Water is the fluid most often used in cleaning, whether it is for personal, household, industrial, or the manufacture of high-value product. Water is the most abundant cleaning chemical. With appropriate additives, water-based cleaning has proven successful. However, particularly in critical manufacturing, the wrong water quality can derail the process and undermine product quality. How do you get water to the right quality? Why might you have to treat water?
Not all water is the same
There is no “pure” water. Naturally occurring water is not pure, whether it comes from underground wells, surface reservoirs, or even freshly fallen rain. This is because water is a very aggressive solvent and picks up minerals from the ground or gases from the air.
• Tap water
Tap water is water coming into the facility from the local water district. It may come directly from a well or reservoir and generally has been treated so that it is safe to drink or bathe in and, in many communities, materials such as fluorides have been added. One of the primary issues with using tap water for industrial processes is that the consistency varies greatly from location to location and from season to season.
• Hard water
Water that contains higher levels of dissolved minerals such as calcium and magnesium is considered to be hard. Although hard water may taste better due to the included minerals, it is not as effective for cleaning. Dissolved minerals interfere with the effectiveness of soaps and may remain as residues when water evaporates or precipitates from the water to form scale.
Water purification treatments
For personal and household use, many people employ some degree of water conditioning. However, the quality of water required for industrial and high precision applications is generally higher. In order to determine the optimal treatment, it is important to understand both the terminology and the methodology of the various water treatment techniques.
• Soft water
Soft water is not necessarily mineral free. Hard water is softened by using an ion exchange process. Many water softeners employ a cation exchange resin where the negatively charged resin beads are bonded to positively charged sodium cations. Calcium and magnesium cations, being more strongly bonded to the resins, displace the sodium and thus are removed from the water. The exiting water contains lower concentrations of the hard minerals but now contains sodium. Although sodium interferes less with soaps and does not precipitate as scale, the water still has the same ionic content.
• Deionized water (DI)
Deionized water is produced in a process in which ion exchange removes both positively charged cations such as calcium or sodium, and negatively charged anions such as chlorine. In the exchange process, the cations are captured, releasing hydrogen (H+) ions; the anions exchange with hydroxyl (OH-) ions. Since the exchange ions recombine to form water, the result is a removal of most ionic impurities. With a reduced ionic content, the conductivity decreases (the electrical resistivity increases). Measuring the conductivity is the most common method of confirming the effectiveness of a DI process.
• Reverse osmosis (RO)
Osmosis is a filtration process. Suppose water is on one side of a membrane and water with dissolved material is on the other side. If the membrane is semi-permeable, meaning that water more easily passes through it than dissolved materials, water passes through in both directions until the concentration of water on both sides is the same (isotonic). Normal osmosis thus results in a net flow of water from the purer side of the membrane to the more dissolved material side. However, if sufficient pressure is applied to the dissolved material side, the process is reversed and water can be “purified” by flowing from the dissolved material side to the purer side.
• Distilled water
Distilled water is produced by evaporation, with the vapor condensing (distilling) into another container. Since most ionic and inorganic impurities have a much lower vapor pressure than water, the distillate is purer and has a lower conductivity. Distilled water is not necessarily pure. If there is more than one volatile chemical present, such as water containing alcohol or oil, each will distill proportional to its respective vapor pressure and will still be in the distillate.
• Water for injection (WFI)
WFI water is treated to meet USP specifications that limit organic, inorganic and, particularly, biological impurities. Distillation and RO processes are most frequently used to produce WFI water. Where ionics are an issue, WFI may not meet manufacturing requirements. Acceptable ionic content can result in a resistivity less than 1 Mega-ohm, and limits on calcium, magnesium, and chlorides refer to the color of the liquid, not specific amounts.
No single water treatment process meets all manufacturing requirements. Water softening replaces one ionic material with another. DI processes do not remove non-ionic materials, either dissolved or particulate. One industrial facility in the Northwest U.S. found fragments of salmon eggs in the effluent of a DI system. Distillation will not eliminate impurities such as organic solvents with substantial vapor pressure. RO processes separate most, but not all, dissolved material. Ultra-violet (UV) light may kill bacteria and other life forms but does not separate the dead bacteria from the water.
Getting water to be purer and keeping it pure are two separate issues. Because water is such an aggressive solvent, ultra-pure “18 Mega-ohm” water generally does not remain pure for long. Ions can leach into pure water from containers and pipes. Even small amounts of atmospheric gases, including carbon dioxide or laboratory acid fumes, can rapidly degrade the purity (and resistivity) of water.
What process is best for my application?
The answer, again, depends on knowing what you need. For cleaning of gross amounts of contaminants at an early stage of assembly, tap water may be adequate, provided that the seasonal consistency is acceptable. Keep in mind that if the process is moved to another location, the tap water may be different. Also, because dried-on contaminants tend to be more adherent, any impurities in the tap water that remain when the part is dry may be more difficult to remove at a later stage of operation. For general cleaning, perhaps a single stage DI or RO system will provide the purity and consistency needed. Many high performance applications employ systems that incorporate more than one treatment process, or have a multi-pass system to increase the purity. Any system must be properly monitored and maintained.
Barbara Kanegsberg and Ed Kanegsberg (the Cleaning Lady and the Rocket Scientist) are independent consultants in critical and precision cleaning, surface preparation, and contamination control. They are the editors of The Handbook for Critical Cleaning, Second Ed., CRC Press. Contact: firstname.lastname@example.org
This article appeared in the May 2013 issue of Controlled Environments.