Abstract
Since the mid 1980's ozone as a cooling tower water treatment has evolved from a curiosity to an accepted water treatment practice. The Cooling Tower Institute (CTI) at the 1992 Spring meeting in Monterey, CA devoted one full afternoon to the topic of ozone in cooling water systems. Many of the water treatment consultants who at first were more than skeptical about the heralded benefits of ozone, now consider it another accepted tool in water treatment. The CTI has already dedicated a chapter on the use of ozone in its compendium on Oxidizing Biocides.
The National Association of Corrosion Engineers (NACE) has also recognized the growth of the use of ozone in cooling tower water applications. No less than eight committees will consider issues ranging from monitoring ozone levels, to scale and corrosion rates, to toxicity issues at the 1992 Fall Committee Week in Calgary.
Obviously ozone will not work well in all applications. High temperature and high organic loading can eliminate ozone as a water treatment consideration. Ozone alone is also not the answer to every total treatment question. Due to the individuality of every cooling water system, ozone as the biocide must be considered in the light of full treatment: corrosion and scale control, as well as bacteria control and water conservation.
In many of the success stories involving ozone, water with low levels of hardness was used. This eliminated the problems of scale formation which can occur in supersaturated water without a chemical scale inhibitor. Although ozone has often been touted as a scale inhibitor, there have been several well documented cases of exceptional scale formation. Occasionally this has resulted in system failure and replacement.
This paper will consider three locations where high levels of calcium in the feed water to the tower presented extremely high Langelier's Indices at typical cycles of concentration. To avoid the precipitation of calcium carbonate, a reverse osmosis system was used in conjunction with the ozone system. Ozone as a biocide will maintain bacteria levels on the order of municipal water, as required for the proper operation of the membranes. Reverse osmosis will eliminate the supersaturated conditions that often lead to scale formation.
