- Chart: Osmosis vs. Reverse Osmosis
Ways to Wash Water
by Dennis Cheeseman
When a lithographic press experiences problems due to the water quality in its fountain solution, some form of water treatment is needed. This article will discuss what salts are present in typical city water, their effect on the lithographic process, the case for water treatment and the various water-treatment alternatives.
Typical salts used in fountain solutions include magnesium (for scratch curing), chromium (anchors gum to the plate), phosphates and citrates (prevent restart scumming), sulfates (prevent scumming), aluminum (helps phase equilibria), chloride (kills germs) and silicates (prevents tinting).
All of these salts also occur naturally in untreated water. Therefore, in order to control their levels (measured in parts per million), you must treat your water.
For example, hardness levels exceeding 200 ppm of calcium generally lead to the formation of calcium soaps or salts. Calcium soaps, because of their oil-loving nature, lead to deposits of ink in unwanted areas such as water form rollers and molleton-covered rollers. Calcium salts, because of their water-loving nature, prevent ink transfer and lead to stripping on ink rollers.
Also, severe fluctuations in the conductivity of incoming water can cause variations in the interaction between the fountain solution and ink. The dynamic surface tension of fountain solution depends on the concentration of the etch used. The higher the concentration, the lower the surface tension. Some automated feed units control the concentration of the etch to a preset conductivity. Large fluctuations in incoming water conductivity lead to variations in etch concentration, which lead to changes in dynamic surface tension. This results in changes in ink-to-fountain solution ratio, creating lithographic problems.
There are many water-treatment processes available, such as softening, de-ionization, reverse osmosis, distillation, activated carbon, micro-filtration and ultra-filtration. The first three are the most commonly used in our industry.
The softening process involves exchanging ions in water. Ions are dissolved minerals that contain an electric charge, either positive or negative. The positive ions, in the form of calcium, magnesium, iron and manganese, cause the "hardness" that is associated with water. Replacing these hardness ions with sodium ions is the process used to soften water. Softening does not lower the conductivity of the water.
Deionization is the process of removing ions from water by ion exchange. Since deionization removes both the negatively charged and the positively charged ions, materials capable of attracting both are required.
The quality or degree of deionization is generally expressed in terms of specific resistance (ohms) or specific conductance (mhos). Ionized material in water will conduct electricity. The more ions, the more conductivity and the less resistance. When ions are removed, conductivity decreases and the water quality improves.
Reverse osmosis removes the organic materials as well as ions. To fully understand it, one must first understand that in normal osmosis, water flows from a less concentrated solution through a semipermeable membrane and into a more concentrated solution (see figures).
The reverse-osmosis process uses pressure to reverse the normal osmotic flow. Thus in reverse osmosis, water flows from a more concentrated solution to a less concentrated solution, while the membrane removes contaminants. A typical reverse osmosis system rejects 90-to-95 percent of the impurities found in most water supplies.
Dennis Cheeseman is director of customer services for US Ink Corp. Phone is (201) 935-8666; fax, (201) 933-2291.
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