A Sound Basis of Design

A high purity water treatment system should include technologies designed to remove specific feed water impurities to ensure consistent product water quality.  Since the water source is inherently dynamic and a feed water analysis only provides a snapshot of the impurities present, a worst case approximation of all objectionable impurities should be used as a design basis.  Based on this estimate and the final product requirement, the proper treatment operations can be identified and appropriately designed.  Incomplete knowledge of feed water source can be attributed to:

• No historical feed water data
• Failure to account for seasonal fluctuations
• Unknown changes in municipal treatment techniques
• Failure to review of other treatment systems operating on feed water source
• Inaccuracy of subjective water chemistry tests or incomplete feed water analysis
• Lack of testing for objectionable impurities

The feed water analysis should include those specified for final product water quality or that may contribute to a generalized parameter (e.g. resistivity), but also those which may affect the treatment operations.  For example, the presence of silica in the feed water can significantly impact reverse osmosis (RO), Electrodeionization (EDI), and distillation unit performance, but they may not be critical to the final product water quality for certain applications.

Process Design Considerations

High-purity water systems are challenging to monitor as they contain many different unit operations.  Each step is designed for a specific purpose of changing the chemistry or removing a certain impurity or several impurities.  The successful operation of downstream processes often relies upon sound performance of upstream operations.  The use of pretreatment techniques exemplifies this, as many treatment techniques are included in system design for the sole purpose of protecting expensive downstream primary deionization techniques such as RO and distillation units. 

As such, the overall process design and choice of pretreatment technologies should be based not only on the final product specifications, but also with the purpose of protecting the primary deionization techniques.  Inadequate pretreatment may result in scaling, fouling, oxidation, or biofouling of downstream components.

In turn, polishing treatment techniques downstream of the primary deionization steps, may be required for the following reasons:

• Meet very stringent product quality specifications
• Specific contaminant reduction
• Maintain water quality (high-purity water is aggressive and will degrade)
• Compensate for deficiencies or limitations with the primary deionization technique

Importance of Inter-Component Monitoring

Each operation in a high purity water system is employed for a particular purpose, and their performance should be measured and analyzed on a regular basis to ensure efficient and effective operation.  Although the criticality of each operation may vary, over time poor performance of any operation will have a detrimental effect on some aspect of the system.  The performance of a process is ideally monitored in real time by in-line or on-line instrumentation.  For example, on-line hardness analyzers for softeners and chlorine analyzers for activated carbon units provide valuable data in real time. 

On-line instrumentation can be expensive and wet chemistry tests are a cost-effective alternative for many systems.  When taking grab samples instead of using on-line analyzers, the frequency and location of sampling is critical.  Routine sampling is recommended for most parameters.  If microbial control is a critical attribute of your final water, periodic inter-component sampling for total viable bacteria, bacterial, endotoxin, or other specific microorganisms is also recommended.

Additionally, rigorous monitoring of the performance of each process helps to establish maintenance intervals that are typically arbitrarily chosen when the system is commissioned.  This can be used as a tool to relax certain maintenance functions such as consumable replacement, media filter backwashes, softener and ion-exchange regenerations, cleanings, and sanitizations.  This saves both money and system downtime.

However, the two most critical benefits of inter-component monitoring of unit operations in a high-purity water system are that upsets or excursions in the system are identified before product water quality is impacted, and a comprehensive understanding of each process is realized. 

Final Product Quality

Measuring the effectiveness or efficiency of each process stage leads to an increased knowledge of system operation and continuous process improvements.  Monitoring specific impurities also facilitates real time adjustments to ensure that final product quality is never jeopardized.  If there is ever an upset condition, the root cause is more easily discovered.  With the variable nature of feed waters, consider a holistic system monitoring program that reduces risk and results in reliable production of high-purity water.

This post was originally posted in June of 2018.  It has been updated from the original.