upgraded to thin film composite (TFC) membranes for its RO process. Although the switch to TFC membranes improved silica rejection rates and increased the run time on IX resins, the high chlorine residual (2.0 - 2.5 ppm) and fluctuating water pressure in the municipal water threatened the integrity of the new membranes. Because TFC membranes are highly sensitive to chlorine, chlorine residual must be reduced to less than 0.02 ppm before the water enters the RO units.

The plant has used sodium sulfite since it went on line in the 1970s. But with the switch to TFC membranes, feeding the precise amount of this reductant to protect the new RO membranes became problematic. Every two hours, an operator had to test influent water to the RO units, then adjusted the sodium sulfite injection pump rate in an attempt to maintain proper dechlorination. This, however, failed to keep pace with frequent changes in municipal water pressure and the RO start-stop operations.

To compensate for these fluctuations and to remove all risk of damage to the TFC membranes, personnel routinely overtreated with sodium sulfite. The cost of labor and extra sulfite seemed acceptable if it extended TFC membrane life. Overtreating, however, led to a separate set of problems. Membranes became plugged with residue, requiring frequent cleaning, adding more man-hours to the already time consuming tasks of manual testing and water chemistry adjustments.

Solution

For three months, the staff monitored the performance of an automated Strantrol High Resolution Redox^® (HRR) controller.

• The HRR sensor measures oxidant strength in water and transmits data to a controller.
• Based on the HRR data, the controller modulates the dechlor injection pump rate.

Results

The new control system provided steady, precise dechlor dosages despite variations in flow rates and pressures, resulting in two permanent installations at the facility in 1996. Sulfite overfeed, the only sure method of preventing chlorine damage to TFC membranes prior to installing the controllers, had led to the use of 8,700 lbs. of sodium sulfite per quarter.  After the installation of the HRR systems, sulfite usage dropped 25 percent, to 6,500 lbs. per quarter, resulting in savings of approximately $10,000 per year.

• When manual testing was the only means of monitoring dechlorination operations, an employee spent a total of two hours per shift performing the task.
• Since the plant operates three eight-hour shifts per day, 365 days a year, the cost of labor devoted to this added up to more than $25,000 annually.
• Manual chlorine residual testing is now performed only once per shift. Based on labor and chemical savings, the controllers paid for themselves in about six months. However, the real payback has been in the assurance that the membranes are now being protected 24 hours a day.