An oxidizing microbiocide is as any chemical that is an oxidizing agent. They are chemicals that kill and destroy not only the microorganism but also the nutrients found in cooling water. Their action is essentially "wet oxidation" or "burning" under water. No microbiological organism is immune to this action -- provided that the oxidizing biocide can reach the microorganisms.

In this month's column, I'll continue looking at common questions and answers about oxidizing biocides.

More Questions and Answers

Is hydrogen peroxide a product to consider for cooling water applications?

It is. Hydrogen peroxide is being used more frequently because it is a liquid, it degrades in water, it does not contribute any dissolved solids, and it is easy to handle. If you choose to use it, use 30 percent or more dilute -- not 100 percent active. Hydrogen peroxide has some disadvantages, so be cautious -- evaluation is necessary.

Ozone: Is it a great biocide, scale inhibitor and corrosion inhibitor? And is its use in cooling water systems expanding?

Yes and no. Actually, there is much to say about ozone use in cooling tower systems. It is not what many ozone equipment manufacturers claim. How-ever, it is a very strong oxidant -- the second strongest known to man, with fluorine topping the list. It will control bio-organisms only if it is not consumed reacting with other cooling system components or water contaminants, or stripped from the water. It is not a scale inhibitor and often causes corrosion, particularly in copper and galvanized steel. Its use is expanding slowly in cooling tower systems. Problems and failures make the net result negative expansion.

Is ozone cost-effective vs. other oxidants in cooling water systems?

From a direct cost comparison, including generation, application and maintenance of ozone equipment, the answer is no. However, site-specific restrictions on chemicals can make its use favorable. Any cost-effectiveness analysis should include ozone's impact on cooling system components, including attack of gaskets, rubber hoses, V-belts and other components.

Are oxidants effective for Legionella bacteria control?

Oxidants are the best defense against Legionella bacteria. Ozone is extremely effective when it can reach the Legionella bacteria under deposits in film fill, etc. Chlorine dioxide, chlorine and bromine must be used at free residuals of 1 mg/l or more to be absolutely effective. Continuous dosages are known to be effective, but periodic or lower oxidant levels may not provide adequate control.

What chlorine compounds are used in cooling systems, and what are their available chlorine contents?

All of the following are used:

  • Chlorine gas has 100 percent available chlorine.
  • Industrial-grade sodium hypochlorite has 12 percent available chlorine.
  • Calcium hypochlorite has 12 percent available chlorine.
  • Calcium hypochlorite (HTH) has 65 percent available chlorine.
  • Dichloro isocyanuric acid (Dichlor) has 60 percent available chlorine.
  • Trichloro isocyanuric acid (Trichlor) has 90 percent available chlorine.

To be effective, what bromine levels are needed at pH levels less than 7.5 and greater than 7.5?

Bromine use generally is the same at all pH levels. If added continuously, it often is maintained at 0.1 to 0.2 ppm as a free residual. However, if ammonia is present, then a total available bromine residual (also referred to as a combined available residual, and with ammonia, its bromamines) is maintained at 0.1 to 0.3 ppm.

If added periodically (i.e., duration of 6 to 8 hr, three times per week), the level is much the same -- 0.1 to 0.3 ppm -- because bromine and bromamines have essentially the same activity.

Does bromine deteriorate corrosion- and deposit-inhibitors more than chlorine does?

There is published information that indicates bromine deteriorates other inhibitors more than chlorine does; however, there are a number of factors that must be considered. Our experience shows very little difference. If anything, bromine is used at lower free residuals than chlorine. It is more reactive; thus, it is gone faster than chlorine but held in water (less stripped at cooling tower) longer than chlorine, and bromamines are stronger than chloramines. So, it is site-specific, not a general rule.

What residuals are common for chlorine dioxide use in cooling towers?

Chlorine dioxide residuals are usually at lower levels than bromine -- often 0.03 to 0.2 ppm, with levels of 0.05 to 0.1 ppm not uncommon. This is due to the unique properties of chlorine dioxide: It does not react with ammonia, is not affected by pH, and only reacts with certain organics. However, it is easily stripped from the water at the cooling tower and thus may be totally removed before passing through the tower. It is more expensive to use in most cases and must be generated on-site (at the cooling tower).

Why isn't hydrogen peroxide used more commonly in cooling tower systems?

Hydrogen peroxide is an oxidant that acts more slowly than chlorine, bromine or chlorine dioxide -- it takes a higher residual (usually 1 to 10 ppm). It is more costly and is seldom sold by water treatment service companies because it is available directly from manufacturers to end users.

In past years, hydrogen peroxide was used to clean up oily, gelatinous deposits and only recently has it seen increased use in cooling tower systems, spray ponds and once-through cooling water systems. It adds no dissolved solids and degrades to water and oxygen.

These guidelines are based on typical oxidant levels used. However, when selecting any oxidizing agent, site-specific conditions must be evaluated for actual effectiveness. PCE