Advances In Instrumentation Used To Monitor High-Purity Water Treatment Systems

High-purity water treatment technology has progressed and changed significantly in recent years. Major trends include the wider variety of membrane processes driving an increasing share of purification, more use of reclaimed and recycled water, and treatment systems with more user-friendly interface. In addition, industry pure water requirements have also changed. Pharmaceutical waters now have conductivity and total organic carbon (TOC) requirements that are most easily met with on-line measurements. The power industry has more specific water chemistry guidelines and higher priority "core parameters" to be monitored in all plants. The semiconductor industry has more stringent ultrapure water limits as well as process-specific requirements. All of these changes have raised challenges for the associated instrumentation. Application requirements also frequently point toward additional parameters, lower detection levels and lower costs. Progress in instrumentation technology and innovative design are meeting many of these challenges. Presented here is a summary of instrumentation advances that help to meet these newer application needs.

Membrane Processes
Reverse osmosis (RO) was initially introduced with cellulose acetate (CA) membranes which provided the state of the art separation at the time. Their good separation, low fouling tendency and low operating cost relative to deionization resins produced steady growth of RO in the water treatment field. However, their limited pH range between 4 and 6 generally required pH control upstream to prevent hydrolysis of CA. As a result, an on-line pH measurement and automatic or manual control system to feed acid were typically needed as pretreatment to protect CA membranes.

The development of thin film composite (TFC) membranes using polyamide (PA) polymers improves separation, operates at lower pressure and lowers operating costs further. In addition, they can tolerate a much wider range of pH which eliminates the need for pH control and acid addition in most cases. However, TFC/PA membranes are very vulnerable to oxidation from even the low levels of chlorine in municipal water supplies. Where chlorine is added for disinfection ahead of pretreatment filters there is even more potential for damage. Some deionization resins are also attacked by chlorine.