Cooling Tower Treatment: Changes and Challenges

Cooling Tower Treatment: Changes and Challenges

By Brad Buecker, Senior Technical Publicist

The primary defense mechanism against microbiological fouling in cooling systems is an oxidizing biocide, and chlorine is the biocide most often associated with water treatment. Produced as an element by Carl Wilhelm Scheele in 1774 and confirmed as an element by Humphrey Davy in 1810, chlorine has had an enormous impact on global health since its introduction as a sterilizing agent for drinking water in the early 1900s, and for other applications even earlier. For much of the 20th century, gaseous chlorine was the primary choice of industrial cooling water oxidizing biocide because of its low cost and high effectiveness. However, safety issues and changes to cooling water scale/corrosion inhibitor chemistry have necessitated a change in biocide chemistry. 

When chlorine is blended with water, the following reaction takes place: 

Cl2 + H2O → HOCl + HCl

Hypochlorous acid (HOCl) is the agent that kills microbes. But, as the water pH rises, especially above approximately 7.5 , the following dissociation reaction occurs: 

HOCl  → H+ + OCl

The hypochlorite ion (OCl) is much less effective than hypochlorous acid as a killing agent. 

This was largely not a problem back in the middle of the last century, when the common cooling tower treatment chemistry consisted of sulfuric acid feed for scale control and sodium dichromate use for corrosion control. These programs were straightforward to control, with pH typically maintained within a 6.0–6.5 range. However, issues related to hexavalent chromium toxicity, which emerged in the 1970s and 80s, led to the abandonment of that treatment method for all open cooling water and most closed cooling water systems. In most cases, the replacement programs were based on a core chemistry of inorganic and organic phosphates (phosphonates), which typically operate at a pH range of approximately 8.0–8.5. Chlorine and industrial-grade bleach became much less effective in these more alkaline waters.  

Which brings us to today. Phosphate/phosphonate programs for scale and corrosion inhibition are now being phased out because of environmental concerns regarding phosphorus discharge, as evidenced by the many toxic algae blooms cropping up throughout the United States. Also, much more effective, phosphate-free scale/corrosion control methods, such as ChemTreat’s FlexPro® technology, are now available. However, these programs still operate at a moderately basic pH, which inhibits the effectiveness of chlorine and bleach.  

A common replacement for many years has been bleach-activated bromine, which produces the analogous hypobromous acid (HOBr). HOBr dissociates at a higher pH than HOCl and can be more effective in alkaline waters. Compounds like monochloramine (NH2Cl) (such as ChemTreat proprietary CL4515) and monobromamine (NH2Br) have also been developed. Although weaker than HOCl or HOBr, they are better at penetrating the slime produced by microbiological colonies and killing the underlying organisms.  

ChemTreat has also developed a new stabilized halogen chemistry that offers better contact properties of the biocide with improved killing efficiency. (A paper on this chemistry from a Midwestern utility was offered at the recent 39th Annual Electric Utility Chemistry Workshop.) These programs can also be supplemented with nonoxidizing biocides that help offer a “one-two” punch against cooling system microbiological fouling.   

Results are examples only. They are not guaranteed. Actual results may vary.