By Brad Buecker
In the October 7 Power Engineering electronic newsletter, I wrote about the increasing popularity of alkaline treatment programs for recirculating cooling systems as a method to reduce or eliminate sulfuric acid feed. As with virtually anything in life, there is no free lunch. The solution to one problem generates other issues of concern. Such is the case with alkaline cooling water programs and control of microorganisms.
Cooling systems provide an ideal environment, warm and wet, for microbes. Bacteria will grow in condensers and cooling tower fill, fungi on and in cooling tower wood, and algae on wetted cooling tower components exposed to sunlight.
A problem with bacteria is that once the organisms settle on the surface of condenser tubes, they secrete a polysaccharide layer for protection. This film then collects silt from the water, growing even thicker. Even though the bacteria at the surface may be aerobic, the secretion layer allows anaerobic bacteria underneath to flourish. These bugs in turn generate acids and other harmful compounds that directly attack the metal.
Microbial deposits also establish concentration cells, where the lack of oxygen underneath the deposit causes the locations to become anodic to other areas of exposed metal. Pitting is often a result. Equally important, especially during peak operating periods, is that the biofilm is an excellent insulator. Heat transfer is greatly reduced in condensers when biofilms are present, sometimes to the point where a unit has to be derated. The loss of even a few megawatts on a hot summer day can cost a utility many thousands of dollars.
Fungi will attack cooling tower wood in an irreversible manner, which can eventually lead to structural failure. Algae will foul cooling tower spray decks, potentially leading to reduced performance and unsafe working locations.
The core of any microbiological treatment program is feed of an oxidizing biocide to kill organisms before they can settle on condenser tube walls, cooling tower fill, and other locations. Chlorine was the workhorse for many years, where when gaseous chlorine is added to water, the following reaction occurs.
Cl2 + H2O ⇔ HOCl + HCl
HOCl, hypochlorous acid, is the killing agent. The functionality and efficiency of this compound are greatly affected by pH due to the equilibrium nature of HOCl in water.
HOCl ⇔ H+ + OCl-
OCl- is a much weaker biocide than HOCl, probably due to the fact that the charge on the OCl- ion does not allow it to penetrate cell walls. The killing efficacy of chlorine dramatically declines as the pH goes above 7.5. Thus, for modern alkaline scale/corrosion treatment programs, chlorine chemistry may not be efficient. Chlorine demand is further affected by ammonia or amines in the water, which react irreversibly to form the much less potent chloramines.
Due to safety concerns, liquid bleach (NaOCl) feed has replaced gaseous chlorine at many facilities. The major difficulty with bleach is that the product contains small amounts of sodium hydroxide, thus when it is injected into the cooling water stream it raises the pH, if by only a small amount.
A popular alternative is bromine chemistry, where a chlorine oxidizer and a bromide salt, typically sodium bromide (NaBr), are blended in a makeup water stream and injected into the cooling water. The chemistry produces hypobromous acid (HOBr), which has similar killing powers to HOCl, but functions more effectively at alkaline pH. Another factor in favor of bromine is that it does not react irreversibly with ammonia or amines.
The latest improvement in this technology is development of stabilized bromine, where a supplemental chemical included with the sodium bromide helps to maintain the staying power of the biocide in the cooling water.
A supplemental method for controlling microbes is feed of a non-oxidizing biocide on a temporary but regular basis, perhaps once per week.
Careful evaluation of the microbial species in the cooling water is necessary to determine the most effective biocides. Personally, I have seen very positive results with DBNPA on lake water supplies, but this chemical should not be considered a universal killing agent. Also, none of these chemicals should be used or even tested without approval from the appropriate regulating agency. They must fit in with the plant’s National Pollutant Discharge Elimination System (NPDES) guidelines.
As with all chemicals, safety is an absolutely critical issue when handling the non-oxidizers. Adherence to all handling guidelines and use of proper personal protective equipment is a must. Many of these chemicals will attack human cells as well as those of microbes.