stored. The ozone does not persist, and it decomposes to benign molecular oxygen. The
likelihood of forming harmful byproducts is very low.
Water for Industrial Use
Water destined for industrial use may range in quality all the way from cooling
water, for which the major requirement is that it be wet, to hyperpure water used in the
semiconductor industry. For economic reasons, water destined for industrial applications
is usually treated only to meet the requirements of the intended use. As examples, water
used in food processing must be processed to destroy pathogens and boiler feedwater
must be treated to remove corrosive and scale-forming solutes.
There are a number of specific treatment operations to which industrial feedwater may
be subjected to make it suitable for a large variety of industrial applications. Dissolved
oxygen that may be corrosive can be removed with hydrazine or sulfite. Precipitants
may be added to remove specific contaminants, the most common of which is calcium
ion, Ca2+. Calcium can form harmful deposits, especially when the water is heated. The
addition of phosphate precipitates calcium, reducing the levels of soluble calcium to
very low values. In some cases, the calcium can be tolerated in the water if chelating
agents are added to prevent the precipitation of calcium solids. Scale can be inhibited
by the addition of dispersants. It may be necessary to adjust pH by adding acid or base.
Even water not destined for food use may have to be disinfected to prevent growth of
harmful bacteria, such as in cooling systems.
An important consideration with industrial water is the possibility for recycling and
sequential use. As the name implies, recycling refers to running water back through a
system for essentially the same use. Sequential use recognizes that several applications
may require water of successively lower quality. The water is first used for the application
requiring the best quality of water. The next use of the water requires a somewhat lower
quality, and the last use before discharge requires the least quality. In some favorable
cases, the water leaving the system can be applied to grass or fields for irrigation.
Recycling and sequential use are important aspcects of the green utilization of water.
Both organic and inorganic (salt) solutes can make water unsuitable for recycling.
Biodegradable organic materials can be degraded by biological waste treatment measures,
which are discussed under the category of sewage treatment below. Filtration over
activated carbon mentioned earlier in this section can remove harmful organic solutes.
The most straightforward way to remove dissolved salts is distillation in which water
is evaporated leaving the salts behind, then condensed to a pure H 2 O product. Although
used to get fresh water from seawater in some energy-rich arid Middle Eastern countries,
this approach is too costly for water recycling in most parts of the world. A more cost-
effective method of water purification is reverse osmosis, the basic principle of which
is shown in Figure 7.9. With this method, water is forced under pressure through a
semipermeable membrane that has an attraction for pure water, but a tendency to reject
dissolved salts. The purified water is readily recycled for a variety of uses, but disposal
is required of the concentrated salt brine left behind.
188 Green Chemistry, 2nd ed