Based on typical oxidant levels used, this table provides guidelines on the effectiveness of the major classes of oxidizing microbiocides with the various types of microorganisms found in cooling water systems.

Cooling water microbiological control is important all year long -- and even more important during the warm days of summer. This is not only due to warmer water temperatures but also due to more nutrients and microbiological organisms entering the cooling tower system from the air.

There are important reasons for good microbiological control in all types of cooling systems such as in cooling towers, evaporative condensers and even closed cooling water systems. Uncontrolled or ineffectively controlled microbiological organisms can cause:
  • Biomass or biodeposits, which can lead to plugging of cooling tower fill, loss in heat transfer, creation of a hiding area for Legionella, and contamination in stagnant areas.
  • Bio-corrosion of mild steel, stainless steel, copper alloys and galvanized steel.
  • Bio-deterioration of wood, plastics and concrete. Diseases such as Legionella, Legionella-like bacteria and fungi.
Thus, it is important to know what microbiocides can or cannot accomplish under various cooling water qualities.

Oxidizing Biocides. Oxidizing biocides are very common, effective and, when properly used, often the lowest cost microbiocides in cooling water systems. Though non-oxidizing microbiocides also are effective, I will only discuss oxidizing biocides in this article.

What is an "oxidizing" microbiocide? 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.

Questions and Answers

Can chlorine compounds provide bio-control above a pH of 7.5? How about pH 9.3?

Absolutely. Chlorine is effective over a wide pH range (3.0 to 10.0) but loses its potency above a pH of 7.5. It can control bio-organisms at pH levels of 7.5 to 9.0 (even higher). Keep in mind that more chlorine -- 10 percent to 300 percent or more-- is needed at pH levels above 7.5. Chlorine also is slower in disinfecting at those higher pHs.

Are chlorine compounds more cost-effective than any other oxidant?

They are, for several reasons. The lowest cost oxidant is chlorine gas, followed by sodium hypochlorite. Even calcium hypochlorite and chloroisocyanuric acid, which are dry products, often are low cost oxidants. They are considered commodities rather than specialty chemicals and therefore are lower cost items. They work at higher pHs, with ammonia, and also in the presence of many organics.

Why are bromine systems so popular today for cooling water treatment?

Because many cooling water treatments operate at pH levels above 7.5, bromine has greater biocide effectiveness than any other oxidant. Also, when ammonia is present, the formation of bromamines, which are more active than chloramines, is advantageous.

Another advantage in using bromine is that because bromine is heavier, it is not as easily stripped by the air in cooling towers and thus stays in the cooling water longer than chlorine, and with fewer odors.

What are the different bromine systems available for cooling water treatment?

There are quite a few in use or available today, and the most common are:

  • Generation of bromine with oxidants such as chlorine compounds by reaction with bromide salt.
  • Release of bromine from an organic compound when dissolved in water (i.e., bromochlorodimethylhydantoin (BCDMH).brp>
  • Use of a liquid, stabilized bromine product.

Are there other bromine systems in use today?

Yes, generation of bromine from a bromide salt is accomplished not only with chlorine compounds but also with ozone, hydrogen peroxide and ultraviolet light. Other bromine generators are a mix of a bromide salt with a chlorine release agent. Several liquid bromine products release bromine without the need for activation, and several dry bromine compounds do the same.

When you speak of bromine, do you mean the chemical bromine (Br2)?

Actually, the term "bromine" is used incorrectly in many cases. When bromine is generated (or released) into water, it forms a mixture of hypobromous acid (HOBr) and hypobromite (OBr), depending upon the pH of the cooling water. These species of bromine are very effective biocides. We do not form "bromine" per se.

Chlorine dioxide is a strong oxidant. Is it used much in cooling water systems?

Chlorine dioxide it is not very common in cooling water systems due to relatively high costs. It must be generated on-site and is easily stripped from water. Chlorine dioxide does not react with ammonia, is a strong oxidant for some organics, and is not greatly affected by pH. It is cost effective but its use is dependent upon specific site conditions.

Look for more on bio-control in the next issue. PCE