July 2020

Water Management

Cooling water microbiological control

The authors’ previous article, “Advanced cooling tower water treatment,” published in the June issue of Hydrocarbon Processing, outlined modern chemical treatment methods for scale and corrosion control in cooling towers and associated cooling systems, which are integral components of refineries, petrochemical plants and similar facilities.

The authors’ previous article, “Advanced cooling tower water treatment,” published in the June issue of Hydrocarbon Processing, outlined modern chemical treatment methods for scale and corrosion control in cooling towers and associated cooling systems, which are integral components of refineries, petrochemical plants and similar facilities. However, an issue that can sometimes dwarf other problems is microbiological fouling.

Cooling systems provide an ideal environment—warm and wet—for microbes to proliferate and form colonies. Bacteria will grow in condensers and cooling tower fill, while fungi will grow on and in cooling tower wood, and algae will appear on wetted cooling tower components exposed to sunlight. A major problem with microbes, particularly many bacteria, is that once they settle on a surface, the organisms secrete a polysaccharide layer (slime) for protection. By itself, this film can severely inhibit heat transfer, but it also collects silt from the water and grows thicker, further degrading heat exchange (FIG. 1).

FIG. 1. Heat exchanger tubes fouled with microbes and slime.

However, this is just part of the problem. Even though the bacteria near the surface might be aerobic, the slime layer allows the anaerobic bacteria underneath to flourish. These organisms generate acids and other harmful compounds that can directly attack metals. 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. Metal loss occurs at anodes, resulting in pitting (FIG. 2).

FIG. 2. A large under-deposit corrosion pit (with deposit removed) in a stainless-steel heat exchanger tube.

Fouling is not limited to heat exchangers; cooling towers can also be very susceptible to fouling (FIGS. 3 and 4). Numerous cases of a partial or complete cooling tower collapse have been recorded over the years due to weight gain in tower fill from fouling. Treatment programs must be carefully planned and implemented to proactively prevent fouling and to maintain cooling systems in proper condition.

FIG. 3. Fouled cooling tower film fill.
FIG. 4. Severe algae growth in a cooling tower.

The first compound: Chlorine gas

Around 200 yr ago, chlorine was first used as a disinfectant in water. Although microbiology was still in its infancy, scientists began to recognize that water-borne diseases were greatly reduced when water consumed by humans was treated with chlorine. As understanding of microbiology continued to grow, chlorine’s benefits for cooling water chemistry also emerged.

Chlorine gas was the workhorse for cooling water treatment for many years. When the chemical is added to water, the following reaction occurs (Eq. 1):

Cl2 + H2O ⇔ HOCl + HCl                          (1)

Hypochlorous acid (HOCl) is the killing agent. It functions by penetrating cell walls and then oxidizing internal cell components. The efficacy and killing power of this compound are greatly affected by pH due to the equilibrium nature of HOCl in water, as shown in Eq. 2.

HOCl ⇔ H+ + OCl                                   (2)

OCl is a much weaker biocide than HOCl, probably because the charge on the OCl ion does not allow it to effectively penetrate cell walls. The dissociation of HOCl dramatically increases as the pH goes above 7.5. Since most cooling tower scale/corrosion treatment programs operate at an alkaline pH, chlorine chemistry may not be the best choice in some applications. Chlorine efficiency is further influenced by ammonia and organics in the water that react irreversibly with the chemical and increase chlorine demand.

Due to safety concerns, liquid bleach (NaOCl) feed, although more expensive, has replaced gaseous chlorine at many facilities. Bleach often contains a small amount of sodium hydroxide. When it is injected into the cooling water stream, it raises the pH, perhaps only slightly, but, if the water is alkaline to begin with, most of the reactant will exist as the OCl ion. An alternative is onsite generation of hypochlorite, which has proven to be effective in several applications.

Several factors influence the performance of chlorine or bleach-generated chlorine, and have led to the evolution of more advanced technologies. First, oxidizing biocides, such as chlorine, are very effective on free-floating organisms (e.g., planktonic bacteria). However, if gaps in the treatment, or problems with the treatment program, allow organisms to settle, some of these sessile bacteria will quickly begin to form a protective glycocalyx (slime) layer for protection (FIG. 5). The colonies may contain a variety of organisms, including aerobic, anaerobic and facultative bacteria. The slime layer can be very protective, and powerful oxidizers, such as chlorine, are consumed by the slime and do not reach the organisms underneath.

FIG. 5. Development of sessile bacteria colonies, which release organisms that can then establish colonies elsewhere in the cooling system. Photo is by an unknown author licensed under CC BY-NC.

Accordingly, it is quite important, regardless of the oxidizing biocide chosen for the application, to be proactive in preventing deposition and buildup of microbiological colonies. If these colonies become established, it can be difficult to remove them. For example, one of the authors participated in a shock chlorine treatment of a steam surface condenser at a former power plant. The condenser had become microbiologically fouled due to an upset in the biocide feed system, and condenser heat transfer had noticeably declined. The shock treatment killed the microbes, but the slime layer was so adherent that only a portion of it detached during the cleaning and subsequent rinse, such that condenser performance1 did not fully recover from the upset. A mechanical tube scraping was required shortly after to remove the remaining slime.

A serious issue that has increasingly come to the public’s attention (and certainly to the water technology community’s attention) in the last 40 yr is that of airborne pathogens—most notably Legionella, which was responsible for the original Legionnaires’ Disease outbreak in 1976, and causes infection via inhalation of water droplets or mist (< 5 micron diameter) containing the organisms. Such small droplets may come from many sources, including cooling towers, decorative fountains, potable hot water systems and shower heads, humidifiers, and whirlpools and spas, among others.2 These organisms do not grow independently, but proliferate within sessile colonies of other microbes. This makes it quite imperative to keep cooling systems clean, with an oxidizing biocide as a core treatment method. It is also important to eliminate “dead legs” in any system, where low flow conditions can keep biocides from contacting and killing microorganisms.

Another issue that has caused concern from chlorine treatment is the potential for the chemical to react with organic compounds in the water to form halogenated organics. Some of these compounds are suspected carcinogens, and guidelines have been formulated that restrict the concentration of these substances. This issue has only grown in importance, given the diminishing availability of freshwater supplies for new industrial plants, including those for power production. Common in some areas of the U.S. (California is a notable example) are mandates for the use of treated municipal wastewater plant effluent as industrial facility makeup. These supplies can introduce a variety of impurities to the cooling water, including ammonia, organics and phosphorus, among others.3

Chlorine alternatives

As previously mentioned, the killing power of chlorine falls off with a rise in pH, which is problematic, given that most scale/corrosion inhibitor programs operate in a mildly basic pH range. A popular answer to this challenge has been bromine chemistry, where a chlorine oxidizer (bleach is the common choice) and 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 an alkaline pH level. FIG. 6 compares the dissociation of HOCl and HOBr as a function of pH.

FIG. 6. Dissociation of HOCl and HOBr vs. pH. Note: At a pH of 8, approximately 80% of the HOBr remains undissociated, while only about 25% of the HOCl is still intact.

Another strong oxidizer that has seen some success is chlorine dioxide (ClO2). Unlike chlorine, ClO2 is not consumed by ammonia or organics in the water—thus, it is free to attack organisms. However, ClO2 must be generated onsite, which adds to the expense of this chemical.

Some promising alternatives include monochloramine (NH2Cl) and monobromamine (NH4Br). While these compounds are weaker oxidizers than the compounds previously outlined, they appear to be more effective at penetrating the protective slime layer that is produced by bacteria, which enables them to then directly attack these organisms.

Recently developed is a new halogen stabilizer/biodetergent that is applicable for bleach-only oxidizing treatments. This product has no biocidal properties and, therefore, does not fall under regulatory guidelines, but it is effective in stabilizing chlorine and reducing losses from irreversible reactions. The critical portion of the formulation is the biodetergent, which disperses the biofilm formed by the organisms and allows the biocide to contact the microbes directly.

In many cases, oxidizer feed is limited to 2 hr/d, which gives microbes time to settle and form colonies during off times. Accordingly, a supplemental feed of a non-oxidizing biocide on a once-per-week basis can be quite successful in controlling biological growth. These non-oxidizers, in conjunction with biodetergents, reduce overall chlorine usage and do not produce halogenated organic byproducts. TABLE 1 lists properties of some of the most common non-oxidizers.

Careful evaluation of the microbial species in the cooling water is necessary to determine the most effective biocides. Antimicrobial compounds should not be used or even tested without approval from the appropriate regulatory agency. They must be incorporated into the plant’s National Pollutant Discharge Elimination System (NPDES) permit. In addition, as with all chemicals, safety is a critical issue with biocides. Safety data sheet guidelines should be followed when handling these products.


This article provides an overview of some of the most important concerns and treatment methods for microbiological fouling control in industrial cooling water systems. It is not designed to be an absolute reference, but rather to give plant personnel a starting point for further inquiry into ideas for establishing the best program at their plant. HP


  1. Buecker, B., “Condenser Chemistry and Performance Monitoring: A Critical Necessity for Reliable Steam Plant Operation,” International Water Conference, October 18–20, 1999, Pittsburgh, Pennsylvania.
  2. Post, R., B. Buecker, and S. Shulder, “Power Plant Cooling Water Fundamentals,” pre-conference seminar, 37th Annual Electric Utility Chemistry Workshop, June 6–8, 2017, Champaign, Illinois.
  3. Post, R. and B. Buecker, “Grey Water—A Sustainable Alternative for Cooling Water Makeup,” International Water Conference, November 4–8, 2018, Scottsdale, Arizona.

The Authors

From the Archive



{{ error }}
{{ comment.comment.Name }} • {{ comment.timeAgo }}
{{ comment.comment.Text }}